Dr. Dan Talks Agronomy

Kick Up Organic Matter

Cover crops are one way to introduce more organic material into soils to support soil health. (DTN photo by Jim Patrico)

My definition of soil health: Soil pH above 6.2; soil electrical conductivity (EC) of 1 deciSiemens per meter (dS/m) or less; and 3% or more organic matter. The soil should be porous with good structure and low bulk density, and have available supplies of soluble carbon and nitrogen that come from leaking roots.

When it comes to increasing soil health, we most often focus on increasing organic matter. Doing that requires setting up the right soil conditions to support an active biological system that will recycle carbon and mineralize nutrients.

Organic matter has doubled over the last 10 to 12 years on our Nebraska family farm. It's come by adopting a 100% no-till system and a two-thirds corn to one-third soybean rotation. Organic matter now routinely measures over 3% on soil tests. I'm happy with that number, but it took over a decade to get there.

My point is, increasing soil health takes a long view. It is easy to think that transitioning to no-till and leaving crop residue on the surface is enough to increase organic matter, but there is more to the story.

Natural organic matter is composed of the remains of plants and animals that live in the soil or lie on the surface of the soil. That includes crop residue above ground, roots below ground, microbial biomass and the remains of worms and other soil insects.

Everything that we think of as organic matter starts first as organic materials that must decompose into soluble carbon, soluble nitrogen, mineralized nutrients and eventually humus. Humus is the natural organic matter in soil. It is high in passive carbon and resistant to further breakdown. Humus is very stable and only about 5% decomposes further each year.

An acre of soil 6 inches deep weighs about 2 million pounds. So if your soil measures 2% organic matter, it contains about 40,000 pounds of organic matter. The 2,000,000 pounds includes:

-- 1,953,000 lbs. soil minerals

-- 40,000 lbs. soil organic (passive) carbon (2%)

-- 4,000 lbs. soil organic nitrogen (0.2%)

-- 2,000 lbs. soil organic phosphorus (0.1%)

-- 1,000 lbs. microbial biomass (0.05%)

For each 1% increase in organic matter, you are adding more active and passive carbon, nitrogen and phosphorus and other nutrients back to the soil.

BUILDING MATTERS

As you set out to build organic matter, it's also important to set goals to maintain levels. In a 50:50 corn-soybean rotation, there will be a net gain in organic matter after corn, but a net loss after soybeans. Corn has a carbon-to-nitrogen ratio (70:1) and decomposes slowly and much of the carbon ends up in humus. Soybeans have a low carbon-to-nitrogen ratio (30:1) and much of that carbon ends up being consumed. So having more wheat (ratio 80:1) and corn in the rotation will slowly build organic matter.

Other practices that help build organic matter:

-- Eliminate or reduce tillage.

-- Prevent erosion.

-- Maintain near-neutral soil pH and moderate soil test levels to sustain nutrient cycling.

-- Plant cover crops to add back some organic matter, soluble carbon and nitrogen to sustain microbial activity.

-- Add compost and manures to support soil health, nutrient cycling and organic matter levels.

DO THE MATH

It takes 10 pounds of organic material to produce 1 pound of organic matter. The other 9 pounds will be consumed during the decomposition process. Increasing organic matter 1% requires adding 200,000 pounds of organic material per acre to end up with 20,000 pounds.

For example, 200-bushel corn generates about 10,000 pounds of above-ground residue per acre and another 10,000 pounds of below-ground residue. That is 20,000 pounds per acre per season that can eventually yield 2,000 pounds as organic matter, a 0.1% increase. A decade of continuous corn could result in a 1% increase in organic matter, but we lose other benefits of rotation. That's why many of us depend on rotation of two years of corn (and sometimes three years) followed by a year of soybeans, which can build organic matter too.

Wheat in the rotation and cover crops can build organic matter if you select the right species. Cereals and grasses allowed to produce stems and/or boot out have a high carbon-to-nitrogen ratio and much more carbon will end up as stable organic matter. A mix of species with a low carbon-to-nitrogen ratio terminated prematurely reduces the opportunity to build organic matter.

Understanding some of these basics is the first step in building organic matter on the road to soil health.

Dan Davidson can be reached at AskDrDan@dtn.com

(PS/AG/BAS)