Tight profit margins are making fall decisions on crop inputs difficult for farmers, note University of Minnesota Extension specialists.

Selecting nutrients, and when and where to use them for maximum profitability, are relatively easy management decisions to make for nitrogen used on crops such as corn, wheat and sugarbeets. However, deciding to apply other nutrients such as phosphorus, potassium and sulfur takes more research on the part of the producer.

In a recent Extension crop news blog, scientists Dan Kaiser and Jeff Vetsch answered five frequently asked questions that they get from farmers who are trying to decide whether to apply fertilizer in the fall:

1. Do I need it or not? While the use of P and K fertilizers has increased the fertility levels of some soils, there is a point where additional fertilizer doesn’t give an immediate return on investment, or the chance of an ROI to the nutrient applied is low.

Recent research funded by the Minnesota Ag Fertilizer Research and Education Council (AFREC) has clearly shown that fertilizer is needed to increase yield in fields testing low for P or K.

“This research has clearly shown no inherent benefit to maintaining high to very high P-testing soils,” the scientists wrote. “Some may argue their highest-testing soils are their best-producing fields, but remember, fertilizer is not the only thing dictating yield levels. Reducing inputs on very high-testing fields is the best way to use soil testing to improve a farmer’s profitability.”

2. Is there any difference when using MAP or DAP? While MAP (monoammonium phosphate) and DAP (diammonium phosphate) are somewhat different, they both should equally supply P to a crop if the same total rate of P is applied per acre with both products. The products will differ slightly when they react in the soil due to their pH of dissolution, which is more acidic for MAP and basic for DAP.

“MAP should be better in high-pH situations, as it should acidify the soil around the granule, but we have not seen a greater benefit from MAP in Minnesota,” Kaiser and Vetsch wrote. “More often, the decision of what to use is dictated by what is commonly carried by the retailer.”

3. How about co-granulated products? The presence of co-granulated products, such as MicroEssentials S10, S15 or SZ, has increased over the years. The main questions here are whether the additional nutrients are needed, and if having them all in one granule is better than blending them together from multiple fertilizer sources.

Having a consistent amount of nutrients per fertilizer granule can help with distribution, but do you need all the nutrients?

“Sulfur is more widely needed across the state, so if some of these products that contain S can offset the need for additional S, then they can be economically feasible,” the scientists wrote. “The same is true for zinc if you need it.”

4. Do I need to apply potassium? Research in the use of potassium for corn and soybean production has been ongoing for the past decade. Projects funded by AFREC and the Minnesota Corn and Minnesota Soybean research and promotion councils have shown that potassium can be a yield-limiting factor.

There are some clear issues identified by this research. For soils higher in clay, like loam and clay-loam soil, the potassium soil test can overpredict availability of K in soils, which can lead to underapplication of K fertilizer.

“We suggest growers in central and western Minnesota use 200 ppm as a critical level for soil-test K if they are using a maintenance strategy,” Kaiser and Vetsch wrote. “For the same areas where K tests are around 160 ppm, do not forget about K, particularly ahead of a corn crop. On silt loam soils in the southeastern part of the state, a critical level of 160 is enough. On very sandy soils with cation-exchange capacity of 8 or less, our research has seen no yield response in corn or soybean when soil-test K is greater than 100 ppm.”

5. What about base saturation? The idea of an ideal base saturation for K is not new and keeps popping up from time to time. In some current trials, the scientists wrote that they have looked at the range of base saturations. The sites all started at medium to high soil-test levels, and after 10 years, there have only been sporadic yield responses at the sites. One caution on base saturation is that the cost of the fertilizer needed to change the base saturation is high, particularly on soils high in clay content. For soils testing 200 ppm in K or greater, there is very little chance that additional K will increase yield, even if it changes the base saturation of the soil. Some concepts like base saturation do have roots in scientific literature, but the concepts were developed in other areas of the country with different soils, so the results may not apply to specific Minnesota conditions.

Source: U-M Extension