Production | Spring 2010
Supplemental ‘N’ for soybeans? Save your money.
By Scott Garvey
Some growers see a payback some years, but there’s little evidence to support routine application of extra nitrogen in soybeans
For 5,000 years, farmers in China used soybeans to improve their soil’s nitrogen (N) content and improve yields in subsequent crops. Today, soybeans enjoy worldwide popularity and are widely used in everything from tofu, soymilk and margarine to industrial products such as hydraulic oil and adhesives.
And with new and exciting uses on the horizon for the humble soybean it’s little wonder farmers in non-traditional growing regions are experimenting with the crop.
According to published data from Manitoba Agriculture, Food and Rural Initiatives (MAFRI), newer varieties of the crop require only 2,375 to 2,675 corn heat units. That opens up much of the southern prairies to potential soybean production.
But introducing the crop to new regions requires that the proper strain of rhizobia — soil bacteria that fix N into a plant-available form — be established in the soil. Rhizobia colonize the roots of young soybean plants and form mini N-fixing factories known as nodules. Without them, soybeans are dependent on applied N. "We haven’t grown soybeans here for very long," says Bruce Brolley, MAFRI’s acting manager of the industry focus section. "In some cases we don’t have the proper bacteria — Bradyrhizobium japonicum — in the ground because it’s not native to the prairies."
He recalls one 2001 case in Manitoba where healthy plants came up only on the hills. Everywhere else they grew without nodules. "We had to apply a rescue treatment of N at the R3 stage, because plants in the low spots were small and yellow. Excess water had caused the rhizobia to temporarily die off."
So if top-dressing N can rescue a failing soybean crop, does it pay to apply it routinely to improve yields? Some published research suggests it does. According to a few research papers published online, seeding soybeans with additional N showed marked improvements in yield. However, the plants began to rely less on nodules to fix N and started to absorb it directly from the soil, minimizing one of the crop’s production-cost advantages.
But most of that research originates in China and other Asian countries. According to Horst Bohner, a provincial soybean specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) those results aren’t likely to be relevant in North America.
"The strains [of rhizobia] they have in Asia are extremely inefficient at fixing N," he explains. "The strains we have here were all introduced for that purpose and are relatively efficient at getting the job done. In the scientific community it’s fairly well accepted that N fertilizer on soybeans is a no go."
Brolley agrees with that assessment, "The research work I’ve seen doesn’t show it as conclusively successful," he says. "The only reason we would recommend application of N in-crop is to rescue soybeans that had poor nodulation."
Nevertheless, Bohner says, OMAFRA decided to experiment with the concept last summer in five different Ontario fields. At seeding, N was broadcast and deep banded five inches below the root zone at the rate of 100 pounds per acre.
Some results were contradictory. Overall, though, combined results showed a yield difference of less than one bushel per acre between fertilized and non-fertilized plots. "It was a complete failure," says Bohner.
However, Kevin Marriott, a grower at Petrolia, ON, believes limited application of N has improved his yields some years. It depends on the season, he says. "In cool weather a small amount of N would pay you back." But his experience has shown no benefit when the weather is warm and growing conditions are good.
"If you only knew what the weather would be like four to six weeks after planting, deciding whether or not to apply it would be easier," he says.
Marriott typically uses a broadcast application of mono ammonium phosphate mainly for the phosphate component, but the blend includes a small amount of N. "I’ve used upward of 20 pounds per acre," he says. "It’s a very low cost for that much N. It feeds soybean plants early, gives them stronger roots and gets them going." That way, he believes, crops get ready access to N before nodules develop, giving young plants a head start.
But Marriott admits it’s difficult to put an exact number on any yield advantage. "I’ve had extremely good yields in the past five years. It’s hard to know if that was from the nitrogen or the weather." Based on what he’s seen in his fields, however, he intends to continue the practice as long as fertilizer prices remain affordable. "I really feel I get a pretty good payback," he adds. FF
