Soil fertility to the ability of the soil to supply essential plant nutrients and soil water in adequate amounts and proportions for plant growth and reproduction in the absence of toxic substances which may inhibit plant growth.
1. Loss of top soil by erosion: The top soil is rich in nutrients and organic matter. Loss of the fertile topsoil components through erosion by water and wind results in decreased fertility. Soil erosion is very common in many parts of the country. The basic causes of soil erosion are the result of human activities such as deforestation, overgrazing and poor soil management.
2. Nutrient mining: Removal of more nutrients by crops than added through manures or fertilizers is called as nutrient mining or depletion. At present, nutrient mining is a major threat to productive sustainable farming. It is a widespread problem in low- and medium input agriculture. Nutrient mining is accelerated by imbalanced fertilization.
3. Physical degradation of soil (poor structure, compaction, crusting and waterlogging etc.: Physical degradation of soil (poor structure, compaction, crusting and waterlogging) reduces soil fertility. Soil structure is a variable soil property that, from an agronomic point of view, can improve or deteriorate. High disturbance of soils (tillage) often results in some structural deterioration, which can be reversed to different degrees.
4. Decrease in organic matter content and soil bioactivity: The decreased levels of organic matter causes a strong reduction in soil fertility as it plays several roles in soil. Decreased organic matter levels results in poor physical, chemical and biological properties. The microbial activities in soil decrease due to reduced levels of organic matter. These microbial activities play significant role in nutrient availability and recycling.
5. Loss of nutrients through various routes: Losses of nutrients from soil can be caused by soil erosion, leaching, crop removal or in the form of gases (as in case of N and to a lesser extent S). Nutrient removal by crop products compared with external nutrient inputs can be similar, higher or lower. Negative nutrient balances result where nutrient removals exceed nutrient additions. Excessive rainfall, or excessive irrigation, resulting in the passage of water through the soil profile through deep percolation will carry with it soluble nutrients, particularly nitrate, sulphate and boron, etc.
Waterlogging causes loss of N through denitrification of nitrates. Ammonia volatilization from urea and some ammonium-containing fertilizers results in losses of nitrogen from soil.
6. Soil acidification, salinization and alkalinization: Acidification, salinization or alkalinization causes reduction in soil fertility, and eventually lead to problems of nutrient deficiencies, toxicities and imbalances. Factors responsible for soil degradation are generally interrelated.
7. Inefficient soil management: Poor or inefficient soil management results in decreased soil fertility. Improper crop rotations followed may decrease the soil fertility tremendously. Excessive soil tillage leads to erosion of the soil, which leads to reduced soil fertility.
8. Soil pollution: Soil pollution caused by indiscriminate use of agro-chemicals and heavy metals reduces fertility of soil by affecting the soil biological properties. The growth of useful soil organisms is adversely affected, which eventually causes a decline in biological soil fertility.