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Introduction to Soil Fertility

Soil fertility is concerned with the inherent capacity of soil to provide nutrients in adequate amounts and in proper balance for the growth of specified plants when other growth factors such as light moisture and temperature and the physical condition of the soil are favourable. Soil fertility is an aspect of the soil plant relationship viz. plant growth with reference to plant nutrients available in soil. A fertile soil is considered to be one that produces abundant crops under suitable environmental conditions. Only a very small fraction of the total nutrient content of soil can be utilize by plants. To get quick measure of nutrient availability following soil fertility evaluation methods used ---

1. Biological
2. Visual symptoms of nutrient deficiency or toxicity
3. Plant analysis
4. Soil analysis.

Soil factors influencing nutrient availability

Soil reaction (pH) is the most important factor, which governs availability of nutrients in soil. The pH range of 6.5 to 7.5 is the optimum for availability of most of the nutrient availability.

• Nitrogen

The ammonifiers and nitrifies are active at pH 5.5 to 6.0, below which nitrogen availability decreases.

• Phosphorus

Phosphorus availability is maximum in the pH range of 6.5 to 7.5. In red and lateritic soils the phosphate availability depends on the amount of sesquioxides. Liming of acid soils increase phosphate availability.

• Potassium

Potassium availability depends on exchangeable potassium, potassium saturation of exchange complex, CEC and pH.

• Calcium, Magnesium

Calcium and magnesium are available in cationic form and their availability is decreased governed by the factors which control exchange equilibrium in soil at low pH (below 6.0) their availability decreases.

• Micro Nutrient Availability

These are available to at low pH range. Alternate drying and heating the soil increases availability soil manganese. Availability of boron decreases below pH 5.0 and above 7.0 but above pH 8.5 it again increases. Availability of molybdenum increases above pH 6.5.

Maintenance of soil fertility

Nutrients are continuously removed from the soil by crops in addition to losses by leaching and erosion. It is therefore, essential that sound soil and crop management practices are adopted to improve and maintain soil fertility and soil physical conditions for the purpose of sustained crop production. The management practices that play an important role in the maintenance and improvement of soil fertility are given below: -

Crop rotation

The benefits of a good crop rotation are, increased organic matter, nitrogen supply and improved soil structure. Especially deep rooted legumes or crops capable of feeding themselves efficiently at various soil depth. Deep rooted crops increased permeability of soil at lower depths to air and water. The other benefits of crop rotation are keeping soil under crop, control of run off, soil erosion and efficient use of fertilizers e.g. cereals-legumes.

Manures and Fertilizers

Balance fertilization with nitrogen, phosphorus and potassium fertilizers helps in maintaining soil fertility. Application of organic manures and chemical fertilizers helps to improve and maintain soil fertility, soil productivity and soil physical conditions. Soil fertility and agricultural production can be maintain only by efficient and judicious management of nutrient addition to the soil from external sources.

• Manures

Organic manures bind the sandy soil and improve its water holding capacity. They open the clayey soil and help in aeration better root growth. They add plant nutrients in small percentage and also add micro nutrients which are essential for plant growth the microbial activity is increased which helps in releasing plant nutrients in available for e.g. bulky organic manures- FYM, compost from organic waste, night soil, sludge, sewage, sheep folding, green manures, concentrated organic manures- oilcakes (edible, non-edible), blood meal, fish meal, bone meal. Organic manures should be incorporated before the sowing or planting because of slow release of nutrients.

• Fertilizers

Chemical fertilizers play an important role in crop production as the nutrient elements in fertilizer are present in higher concentration and in forms which can be readily utilize by plants directly or after rapid transformation. Their dose can be adjusted to suit the requirement as determine by soil fertility evaluation. fertilizers applied through straight fertilizers- providing single nutrient or complex and mixed fertilizers- supplies two or more nutrients. While application of fertilizers following consideration should be taken in account-

• Soil factors

The coarse texture soils are poorer in available nutrients than fine texture soils. Hence, it is necessary to apply nitrogenous, phosphatic and potassic fertilizers more frequently than in fine texture soils. Higher the soil fertility the lower is the response of crops to fertilizer. Soil reaction is important in selection of right type of phosphatic fertilizer. Efficient use of fertilizers by the crop is more in the higher organic matter status of soil.

• Climatic factors

These include temperature, rainfall, evaporation, and length of day and growing season. The rate of nitrification is slower in a cooler climate that in a warmer climate. Hence, more ammonical nitrogenous fertilizers will needed in a cool climate. In region of high rainfall because of higher yield potential and leaching loss of soil and fertilizers nutrients, require higher fertilizer. In arid region soil moisture is the limiting factor in fertilizer use. If soil moisture is conserve efficiently fertilizer becomes very effective.

• Application methods

Application of fertilizer is by using proper methods and time is important in crop production. Nitrogenous fertilizer should be applied in split doses while slow releasing, phosphorus and potassium should be applied at the time of sowing or planting. Instead of broadcasting fertilizers should be applied by placement method or through fertigation.

Reclamation of problem soils

Problematic soils; acidic, saline and alkaline soils can be bring in proper condition for raising the crop satisfactorily and economically, special measures have to be adopted. Acidic soils can be corrected by liming. Saline and alkaline soils can be reclaimed by improving drainage scrapping off surface soils and using gypsum, sulphur, molasses and adopting agronomical measures like green manuring, adding organic manures, mulching and suitable crops rotation.

See also...

Identifying Your Soil Type
How pH Affects Plant Foods
Finding Soil's pH
Raising Soil pH
Lowering Soil pH
pH Plant Preference Lists