These soils have large air spaces which allow water to move through. The C horizon is composed chiefly of bedrock. Clay minerals are small and have a large surface to volume ratio and hold a negative electrical charge. Dissolved substances with a postive charge can form ionic bonds with these particles. Clays play an important role in holding nutrients that have been released from parent rock of are the products of decompositional cycles.
Ammonium is a cation, and therefore is held by the clay.
In clay rich soils, ammonium is more liely to be an effective fertilizer of plants that nitrate, and nitrate is more likely to wash from the land a pollute local streams and lakes. O are more important to plants than Nitrogen. Most of it occurs in proteins. N must be absorbed from the soil in its oxidized formed and then reduced to be used in proteins. Atmospheric nitrogen is useless to plants and must be fixed by microbes, some of which exist in the roots of certain plants or by industrial processes to form fertilizers.