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Soybean nutrient use analyzed

A recent study done by John Heard of Manitoba Agriculture, Food and Rural Initiatives (MAFRI) strives to understand what soybeans do with the nutrients they gather. The goal is to generate a better understanding of nutrient use so that better fertility recommendations can be developed when growing soybeans.

According to the study, soybean production values are often established from work done in other parts of the soybeans’ growing range in through the US and southern Ontario. These are regions with greater yield potential due to more accumulated heat units. This study was done to see if these data held true in Manitoba.

“I thought we should learn something more about the crop and how it grows,” Heard explains. “What we found here may not make a guy a buck today, but at least he now knows where his bucks are going.”

Another purpose of the study was to look at the residual nitrogen (N) available to next year’s crop. It is a common rotational practice to grow legumes in order to replenish soil nitrogen and soybeans are given nitrogen credits in other parts of Canada and the US (this ‘credit’ may actually be that soybean residue does not immobilize or ‘tie-up’ as much nitrogen as does corn stover or other crop stubble). Studies done in Manitoba suggest soybeans’ N contribution to next year’s crop may be quite small.

The plot was located in central Manitoba near Carman. The soil is classified as a Reinfeld clay with 5.7 percent organic matter. OAC Prudence soybeans were sown on May 22 in 2005 and spaced in eight inch rows. The field was fertilized with 40 pounds of N, 20 pounds phosphate (P) and 20 pounds potassium (K) per acre. Plants were sampled at six critical growth stages and the overall uptake and partitioning of three primary nutrients, N, P and K were examined.

“One of the curiosities here is that research studies and soil test summaries from commercial labs indicate there is precious little nitrogen contribution to next year’s crops,” Heard says. “The soybean plant makes a lot of nitrogen but most of it is in the seed.”

As nitrogen fertilizer prices go up, including soybeans in the rotation may help reduce N requirements, but a reduction in N requirements for the following crop may not hold true. “Growing soybeans is a good way to reduce your nitrogen fertilizer bill for one year but not for two years,” Heard says. “You save all your money the year you grow soybeans but you’re stretching your inoculant dollar a little too far if you expect two years of N when you only grow it once.”

However, the N fixed in the soybean nodules is large, with the total N uptake by the crop in the study coming out to 199 pounds per acre with a full 88 percent of it going into the seed. During grain fill, the seed accumulated 3.7 pounds of nitrogen per day between the R6 and R8 stages. These data were fairly close to other numbers generated in studies done by the Canadian Fertilizer Institute (CFI).

On the other hand, P uptake and removal was higher in the Manitoba study than it was in the CFI data. With concerns about P and water running high these days, a soybean field may be an effective way to remove soil P.

This study also found that soybeans are big consumers of K. Although this is generally well known to experienced growers, it might not be to new growers. “They’re finding lower yields and K deficiency showing up in the US cornbelt and Ontario,” Heard says. “I don’t think we’re at that stage where we’ve actually seen it manifest itself in the field yet, but our soybeans tend to be grown on heavier textured soils with high soil K levels.”

Potassium deficiency should be observed in the soil test result before it causes visual plant deficiency symptoms. If it does, farmers should see some yellowing or browning on leaf margins. So soil testing remains a priority when including soybeans in your rotation.

Based on the study, it is evident that soybean growers should ensure inoculation is done properly so that the soybean plant develops many large, active nodules. Ultimately, that will help ensure that the soybean crop not only has an adequate supply for optimum yield, but also does not use soil N at the expense of next year’s crop.