Moisture Content and Availability. Soil organic matter has a variable effect on available water in soil. It is generally a positive relationship, but whether or not the effect is significant depends on other soil properties, notably texture. It is important to distinguish between water retention and water availability. Increased soil aggregation can have a similar effect.
Organic matter has a strong, positive effect on infiltration of water into soils.
This effect is due mainly to a decrease in bulk density, and improvements in aggregation and structure. High-nitrogen materials such as legumes, on the other hand, cannot maintain organic matter levels because they decompose too rapidly. Varied reports from the literature suggest that each plant species should be considered on its own merits and in the context of the farming systems in which it will be used. Table 1 presents a sample of how diverse the literature is on this subject. Any interpretation of these varied results should also take into account soil type, soil nitrogen levels, native soil organic matter levels and microbial activity.