The amount of nutrients in the soil solution at any one time is just a fraction of that needed by plants over the course of the year. The soil solution must be continually replenished from the store of nutrients in minerals and organic matter. Many farmers also contribute nutrients to the soil solution by adding readily available fertilizers once or more each year. Soil particles and plant residue are made of large quantities of nutrients that are unavailable to plants. Plant residue eventually decomposes into mineral ions or reforms into humus.
Plant nutrients exist in several forms .
Nutrient ions attached to the exchange sites on clay and humus are released into the soil solution for use by plants and soil organisms such as bacteria and fungi. As plants draw nutrients out of the soil solution, more may be released into solution from exchange sites on clay and humus. Fertilizers added to the soil solution do not remain unchanged, waiting to be used by plants. Like minerals from other sources, they become attached to exchange sites, are used by microorganisms, and are transformed. This means that prolific root growth and adequate water are essential to plant nutrition.