A 60/40 ratio probably makes you think of crop-share lease arrangements. But that ratio takes on new significance in terms of crop production. A ratio of 60% water and 40% air in your soil’s macropores creates optimal conditions for plant roots and microorganisms that help plants thrive. A 60/40 ratio means your soil is healthy.
All soils are not created equal in terms of the number and size of macropores nor in clay content, the two primary components of water-holding capacity. You can’t expect a droughty sand to yield as much as a silty clay loam; but you can improve or maintain the health of the soils that you farm, optimizing air and water access for each one.
"We can think about water and air almost interchangeably," says Farm Journal Field Agronomist Ken Ferrie. He has been studying both since his college days, through digs on the farm of the late world champion corn grower Herman Warsaw, to his current role in which he consults for farmers and conducts Farm Journal’s on-farm test plot studies.
"Pore spaces in soil hold both water and air," Ferrie says. "Therefore the same things that affect water-holding capacity affect air. When the ratio gets out of whack, the crop has problems. If soil becomes saturated with water, there’s no room for oxygen; if there’s a drought, there’s no water."
If plants run short of oxygen, the detrimental effects show up faster in warmer conditions, Ferrie notes. That’s because plants’ respiration rate is higher in warm weather. "If soil gets saturated in the spring, when the weather is cooler, the small plants might survive for three or four days," he says. "But in warm conditions, they might die in 48 hours."
Physical, biological, chemical. As we explained in our first soil health article ("What Makes Healthy Soil?" January 2013), soil health involves physical, biological and chemical aspects. Water and air illustrate all three.
"The water-holding capacity involves soil texture, which is a physical property," Ferrie explains. "But it is affected by structure, which is a biological aspect, and pH, which is a chemical component.
"To a farmer," Ferrie says, "the issue is having enough air and water to grow a crop. That requires the right balance of oxygen to promote cell respiration and root growth and water to keep the plant efficient through the entire growing season. But it’s not just the crop that needs the right balance, so do soil microbes.
"Beneficial microbes are aerobic, so they need oxygen. Under saturated conditions, the aerobic microbes die off and anaerobic populations build up. The anaerobic microbes can be detrimental to the soil and the crop," Ferrie says. "The beneficial microbes, which recycle and release nutrients and assist in nutrient uptake, also need adequate water."
What you can and can’t change. Managing water and air begins with respecting your soil type—playing the hand Mother Nature dealt you.
"Texture is the percentage of sand, silt and clay particles (from largest to smallest) in your soil," Ferrie explains. "Clay particles have the ability to hang onto water molecules. Negative electrical charges in clay particles allow clay to attract and hold water because water molecules are positively charged. Made up of lattices, clay particles hold water between the lattices, as well as around the outside of the particles. Organic matter also has this ability; but clay content is the biggest factor in water-holding capacity."
What farmers are really concerned with is not merely water-holding capacity but useable water. "Unusable water is bound so tightly in the micropores of clay particles that plant roots cannot extract it," Ferrie says. "Useable water is found in larger pores, the macropores between particles of healthy crumb-like soil."
"Soil texture is the biggest factor in infiltration rate. Water infiltrates fastest in sand, which has the most and largest macropores but no clay and organic matter to hold onto the water, so it percolates down through the profile. Because of texture differences, we can’t make all soil types equal in performance; but we can manage the physical limitations imposed by texture to improve the water-holding capacity, pore size and infiltration rate of the soils we farm."
Manage the water table. Drainage is a component of soil health because it lets you manage water-holding capacity.
"Removing the excess water lets in oxygen and allows soil microbes and plant roots to breathe and survive," Ferrie says.
"Lowering the water table at the appropriate time stimulates crop root growth. Better-drained fields held out longer against the drought conditions in 2012 because roots were able to grow deeper early in the season."
Of course, you don’t want to lower the water table farther than necessary. "You can manage the depth of the water table by gating tile outlets," Ferrie notes. "The concept is called drainage water management."
Another way to increase the amount of useable water is to make sure water can move up and down through the soil profile. "That requires removing compacted layers or sudden density changes that cause a break in soil water fronts," Ferrie says. "Separating water into fronts puts an end to the wicking action that pulls water upward through the soil profile.
"The break occurs when water hits a layer of soil because it moves outward rather than up or down through the layer. You can remove horizontal layers with vertical tillage or with deep-rooting cover crops, such as radishes."
On the other hand, if you farm sandy soil with little water-holding capacity, you can supplement rainfall with irrigation. "Irrigation, like drainage, is a soil health tool," Ferrie says.
"Like drainage, irrigation must be managed. When you irrigate, you are not just ‘watering’ a crop—you are keeping plant and soil microbial systems functioning efficiently," he adds.
"The keys to success are watering on a timely basis and applying enough water but not too much. Apply enough water to your water fronts together. Then water will be wicked upward through the soil, as it transpires into the atmosphere by the growing plants or evaporates off the soil surface."
Deep-rooting radish cover crops and vertical tillage remove compaction and dense soil layers, which impede water movement in soil.
Improve structure. If soil has good structure for its type, it will have macropores that contain water and oxygen. You can envision soil structure by thinking of a snowball consisting of individual soil "peds," or crumb-like bits, which are made up of sand, silt and clay particles. Inside the snowball, between the soil peds, are macropores. "Improving structure means creating more macropores," Ferrie says.
You can see what healthy structure for your soil type looks like by digging in an undisturbed area.
"As soils health improves, the structure improves downward through the profile," Ferrie says. "The deeper we improve the structure, the healthier and more resilient our crop will become."
Unfortunately, building better soil structure isn’t easy or accomplished quickly. "It takes a long time and serious commitment," Ferrie says.
In sandy soil about all you can do is return organic matter, or carbon, to the soil by irrigating and growing cash crops and cover crops. "That will help build some water-holding capacity," Ferrie says.
In other soils, you must eliminate any production practices that might destroy soil structure. "Almost any tillage has a detrimental effect," Ferrie says. "The more abrasive the tillage tool, such as a large offset disk (as opposed to a chisel plow), the more you will damage soil structure."
Avoiding abrasive tillage helps build soil structure by slowing the downward movement of fine soil particles.
"If you have a poorly structured soil containing a little sand and a little silt, too much abrasive tillage can cause the fine silt particles to migrate downward and form a tight layer," Ferrie says. "That silt layer will interfere with water movement through the soil profile."
Grass crops are a must. Improving structure also requires the use of some type of grass crop, such as wheat or barley, or cover crops. "Grassy root systems and soil microbes produce a kind of glue that bonds soil particles together in a desirable crumb-like structure," Ferrie says.
As you build structure with grass, you must change your farming practices so you don’t destroy what you just created. "That means reducing or eliminating tillage," Ferrie says.
Another benefit of cover crops and reduced tillage is that more crop residue will remain on the soil surface.
"Residue protects the soil from heavy rainfall that destroys the surface structure and cuts off water infiltration," Ferrie says. "But maintaining enough residue cover to protect the soil is a challenge in a corn–soybean rotation, even with no-till practices. It’s even more difficult if you till your soybean stubble."
Returning to the chemical aspect of soil health, Ferrie adds: "As you attempt to build structure, you must maintain a sufficient calcium load in your soil. Calcium is required to flocculate, or build electronic bridges, between clay particles. Without it, the particles collapse together.
"This is a function of soil calcium levels. In acid soils, hydrogen ions displace calcium ions. So if a soil has become acid, apply calcium carbonate (limestone). The carbonate flushes out the hydrogen ions.
"In poorly drained soils, sodium replaces calcium. If sodium content is high, you may need to try to improve drainage and apply calcium sulfate. The sulfur will flush out the sodium, and the calcium will flocculate the soil particles," he says.
A healthy systems approach. Improving soil air and water, in most cases, requires a systems approach that spans several years, Ferrie summarizes.
"You must put everything together because changing just one practice, such as growing a cover crop, won’t make much improvement in soil structure," Ferrie says. "Set the stage by testing your soil and balancing the pH. Improve drainage, if necessary.
Remove dense or compacted layers. Add irrigation, if it’s needed, and if it’s economically feasible.
"Then reduce or eliminate tillage. That’s a big step because changing your tillage practices begets other changes. Think the system through. You’ll need to equip your planter to handle more residue. You will need to modify your fertility program and use different herbicides, with different
It might be years before you can see improvement in your soil. But, ultimately, the payoff will be that desirable 60/40 ratio of soil air and water, leading to higher, more consistent crop yields for you, or for your kids when they take over the farm.
"Meanwhile," Ferrie concludes, "if you’re farming soils with good or excellent health, do everything you can to maintain them. If you destroy them, it will take a really long time for them to recover."
You can e-mail Darrell Smith at firstname.lastname@example.org.