Organic Matter in Soil

While the inorganic portion of soil comes from the weathering of non-living rocks, the organic portion of soil is derived from living organisms. There are two types of organic matter in soils:

  1. Humus, which is stable and resistant to further decomposition
  2. All other organic material in various stages of decomposition

There are many benefits to having organic matter in your soil. For example, it improves the physical condition of the soil such as its texture and structure. Organic matter improves the ability of heavy clay soils to drain, while improving the ability of sandy soils to retain water and much-needed nutrients. The decomposition of organic matter releases nutrients like nitrogen, phosphorus, magnesium, calcium, and more into the soil, that can then be absorbed by plants to grow. There are countless ways that organic matter benefits soil composition, but in general it improves the overall tilth of the soil.

In some sections of the country, like the South, there may be very little organic matter in soil. In such cases, you may need to add organic matter each year in the form of manure, compost, saw dust, leaf mold, and cover crops for gardens to have the necessary amount of organic matter.

How to Add Organic Matter with Cover Crops

A cover crop is a crop planted in a garden during the off season to improve the overall productivity of the soil. While they are alive, cover crops help prevent the existing garden soil from eroding and assist in controlling weeds. When it is

time to plant the garden each year, the cover crop is plowed under to providing the soil with organic matter to improve soil structure, the ability of the soil to drain properly, and the ability of the soil to hold on to water and nutrients. As this cover crop decomposes, it releases important nutrients in the soil which can then be absorbed by the garden’s plants. The type of cover crop and length of time it is allowed to grow determines the amount of organic matter and types of nutrients released during decomposition.

There are two basic types of cover crops: legumes and non-legumes. Within each of these two basic types of cover crops, there are cool-season and warm-season versions, resulting in a total of four categories of cover crops.

Legumes have nitrogen-fixing rhizobia bacteria in their roots which capture nitrogen from the air for absorption by the legumes. While legumes may provide the soil with more nitrogen than non-legumes, they are also at higher risk of harboring viral diseases and soil-borne diseases. Nonlegume cover crops provide less nitrogen, but more organic matter when plowed under.

If cover crops are used over several years, the percentage of organic matter in the soil will slowly increase. Increased organic matter will increase the activity of earthworms, fungi, and other organisms in the soil which break down the organic matter improving the overall tilth and structure of the soil.

How Organic Matter Decomposes and the Carbon-Nitrogen Ratio

Regardless of whether organic matter is added to the soil via cover crops, manure, sawdust, or naturally over time, it is decomposed, or broken down, by living organisms in the soil such as earthworms, bacteria, and fungi. Other outside factors affect the rate at which that organic matter decomposes such as the amount of moisture present, the temperature of the soil, and the carbon-to-nitrogen (C:N) ratio.

The carbon-to-nitrogen ratio of soil affects how organisms decompose organic matter. For organic matter in stable soil, this C:N ratio is typically in the range of 10:1 to 12:1. Before they decompose, alfalfa and manure can have a C:N ratio of about 10:1 and 20:1, respectively. However, this ratio can be quite a bit higher for certain types of undecomposed organic matter like straw, which has a C:N of about 80:1 and sawdust, which has a C:N ratio of about 500:1.

Once the carbon-to-nitrogen ratio in organic matter exceeds 30:1, there is very little organic nitrogen in comparison to the carbon. So, the microbes will start robbing inorganic nitrogen from the soil that would otherwise have been consumed by plants growing in the soil. This inorganic nitrogen is found in the soil in the form of ammonium (NH4+) and nitrate (NO3-). The microbes convert the inorganic nitrogen compounds into organic nitrogen through a process called immobilization. As a result, plants have less nitrogen available to absorb from the soil.

The gap between the amount of carbon and nitrogen in the soil decreases or narrows as decomposition continues and the organic matter gets closer to forming humus. The organic microbes die as a result of the diminished supply of food for energy, and their proteins are broken down by the next wave of microorganisms (primarily bacteria) which as a result increase the amount of inorganic nitrogen (NH4+) in the soil. This process where predecessor organisms die and provide food for the next wave of organisms is repeated until the material is decomposed producing inorganic nitrogen compounds that can be consumed by plants and is called mineralization. Over time, this will improve the tilth and fertility of the soil.

If you incorporate undecomposed organic matter with a high carbon-to-nitrogen ratio into the soil, as described above, this can cause a deficiency of inorganic nitrogen in the soil. For this reason, it is best to add inorganic nitrogen to the soil in the form of fertilizer before planting or during the growing season.

If you add undecomposed organic matter with a high carbon-to-nitrogen ratio on top of the soil (for example, spreading mulch around plants in flower beds), this generally does not lead to any type of noticeable deficiency in nitrogen. For this reason, doing so would typically not require that you boost the levels of inorganic nitrogen with fertilizer.

If you incorporate organic matter with low carbon-to-nitrogen ratios into your soil, this organic matter tends to decompose very fast and does not cause inorganic nitrogen to be “robbed” from the soil (and plants growing there). There is plenty of organic nitrogen in the decaying organic matter to feed the microorganisms. The decomposition process results in the production of inorganic nitrogen-based compounds – ammonium (NH4+) and nitrate (NO3-) - which can be absorbed by plants. Immobilization does not occur in this instance.

What is Humus?

Once organic matter in soil reaches a point where it begins to resist further decomposition, it is referred to as humus. Like clay particles, humus has negatively charges spots on its surfaces that adsorb positively charged plant nutrient cations like ammonium, calcium, hydrogen, magnesium, and potassium. This improves the soil’s overall cation exchange capacity, increasing its ability to retain plant nutrients.

Once nutrient cations have been adsorbed by soil particles in the root zone, this allows for a constant exchange of cations between the surface of the clay and humus soil particles and the soil solution in the pores of the soil. Once these plant nutrient cations are in the soil solution, they can be absorbed by plants.