You’re at a meeting for clients of your local certified crop adviser. It’s just after a big lunch, and the speaker is talking about soil fertility. When he gets to cation exchange capacity, your eyes glaze over and then shut.
Pete Maybach is aware of the phenomenon. But he’s also aware it’s easier to make decisions about which fertilizer nutrients to invest in if you understand basic concepts. That includes what happens to nutrients once they’re applied in various soil types.
“Cation exchange capacity is a measure of how many cations can be retained on a soil particle’s surfaces. Negative charges on the surfaces of soil particles bind positively charged atoms or molecules — the cations — but allow these to exchange with other positively charged particles in the soil water.” That’s how Wikipedia defines cation exchange capacity. Have your eyes glazed over?
A Purdue University publication by Dave Mengel in the 1980s explains how CEC is calculated for reporting on soil test results: “More commonly, the soil testing labs estimate CEC by summing the calcium, magnesium and potassium measured in the soil testing procedure with an estimate of exchangeable hydrogen obtained from the buffer pH. Generally, CEC values arrived at by this summation method will be slightly lower than those obtained by direct measures.”
Are you asleep yet?
“Think of various soil types like different sizes of tractors,” Maybach says. “If a soil type represents a tractor, then CEC represents the fuel tank. If you have sandy soils, it’s a small tank. Maybe it only holds a couple gallons.”
That means nutrients must be added more often but in smaller amounts, he says. The “fuel tank” simply isn’t big enough to hold much fuel, or in this case nutrients, at one time without the tank running over or nutrients leaching away.
Many soils in the rest of Indiana have CEC values around 10 to 12. “It’s like having a 75-hp tractor with a medium-sized fuel tank,” Maybach says.
That means you can apply more nutrients without risk of losing them compared to sandy soils, but perhaps not as much as if you farmed prairie soils.
Where other nutrients fit
If CEC defines the size of the tank, calcium is the tractor frame, Maybach suggests. Magnesium becomes the paint and chrome that goes over the frame. He believes in keeping those two minerals in balance.
In his analogy, potassium represents the air conditioning and cab package. Nitrogen is the fuel that drives the tractor and must be supplied to meet the needs of the crop. Phosphorus is the transmission that allows the tractor to move and function.
One part is still missing, he says. “Sulfur is the key that turns on the tractor,” Maybach says. “We’re finding that we need sulfur more often today because we’re getting much less from the atmosphere.
“If any of these parts aren’t in good shape, fix them They’re fundamental nutrients, just like the basic parts of a modern tractor.”