Soil Organic Matter – An Overlooked Resource
Many farmers have only a general understanding of their soil. Provided it grows plants and feed for livestock, then what else do you need to know? The better your understanding of your soil, the better your ability to make good fertiliser decisions!
What is soil?
In broad terms soil can be defined as a series of horizontal layers on the earths surface containing a mixture of inorganic material (derived from the soil parent material and fertiliser additions) and organic matter - comprising both living organisms (plant roots, earthworms, insects, fungi, bacteria etc) and the remains left over from living organisms after they die. It also contains the residue from plants (stems, leaves, shoots) as well as plants that have been eaten by animals (dung etc).
Why is organic matter important?
Organic matter is a vital component of any good soil. It improves soil stability. It increases soil water holding capacity as well as preventing the leaching of nutrients. It increases the supply of essential plant nutrients, especially of nitrogen, phosphorus and sulphur as well as trace elements like boron and copper. It does this primarily by providing the food and energy required by soil micro-organisms. These in turn release nutrients from the organic matter back into soil solution. It improves the cation exchange capacity of a soil and, because of its dark colour; it helps to absorb more of the suns heat. In fact, it is not overstating the case to say that organic matter is critical for a healthy, living and productive soil.
What is soil fertility?
In a general sense, this is an estimate of a soils ability to supply essential plant nutrients for a particular land use i.e. agriculture, horticulture or forestry. It incorporates both the inorganic and the organic component of any soil but should also take account of other factors: soil physical properties (structure, texture), climate, slope etc. Obviously, the most fertile soils tend to underlie the best and most valuable productive land, whilst soils of lower fertility are less valuable i.e. they offer the farmer fewer production options and lower returns.
What impact does organic matter have on a soil’s fertility?
Farming extracts substantial amounts of nutrients from a soil. For productivity to be maintained, fertility also needs to be maintained or improved. The obvious question is “How?” The obvious answer is “by applying fertiliser.” Two questions need to be addressed before righting out a fertiliser order. Firstly, what is the overall nutrient status of the soil. Basic soil tests like Olsen P and pH do not provide enough information. Other nutrient(s) may be lacking and thus limiting productivity. You need the right nutrients in balance for good productivity.
Secondly, many soil testing agencies only focus on the inorganic component of the soil and thus provide no information on the critical organic matter fraction. Because the organic matter can also supply nutrients, clearly it makes good economic sense to test for organic parameters as well i.e. organic matter, total phosphorus, total sulphur etc.
Conclusion
Organic matter is a valuable resource that is often overlooked and unappreciated. The fertility of a soil is dependent on the commonly tested inorganic fraction and the organic matter as well. Farmers should get soil test information on both the inorganic and the organic components of their soil so they can make better decisions on fertiliser requirements.
