Soil Fertility: Definition, Types and Factors | Soil Science

In this article we will discuss about:- 1. Definition of Soil Fertility 2. Types of Soil Fertility 3. Factors Affecting Soil Fertility.

Definition of Soil Fertility:

Soil fertility may be defined as the inherent capacity of soil to supply plant nutrients in adequate amount and suitable proportion and free from toxic substances. Soil productivity is the ability of the soil to produce crop per unit area. Thus a fertile soil may or may not be productive depending upon crops, marketing condition and several other factors (i.e. excessive acidity or alkalinity, the presence of toxic substances, poor physical properties or an excess or deficiency of water. But every productive soil has to be fertile. Soil productivity mainly depends on soil fertility to a great extent.

Types of Soil Fertility:

(i) Inherent or Natural Fertility:

The soil, as a nature of them, contains some nutrients which is known as ‘inherent fertility’. Among the plant nutrients, nitrogen, phosphorus and potassium is essential for the normal growth and yield of crop. Indian soil contains 0.3 to 0.2 per cent nitrogen, 0.03 to 0.3 per cent phosphorus and 0.4 to 0.5 per cent potassium. The inherent fertility has a limiting factor from which the fertility is not decreased.

(ii) Acquired Fertility:

The fertility developed by application of manures and fertilizers, tillage, irrigation etc. is known as ‘acquired fertility’. The acquired fertility has also a limiting factor. It is found by experiment that the yield does not increase remarkably by application of additional quantity of fertilizers. So it is necessary to apply fertilizer on the basis of nutrient content of a soil and it is estimated by soil testing.

Losses of Plant Nutrients from the Soil:

Soil is the store house of plant nutrients. Losses of plant nutrients from the soil are the main cause of decreasing the fertility of soil.

Plant nutrients are lost from the soil by the following ways:

(i) Removal of Plant Nutrients by Harvested Crops:

Plant absorbs nutrients from the soil and stores them in their different parts. The crops remove large quantity of nitrogen and potassium and relatively small quantity of phosphorus. Four to five per cent of total nitrogen is lost from soil per acre annually through the harvested crops. The loss can be reduced by adding farm waste materials to the soil.

(ii) Removal of Plant Nutrients by Weeds:

Weeds compete with crops for mineral nutrients. Competition begins when the supply of plant nutrients falls below the requirements of both weeds and crops. Weeds by nature of them grow fast and remove the plant nutrients from the soil. This will be acute if weeding is not done as soon as weeds emerge or germinate.

(Iii) Losses of Nutrient by Soil Erosion:

Erosion is the physical removal of top soil by water and wind. Plant nutrient, particularly nitrogen remains on the upper layer of soil. When erosion is severe, the nutrient is lost along with soil and the fertility of soil decreases accordingly.

(Iv) Losses of Nutrients by Leaching:

Fertilizers, both straight and mixed, are soluble in water and as such they are liable to loss by leaching in rain water or irrigation water. Leaching loss is more acute in sandy soil and bare soils. Nitrogen is mainly lost from soil by leaching.

(V) Losses of Nutrients in Gaseous Form:

Nitrogen is generally subjected to loss in gaseous form.

Losses of nitrogen occur due to following reduction:

(a) Denitrification:

The biochemical reduction of nitrate nitrogen to gaseous compounds is called ‘denitrification’ and is thought to be the most widespread type of volatilization.

The reactions are believed to occur as follows:

All these changes apparently are directly biochemical and are encouraged by poor aeration and drainage and the presence of abundant amounts of easily affected nitrogenous compound. The biochemical reduction of nitrate (NO₃) and nitrite (NO₂) to gaseous nitrogen is often termed denitrification.

(b) Nitrites in slightly acid solution will evolve gaseous nitrogen when brought in contact with certain ammonium salts with simple amines such as urea and even with non- nitrogenous sulphur compounds and carbohydrates. The following reaction is suggestive of what may happen to urea-

This type of gaseous loss is strictly chemical and does not require either the presence of microorganism or adverse soil condition.

Pseudomonas fluorescence, P. denitrificans, P. stutzeri, Bacillus subtitis, Thiobacillus denitrification etc. are associated with denitrification.

Factors Affecting Soil Fertility:

The factors affecting soil fertility may be of two types; i.e. – (a) Natural factor and (b) Artificial factor. The natural factors are those which influence the soil formation and the artificial factors is related to the proper use of land.

The factors affecting the fertility of soil are as follows:

i. Parent Materials:

The property of soil depends on the property of parent rock. If the parent rock contains more nutrient, the soil developed from rock contains more nutrient. The soil developed from calcareous rock contains more phosphorus than the soil which is developed from granite rock. The soil developed from acid igneous rock (i.e. quartz) and basic igneous rock (i.e. norite and dolerite etc.) become sandy and clayey in nature respectively.

ii. Climate and Vegetation:

Plant kingdom is closely related with climate. Rainfall and temperature has an effect on soil fertility. In heavy rainfall areas, the nutrients are lost by leaching. As a result of which the fertility of that soil becomes low. Besides these, the upper layer is eroded which decreases the soil fertility. Organic matter is oxidised in high temperature. For this, the fertility of soil in temperate region becomes low.

iii. Topography:

The fertility of soil is also dependent on the topography of soil. Leaching and erosion is most common in sloppy land. As a result of which, the fertility of that soil becomes low. On the other hand, the fertility of level and becomes more, because the nutrient of high land in soluble form deposited in the level land, specially in low land.

iv. Inherent Capacity of Soil to Supply Plant Nutrients:

The nutrient contents of a soil vary according to the nature of soil. The soil which contains much quantity of nutrient becomes more fertile. In an experiment in central farm, Coimbatore, it was found that the garden soil of nine inches depth contains 1400 lb (630 kg) potassium per acre. So the fertility of soil depends on the inherent capacity of soil.

v. Physical Condition of Soil:

Aeration and movement of water is good in the soil containing adequate amount of organic matter and this type of suitable condition of soil is beneficial for the growth of plant. The physical condition of soil should be suitable for the growth and development of plant. It is essential for proper supply of oxygen in the soil. Improper supply of oxygen is unsuitable for the growth of plants as well as for the proper function of soil organism.

As a result of which, organic matter do not decompose properly and the nutrients of organic matter does not transformed in available form of the plant. Suitable physical condition of the soil increases the water holding capacity of soil which is favourable for the growth of plant. The fertility of soil depends mostly on the texture and structure of soil.

vi. Soil Age:

The soil developed earlier losses its fertility gradually. Because the fertility of the soil decreases by the process of leaching and weathering in course of time. Besides this, cultivation of crops without manuring decreases the fertility of soil.

vii. Micro-Organism and Soil Fertility:

Various types of organism live in the soil. The soil organism brings the unavailable nutrients into the available form. Different types of bacteria, fungi and algae live in the soil. The nitrifying bacteria fix nitrogen from air. Dr. P.K. De, in an experiment showed that blue green algae fix 50 kg nitrogen per hectare in the paddy land having good amount of water.

viii. Availability of Plant Nutrients:

The nutrient of the soil must be in the available form of plant. The plant do not absorb nutrient if it is not soluble in water. Super phosphate applied in acid soil is converted into iron or aluminium phosphate which is not soluble in water. As a result, phosphate remains in the soil in unavailable form to plant.

The availability of nutrients depend mainly on the following factors:

(a) Soil aeration.

(b) Soil pH.

(c) Activity of microorganism.

ix. Soil Composition and Fertility:

The plant absorbs the nutrient from the soil. The nutrient of minerals becomes available by weathering. The soil containing more organic matter becomes more fertile. The sandy soil is less fertile, whereas loamy soil is more fertile.

x. Organic Matter and Soil Fertility:

The fertility of soil increases if the soil contains more organic matter. Organic matter contains the plant nutrients. Besides this, organic matter improves the physical condition of soil. Decomposition of organic matter increases the nitrogen content of soil. Thus the fertility of soil increases.

xi. Soil Erosion:

Erosion is the physical removal of top soil by water and wind. As such it decreases the fertility of soil. Because the nutrients remaining in upper layer of soil is lost by erosion and the fertility of soil decreases accordingly.

xii. Cropping System:

Cultivation of same crop year after year in the same field decreases the fertility of soil. There are various types of cropping system in India such as mono-cropping, mixed cropping, relay cropping and crop rotation. Crop rotation increases the fertility of soil.

xiii. Favourable Environment for Root Growth:

The suitable condition for growth of plant depends on physical, chemical and biological condition of soil. Soil contains 25 per cent water and 25 per cent air by its volume and this condition is favourable for good aeration. The bad aeration in the soil is not good for the growth of the crop.