Relative Corn Yield Increased as Soil Phosphorus Levels Rose, But the Improvements Plateau
Researchers wanted to determine whether current phosphorus recommendations accurately reflect corn's response to fertilizer in modern farming. Their findings provide useful information for managing fertilizer expenses while maintaining productive crops.
Soil testing helps farmers understand nutrient availability and make informed fertilizer decisions before planting. By identifying existing nutrient levels, producers can determine where fertilizer applications are most likely to improve crop performance.
Research conducted across South Dakota examined how soil phosphorus (P) levels influence corn yield responses and whether current fertilizer recommendations remain suitable for modern farming practices.
Jason Clark, Assistant Professor and SDSU Extension Soil Fertility Specialist, worked with Jeffrey D. Svedin, Curtis J. Ransom, and David E. Clay on the research. Their study evaluated corn responses to phosphorus fertilizer across different soil conditions to determine how well existing recommendations predict the benefits of additional phosphorus applications.
Agricultural management has changed considerably over the past two decades. More farmers have adopted no-till farming, expanded cover crop use, and introduced practices designed to improve soil health. Corn genetics and average crop yields have also advanced. These developments may influence soil nutrient availability and how efficiently corn plants use phosphorus.
Research across 35 locations
The study covered 35 research locations across central and eastern South Dakota from 2019 through 2022. Researchers established 117 monitoring sites that represented a range of soil types, landscapes, and farming management systems.
At each site, researchers compared corn grown without phosphorus fertilizer with corn receiving phosphorus at a rate intended to maximize the likelihood of detecting a yield response.
Before planting, they collected soil samples and tested phosphorus concentrations and other important soil properties. At harvest, they measured grain yields from fertilized and unfertilized treatments.
The researchers examined relative yield, the frequency of yield responses, and the differences in grain production between the two treatments. These measurements helped them determine how existing soil phosphorus levels affected the benefits of fertilizer applications.
Soil phosphorus levels influence yield response
The results showed a clear relationship between soil test phosphorus and corn performance. Relative corn yield increased as soil phosphorus levels rose, but the improvement eventually reached a plateau at approximately 18 parts per million (ppm).
This value closely matches South Dakota's existing critical soil phosphorus level of 16 ppm. The critical level helps guide fertilizer decisions by identifying the soil phosphorus concentration above which additional phosphorus is generally less likely to increase crop yields.
Researchers found that 57% of the monitoring sites had soil phosphorus levels below the current critical value. These locations were generally more likely to show a positive yield response to phosphorus fertilizer.
The likelihood of a yield response declined as soil phosphorus concentrations increased. Sites testing below 4 ppm recorded a 100% response rate to phosphorus fertilization. However, sites with phosphorus levels above 30 ppm showed a response rate of only 33%.
These findings demonstrate that fertilizer benefits depend strongly on the amount of phosphorus already available in the soil. Fields with very low phosphorus levels have a greater chance of benefiting from added nutrients, while fields with higher concentrations are less likely to experience the same response.
Yield gains decrease as phosphorus increases
Researchers also measured the average increase in corn grain production following phosphorus fertilizer applications. The results varied across soil phosphorus categories.
Fields classified as having very low phosphorus levels gained approximately 16 bushels per acre. Fields in the low category gained about 21 bushels per acre, while fields in the medium category gained around 16 bushels per acre. Fields in the high category gained approximately 10 bushels per acre.
In the very high phosphorus category, the yield benefits became very small. Overall, the average yield difference between fertilized and unfertilized corn declined as soil phosphorus increased, reaching zero at approximately 27 ppm.
This result suggests that additional phosphorus may provide little or no yield advantage in many fields with high soil phosphorus levels. Applying fertilizer in these situations may increase production costs without providing enough additional grain to justify the expense.
However, the results do not mean every field above a particular phosphorus level will respond in the same way. Differences in soil conditions, environmental factors, and biological processes can affect nutrient availability and crop growth.
Current recommendations remain reliable
The research also evaluated whether South Dakota's existing phosphorus recommendation system needs major changes. Although statistical models suggested that the critical phosphorus level might be slightly higher than 16 ppm, the difference was small.
Researchers concluded that the current recommendation remains accurate and continues to provide reliable guidance for phosphorus fertilizer decisions. The findings support using soil test results to determine nutrient needs rather than applying additional phosphorus without considering existing soil conditions.
One important finding was that soil phosphorus concentration did not explain every yield response. Some fields with phosphorus levels above the critical value still responded positively to fertilizer. This indicates that factors beyond soil test phosphorus can influence how effectively corn accesses nutrients.
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