During the international year of soil in 2015, global attention was drawn to the importance of a healthy soil. One of the key facts emphasized by the food and agricultural organization was that sustainable management of soils can lead to a 58% increase in food production (FAO, 2015). Soil nutrient composition plays a key role in determining the goodness of a soil. A healthy soil will have all the essential elements in the right proportions to support healthy plant growth throughout its life cycle. However, when a soil has been cultivated it may require the addition of organic or inorganic fertilizers.
This article will discuss the soil nutrient composition, the sources of these nutrients and factors influencing their availability.
Soil Mineral Elements
Soil is a major source of nutrients for plant growth. Nutrients supplied by the soil are called mineral nutrients. The non-mineral nutrients such as carbon (C), hydrogen (H) and oxygen (O) come from air and water during photosynthesis. Soil mineral nutrients are separated into two groups the macro and micronutrients. The macro nutrients are further broken down into two groups the primary and the intermediate nutrients. The primary nutrients are required by plants in relatively large proportions. These are the most famous; the nitrogen (N), phosphorus (P) and potassium (K) commonly referred to as NPK. The intermediate nutrients are required by plants in medium quantities, these are calcium (Ca) magnesium (Mg) and sulphur (S).
The micronutrients are required in relatively small proportions. They include the iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl). It is important to note that though the soil nutrients are separated into different groups (based on the quantity required by the plant), each nutrient is equally important. A shortage of any nutrient can limit the growth and yield of a plant. This is in accordance with Liebegs law of the minimum.
Table 1. Forms by which nutrient elements are taken up by plants
Soil Nutrient Sources
Sources of soil nutrient include:
1. Organic matter decomposition
3. Biological nitrogen fixation
4. Inorganic fertilizer application
5. Weathering of soil rocks and minerals
What affects the availability of nutrients in soil?
Factors that affect the availability of soil nutrients include leaching, soil erosion, soil pH, denitrification, volatilization, nitrogen immobilization and crop nutrient uptake. This article will discuss some of these factors. For an exhaustive discussion of all the factors, there is a list of publications or weblinks at the end of this article.
Soil Erosion disrupts the soil structure, washes away organic matter in the soil and therefore reducing soil fertility. This often increases the need for additional and costly fertilizers to compensate for nutrient losses.
Some causes of soil erosion include:
1. Soil gradient: The steeper the gradient of a soil, the more vulnerable it is to soil erosion.
2. Soil erodibility: The more erodible a soil, the more sensitive it is to erosion. This is influenced by the soil characteristic and nature. A soil that has experienced previous erosion will have higher chances of erosion.
3. Vegetative cover: Soil vegetation can protect the soil from wind or water erosion by creating vegetative cover to the soil.
This is the washing downward of nutrients in the soil below the root zone. Some of the factors influencing leaching include:
1. Mobility of nutrients: When there is sufficient water in the soil, nutrients in soil solution can be easily washed down beyond the root zone. An example is nitrogen present in the form of nitrate; a highly mobile negatively charged ion.
2. Soil texture: Leaching occurs in soils which have high water infiltration rates and low ability to hold nutrients. Examples of such are the sandy soil and clay soil.
This process results in the gaseous loss of nitrogen to the atmosphere. Denitrification occurs in warm and anaerobic (saturated) soils usually having high nitrate levels. During this process soil microbes break down nitrate to obtain oxygen for their respiration. The end product is nitrogen gas released to the atmosphere. Ways of avoiding loss of nitrogen through denitrification include proper timing of organic or inorganic fertilizer application; such that the soil receives it when it really needs it. Crop producers are advised to apply fertilizers in splits in order to match the crop demand for nitrogen with supply.
This process also involves the gaseous loss of nitrogen to the atmosphere. When nitrogen fertilizers are applied in urea form (this could be inorganic fertilizers of animal manure), an enzyme urease catalyzes the reaction of urea with water resulting in ammonia gas released to the atmosphere. Volatilization occurs in warm and moist soil conditions. Volatilization also increases with high soil pH. To reduce nitrogen loss through this means, crop producers are encouraged to adapt methods of applying urea or ammonium based fertilizers below the soil surface.
Bibliography and Further Reading
FAO (2015) Healthy Soils are the Basis for Healthy Food Production https://www.fao.org/3/a-i4405e.pdf
Morgan RPC (2005) Soil Erosion and Conservation. 3rd Ed, Blackwell publishing, UK
Havlin J, Tisdale SL, Beaton JD, Nelson WL (2014) Soil Fertility and Fertilizers: An Introduction to Nutrient Management 8th ed. Boston: Pearson