Soil organic matter (SOM) provides many benefits for crop production in Kansas. Higher SOM levels mean better soil aggregation, improved water infiltration, less compactability, less erodibility, and a generally higher level of productivity due to more available water and nutrients.
About how much of an increase in available water and nutrients can producers expect from each 1% increase in SOM?
Water: Numerous studies have shown that SOM increases soil water holding capacity. In one such study, the authors concluded that available soil water increased from 1-10 grams for every 1 gram increase in SOM. Stretching out the math from there, the end result is that each 1% increase in SOM results in about 20,000 to 25,000 more gallons of available soil water per acre, or about one acre-inch. Only a portion of the total water in a soil is available, so the increase in total water content for each percentage increase in SOM is even higher than that.
Nitrogen: A soil with 3% SOM contains about 3,000 pounds of total nitrogen per acre. Each percent increase in SOM adds about 1,000 pounds of nitrogen per acre. Most of this nitrogen is unavailable until it is mineralized by soil microbes. In Kansas, SOM mineralization rates range from 1% to 3% per year, depending on soil temperatures, soil moisture levels, soil pH, and tillage systems.
With a mineralization rate of 2%, typical of central Kansas, this would mean that one acre with 3% SOM may provide about 60 of available nitrogen. Each percent increase in SOM would increase available nitrogen by about 20 pounds per acre. In eastern Kansas, the increase in available nitrogen would be higher because there is more soil moisture and the mineralization rate is higher.
Phosphorus: In general, the ratio of available nitrogen to available phosphorus in SOM is 10:1. Therefore, each percent increase in SOM would increase available phosphorus by about 2 pounds of elemental P, or 4.5 pounds P2O5 per acre in regions where the annual mineralization rate is about 2%. In eastern Kansas, the increase in available phosphorus would be higher.
Potassium: Potassium release by SOM is negligible.
Sulfur: In Kansas, sulfur release from organic matter is 2.5 pounds per 1% SOM.