Increasing the Available Nutrients in Soil

This article throws light upon the six main factors to be considered for increasing the available nutrients in the soil. The factors are: 1. Physical Conditions of Soil and Plant Nutrients 2. Management of Inhibitory Factors like Acidity, Salinity and Alkalinity 3. Moisture and Nutrients 4. Fertilizer and Nutrients Management 5. Organic Manure and Nutrient Management 6. Bio-Fertilizer and Nutrients Management.

Factor # 1. Physical Conditions of Soil and Plant Nutrients:

Physical condition of soil like poor aggregation (structure), poor aeration, cloddy surface soils may reduce nutrient availability for one of these reasons:

(a) By restricting the exchange of oxygen and carbon dioxide in the soil, the ability of plant roots to translocate nutrients to the leaves is restricted.

Continued soil aeration is essential to remove the carbon dioxide produced by plants and microorganism and to supply the oxygen needed by plant roots and the microorganisms in the soil. It is an established fact that oxygen is essential for normal root growth and oxygen is essential to the root if it is to carry out its normal function of absorbing nutrients and water.

Well aerated soils, by providing more oxygen, would stimulate respiratory activity and encourage a large uptake of nutrients by roots. Due to restricted aeration (poor aeration) absorption of nutrients like potash, calcium, magnesium, nitrogen and phosphorus is reduced.

(b) By physically restricting root elongation, especially deep root penetration, the volume of soil in contact with plant roots and the total nutrients absorbed by the roots will therefore, be limited.

(c) By reducing water infiltration, water may become the first limiting factor in plant growth even though all nutrients are present in adequate amounts.

The lack of balance in air and water within the soil brings about a number of effects, e.g., water-logging due to poor drainage brings about reduction of sulphate and decrease in availability of sulphur under water-logged condition due to poor aeration, nitrification is decreased and de-nitrification is stimulated. In this way, nitrogen is lost. This loss may be checked by proper drainage.

The ability of plants to absorb water and nutrients depends on permeability of root surfaces which is in turn influenced by the metabolic activity of the roots. The metabolic activity is increased by supplying oxygen to the roots and removing carbon dioxide. Higher soil temperature in the root zone in winter months tends to increase growth primarily through higher uptake of nutrients. The higher temperature in winter may be maintained by mulching.

Factor # 2. Management of Inhibitory Factors like Acidity, Salinity and Alkalinity:

Soil that is about neutral (pH-6.0-7.0) is suitable for the availability of almost all nutrients. In very acid soils, one might expect toxicities of iron and manganese and deficiencies of phosphorus and molybdenum. These can be corrected by liming. Saline and alkaline soil may have deficiencies of iron, manganese, zinc, and copper, and in a few cases, a toxicity of molybdenum . Gypsum or sulphur may be used for lowering the soil pH.

Factor # 3. Moisture and Nutrients:

Plant absorbs nutrients from the soil only in a dissolved state and sufficient moisture is therefore, required for utilizing the nutrients of the soil. Management of moisture in the soil by irrigation, where the rainfall is low, and draining the soil, where it is subject to stagnation of water, help to raise the nutrients availability.

Improving the drainage of acid soils will encourage the formation of the oxidised forms of iron and manganese. These are less soluble under acid conditions, less toxic than are the reduced forms. High pH soil (saline and alkaline) may be improved by proper irrigation.

Factor # 4. Fertilizer and Nutrients Management:

The most common management practice is to add chemical nutrients in the form of fertilizers; nitrogen, phosphorus and potassium are important elements and are required in large quantities by crops. These are most commonly deficient in soils. Therefore, concentrated fertilizers of each of these nutrients are applied to the soil to increase the nutrients status of the land.

Fertilizers influence the reaction of the soil. For instance, ammonium sulphate is an acidic fertilizer. When constantly applied to neutral soil it will show a tendency to turn the soil into an acidic one. Sodium nitrate is basic fertilizer and its continuous use on some of the soils turn them alkaline.

The physical condition of the soil becomes bad due to compactness. It is, therefore, common to recommend the use of appropriate fertilizers which will suit the soil and bring its reaction as close to neutral as possible, because all elements essential for plant growth are mostly available at neutral reaction.

Factor # 5. Organic Manure and Nutrient Management:

Under constant cultivation, soils are losing organic matter faster than it can be replaced. A decrease in soil organic matter results in compact soils, which has poor aeration and low infiltration capacity. Soil organic matter can be maintained by the use of organic manures such as farmyard manure, compost, green manuring and by including legume crops in rotation. Organic matter is a good source of almost all plant nutrients. It provides food for soil microorganism and macro-organism.

Organic matter improves the physical properties of the soil (such as structure by promoting granulation) and increases water-holding capacity and permeability. Soil organic matter improves the physico-chemical properties of soil such as cation exchange and buffer action. Because of these beneficial effects of organic matter, organic manure should be used in abundance.

Factor # 6. Bio-Fertilizer and Nutrients Management:

Bio-fertilizers are the cultures of microorganisms used for inoculating seed or soil or both under ideal conditions to increase the availability of plant nutrients. The conversion of molecular nitrogen or inert nitrate or ammonia by microorganisms is known as Biological nitrogen-fixation. Biological nitrogen-fixation is probably one of the chief sources of nitrogen for sods.

The three groups of such organism are:

(a) Bacteria

(b) Blue-green algae and

(c) Azolla.

There are two main classes of bacteria that fix atmospheric nitrogen, non-symbiotic and symbiotic. Non-symbiotic nitrogen-fixing bacteria include Azatobacter and Clostridium. Azatobacter is strongly aerobic and oxidizes carbohydrates and other sources of carbon completely to CO₂ and water. The efficiency of nitrogen-fixation usually vary from 15 to 20 mg of nitrogen per gm. of carbohydrates. Clostridium is less efficient than Azatobacter.

About 2-3 mg nitrogen are fixed per gram of carbohydrate fermented. Low efficiency of Clostridium can be explained on the basis that while anaerobic fermentation (Clostridium is anaerobic) releases only a small amount of energy, aerobic changes produces tremendously large amounts of energy which help to fix more nitrogen.

Symbiotic nitrogen-fixing bacteria e.g., Rhizobium is associated with root nodules of leguminous plants. Leguminous crops can add nitrogen to the soil by addition of root nodules and by excretion of soluble nitrogen compound.

Blue-green algae and Azolla fix atmospheric nitrogen in the flooded rice fields. The best pH range for this fixation is between 7.0 and 8.5. When the water dries up algal material add organic matter to the soil. In nitrogen-fixation by microbes, enzyme (nitrogenase) is essential. Azolla can be inoculated in field (rice) with enough standing water at the rate of 500-1000 kg/ha. It saves nitrogen fertilizer as much as 30 kg per hectare.

The Indian soils are poor in organic matter; therefore, availability of inorganic compounds such as phosphorus to the growing crop is a problem. Recently it was observed that the soils are showing deficiency of micronutrients like zinc and iron. Every year, soils become saline and alkaline by the constant use of chemical fertilizer. Physical condition of the soil is also deteriorating.

Farm yard manure, compost and green manure contribute variable amounts of Nitrogen, Phosphorus, Potash and other nutrients to the crops, but these sources alone are not sufficient to maintain high yields in crops in intensive sequences.

This is especially true for nitrogen, a nutrient that is lost from the system by de-nitrification, leaching and volatilisation as well as by crop removal. It was observed that combined use of organic, biological and inorganic amendments gave higher yield without any deterioration in the soil. Therefore, integrated nutrient management is the only way to maintain and improve the nutrient status of Indian soils.