Getting enough nutrients into a soybean crop is more complicated than simply putting fertilizer in the soil.
Soybeans have specific nutrient needs throughout the growing season, and factors such as soil type, moisture, pH and root health can influence nutrient availability.
For growers, the challenge is finding the right balance between nutrient availability, crop demand and efficient uptake.
As researchers continue to examine soybean nutrition, they are looking for ways to help crops make better use of available nutrients while identifying where additional inputs can make a difference.
Tissue testing
Hugh Earl, associate professor and department chair of plant agriculture at the University of Guelph, is exploring critical micronutrient tissue-test levels and ways to fill the nitrogen gap in Ontario soybeans.
The project aims to establish critical tissue-test levels for key micronutrients, including boron, iron, manganese and zinc, with the long-term goal of developing an AI-based diagnostic tool for micronutrient deficiencies.
“I’d say that micronutrients tend not to be on our radar, except in the cases where a really clear, visually apparent issue arises, and so then there is an obvious problem to investigate,” says Earl, noting visual symptoms for a number of deficiencies can look similar, which complicates diagnosis.
He said his research results suggest that yield-limiting deficiencies and toxicities can be present even when there are no clear visual symptoms.
“So you really need more deliberate action than just observing the crop to know for sure if you have a micronutrient issue to solve,” he says.
Earl believes tissue testing is likely the best approach. Ideally, with a corrective application, farmers should also leave a test strip to verify that the diagnosis was correct and that the prescribed intervention was beneficial.
Micronutrient ROI
He said the goal of the project is to help farmers better determine when a micronutrient application is worth the investment.
Past research shows a correlation between tissue-test micronutrient levels and yield. He says establishing that relationship requires a range of micronutrient levels to develop response curves.
“Getting the tissue test levels up for experimental purposes is easy, you just apply more of the nutrient, but getting lots of examples of naturally occurring low, yield-limiting levels is harder,” Earl says, adding that micronutrient deficiencies are not as common as deficiencies of other nutrients and that, when they do occur, they are often caused by soil conditions that affect multiple micronutrients at the same time.
“The advantage of our approach is that we are able to manipulate each micronutrient independently across the range of deficient to toxic levels, in an overall system that is otherwise optimized and supports high yields, so we know that the crop performance differences we see are due only to the nutrient that we altered,” he says.
Predicting yield loss
As a result, the research should allow researchers to develop more accurate predictions of yield loss based on tissue-test levels than has been possible through traditional field studies.
“The current project doesn’t test the effectiveness of foliar applications for correcting deficiencies and restoring yield potential, but we hope to advance to that step in future work, as well as testing the reliability of those methods in actual commercial field scenarios,” he says.
Earl says one of the biggest challenges his team faces in accurately reproducing and identifying these symptoms is developing a culture system that can reliably induce clear deficiencies.
“We have definitely achieved that for boron (as well as for boron toxicity), manganese and zinc. So far we haven’t been able to get iron levels low enough to produce really clear visual symptoms — there is just too much iron in the soil substrates we have used,” he says, adding that even though the visual symptoms are not there, his team has been able to produce iron deficiencies and toxicities that affect crop growth, based on seed yield or biomass data.
Earl hopes the research will determine whether nutrient deficiencies can be detected through AI-based image analysis, even before they are visible to the human eye. Another limitation is that the work so far has focused on one soybean variety, and deficiencies may not present in the same way across different varieties. As a result, there is still considerable work to be done to develop, refine and fully test the technology.
Drought impact on nitrogen fixation
Earl says it’s difficult to determine if there are particular stages of soybean development when the crop is most vulnerable to drought.
He says this is usually dependent on the specific situation and how dependent the crop is on biological nitrogen fixation versus scavenging soil nitrogen.
“Our experiment is providing some insight into this, because we are testing all eight factorial combinations of high soil N vs low soil V, water replete vs. drought, and inoculated vs. uninoculated (there are no native rhizobia in the culture system, so uninoculated = unnodulated),” he says.
As a result, he says his team has had cases where they expected the crop to be highly dependent on biological nitrogen fixation under low soil nitrogen, and others where soil nitrogen is fully adequate to support the crop.
The team is discovering that under low soil nitrogen, successful nodulation adds to yield under both water-replete and severe drought conditions.
“We saw some things in that trial that surprised us, regarding how crops that were dependent on biological N fixation compared to those that weren’t, but we need more data to support that before I am comfortable sharing it,” Earl says.
Ongoing research
Research is ongoing to determine whether the foliar-applied biological product Envita can help compensate for reduced nitrogen fixation under drought stress.
He says his research shows that Envita can increase yields under conditions where other data indicate that nitrogen is limiting. For example, under low soil nitrogen and water-stress conditions, the researchers found that nodulated soybeans yielded significantly better than unnodulated plants, suggesting a benefit from the additional nitrogen provided through biological nitrogen fixation. If Envita application can close the yield gap between those two treatments, it would provide strong evidence of the product’s effectiveness under these conditions.
Looking ahead, he says there is potential for yield improvement through micronutrient applications, even when visual deficiency symptoms are not obvious.
“However, in my opinion this should only be approached through a management strategy that incorporates diagnostic methods like tissue testing, not by just applying micronutrients preventatively, because yield reductions from micronutrient toxicities are also possible, again, even in the absence of obvious visual effects,” Earl says.
The post New research aims to improve soybean nutrient management appeared first on Farmtario.
