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Fertilizing soybeans: is it needed now?

Several factors during the past few years may have left some farms with soil that is more nutrient deficient than the growers who own them would generally have believed. That could soon lead to problems with soybean health and yield. “Fertilizer prices have been very high over the last few years, which has meant lean application levels,” says Ken Currah, Pride Seeds market development agronomist. “At the same time, growers have tried to maximize income with high-nutrient-removal, high-yield corn, sometimes growing it back to back.”

Compounding the problem is the fact that “for years, producers have fertilized ahead of corn and believed that’s enough for the next crops in the rotation,” says John Waters, a crop advisor with Lakeside Grain in Forest, Ontario. Even though soybean yields have been very good the last while, Currah is quite certain that all these factors have resulted in some soil degradation, and that may well start affecting soybean yields very soon. He is particularly concerned about potassium.

Soybeans differ from corn in that they, as legumes, generally can supply their own nitrogen, but they require optimum levels of phosphorus, potassium and other nutrients to do so, says Dr. Tom Bruulsema, northeastern region director (North America program) at the Georgia, USA-based International Plant Nutrition Institute and an adjunct professor at the University of Guelph. “In addition, if plants don’t have adequate potassium nutrition,” he says, “they may suffer from higher incidences of mouldy seed arising from pod blight (Phomopsis) and purple seed stain caused by Cercospora.” He notes that potassium also helps the crop mature uniformly and can improve market grade by reducing the percentage of shrivelled seed.

Currah also thinks growers need to consider the role of potassium in the plant as it relates to various plant health issues, such as drought tolerance. If the previous year’s high-yielding crops have mined down soil potassium levels, then the necessary reserves are no longer in place to ward off the effects of environmental stresses during the next growing season.

Crops also to blame?
Some also question whether corn crops are mining more potassium from the soil than is generally supposed. That is possible, in Bruulsema’s view. While removal can vary depending on the potassium fertility status of the soil, he says silage corn removes by far the largest amounts. “A 24-ton-per-acre silage corn cut removes 130 to 240 pounds of K2O per acre,” he notes. “A harvest of 50 bushels per acre of soybeans removes around 70 pounds, while a hefty 200-bushel per acre yield of grain corn removes about 55 pounds.”

To investigate whether the potash that corn crops are mining out of the soil might be more than believed, and thus how much it could be affecting soybeans adversely, Waters conducted some small trials in 2010 that involved the application of high amounts of potash during the spring, just before no-till soybean planting in fields where corn was grown in 2009. “This season, we saw no difference with potash levels of 100, 200 and 300 pounds per acre,” he says. “However, we’re going to see what happens next year (2011) with some of those plots after the winter moisture has broken down the potash to some extent.” Redistribution through the soil profile, and release or fixation by clays in response to freeze-thaw and wet-dry cycles can influence potassium availability. “We’re also doing similar plots, but with the potash having been applied this fall (2010).”

However, Dr. Palle Pedersen, technical manager with Syngenta Crop Protection (US), thinks it is the soybean crop that is removing more nutrients than are generally believed, not the corn. “The newer soybean varieties are providing bigger yields than years ago, bigger than anything we’ve seen, and we are taking more nutrients out than are being added in,” he says. “You can easily fertilize with phosphorus and potassium every other year, but it is important that you apply what you are removing. You simply cannot use the same rates as you did 20 years ago. You may be able to mine the soil for a few years, but this is not sustainable.”

Soybeans, like any crop, are best fertilized on the basis of a soil test, say Pedersen and Bruulsema. “If I were a grain farmer with no manure to add to my soil, I would do soil testing every year,” adds Pedersen, “and most likely fertilize every year. Take your samples and get your soil nutrients up to recommended levels. Commodities are worth so much these days, and to get the best yields, you must fertilize adequately.”

The fact that growers are planting soybeans earlier to maximize yield means that agriculture has moved the bar from where it was 20 years ago. Getting the seedlings the nutrition they need is more important than ever to get a high yield, in Pedersen’s view. “Fertilizer is not,” he stresses, “a place to cut corners.”