A wet spring, late planting and variable weather for the growing season resulted in a wide range of corn and soybean yields within and between fields in 2019. Entire fields or parts of fields were planted later than usual or not planted, resulting in variable amounts of phosphorus and potassium removed during harvest. Reduced yields and low market prices for corn and soybeans create uncertainty about P and K fertilization decisions for the 2020 crop — especially for corn. 

The high number of prevented planting fields in some areas, a late harvest and the inability to get P and K fertilizer applied as planned in fall raises even more questions about how much to apply per acre this spring.  

If P and K fertilizers were applied last fall or last spring, but no crop could be planted, there’s no reason not to count all of the applied P and K as available for the 2020 crop. The same goes for any lime applied over the past 12 months. Any nitrogen that was applied in dry ammonium fertilizers (MAP or DAP) is likely no longer available and shouldn’t be counted on in 2020. 

P, K removal with crops 

At our Crop Advantage meetings across Iowa in January, fertilizer management decisions are one of the topics being discussed. ISU Extension agronomist Antonio Mallarino, a specialist in soil fertility management, spoke at our meetings. Here are some of his key points, and answers to questions farmers are asking. 

In fields where crops grew in 2019 and grain or corn silage was harvested, soil testing and figuring out the amounts of P and K removed with harvest are useful to decide P and K fertilizer application rates, especially with variable yields and low crop prices. Research results from hundreds of field trials were used to develop guidelines for P and K soil test interpretations and fertilization rates recommended by ISU. Refer to ISU Extension publication, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, PM 1688.

Soil testing is not a perfect diagnostic tool, but research has shown that crop yield increases from P and K fertilization are large and likely to occur when soil test results are in the very low and low interpretation categories. Yield increases are small and less likely for soils testing in the optimum category, and very unlikely to occur when fertilizer is applied to soils testing in the high and very high categories for P and K.  

ISU’s suggested fertilizer application rates for low-testing soil are based on yield response data and will result in maximum yield in most conditions and yield potential situations. These rates will build the soil test levels gradually depending on soil properties and actual yield levels. 

In soils testing optimum for P and K, suggested application rates are based on nutrient removal with harvest (grain, silage, crop residue, hay or pasture). To estimate the amount removed, publication PM 1688 includes average P and K concentrations and indicates that farmers and crop consultants should provide the soil testing lab with site-specific information of the plant part harvested (just grain or the entire plant?) and the prevailing yield level when submitting soil samples for analysis. 

The yield level in the publication for removal-based rates uses “default” yield levels (approximately state averages) because soil testing labs often don’t receive yield information. You should use your actual yields to estimate P and K removal, Mallarino says.  

No P and K fertilizer application is suggested for soils testing in the high and very high categories — other than applying P and K in starter fertilizer to help in fields in unseasonably cold and wet springs with thick crop residue cover.  

Economic return to P, K 

Research also has evaluated economic returns to P and K fertilization of corn and soybeans by using results from field trials. Mallarino has summarized net returns from fertilization for different soil test values and scenarios for three grain prices while maintaining the same fertilizer prices. In soils testing very low, economic benefit from fertilization is very high and very likely for all price scenarios. In soils testing low, benefits are lower but likely and most of the time well above breakeven, even with lower grain prices than today. 

Thus, producers are advised against reducing recommended P or K application rates for low-testing soils, because the benefits from investing in fertilization are large and likely to be gained even with current low grain prices. Application of removal-based rates of P and K to high-testing soils results in mostly negative returns for all price scenarios (losses to investment in fertilization), so producers should not apply removal-based rates at those test levels. 

In soils testing optimum (for which removal-based application rates are suggested), fertilization decisions aren’t as straight-forward as for low-testing or high-testing soils. The benefits are much smaller, but on average, are still above breakeven for the highest price scenario for all yield levels (using prices of a few years ago). For the other price scenarios, however, returns bounce around breakeven, with about equal probability of getting small benefits or small losses to investment in fertilization. 

Iowa farmers typically apply P and K each year or a two-year rate for the corn-bean rotation once before corn. With unsafe land tenure (if you think you may lose rented ground) or seriously bad economic conditions, farmers who fertilize each year will increase profit with little or no chance of yield loss by reducing the removal-based rate, applying only starter fertilizer, or even skipping P and K fertilizer application.  

Mallarino points out that skipping a removal-based application will not maintain your soil-test values; however, and in the future, you will have to apply a higher rate. Farmers who fertilize every two years with application due for the 2020 crop year could do the same thing to increase profits from the first crop, but the cost of applying fertilizer again for 2021 might offset any gain unless crop prices are much better. 

Another issue for maintaining soils testing optimum for P and K is many farmers are confused about the yield level that should be used to estimate P and K removal. The suggestion in publication PM 1688 is to use the “average or prevailing yield level” with the provided average P and K concentrations for different crops and harvesting systems. This suggestion means considering the yield levels during the last three or four years — not a yield goal, and not just the 2019 yield, even if it is lower or higher than normal. This is because there is a good relationship between removal with harvest and P or K soil test levels only over several years. 

Prevented-planting fields 

P and K management for 2020 in fields or portions of fields where corn or beans couldn’t be planted or drowned out in 2019 present additional challenges. If P and K couldn’t be applied last spring or last fall, you can apply the same rates you were planning to apply, and taking a new soil test will not be needed to apply for 2020 as long as volunteer weeds or seeded cover crops were not harvested for hay or silage, and there wasn’t severe erosion by rain or flooding. In Iowa soils, P and K from previous application (and no crop grown) will still be available for the 2020 crop. 

If P and K were applied for the 2019 crop, or biomass (silage or cornstalks) was removed or there was severe soil erosion, there is more uncertainty. In these cases, new soil sampling and testing may be the most cost-effective solution. There is no reliable information about P and K removed by weeds or by several kinds of cover crops, and analysis of sampled biomass will provide quite variable and likely untrustworthy removal estimates. With severe soil erosion, much of the applied P and K may have been lost with surface runoff, especially in no-till fields. Other options are to apply a conservative rate, perhaps the average annual crop removal from the previous three or four years or use starter fertilizer. 

What about fields with “flooded soil syndrome,” or sections of fields where no crop was planted and there were no weeds or crop growth since the flood? In these cases, soybeans may require inoculation, and when P had not been applied, perhaps a higher P rate than planned or additional starter P may be justified for corn. The fact sheet Flooded Soil Syndrome provides information. Also for additional information, visit the Iowa State University Soil Fertility website. 

Kassel is the ISU Extension field agronomist at Spencer in northwest Iowa. Contact him at [email protected].