How much fertilizer someone recommends based on soil test results may depend upon where you live and the philosophy used to determine fertilizer rates. There are differences in soil types from one region to another, but Matt Clover suggests you ask enough questions to understand the philosophy behind your fertilizer recommendations. You can tweak your rates yourself based on your goals.
“There is no magic in state lines that makes soil testing recommendations change,” says Clover, a Pioneer agronomist based in Iowa who specializes in soil fertility. “This applied to areas near state lines with no natural borders, like eastern Illinois vs. western Indiana. The farther away form a state line, the more areas may truly differ.
“Specialists in different parts of the country apply different philosophies, and that’s why you may see varying recommendations. Once you understand how recommendations are made, you can decide which strategy makes the most sense for you.”
Most state university specialists ascribe to either a buildup and maintenance, or sufficiency philosophies. Some people base recommendations off base-cation saturation ratios, usually involving calcium and magnesium. No universities recommend this approach, Clover notes.
Build and maintain
If your state recommendations follow a build-and-maintain approach, the goal is adding fertilizer to feed the soil, Clover says. In either this or the sufficiency method, specialists will establish a critical level for each nutrient, including phosphorus and potassium. “The idea in build-and-maintain is applying enough fertilizer to either raise the soil test level to the critical level, or maintain it in the optimum range, plus an extra amount to help build soil test levels over time.”
Tri-state recommendations for Indiana, Ohio and Michigan, updated for 2021, follow this approach. If an area tests below the critical level, recommended fertilizer rates will be higher, and will level off where soil test levels are already at or above the critical level. What’s the correlation between soil test results and seeing a yield response in the year fertilizer is applied?
“It’s usually fairly high, 60% to 80%, if soil tests are below the critical level,” Clover says. “If they’re in the optimum range, odds are lower, probably 15% to 25%.”
You may want to drop rates or skip a year if soil test levels are above critical levels, depending upon cash flow and yield goals. These percentages for response tend to apply in both build-and-maintain vs. sufficiency systems.
Feeding the plant is the philosophy behind the sufficiency approach. It tends to rely on nutrients already in the soil to help augment what you apply through fertilizer. Especially once the critical level is reached, recommendations don’t include an amount to build soil test levels.
There are pluses and minuses between the two approaches, Clover says. For build-and-maintain, there is higher risk of not seeing full payback for fertilizer expense in the short term because you will likely apply more than in a sufficiency-based system. However, there is minimal risk of underapplying fertilizer.
The risks flip-flop if you follow a sufficiency approach. There is low risk of not getting your money’s worth in the short term, but a higher risk of not applying as much as the crop could need.
The best advice is to know which method your consultant, retailer or lab uses, and adjusting to meet your own goals and comfort level, Clover concludes.