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On the Mend

March 9, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor

Steps to determine what ails your soil and nurse it back to good health

If you haven’t encountered the tale of two fields, you know of fellow farmers who have: One field is easy to farm and hits yield goals every year, weather permitting, but the other field seems to fight every step of the way and almost always yields less. What causes yield variation when the fields are the same soil type? One field is the picture of perfect soil health while the other is plagued with sickly soil.

Soil health involves chemical, biological and physical aspects. For now, let’s examine how physical aspects can be degraded over time and how you can improve them.

"Understanding physical aspects of soil lets you set realistic yield goals, based on water-holding capacity," says Farm Journal Field Agronomist Ken Ferrie. "It also tells you how to fertilize and manage the soil to achieve those goals and how to improve the health of poorly producing soils."

Physical aspects of soil health include texture, structure and aggregate stability. Texture is the percentage of various particle sizes—sand, silt and clay, from largest to smallest—in the soil profile. Most soil types contain more than one type of particle, such as clay loam, silt loam or silty clay loam.

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While abrasive tillage degrades soil, vertical tillage, done properly, can remove compaction and/or sudden density layers while improving water infiltration. No-till and strip-till have the ability to preserve and improve soil structure. More grass crops and organic matter also help, but recovery might take time.

You can determine texture by squeezing moist soil into a ribbon, consulting a soil survey in a book or online or using a cell phone app when you’re standing in the field. Once you know the mixture, use the USDA soil texture pyramid to identify the soil type (clay, sandy clay loam, etc.)

The most accurate way to classify soil is to have it analyzed by a laboratory, which will tell you the percentage of sand, silt and clay.

Understand soil texture. The biggest variable that affects water- and nutrient-holding capacity is the percentage of clay (and various types of clay). "Negative electrical charges in clay particles allow it to attract and hold water and nutrients, which are positively charged," Ferrie explains. "Made of lattices, clay particles can hold water between the lattices, as well as around the outside of the particle.

"Sand and silt don’t have lattices nor negative charges, so they have no water-holding capacity. But they do have a coating around the particles called organic matter. Organic matter contains carbon, and each carbon ion has four negative charges. That gives organic matter some ability to hold water and nutrients. But in the mineral soils that most farmers deal with, the main thing that holds water and nutrients is clay."

After you learn the textures of your various soils, you can respect them and manage accordingly. "Some areas, such as the Mississippi Delta, basically have uniform soil texture," Ferrie says.

"That makes management relatively easy because you can treat each field the same way. But in the Midwest, there are various soil textures in the same field. It’s as if the field has multiple personalities, and you have to farm all of them differently."

For example, texture tells you how to manage soil fertility to keep the soil balanced and healthy. "Two tons of lime per acre would be a low rate on a heavy soil but excessively high on sand," he says. "Applying 200 lb. per acre of nitrogen is OK on a clay soil; but on sand, which has less nutrient-holding capacity, much of that nitrogen could be lost to the environment, where it would become a pollutant."

The more clay in your soil, the less you have to worry about overapplying nutrients and application timing, Ferrie adds. If the clay content is exceedingly high, you might need to worry about nutrient availability (requiring multiple applications, so nutrients don’t become tied up and unavailable to plants) and drainage to remove excess water. Sandy soils become acid faster, so they need to be limed more frequently, and in smaller amounts, to keep microbial populations and overall soil conditions healthy.

The role of structure. Soil structure describes how particles of sand, silt and clay are held together in clumps, or what soil scientists call "peds."

"Unlike texture, structure is something farmers can change—for better or worse," Ferrie says. "Think of the ideal structure as being crumb-like. That kind of structure creates macropores, large pore spaces where water can be stored and removed by plant roots. Macropores also contain oxygen, which is required by soil microorganisms, which are essential for healthy soil."

Soil scientists break down structure into categories. Granular structure is imperfect spheres, all about the same size, and visible with the naked eye. Granular aggregates contain all three sizes of soil particles—sand, silt and clay. Blocky structure breaks apart into small cubes with angular edges, resembling broken glass.

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FEATURED IN: Farm Journal - March 2013

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