Materials such as manure and peat have already undergone decomposition and modification to produce highly resistant humic materials. In forests higher SOM levels are observed under N2 fixing trees than under non-fixing trees. When previously undisturbed soils are converted to agriculture and are subject to frequent tillage, there is a rapid phase of organic matter loss with an eventual stabilization at lower organic matter levels. The increase in soil organic N with increasing precipitation probably reflects increased inputs of plant material to soil as water availability increases. In flooded soils or in very cold climates organic matter inputs exceed rates of decomposition, and peat soils, Histosols, are formed.
Sandy soils have mostly large pores, while loams and clays have a wide distribution of pore sizes.
Micropores may render soil organic matter inaccessible to decomposers, and bacteria may be safe from predators in slightly larger pores. The negative charges on clay surfaces interact with cations, humus, microbial secretions and cell walls, and other clay particles to form stable aggregates. Coatings of clay reduce the accessibility of SOM to microbial attack. A type of soil mineral found in volcanic soils, allophane, is composed of amorphous aluminosilicates which are smaller than clay particles. Thus the large reactive surface of allophane reduces availability of SOM to microbial attack.