Ammonium (NH4+) is rapidly taken up in the soil by soil life and then converted to nitrate (NO3-). The proportion of ammonium in soil is generally low. Higher levels are, for example, found after applying animal manure, and slow-acting fertilizers often also show higher ammonium levels. A crop can absorb ammonium. Ammonium could also attach to the clay humus complex (CEC), but this is unusual because soil life converts it too quickly.