SOIL SCIENTIST

What is organic matter?

Organic matter is the vast array of carbon compounds in soil. Originally created by plants, microbes, and other organisms, these compounds play a variety of roles in nutrient, water, and biological cycles. For simplicity, organic matter can be divided into two major categories: stabilized organic matter which is highly decomposed and stable, and the active fraction which is being actively used and transformed by living plants, animals, and microbes. Two other categories of organic compounds are living organisms and fresh organic residue. These may or may not be included in some definitions of soil organic matter.

Stabilized organic matter

Many soil organisms decompose plant and animal tissues, and transform the organic matter into new compounds. After years or decades of these transformations, what remains are large, complex compounds that few microbes can degrade. Other compounds become bound inside soil aggregates where microbes cannot reach. These hard-to-decompose, or stabilized, substances make up a third to a half of soil organic matter. Scientists often divide stabilized organic matter into three chemical groups: humic acids, fulvic acids, and humins. Fungi and actinomycetes create many of the humic acids that combine in soil to become stable compounds. Much of the stabilized matter in your soil originated from plants that grew one or more centuries ago. Some of these old compounds are bound to clay, and are important in gluing together tiny aggregates of soil particles.

Stabilized organic matter acts like a sponge and can absorb six times its weight in water. In sandy soils, water held by organic matter will make the difference between crop failure or success during a dry year.

Organic compounds in soil
This pie chart represents organic matter in soil before cultivation. After land has been cultivated for one or two decades, much of the active fraction is lost and stabilized organic matter makes up more than half of the soil organic matter.

Both organic and clay particles can hold on to nutrients electrochemically - like a magnet holds on to iron filings. The amount of nutrients that the organic compounds and clay could carry and make available to plants is called the soil’s cation exchange capacity (CEC). In Minnesota, a pound of stabilized organic matter can have five times as much CEC as a pound of clay. In other words, organic matter can hold five times as much nutrients for plants to use. Half or more of the CEC in Minnesota topsoil comes from organic matter. Although you cannot change the amount of clay in your soil, you can easily decrease or (with more difficulty) increase the amount of organic matter in your soil.

In addition to nutrients, stabilized organic matter holds on to pesticides. This prevents pesticides from moving into water supplies and improves the decomposition of the compounds, but it also makes pesticides less effective by preventing their contact with the target organism.

The active fraction

Up to 15% of soil organic matter is fresh organic material and living organisms. Another third to one half is partially and slowly decomposing material that may last decades. This decomposing material is the active fraction of soil organic matter.

The active organic matter, and the microbes that feed on it, are central to nutrient cycles. Many of the nutrients used by plants are held in organic matter until soil organisms decompose the material and release ammonium and other plant-available nutrients. Organic matter is especially important in providing nitrogen, phosphorus, sulfur, and iron. A soil with 3% organic matter contains about 3,000 pounds of nitrogen per acre. Depending on the rate of decomposition, 25 to 100 pounds may become available to plants in a year, but it is difficult to predict the decomposition rate. In Minnesota, decomposition rates are typically about 2%. This means that one acre of crop plants on a soil with 3% soil organic matter may get 60 pounds of their nitrogen and 6 pounds of their phosphorus from soil organic matter.

The Changing Forms of Soil Organic Matter Additions. When roots and leaves die, they become part of the soil organic matter. Transformations. Soil organisms continually change organic compounds from one form to another. They consume plant residue and other organic matter, and then create by-products, wastes, and cell tissue. Microbes feed plants. Some of the wastes released by soil organisms are nutrients that can be used by plants. Organisms release other compounds that affect plant growth. Stabilization of organic matter. Eventually, soil organic compounds become stabilized and resistant to further changes.

Organic matter also affects nutrient cycles by chelating (chemically holding on to) nutrients, and preventing them from becoming insoluble and therefore unavailable to plants. For example, humic substances help make iron available to plants, even in medium-to-high pH soils.

Regular additions of organic matter are important as food for microorganisms, insects, worms, and other organisms, and as habitat for some larger organisms. Soil organisms degrade potential pollutants, help control disease, and bind soil particles into larger aggregates. Well-aggregated, crumbly soil allows good root penetration, improves water infiltration, makes tillage easier, and reduces erosion.

Very fresh organic matter can cause problems to crops in two ways - nitrogen tie-up and allelopathy. A temporary nitrogen deficiency for crops occurs if the organic matter is low in nitrogen. "Allelopathic" chemicals are formed when some residues decay, and can inhibit plant growth. (See pages 5 and 6 for more information.)

Organic matter is more than fertilizer

Organic matter is not just N, P, K, and carbon. Two sources of organic matter with the same nutrient content or total organic matter content might not have equal effects on your crops and soils.

In one research trial, fields treated with animal manure had different microorganisms and enzymes than fields where green manure or mineral fertilizers were used. The importance of these differences are not well studied, but they probably affect nutrient cycling and pests. In your system, manure may mean positive effects such as reducing some diseases, or negative effects such as increasing weed growth.

Plant residues also differ greatly as a source of organic matter. Above-ground growth has a different action in soil than roots, even when it is tilled into the soil. All roots do not act the same. For example, tap-rooted plants such as alfalfa create vertical pores in the soil, whereas the finely branched roots of grasses enhance soil aggregation.

What Does Organic Matter Do?

Nutrient cycling

• Increases the nutrient holding capacity of soil (CEC).
• Is a pool of nutrients for plants.
• Chelates (binds) nutrients, preventing them from becoming permanently unavailable to plants.
• Is food for soil organisms from bacteria to worms. These organisms hold on to nutrients and release them in forms available to plants.

Water dynamics

• Improves water infiltration.
• Decreases evaporation.
• Increases water holding capacity, especially in sandy soils.

Structure

• Reduces crusting, especially in fine-textured soils.
• Encourages root development.
• Improves aggregation, preventing erosion.
• Prevents compaction.

Other effects of soil organic matter

• Pesticides break down more quickly and can be "tied-up" by organic matter (and clays).
• Dark, bare soil may warm more quickly than light-colored soils, but heavy residue may slow warming and drying in spring.
• Many of the effects of organic matter are related to the activity of soil organisms as they use soil organic matter. See Soil Biology (BU-7403 in this series) for more information.
• Plant residues and other organic material may support some diseases and pests, as well as predators and other beneficial organisms.

What Determines Soil Organic Matter Levels?

The amount of organic matter in soil is the result of two processes: the addition of organic matter (roots, surface residue, manure, etc.), and the loss of organic matter through decomposition. Five factors affect both additions and losses.

Management. Practices that increase plant growth on a field (cover crops, irrigation, etc.) will increase the amount of roots and residue added to the soil each year. On the other hand, intensive tillage increases the loss of organic matter by speeding decomposition. While tillage primarily burns younger organic matter, older, protected organic compounds can be exposed to decomposition if small aggregates are broken apart. In addition to changing the amount of soil organic matter, tillage practices affect the depth of soil organic matter.

Soil texture. Fine-textured soils can hold much more organic matter than sandy soils for two reasons. First, clay particles form electrochemical bonds that hold organic compounds. Second, decomposition occurs faster in well-aerated sandy soils. A sandy loam rarely holds more than 2% organic matter.

Climate. High temperatures speed up the degradation of organic matter. In areas of high precipitation (or irrigation) there is more plant growth and therefore more roots and residues entering the soil.

Landscape position. Low, poorly-drained areas have higher organic matter levels, because less oxygen is available in the soil for decomposition. Low spots also accumulate organic matter that erodes off hill tops and steep slopes.

Vegetation. In prairies, much of the organic matter that dies and is added to the soil each year comes from grass roots that extend deep into the soil. In forests, the organic matter comes from leaves that are dropped on the surface of the soil. Thus, farmland that was once prairie will have higher amounts of organic matter deep in the soil than lands that were previously forest.

Differences in organic matter appear in the color changes across a bare field. Less organic matter is produced on the drier hilltops, and some is lost to soil erosion and deposited in low spots.

How do organic matter levels change?

To build organic matter levels in topsoil, more organic matter must be added than is lost to decomposition and erosion. Like a person trying to lose or gain weight, increasing organic matter is about changing the balance between how much energy goes in and how much is burned off.

Another way to think of soil is like a giant wood stove. You continually add organic matter (wood), and it burns to release energy and nutrients that will be used by plants and microorganisms. Ideally, you want a slow, steady burn that releases nutrients to plants as needed.

Intensive tillage aerates the soil and is like opening the flue or fanning the flames. Decomposition is desirable because it releases nutrients and feeds soil organisms. But if decomposition is faster than the rate at which organic matter is added, soil organic matter levels will decrease.

Reducing decomposition is only half the equation. It is just as important to increase the amount of organic matter added to the soil. Organic matter can be either grown in the field or brought to the field.

How long does it take? Building organic matter is a slow process. First, the amount of residue and active organic matter will increase. Gradually, the species and diversity of organisms in the soil will change, and amounts of stabilized organic matter will rise. It may take a decade or more for total organic matter levels to significantly increase after a management change. Fortunately, the beneficial effects of the changes appear long before organic matter levels rise. These improvements, however, can be reversed in a year or two by returning to previous practices.

Why does it take so long for organic matter levels to increase? An acre of soil six inches deep weighs about 1000 tons, so increasing the proportion of organic matter from two to three percent is actually a 10 ton change. However, you cannot simply add 10 tons of manure or residue and expect to measure a one percent increase in soil organic matter. Only ten to twenty percent of the original material becomes part of the soil organic matter. Much of the rest is converted over several years into carbon dioxide.

An illustration of soil organic matter losses and gains in response to tillage.
Most organic matter losses in soil occurred in the first decade or two after land was cultivated. Native levels of organic matter may not be possible under agriculture, but many farmers can increase the amount of active organic matter by reducing tillage and increasing organic inputs.