Soil Organic Matter
- What is organic matter?
- What are the 4 main steps in degradation of organic matter?
- What is the impact of incorporating organic matter into the soil?
- What is humus?
Question 1: What is organic matter?
Organic matter is anything that contains carbon compounds that were formed by living organisms. It covers a wide range of things like lawn clippings, leaves, stems, branches, moss, algae, lichens any parts of animals, manure, droppings, sewage sludge, sawdust, insects, earthworms and microbes.
There are 3 main components of organic matter in soils:
- dead forms of organic material - mostly dead plant parts
- living parts of plants - mostly roots
- living microbes and soil animals
By far the largest component is the dead matter - it constitutes about 85% of all organic matter in soils. Living roots make up about another 10% and the microbes and soil animals make up the last few percent. Partly decayed organic matter is called humus.
Organic matter in soil
Soil sand - no organic matter
In answering this question, we concentrate mainly on dead plant matter as this constitutes the major part of organic matter in soil (apart from living roots). However, the broad principles also apply to the degradation of animal and microbial matter.
Living organisms are made up of thousands of different compounds, so when they die there are thousands of compounds in the soil to be decomposed. As these compounds are decomposed, the organic matter in soil is transformed and gradually changes so that eventually it is no longer recognizable as part of the original plant. These stages are:
- Breakdown of compounds that are easy to decompose - like sugars, starches and proteins
- Breakdown of compounds that take several years to decompose like cellulose (an insoluble carbohydrate found in plants), lignins (a very complicated structure that is part of wood)
- Breakdown of compounds that can take up to 10 years to decompose - like some waxes and the phenols. This also includes compounds that have formed stable combinations and are located deep inside soil aggregates and are therefore not accessible to soil organisms
- Compounds that take tens, hundreds or thousands of years to decompose include humus-like substances which are the result of integration of compounds from breakdown products of plants and those generated by microorganisms humus-like substances.
Compounds in the first group are quick and easy for fungi and bacteria to decompose, so the carbon and energy they provide is readily available. Most of the microbes living in the soil have the enzymes needed to decompose these simple compounds. This type of decomposition is the first stage during the degradation of organic matter. Mites and small soil animals often help this stage by breaking up the organic matter into smaller pieces exposing more of the material to colonisation by bacteria and fungi.
The second stage involves the microbes decomposing more complicated compounds - the second group listed above. Many, but not all, fungi and bacteria can decompose these compounds. The compounds take longer to decompose because they are much larger and they are made up of more complicated units than the compounds in the first group. Specific enzymes, not commonly produced by most microorganisms are required to break down these compounds.
hyphae asociated with organic matter
(photo van Vliet and Gupta)
It's important to realise that decomposition will only take place if conditions are suitable. The rate of breakdown is greatly affected by the conditions. There must be some moisture available, soil temperatures must be suitable (usually between 10 and 35°C) and the soil must not be too acidic or alkaline. Decomposition also occurs at higher temperatures, as in composts, or under waterlogged conditions. The types of organisms involved in breaking down the organic matter will depend on the conditions.
Incorporating organic matter into soil can have several impacts because it disturbs the physical, chemical and biological balances in the soil. It can:
- change the amount of nitrogen that is available to plants
- change the amount of other nutrients available
- change the way the soil sticks together (soil aggregation)
- change the number and type of organisms present in the soil
All of these changes are related to the way organic matter is decomposed when it is incorporated into soil and to the particular type of organic matter used.
When organic matter is incorporated into soil, the larger organisms like mites and soil animals break it into smaller pieces. Then the fungi and bacteria start to decompose it (they secrete enzymes to break up the chemical compounds it is made of). When the enzymes break the molecules into smaller sections, the bacteria and fungi can use some of energy or nutrients released for their own growth. For example, when an enzyme stimulates the breakdown of a protein, the microbe may be able to use the carbon, nitrogen and sulphur (if there is some) for its own physiological processes and cell structure.
If there are nutrients that the microbes do not use, they will be available for other soil organisms or plants to take up and use. When microbes die, their cells are degraded and nutrients contained within them become available to plants and other soil organisms.
Microbes can access nitrogen in the soil more easily than plants can, so the plants sometimes miss out. This means that if there is not enough nitrogen for all the organisms, the plants will probably be nitrogen deficient. This is why incorporating organic matter into soils can change the amount of nitrogen (and other nutrients) that is available to plants. These will be a short-term effect that happens when soils do not have high levels of organic matter and soil microbes.
If the number of fungi and bacteria associated with the breakdown of breakdown or organic matter increases, there may be some improvements to the soil structure. Adding organic matter can also increase the activity of earthworms, which in turn can also improve soil aggregation. If organic matter is retained in the soil, the number of microbes in the soil increases. This is because the microbes can use the organic matter as a source of energy and so they can grow and multiply.
Humus is composed of organic compounds that are highly complex in structure. Like other organic matter, humus originates from dead organic material. The compounds that make up humus are highly complex organic compounds that have resisted decomposition and have accumulated in the soil. Humus organic matter is so altered that it can no longer be recognised as plant material.