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Soil quality is a term that is hard to explain, but reflects an understanding of the usability of a given soil.
In most situations, the most important use of a soil is as to grow food, so the quality of a soil is understood as the ability to grow the highest yielding crops.
A soil can be impaired in a large number of different ways. This can include the effect of the soil texture and mineralogy, the amount of soil organic matter, the droughtiness (how susceptible it is to drought), the risk of flooding, the local climate and so on. Some of these factors are discussed here.
It may also be affected by the way it has been managed and be impaired by high levels of Soil erosion, high salinity/sodicity, pollution, etc. It may also have known susceptibility to specific plant pathogens which prevent growth of crops.
It may also be considered to be of low quality due to a range of local cultural and religious factors.
Hence it is only really possible to understand the term in a holistic multi-dimensional way, including soil properties, biology, chemistry and physics, previous management and local perception.
In simplest form, the Soil Science Society of America defines Soil Quality as “the capacity (of soil) to function”. Of course, the problem with this definition is that the ability of a soil to function changes depending on what you want it to do, and one soil which functions well as an agricultural soil may be poor at another function.
Generally speaking, the highest quality soils are used for the highest valued soil activities in a given area. There is usually some kind of pyramid of behaviour, with soils that are valued less being used for lower value activities, sometimes if they are not even particularly suitable for that purpose.
It is therefore entirely possible that the whole concept is bunk.
In contrast, soil and plant nutrients are well understood chemical ions which have a measurable effect on plants. So it is possible for a soil to be highly fertile (high in measured plant nutrients) whilst being low quality for other reasons. It is also possible for a soil which is low in nutrients to be considered to be relatively high quality.
Determining the quality of the soil in practice
Before getting started to determine any soil of any particular land at at all, you may wish to first look into the plants you wish to grow. Different vegetables (assuming that is what you want to grow) will require different types of soil and (but this is a next step), different quantities of humidity/water and nutrients. Humidity/water availability can be changed by cultivation of the soil and/or by adding means to remove water from the soil more rapidly (for example, you can raise soils/beds on some parts of the land, dig trenches or ponds elsewhere, make slopes, add pebbles or sand, ...) The type of soil however, can not be changed at all or only to a very small amount.
If you work the other way around (getting any land you can get hold of for a low price or near in the vicinity of a house, ...) then obviously you don't need to look at the plants you want to grow. Actually, you won't be able to choose this at all, rather you'll need to plant whatever works on the specific type of soil you'll end up with after you purchased the land.
To get a rough estimate of the soil qualities/specifics in a country, we can first search online for maps. Basic maps should exist/be available which gives us a rough estimate of the types of soils. Such maps are only estimates however, and even in an area marked on a map, soil can be quite different in practice from what is indicated. So to get more accurate data, we'll go on-site and perform the shaking test with soil from the land. It goes as follows:
- take some soil and humidify it by adding some water, don't add too much water, only enough so the soil just doesn't stick to your fingers.
- if you can roll a roll from it, it's a loamy soil
- if you can't roll anything from it (it just breaks up to a pile), it's a sandy soil
- if you can roll a ball from it, it's a clayish soil
To determine even a bit more accurately:
- if you can make a pile with some structure (droplet) from it, it's a loam-sandy soil
- if you can roll a 10 cm long roll with cracks from it, it's a sand-loamy soil
- if you can roll a 10 cm long roll without cracks from it, it's a loamy soil
- if you can roll a horseshoe with cracks from it, it's a clay-loamy soil
- if you can roll a horseshoe without cracks from it, it's a loam-clayish soil
To determine how much the pH of the soil is, as well as how moist it is, and how much nitrogen is in it, you can
- examine the pioneer crops that came unto your soil naturally
- or buy a DIY soil tester
Tabels should exist for native pioneer plants that come on soils with different pH-levels for your country (depending on the region, there will be different species, so there's no uniform table). For central europe, you could look into the book "Handboek ecologisch tuinieren by Herman van Boxem".
For the DIY testkits, you can use a test kit that tests soil for pH, N, P and K. The tetra-test for pH you can find at fish tank/aquarium shops is also accurate. pH-teststrips and column/rod-shaped pH testers have respectively bad and moderate accuracy.
More discussion of this concept
- Groenland 1 by Bartel van Riet
- Stappen naar een ecologische tuin by Geertje Coremans
- The Shaking test