Increasing the organic matter content of soil is one of the single-most effective means of improving soil quality.
Leaves can be directly incorporated into the soil or composted. Photo credit: Dan Machold Flickr CC BY-NC-SA 2.0
Double digging prior to organic matter incorporation. Photo credit: laurenipsum Flickr CC BY-NC-SA 2.0
Organic matter is derived from the breakdown of plant and animal residues by soil organisms such as earthworms, bacteria, fungi, and actinomycetes. The richest form of organic matter is humus, which is what remains after extensive decomposition of organic matter.
Organic matter supplies plant nutrients to the soil, such as nitrogen, phosphorus, and sulfur. It improves soil structure and aeration, increases nutrient retention and plant-available water, and reduces erosion.
Incorporating organic matter speeds up its decomposition, reducing the amount of time needed to gain all the benefits of soil organic matter and humus. Surface application of organic matter can serve as a mulch, and may be preferable when using woody materials because of the potential need to add nitrogen fertilizer if this hard-to-decompose material is tilled in.
When slow-to-decompose organic matter, such as corn stalks, straw, and especially wood wastes, are added to the soil, the soil organisms begin to break down the organic matter. Microorganisms require nitrogen to complete this process. The sudden addition of plenty of food for the microrganisms (the carbon in the organic matter) and the comparatively low concentrations of nitrogen in the organic matter (high carbon to nitrogen ratio) results in the nitrogen in the soil being used up.
Nitrogen, which is the key plant nutrient, is no longer available for plants. Because of this, it may be necessary to add nitrogen fertilizer to the soil/organic matter mix. Typical fertilizer rates for soils when using high carbon to nitrogen ratio organic matter is one pound of nitrogen per one hundred pounds of organic matter.