Before cutting phosphorus, consider the numbers

Since phosphorus prices reached historic highs, producers looking to trim fertility costs have often started with this critical nutrient. Although grain prices have rebounded somewhat, the grain-to-fertilizer price ratio continues to put strain on operational budgets, and phosphorus is frequently the largest single expense in a fertility program. Decisions to reduce phosphorus rates are typically based on three assumptions: that current economics justify the cut, that soil phosphorus reserves are available to draw down, and that newer products can maintain yields with reduced application rates.

The reserves assumption is the easiest of the three to check. Data from The Fertilizer Institute (TFI) show 56% of Missouri soil samples test below critical level for phosphorus. In contrast, potassium shows 33% below critical level. The “critical level” marks the point at which additional phosphorus applied above crop removal is unlikely to generate a yield response. Above this threshold, fertilizer recommendations generally move from building soil phosphorus reserves to maintaining them. The TFI tests indicate that the reserves are not there on better than half of the acres being sampled.

Sufficient levels of phosphorus also affect the efficiency of every other input. In a soil with adequate phosphorus, a corn crop takes up 40% to 60% of applied nitrogen. In deficient soil, that uptake is 25% to 40%. Assuming 60 cents per unit of nitrogen from anhydrous applications and 80 cents per unit of phosphorus, cutting 40 pounds of P saves $32 per acre. Losing 20% of the nitrogen efficiency on a 200-pound rate spends $24 of that on N that’s never taken up. The net is $8 per acre, before any yield effect is considered.

Rebuilding a soil test is more expensive than maintaining one. MFA recommendations put soil test optimum between 22.5 and 27.5 parts per million (ppm), and within that range the recommendation is straight replacement of crop removal. Below 22.5 ppm, the recommendation becomes replacement plus build, and build requires 16 pounds of phosphorus to move a soil test 1 ppm—roughly 8 pounds of fertilizer for every pound that stays in the soil. Producers depleting nutrient levels they spent years building recovers about an eighth of what they put in.

Current economic conditions have led to a rise in efficiency claims from higher-priced products of P per unit but applied at a lower rate. Similarly, there are claims for fertilizer additives that will require less P to be added to the system. The truth about these “efficiency” products must be evaluated against how the phosphorus cycle actually works. Much still needs to be answered with these products.

For one, they cannot be evaluated on yield alone but how they impact P reserves in the soil. What are the long-term impacts? Crops must have P, and what we do know is that none of these products adds more phosphorus to the system. Current evidence says some of these products may increase the availability of other P in the soil marginally at best. Even then, draw-down of soil tests will occur when using “at replacement” or greater P rates.

Crop yields increase when phosphorus is sufficient. Nitrogen use efficiency increases with sufficient phosphorus, and so does profitability. Drawing down and then rebuilding a soil test will be far more costly than maintaining those levels, even in the face of record P prices. Grid soil sampling, yield removal data and realistic yield goals remain the most effective tools available, and MFA’s Nutri-Track remains the best way to ensure these goals are met. Visit with your local MFA agronomists or precision specialists for more information. 

Editor’s note: Phosphorus management was the topic of a recent “Made for Agriculture” podcast. Find it and other episodes at mfa-inc.com/podcast.