A fertile soil should be capable of supplying all the elements plants need for growth. Fertility is an essential component of soil health and productivity. Not only must nutrients be present in the soil, they need to be in a form the plant can use. Since these ionized forms of nutrients are soluble in water, plant roots can absorb them along with the water they take up. The rate at which nutrients become available is affected by weather, irrigation, soil type, pH, and fertilizer applications. Nutrients present in forms other than ions are not directly available to plants although they do represent reserves that can become available in the future.

Five different pools of soil nutrients can be found in soil. The “rocks and minerals” pool is also called parent material. This parent material can be a rich source of nutrients, but they release very slowly, generally over decades to hundreds of years depending on climate and rainfall. Organic matter is also a pool of nutrients that slow-release over just a few seasons. When cover crops, manure, compost, or organic fertilizers are added to the soil their nutrients are not immediately available to plants. The organic matter must first be broken down by microorganisms. As the organisms feed and die, nutrients release gradually into the ion form plants can take up.

Two other pools that concentrate nutrients are the surfaces of clay and humus particles. Because of their extremely small size, humus and clay particles have large amounts of surface area relative to their volume. Their surfaces have negative charges which make them very attractive to positive charged cations, which means clay-rich soils have a larger cation exchange capacity (CEC) than sandy soils. However, those with sandy soils can greatly increase their cation exchange capacity through the addition of organic matter and the creation of humus.

The fifth pool represents the nutrient pool immediately available for plant uptake: those nutrients currently in the soil solution, the mixture of water and nutrients in the soil. This pool is in equilibrium with the clay and humus nutrient pools. As nutrients are taken by plants from the soil solution, they are replaced by the nutrients adsorbed to the clay and humus particles. Increasing soil organic matter can increase cation exchange capacity, especially in sandier soils. Research continues on adequate crop nutrition in organic and sustainable systems.

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