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Plant nutrients are the chemicals that plants need to grow. Most plant derive the majority of their essential plant nutrients from the soil.
Macronutrients are the chemical ions (charged molecules) that plants need in order to grow. In order of amount required they are:
Nitrogen (N) Phosphorous (P) Potassium (K) Sulphur/Sulfur (S) Calcium (Ca) Magnesium (Mg) Silicon (Si)
Micronutrients are ions that plants need in relatively small amounts. The most important micronutrients are: Iron (Fe) Boron (B) Copper (Cu) Manganese (Mn) Zinc (Zn)
Nutrient availability in the Soil
An important factor in the fertility of a soil is the availability, rather than the total amount, of a given nutrient in the soil. This is a concept which is hard to understand - but in brief, the chemical ions in the soil are not always present in a form that the plant is able to use. It therefore makes most sense to talk about the availability of a particular nutrient to a crop rather than the total amount of it present in the soil. This can be a serious issue with regard to the macronutrients, where there may be large amounts of the nutrient in the soil, but the availability is very low, leading to deficiencies in plants.
Deficiencies are growth problems in crops which are caused by the low availability of plant nutrients. This can lead to poor growth, low numbers of seedlings growing from seed, the lack of a seed head, poor yields and other problems. Sometimes the high availability of undesirable chemical ions in the soil cause growth problems because they are toxic to the growing crops. Crop scientists often diagnose signs of deficiencies in plant nutrients from growth problems in plants - for example see this guide from the University of Arizona.
Solving deficiencies of plant nutrients in soil
With so many different essential nutrients, it can be difficult to manage problem soils to prevent deficiencies. However, low fertility is often solved using inorganic chemical fertilisers, often formulated to address N P and K availability. Where specific micronutrients are the cause of low crop yields, government programmes focussing on them sometimes show dramatic results, such as this one in Karnataka, India.
Other approaches can be to use organic fertilisers, which frequently contain high levels of N. However, these can be problematic as the available content of the nutrients can be relatively low - unlike the inorganic fertilisers, which have been designed to have a high immediate availability of nutrients. On the other hand, organic fertilisers often will release (make available) the nutrients over a longer period of time and will have a longer term effect.
A low soil pH may have a dramatic effect on the availability of plant nutrients, so liming is often an effective management tool. The availability of nutrients may also be affected by the drainage, aeration, salinity/sodicity and Cation Exchange Capacity of the soil.
Cation Exchange Capacity (CEC)
The CEC is an important characteristic of any soil because it affects the way that positive nutrient ions are held in the soil. Whilst a detailed explanation of this is not suitable for this page, in general a higher CEC leads to a higher potential availability of nutrients and a potentially more fertile soil. The CEC is mostly affected by the amount (and type of) clay minerals in a soil and the organic matter, so long term additions of organic matter can have a dramatic effect on the CEC and the amount of nutrients it can hold.