Posted at 19:46h
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From the October Brookside Labs Newsletter…
Value of Soil Organic Matter – by Luke Baker, Ph.D. – Agronomist/Lab Specialist
Have you ever thought about the value of soil organic matter? I am sure that you have because as we all know it plays so many key roles in our soils. Some of these roles include, but are not limited to: 1) Gives the soil its dark color, which helps to facilitate warming in the spring. 2) Improves the water retention of the soil (organic matter can hold up to 20 times its weight in water). 3) Cements soil particles into structural units called aggregates (gives soil its structure) 4) Forms stable complexes with copper, manganese, zinc and many other polyvalent cations which improve plant availability of these nutrients. 5) It increases the soil’s buffering capacity, which helps to maintain uniform reaction in the soil. 6) Soil organic matter has a high cation exchange capacity that can range from 300 to 1400 milli-equivalents per 100 grams of soil. Thus it has the ability to raise a soil’s cation exchange capacity and often accounts for 20 to 70% of the cation exchange capacity in most soils. 7) Decomposition of organic matter yields carbon, nitrogen, phosphorus, sulfur, and boron. Therefore, it is a major source of plant nutrients. 8) Organic matter combines with organic chemicals that are added to the soil, which modifies the application rate of pesticides for effective control. It also, combines with these chemicals to make them less harmful to the environment. These are just some of the many benefits that organic matter can have on soil. However, how do we value something like this? First of all, we should start by saying that it is invaluable. Without organic matter most soils would have very limited production for a multitude of reasons (lack of microbes present, lack of water holding, lack of mineralization, etc…). In this exercise I am going to take a look at organic matter strictly from a nutrient perspective. If we assume that the top 6 inches of soil contains 2,000,000 pounds of soil then 1% organic matter would be 20,000 pounds of organic matter per acre. Most organic matter is 50% carbon, so 1% organic matter would be 10,000 pounds of carbon or 5 ton. Carbon is valued at around $4.00 per ton, so we have $20 worth of carbon. Next, if we assume the carbon to nitrogen ratio is 10:1 (which is very common) this would be 1% organic would contain 1,000 pounds of nitrogen. Nitrogen values change daily, but for now let us assume nitrogen is running about $0.50 cents per pound. Therefore, we would have $500 of nitrogen. For the other elements we are going to assume they are in the relative ratio of 100 parts carbon/10 parts nitrogen/1 part phosphorus/1 part potassium/1 part sulfur, which would give us 100 pounds of phosphorus, potassium, and sulfur in 1% organic matter. Again, we can assign values to these nutrients ($0.60 for phosphorus, $0.50 for potassium, and $0.50 for sulfur), which would give us $60 of phosphorus, $50 of potassium, and $50 of sulfur. If we add all of this up that is $20 for carbon + $500 for nitrogen + $60 for phosphorus + $50 for potassium + $50 for sulfur = $680 per acre for each percent of organic matter. Now if we have 4% organic matter in our soil it would be $680 x 4 = $2,720 dollars of nutrients per acre. Now if one would include minor elements, we could be well over $3,000 per acre just in nutrient content. Now it is difficult to assign dollar values to all of the other things that organic matter can do (see the list above), but if we could we can easily see that organic matter is invaluable. It is a precious resource that we need to maintain and continue to develop in our soils.