Lime, Phosphorus, and Potassium Fertilizers Decisions in Times of Limited Resources

By Virgil Schmitt

Once you have taken soil samples, Taking Good Soil Samples, the First Step in a Good Fertilizer Decision (https://crops.extension.iastate.edu/blog/gentry-sorenson/taking-good-soil-samples-first-step-good-fertilizer-decision) and received your soil test report, how do you interpret it and any fertilizer application suggestions that may be included? And if the markets have left you without enough resources to do everything that should be done, how should you set your priorities?

The Iowa “bible” on soil fertility for phosphorus (P), potassium (K) and lime is Iowa State University (ISU) Extension and Outreach publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa(https://store.extension.iastate.edu/product/5232), which can be purchased on-line or at any county Extension office for $2.50 or downloaded at no charge. Following the guidance in PM 1688 will lead to the best long-term profitability.

In PM 1688, on pages 5 and 6, you will note that soil test values for P and K are categorized as Very Low, Low, Optimum, High, and Very High. Note that not all soil test laboratories follow the categorization parameters listed in PM 1688, so the first thing you will want to do is make any needed adjustments to the categories listed on the soil test report.

The second thing you will want to do is verify that any fertilizer recommendations listed on the soil test report are consistent with application rates in PM 1688. Some laboratories are “spot on” while other are “out there a ways.” You should make any needed adjustments to the fertilizer recommendations listed on the soil test report.

In PM 1688, you will note that no fertilizer is recommended when soil tests are High or Very High, and crop removal is recommended if the soil test is Optimum. The concepts behind suggested application rates in Tables 3 through 14 are described on pages 3 through 5. Especially note the likelihood of a positive yield response within each category is listed on page 3 and are 5% and less than 1% for High and Very High testing soils, respectively, and with low commodity prices, the chances of a positive Return on Investment (ROI) are even less. Without getting into specifics, the recommendations for soils testing Very Low and Low are based on yield responses in numerous field trials and will result in maximum yield in most conditions, including high yield environments. For the Optimum category, default yield levels provided in footnotes to Tables 3 through14 should be adjusted to prevailing yield levels in order to estimate removal using the P and K concentrations of harvested plant parts provided in Table 2. A fall 2023 ICM News article, Fall Phosphorus and Potassium Fertilization Decisions with Variable Prices and Updated Soil-Test Interpretations ( https://crops.extension.iastate.edu/cropnews/2023/09/fall-phosphorus-and-potassium-fertilization-decisions-variable-prices-and-updated) discussed updates to PM 1688 and provided examples of economic benefits with variable crop and fertilizer prices.

At the end of PM 1688 (pages 10 through 12), you will note that a soil pH of 6.0 is considered sufficient for corn and soybean production in the area essentially west of I-35 and north of I-80 plus the area essentially west of Highway 48 and south of I-80. For the rest of Iowa, a pH of 6.5 is considered to be sufficient for corn and soybean production. When the soil is more acidic, the amount of lime to apply is decided on the basis of the Buffer pH test and provided tables or equations. Again, following the guidance in PM 1688 will lead to the best long-term profitability.

But if the markets have left you without the resources to do everything that following PM 1688 recommends, how could you reduce recommended inputs while avoiding large yield losses? You could use one or more of the following points.

1. You may postpone lime application if soil pH is about 5.8 or higher. Many field trials have shown that the small corn or soybean yield increase when liming soils with a pH of 5.8 or above is unlikely to provide a positive ROI with the low crop prices predicted for the near future. Furthermore, results of field trials have shown that if the pH is less than 5.8 and the Buffer pH calls for more than about 2.5 tons of Effective Calcium Carbonate Equivalent (ECCE) per acre, you may apply only 2.5 5tons of ECCE and plan to test soil again in two years to apply more if needed.
2. If available funds do not allow for application of all the P and K fertilizer that is recommended for your fields, you should apply the recommended rates for areas testing Very Low, even if you rent, because research showed large probabilities of yield increases and large positive ROIs.
3. Then, use any leftover funds for applying at least some fertilizer to all areas testing Low, if available funds to not allow for application of the total fertilizer amount recommended.
4. Postpone any major P and K to areas testing Optimum, except for starter if you use it. This is because the likelihood of having a yield increase in the year immediately following the application of fertilizer to soil testing Optimum is only about twenty-five per cent and is even less for a positive of ROI.
5. Plan to re-test your soil in two years to monitor movements in pH and soil test levels to determine your next fertilizer and liming steps.

In times with limited available financial resources and a need to reduce suggested lime, P and K inputs, following one or more of the suggested changes will provide the maximum ROI on the resources that are available.