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Fall Approaches – Time to Think about Crop Fertility for 2022

By Joel DeJong and Antonio Mallarino 

As we enter the harvest season, we know we need to be looking forward to the next growing season at the same time. Having plans in place for your 2022 fertilization program before harvest starts is a good idea. Making these plans is tougher this year due to variable yields across the state due to drought stress this season, and the fact that fertilizer prices have risen a lot since this time last year.

Interestingly, some recent calculations comparing the relationship of corn price and fertilizer costs shows nearly the same price ratio now as we saw at this time last year – but the prices of both are significantly higher. Some producers in Iowa will have reduced yields due to drought or other weather problems. Others might be experiencing cash flow restrictions. Still others are just concerned about spending that much money on fertilizer per acre. Those are a few examples of why there might be consideration given to reducing the amount of fertilizer applied to 2022 crops. If that is you, here are a few questions to consider:

1. What was the “cost of doing business” in 2021? In other words, how much phosphate and potash did I apply before this crop and how much did I remove from each field when I harvested my crop? Don’t forget to start by doing that math. Read PM-1688, “A General Guide for Crop and Limestone Recommendations in Iowa,” and use Table 2 to calculate removal from your fields. If yields were reduced, and your normal application rate is based on nutrient removal, you might be able to reduce fertilizer inputs and subsequently the cost of fertilizer inputs.

2. Can I reduce my fertilizer inputs for 2022? It depends. The best way to determine that is to know your soil test levels for P and K. As stated in PM-1688, the percentage of P and K applications expected on average to produce a yield response within each soil test category is 80 percent for Very Low, 65 percent for Low, 25 percent for Optimum, 5 percent for High, and less than 1 percent for Very High. This means that as soil test levels increase, the probability, and the amount, of a positive yield response to fertilization decreases. Additionally, net return decreases and usually becomes negative at High and Very High soil test levels. Consider avoiding applications to fields or field areas that do not need P and K or lime. But do not skip areas with low soil test levels – the odds certainly favor a positive economic response!

3. If soil-test levels are low, do I need to fertilize at a level to rapidly move those soil test numbers up? The ISU recommended rates of fertilizer application in the Low and Very Low soil test categories are greater than crop removal, but do not rapidly build soil test levels. These application levels should produce maximum yield in most conditions, provide profitable crop responses, and will gradually increase soil test levels over time. Applying rates higher than those recommended in PM-1688 will increase soil-test levels faster, but seldom will increase yield further.

4. If it continues to be dry, will my soil-test results be accurate? If possible, try to delay soil sampling until meaningful rainfall because it will result in a better sample and more reliable soil-test results. If you must take soil samples under the current dry conditions, be careful with sampling depth control and that you get the complete soil core in the bag. Note that soil pH test results may be a bit more acidic than it would in normal conditions. Also, soil K test results may be lower than they would be under normal conditions due to less recycling to the soil and less replenishment of soluble or easily exchangeable soil K pools. However, soil P test results probably will be affected little by the recycling issue. 

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