1. Reasons for decline in soil fertility

2. Reasons for decline in soil fertility
Introduction
Soil fertility is a component of overall soil productivity that deals with its available nutrient status, and its ability to provide nutrients out of its own reserves and through external applications for crop production.

3. Reasons for decline in soil fertility
Causes of decline in soil fertility
Loss of top soil by erosion
Nutrient mining
Physical degradation of soil (poor structure, compaction, crusting and water-logging etc.
Decrease in organic matter content and soil bioactivity
Loss of nutrients through various routes
Soil acidification, salinization and alkalinization
Inefficient soil management
Soil pollution

4. Reasons for decline in soil fertility
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.

5. Reasons for decline in soil fertility
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.

6. Reasons for decline in soil fertility
2. Nutrient mining
It results in the exhaustion of any nutrient required in moderate to large amounts.
Nutrient mining could be severe in the case of N, P, K and S depending on soil nutrient reserves and the amounts replenished.
The problem is more acute in areas where high yielding varieties of crops are being cultivated

7. Reasons for decline in soil fertility
3. Physical degradation of soil
Physical degradation of soil (poor structure, compaction, crusting and water-logging) reduces soil fertility.
Soil structure is a variable soil property that, from an agronomic point of view, may improve or deteriorate.
High disturbance of soils (tillage) often results in some structural deterioration, which can be reversed to different degrees.

8. Reasons for decline in soil fertility
3. Physical degradation of soil
Soil compaction decreases the number of large pores (> 100 μm) and, as these are the ones through which roots grow most easily, compaction can have an adverse effect on root growth
Water-logging causes loss of N through denitrification of nitrate.
Figure : (a) Root growth in a soil with no mechanical impedance problems and (b) Root growth in a soil with mechanical impedance problems

9. Reasons for decline in soil fertility
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.

10. Reasons for decline in soil fertility
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.

11. Reasons for decline in soil fertility
5. Loss of nutrients through various routes
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.
Water-logging causes loss of N through denitrification of nitrates.
Ammonia volatilization from urea and some ammonium-containing fertilizers results in losses of nitrogen from soil.

12. Reasons for decline in soil fertility
Picture: Losses of nutrients through different routes from soil
Source:

13. Reasons for decline in soil fertility
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.
Iron deficiency in rice at higher soil pH

14. Reasons for decline in soil fertility
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.

15. Reasons for decline in 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.