Soil consists of many negatively charged mineral and organic particles. A measure of the total negative charge in soil is called its cation exchange capacity, or CEC. Most soils have enough exchange capacity to absorb and hold all positively charged particles or nutrients in soils. While the soils exchange capacity can hold ammonium ions, nitrate is a negatively charged nutrient and the negatively charged soil particles generally repel it, causing it to remain in the soil solution.
Water moving through soil will leave most nutrients “stuck” on the soil cation exchange sites. Nitrate is extremely soluble in water, so water picks it up and carries it along while moving through the soil.
Coarse-textured soils have large pore space, resulting in lower water holding capacity. Therefore, these soils have a higher potential to lose nitrate via leaching, when compared with fine-textured soils. Some sandy soils, for instance, may retain only ½ inch of water per foot of soil, while some silt loam or clay loam soils may retain up to 2 inches of water per foot. However, nitrate can be leached from any soil if excess rainfall or irrigation saturates the soil and causes water to move through the root zone.
Ammonium nitrogen has properties that are of practical importance for nitrogen management. Ammonium has a positive charge, and the negatively charged soil and soil organic matter attract or hold it. This means that NH4+ does not move downward in soils. However, NH4+ that plants do not take up is subject to change (conversion to NO3–) by the biological transformation occurring within days of NH4+ application to the soil system.
Inhibiting the conversion of NH4+ to NO3–, or nitrification, can result in less nitrogen loss and more plant uptake. However, preventing nitrification totally is not possible. Nitrification inhibitors are available commercially. These inhibitors are added to nitrogen fertilizers, including manure, and they work by inhibiting the growth of the bacteria that cause nitrification.
No way is available to totally prevent the loss of nitrogen to the atmosphere via volatilization or by the movement of some NO3– to water supplies, but sound management practices can keep losses within acceptable limits. These practices can include proper application timing, placement and incorporation, as well as appropriate storage and handling of manure or commercial fertilizers.