Crops need nutrients just like people do. In Soil fertility A fertile soil will contain all the major nutrients for basic plant nutrition (e.g., nitrogen, phosphorus, and potassium), as well as other nutrients needed in smaller quantities (e.g., calcium, magnesium, sulfur, iron, zinc, copper, boron, molybdenum, nickel). Usually a fertile soil will also have some organic matter that improves soil structure, soil moisture retention, and also nutrient retention, and a pH between 6 and 7. Unfortunately, many soils do not have adequate levels of all the necessary plant nutrients, or conditions in the soil are unfavorable for plant uptake of certain nutrients.
Soil fertility: Soil fertility may be defined as the ability of soil to provide all essential plant nutrients in available forms and in a suitable balance whereas soil productivity is the resultant of several factors such as soil fertility, good soil management practices availability of water supply and suitable climate.
Some soils are productive and they support luxuriant growth of plants with very little human effort whereas others may be unproductive which support almost no useful plant life regardless of every human effort In order for soil to be productive, it must:
(i) Be easily tillable and fertile
(ii) Contain all essential elements in the forms readily available to plants in sufficient amount, and
(iii) Physically good to support plants and contain just the right amount of water and air for proper root growth. The soil must supply these essentials every day in the life of the plant.
Soil Fertility Factors :
Several factors are known to govern the fertility of soil. Some of the important factors are discussed below:
1. As a result of cropping, a large amount of organic matter and soil minerals are removed and if the normal cycling of mineral elements is retarded, loss in soil fertility may result.
2. Besides cropping, soil erosion and loss of water also causes tremendous loss of plant nutrients from the top soil.
Generally, water is lost through leaching, drainage, evapotranspiration and runoff.
Generally, water is lost through leaching, drainage, evapotranspiration and runoff.
The following adverse affects are observed in Soil fertility due to water loss
(i) Soil becomes very hard.
(ii) The seed germination percentage decreases.
(iii) The nutrients in the soil leach or evaporate.
(iv) The root growth retards, so that plants become stunted and, as a result, the yield is reduced.
(v) Stomata become closed, as a result of which accumulation of gases or metabolic wastes increases in plant tissues leading to death of the plant.
(vi) The activity of soil-micro organisms decreases.
3. Conversion of organic forms of nitrogen locked in humus into ammonia gas and nitrogen gas and leaching out of soluble nitrates and nitrites from surface soil greatly affect the fertility status of soil.
4. Like deficiency, the abundance of certain nutrient elements in soluble form may also be toxic and even the elements, say alkalies, essential for plant growth may be toxic if present in excess. Flowering plants do not grow in the soil containing more than 6 per cent NaCl and other salts. The elements are not equally toxic and the various species of plants differ in their susceptibility to different elements.
5. Toxic chemicals and pesticides in Soil fertility
Several agricultural chemicals being used for controlling various diseases and insect pests are highly toxic and their application adversely affects the soil micro flora and fauna. Prolonged persistence of these pesticides in soil is bound to lower the soil fertility both directly and indirectly.
The soils may be alkaline or neutral or acidic in their reaction. Some plants find acid soil unsuitable for growth and other plants find alkaline ground un-favourable. pH value of soil solution determines the availability of certain plant nutrients and thus it has bearing on soil fertility problem. Increase in the acidity of the soil makes mineral salts more soluble in soil solution and thus salts may become available in concentrations that may be highly toxic or may damage plants growing in such soils. Janick et al. (1969) have demonstrated that high concentration of both iron and aluminium may damage plants growing in acid soils.
Maintenance of Soil Fertility:
Soil fertility is the most important asset of a nation. Maintenance of soil fertility is an important aspect of agriculture. The soil fertility problem has been studied in many countries and scientists have brought to light several facts concerning soil fertility and its maintenance.
Soil fertility is of two types;
(a) Permanent soil fertility:
It is derived from the soil itself. It can be improved, maintained or corrected by soil management practices.
(b) Temporary soil fertility:
It is acquired by suitable soil management but the response of built up soil fertility is highly dependent on the degree of permanent fertility which is already there. Several methods are known for controlling the loss of soil fertility. Here only the important methods are discussed.
1. Application of Organic Manures and Chemical Fertilizers:
Plants absorb water and minerals from the soil, which is essential for growth, flowering, crop yield, and other vital activities. Soil is a store house for organic and inorganic plant nutrients. Some soils are rich in organic and humus content and are considered to be fertile and more productive while others that are deficient in humus and minerals are less productive.
The soil is subjected to a continuous depletion of nutrients due to its continuous use by crops. This requires the addition of mineral resources. The various soil components are being removed by living organisms and are returned to the soil by death and decay of organisms. If the rate of removal or loss of minerals is greater than the rate of addition, the soil will naturally become less fertile. The minerals of the soils are lost due to crops, leaching or soil erosion.
The minerals are often removed from the top layer by rainwater. Cultivation of crops regularly, year after year, makes the soil less productive. In intensive cultivation there are little chances for the restoration of lost nutrients in the soil until they are supplied from outside. The leguminous plants, however, compensate the loss of nitrogenous compounds. Besides this, manure and fertilizers are to be supplemented to restore the fertility of the soil.
The deficiency of mineral nutrients in the soil either can be compensated through organic manures such as green manuring, compost etc. or it can be supplemented by the application of chemical fertilizers from outside sources.
The organic content of the soil which is a good source of plant nutrients contributes most to the fertility of the soil.
(i) They modify the physical properties as increase in granulation of the soil and increase in permeability and moisture holding capacity of soil.
(ii) They provide food for soil microbes and thus enhance microbial activities.
(iii) Decomposition products of organic manures help to bring mineral constituents of soil into solution.
(iv) They improve physico-chemical properties of soil, such as cation exchange and buffering action.
B. Chemical Fertilizers in Soil fertility
Of the elements known to be essential for plant growth, nitrogen (N), phosphorus (P) and potassium (K) are required by plants in pretty large amounts, and are therefore, designated as major ox primary nutrients while calcium, magnesium and sulphur are secondary nutrients. For acid soils, use of Ca and Mg is necessary. Seven elements iron, manganese, boron, molybdenum, copper, zinc and chlorine are required in trace amount and hence called micro-nutrients.
2.Application of Soil Conservation Practices for Soil fertility
Loss of plant nutrients and water from the soils due to soil erosion can be checked effectively and the fertility of soil can be maintained by application of various biological and engineering methods of soil conservation. A detailed account of these methods has already been given in soil conservation topic.
3. Water management Practices for Soil fertility
Water supply is critical factor in crop production in most areas of the world. Soil moisture greatly affects the availability of mineral nutrients in the soil. It has been proved beyond doubt that fertilizer response is much higher with adequate irrigation.
Drainage and moisture control influence micronutrient availability in soils. Improving the damage of acid soils encourages the formation of less toxic oxidized forms of iron and manganese.
4. Prevention and Elimination of Inorganic Chemical Contamination of Soil:
Loss of soil fertility due to application of toxic chemicals as pesticides can be eliminated if:
(i) Application of toxic chemicals to soil is reduced and
(ii) The soil and crop are so managed as to prevent cycling of toxic chemicals.
5. Stabilization of Soil pH for Soil fertility
The stabilization of pH through application of soil amendments and buffering seems to be an effective guard against the problems of non-availability of certain plant nutrients and radical changes in microbial activities arising due to change in soil pH.
Irrigation Systems in Soil fertility
Water is a very important natural resource, which is the basis of all life forms. For sustainable agricultural production, water is one of the most precious important inputs. Plants need water in huge amount throughout their life. Water is also one of the main factors that influence most of the metabolic process such as photosynthesis, respiration, adsorption, opening and closing of stomata and translocation of food material. The growth and yield of crop plants is very much affected by the availability of water.