THE SOIL ORGANIC MATTER, HUMUS AND THE FOODWEB OF SOIL

Critical to any model for sustainable soil management, it is important to understand the role that soil organisms’ play and that farmer focus on strategies that build both their numbers and their diversity. As with cattle and other farm animals, soil livestock require proper feed. That feed comes in the form of organic matter to soil.

In farming and to the farmer, organic matter and humus are terms that describe somewhat different but related things. Organic matter in its qualities is refers to the organic fraction of the soil that is composed of both living organisms and once living residues in various stages of decomposition. Humus is only a small portion of the totality of organic matter. It is the end product of organic matter decomposition and is relatively stable. Further decomposition of humus occurs very slowly in both agricultural and natural settings as well. In natural systems, a balance is reached between the amount of humus formation and the amount of humus decay. In most agricultural soils, this balance also occurs, but often at a much lower level of soil humus. Humus contributes to the well structured soil that is, in turn, produces high quality and productive plants. It is clear that management of organic matter and humus is essential to sustain the whole soil ecosystem.

To the farmer and soil scientist, the benefits of a soil rich in organic matter and humus are many. These benefits includes; rapid decomposition of crop residues, granulation of soil into water stable aggregates, decreased crusting and clodding, improved internal drainage, better water infiltration, and increased water and nutrient holding capacity in the soil. Improvements in the soil's physical structure facilitate easier tillage operation, increased soil water storage capacity, reduced soil erosion, better formation and harvesting of root crops, and deeper, more prolific plant root systems. Improvements in nutrient cycling also reduce the fertilizer bill as well.

Ultimately, building organic matter and humus levels in the soil is a matter of managing the living organisms which are present in the soil. This entails working to maintain favorable conditions of moisture, temperature, nutrient status, soil pH, and soil aeration. It also involves providing a steady food source. All the soil organisms mentioned previously, except algae, depend on organic matter as their food source. Therefore, to maintain their populations, organic matter must be renewed from plants growing on the soil, or from animal manure or other materials which are imported from off site. By feeding the soil livestock, fertility is built up in the soil and the soil will feed the plants with its necessary plant nutrients.

Physical condition (Tilth) of soil and organic matter

Any soil that drains well, does not crust, takes in water rapidly, and does not make clods is said to have a very good tilth. Tilth is the physical condition of the soil as it relates to tillage ease, seedbed quality, easy seedling emergence, and deep root penetration. A good tilth is greatly dependent on aggregation, the process whereby individual soil particles are joined into clusters or aggregates.

Aggregates are formed in soils when individual soil particles are oriented and brought together through wetting and drying, freezing and thawing, and by plant growth and earthworm activities. The weak electrical forces from calcium and magnesium hold the soil particles together when the soil dries. When the aggregates become wet again, their stability is then challenged and they may break apart once again. In the case of earthworm created aggregates, they are stable once they come out of the worm. An aggregate formed by physical forces becomes stabilized which will remain intact when wet through microbial processes involving organic matter decomposition and its by-products, waxes, and other glue like substances. These by-products cement the soil particles together forming water stable aggregates. The aggregate of the soil is then strong enough to hold together when wet hence the name water-stable.

Tillage, organic matter, and plant productivity and growth

There are several factors affect the level of organic matter that can be maintained in a soil. Among these factors are organic matter additions, moisture, temperature, tillage, nitrogen levels, cropping, and fertilization. The level of organic matter which is present in the soil is a direct function of how much organic material is being produced or added to the soil versus the rate of its decomposition. The objectives of this balancing act entail slowing the speed of organic matter decomposition, while increasing the supply of organic materials which are produced on site or the addition of organic matter from off site.

In the soil, moisture as well as temperature also profoundly affects soil organic matter level. One only has to compare organic matter which is present in soils to see the decline in organic matter due to higher temperature and moisture in the soil. The most dramatic example of the effects of temperature and moisture may be seen by comparing the organic matter rich soils of the arctic region with the organic matter poor soils of the tropics. The warmer and wetter it gets, the more difficult to maintain soil organic matter.

Tillage can be beneficial or harmful to a biologically active soil depending on what type of tillage that is used and when it is done. Tillage affects both soil erosion rate and soil organic matter decomposition rate. Crop lands having organic matter levels below one percent are biologically dead. These low levels are due primarily to tillage. Clean tillage involving moldboard plowing and disking breaks down soil aggregates and leaves the soil prone to erosion from wind and water. The moldboard plow can bury crop residue and topsoil to a depth of fourteen inches. At this depth, the oxygen level in the soil is so low that decomposition cannot proceed adequately in the soil.

Surface dwelling decomposer organisms suddenly find themselves suffocated and soon die due to suffocation. The crop residue that was originally on the surface but has now been turned under will putrefy in the oxygen deprived zone as it rots in the absence of oxygen.