Soil Nutrient Cycling

Soil nutrients are essential for plant growth and subsequent food production. These nutrients are of macro- and micronutrient categories based on their required amount for plants. Primary macronutrients required in large amount are nitrogen (N), phosphorous (P), and potassium (K). Secondary macronutrients required in moderate amounts are calcium (Ca), magnesium (Mg), and sulfur (S). The nutrients like iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl) are required in very small amounts for plants and are considered micronutrients. Carbon (C), hydrogen (H), and oxygen (O) are not soil nutrients but are required for plant growth, and they are available through air, soil, and water. Other nutrients in soil that are not directly essential for all plants but are required for other purposes, such as sodium (Na), silicon (Si), and nickel (Ni), are vital for some plants and may stimulate growth in other plants; cobalt (Co) is a vital nutrient necessary for N fixation in soil by legume plants to support other crop growth; and selenium (Se), arsenic (As), and iodine (I) are not essential for plants as such but are required for humans and other herbivores who eat plants.

[Page 403]These soil nutrients are available in the soil as a result of the weathering of soil minerals; decomposition or breakdown of dead plants, animals, and microbes in the soil; application of fertilizer, manure, compost, lime, and other biosolids; fixation of N in soil by legume plants; and movement of soil from other locations by wind and water erosion, and so on. Decomposers present in soil, such as bacteria, fungi, and termites, process and break down decaying matter to form soil nutrients. The decomposition process varies from tropical climate to temperate climate as the amount of decomposer varies with soil and atmospheric temperature. Therefore, soils in the Amazon forest and other tropical areas are rich in soil nutrients. These nutrients are also lost in soil as a result of soil erosion, leaching or percolation, volatilization, and crop uptake. In slope land, soil nutrients can be lost with water runoff.

It is essential for a soil to have high nutrient concentrations at the time of high crop growth rate, but the soil should have a high storage capacity to retain nutrients in it at low crop need. Cation exchange capacity of soil is the property that helps in attracting and holding positively charged ions from leaching. A high cation exchange capacity helps in storing the nutrients (K+, Ca2+, Mg2+, NH4+), and other micronutrient trace metals are held together by soil clay and negatively charged organic matters, known as a reservoir of plant nutrients.

Biological cycle and chemical transformations are two basic plant nutrient cycling processes. The soil food web and the soil-plant-animal system are part of the biological cycle that makes the soil fertile. Soil organic matters store plant-available nutrients. Plants use those nutrients to grow food. Humans and animals consume these foods, and their excreta, including plant by-products, are used to prepare compost and manure. The manure and compost is used in soil to increase the soil nutrients amount. This cycle is known as the soil nutrient cycle.

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