Issue November 2006 Behind the Lines Cover Story Short Takes Stories Crops Business     Letter From Europe Rural Roots   Contact Us

Seedbed   Soil organic matter – the living, the dead and very dead These three groups of organic matter have very different properties, but they all play a crucial role in creating a healthy, productive soil by KEITH REID As near as I can tell, this description of soil organic matter was coined by Dr. Fred Magdoff of the University of Vermont. It is a simple description, yet it sums up very nicely the three main categories of organic matter in the soil. These three groups have very different properties and behave very differently, but they all play a crucial role in creating a healthy, productive soil. The living. This group includes the complete collection of living organisms in the soil -- from bacteria and viruses to earthworms and insects. Some of the living, like plant roots and algae, fix carbon out of the air into organic compounds, but most use organic materials as food sources. These organisms drive most of the chemical transformations that happen in the soil, so the nitrogen cycle and the sulphur cycle, as well as the availability of many micronutrients, depend on the life in the soil. Various soil organisms play different roles in the breakdown of organic materials and the cycling of nutrients. The larger organisms, like earthworms, nematodes, mites, millipedes or insects act as shredders, physically breaking down large pieces of residue into smaller bits that the microbes can get at. Fungi help this process as their hyphae invade the larger pieces of residue, excreting enzymes to break down the bonds between plant cells. Some bacteria are generalists, using a wide range of materials as food and breaking them down further, while others play very specific roles in the soil. Nitrobacter, for example, is a bacterium that converts ammonium into the nitrite form, while a different bacterium, Nitrosomonas, converts the nitrite to nitrate.  Adding to the complexity of life in the soil is another population of organisms that behave as predators, eating the smaller organisms while trying to avoid being eaten themselves. These predators also help to keep the numbers of pathogenic microbes in check within the soil. The dead. This is the food for the living, and includes crop residues and root excretions as well as dead roots, soil organisms that have reached the end of their lives and added organic materials like manure. These materials contain many organic compounds that are easily broken down (labile), so they are very attractive to soil organisms. Adding organic materials to the soil is like providing an all-you-can-eat buffet. But rather than getting fat, the soil organisms multiply to take advantage of the new food supply. As the labile compounds are consumed, most of the carbon is released back into the air as carbon dioxide.  In this process, plant nutrients like nitrogen, sulphur or manganese are released into the soil solution and are then available to crops. A small portion is converted into more complex compounds, and this is the process of humus formation.  The very dead. Not all of the organic material added to the soil is easily broken down by soil organisms, so there is a gradual accumulation of complex molecules like lignin and humic acids. More complex compounds are created within the soil organisms themselves. These materials do not contribute to the cycling of nutrients, because they are not easily broken down. Instead, these complex molecules play a huge role in maintaining soil structure and in holding water and nutrients. They coat the soil minerals, acting as a glue to hold them together in stable aggregates. They also have a huge surface area and electrical charge, so they increase the cation exchange capacity of the soil. In many sandy soils, the entire capacity of the soil to hold on to nutrients is in the organic matter. These materials are resistant to breakdown, but not completely immune. If there is no other food source for soil organisms, they will turn to these compounds like a starving man eating a leather boot. Adding a lot of air to the soil through excessive tillage will also break down some of these compounds through oxidation. Once this humus is lost, it takes a very long time to replace. BF Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture and Food and Rural Affairs, based in Stratford. E-mail: keith.reid@ontario.ca