Author: Marsha Campbell Mathews, University of California CE Agronomist
Most nitrogen in manure is in either the ammonium or organic form. The ammonium form functions very similarly to commercial fertilizer, whereas the organic form must first be broken down by bacteria before it can be taken up by a crop. When the organic nitrogen in manure is initially broken down, it is released as ammonium. Ammonium nitrogen is positively charged and generally immobile in soils since it is attracted to the negatively charged clays and organic matter in soils.
Under warm conditions in moist soil, ammonium is converted by bacteria into nitrate nitrogen. Nitrate nitrogen has a negative charge which prevents it from adhering well to soil particles, leading to the risk of the nitrate leaching out of the soil profile. Therefore, nitrate nitrogen is the form of nitrogen that is most commonly found in groundwater. When water moves through the soil profile past the roots, this moving water can carry the free-floating nitrate with it beyond the reach of plant roots and potentially into the groundwater.
