On the Mend

March 8, 2013 08:16 PM

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.

Platy structure appears flat and compressed; it breaks apart as if it was stacked in layers. Single-grain structure is sand, like you find on a beach. Massive structure refers to soil that is in a cohesive mass, like blocks, with no aggregates visible.

"Each soil type naturally has a certain structure," Ferrie says. "You can learn what structure should be present by consulting a soil survey in a book or on the Internet.

"For example, in Illinois, if you dig from 0" to 11" deep in a Catlin silt loam soil, you should encounter a granular structure. When you crumble the soil in your hand, soil should aggregate as round, imperfect spheres, with a visible crumb-like structure."

Is your soil healthy? To assess your soil’s health, dig a pit and see if it matches the characteristics in the soil survey. "If the soil type is supposed to have blocky structure, but it actually is massive, soil health has been degraded," Ferrie says. "It suggests the soil has been farmed with too much abrasive tillage—such as heavy disking over many years. In a massive soil structure, there are only micropores, with no room for water or nutrients. That limits the biological potential of the soil."

You can make the same type of comparison by digging up soil in a fenceline and comparing it to farmed soil in the field. After many years in grass, the structure of the fenceline soil will be the way nature intended. "It shows you what your soil is like in a healthy state," Ferrie says.

"By managing compaction and reducing tillage, you can have a positive effect on soil structure," Ferrie says. "Restoring structure is not an easy fix; if it took 20 years to degrade the soil, it may take that much time or more to restore its health. The moral is, don’t destroy soil structure in the first place."

"You can make faster improvements by putting land into pasture, adding grass crops and cover crops to your rotation and adding organic matter, such as manure, for microorganisms to feed on," Ferrie continues. "If you farm a naturally weak-structured, platy soil, try to limit the amount of tillage."

What you really are attempting to do is improve aggregate stability. That means maintaining a healthy crumb-like structure, in which sand, silt and clay particles are held together by organic matter and glues given off by mychorrizal fungi.

Build soil aggregates. "To build aggregates, you must flocculate clay particles," Ferrie says. "That requires building bridges from one particle to another with calcium. Calcium is the key to flocculation because it has two positive charges. That lets it hold particles together, while also holding them apart.

"Flocculation breaks down, and structure is lost when you replace calcium ions with a single-charged ion such as hydrogen or sodium," he says. "So good aggregate stability requires managing soil acidity and salt loads, as well as timeliness and type of tillage."

While reducing tillage is an important goal, "bear in mind that soils with poor aggregate stability might run together so badly that they are sort of addicted to tillage," Ferrie says. "In that case, work on other aspects of soil health first."

Improving aggregate stability will let water infiltrate the soil faster, reduce surface sealing and improve early season emergence. "Until aggregate stability improves, even a 1⁄2" rain may create stand and emergence problems," Ferrie says. "So don’t plant those problem fields when rain is in the immediate forecast."

It boils down to knowing the texture of your soil to know how to manage fertility. "Structure tells you whether you soil is healthy or needs improvement. Improving aggregate stability will improve early season emergence," Ferrie says. "It all starts with giving your soil a physical examination, so you know where problems exist and how to make improvements."

Cation Exchange Capacity Links to Soil Texture

Besides the ribbon test, soil survey book or website, there’s another way to determine texture: the Cation Exchange Capacity (CEC) rating on your soil analysis. CEC is a measure of the soil’s ability to hold and store nutrients. Some soils are less able to store nutrients because they contain more sand particles than clay. So the lower the CEC, the sandier the soil texture.

"Think of Cation Exchange Capacity like a dinner plate," says Farm Journal Field Agronomist Ken Ferrie. "If you have a big plate, you can fill it with enough food to last you through the day. But if you heap too much food on a small plate, some of it will spill off and be wasted; and, after awhile you’ll need to eat again. It’s the same with crops: If your soil has a high CEC, you may be able to apply enough nutrients to last the growing season. But if it has a lower CEC, you’ll need to apply smaller amounts more often."


Building on the Systems Approach, the Soil Health series will detail the chemical, physical and biological components of soil and how to give your crop a fighting chance. www.FarmJournal.com/soil_health

You can e-mail Darrell Smith at dsmith@farmjournal.com.

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