BLOOMINGTON, Ill. — “Good” soybean yields don’t necessarily mean the soil fertility is where it needs to be.
Andy Wycislo, Waypoint Analytical Midwest region agronomist, said farmers don’t want to make too many assumptions about their fields’ fertility, and soil sampling is the first step.
“I see this more in the fertile areas of the Corn Belt where they say they have deep soils and don’t have to worry about fertility because the soil ‘is always going to back me up.’ Your yields may be good,” Wycislo said in an Illinois Soybean Association-hosted webinar.
“You may have average or above average yields, but if you’re not properly managing your fertility how much better could they be. Some areas of your field could have very good fertility. Some areas could have very poor fertility based on some of those variables. If you’re looking at a whole field approach you may not even notice that you have lower-yielding areas in your field.”
Fertility levels decline over time with no added fertilizer. Nutrients are removed at harvest and, if not replaced at the right rates over time, the soil will starve.
“This is particularly important with micronutrients more and more. I think the micronutrient fraction has been grossly overlooked in recent years because people were focusing on the macronutrients and ignoring the micros,” Wycislo said.
Fertility programs should be based on hard numbers, not assumptions, to improve plant vigor and stand quality while reducing disease and weed pressure.
Soil sampling provides the answers to what is and isn’t needed to boost bushels through efficient fertility plans.
Composite, grid and zone sampling are soil test options. For composite, one sample represents a large area, and multiple cores are combined into one sample, providing a low-resolution method that does not account for in-field variations.
Grid sampling has become a more popular method where many samples are taken per field in a defined grid, the prevalence being 3.3- or 2.5-acre grids. This provides a high-resolution method that helps capture in-field variations.
In zone sampling, one sample represents a pre-defined zone that can be determined by yield maps, soil texture, drainage patterns, previous soil test results or other factors.
“Maybe you don’t need to do a 2.5-acre grid in a certain area because this entire 12-acre block responds similar to fertilizer and has similar yields,” Wycislo said.
“I recommend if you’ve been composite sampling or you’re not sampling, start out with a grid at least for one or maybe two sampling cycles to get a good feel for your field’s variability and over time you can analyze those soil tests results, look at your yield maps, look at your drainage patterns and soil textures and make a grid that allows you to monitor things in a more concise matter than manner than a zone.”
Sampling typically is done in the fall or spring and results fluctuate seasonally. To compare multiple samples, make sure they are collected at the same time of the year.
Take samples as often as necessary to detect variation. Sandy soils need to be sampled more frequently because they don’t hold nutrients, as well.
“Every four years is kind of the standard in this part of the world, and that may be fine, but do we really know if that’s frequent enough. More frequent sampling could help determine if your fields fluctuate from year to year or stay fairly stable,” Wycislo said.
“Different soil types can fluctuate from year to year. If you determine your field is fairly stable, then maybe four years is fine, but if for some reason you get wild fluctuations in pH or mineral nutrients, then maybe you need to sample more often.”
Wycislo said samples should be sent to a quality laboratory.
“Make sure they have rugged internal quality control, find out if they participate in proficiency testing programs such as North American Proficiency Testing or Agricultural Laboratory Proficiency, so see the facility, and ask about turnaround time so you can get timely results,” he said.
More Is Best
Soil analysis should include pH, buffer pH — closer the buffer pH is to soil pH the greater the buffering capacity of the soil — carbon exchange capacity, organic matter, phosphorous, potassium, calcium and magnesium. All are necessary for basic fertilizer and lime recommendations.
“Micronutrient analysis is becoming more critical as levels continue to decline and this analysis is also now affordable with some labs,” Wycislo said.
“Make sure your analysis package gives you all the information you need to make good recommendations. Recommendations are a guide. Many factors play into a proper fertility programs.”
Soybeans can sometimes be overlooked on fertility programs because nitrogen is not applied, but a solid fertility program can boost soybean yields just as effectively as with corn.
“You’re not applying nitrogen to soybeans, and I think sometimes that can maybe contribute to soybeans being overlooked, but you can get bumper crops in soybeans just as much as you can with corn if you’re managing your fertility properly,” Wycislo said.