Size Up Your Soil

November 14, 2008 06:00 PM
 
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.”
Fast Facts
  • To set a realistic yield goal, you must know how much usable water your field can provide.
    Usable water depends on soil type, texture and water movement.
  • Water flows through macropores.
  • The number of macropores is dictated by the soil's texture, structure and bulk density.
  • You can't change soil texture; just understand what you have and respect it.
  • It's easier to maintain structure than restore it after it is lost.
  • To maintain or improve soil structure, apply calcium as it is needed, protect the surface with residue cover and use the least abrasive tillage tools.

Usability factor.
Water moves through soil by capillary action, which involves bonding processes called adhesion and cohesion. Texture and pore space influence the water's movement.
"If we set a volume of soil in a container with an open bottom and pour water into it, eventually water will drip out the bottom,” Ferrie says. "That is saturation—100% of pore spaces are filled with water. If we stop pouring water, it eventually stops dripping out the bottom. At that point, the soil is at field capacity. The adhesion/cohesion concept is the reason the soil holds that much water.”

Each water molecule includes two hydrogen ions, giving it a positive side and a negative side. "The positive side of the water molecule attaches tightly to the negative sites on a clay particle,” Ferrie explains. "This process is called adhesion. The water is bound so tightly that plants can't extract it, so that water isn't usable.”As a result, soils that contain high amounts of clay can have high amounts of total water but only low amounts of usable water. "In these soils, corn can be wilting during drought stress, but 4" or 5" below the surface, the soil will be moist enough that you can roll it into a ball,” Ferrie says.

Because water molecules have a positive end and a negative end, water also attaches to itself. This is cohesion, a very light bond that can be broken by plants and microbes. Water held by cohesion is usable water.

"For example,” Ferrie says, "when water runs down your windshield in a streamlike fashion, it is doing so because of cohesion; the water molecules are attached to each other.”
Water rises through the soil—in a process called wicking action, or capillary rise—because of the pull from negative charges on soil particles. "When you pull water to soil particles through adhesion, cohesion pulls other water molecules with it,” Ferrie says. "This is what happens when you stick the corner of a paper towel into a glass of water. The water rises up the towel. The more negative charges the towel has, the more water it absorbs.”

Coarse sand consists of few negative charges, and they are far apart (because of sand's large macropores). "Sand can only lift water so far, and then gravity stops it,” Ferrie says. "Fine sand has smaller macropores, so the capillary rise goes higher.”

Irrigation water moves faster through coarse sand than fine sand because the coarse sand has larger pores and fewer negative charges. Clay loam can pull water upward 2' from the water table, while sand can pull it only 6" to 8". This is a big factor in determining usable water. Capillary movement of water through soil depends on the amount and type of micropores and macro-pores. With large macropores and few negative sites, as with sand, you have poor water-lifting capability, but, during a rainstorm, for example, you will have high infiltration rates. At the other end of the extreme, clay soils—which contain smaller macropores and a higher volume of negative charges—have a strong ability to lift water, but only a low percentage of the stored water is usable because it holds tight to the negatively charged clay particles. Such soils have poor infiltration rates and are more likely to have increased amounts of runoff.

What you can do. "You can't change soil texture,” Ferrie says. "All you can do is understand what you have and respect it. Changing soil structure is a long-term process—it's much easier to keep good soil structure than to correct poor structure.”Anything you can do to maintain or improve soil structure increases usable water capacity. "Keep clays flocculated by applying calcium,” Ferrie advises. "Keep them protected with residue cover. Use the least abrasive tillage tools. "Probably the most abrasive tool to soil structure is a large disk,” Ferrie continues. "It can break the bridge that connects soil particles and forms structure. That's why big disks are used to make roads—they pulverize soil, break its structure and make it pack.” If you have a coarse soil structure with high infiltration rates and poor lifting capacity, be sure to manage all nutrients, especially nitrogen, carefully to prevent leaching.

Be timely with your irrigation. "Suppose you have a field of corn rolling,” Ferrie says. "You run your center pivot fast to cover the field as quickly as possible, but you only dampen the surface. The water you apply evaporates in an hour, and the corn continues to roll. What you need to do is apply enough water to make the water fronts meet, from above and below. Then, when evaporation begins, you'll use cohesion to pull water up from below.”

If you have clay soil with poor structure, a low percentage of macropores and water held tightly by many negative charges, try to improve the drainage by putting tile lines shallow and closer together. You'll also need surface drainage to handle the runoff. Manage nutrients to prevent them from being tied up, or fixed, by the negative soil charges. Pay close attention to the type and timing of tillage because of compaction concerns. "Tillage has a drastic effect on mac-ropores and micropores,” Ferrie says. "When you lift and shatter soil, you change its bulk density, creating a massive amount of macropores. That lets water evaporate easier—as when you're opening up the surface of a field to dry it out for planting. This allows for considerably higher infil-tration rates, to the depth of the tillage, in the short term. "But when you use horizontal tillage to loosen the soil surface, the water can be evaporated out of the large macropores you created much faster than it can be drawn up through the untilled soil beneath. That breaks the cohesion of the water fronts. "While this dries the field tempo-rarily, the drastic change in the size of the macropores creates a layer of soil with a different density,” Ferrie says. "That interferes with the movement of water up and down, as well as rooting.” You can explore this effect in your own field by digging a soil pit. We'll tell you how in a future issue.


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




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