Soil properties are strongly influenced by the activity of the organisms that inhabit the soil. A healthy soil, also called a soil of high quality, supports incredible numbers of organisms. These may be the macroscopic organisms such as earthworms and nematodes or microorganisms such as fungus, actinomycetes and bacteria. These organisms breakdown the plant material into soil organic matter.
Earthworms benefit soil in several different ways. They begin the cycle of decomposition for organic matter. They eat organic matter that is difficult for other organisms to break down, like lignin and cellulose and excrete compounds that other organism can digest. They ingest soil as they dig and excrete small peds that form channels for water to enter and gasses to be exchanged.
The soil microorganism, fungi, actinomycetes, and bacteria, differ in their respective abilities to decompose organic matter, tolerate drought and other forms of stress, their numbers and biomass in the soil, and in the other functions that they perform in the soil. Fungi can breakdown lignin and cellulose and also begin the decomposition of organic matter. They are more resistant to drought than the other microorganisms. Fungi are composed of long strings cells called hyphae that create mycells. Fungi are the least numerous of the soil microorganisms. Actinomycetes look like fungus but their cells are more like bacteria. Their numbers are intermediate between fungi and bacteria. They can digest some of the hard to digest organic compounds and are somewhat drought resistant. Actinomycetes isolated from soil have provided a number of antibiotics that we use like streptomycin. Bacteria are the most numerous soil organisms. They are not drought tolerant and cannot decompose complex organic compounds. They are very important in N cycling, sulfur chemistry in mine spoils, and global climate change.
Soil organic matter is composed of organic compounds Soil organic matter is divided into three forms: water soluble fulvic acid, humic acid that is soluble at high pH but not at low pH, and water insoluble humin. Fulvic acid is readily used by microorganisms for building tissue and for energy. Humic acid can also be used by microorganisms, but it contains compound that cannot be used by all organisms. Humin is very resistant to use by the microorganism.
Soil organic matter provides a reservoir for nutrients. Soil organic matter is an important source of nitrogen, sulfur, phosphorus and many of the micronutrient to plants. Nutrients in soil organic matter cannot be leached from the soil. This is especially important for nitrogen because nitrate moves with the water and can be lost for the root zone. Nitrate is a pollutant in waters, creating high algae concentration and causing health problems. Soil organic matter increases water holding capacity and infiltration. Organic matter binds soil particles together to form structure.
All nutrients move through biogeochemical cycles in the environment in which the nutrient exists for a time in soil organic matter, living tissue, and as an inorganic compound. Soil organic matter is constantly changing as it decomposes. For soil productivity to be maintained fresh organic matter must be added to the soil. There are many possible organic amendments that could be used: plant residue, animal manures, industrial or municipal wastes, food processing residues, wood processing waste, and green manure. A green manure is a crop that is grown for the sole purpose of incorporation into the soil to increase organic matter and organic nitrogen. The decomposition of organic matter is strongly influenced by the carbon:nitrogen (C:N) ratio of the material. When the C:N ratio of the organic matter is greater than about 13:1, nitrate and ammonium in the soil are converted to microbial biomass. The C:N ratio of microbial tissues is about 10:1 for fungus and 5:1 for bacteria. Nitrogen is released when the C:N ratio of the organic matter is about 13:1 because some of the decomposition is used for energy and the organic carbon is converted to CO2.
| Organic Matter | C:N |
|---|---|
| Alfalfa | 13:1 |
| Bluegrass | 20:1 |
| Corn Stalks | 40:1 |
| Wheat straw | 80:1 |
| Sawdust | 225:1 |
When wheat straw (or any other small grain straw) is added to the soil, the population of microorganisms increases. The amount of nitrogen available to plants is decreased by microorganisms that take up the nitrogen as the straw is decomposed. Eventually the fresh organic matter is used up and the microorganism population decreases. As the surviving microbes use the tissues of the dead organisms for building biomass and energy, N is released back to the soil and to the plants. There is a lag time from incorporation of fresh organic matter with a high C:N ratio and the release of N. Crops should not be planted until the lag time has nearly ended, or unless N fertilizer is added to reduce the C:N ratio.
In third world countries where N fertilizer is relatively expensive, straw is burned to make the N immediately available. This has several negative effects on the environment. First, fresh organic matter is not added to the soil and the benefits to structure, aeration, resistance to erosion are lost. Second, the nitrogen released from the burning may be lost by wind or water erosion, or leached from the soil. Third, the burning creates air pollution.
Liebigs Law of the Minimum states that plant growth is limited by the factor in shortest supply. The factor may be water or a nutrient. Nitrogen is a nutrient that is often limiting to plant growth. The nitrogen cycle is very important to soil quality. Nitrogen only has two inorganic forms, NO3- and NH4+, and its organic forms are mainly amino acids. Some elements like P, take many more organic and inorganic forms.