Agronomy Fact Sheet Series Fact Sheet 41 Soil Organic Matter Soil organic matter is the fraction of the soil Benefits of Stable Soil Organic Matter that consists of plant or animal tissue in There are numerous benefits to having a various stages of breakdown (decomposition). relatively high stable organic matter level in an Most of our productive agricultural soils have agricultural soil. These benefits can be grouped between 3 and 6% organic matter. into three categories: Soil organic matter contributes to soil productivity in many different ways. In this Physical Benefits fact sheet, we describe the various Enhances aggregate stability, improving components of organic matter and the water infiltration and soil aeration, different roles organic matter plays in soil reducing runoff. productivity. We also discuss field Improves water holding capacity. management practices that will help preserve Reduces the stickiness of clay soils or increase soil organic matter levels over making them easier to till. time. Reduces surface crusting, facilitating seedbed preparation. What is Soil Organic Matter? Organic matter is made up of different Chemical Benefits components that can be grouped into three major types: Increases the soil’s CEC or its ability to hold onto and supply over time essential 1. Plant residues and living microbial nutrients such as calcium, magnesium biomass. and potassium. 2. Active soil organic matter also referred Improves the ability of a soil to resist pH to as detritus. change; this is also known as buffering 3. Stable soil organic matter, often capacity (see Agronomy Fact Sheet #5). referred to as humus. Accelerates decomposition of soil The living microbial biomass includes the minerals over time, making the nutrients microorganisms responsible for decomposition in the minerals available for plant (breakdown) of both plant residues and active uptake. soil organic matter or detritus. Humus is the stable fraction of the soil organic matter that is Biological Benefits formed from decomposed plant and animal Provides food for the living organisms in tissue. It is the final product of decomposition. the soil. The first two types of organic matter Enhances soil microbial biodiversity and contribute to soil fertility because the activity which can help in the breakdown of these fractions results in the suppression of diseases and pests. release of plant nutrients such as nitrogen, Enhances pore space through the actions phosphorus, potassium, etc. of soil microorganisms. This helps to The humus fraction has less influence on increase infiltration and reduce runoff. soil fertility because it is the final product of decomposition (hence the term “stable organic Organic Materials matter”). However, it is still important for soil Over time, the application and incorporation of fertility management because it contributes to organic materials can result in an increase in soil structure, soil tilth, and cation exchange stable soil organic matter levels. Sources of capacity (CEC, see Agronomy Fact Sheet #22). organic materials include: This is also the fraction that darkens the soil’s Crop residues. color. Animal manure. Department of Crop and Soil Sciences 1 College of Agriculture and Life Sciences Compost (Figure 1). material added, and tillage and rotation Cover crops (green manure) practices, also depend on: Perennial grasses and legumes. Soil temperature. The quickest increases are obtained with Precipitation and soil moisture holding sources that are high in carbon such as capacity. compost or semi-solid manure. Soil type and drainage class. Existing microbial community. Soil fertility status and soil pH. Monitoring Soil Organic Matter To get an idea of the effect of farm management practices on soil organic matter buildup or decrease, soil samples should be taken over time. Consistency in sampling time is important to build records for fields over time (see Agronomy Fact Sheet #1). Although other tests are available, most laboratories will do a loss-on-ignition (LOI) test to estimate the organic matter content of the soil. At Cornell University, soil is exposed to 105oC (221oF) for 1.5 hours to remove soil moisture and then to Figure 1: Compost application can increase soil 500oC (932oF) for 2 hours to determine LOI. organic matter levels over time. Not all laboratories us the same method so for accurate records over time, it is important to Organic Matter Management consistently use the same laboratory service. Farm practices that help to maintain or increase soil organic matter levels: In Summary Use of conservation tillage practices (for With careful management the preservation and example zone tillage or no-till). Tillage accumulation of soil organic matter can help to exposes the organic matter to air and improve soil productivity resulting in greater will result in the lowering of stable farm profitability. organic matter due to increased mineralization rates and erosion losses. Additional Resources o Cornell University Agronomy Fact Sheet series: Rotation of annual row crops with nmsp.css.cornell.edu/publications/factsheets.asp. perennial grass or legume sods will reduce erosion and build up organic Disclaimer matter as a result of the decomposition This fact sheet reflects the current (and past) authors’ best of the rootmass. effort to interpret a complex body of scientific research, Establishment of legume cover crops and to translate this into practical management options. Following the guidance provided in this fact sheet does not will enhance organic matter assure compliance with any applicable law, rule, regulation accumulation by providing the nitrogen or standard, or the achievement of particular discharge (N) needed for decomposition of freshly levels from agricultural land. added organic materials, especially For more information those with a high C to N ratio (corn stover, cereal straw, heavily bedded manure, etc.). Avoiding soil compaction which increases waterlogging, and maintaining proper pH to enhance microbial activity and decomposition of Nutrient Management Spear Program freshly added materials. http://nmsp.css.cornell.edu Actual buildup of stable organic matter will, in Megan Fenton, Carl Albers, Quirine Ketterings addition to the amount and source of organic 2008 Department of Crop and Soil Sciences 2 College of Agriculture and Life Sciences .