Soil organic matter (SOM) is an indicator of how healthy and productive a soil is and serves many important functions. The higher the SOM the better the soil is.
It is the part of the soil that is composed of anything that once lived. It includes plant and animal remains in various states of decomposition, cells and tissues of soil organisms, and substances from plant roots and soil microbes.
After this material is well decomposed through time, about 10% of it forms into stable SOM, or humus.
This is the porous, spongy material of the soil that has a pleasant, earthy smell that is dark in color. SOM forms slower in warmer and drier climates where the amount of plant biomass that can be grown is limited.
Soil organic matter is a reservoir of nutrients that are mineralized by microorganisms and released in the soil that plants can use. For each percent of SOM in the soil about 25 pounds of nitrogen, 5 pounds of P2O5, and 3 pounds of sulfur become plant available annually. In high SOM soils this can lead to significant fertilizer savings. The warmer the soil temperature the more nutrients are released, so summer crops benefit more from SOM mineralization than winter crops.
SOM behaves like a sponge and increases the water holding capacity of the soil.
It can and hold up to 90 percent of its weight in water. This is especially important in drier climates where soil moisture is almost always the most limiting factor when it comes to crop yields.
High SOM soils have a higher percentage of stable soil aggregates which don’t break down, consolidate or condense when it rains. This improved soil structure increases the pore space in the soil which helps the soil take in more water. It also decreases runoff and erosion.
It is not easy, and it takes years to see a significant improvement, but there are ways that we can increase SOM levels in the soil. Here are just a few.
Tillage aerates the soil and causes a flush of soil microbial activity that speeds up the decomposition of organic matter. Tillage also increases erosion so when soil leaves the field organic matter goes with it, since most of the SOM is near the surface of the soil. Before our prairie soils were broken out, they naturally had about 3 to 4% SOM.
The use of intensive tillage over several decades has reduced soil SOM levels by about ½ of what they originally were. By decreasing the amount of tillage, we can stabilize, and through time, increase SOM levels.
Soil test on a regular basis and fertilize accordingly. Proper fertilization, based on a soil analysis and realistic yield goals, encourages plant growth and more root development. This can help build or maintain SOM levels, even when top growth is removed for forage.
Intensify the crop rotation with additional cash or cover crops. Without a green growing plant that provides plant residue and roots that feed the soil biology, there is nothing there to build organic matter.
It makes economic sense to do what you can to maintain and improve organic matter levels in your soils. The higher the organic matter the less fertilizer you will need because more plant nutrients are released in the soil.
High organic matter soils can help keep a crop growing and healthier longer during prolonged dry periods because of the higher water holding capacity.
In some years this can be the difference between growing a profitable crop and not growing one so it can affect the bottom line.
Consider using these practices on your operation to build and maintain organic matter levels in your soil.