Phosphorus Fertilizer Management Tips For The Upcoming Growing Season
DR. NUTIFAFA ADOTEY
JACKSON, TENNESSEE
Fertilizer continues to be one of the largest expenses in row crop production, according to University of Tennessee Field Crop Production Budgets. In season with high-than-normal input cost, nitrogen (N) and phosphorus (P) management offer some of the greatest opportunities for reducing expenses if these fertilizers are managed efficiently. As we head into a new growing season, this blog post outlines key considerations for improving P fertilizer efficiency and maximizing return on investment (ROI).
Nutrient management decisions should start with the soil pH. Soil pH regulates nutrient availability, especially for phosphorus (Figure 1). The chart illustrates how phosphorus fixation varies across pH. Phosphorus fixation occurs when orthophosphate released from fertilizer reacts with other minerals to form insoluble compounds, which make P unavailable for the plant. The chart shows three major peaks of high fixation: iron-related fixation in very acidic soil, aluminum-related fixation in moderately acidic soils, calcium-related fixation in alkaline soils. The highest P availability occurs near neutral pH (Valley no. 2). This chart shows why maintaining soil pH between 6 to 7.5 is essential for maximizing the efficiency of P fertilizer.
Correcting low pH soil is not something that can be done quickly. Lime can take several months to react to make the desired pH changes—particularly in no till systems—so the window just before planting is typically too short to make meaningful changes. A short-term strategy is applying P after planting or band P. These approaches can help reduce fixation in the soil, though timing rainfall becomes critical in rainfed systems.
Phosphorus fertilizer commonly used in Tennessee include DAP (18% N; 46% P2O5), MAP (11% N; 46% P2O5), and TSP (0% N; 46% P2O5). While DAP contains extra nitrogen, research in dryland no till corn in Tennessee showed that this does not always translate to extra yield benefits over TSP in either spring or fall applications. The N credits from DAP can vary significantly depending on weather and other environmental factors. Nitrogen is not very stable in the soil systems so it can be lost to the atmosphere or soil systems. Growers with access to P sources with lower N content may be able to save money without sacrificing yield.
The amount of fertilizer to apply should always be based on soil test levels. Phosphorus fertilizer recommendations by University of Tennessee depends on soil test levels. Currently, there are four categories: Low (L), Medium (M), High (H), and Very High (VH). UT recommends applying P fertilizer on low and medium soils, where the likelihood of yield response and improved profitability is greatest. Across several rate trials between 2020 and 2025, there was very little to no economic return from applying P to high-testing soils in Tennessee. Skipping applications in high or very high soils can significantly reduce fertilizer costs without affecting yield. Click on the agronomic crop you are interested in to view its specific fertilizer recommendations. Available crops include Corn (field)/grain, Cotton, and Soybeans. On-going research is underway to prescribe fertilizer recommendations using Mehlich 3.
How far can P applications be delayed? Research by Bender et al. 2013 shows that rapid P uptake in corn begins around the V8 growth stage (Figure 2). Once applied, P is converted to orthophosphate and can quickly become tied up with aluminum, iron, or calcium if it sits too long in the soil without plant uptake. This means Applying P too early increases the risk of tie up so applying P close to planting or soon after emergence, followed by rainfall, is ideal. Farmers should consider management practices that minimize fixation but fit seamlessly into their operations.
In conclusion, using soil test results, understanding the role of pH, choosing cost effective P sources, and timely applications properly can improve both fertilizer efficiency and overall profitability. ∆
DR. NUTIFAFA ADOTEY
UNIVERSITY OF TENNESSEE