- Organic matter soils form from the partial decomposition of plant materials. These soils are common in wetlands, where there is not enough oxygen for the microbial communities to decompose organic materials. Top soil is formed through weathering of rocks and minerals.
- Bulk density is a measure of the mass of a given volume of soil. Organic soils often have lower bulk densities relative to the top soils. Top soils have bulk density values ranging from approximately 1 to 2 g per cubic centimeter. Organic soils often are much lower--as low as 0.2 g per cubic centimeter.
- The ability of a soil to hold water is directly related to its bulk density. Because of their pore space, organic soils have a lower mass per cubic centimeter than mineral soils. The pore space gives organic soils a higher water retention capacity.
- Because organic soils originate from living biomass, they have higher nutrient stocks compared with mineral soils. However, in natural systems, the nutrients in the organic soils often are unavailable to plants because the material hasn't decomposed fully. Even though the level of nutrients in mineral soils is lower, it is more available to plants. Organic soils sold for home gardens usually are dried, and the nutrients are more available to the plants.
- The low bulk density and high percentage of pore space associated with organic soils lends to a decreased physical stability for plants compared with to top soil. Because top soil is composed of weathered rock and has a more definitive physical structure, it better supports root systems and the weight of vascular plants.