The acid exudates from plant roots release hydrogen ions that can replace nutrient cations on the clay particles. These can then be absorbed by the plant root through the soil water. The most important ion-exchange reactions can be summarized by Figure 8.15. The net results of the adsorption and exchange of nutrient ions on clay particles in soil are similar to those from the solution of nutrient ions in aquatic ecosystems. But as is typical of terrestrial systems, the precise mechanism is infinitely more complex.
Organic material enters the soil as leaf litter, dead grasses and roots, animal wastes, and so on.
It is broken down into smaller particles by the fungi, bacteria, and animals of the detritus food chain and mixed and aerated by the movement of the larger soil animals such as earthworms and soil-dwelling mammals. If conditions for decomposition were perfect, and if detritivore activity lasted long enough, all organic material would be completely broken down to carbon dioxide and water. This does happen in some cases, most notably in the wet tropics, but it is not common. The end product of organic breakdown is typically a series of organic molecules collectively termed humic acids, or simply humus. The molecules are tiny, irregular in shape, and characterized by large surface area.