Last year’s excellent fall weather enabled many Minnesota farmers to apply phosphorus and potassium to their fields. If that didn’t happen, farmers hopefully were able to apply these nutrients in early spring after snow melted and soils thawed.

There were also many acres where nitrogen was applied in our state last fall, in the form of anhydrous ammonia or urea. Early this spring, I didn’t see a lot of standing water in fields following snow melt, meaning that a good amount of that moisture was absorbed into the soil, along with any applied nitrogen that was in mobile form.

If you’re still assessing your in-season fertility program, here are some steps to help formulate or refine your sidedress strategy:

Assess nitrogen availability. Use pre-sidedress nitrate test (PSNT) samples and tissue samples to determine how much and where the nitrate-nitrogen is located in your soil, and if your plants are getting access to it. Use of a nitrogen stabilizer can ensure nitrogen is where it should be. The time to take PSNT samples is between crop emergence and V5. Samples can be taken at different depths to determine where the nitrates are located. Use these results to help determine the amount of your sidedress application. As a rule of thumb, the deeper your nitrogen is located, the earlier you should start sidedressing.

Time sidedressing properly. Most sidedressing begins anywhere from the V2 to V12 corn growth stages, and timing depends on your current nitrogen program. Did you use a nitrogen stabilizer? What source of nitrogen did you use? Did you apply fall and/or spring nitrogen? How much? What is the crop rotation? Talk with your agronomist about these questions to determine when and how much to sidedress.

Don’t forget the sulfur. If you are sidedressing nitrogen, you should include sulfur in that application. How much will depend on the rest of your sulfur program. We know that nitrogen and sulfur roles often crisscross in the plant, with sulfur playing a specific role in nitrate metabolism to help nitrogen be more efficient, which could provide a yield response. Some sulfur fertilizer sources may also prevent urea from volatilization and denitrification. However, sulfur should not take the place of a nitrogen stabilizer.

Also, data show that some newer corn hybrids take up nitrogen later in their life cycles than older genetics do — usually post-tassel. Using a stabilizer will allow nitrogen to stay in the soil longer, so it will be available for the plant during late-season uptake.

A word about soybeans: Soybeans use more nitrogen than corn does but can fix their own nitrogen. However, this doesn’t mean they don’t need sulfur. If you have a high-yielding soybean field that has the rest of its fertility in order, do some sulfur trials to identify if it may be the next limiting factor.

Assess micros for macro impact. Other nutrient deficiencies we saw in corn in 2020 were the micronutrients zinc and boron. Zinc is not mobile in the plant or in the soil, so it’s very difficult to sidedress. Boron is mobile in the soil and could potentially be sidedressed. However, boron is mostly needed during corn plant germination and then during reproduction — much earlier and much later than our typical sidedress timing.

In high-yielding corn, I recommend using tissue samples to monitor any zinc and boron deficiencies after nitrogen and sulfur applications. Data tell us that when we get nutrients like nitrogen, sulfur, phosphorus and potassium right, we have a higher likelihood of receiving yield responses from micronutrient applications.

Zuk is a regional agronomist with WinField United in southern Minnesota. Contact him at [email protected].

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