Soil organic matter By Robin Boom
Organic matter in the soil consists of plant and animal matter which has gone through a decomposition process by soil organisms to break it down into humic substances. These become stable in the soil itself and eventually reach a state where they are not
Soil organic matter contains just under 60% carbon and its accumulation in the soil, known as sequestration, helps reduce carbon dioxide in the atmosphere and is regarded as a solution for global warming and climate change. Conversely the loss of soil organic matter increases green house gases. In New Zealand one of the positive attributes of our better class soils is their relatively high organic matter content compared to many soils around the globe.
Different land uses can affect organic matter losses and gains. Under forestry, soil organic matter builds up from leaf fall and from plant root exudates over time, but some of this can be lost when the forest is harvested. Under permanent pasture organic matter normally builds up over time, but recent research has shown that under intensive dairying where stocking rates are high and nitrogen fertiliser use has increased, that there is a net annual loss on average of approximately a tonne of carbon on most soils except on the volcanic andesitic ash soils where humus levels continue to accumulate. Under extensive sheep and beef there is on average about half a tonne of carbon which is sequestered annually, and under permanent horticultural tree and vine crops there is a net accumulation of organic matter. Continuously cropped ground can lose many tonnes of organic matter in a single year, depending on the cultivation method used, and there are moves internationally to reduce the amount of carbon loss through using conservation tillage techniques such a no till, minimum tillage and strip tillage.
Having a good amount of organic matter (humus) is right at the heart of having healthy, fertile and productive ground, as it provides many functions to do with the physical, chemical and biological properties of the soil itself. Soil organic matter is that dark substance that is found most densely in the topsoil, and is responsible for giving soil its tilth, making it easy to cultivate, and it also maintains good soil structure so that the soil colloids are crumbly and hold together well in nutty granular aggregates, creating space for water and oxygen to move freely through the soil profile. Soils low in organic matter become sticky and solid, and water just sits on the surface not being able to perculate down through the soil. With a winter like we have just come through, soils low in organic matter remain at saturation point a lot longer, and when digging a spade spit of dirt, it appears bound together as one gluggy lump.
The repeated cycle of carbon loss from cultivation causes the soil to become more compacted and more prone to erosion and water surface run-off, and ever increasing amounts of fertiliser are required to maintain crop production. Beneficial soil organisms such as earthworms, fungi, bacteria and actinomyces species which thrive on the organic matter decline as humus levels drop and crops consequently become more susceptible to insect pests and soil borne diseases. Consequently there is a greater need to use more pesticides, herbicides and fungicides which further depletes the positive soil biota