Should both the amount and quality of organic matter in the soil be tested?
In terms of soil testing, Organic Matter (OM) is the term loosely given to the total organic carbon content in the soil. It is what makes the difference between dead dirt and living soil. It provides CEC, water holding capacity, and free carbon to the soil. Ideally, you would like various forms of organic matter in the soil at all times: fresh, active, and stable. In its various forms it feeds soil microbes, aggregates soil, slows down (buffers) chemical reactions, retains moisture, slows nutrient leaching, and more. There are various ways to measure OM and it can be worthwhile to test it using more than one method. By doing so, a more complete picture of its activity and its potential can be formed.
The most common test for OM is the TOC (Total Organic Content) or LOI (Loss On Ignition). This is a simple measure of total organic carbon achieved by burning off all of the carbon in a soil sample and weighing what is left. This number gives the grower a good measure of all forms of carbon combined but does not give information about what portion is interacting closely with the soil. To get this information, an additional test for humus or humic activity is necessary. There are tests (Embrapa, Walkley-Black, and Mebius) that have been developed for this purpose, with each giving a somewhat different number. With the rapid advances in testing technology and the increasing interest in soil humic levels, new tests are being developed that may make this type of measurement cheaper and more accurate. Until then, with experience, farmers can get pretty good at identifying active humic levels in their soil by sight and smell. Aerated soil with decent organic matter levels will support diverse and numerous soil organisms (‘soil biology’). This will affect the color and odor, creating a darker and more ‘earthy’ smelling soil.
Testing Soil Biology?
Quantity, quality, and diversity of Soil Biology is becoming more recognized as an important characteristic of the soil and especially ‘soil health’. There are numerous labs around the U.S. that will test soils (and ‘compost teas’) for diversity of microorganisms. These tests are somewhat expensive and are not used on a regular basis by most farmers. Those who believe in their value often use them to evaluate new ground, or to rule out a poor biology profile as a cause for problems in crop growth.
Beside the ‘smell test’, another way of estimating the amount of active organic matter and soil biology is through use of the Draeger-Tube® method. This test measures the carbon dioxide (CO2) produced in the soil, or ‘soil respiration‘. In this case, higher production of carbon dioxide indicates higher levels of organic matter and soil organisms. However, when performing this test, it should be taken into account that biological activity is affected by temperature. A given soil will always respire less CO2 in the winter than in the summer.