Unfertilized soybean fields with lower soil fertility should be planted earlier than high fertility fields, according to a University of Illinois Urbana-Champaign study that re-evaluates longtime soil testing.
- Megan Allen
This result comes as a bit of a surprise, says Fred Below, senior author of the study.
"Our findings were somewhat counterintuitive. The expectation would have been that higher soil fertility is more important if you plant early and have a higher yield," said Below, professor in the Department of Crop Sciences in the College of Agricultural, Consumer and Environmental Sciences at Illinois.
In recent years, there has been a shift towards earlier soybean planting in Illinois. Earlier planting increases yield potential because a longer growing season gives plants more time to photosynthesize, leading to bigger plants with more branches, more nodes, more pods, and therefore, more seeds. However, with greater yields comes a greater need for soil nutrients.
"One of the reasons this is so important is that growers tend to not fertilize soybean. They usually fertilize corn, and then they rely on soil testing to decide whether to fertilize or let the soybean use the residual from the prior corn crop. So for economic reasons, there's a much greater tendency to rely on soil tests when it comes to producing soybean," said Below.
Soil tests are commonly used to measure the level of nutrients in the soil and gauge how easily the crop can access those nutrients. Farmers use soil test sufficiency values to help decide whether or not they should add fertilizer. However, soil test sufficiency values have not been regularly calibrated since the 1960s, when state universities handed off soil testing to commercial laboratories.
"We wanted to see how reliable the old soil test values were with today's production practices. A lot has changed in soybean production since then, both yield potential and how the crop is managed," said lead author Marcos Loman, who completed the study as a doctoral student in crop sciences.
From 2014 to 2021, the researchers took pre-planting soil samples from 133 unfertilized soybean trials across five Illinois counties. They then correlated the soil test values to yield. When the data was grouped by planting dates of very early, early, late, or very late, the correlation between soil test values and yield was much stronger for the late or very late planted soybean groups. In other words, high fertility fields are more likely to have high yields when soybean is planted late but not when it is planted early.
The team confirmed this result with additional statistics, drawing out the most important soil factors and running them through a predictive model.
"When farmers use a soil test, they're trying to predict the yield response. The key thing here was, if you put the planting date factor in, the prediction changes," Below said.
The analyses told the researchers that a high soil test value - more fertility - is less important for early planted soybean. They suspect this is because the crop has more time to collect the nutrients it needs. Plants can also move some nutrients from their leaves to help produce heavier seeds, and earlier planting leads to bigger plants with more nutrient reserves to redirect toward yield.
"If I have to grow 80 bushels early or late planted, the amount of nutrients that I need are the same. But if I'm planting early, I'll have a longer season to take that up. It's like the tortoise and the hare - slow and steady," said study co-author Connor Sible, research assistant professor in crop sciences.
Sible notes that their results are only for unfertilized soybean, and adding in fertilizers or other management practices like irrigation would likely change the story.
"If you really want to push yields, management is still very important. You can still manage early soybean to go higher, and you can still manage late soybean to go higher," Sible said.
"Perspective is also important. Do you want to maximize the yield of this field or of the whole farm? If you're considering the whole farm, trying to plant your earlier fields on the low soil test and your late ones on the high soil test will probably maximize whole farm yield," he added.
The study, "Soybean planting date affects the relationships between soil test values and grain yield," is published in the Soil Society of America Journal [DOI: 10.1002/saj2.20753]. Authors include Marcos Loman, Connor Sible, and Fred Below. The research was supported by the USDA-NIFA [NC1200] and the Illinois Agricultural Experiment Station [ILLU-802-990].
