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."
If soil has good structure for its type, it will have macropores, which contain water and oxygen. As soils get healthier, structure improves downward through the profile, creating the opportunity for healthier and more resilient crops.
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."
Oxygen promotes cell respiration and root growth and water keeps the plant efficient
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."
- Early Spring 2013