University of Maryland Extension

Soil Organic Matter

Author: 
Patricia Hoopes

As part of soil testing, we have an option to choose to test soil level Organic Matter (OM).  In general, folks know that higher OM levels are more desirable. However, in many cases, there is an extra charge. So why is it important to know the levels of organic matter in our soils?

Organic Matter Defined

Jenny Hans, a late pioneer soil ecologist, when asked to define organic matter said: “Any organic carbon assembly, large or small, dead or alive, is classified as soil organic matter.”

There are three different groups of organic material:

  • Biomass: This includes all living soil organisms. That is all creatures from the microscopic viruses and bacteria to earthworms and millipedes.
  • Residues and by-products of living creatures: These are old plant roots, crop residues, manure and dead soil organisms. All this supplies soil organisms with food. The soil organisms leave behind nutrients they don’t need in a process called mineralization. These nutrients are available to the plants. However, if nutrients are inadequate for the soil organisms, they will take up the nutrients that are there and no additional nutrients will be left behind. In this case, nutrients will be locked up through a process called immobilization, and nutrients will not be available to plants. Soil organisms also give off by-products that are sticky or gummy. These materials hold soil particles together in clumps, or aggregates, and form the basis for good soil structure and tilth. This is great for healthy soil.
  • Humus: Residues that are difficult for soil organisms to decompose contribute to the formation of humus. The end product of the humification process is the result of the activity of soil organisms upon residues. Humus is stable and resists further decomposition. It is not a source of nutrients for soil organisms. It is a mixture of very small and very reactive particles. It forms the majority of organic matter. It enhances the water-holding and nutrient-supply capacity of the soil, which significantly benefits crops.

Managing Organic Matter Levels in the Soil

Organic matter is affected by soil texture, soil type, tillage, crops, and crop residues. Organic matter can be managed by adding additional organic matter and/or by reducing the loss of organic matter.

Add organic matter

  • Create more organic matter by growing healthy and productive crops and planning a high-residue rotation. That may include sod crops that leave lots of roots in the soil (like small grains or forages), crops that leave a lot of surface residue (like grain corn), and cover crops that supply both.
  • Apply livestock manure.
  • Bring in off-farm sources of organic matter, such as food processing waste or manure from neighbors.

Reduce organic matter losses

  • Reduce tillage, meaning to leave more residue, and till less often and less intensively than conventional tillage. No-till is the most extreme version of reduced tillage.
  • Control erosion.

Maintaining Adequate Organic Matter Levels:

  • increases the nutrient holding capacity of soil (CEC).
  • provides a pool of nutrients for plants.
  • chelates (binds) nutrients, preventing them from becoming permanently unavailable to plants.
  • provides food for soil organisms.
  • improves water infiltration.
  • decreases evaporation.
  • increases water holding capacity.
  • reduces crusting, especially in fine-textured soils.
  • encourages root development.
  • improves aggregation, preventing erosion.
  • prevents compaction.

 Dr. Strickling was a soil physicist at the University of Maryland from 1950 to 1984. He studied cropping systems or rotations on soil organic matter and aggregate stability. We have him to thank for the insights on the benefits of OM on soil and aggregate health. He pointed out that some combinations of crop rotations and tillage methods can critically deplete organic matter and have a negative effect on soil physical properties. Nutrient applications cannot compensate for poor soil tilth. To respond to recommended nutrient management, a soil must first be in good physical condition.

Keeping tabs on soil health by monitoring the soil’s organic matter is a critical step to determine effects of management systems on soil health. The value of testing and keeping tabs on soil organic matter probably outweighs the expense. What do you think?

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