Soybean seeds harvested each fall remove about 0.8 lbs of phosphorus oxide or P fertilizer (as P2O5) per bushel. With a yield of 50 bu/ac that is 40 pounds of phosphorus (P) fertilizer. You need to provide adequate P fertilizer to your soybean crop to maintain yield over time. How much P fertilizer do you need? Soil testing is the first place to start.
Soil Test Phosphorus (STP)
The soil test for phosphorus (STP) is calibrated and correlated to soybean yield response. Most states (land-grant institutions) have done this calibration and correlation work based on soil samples taken from a 6 or 8 inch depth. It is important to know what depth your soil sample was taken at. Typically, STP concentration decreases with depth, at least in the top 8 inches. Therefore, a soil sample collected from the same location in the field will likely have a slightly higher STP value if collected from a 6 inch depth versus an 8 inch depth. I suggest following the recommended soil sampling depth from the land-grant institution in your state so that you can utilize their phosphorus fertilizer rate recommendations.
There are several extractants (Mehlich-3, Bray-1, Olsen, etc.) and measurement methods (colorimetric and ICP) used to get a soil test value for phosphorus. Again, I recommend the method for STP be the same one used by your land-grant institution. Please check with the soil testings labs near you (check their website or give them a call) and find out which phosphorus soil test they use.
For a 0- to 6-inch soil sampling depth, a Mehlich-3 STP of 20 parts per million (ppm) can provide greater than 95% of relative maximum soybean yield in Kansas. There is some variability in this critical level of 20 ppm across various States (Iowa, Nebraska, etc.) based on differences in soil sampling depth (6 or 8 inches), soil test method (Mehlich-3, etc.), and inherent variation in soils across the U.S. However, in Kansas, phosphorus fertilizer application is typically suggested when STP falls below 20 ppm. The probability of a yield increase to additional P fertilizer is very small when STP > 30 ppm. Building STP above 30 ppm is not recommended. The variability of STP within a field can be quite large. Therefore, it is ideal to have STP in a field range between 20 and 30 ppm. Soil sampling by grid or management zones to locate low STP areas of the field is helpful in doing this. Just remember, a soil sample is a composite of individual cores or point locations, typically 15 -20 cores. The composite sample may have a STP of 20 ppm, and it would contain individual cores or locations below and above 20 ppm. P management for fields can be adjusted based on land ownership or confidence in remaining the operator of rented ground. If the land is owned, building STP to 25 to 30 ppm can provide some purchasing flexibility during times when fertilizer prices are high. A sufficiency approach or annual applications of P to rented land is likely a more feasible option. P fertilizer rates for soybeans based on STP can be found at the following link.
Leaf Phosphorus Concentration
Another way to assess or monitor how your phosphorus fertilizer program is working is through leaf analysis. This can be a nice compliment to your soil testing efforts. A soybean leaf has three leaflets, called a trifoliolate. Thirty to fifty fully expanded trifoliolates from the top of plants (usually the third leaf from the top) can be collected when the soybeans are at beginning to full bloom without the petiole (the petiole is the stem that connects leaf to the main stem). The generally accepted phosphorus sufficiency range for these trifoliolates is 0.30% on the low end to 0.60% on the high end on a dry matter basis (the P content of leaves after they are dried at 140 degrees F and ground into small pieces). Concentrations below 0.30% would be considered low (yield limiting) while 0.60% would be considered high (but not yield limiting until 0.80%). There have been documented cases where concentrations from 0.30 to 0.39% are not enough to maximize yield. It is generally recommending to have trifoliolate phosphorus concentrations in the middle of the sufficiency range if practical and economical to do so. In most cases, adequate STP and fertilizer rates should lead to adequate soybean P trifoliate concentrations.
Harvesting soybeans means exporting significant amounts of P that must be replaced through regular P fertilizer applications. To reduce the chance of significant yield loss, follow your land-grant insititution's P fertilizer recommendations. Soybean trifoliate sampling for P along with other essential plant nutrients is a nice compliment to your soil testing efforts.
My Two Cents
I follow a build-and-maintain approach for phosphorus. I try an maintian my STP between 25 and 30 ppm based on a six-inch sampling depth using the Mehlich-3 extraction (colorimetric). Maintaining a 25 to 30 ppm STP allows me to have some P fertilizer purchasing flexbility (skip a year) while maximizing yield relative to soybean phosphorus nutrition. To ensure my soybeans are sufficient in P, I do take some trifoliolate samples, with the goal of being between 0.40 to 0.50% P.
Questions and Feedback
Let me know what your thoughts are on soybean phosphorus management? Do you follow a build-and-maintenance or sufficiency approach. Do you fertilizer your soybeans every year? How often do you soil sample? Have you taken a soybean leaf sample before?