Ammonium has limited mobility and is not at risk of leaching. Nitrate is Mobile in the soil and rapidly absorbed by plants. Ammonium, UAN and UREA are converted into nitrate before absorption.
Most of nitrogen is directly absorbed by plants. A little part of it is immobilized in soil organic matter. The nitrogen in soil organic matter is not lost, but it needs to be mineralized before it can be taken up by plants.
Part of immobilized nitrogen is mineralized during the growing season. Part of it is mineralized later. Split nitrogen application would ensure rapid uptake of available nitrogen by plants and thus reduce immobilization.
With good agricultural practice, only a very small portion of applied fertilizer nitrogen can be found in the residual nitrate pool upon harvesting. Manure contains nitrogen mainly as organic compounds, building up soil organic matter. This nitrogen is not immediately available. Manure, especially slurry, also contains some mineral nitrogen, mainly as ammonium. The proportion of mineral nitrogen in manure varies depending on its sources.
Mineralization of soil organic matter contributes to soil nitrogen supply. Mineralization rates are variable, For example, mineralize nitrogen for grassland can exceed 300 k/ha ( kg per hectare).
After harvest, however, microbes find optimum conditions in the humid and warm autumn soil. Since there are no more plants to use the nitrate produced, it is prone to leaching.
During the growth period, mineralized nitrogen contributes to plant nitrogen uptake. Determining soil nitrogen supply accurately by sampling and use of precision farming tools avoids over-fertilization and reduces leaching.
Autumn and winter rainfall stocks up the water table. Most of the residual nitrate is laeched in these two seasons. During spring and summer there s usually little rain, and evaporation from the plough layer counteracts leaching. Keeping the residual nitrate pool as small as possible is the best strategy against laechinga
Leaching occurs when un-used mineralized nitrogen is washed out in ground water during winter time. Nitrogen from mineral fertilizer, especially when applied as nitrate, is immediately available for plants, Tailored and split application of mineral fertilizer reduces the residual nitrate pool after harvest and thus prevents leaching