Research Shows Starter Phosphorus Performs Well in South Dakota Fields
Phosphorus (P) is an essential nutrient that supports healthy crop growth. Unlike nitrogen (N), phosphorus moves slowly through the soil, allowing some of the unused nutrients to remain available for future crops. Because phosphorus can stay in the soil from one season to the next, many farmers apply extra fertilizer when prices are low and use the stored phosphorus when fertilizer costs increase.
Research by Sarah Sellars, Assistant Professor and SDSU Extension Sustainable Farm & Food Systems Specialist, along with Evelyn Prempeh, Whoi Cho, Jason Clark, and Clarence Winter, examined whether this approach provides a profitable return in alkaline South Dakota soils.
Corn and soybeans are major crops across South Dakota. In 2025, the two crops accounted for approximately 69% of the state's total harvested acreage. Both crops require phosphorus, although corn generally has a greater nitrogen requirement.
High pH Soils Can Limit Phosphorus Availability
Central and western South Dakota contain many naturally alkaline soils with pH levels above 7. In these soils, phosphorus can react with calcium and form compounds that are difficult for crops to access.
The study found that only about 45% of applied phosphorus carried over into the following year. Phosphorus fixation in the high pH soil reduced the amount available for subsequent crop production.
Researchers then examined whether producers would receive better economic returns by building soil phosphorus over time or by applying phosphorus primarily to support the crop being grown.
Three Phosphorus Strategies Were Compared
The research used four years of corn and soybean data from an alkaline field at the Dakota Lakes Research Farm near Pierre, South Dakota. The field had an average pH of 7.7.
Researchers compared three management strategies. The first applied no phosphorus fertilizer. The second used a small starter phosphorus band at planting. The third combined starter phosphorus with additional fertilizer intended to raise and maintain soil test phosphorus levels.
Adding extra phosphorus increased soil test phosphorus, but the increase was relatively small. The additional fertilizer expense was greater than the economic value of the resulting yield improvement. As a result, the build-and-maintain strategy did not generate enough financial return.
The no-phosphorus strategy was the least profitable. Continued removal of phosphorus through crop production caused soil phosphorus levels to decline and eventually reduced yields.
Starter Phosphorus Led the Economic Results
Starter phosphorus produced the strongest profitability among the strategies tested. Applying phosphorus near the seed at planting helped provide the nutrient to developing crops before more of it could become fixed in the soil.
The study examined a starter application rate of 18 lb P2O5 per acre. This application provided phosphorus directly to the crop during planting and produced better economic results than the other strategies.
Researchers evaluated both a short-term planning period of 4 years and a long-term planning period of 10 years. They also considered phosphorus fertilizer prices of $0.68 and $0.85 per lb P2O5.
Starter phosphorus remained the most profitable strategy across both fertilizer price scenarios and both planning periods. The results suggest that lower phosphorus prices alone do not necessarily make it economical to build soil phosphorus in alkaline fields.
Soil Testing Can Help Guide Decisions
For South Dakota farmers producing corn and soybeans on high pH soils, starter phosphorus may provide a more practical economic approach than applying large amounts of fertilizer to build soil phosphorus.
However, the researchers caution that the findings have limitations. The study was conducted at one high pH site using one four-year crop rotation. The economic results also depended on assumptions involving phosphorus uptake, crop yield response, and the discount rate.
Because soil characteristics and crop responses vary between farms, producers should use soil test results when making phosphorus fertilizer decisions. Soil testing can help determine existing phosphorus levels and support decisions about appropriate fertilizer rates.
Farmers can review the SDSU Extension Fertilizer Recommendations.
Photo Credit: gettyimages-sasiistock