Decisions about what, when, how and whether to apply fertilizer in the fall usually come down to the bottom line.

Is the return worth the investment in the input? How do I prioritize what to apply within a field? Should I be thinking about micronutrients? Daniel Kaiser, University of Minnesota Extension soil fertility specialist, shares what the research says.

Start with the soil test
The soil test is the foundation to smart fertilizer decision-making.

“The goal of the soil test is to know where to target fertilizer applications for a greater chance of return on investment. It brings a level of accuracy and confidence to decision-making,” Kaiser says. “That also means knowing when to reduce applications in areas where you’re not likely to see a positive return.”

Dan Kaiser

Some soil test methods are better to use in certain conditions than others. For example, the Olsen P test should be used for soils with pH greater than 7.4, while the Bray P test can be affected by free carbonate commonly found in high pH soils, so should be used only in soils with a lower pH.

With so many choices of soil testing laboratories to submit samples, be sure to choose a lab that has research backing its predictions to crop response that are appropriate for Minnesota soils. Find out whether the lab provides fertilizer recommendations or simply reports soil test results. If it provides fertilizer recommendations, be sure they are based on U-M guidelines, or find out how to interpret them if they come from a different source.

Prioritizing nutrients
Even with clear soil test results, most fertilizer decisions require a good deal of prioritization in order to reach an ideal return on investment. Kaiser says it’s essentially a function of knowing which nutrients a crop will be most responsive to.

“For corn, typically the greatest return on investment will come from nitrogen,” he says. “Sulfur can also be profitable, but in medium- and fine-textured soils, you’ll run into issues with soil tests that may not accurately predict response, so organic matter concentration is typically a better indicator of response to sulfur.” The potential for a sulfur deficiency increases as soil organic matter decreases, but the response can also depend on factors like crop rotation and drainage. 

Recent research funded by the Agricultural Fertilizer Research and Education Council found that when soil tested high for phosphorus, there was a 13% chance of increase in corn grain yield from a P application, but the average yield reduction when no P was applied was less than 1%. The research found that P and potassium will provide the greatest return on investment for medium- and low-testing soils, and that maintaining high-testing soils will generally ensure a low risk for yield loss occurring from P or K.

Kaiser says the best way to look at economics when dealing with P and K is to focus on the chance of a positive return, and target field areas with the greatest probability of response to each nutrient. In those scenarios, the increase in yield will more than cover the cost of the nutrients.

Managing micronutrients
For growers thinking about micronutrients for fall, keep the focus on zinc for corn and use the DTPA (diethylenetriaminepentaacetic acid) soil test to determine when a response to zinc is likely — results will be 0.5 parts per million or less.

“Research has not demonstrated the need for any micronutrient in corn production other than zinc,” says Kaiser. “Soybean research shows no response to micronutrients either, so I would recommend keeping the focus on macronutrients when margins are tight.”

Again, be sure to know which test the lab is using and how to properly interpret it for local conditions and recommendations. “We see more issues with inconsistencies among soil tests when measuring micronutrients,” says Kaiser. “They tend to vary in the amount of the nutrient extracted, and interpretations are not necessarily interchangeable.”

When making fertilizer decisions, always remember that the majority of yield deficiencies will be due to macronutrients, as crops will generally have a greater demand for nitrogen, P, K and sulfur. If margins are tight and you’re considering a reduction in application, start with an accurate soil test, consider the crop’s potential response to applied nutrients and move forward knowing you’ve made a well-researched decision about fall fertilizer.

For the latest on fertilizer and nutrient management, follow @UMNNutrientMgmt on Twitter.

Source: University of Minnesota