Original:Traditional Field Crops 8
Traditional Field Crops (Peace Corps, 1981, 283 p.)
Planning and preparation
This chapter deals with reference crop production fundamentals and current recommendations concerning cropping systems, land preparation, seed selection, and planting. The production fundamentals section describes the how, what, and why of these farm operations. The compendium section provides a current summary of reference crop production recommendations based largely on information from international research institutes and some national extension services. Although the compendium section does offer general suggestions for the various crops, agriculture is a location-specific endeavor. This section is mainly designed to show how recommendations vary according to differences in each area's physical environment and specific infrastructure.
As explained earlier, the term "cropping system" refers both to a farmer's or region's overall cropping pattern, and to the specific crop sequences and associations involved, namely:
1. Monoculture: The repetitive growing of a single crop on the same field year after year.
2. Crop Rotation: The repetitive growing of an orderly succession of crops (or crops alternating with fallow) on the same field.
3. Multiple Cropping:
a. Sequential cropping: Growing two or more crops in succession on the same field per year or per growing season, sometimes referred to as double or triple cropping. Example: Planting maize in May, harvesting it in August, and then planting beans. Only one crop occupies the field at a time.
b. Intercropping: This is the most common definition of multiple cropping and involves growing two or more crops at the same time on the same field. There are four basic variations:
Â· Mixed intercropping: Two or more crops without a distinct row arrangement.
Â· Row intercropping: Same as mixed intercropping but with a distinct row arrangement.
Â· Relay intercropping: Growing two or more crops simultaneously during part of the life cycle of each. The second crop is usually sown after the first has reached its reproductive stage (i.e., around flowering time) but before it is ready to harvest. Example: Planting a climbing bean variety alongside maize that has recently tasseled.
Â· Strip intercropping: Growing two or more crops in separate strips wide enough for independent cultivation, but narrow enough to react agronimically.
Monoculture vs. Crop Rotation
It is difficult to compare the pros and cons of monoculture versus crop rotation since much depends on the crops, soils, management practices, climate and economics involved. Monoculture is frequently blamed for soil "exhaustion" (erosion problems and declining fertility and filth) and a buildup of insects and diseases, yet this is not always the case. Some very productive areas of the U.S. Corn Belt have over 50 percent of their cropland devoted to continuous maize, which yields as well as that grown under crop rotation. In fact, Corn Belt research has shown that continuous maize grown under that region's conditions results in a less serious insect buildup than when maize is grown in a crop rotation with soybeans or pasture and hay. On the other hand, monoculture cotton in the southern U.S. in the 19th and early 20th centuries led to serious soil degradation and insect problems. Monoculture is uncommon under small farmer conditions in developing countries, since intercropping is prevalent and a variety of crops must be produced for subsistence needs. It is mainly confined to perennial cash and export crops such as coffee, sugarcane, citrus, and bananas. Whether or not monoculture is harmful depends on the type of crop and soil management and climate factors. Type of crop:
Â· Row crops which provide relatively little ground cover or return only small amounts of residues (stems, leaves, branches and other debris left in the field after harvest) to the soil are poorly suited to monoculture (i.e. cotton, peanuts, maize or sorghum grown for fodder or silage).
Â· Some crops like beans, potatoes, and many vegetables are especially prone to insects and soilborne diseases which usually build up under monoculture.
Soil Management and Climate Factors: A soil's physical condition (filth and permeability), natural fertility, and nutrient-holding ability are directly related to its organic matter (humus) content.
Â· Row crop monoculture will seriously lower soil humus levels unless all crop residues are returned to the soil along with supplemental additions of manure in sizeable amounts (around 30 metric tons/ha or more per year).
Â· The tillage and cultivation operations associated with mechanized (or animal traction) row crop production aereate the soil, which accelerates the microbial breakdown and loss of humus. That is part of the reason why many farmers in the U.S. and Europe have switched to minimum tillage systems such as plowing and planting in one operation. Minimum tillage leads to problems with weeding and herbicidal use.
Â· The problem of humus loss is especially serious in the tropics due to higher temperatures. Decomposition takes place three times as fast as 32Â°C than at 15.5Â°C.
Â· Erosion problems associated with row crops are more serious in the tropics due to higher intensity rainfall (even in semi-arid areas).
Crop rotation may or may not be beneficial in terms of soil condition, insects, and diseases. In terms of soil condition, the ideal would be to rotate low-residue crops like cotton and vegetables with medium-residue crops like corn, sorghum and rice or, better yet, with pasture, but few small farmers can afford this type of flexibility. Including a nitrogen-fixing legume crop like peanuts or beans in the rotation will not necessarily boost the soil's nitrogen content significantly, since much of the nitrogen produced ends up in the harvested seeds themselves. Some areas have experimented with green manure (legume) crops like cowpeas, which are plowed under around flowering time to add humus and nitrogen to the soil (no harvest is taken), but there are several problems with this approach:
Â· Few farmers are willing to tie up their land growing a non-harvested crop.
Â· The effect of green manure crops on soils is short-lived under tropical conditions.
Â· The green manure crop may use up soil moisture needed by the next crop.
Suggested Crop Rotation for the Reference Crops
The variables are too great to make specific recommendations of wide applicability. Much depends on the area's soils, climate, prevalence and type of intercropping, and common insects and diseases. Some general recommendations can be made:
Â· Crops which share similar diseases (especially soilborne ones like root rots) should not be grown on the same field within three years of each other. For example, peanuts, tobacco, beans, soybeans, and sweet potatoes are all susceptible to Southern Stem Blight (Sclerotium rolfsii), as well as to the same types of nematodes, and should not be grown on the same field in succession.
Â· A crop like peanuts or beans which is especially susceptible to soil-borne diseases should not be grown on the same field more than one year out of three. Again, intercropping may lessen these problems, but not always.
Â· Monoculture is less of a problem if disease-resistant varieties are available and are being continually developed in response to new disease strains.
Intercropping (Multiple Cropping)
Intercropping combinations involving two or more of the reference crops (sometimes along with others) are very common on small farms in the developing world.
Intercropping is not ordinarily suited to mechanized farming, but strip intercropping is sometimes used when multiple-row machinery can be operated.
The Pros and Cons of Intercropping
Â· Less risk since yields do not depend on one crop alone.
Â· Better distribution of labor.
Â· Some diseases and insects appear to spread less rapidly under intercropping.
Â· Better erosion control due to better ground cover.
Â· Any legumes involved may add some nitrogen to the soil.
Â· Mechanization is difficult.
Â· Management requirements are higher.
Â· Overall costs per unit of production may be higher due to reduced efficiency in planting, weeding and harvesting.
The type of muliple cropping is closely related to rainfall and length of rainy season as shown below:
Prevalent Type of Multiple Cropping
Simultaneous mixed intercropping with crops of similar maturities
Crop mixes of different maturities
Over 1000 mm
Three types of multiple cropping: sequential, simultaneous, and relay
Advances in Intercropping Systems
Multiple cropping is a diverse and complex subject whose guidelines are often very location-specific. Research interest in multiple cropping has increased markedly over the past decade with most attention being focused on cereal-legume combinations which appear to have the greatest potential, particularly maize or sorghum with beans or cowpeas. The following research results are presented not to imply their direct applicability to a given area but to provide ideas of the many factors involved in intercropping and the state of the art of these complex systems.
The National Maize Program in Zaire has been looking into maize rotations and intercropping with legumes to improve soil fertility without commercial fertilizer. Rotations using soybeans and Crotalaria (a green manure crop poisonous to livestock) have been tried. So far, Crotalaria looks superior in nitrogen-fixing ability with the succeeding maize crop, yielding up to 9000 kg/ha. Maize following a soybean green manure crop has yielded up to 6700 kg/ha. The National Maize Program has also worked with an intercrop combination of cowpeas and maize, but has not yet found suitable cowpea varieties.
Both rotations and intercropping of maize with legumes appear to offer some promise in Zaire, but there are two main problems:
Â· Legume seeds are harder to store from one year to the next under humid conditions.
Â· Even though legumes used as green manures may contribute a good deal of soil nitrogen, farmers are still likely to need fertilizer, since legumes will not do well on the lowphosphorus soils prevalent throughout much of the tropics.
Pearl millet-peanut intercropping trials by the International Crop Research Institute for the Semi-Arid Tropics (ICRISAT) in India showed yield advantages of up to 25-30 percent. An arrangement of one row millet to three rows peanuts appeared to provide the optimum balance of competition. Maize-Bean Intercropping Research: The International Center for Tropical Agriculture (CIAT) has run numerous maize-bean intercropping trails at various locations in Colombia. The trials involve simultaneous or near-simultaneous planting of the two crops rather than relay planting. Results were as follows:
Â· For the farmer, the optimum ratio of maize plants to bean plants depends not only on the relative yields but also on the maize-bean price ratiowhich ranges from from 1:2 up to 1:7 in some Latin American countries.
Â· A large number of trials involving simultaneous or nearsimultaneous plantings of maize with beans showed that bush bean yields were decreased by about 30 percent and climbing bean yields about SO percent compared to when grown alone.
Â· Maize yields were usually not adversely affected by the association with beans at a maize population of 40,000 plants/ha. Maize plant densities over 40,000/ha decreased bush bean yields by shading, while densities below 40,000/ ha lowered climbing bean yields because of inadequate support.
Â· At 40,000 maize plants/ha, relative yields of the two crops were best at bush bean densities of 200-250,000 plants/ha and at climbing bean densities of 100-150,000 plants/ha.
Â· Yields of climbing beans were highest when they were planted simultaneously with maize; bush bean yields were highest when the beans were planted one to two weeks before maize, although this caused a significant yield decrease in the maize. Results varied with temperature and the relative early vigor of the bean and maize seedlings.
In a 1976 Center for Tropical Agriculture, Research and Training (CATIE) trial in Costa Rica, intercropped populations of 50,000/ha for maize and 200,000/ha for bush beans were found to be the best combination and produced yields of 3400 kg/ha and 1800 kg/ha respectively. A 1976 study in the Minas Gerais area of Brazil by the Universidade Federal de Vicosa focused on relay intercropping of maize and beans. Maize populations of 20-, 40-and 80,000 per hectare were intercropped with climbing beans at 100-, 200-, 300- and 400,000 plants/ha. The maize was planted in the wet season, and the beans were planted between the maize rows when the maize was nearing maturity. The following results were obtained:
Â· Maize yield was not affected by the beans and was highest at 60,000 plants/ha.
Â· Bean yields were highest at the lowest maize population and were not affected by bean plant density.
Â· Even though the beans were planted as the maize was starting to dry out, the maize still exerted a strong competitive effect, mainly due to shading. When grown alone under trellising, the bean variety normally yielded 1200-2000 kg/ha at a density of 250,000/ha, but yielded 800 kg/ha when grown with a maize population of 20,000/ha. Cowpeans-Millet-Sorghum: Experience in Africa has shown that cowpea yields are reduced about 45-55 percent when intercropped with millet and sorghum. However, when grown alone, the improved cowpea varieties become more prone to serious insect attack and often require chemical pest control. Furthermore, intercropped cowpeas are not usually sown until later in the wet season and are viewed more as a bonus crop which does not reduce the millet and sorghum yields.
Improving Traditional Multiple Cropping Systems
In southeast Guatemala, small farmers usually plant maize, sorghum, and beans by hand on steep to rolling rocky land, and yields average around 530, 630 and 410 kg/ha respectively. Due to a severe labor shortage for planting at the start of the season, the farmers plant the beans in dry soil. They then overplant the maize and sorghum once the rains arrive without regard to where the yettogerminate beans are. With the local varieties used, if the beans emerge first, they will dominate the maize and sorghum; the reverse will happen if the maize and sorghum germinate first. Hoping for a balanced harvest, the farmers are in a race agains time to finish planting the maize and sorghum before the beans germinate. The main disadvantage of this traditional system is the risk that the dry-planted beans may receive only enough rainfall to germinate without sufficient additional precipitation to sustain growth (i.e., a wet season "false start").
Researchers have experimented with several alternatives. The most promising one involves strip intercropping of maize, sorghum, beans and cowpeas.
At the start of the wet season, beans are planted in strips consisting of three rows spaced 30cm apart. Sufficient space is left between these strips to accommodate sets of double (twin) maize rows with two "varas" (164.0 cm) between the centers of the twin rows. Two or more of these twin row sets of maize can be planted between bean strips depending on the desired cropping mixture. The 30 cm bean row spacing is unusually narrow but gives better weed control due to earlier inter-row shading. Also, the strips are narrow enough to be hand-weeded from the sides to avoid soil compaction or trampling the plants.
Once the beans emerge, the maize rows are planted. If the rains stop for a while after bean planting, maize sowing can-be delayed without danger of the beans dominating the young maize seedlings (one advantage of strip intercroping). The beans are a short season variety that matures in 60-65 days.
As soon as the beans are harvested, a short season sorghum variety is planted in the space between the sets of twin maize rows. Later, the nearly mature maize plants are doubled over to reduce any shading of the young sorghum plants, which are slow starters. This points the ear tips downward, preventing water entry (which favors fungal grain rots) and reducing bird damage.
About two weeks before the maize is doubled, cowpeas are sown along the outer edges of the twin maize rows (i.e., along the edges of the harvested bean strips). The leaves of the maize plants are stripped off as they die off with maturity and used as a mulch (soil covering) to conserve soil moisture. The cowpeas use the maize stalks to climb on and cause no competition due to their late planting.
Shifting Cultivation As a Cropping System
Shifting cultivation (slash and burn agriculture) is a traditional cropping system that was once widely practiced throughout the humid tropics. Due to increasing population pressure on land, the system is now mainly confined to the dense forest areas of the Amazon Basin, Central and West Africa, and Southeast Asia.
While there are some variations, shifting cultivation consists of three major steps:
1. The land is incompletely cleared by hand cutting and burning trees and other vegetation. The burning has several effects:
Â· All the vegation's nitrogen and sulfur is lost to the atmosphere as gasses. However, the other nutrients (phosphorus, potassium, calcium, etc.) are deposited on the ground as ash.
Â· Even though much organic matter is lost, a lot has already been added to the soil over the years by leaf fall and root decomposition.
Â· Burning only kills some insects, diseases, and weed seeds, not all of them.
2. Crops are grown on the land for two or three years, usually under some form of intercropping that may include long-cycle crops such as manioc (cassava) and yams in humid regions. Little, if any, tillage (hoeing, etc.) is required for seedbed preparation, since the soil is usually in good physical condition as a result of the previous fallow. The crops utilize the naturally accumulated nutrients from the fallow period. Yields are fair the first year, but then rapidly decline, causing the land to tee temporarily abandoned after several years of cropping.
3. The land is then allowed to revert to a natural vegetation fallow for 5-10 years in order to "rejuvenate" the soil in several ways:
Â· The vegetation, especially if it consists largely of trees and other deep-rooted species, recycles leachable nutrients like nitrogen and sulfur that may be carried into the soil by rainfall during the cropping and fallow periods. Some of the fallow vegetation may be leguminous and actually add nitrogen to the soil.
Â· The fallow increases the amount of soil humus which is a vital storehouse and source of nutrients, as well as being a great improver of soil physical condition.
Â· Small, but significant, amounts of nitrogen are produced by lightning, and these are added to the soil by associated rainfall.
The fallow period also helps avoid a buildup of pests and diseases. Shifting cultivation requires no outside inputs and is in complete harmony with the natural environment of the humid tropics. However, the system's success depends heavily on maintaining an adequate length of fallow cycle. As the frequency of clearing and burning increases, trees and brush are eventually killed off . and give way to a very inferior grass (savanna) fallow, which is shallowrooted, inefficient at recycling and accumulating nutrients, and very difficult to clear off for cropping. (Many tropical grass species are actually stimulated into dense regrowth by burning.) Under these conditions, slash and burn agriculture becomes a menance to the environment, causing severe deforestation, erosion, and soil exhaustion. Many areas of Central America have been denuded in this manner.
Improving Shifting Cultivation:
As explained, the system is basically suited only to the humid tropical forest zones under low population density. European attempts to replace shifting cultivation in parts of Africa with "modern" agriculture usually met with disaster (erosion, pests, diseases, and a serious decline in soil condition). Some tropical soils have an iron-rich laterite layer which may become exposed through erosion. Unless such soils are kept under continuous shade, the laterite can harden irreversibly, making them useless.
Listed below are some of the most promising possibilities for improving shifting cultivation:
Â· The "Taungya" system of Burmese origin involving agriculture and forestry; it basically consists of clearing land for a cropping cycle followed by planting fastgrowing trees to provide lumber and rural improvement. Both phases would be operating simultaneously within an area.
Â· Using fertilizers (chemical or organic) to increase yields during the cropping period.
Â· Seeding the fallow area with specially selected plants that may be more beneficial than the natural species; the improved fallow might include dense growing vining legumes or leguminous trees and shrubs.
Land preparation for cropping
On small farms, land preparation methods for the reference crops may or may not involve actual tillage (working the soil with hoes, plows or other equipment) or seedbed shaping (leveling land or making raised beds or ridges).
Methods Involving No Tillage or Seedbed Shaping
Under conditions of shifting cultivation, low management or steeply sloping or rocky soils, land is often cleared by simply slashing and/or burning, followed by making the seed holes with a planting stick or hoe. No attempt is made to actually till the soil or to form a specific type of seedbed.
Â· Slash, burn and plant: This method is most suitable for sandy soils which are naturally loose or for other soils that are maintained in good filth (a loose, crumbly condition) by a lengthy vegetative fallow which produces soil humus. It may be the only feasible method for rocky soils or those with pronounced slopes where tillage would accelerate erosion.
Â· Slash, mulch and plant: This method is suited to the same conditions. The vegetation is slashed down or killed with a herbicide and then left on the surface to form a mulch (a protective covering). The seeds may be planted in the ground or may even be scattered over the ground before slashing. The mulch is valuable for erosion and weed control, conserving soil moisture, and keeping soil temperatures more uniform. The International Institute for Tropical Agriculture (IITA) has found this system very beneficial for maize and cowpeas and has developed two types of handoperated planters capable of planting seed through a mulch.
There is nothing basically wrong with either of these methods. However, in some cases, tillage and seedbed shaping may have some important advantages:
Â· Soils prone to drainage problems due to topography, soil conditions or high rainfall usually require the use of raised beds or ridges for successful crop production (except for rice).
Â· If liming is needed to correct excessive soil acidity, it must be mixed thoroughly into the top 15-20 cm of soil to be fully effective.
Â· Chemical fertilizers containing phosphorous and potassium and organic fertilizers should be incorporated several centimeters into the soil for maximum effectiveness. Under non-tillage methods, they can still be correctly applied using a hoe or machete, but it is definitely more work. Chemical fertilizers containing phosphorus are best applied to the reference crops in a band 7.5-10 cm deep that parallels the crop 5-6 cm to one side. A fertilizer furrow can be made easily with a wooden plow or other animaldrawn implement.
Â· Most animal- or tractor-drawn planters require a tilled seedbed for successful operation. There are exceptions, however, such as the IITA planters.
Methods Involving Tillage
Tillage refers to the use of animal- or tractor-drawn equipment or hand tools to work the soil in preparation for planting and has five main purposes:
Â· To break up and loosen the soil to favor seed germination, seedling emergence, and root growth
Â· To chop up and/or bury the previous crop's residues so they will not interfere with the new crop
Â· To control weeds (an ideal seedbed is completely weedfree at planting time)
Â· To incorporate (mix into the soil) liming materials and fertilizers (chemical or organic)
Â· To shape the kind of seedbed most suited to the particular soil, climate, and crop (i.e., raised beds, ridges, flat seedbeds).
Primary tillage refers to the initial breaking up of the soil by plowing or using a heavy-duty digging hoe. Depth of plowing usually ranges from about 15-30 cm, depending on the type of plow used, its traction source, and the soil. For example, an ox-drawn wooden plow will not have the penetration ability of a tractor-drawn moldboard plow, especially in heavy soils. Secondary tillage refers to any additional tillage operations between plowing and planting to break up clods, cut up trash, kill weeds, and smooth out the seedbed. It is most commonly performed with some type of harrow (an implement used to pulverize and smooth the soil).
Secondary tillage is shallower than planting and requires less power. Ridging and bedding (forming ridges or beds for raised planting) also can be included in this category.
Reference Crop Tillage Systems
The reference crops share the same basic tillage methods, but these vary with the particular soil, the available tillage equipment, and the need for incorporating lime or fertilizer. There are three basic tillage systems, each with advantages and disadvantages:
Â· Plow (or hoe)/Plant: If plowed at the right moisture level, some soils (especially loams and sands) may be suitable for sowing with a planter without any secondary tillage to break up the clods. Most soils can be hand-planted after plowing, since the farmer has better control over seed depth than when a mechanical planter is used. He can also push any big clods aside or break them up while walking down the row. This type of rough seedbed is actually very advantageous in terms of weed control since the cloddy surface discourages their growth. It also favors moisture penetration and reduces runoff. On the other hand, if bedding or ridging is needed, a better job can be done if any large clods are first broken up by harrowing (cultivating).
Â· Plow/Harrow/Plant: This is the most common system where animal- or tractor-drawn planters are used, unless the soil breaks up well enough under plowing alone. If soil conditions are conducive to weed growth, the ground should be harrowed as close to planting as possible to give the crop a head start on the weeds.
Â· Minimum Tillage: Farmers with access to tractor- or animaldrawn tillage equipment may overdo tillage, especially through repeated harrowings to control sprouting weeds or break up clods. Killing one crop of weeds by stirring the soil only stimulates another by moving other weed seeds closer to the soil surface. Excessive tillage stimulates the microbial breakdown of humus and may further destroy good soil physical condition by over-pulverizing the soil, The machinery, animal, and foot traffic also compact the soil, impairing root growth and drainage. Tillage is seldom excessive when hand tools are used to prepare ground for the reference crops, because of the amount of labor it would involve. Slash-and-burn and slash-and-mulch methods fall under zero tillage, as do methods using specially adapted mechanical planters to sow seed into unplowed ground (common in the U.S.). The plow/plant system described above or plowing and planting in one tractor pass are examples of minimum tillage. The savings on equipment wear and fuel are advantages where tractors are used.
Tillage and Seedbed Fineness
The degree to which clods need to be broken up depends mainly on seed type and seed size and whether hand planting or mechanical planting will be used.
1. Seed type: Maize, millet and sorghum are monocots with seedlings that break through the soil with a spike-like tip. This reduces the need for a clod-free seedbed. Peanuts and other pulses are dicots, and emerge in a blunt form, dragging the two seed leaves with them; they tend to have more trouble with clods.
2. Seed size: Large seeds have more strength than small seeds, enabling young shoots to push more effectively through rough seedbeds. Maize seeds are large monocots, which gives them especially good clod handling ability. Peanuts and the other pulses are large-seeded, but this advantage is partly offset, since they are dicots. The small seeds of sorghum and especially millet are less powerful, but this is offset by the fact that they are monocots. Small seeds require shallower planting than larger ones, and cloddy soils do not allow this type of precision if mechanical planters are used.
3. Farmers can usually get by with cloddier seedbeds when hand planting. They have more control over planting depth and can push any large clods aside. In addition, it is very common under hand planting to sow several seeds per hole, which gives them a better chance of breating through.
Clayey soils, especially those low in humus, are usually in a cloddier condition after plowing than loamy or sandy ones. Most plowing takes place at the end of the dry season, when soils are very dry, which accentuates the problem. Rainfall following plowing may significantly reduce clod problems on some soils by breaking up the clods.
A plowing depth in the 15-20 cm range is usually adequate, and there is seldom any advantage in going deeper. In fact, shallower plowing is often recommended for low rainfall areas like the Sahel to conserve moisture.
In some areas, tractor-drawn sub-soilers (long narrow shanks that penetrate down to 60 cm) are used in an attempt to break up deep hardpans (compacted layers). Results are fair to poor, depending on the type of hardpan; those consisting of a dense clay layer often re-cement themselves within a short time.
About 65-80 percent of the reference crops' roots are found in the topsoil, since this layer is more fertile (partly due to its higher organic matter content) and less compacted than the subsoil. However, any roots that enter the subsoil can utilize its valuable moisture reserves, making a critical difference during a drought. Proper fertilization of the topsoil will encourage much deeper root development. On the other hand, poor drainage and excessive acidity in the subsoil will hinder or prevent root penetration.
Handling Crop Residues
There are three basic ways of handling the previous crop's residues (stalks, leaves, branches) when preparing land: burning, burying and mulching:
1. Burning--This destroys the organic matter contained in the residues, but may be the only feasible solution where suitable equipment is lacking or where time is short.
2. Burying--chopping residues up with a disk harrow or slasher and then plowing them under is a common practice in mechanized farming.
3. Mulching--Chopping up residues and leaving them on top of the ground has some definite benefits such as greatly reducing soil erosion caused by rainfall and wind as well as water losses due to evaporation. However, there are two disadvantages to mulching which should be considered:
Â· Residues are left on the surface and can interfere with the operation of equipment such as planters, plows, and cultivators which may plug up.
Â· Mulching is not recommended for peanuts, especially in wet regions, since they are very susceptible to Southern stem rot (Sclerotium rolfsii), which can incubate on unburied residues from any type of plant (see page 243).
Animal versus Tractor Power: Some Considerations for the Small Farmer
In the developing countries, tractor power and its associated equipment are mainly confined to large farms and to areas where labor costs are high. The large investment, fuel and repair costs, and maintenance requirements all weigh heavily against the purchase of such machinery by small farmers. Spare parts and the necessary repair facilities are commonly lacking, meaning that a breakdown can be disastrous. A study by ICRISAT on the economics of full-size tractors in India showed new evidence that they significantly increase yields, cropping intensity, timeliness or gross returns per hectare. Money can usually be much better spent on animal traction equipment, improved seeds, fertilizers, and other highreturn inputs.
However, there are two situations where tractor power can be justified:
Â· Animal-drawn equipment may not be sufficient to meet the production needs of the intermediate farmer who has about 5-20 ha of land. In this case, small horsepower equipment may be very suitable. The International Institute for Tropical Agriculture (IITA) farming systems program has developed a 5 hp gasolinepowered multipurpose equipment unit that can plant field crops with a two-row "punch" planter, haul 500 kg in a trailer, and convert to a walk-behind tractor for rotary tillage, ridging, brush slashing, and plowing rice paddies. Other types of low-horsepower units are available from other manufacturers.
Â· The small farmer can sometimes benefit by hiring tractor work on an as-needed basis during peak periods when his normal labor supply is insufficient to meet demands.
Basic Tillage Equipment for Plowing and Harrowing
Hand Implements: Heavy duty digging hoes can be very effective for small areas. In Kenya, for example, nearly all small holdings are prepared this way, although an average family cannot handle much more than 0.5 ha with this method. In a wet-dry climate, most land preparation takes place when the soil is hard and dry, which poses added obstaces for hand tools. Some extension services recommend that land be prepared at the end of the previous wet season before the soil dries out. However, this is not always possible due to standing crops.
A digging hoe and a heavy duty hoe blade.
One common type of wooden plow. Most of them have metal tips to reduce wear.
Wooden Plow: Designs of wooden plows go back many centuries. They often are animal drawn, and some have a metal tip. They do not invert the soil or bury crop residues but basically make grooves through the soil. Their effectiveness depends a lot on soil type and moisture content. The grooves they make also can serve as seed and fertilizer furrows.
Moldboard plow: This is the ideal plow for turning under grass, green manure crops, and heavy crop residues such as chopped-up maize stalks. It also buries weed seeds deeper and damages perennial weeds more than other equipment. Moldboard plows are available in animal-drawn models (usually just one plow bottom) and tractor models (usually two to six bottoms). Depending on plow size (width of the moldboard as viewed from the front or back) and soil condition, they will penetrate to 15-22 cm.
Unless equipped with a spring trip device, moldboards do not handle rocky soils well. They are not as well suited to drier areas as disk plows. They also encounter problems in sticky clay soils and may form a plow pan (a thin compacted layer that can hinder root growth) if used at the same depth year after year.
A moldboard plow. The moldboard section is curved so that it turns over the soil slice that is cut by the plowshare.
Disk Plow: Better suited than the moldboard to hard, clayey, rocky or sticky ground, but does not bury residues as effectively. This is an advantage in drier areas where surface residues reduce wind and water erosion and cut down moisture evaporation. Disk plows are not recommended for peanut ground where Southern stem rot is a problem, because surface plant residues harbor the spores. They also will not do an effective job turning under grass sod. Disk plows are mainly available in tractor-drawn models. Unlike moldboards plows, they are less likely to form a plow pan if used at the same depth year after year.
A ridging plow or middlebreaker for making raised beds or ridges
Ridging Plows (Lister Plows or "middlebreakers"). These basically consist of a double-sided moldboard that throws soil both ways. This will produce a series of alternating furrows (trenches) and ridges when operated over a field. Depending on the climate and soil, the crop is either planted in the furrows (in low rainfall areas with no drainage problems) or on top of the ridges (in high rainfall areas or those with drainage problems). Such furrow planting is advantageous in drier areas for cereal crops, since it conserves moisture. Soil is thrown into the row as the season progresses for weed control, and this also sets the roots deep into the soil, where moisture is more adequate. Such furrow planting is not recommended for peanuts and often not for beans due to increased root rot and stem rot problems.
These are available in tractor-powered models. They throroughly pulverize the soil and partially bury crop residues. Heavy duty models can be used for a once-over complete tillage job. The disadvantages are that power requirements are very high and the soil can be easily overworked with this implement. In fact, rototillers do a far more thorough job of seedbed preparation than is needed for the reference crops and are best used for vegetable ground.
A rototiller or rotavator. Note the revolving blades under the hood behind the wheels.
Animal-drawn disk harrow
Disk Harrows: Disk harrows are commonly used after plowing to break up clods, control weeds, and smooth the soil before planting. They are also used to chop up coarse crop residues before plowing (especially if a moldboard or disk plow will be used), but heavier models with scalloped disks (disks with large serrations) are most effective for this purpose. Both animal and tractor-drawn models are available but they are expensive and prone to frequent bearing failure unless regularly greased. Large, heavy duty versions pulled by tractors are often called Rome plows and can sometimes substitute for plowing. The gangs of disks are offset to the direction of travel so that they cut, throw, and loosen the top 7.5-15 cm of soil but pack down the soil immediately below that. Repeatedly harrowing a field prior to planting can actually leave it harder than before plowing if done when the soil is moist.
Spike-Tooth Harrows: These consist of a metal or wood frame studded with pegs or spikes; extra weight in the form of stones or logs may be needed under some conditions for maximum effectiveness. They are used to smooth the seedbed and break up clods (at the right moisture content), and are especially suited for killing small weed seedlings that may emerge before planting.
Spike-tooth harrows are made in many widths and are classified by weight and the length of the tines. In some cases, this type of harrow can be run over the actual crop rows from several days after planting up until the seedlings are a few centimeters tall to control early geminating weeds or to break up any soil crusting. Spike-tooth harrows will clog up if trash is left on the soil surface.
A spring tine harrow
Spring-Tooth Harrows: These have tines made from spring steel that dig, lift, and loosen the top 7.5-10.0 cm of soil, break up clods, and smooth out the seedbed. Both animal and tractor-drawn models are available. They are not suited to hard or trashy ground but handle stones well.
Field Cultivators: These are similar in appearance to chisel plows, but usually are not as built as heavy. They can be used for initial tillage on ground with little surface residue, but are mainly used as a secondary tillage implement for weed control. Most models are designed for tractor use.
(Additional information on the use of animal-drawn equipment can be found in Animal Traction, U.S. Peace Corps Appropriate Technologies for Development Manual Series #12, by Peter Watson, 1981.)
Beans grown on raised beds
The best seedbed shape depends more on the climate and soil involved than on the particular reference crop.
Flat Seedbeds: This shape is used where soil moisture is adequate for crop growth and where there are no drainage problems. Under such conditions, the reference crops are often planted on a flat seedbed and then "hilled up" with soil (soil is moved into the crop row and mounded around the plants) as the season progresses to control in-row weeds, provide support, and improve drainage. In warm, humid areas where stem rot is a problem, this practice is not recommended for peanuts.
Raised Seedbeds (Ridge or Bed Planting): Under heavy rainfall and/ or poor drainage, the reference crops are usually planted on ridges or raised beds to keep them from getting "wet feet". This also helps minimize soil-borne disease problems like root rots and helps control water erosion if the ridges are run on the contour. Water infiltration is encouraged and runoff minimized. In addition, ridge planting makes for easier entry of digging equipment when peanuts are harvested. Finally, more topsoil is provided for crop growth under this system. The main disadvantage of ridge planting is the accelerated loss of soil moisture from the mounds--normally not a serious problem in wet areas except during dry spells. In drier areas mulching would be beneficial. In regions where the wet season starts out slow, the crops may be flatplanted and then later "hilled up" as the rains increase. Furrow irrigation always requires ridge planting.
Furrow Planting: Under conditions of low rainfall or poor soil water-holding capacity (i.e., sandy soils), crops are often planted in the furrow bottom between ridges where soil moisture is greater. Soil can then be thrown into the furrows to control in-row weeds and improve drainage (if rainfall picks up) as crop growth progresses. This type of sunken planting is not recommended for peanuts in moist areas, since it encourages stem and root rots, particularly if soil is thrown into the row.
Note: Local farmers usually have good seedbed experience, so beware of tampering with time-tested methods without first considering all the angles and running some trials.
Equipment for Seedbed Shaping
Flat seedbeds usually require no special efforts beyond plowing and possibly harrowing. If additional land leveling is required, the small farmer without access to special tractor-drawn leveling equipment can do a satisfactory job dragging a heavy board hitched to two draft animals over the field.
Ridges or beds can be made with digging hoes, special ridging plows (see tillage equipment section) or tractor-drawn disk-bedders (rolling disks arranged at opposing angles to throw soil up to form beds). The crop can be planted either on top of the ridges or in the furrows, depending on the soil and climate.
Summary of land preparation recommendations for the reference crops
Land preparation is a very location-specific practice varying with climate, soil type, crop, management level, and available equipment. The following is a summary of the principal factors involved in choosing the most feasible and appropriate land preparation method and seedbed shape for the reference crops:
1. Seedbed Fineness (thoroughness of preparation)
Â· Maize's large seeds and spikelike emergence gives it the best clod-handling ability of the reference crops.
Â· Rough (cloddy) seedbeds discourage weed growth and reduce erosion caused by rain or wind; they also increase water retention by cutting down water runoff.
Â· The reference crops can tolerate a rougher seedbed when planted by hand than when typical mechnical planters are used.
Â· To cut down on soil compaction and other effects of overworking the soil as well as to reduce labor, machinery and fuel costs, it is best to use the minimum amount of tillage consistent with adequate seedbed preparation.
2. Tillage Depth
Â· There is seldom any advantage to plowing deeper than 15-20 cm.
Â· Shallower plowing may be advisable in drier areas to reduce wind erosion and moisture losses.
3. Crop Residue Management
Â· Leaving crop residues on the soil surface is especially advantageous in drier areas since it reduces moisture losses and wind erosion. It also reduces erosion due to rainfall and increases water retention.
Â· When growing peanuts (and sometimes beans), complete residue burial is usually recommended where Southern stem rot (Sclerotium) is a problem, since the disease can incubate on surface plant residues.
Â· With the other reference crops, surface residues may sometimes aggravate certain insect and disease problems.
4. Suitability of Equipment
Â· The moldboard plow is the most effective implement for burying crop residues and grass sod.
Â· A disk plow is better suited than the moldboard to hard, clayey, rocky or sticky ground but does not bury residues or grass sod effectively.
Â· Chisel plows are best suited to lower rainfall areas and leave trash on top of the soil. They are fairly ineffective on wet soils.
Â· Disk harrows handle clods better than spike- (peg) tooth and spring-tooth harrows but are more costly and prone to repair problems.
5. Seedbed Shape
Â· Ridge planting is recommended for all the reference crops under high rainfall or poor drainage.
Â· Flat planting is best suited to soils with good drainage. However, soil can be mounded into the crop row as growth progresses to control weeds and improve drainage if rainfall increases.
Â· Furrow planting is best suited to low rainfall areas since it conserves moisture.
Â· Peanuts and beans are especially susceptible to root rots favored by excess moisture. They should be either flat-planted or ridgeplanted.
Factors Affecting Variety Selection
The selection of a locallyadapted variety with good yield potential and acceptable grain characteristics is fundamental to successful crop production. There are several important varietyrelated characteristics that should be considered when selecting seeds:
1. Yield potential: This is related to inherent natural vigor and other characteristics listed below.
2. Time to maturity: Varieties fall into three general maturity classes: early-, medium-and late-maturing (when grown under similar temperatures). Early varieties produce a crop more quickly, but yields may be about 10-15 percent lower compared with slower-maturing types if both receive adequate moisture. However, early varieties are especially well suited to short rainy seasons or sequential cropping. Since temperature has a strong influence on a variety's actual length of growing period, some countries like the U.S. are now labeling maize varieties in terms of the growing degree days (total heat units) required for maturity rather than calendar days.
3. Elevation adaption: This has to do with a variety's time to maturity and growth ability at different elevations and temperatures. In regions with pronounced variations in elevation such as Central America, the Andean countries, and Ethiopia, maize and sorghum varieties are classified according to their elevation adaption (i.e., 01000, 10001500, etc.); a similar system may also be used for beans and other pulses.
4. Heat or cold tolerance: Varieties vary in their tolerance to excessive heat or cold.
5. Drought tolerance: Even varieties within a crop can vary considerably in this respect. In a 1978 International Maize and Wheat Improvement Center (CIMMYT) maize trial, a variety selected for drought tolerance outyielded the best fullirrigation variety by 64 percent under conditions of severe moisture shortage.
6. Resistance (partial tolerance) to insects, diseases, and nematodes, as well as to bird damage and soil problems such as excessive acidity and low phosphorus levels. Reference crop varieties can differ considerably in their tolerance to these problems, which are some of the major concerns of plant breeding work. Resistance to lodging also is an important consideration in selecting a maize variety.
7 Growth habit and other plant characteristics: For example, bean varieties can be bush, semi-vining or vining in their growth habit; millet varies in tillering ability and sorghum in its ratooning potential (see page 55). Plant height and the ratio of leaf and stalk also varies with variety.
8. Daylength sensitivity (photosensitivity) varies markedly among sorghum and millet varieties (see page 45).
9. Seed color, shape, size, storability, etc.
Traditional Versus Improved Varieties
In selecting a variety, it is important to understand the differences between traditional varieties, hybrids, synthetics, and other improved varieties.
1. Traditional (local) varieties: They tend to be relatively lowyielding but are usually hardy and have fair to good resistance to local insect and disease problems. However, most are adpated to low levels of soil fertility and management and often do not respond as well as improved types to fertilizer and other improved practices. Native varieties of maize, sorghum and millet tend to have an overly high ratio of stalk and leaves to grain, but this may be an advantage where livestock are important.
Despite certain disadvantages, local varieties may be the best choice in some situations During the first years of the Puebla maize project in Mexico (see page 80), some of the local varieties consistently outyielded anything the plant breeders could come up with.
2. A hybrid is a type of improved variety produced by crossing two or more inbred lines of a crop. This is relatively easy to do with maize and sorghum, and a number of hybrids are available to these two crops. Hybrid development in peanuts, beans and the other pulses has proven more difficult, and they are not yet generally available. Millet research is still at too early a stage for hybrids to assume much importance.
When grown under similar conditions, an adapted hybrid may out-yield the best adapted, normally-produced varieties by 15-35 percent, but not always. Despite these possible yield benefits hybrids have several disadvantages:
Â· Unlike naturally produced varieties, the seed harvested from a hybrid should not be replanted by the farmer. If reseeded, a hybrid begins to degenerate and revert back to the original (and usually less desirable) lines from which it was developed. Yields may drop as much as 15-25 percent each successive crop. Many small farmers lack the inclination or the money to buy new seed for each planting unless special arrangements and educational efforts are made.
Â· Hybrid seed may be several times more expensive than that of other types.
Â· Hybrids require good management or they may not yield much more than other types.
Â· Hybrids show a narrower range of adaptation to different growing conditions than other varieties; this makes finding a suitable hybrid more difficult. It is estimated that 131 different hybrids had to be developed to suit varying maize growing conditions in the U.S.
3. Synthetics are improved varieties that have been developed from crosspollinating lines (naturally pollinated with no purposeful inbreeding as in hybrids). These lines are first tested for their combining ability and then crossed in all combinations. Synthetic varieties often yield as well as hybrids under small farmer conditions and have several advantages over them:
Â· They have greater genetic variability than hybrids, which makes them more adaptable to different growing conditions.
Â· The seed costs less than that of hybrids.
Â· Unlike hybrids, seed harvested from a synthetic can be replanted without loss of vigor as long as farmers are willing to select it from the plants with the best characteristics.
4. Varieties improved through mass selection: This is the most elemental form of varietal improvement and consists of natural crossing between lines with no attempt made to test for combining ability (as with synthetics) and continually selecting seed from plants showing the best combination of desirable characteristics. While yields may not be as good as those from hybrids or synthetics, the seed is cheaper and also can be replanted.
Guidelines for Selecting Quality Seed
Seed quality can be influenced
by the following factors:
1. Varietal purity: Farmers who use their own harvested seed for replanting can be reasonably assured of varietal purity, especially with crops that are naturally selfpollinated (millet, sorghum, peanutes, cowpeas, beans, and most other pulses). Since maize is cross-pollinated, there is opportunity for "contamination" from other nearby maize varieties. This can be minimized by selecting seed for replanting from the inner part of the field.
Commercially available seed may or may not have good varietal purity, depending on its source and the country's commercial seed standards. In some areas, certified seed is available with guaranteed genetic purity and tested germination.
2. Germination and vigor depend largely on the seed's age and the conditions under which it has been stored. High temperature and humidity as well as insect damage (weevils, etc.) can drastically reduce both germination and vigor. Certified seed is usually labeled with a tested germination percentage, but post-tested storage conditions can make this a worthless guarantee. You should encourage farmers to run their own germination test (see page 121) before planting any seed, regardless of source.
3. Visual traits: Mold, insect damage, cracking, and shrunken or shriveled seed mean trouble.
IMPORTANT NOTE: Beans, soybeans and shelled peanuts are very susceptible to damage from rough handling of dry seeds in harvesting, processing, and shipping. Dropping a sack of beans on a cement floor is enough to harm them. Both the seedcoats and sects crack very easily; careless handling can also cause invisible damage. In both cases, these injuries can lead to stunted, malformed seedlings lacking in vigor.
4. Impurities such as weed seeds; These are more of a problem in crops with small seeds like millet and sorghum, where separation is more difficult.
5. Seed-borne diseases: Some diseases like anthracnose may show visible symptoms on contaminated seed, while many others do not. Certified seed, if grown under the proper procedures of inspection and roguing (elimination of diseased plants), is free from certain seed-borne diseases and is especially recommended for beans when available. Some common fungal diseases are carried mainly on the seed coat surface and can be controlled by dusting the seed with a fungicide; others (especially viruses) are internal and have no control (see page 250).
How to Select Home Grown Seed
Most farmers not using hybrid will set aside some of their harvested seed for replanting future crops. This is fine as long as the variety is suitable, storage methods are adequate, and seed-borne diseases are not a problem. If the guidelines below are followed, the farmers actually may be able to improve the varieties they are using or at least prevent a decline in their performance:
1. Seed selection should start while the crop is still growing in the field: Most farmers wait until after harvest to select seed for replanting and go largely by seed or ear size. Selecting maize seed from the largest ears may have little, if any, value. This is because the ear's size may be due less to the plant's inherent genetic ability than to environmental or management factors like fertility, plant density, and available moisture.
2. When selecting plants as potential seed sources, keep in mind the important plant characteristics that favor good yields:
Â· General: Resistance to disease, insects, drought and nematodes; general plant vigor, ratio of stalk and leaves to grain, and time to maturity.
Â·Maize: Resistance to lodging, extent and tightness of husk covering (for insect, bird, and water resistance), and number of well-formed ears per plant.
When selecting maize plants, make selections from well within the field to avoid possible cross-pollination, so this is not a problem with them.
3. Mark the selected plants with cloth or stakes.
4. Additional guidelines for maize: When choosing among good ears after harvest, physical differences like the number of kernel rows, kernel size, and filling of the tips and butts of the cob are relatively useless as indicators of yield potential. However, the very small, misshapen seeds at the extreme ends of the cob should be discarded. Check also for uniformity of kernel color and for insect damage.
How to Conduct a Germination Test
Farmers should be encouraged to run a germination test on seed before planting, regardless of the source. The same holds true for extension workers receiving shipments of improved seed. Germination figures that appear on seed labels can be inaccurate even if the tests were conducted fairly recently. Warm, humid conditions in the tropics can rapidly lower the germination rate. To run the test:
Â· Count out 100 seeds and place them on top of several thicknesses of moist newspaper. Spread them out enough so you can distinguish which ones have germinated.
Â· Carefully roll up the moist newspaper so that the seeds remain separated from each other and stay pressed against the newspaper. Place in a plastic bag or periodically re-moisten the newspaper to keep it from drying out.
Â· Sprouting time will vary with temperature, but you should be able to get a good idea of germination within three to five days unless it is unusually,cold. Good seed should have a germination rate of at least 80-85 percent under these conditions. Up to a point, you can compensate for low germination by planting more seed, but below a rate of 50 percent or so seedling vigor may suffer also.
It is a good idea where possible to supplement this type of test with an actual field test, since soil conditions are not usually as ideal. Plant 50100 seeds, keep the soil moist enough, and then count the emerged plants. If germination is very much lower than with the newspaper method, do some troubleshooting to see if insects or seeds may have caused problems.
The Goals of Successful Planting
When planting, farmers must accomplish four objectives in order to promote good crop yields:
1. Attain an adequate stand (population) of plants. This requires seed with good germination ability, adequate land preparation, sufficient soil moisture, correct planter calibration (adjustment) if mechanical planters are used, proper planting depth, and control of soil insects and diseases that attack seeds and young seedlings. In some areas, birds and rodents also cause problems.
2. Attain the desired plant spacing both in the row and between the rows.
3. Observe timeliness in land preparation and planting. The right time to plant depends on the crop's characteristics (i.e., peanuts should be planted so that harvesting will be likely to occur during reasonably dry weather), the onset of the rains and overall rainfall pattern, the influence, if any, of planting date on insect and disease problems such as sorghum head mold.
4. Use the right type of seedbed for the particular crop, soil, and climate (see page _).
Planting Methods And Equipment
1. Hand planting with a planting stick, hoe or machete: This is the most common method used by small farmers in the developing world.
Â· Equipment costs are negligible.
Â· Less thorough seedbed preparation is needed than for most mechanical planters. The farmer who hand plants can push large clods out of the way while walking down the row or can plant directly into untilled soil.
Â· Time and labor requirements are high: it takes three or four person-days to plant a hectare by hand.
Â· When hand planting, farmers usually put several seeds in each hole and space the holes rather far apart, partly to save labor. This practice can often reduce yields by resulting in too low an overall seeding rate and too much competition among the plants that emerge from the same hole.
2. Improvements in Hand Planting
Â· Hand-operated, mechanical "punch" or "jab" planters are available that make the planting holes and drop in the seed in one movement (the seed is automatically metered out from a reservoir). They are operated like an ordinary planting stick (jabbed or punched into the ground) but are much quicker and are also very useful for filling in any "skips" (vacancies) in larger fields. A hectare of maize can be planted in 15-20 personhours. The farming systems program of IITA in Nigeria has designed a very successful punch planter suitable for planting maize, sorghum, cowpeas, beans, and soybeans into untilled ground. It is also capable of planting through a dried mulch. The IITA punch planter can be built in a workshop with access to metal shears (no welding is needed). (Write IITA for plans.) Other types of punch planters are available commercially in some countries.
Â· Hand-pushed planters: Most models require a fairly loose clod-free seedbed for satisfactory operation. However, IITA's farming systems program has designed a very effective rolling punch planter (called a rotary injection planter, see illustration) that can be built in any workshop with welding and metal-shearing capabilities and is being manufactured by Geest Overseas Mechanization Ltd., West Marsh Road, Spalding, Lincolnshire PE11-2BD, England (price is about U.S.). The rotary injection planter uses the same principles as the hand punch planter, but has six punchinjection devices on a rolling wheel plus a following press wheel to firm down the seed row. The standard design produces a seed spacing of 25 cm, but alternate rollers can be made for different spacings. The rotary injection planter is also available as a four-row, hand-pulled model for planting direct-seeded rice.
Â· Hand planting into furrows made with an animal- or tractordrawn implement: A wooden plow, cultivator shank or other implement can be used to make seed furrows in plowed ground. If certain precautions are followed, the fertilizer can be placed in the same furrow (see page 157).
Â· Reasonably parallel crop rows are required if weeding is to be done with an animal- or tractor-drawn cultivator. Farmers can easily construct a parallel row "tracer" consisting of a wood or bamboo frame with hardwood or steel teeth for marking out rows. (A design for this handy implement can be found in the Peace Corps' Animal Traction manual.)
Â· Improved seed-spacing accuracy can be achieved by running a rope or chain down the row with knots or paint marks to indicate the proper spacing. Otherwise, farmers commonly make large errors in spacing when using planting sticks or dribbling out seeds into a plowed furrow.
3. Animal- and tractor-drawn mechanical planters are available in many different models. A farmer using a onerow animal-drawn planter can sow about 1-1.5 ha in a day and about 5-8 ha using a two-row tractor-drawn planter.
Here are some important considerations concerning these types of planters:
Â· Most mechanical planters require a more thoroughly prepared seedbed than is needed for hand planting. Some models have special soil openers that permit satisfactory operation in hard or cloddy soil.
An animal-drawn planter with a separate hopper for banding fertilizer
The IITA designed hand-operated "jab" or "punch" planter, which can be made in a workshop. The attached metal bracket firms the soil over the seed and spaces the next seed insertion.
The rolling "punch" planter developed by IITA and now manufactured commercially. It also can be built in a workshop.
A hand-pushed fertilizer band applicator. This model places the fertilizer below the soil surface, which is essential for phosphorusbearing fertilizers. The attachment at the left is used to close the furrow but usually is not needed.
Â· The farmer must be able to calibrate (adjust) the planter so that it drops the seeds at the correct intervals along the row (see page 135).
Â· Some models have attachments for applying fertilizer in a band beneath the soil and slightly to the side of the seed row. This is an especially effective method for fertilizers containing phosphorous.
Farmers using planters without fertilizer applicators will often broadcast the fertilizer and plow it under before planting or leave it on top of the ground; this should not be done with fertilizers containing phosphorus! Farmers buying mechanical planters should be encouraged to purchase a fertilizer attachment if available and effective. (NOTE: The applicator should not dribble the fertilizer on top of the ground or place it in direct contact with the seed.)
Plant Population And Spacing
Both plant population and spacing affect the yields of the reference crops, and extension workers should understand the relationships.
Plant Population and Its Effects on Crop Yields
Â· Up to a point, crop yields will increase along with increases in plant population until the competition for sunlight, water, and nutrients becomes too great.
Â· Excessively high populations will reduce yields, encourage diseases, and seriously increase lodging in maize, sorghum and millet by promoting spindly, weak stalks.
Â· Excessively low plant populations will cut yields due to unused space and the limitations on maximum yield per plant.
Â· Under most conditions, changes in plant population will not affect yields as much as might be expected. This is because most crops have a good deal of built-in buffering capacity, especially if the population is too low. In this case, the plants respond by making yieldfavoring changes such as increased tillering (millet, sorghum) and branching (peanuts, other pulses), more pods or ears per plant or larger ears or grain heads. In maize, a plant density that is 40 percent below the optimum for the given conditions may lower yields by only 20 percent.
Â· Plant population changes have a more pronounced effect under conditions of moisture stress
What is the ideal plant population?
There is no easy answer to this, because the optimum plant density depends on several factors:
Â· Type of crop and variety: Because of differences in plant size and architecture, crops and their varieties vary in their tolerance to increasing plant populations. For example, early maturing maize varieties are usually shorter and smaller than later maturing ones and therefore may benefit from higher plant densities. Beans and cowpeas respond well to populations three to four times higher than for maize due to their smaller plant size and a growth habit that favors better light interception.
Â· Available soil moisture: The optimum plant population density varies directly with rainfall and the possibility of moisture stress. Plant population has a stronger effect on yields under low moisture conditions than when moisture is adequate. This is because increased populations also increase water use, although plant spacing can make a difference. This is particularly true for maize and sorghum, because yields can be significantly reduced by relatively small increases in plant population when grown under moisture stress.
Â· Available nutrients: Adequate soil fertility is especially essential with high plant populations. In fact, fertilizer response is often disappointing when plant populations are too low for the given conditions. In fact, this is one of the main reasons that small farmers often do not get their money's worth out of fertilizer. An ear of maize can only grow so big, and even high rates of fertilizer can not make up for too few ears produced by a small number of plants.
Â· Management ability: High populations require more soil fertility and moisture as well as better overall general management.
Plant Spacing and Its Effects on Crop Yields
The reference crops are row crops for some very good reasons. A row arrangement permits quicker and easier weeding and facilitates most other growing operations. Row cropping with its handy space for equipment, animal, and human traffic allows for ease of mechanization and handling, no matter what the level of sophistication. Distributing a given plant population over a field involves both plant spacing within the row and the distance between the rows (row width).
Plant spacing within the row: The number of seeds that need to be planted per meter or foot of row length depends entirely on the plant population and row widths that have been chosen according to recommendations. The main concern then becomes whether hill planting or drill planting should be used. In drill planting mechanical planters drop seeds out one at a time along the row. Small farmers who hand plant their crops usually use hill planting, sowing several seeds per hole and spacing the holes rather far apart. This reduces time and labor and also may improve seedling emergence under crusty soil conditions, but it may lower yields somewhat because of inefficient use of space and increased competition between the plants within a hill for sunlight, water, and nutrients.
Row Width: Space between rows is determined by the type of equipment used as well as by plant size or "spread". The use of tractor-or animal-drawn equipment requires more space between rows (wider row widths) than when only hoes and backpack sprayers are used. Beans can be spaced in narrower rows than maize or other tall crops and still be weeded with an animal-drawn cultivator without knocking the plants down. Row width influences crop yields in four ways:
Â· As row width is narrowed, the plants can be spaced farther apart within the row and still maintain the same population. Up to a point, this makes for better weed control due to earlier and more effective between-row shading by the crop.
Â· Narrower rows allow for higher plant populations without overcrowding.
Â· As row width is widened, plants have to be crowded closer together within the row in order to maintain the same population. This may lower yields.
Should the use of narrower rows be encouraged? Here are some things to consider:
1. Switching from 100 cm to 75 cm rows in maize and sorghum may increase yields by as much as 5-10 percent when total plant population is kept constant.
When grown alone, bush beans and bush cowpeas are ususally planted in narrow rows (45-60 cm) by most small farmers. Under good management and yields, most studies have not shown much advantage to reducing peanut row width below 75-100 cm. Given the marginal moisture conditions under which millet is grown, row widths less than 75-100 cm are unlikely to be advantageous.
2. Row width and moisture use: Although narrower rows cut down water evaporation from the soil surface because of earlier and more complete soil shading, this is often negated by increased plant water use (transpiration) due to better leaf exposure to sunlight. Under low moisture conditions, plant population has a much greater influence on water use than row width.
3. It is doubtful that a 5-10 percent yield increase will have much of an influence on small farmers whose yields are fairly low. Even if yields are good, switching to narrower rows may cause more problems than it is worth:
Â· Narrow rows cost the farmer more in terms of time, seed, and pesticide. That is because the narrower the row width, the greater the total amount of row length per hectare or other land unit, since there are simply more rows to deal with.
Â· If tractor-drawn equipment is used, overly narrow rows may increase plant damage from tractor tires and passing equipment as well as increase soil compaction near the row zone. If several crops are being grown under tractor cultivation, it is more convenient to settle on a standard row size rather than be constantly readjusting tire spacing, tire size, and cultivator tine spacing. Remember, too, that row width must be kept wide enough to permit tractor cultivation (weeding). This cannot be done by relying solely on herbicides!
Summary of Plant Population and Spacing Studies Conducted with the Reference Crops
MAIZE: Overly high populations cause increased lodging, barren stalks, unfilled ears, and small ears. Dry, husked ears weighing more than 270-310 g indicate that plant population was probably too low for the conditions and that yields might have been 10-20 percent higher. Ear size of prolific (multiple-ear) varieties will not vary as much with changes in plant density as will single-ear varieties; rather the number of ears per plant will decrease as density increases.
Hill versus drill planting: Numerous trials with maize have shown yield increases of 0-13 percent when drill planting (one seed per hole) was substituted for hill planting at two to three seeds per hole. However, lodging appears to be more of a problem with drill planting. Farmers who are hand planting four to six seeds per hole should be encouraged to switch to two to three seeds per hole and space the holes close enough together to achieve the desired plant population. It is doubtful that switching to drill planting is worth the extra labor involved under hand planting.
Under adequate moisture and fertility, optimum plant populations vary from about 40,000 to 60,000 per hectare. Plant size, management, fertility, and the varietiy's tolerance to plant density and available moisture must be considered when making population changes. Studies also show that overly high populations have a negative effect on maize yields when moisture is low.
SORGHUM: Optimum plant population varies markedly with available water, plant height, tillering ability, and fertility. In varieties that tiller well, plant population is less important than with maize since the plants can compensate for overly low or high populations by varying the production of side shoots.
In West Africa, the improved long-season photosensitive and the improved short-season non-sensitive varieties are sown at the rate of 40,000-80,000/ha under good management; the dwarf photosensitive, long-season varieties are sown at rates of 100,000/ha or more.
All the above populations are based on monoculture.
PEARL MILLET: In West Africa, millet is planted in hills usually a meter or more apart; many seeds are sown per hill, and thinning takes place two or three weeks later. This involves much hand labor and is seldom finished before serious competition has taken place. Trials in Upper Volta by ICRISAT showed that millet germinated best when planted at many seeds per hill and that hill planting outyielded drill planting. However, other ICRISAT work in West Africa showed no difference between yields from hill and drill plantings.
Population and spacing: In West Africa, pearl millets of the Gero type are often interplanted at populations of 7500-8500 plants per hectare with two to three other crops. The taller, long-season Maiwa types are sown at 40,000-80,000 plants/ha when planted alone under good management. For improved dwarf Geros, populations over l00,000/ha are recommended.
Most varieties have a strong tillering ability and adjust themselves to varying densities through changes in tiller production. Within limits, yields are not greatly affected by plant population changes.
PEANUTS: In parts of West Africa, peanuts are frequently interplanted in combination with sorghum, millet, and maize. Because peanuts are the most valuable, the tendency is to keep the cereal population down to about 3000-6000 hills per hectare and the peanut density high at about 30,000 hills per hectare, or about the same as under sole cropping.
In West Africa, the recommended plant population for improved varieties grown alone ranges from about 45,000-100,000/ha. Rows vary from 24-36 inches (60-40 cm) and seed spacing in the row from 15-25 cm. For the Virginia types populations of 45,000-60,000/ha have been found to be optimum, with higher populations recommended for the Spanish-Valencia types.
Early studies in the U.S. in the 1940's and 1950's obtained yield increases of 30-40 percent by switching from row widths of 90 cm to 4560 cm. At that time, however, average yields were relatively low (1550 kg/ha). As yields have increased over the years, the importance of row width has diminished considerably, and most U.S. growers are using 75-95 cm widths with one seed every 10 cm. A stand of one plant every 15-20 cm is felt to be adequate, but overplanting is needed to make up for any losses.
Two new developments may influence row widths: 1) smaller-size, dwarf varieties which will not fully spread out to cover a 90 cm row width. 2) Plant growth regulators like Alar, which are internode shorteners (internodes are the spaces between nodes on the stem and branches) which decrease plant size and are especially suited to runner types.
BEANS: The International Center for Tropical Agriculture (CIAT) studies in Colombia have shown that bush beans grown alone give highest yields in spacings of either 30 cm between rows and 9 cm between plants 45 cm between rows and 60 cm between plants (equivalent to about 400,000 seeds/hectare). A yield plateau is usually reached around 200,000250,000 actual plants per hectare, but stand losses from planting to harvest are often in the 25-40 percent range, meaning that considerable overplanting is necessary. High density plantings also appear to increase the height of the pods from the ground, which lessens rotting problems. However, very narrow rows aggrativate Sclerotium stem rot where it occurs.
Studies by CIAT and the Center for Tropical Agriculture, Research and Training (CATIE) indicate that plant populations for bush beans in the range of 200,000250,000/ha are also ideal when grown together with maize.
Trials with climbing beans show that final plant populations of 100,000-160,000/ha are optimum, whether grown alone with trellising or with maize.
COWPEAS: In West Africa, improved cowpea varieties of the vining type are grown at population densities ranging from 30,000100,000/ha in rows 75-100 cm apart.
CHICKPEAS: An ICRISAT study showed that yields remained relatively stable over wide ranges of plant density (4-100 plants per square meter).
Guidelines for Attaining a Good Stand and The Desired Spacing
Eight key factors largely determine whether a farmer actually ends up with a good stand of plants and the right spacing for the conditions:
Â· Seed-germination ability
Â· Percent of overplanting
Â· Planting depth
Â· Seedbed condition (clods, moisture, etc.)
Â· Seedbed type (flat, furrow or ridge planting)
Â· Accurate measurement when hand planting and calibration of mechanical planters
Â· Soil insects and diseases
Â· Fertilizer placement.
Always run a germination test (see page 121 before planting; good seed should test out at close to 90 percent. Up to a point, overplanting will compensate for lower germination, but seed testing below 50 percent germination should not be used since seedling vigor is also likely to be affected.
Percent of Overplanting
No matter how well the seed germinates, the farmer should still overplant to make up for any added plant losses due to insects, diseases, birds, and weeding operations. When using good seed, it is usually a sound practice to overplant by 15-20 percent in order to assure the recommended final stand of plants at harvest. Of the reference crops, beans, cowpeas and peanuts are likely to suffer the greatest plant losses and will usually benefit from even higher overplanting. Much depends on the specific growing conditions. High rates of overplanting (500 percent or more) followed by heavy hand thinning are a standard practice when field planting small vegetable seeds like cabbage, tomatoes, and lettuce. This is not recommended for the reference crops, since their seeds are larger, hardier, and more vigorous in their early growth. Labor and seed costs are excessive with high overplanting and thinning. Millet is commonly thinned in West Africa after being overplanted heavily, but this should be discouraged.
Optimum planting depth varies with the crop, soil type (sandy versus clayey), and soil moisture content. Seeds should be placed deep enough so that moisture is available for germination, but shallow enough so that seedling emergence is not impaired. Local farmers should be regarded as the ultimate authority on the best planting depths, but here are some general guidelines:
Â· Seeds can be planted deeper in sandy soils than in clayey soils without reducing plant emergence.
Â· Planting depth should be deeper under low soil moisture conditions.
Â· Large seeds have more emergence strength than small ones, but this is also affected by the seedling's structure. Maize, millet, and sorghum push thorugh the soil with spikelike tips which aid emergence. Peanuts, beans, and the other pulses emerge in a much more blunt form.
Normal Ranges in Planting Depths for the Reference Crops
Peanuts, Beans, Cowpeas.
Cloddiness and soil moisture will affect germination. Some soils, especially those high in silt, tend to form a hard surface crust when drying out after a rain. This can sometimes seriously reduce emergence, especially for the pulses. If necessary, these crusts can be broken up with a spiketooth harrow or other homemade device.
Seeds should be in reasonably firm contact with moist soil. Most tractor-drawn planters have steel or rubber "press" wheels running behind them to help improve seed and soil contact. (See page 105 for more information on seedbed preparation.)
Most of the crops can be flat-, furrow- or ridge-planted according to the particular soil and climate conditions. Good drainage and freedom from ponding (standing) water is especially vital for peanuts, beans, and cowpeas, which are particularly susceptible to root and stem rots. They should be flat-planted where drainage is good or sown on top of ridges or beds where drainage is poor. If flat-planting, care should be taken not to form a depression along the seed row where water could collect. This is a problem where mechanical planters with heavy press wheels are used, but can be avoided by using wider press wheels and throwing extra soil into the row ahead of the planter with cultivator sweeps.
Planter calibration; accuracy of hand planting
Mechanical planters must be calibrated (adjusted) prior to planting to assure that they space the seeds out correctly.
Hand planting is prone to large errors in row width and seed spacing unless some effort is made to assure accuracy. The use of a planting rope or chain along the row with knots or paint marks to indicate the spacing is recommended.
Soil insects and diseases
Seeds may need to be treated with a fungicide dust to help control seed rots which are especially serious under cool, wet conditions. Seed or soil treatment with an insecticide may also be needed to protect against damage from insects that attack the seeds and young seedlings.
Fertilizer placed too close to the seeds or in contact with them may prevent or seriously reduce germination. This depends on the kind, amount, and placement of the fertilizer (see page 161).