By Scout Nelson
Phosphorus (P) is an important nutrient for crop growth. Unlike nitrogen (N), phosphorus is regarded as a nutrient with low mobility in the soil. Most of the phosphorus remaining after crop uptake is known to carry over into the following growing season. Because of this characteristic, many producers follow a common phosphorus management practice by applying more fertilizer when prices are low and using the stored phosphorus when fertilizer prices rise.
Research by Sarah Sellars, Assistant Professor & SDSU Extension Sustainable Farm & Food Systems Specialist, and Evelyn Prempeh, Whoi Cho, Jason Clark, and Clarence Winter examined whether this traditional approach remains profitable in alkaline soils.
Corn and soybeans remain South Dakota's leading cash crops. Over the past 20 years, these crops have continued to increase their share of harvested acreage across the state. In 2025, corn and soybeans represented approximately 69% of South Dakota's total harvested acreage. While corn requires mostly nitrogen fertilizer, it still has a phosphorus requirement. Soybeans are also among the crops that mainly require phosphorus fertilization.
Phosphorus Behaves Differently in Alkaline Soils
Much of central and western South Dakota has naturally alkaline soil. These soils have a pH above 7. In high pH soils, phosphorus fertilizer can react with calcium already present in the soil. This reaction can create compounds that crops cannot easily use. As a result, some of the phosphorus applied to a field can become tied up in the soil.
This creates an important question for farmers. If phosphorus fertilizer is applied to build soil phosphorus for future crops, how much of that fertilizer remains available for plants?
The study found that only about 45% of applied phosphorus carried over to the following year. The high pH of the soil contributed to phosphorus fixation, which reduced the amount available for future crop use. This finding can change how farmers think about building soil phosphorus through fertilizer applications.
The research also raises an economic question. Is it better to spend extra money building soil phosphorus over several years, as recommended by a “build-and-maintain” approach, or should farmers apply enough phosphorus to support the current crop?
Study Examines Three Phosphorus Strategies
Researchers 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.
The economic analysis compared three phosphorus management strategies. The first strategy applied no phosphorus fertilizer. The second used a small starter band of phosphorus at planting to support the current crop. The third applied starter phosphorus along with additional fertilizer designed to raise and maintain soil test phosphorus levels over time.
The results showed that applying extra phosphorus did increase soil test phosphorus. However, the increase was small. The added fertilizer cost was also greater than the value of the yield improvement. This meant that building soil phosphorus did not provide enough economic return.
No phosphorus fertilizer was the least profitable strategy because continued soil phosphorus drawdown reduced crop yield.
Starter Phosphorus Provides Better Returns
Starter phosphorus was the most profitable strategy in the study. Researchers found that placing phosphorus near the seed at planting helped crops access the nutrient before it became fixed in the soil.
The study evaluated a single starter rate of 18 lb P2O5 per acre. This rate provided phosphorus directly to the crop during planting and performed better economically than the other strategies.
Researchers also considered two planning periods: a short-term period of 4 years and a long-term period of 10 years. Two phosphorus fertilizer prices were included, at $0.68 and $0.85 per lb P2O5. Starter phosphorus remained the most profitable strategy under both price conditions and for both planning periods.
The results show that lower fertilizer prices do not automatically make building soil phosphorus the best financial choice in alkaline soils. Even when phosphorus costs less, much of the added nutrient can become fixed and unavailable to crops.
What Farmers Should Consider
For corn and soybean producers working with high pH soils, starter phosphorus placed near the seed at planting can be a practical and profitable option.
However, the research has some limits. The findings come from one high pH site and one four-year crop rotation. The economic results also depend on assumptions about phosphorus uptake, crop yield response, and the discount rate. Therefore, the results may not apply equally to every farm or soil type.
Farmers should test their soil through a laboratory before deciding on phosphorus rates. Soil test results can help identify existing phosphorus levels and guide fertilizer decisions. Producers can also review SDSU Extension Fertilizer Recommendations or contact SDSU Extension office to discuss their results.
Photo Credit: pexels-jan-kroon
Categories: South Dakota, Crops, Corn, Soybeans