Phosphorus and Potassium Management with High and Uncertain Prices This Spring

This spring began with higher and uncertain phosphorus (P) and potassium (K) fertilizer prices, with a higher increase for P fertilizers. Fortunately, corn and soybean grain prices began increasing in February and expectations are for further increases, mainly for corn (Chad Hart - Ag Market Outlook). Although many producers already applied the phosphorus (P) and potassium (K) needs for 2026 last fall, others will apply them this spring and are considering reducing planned rates due to lower expected economic benefits from fertilization. However, reducing P and K rates across all conditions is not a good management decision.

Soil Testing

Soil testing is a diagnostic tool on which P and K fertilization should be based, and its appropriate use results in economic benefits. Iowa State University (ISU) field research results have been used to develop Iowa soil sampling methods and soil-test interpretations for P and K. ISU Extension publications CROP 3108 (Take a Good Soil Sample to Help Make Good Fertilization Decisions), PM 1688 (A General Guide for Crop Nutrient and Limestone Recommendations in Iowa), and PM 689 (Fertilizing Pasture) ) provide information to make sound economic decisions related to fertilization.

Soil Testing Categories and Yield Response

On average, crop yield increases from P or K fertilization are large and highly probable in soils testing within the categories Very Low (80% or higher) and Low (about 65%) but are lower for soils testing in the Optimum category (25% or less), for which only maintenance fertilization based on prevailing yield and removal is recommended. There is a very small probability of small yield increases in the High category (5%), for which a low starter rate only is recommended for corn), and very small and unlikely increases in the Very High category (1% or less).

For corn and soybean crops, Figure 1 shows relationships between profits from P fertilization and soil-test P (similar for dry or moist sample handling methods) and Figure 2 shows similar relationships for K fertilization using the dry or moist (or slurry) soil sample handling methods. The figures show that large economic benefits are likely in low-testing soils using recommended fertilization rates, whereas much lower benefits occur in soils testing Optimum using removal-based rates. Investment in fertilization of soils testing Optimum is riskier except with very favorable prices because of small yield increases that often do not offset the fertilizer cost.

Suggestions

With low-testing soils, reducing recommended fertilization rates is not a good business decision, and use of the recommended application rates will gradually build up soil-test values to levels that should be maintained for long-term productivity and benefits. However, this is not a good time to apply higher rates than recommended to build-up to higher levels and faster.

With soils testing Optimum, producers with safe land tenure can apply the removal-based rates or temporarily reduce them. But those with uncertain land tenure or in a bad economic situation should definitely skip or reduce recommended removal-based P and K rates. This will not affect yield much because expected yield increases are small and will increase economic benefits. Producers with starter attachments can apply starter instead of applying expensive removal-based rates. However, a downside of this practice is higher rates will be needed in the future to raise soil tests back to the Optimum category.

With soils testing High or Very High, producers with high-testing soils will not see a yield decrease by not applying unneeded and expensive fertilizers and will see increased economic benefits from crop production. They can safely wait to restart applying removal-based rates when soil-test levels decrease to the optimum range or prices become more favorable.

For hay or pastures, producers should remember that P and K fertilization are essential for the seeding year, and recommended rates should not be reduced (see publications PM 1688 and PM 689). However, for re-fertilization (topdressing) of established forages or pastures in soils testing Optimum, producers can reduce or withhold removal-based rates except perhaps for the most valuable crops such as alfalfa for hay. With high-testing soils producers should not apply removal-based rates or higher.

What about P and K in animal manures?

For farmers who produce both crops and animals in confinement this is a great time of saving in fertilizer dollars by using manure nutrients. Some crop producers could by manure, although pricing is very variable. Using manure P and K is more complicated than using fertilizers, but useful information and guidelines are included in ISU Extension publications such as PMR 1003 (Using Manure Nutrients for Crop Production), PM 3014 (How to Interpret Your Manure Analysis), and other manure management publications in the Extension Store.

Decline of high soil P and K levels when removal-based fertilization is withheld

Producers should have an idea of how high soil-test P and K levels decline when removal-based applications are stopped. Figure 3 shows that the relationships between P or K removal by corn and soybean and soil-test values are good over many years but not necessarily from year to year. The mismatch in short-term results from variation in recycling with crop residues and large temporal variation in the equilibrium of different soil P and K pools, and these processes are known to be greatly influenced by rainfall and soil moisture patterns.

Since P or K removal is not closely related to soil-tests values in the short term, producers should consider “prevailing” yield levels in a field and long-term soil-test trends when making decisions about soil-test P or K maintenance. Producers should not overreact to higher or lower than normal yield or soil-test value in a specific year because this may not reflect well what’s going on. Large changes in application based on information from only one year may not be the best thing to do for long-term profitability and overreacting to very high or very low values will introduce more variability and confusion.

Results in Figure 2 justify more frequent soil sampling than the usual 4-year interval. Sampling every two years for the corn-soybean rotation is reasonable, since the relative cost of both soil sampling and testing have decreased over time compared to crop production costs. Also, frequent sampling will provide more useful soil-test trends. When it’s time to make a fertilization decision, comparing the current yield and soil-tests values with trends over time will result in a better estimate of the amount of fertilizer to be applied.

Summary

With unfavorable crop and P or K fertilizer prices:

1. Do not fertilize soil testing High-or Very High.
2. Do not reduce recommended rates for low-testing soils.
3. Temporarily reduce or skip maintenance removal-based rates for soils testing Optimum.
4. Use manure nutrients as much as possible.

Additional resources

See the Nutrient Topics Phosphorus, Potassium, Soil/Plant Sampling, Testing, and Recommendations, and Manure Nutrients on the Extension Soil Fertility website.