Adequate fertility is a crucial building block for high soybean yields. "Exceptionally good soybean yields come from fields that are optimum in fertility,” says Farm Journal Field Agronomist Ken Ferrie. "If a field is low in fertility, soybeans will struggle more than corn.”
When you plan your fertilizer program for high-yielding soybeans, forget what you learned from growing corn. Fertility-wise, the two crops are as different as cats and dogs.
"Corn responds more to applied fertility,” Ferrie says. "Soybeans, on the other hand, respond to soil fertility. That means you can't fix problems in one year, as you can with corn. It also means you need a balanced soil fertility profile, from pH to micronutrients.”
Because the soybean plant is a legume, soil pH is important. "We rely on rhizobia bacteria to make nitrogen [N] for soybeans,” Ferrie says. "But in an acid soil, the bacteria can't produce enough N. So low pH gets growers in trouble. You need to keep it in the sweet spot—the 6.3 to 6.5 range.”
In that pH span, soil microbes thrive, letting the plant take up phosphorus (P), potassium (K) and micronutrients.
Potassium pitfalls. Soybeans differ from corn in their ability to take up P and K. "With soybeans, it's more important to have soil phosphorus and potassium levels in the optimum to high range,” Ferrie says.
Soybeans are efficient feeders on P. The plant creates an acidic area around its taproot, which lets it solubolize P from the soil.
But they are less efficient with K. "Potassium is the element we watch most closely in the soil,” Ferrie says. "With potassium, two problems can occur, both related to the clay content of the soil. If the soil has a high clay content, it can fix potassium to soil particles and make it unavailable to plants. In sands, on the other hand, potassium can leach out of the soil.
"In these situations, consider applying potassium every year, rather than applying a two-year supply ahead of corn. Work with an agronomist to identify fixation and leaching problems. ”
- October 2009