Soil is an important part of our lives because it supports our food production, and one of the most important components of soil is organic matter. Plants that we grow provide this organic matter and the nutrients which regenerate the soil, says Gary Wilson, an Ohio State University Extension educator.
"An ideal soil is about 45% solid matter in different ratios of sand, silt and clay," Wilson says. "The amount of sand, silt and clay determines a soil's texture.
"Water represents about 25% of the soil space and air makes up another 25%."
The air is located between the mineral soil particles and is about 78% nitrogen, 21% oxygen and 0.3% to 0.4% carbon. Soil organic matter represents about 5% of the soil volume in an ideal soil, Wilson says.
"At one time, our soils contained about 7% to 10% organic matter, but that content has decreased 2% to 3% for most of Ohio agricultural mineral soils," he says. "As any good farmer or gardener knows, the amount of soil organic matter is critical for raising good crops."
Soil organic matter is formed when using chlorophyll in the leaves, plants combine energy from the sun with water and carbon dioxide to form glucose, or sugars, releasing oxygen back into the atmosphere, Wilson says.
Glucose and other nutrients are combined to form structural compounds like carbohydrates, amino acids (proteins), lipids (fats) and lignin, which add support to the cell walls in a plant. Plants become food for the micro-organisms in the soil.
"Soil organic matter is basically all the organic substances, or anything with carbon, in the soil, both living and dead," Wilson says. "Soil organic matter includes plants, blue green algae, micro-organisms — bacteria, fungi, protozoa, nematodes, beetles, springtails etc. — and all the fresh and decomposing organic matter from plants, animals and all living life.
"If we started with 100 pounds of a dead plant, we would end up, after it was decomposed, with 60 to 80 pounds of carbon dioxide going back up into the air."
Wilson says the remaining 20 to 40 pounds of energy and nutrients are decomposed and turned into 3 to 8 pounds of micro-organisms (the living), 3 to 8 pounds of nonhumic compounds (the dead) and 10 to 30 pounds of humic compounds (the very dead matter which is resistant to further decomposition).
He says soil organic matter is basically a byproduct of the carbon and nitrogen cycle. It changes with the type of tillage performed on the soil.
"With conventional tillage, which is soil plowed or chiseled, soil organic matter is being burned up by the oxygen that is introduced into the soil when we turn it over, and this process releases soil nutrients like nitrogen and phosphorus," Wilson says.
For the first year or two, soil micro-organisms thrive on this release of nutrients and crop yields go up, he says. But, eventually, the micro-organisms start to die off as nutrients are lost and excess nutrients run off the soil.
"Typically, conventional-tilled soils have less organic matter, 2% to 3%, than an ideal soil at 5%. This translates into a huge difference in soil productivity," Wilson says.
He adds that different rates of tillage result in different rates of soil organic matter loss. Moldboard plowing 6 to 8 inches deep releases twice as much soil organic matter as chisel plowing.
In the last 20 years, farmers have increasingly adopted conservation tillage and have started to slow down the rate of soil organic matter loss, he says.
"A farming system that combines no-till with cover crops helps to further restore soil organic matter levels and improves water quality and is more sustainable," Wilson says. "In Ohio, a majority of wheat and soybeans are no-tilled, but less than 10% of corn acres are no-tilled.
"The reason for this difference is since corn is a grass, it requires more nutrients and water, so farmers typically see a 5- to 15-bushel yield decrease for the first 7 to 9 years when they convert from conventional-tilled fields to no-till."
Tilling the soil basically stimulates micro-organisms, especially bacteria, to release more nutrients, Wilson adds. However, the downside is that it also burns up soil organic matter quicker.
"Corn is a big user of soil nutrients, especially nitrogen, so it tends to yield better when tilled," Wilson says. "However, tillage can start a process that causes soil productivity and soil quality to spiral downward.
"Long-term research shows that 7 to 9 years of continuous no-till can produce just as good or higher yields than conventional-tilled fields because it takes 7 to 9 years to restore soil organic matter, improve soil health by getting the microbes back into balance and restoring the nutrients lost by tillage."
