Organic matter matters
Organic matter has a big influence on soil properties.
Add organic matter. That’s the short answer to better managing your soil, says James Cassidy, soil science instructor at Oregon State University and manager of the student-run university farm.
Cassidy, known for holding his student audience spellbound during soil lectures through his enthusiasm and wit, links everything in life back to soil. “It’s all about soil—it all comes from soil and all goes back to soil, sooner or later. Every single atom in your body has been through the soil system.” He believes that a better understanding of soil—how it works and stores nutrients—will lead to growing better quality fruit.
Soil is the most diverse habitat on earth, composed of 45 percent minerals, 5 percent organic matter, and the rest air and water. A single pinch of soil contains more than a billion living organisms, existing in a four-dimensional, complex habitat, he said. Soil, which has formed over time through decomposition, is essentially “rotted rocks and decomposing organic matter,” he explained during a cherry research symposium sponsored by Oregon State University and held at The Dalles, Oregon, earlier this year.
Aggregate of soil
A complete ecosystem is contained within an aggregate of soil. In an aggregate, a speck of soil less than a millimeter in size or about the size of a broken pencil lead, the following are found:
—Different sized rock particles (sand, silt, and clay)
—Water – held by capillary force
“The soil activity is what’s happening in between the soil particles,” Cassidy said. “The thing to be managing conceptually is managing the pore space and size of the pores.”
Diversity is the key to pore space and size. Big, medium, small, and super tiny pore sizes distributed throughout the soil profile help the soil drain and hold water, as well as provide air to the roots.
Macro pore sizes, like worm channels, help pull raindrops, irrigation water, and oxygen together, bringing water and gas exchange to the roots. “The way to manage pore size is to disturb the soil as little as possible,” he said, adding that minimizing soil disturbance is a good way to preserve pore size distribution.
“We have the power with large tractors to work the soil, but resist that urge,” he said. “The more we disturb soils, the less water and oxygen get in. One measure of soil quality is how quickly water penetrates.
“Diversity of pore size leads to diversity of soil habitat that leads to diverse organisms that leads to diversity of function that leads to the breaking down of rock,” said Cassidy. While it’s all about diversity, he acknowledges that in agriculture, growers are trying to grow one thing, which can work counter to building a diverse ecosystem.
Though sand and silt are primary minerals that have been ground down into small pieces (sand is just a larger piece than silt), clay is a secondary mineral created by the dissolution of primary minerals and then recrystallized or synthesized into layered mineral sheets. The silica tetrahedral sheets in the clay are where nutrients like aluminum, silica, magnesium, potassium, and such are held by net negative charges that are a result of isomorphic substitutions in mineral crystal at the time of recrystallization. Sand and silt don’t have a charge, but clay has the all important negative charge.
“And what gets stuck to the negative charge?” he asks. “Positively charged nutrients like potassium, calcium, magnesium, and most everything else a tree needs to grow.” Without the negative charges, he noted that nutrients could not be stored in the soil and would leach away.
A soil’s cation exchange capacity is a measure of the amount of net negative charge per kilogram of dry soil and therefore a measure of how much nutrient can be stored, he said. A soil test number of 20 would be good, below 5 is considered low, and above 40 would be high.
Moreover, the cation exchange capacity determines the value of a soil, he said, as soils with low CEC have a low net negative charge and do not hold nutrients in the soil as well as soils with a high CEC number.
Small portion but mighty
Organic matter, which is only a small portion—at best 5 percent—of the total makeup of soil, packs a mighty punch. Organic matter influences soil properties and plant growth far greater than its low percentage would indicate.
Cassidy said that organic matter adds nutrients to the soil, provides nutrient storage because it’s negatively charged, and is the glue that creates soil structure. Organic matter, with it’s negative charge can help improve soils with low cation exchange capacity. It also provides carbon and energy (food) for the soil microorganisms.
The easiest way to add organic matter to soil is to grow it in place and mow and blow the green manure where it’s wanted. But adding compost is also effective. He advised growers to pay attention to the organic matter percentage in their soil test results and experiment on parts of their orchard to raise soil organic matter levels. Over time, see if water infiltration rates improve and organic matter levels are increased.
Cassidy noted that slow water infiltration rates are undesirable for several reasons: The first two things lost in the runoff are clay particles and organic matter. That causes the soil to become sandier, and because sand doesn’t have a charge, the soil loses some of its negative charge and can’t store nutrients.