The importance of soil organic matter : farming for the environment, society and economy

Abstract

Put simply, soil organic matter, is what differentiates soil from dirt. Soil is composed of weathered mineral material, organic material, water and air; and it is the organic material present that sets soil apart from simple weathered rock, or dirt. The organic material, which is carbon-based molecules, is referred to as soil organic matter (SOM). A very good definition is, “Soil organic matter is soil material that originates from organisms that were once or are currently living” (Magdoff & Weil, 2019). SOM can be divided into two fractions, the living fraction and the “dead” fraction. The “dead” fraction can be further divided into three pools; active, slow and passive (Grubinger, 2015). The living fraction is composed of living plant roots and various soil dwelling animals; from rodents, to earthworms and to the tiniest of microorganisms. This fraction is essential as it plays the vital role of contributing organic residues to the soil and in the formation of more stable pools of organic matter. Soil microbes, also part of the living fraction, are the drivers of the SOM cycle. There are four main processes in this cycle; decomposition of organic residues, release of nutrients, release of carbon dioxide and transfer of carbon from one SOM pool to another. The three pools of “dead” SOM are determined by the time it takes for them to completely decompose. The active pool is primarily made up of root exudates and fresh plant and animal residues that break down in a very short time, from a few weeks to a few years. High biological activity is associated with this pool. The passive pool, also referred to as humus, provides very little food for soil organisms and hence considered biologically inactive. It may take hundreds or even thousands of years for organic material in this pool to fully decompose, which makes this pool very stable. The slow pool is somewhere in between the active and passive pool; and it may take a few years to a few decades for organic material to completely break down. This pool consists primarily of detritus, partially broken-down cells and tissues that are only gradually decomposing (Grubinger, 2015). Considering that the living fraction is essential for the formation and maintenance of all three pools of the “dead” fraction, it is imperative that farmers manage soils towards maximising the living fraction within.