Another Look at Phosphorus Fertilizer Application Timing in Soybean

By Dan Kaiser

In a recent blog post, I discussed the lack of a need for direct application of phosphorus fertilizer directly ahead of soybean. After discussions with crop consultants, I decided to take a look back at the topic focusing on soils with a pH of 8.0 or higher. Building phosphorus in highly alkaline soils is difficult in Minnesota due to the abundance of calcium in the soil profile. High rates of phosphorus fertilizer applied to highly alkaline soils can appear to disappear, as if no phosphorus was applied. In the case of highly alkaline soils, timing of P fertilizer application in a multi-crop rotation may bear special considerations in order to increase soybean yield.

In 2010, funding from the Agricultural Fertilizer Research and Education Council (AFREC) was used to establish phosphorus response strip trials in farmers’ fields across Minnesota. Side by side strips were established where 0 or 200 pounds P2O5 was applied to establish the amount of yield produced for corn and soybean when no P was applied at various soil test phosphorus concentrations. At selected locations, the first year strips were split in half and either 0 or 150 lbs P2O5 was applied to assess the impacts of phosphorus carried over from one year to the next. The rates selected were done so to ensure P was not limiting and may not have represented the optimal rate needed at any of the locations.

A field site was established in Kandiyohi County near Blomkest. The field site has a soil pH of 8.2, Olsen P averaged 7 ppm, and year one was corn and year two was soybean. Corn yield was increased by 14% by phosphorus fertilizer year one and soybean yield was increased by 20% the next year but only from the new application of phosphorus. Soybean yield was not increased where P was applied before corn and an application of P before both crops would be warranted.

In 2016 and 2017, nine soybean phosphorus response trials were established in the Red River Valley with funding provided by the Minnesota Soybean Research and Promotion Council as far south as Moorhead, and as far north as Thief River Falls. Soil pH ranged from 6.5 to 7.7 and Olsen P ranged from 3 to 38 ppm. Soybean grain yield was increased in two of the nine locations, near Mahnomen and Moorhead in 2017. Both responsive sites tested Low in soil test P where a response to P is expected. The remaining sites tested medium or higher. Previous years’ phosphorus application is not known for the 2016 or 2017 sites, so it is not known if a previous years’ application of P was not sufficient to maintain high soybean yield.

So what’s the verdict on timing of P fertilizer application?

For most growers, where soil pH is 7.5 or less, the data is pretty clear that timing is not important. Application of P ahead of beans will not reduce yield but will not consistently increase yield. Therefore, growers can save on application costs by only applying ahead of corn in a two-year corn-soy rotation. If soil pH is 8.0 or greater, then there is more evidence of a need for fertilizer applications every year ahead of all crops to ensure P will be available. For soils with pH from 7.5 to 8.0, there is no clear evidence of P application increasing soybean yield consistently so the decision will need to be made on a site by site basis. If it is very difficult to increase soil test P, then yearly application of P is more warranted. Howe