Soil Organic Matter

Part 1, Section 1: Soil Management

Soil Health

Improving Soil Health

Soil Organic Matter

Soil organic matter content is considered one of the most important indicators of soil health. From an agronomic and environmental point of view, organic matter gives the largest benefits if it is found close to the soil surface. Near-surface organic matter improves soil aggregation, which increases infiltration, improves resistance to erosion, improves workability, and leads to improved seed-to-soil contact when planting. The surface soil also becomes more resistant to compaction and will facilitate root development. To maintain or increase organic matter content one needs to maximize inputs or additions of organic materials, and minimize outputs or losses. Inputs can be increased by growing crops that produce large amounts of residue and fine roots (species such as corn, small grains, grasses), leaving crop residue in the field, growing cover crops during otherwise bare fallow periods and adding compost and manure (especially bedded manure). How much organic material should be returned to the soil to maintain organic matter contents? This question is receiving increased attention because of the interest to harvest crop residue to produce biofuels. A recent review suggests that in a no-till system 4500 lbs/A/yr of crop residues need to be returned to maintain surface organic matter content under conditions pertaining to the Corn Belt. To meet this goal will be very challenging because most annual crops don’t produce that much crop residue, even if no residue removal is allowed (Table 1.1-3 ). Other sources of organic inputs are manure, compost, and cover crops. Liquid manure may not contribute to organic matter if it does not contain any solids. Highly decomposed packed manure or compost may contribute much to organic matter content since it is already partly humified. Organic matter losses include removal of crop residue (such as hay, straw, or silage harvest) and burning. Soil tillage reduces surface organic matter content, so eliminating tillage will help improve soil health. The type of tillage also matters; the moldboard plow tends to cause the largest losses of surface organic matter, while the chisel plow and disk harrow cause smaller surface organic matter losses. Recent research suggests that tillage primarily redistributes organic matter to deeper soil layers. With moldboard plowing, soil organic matter content is uniform throughout the plow layer. In long-term no-tillage or sod, on the other hand, soil organic matter content will be high at the soil surface and decrease rapidly below 2 or 3 inches. Chisel plows and disks tend to lead to soil organic matter distribution that is intermediate between moldboard and no-tillage.