What is organic matter?

Organic materials are the fraction of the soil that includes approximately by weight, according to USDA-NRCS, 2014:

• 5% of living organisms
• 10% crop residues
• 33-50% decomposingorganic matter(the active fraction)
• 33-50% stableorganic matter(humus)

The active fraction oforganic matterreadily changes mass and form as it decomposes; thus, it is unstable in the soil and is most affected by management practices such as tillage, cover crops, and crop rotations (Carter, 2002). The rapid turnover of the active fraction can contribute to nutrient release for crops.

Humus on the other hand, is organic material that has been converted by microorganisms to a resistant state of decomposition. Humus improves soil fertility by acting as a reservoir for nutrients, increasing the water holding capacity of the soil, improving soil structure and friability, and providing a source of energy for living soil organisms.

The Benefits of Organic Materials

Increasing levels oforganic matterimprove the physical, chemical, and biological functions of the soil. The benefits oforganic matterare summarized into the following five functions:

1. Biological Function
There are many benefits toorganic matter, most of which begin with enhancing the biological diversity and activity in the soil. Asorganic matterincreases, microbial activity tends to increase. Organic matter consists of 58% carbon, which is required in combination with other nutrients for microbial activity. Microorganisms excrete compounds that also act as a binding agent for soil particles, which can increase aggregate stability, water infiltration, and water holding capacity.

2. Nutrient Supply
Organic matter is a valuable nutrient source for plants and living organisms. As microorganisms increase their activity during warmer weather that occurs predominantly in the spring and summer, greater amounts of nutrients are cycled from organic forms into those that are inorganic and plant available.

For every % oforganic matterin the top 6-inches of a medium-textured soil (silt and loam soils), approximately 10-20 lbs. of nitrogen, 1-2 lbs. of phosphorus, and 0.4-0.8 lbs. of sulfur are released per acre annually (USDA-NRCS, 2014). In addition,organic matterparticles contain sites of negative charges (i.e., cation exchange capacity [CEC]) that attract and hold positively charged ions such as calcium, potassium, magnesium and ammonium-nitrogen.

3. Soil Structure
Organic matter causes soil particles to bind and form stable soil aggregates, which improves soil structure. With better soil structure, water infiltration through the soil increases and improves the soil’s ability to absorb and hold water as well as reduces the potential for surface crusting of the soil.

Humic Acid Potassium Humate Black Shiny Flakes Or Powder

4. Water Holding Capacity
Soils with higherorganic mattercan infiltrate and store water at greater capacities. Organic matter behaves similar to a sponge, with the ability to absorb and hold up to 90 percent of its weight in water. A great advantage of the water-holding capacity oforganic matteris that it will release most of the water that it absorbs.

Figure 2 shows the increase in plant available water with higherorganic mattercontent across three different soils. Increasingorganic matter1% in the topsoil decreases the bulk density and increases the available water capacity by approximately 0.2-0.3 inches, which can be extremely valuable to help plants manage water through periods of moisture deficits.

5. Erosion Control
Greater aggregate stability is often the result of soils with moreorganic matter, which can increase water infiltration rates and result in reduced potential for water, soil and nutrients to erode. Data used in the universal soil loss equation indicate that increasing soilorganic matterfrom 1 to 3% can reduce erosion 20 to 33% because of increased water infiltration and stable soil aggregate formation.

Management Practices to Increase Soil Organic Matter

Several factors such as climate, soil type, crop grown and specific management practices can each influence the amount of soilorganic matterregionally or in specific fields; therefore, local conditions should be used as benchmarks for comparison when implementing practices to increase soilorganic matter(Figure 3, STS).

Soils that have greaterorganic matter, such as those originally developed in prairies, annually cycled residues to the soil whereas forest soils with lowerorganic mattertook longer to cycle the woody material back to the soil.

Building soilorganic matteris a slow process that takes years or decades as it requires the addition of substantial plant biomass and protection from loss over time.

Three management strategies to build and maintain soilorganic matter are:

1. Minimize tillage or adopt no-till: Slows soil organic residue decomposition and gives greater protection to the soil from erosion
2. Add crop residues by including cover crops, organic matter (e.g., residues, manure, etc.), or growing a high biomass/yielding crop rotation. Crop residues help protect the soil surface from raindrop impact and erosion and add carbon back to the soil.
3. Soil test and apply advanced crop nutrition: Identify and correct yield-limiting factors to encourage greater growth of crops that can be returned to the soil. Adding crop residues that help build or maintainorganic matterand including other potentially limiting essential crop nutrients can create more productive and sustainable cropping systems.