Reasons for decline in soil fertility

As crop yields have increased over the years due to the technological changes, many soils are unable to supply the amounts of nutrients required to obtain higher yields without external inputs, suggesting a decline in soil fertility.

Deficiency of macronutrients is widespread now in India. Micronutrients are also getting limited in many situations.

Declined soil fertility is an important threat to sustainable crop production.

The causes of soil fertility are discussed in the present lesson.

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.

Soil fertility

Causes of decline in soil fertility

1.Loss of top soil by erosion

2.Nutrient mining

3.Physical degradation of soil (poor structure, compaction,

crusting and waterlogging etc.

4.Decrease in organic matter content and soil bioactivity

5.Loss of nutrients through various routes

6.Soil acidification, salinization and alkalinization

7.Inefficient soil management

8.Soil pollution

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.

It is a widespread problem in low- and medium input agriculture.

At present, nutrient mining is a major threat to productive sustainable farming.

Nutrient mining is accelerated by imbalanced fertilization.contd……

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 .

3. Physical degradation of soil

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.

(contd…..)

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 (see Figure below).

Waterlogging causes loss of N through denitrification of nitrate.

Figure (on the left)(a) Root growth in a soil with no

mechanical impedance problems(b) Root growth in a soil with

mechanical impedance problems

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.

(contd…)

5. Loss of nutrients through various routes

Picture: Losses of nutrients through different routes from soilSource: http://wattsupwiththat.com/2009/10/12/fixing-the-nitrogen-cycle-in-climate-modeling/

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.

Picture (on the left): Iron deficiency in rice at higher soil pH

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.

Let Us Sum Up

Scientific reports suggest that soil fertility is declining under many

situations in India, especially under intensive cropping systems. The

reduced nutrient supply reduces crop yields. There are several causes

of decline in soil fertility, e.g. chemical degradation of the soil, losses

of nutrients from soil. 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. Nutrient mining or depletion is a widespread problem in low-

and medium input agriculture. Nutrient mining is accelerated by

imbalanced fertilization.