By Scout Nelson
Soil testing remains one of the most important tools farmers use to make nutrient management decisions. By measuring nutrient levels before planting, growers can determine whether crops are likely to respond to fertilizer applications. A recent South Dakota research project focuses on phosphorus (P), one of the key nutrients needed for strong corn growth and grain production.
Jason Clark, Assistant Professor and SDSU Extension Soil Fertility Specialist, along with Jeffrey D. Svedin, Curtis J. Ransom, and David E. Clay conducted research in which they examined whether current phosphorus fertilizer recommendations for corn remain effective under modern farming conditions and management practices.
Over the last two decades, agricultural practices have changed significantly. Many producers have adopted no-till systems, increased the use of cover crops, and improved soil health practices. Corn genetics and average yields have also improved.
These changes may affect how phosphorus behaves in the soil and how efficiently plants use available nutrients. Because of these developments, researchers sought to determine whether existing phosphorus recommendations continue to accurately predict corn yield responses.
The study included 35 research locations across central and eastern South Dakota between 2019 and 2022. Researchers established a total of 117 monitoring sites representing different soil types, landscapes, and management systems. Each site included a treatment without phosphorus fertilizer and a treatment receiving phosphorus fertilizer at a rate designed to maximize the chance of observing a yield response.
Soil samples were collected before planting and analyzed for phosphorus levels and other important soil properties. Corn grain yields were collected at harvest to compare fertilized and unfertilized areas. Researchers evaluated relative yield, yield response frequency, and differences in grain yield between treatments.
Results showed a strong relationship between soil phosphorus levels and corn response to fertilizer. Corn relative yield increased as soil test phosphorus increased until reaching a plateau at approximately 18 parts per million (ppm). This finding is very close to the current South Dakota critical phosphorus value of 16 ppm.
Researchers found that 57% of the monitoring sites had phosphorus levels below the current critical value. These sites were generally more likely to respond positively to phosphorus fertilizer. As soil phosphorus increased, the likelihood of observing a yield response steadily declined.
Yield response frequency was highest in fields with very low phosphorus levels. Sites testing below 4 ppm showed a 100% response rate to phosphorus fertilization. In contrast, sites testing above 30 ppm showed a response rate of only 33%. These results demonstrate that additional phosphorus becomes less valuable as soil phosphorus levels increase.
The study also examined the average yield increase gained from phosphorus fertilization. Fields with very low phosphorus levels gained approximately 16 bushels per acre. Fields in the low category gained about 21 bushels per acre, while medium phosphorus fields gained around 16 bushels per acre. High phosphorus fields gained about 10 bushels per acre. Yield benefits became very small once phosphorus levels reached the very high category.
Researchers observed that the average yield difference between fertilized and unfertilized corn gradually declined as soil phosphorus increased. The yield advantage reached zero near 27 ppm phosphorus. This suggests that many fields with higher soil phosphorus levels may not benefit economically from additional fertilizer applications.
The findings also support the current phosphorus recommendation system used in South Dakota. Although statistical models suggested the critical level might be slightly higher than 16 ppm, the difference was minimal. Researchers concluded that the current recommendation remains accurate and continues to provide reliable guidance for fertilizer decisions.
An important observation from the study is that soil phosphorus alone does not explain every yield response. Some fields with phosphorus levels above the critical value still responded to fertilizer. This suggests that other soil, environmental, or biological factors can influence phosphorus availability and crop performance.
The research highlights the importance of routine soil testing and data-driven nutrient management. Applying phosphorus where it is truly needed can improve profitability, increase fertilizer efficiency, and help prevent unnecessary nutrient applications. At the same time, maintaining adequate phosphorus levels supports strong crop growth and long-term productivity.
Photo Credit: pexels-binyaminmellish
Categories: South Dakota, Crops, Corn, Sustainable Agriculture