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Size Up Your Soil

November 15, 2008
By: Sara Schafer, Farm Journal Media Business and Crops Editor

The first step in growing profitable corn yields is figuring out how much each field can produce. If you don't start out with a realistic goal, you risk over- or under-applying inputs, both of which are hazardous to your bottom line.

"When I establish a yield goal, one of the first things I look at is usable water per foot of soil,” says Farm Journal Field Agronomist Ken Ferrie. "When I know that, I can plan my plant population, nitrogen amounts and timing and everything else.” To figure out the amount of usable water, you need to understand your soil—its type, texture and how water moves through it. "If you're using variable-rate application, look at each soil type in a field,” Ferrie notes. "If not, base your plans on the percentages of light and heavy soil.”

Pinpoint water location. Sizing up how water moves through your soil starts with finding out how many macropores and micropores there are in your soil. "Macropores are large pores that hold usable water,” Ferrie says. "Micropores are small pores that also contain water, but the water is mostly unusable by the crop.” In addition, there are biopores, which are formed by earthworms and root channels. "Macropores and biopores are the ones associated with moving water up and down through the soil,” Ferrie says. "They determine how easily the water moves. They are associated with infiltration rate, holding capacity per foot of soil and amount of capillary rise—how high up from the water table the water can move.”

Types of particles. The makeup of micropores and macropores is determined by the soil's texture, structure and bulk density. Texture is the type of particles, and the percentage of each kind, in the soil. Types of particles include sand, silt and clay. Sand particles, which you can see, are the largest. Clay particles, the smallest, are microscopic.  "Think of it this way: If a sand particle is the size of a 747, then silt is a Cessna and clay is a hummingbird,” Ferrie says.Clay, unlike silt or sand, is made up of lattices—layers held apart by negative charges that repel each other. "This allows water to get between the lattices,” Ferrie says. "Water can be held between the lattices. It also can be held tightly to the particle because of the particle's negative charge. So clay, more than anything else, gives soil its water-holding capacity.” Sand and silt do not have negative charges within their particles. But they do have organic matter (or carbon) sequestered around them and pick up some negative charges from that. Clay, because of its lattices and the small size of its particles, has tremendous surface area. "In a tablespoon of clay, there is enough surface area to cover a football field,” Ferrie says. "Surface area is what water and nutrients attach to.”

When soil cracks, it's because the water between the clay lattices was wicked out or evaporated, and the lattices collapsed. In contrast, "sand doesn't collapse,” Ferrie says. "It blows. Silt gets very hard.”

Most soils are mixtures of these three types of particles. The highest proportion of particle types is listed first—as in silty clay loam.

Sand has large particles, with big pores in between. Clay is microscopic, so the pores are small.  "When we aggregate sand, silt and clay together in a crumblike structure, it gives us macropores between the crumbs,” Ferrie says. "So soil structure is a big part of determining the space available for water and air. Farmers can influence this process.” The crumb structure and the macro-pores result from the aggregation of soil particles. "It's hard to form aggregates in sand or clay,” Ferrie says. "It's easier to do in loams, which are mixtures of the three types of particles.”  A farmer's goal is to put sand and silt together with clay, in a crumb or a "granular” structure. (You may see descriptions of other clay structures, such as blocky, angular and platelike. These structures usually are found deeper in the soil. Crumb structure is in the top 4" to 6".) To maintain and create crumb structure, clay particles must bridge the clay.

Building the bridge. "Some of the elements in soil have a single variant, or electrical charge,” Ferrie explains. "Magnesium and calcium have a double-variant positive charge. These double-variant positive charges flocculate, or ‘bridge,' the soil. "A positive charge hangs onto the negative charge of one clay particle, and an open positive charge attracts to the negative charge of another clay particle. The bridge (or positive charge) pulls the particles together, but it also holds them apart. Pectin from grass roots and organic matter glue this into a crumblike structure.” Only clay particles are flocculated. "Because they are microscopic in size, it's not hard to hold them together,” Ferrie says. "But sand can get caught up in the gluing process, which then forms the crumb. "Once formed, the crumb structure keeps the soil from running together and provides a place to store oxygen and usable water.”

By pulling soil particles together into crumbs, macropores are created between the crumbs. Micropores are trapped inside the crumbs. "Macropores occur naturally in coarse sands,” Ferrie says. "In other soils, we create macropores by creating crumb structure. By creating macro-pores, we allow water infiltration.” If the soil surface is left unprotected (because of moldboard plowing or fire, for example), a hard rain can break the bridges between the clay particles. The soil runs together and then seals at the surface. "That's nature's way of destroying structure, giving a platelike structure,” Ferrie says. Man also can destroy the structure. One way to do that is to allow the soil to accumulate a large amount of single-variant elements. "Calcium ions make the bridge between clay particles,” Ferrie says. "If we overload the soil with sodium, which replaces calcium and breaks the bridge, we get poor structure. To put the bridge back in, we apply gypsum, which is calcium sulfate. The sulfate marries with the sodium, which is flushed out. The calcium rebuilds the bridge. This is fairly rare—and found in certain parts of the country and in livestock operations when manure is overapplied. "More commonly in the Corn Belt, soils accumulate too many hydrogen ions, which creates acid soil,” Ferrie continues. "To correct the acidity, we apply calcium carbonate. The carbonate flushes out the hydrogen, and the calcium rebuilds the bridge between the soil particles. "Of course, in reality, the process is not quite that simple. If we break all the bridges, getting the glue back and rebuilding the crumb structure takes many years, especially in a corn/soybean rotation. We can speed the process a little bit, with practices such as cover crops; but it's always better not to let the bridges be broken.”

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