Looking ahead to the 2015 growing season, there has not been a dramatic improvement in commodity prices. And, at this time, I’m hearing grain market analysts that offer any hope of major improvement. Likewise, the three major expenses in crop production (fertilizer, seed, land rent) have not changed much when compared to the last two or three years. Crop producers are concerned and are asking if they can reduce rates of broadcast applications of phosphate and potash without having a negative effect on soil test values for phosphorus (P). There has been some research conducted at the regional Experiment Stations that help in addressing this concern. These data become more meaningful because the studies were conducted over a period of years starting in 1986 and terminating in 1993.
For this study, phosphate fertilizer, supplied as 0-46-0, was applied annually in a corn – soybean rotation at the Southern Research and Outreach Center (Waseca) as well as the West – Central Research and Outreach Center (Morris). The 0-46-0 was to supply 50 and 100 lb. phosphate per acre. There was also a control where phosphate was no applied. Other nutrients were applied as needed to provide for optimum crop production. Corn and soybean yields were measured and annual soil samples ( 0 to 6 inches) were analyzed for P.
This blog is written to describe changes in soil test P each year when phosphate application was terminated after the 1985 growing season. Crop yield, of course, varied with location and weather during the growing season. At the Waseca location, corn yields were usually in the range of 180 bu./acre while soybean yield was in the range of 50 to 55 bu./acre. At the Morris location, corn yield was about 160 bu./acre and soybean yield was about 50 bu./acre.
Soil test values for the Waseca and Morris locations are shown in Figures 1 and 2 respectively. Some interpretation of the results is probably appropriate. At Waseca, the soil test value for P (Bray test) was initially about 7 ppm. With no phosphate applied, this value decreased by about 0.6 ppm per year. This decline was about 2.25 ppm per year when the annual broadcast was 50 lb./acre prior to 1986. When the phosphate rate at this site was increased to 100 lb./acre, the decline was 3.1 ppm per year. These declines are shown in Figure 1.
Soil properties were at the Morris location. There was a higher soil pH with elevated levels of calcium carbonate. With no phosphate applied, the rate of decline in soil test P was .26 ppm per year. Annual application of 50 lb./acre prior to 1986 had increased the soil test P value to 17 ppm. Without additional phosphate, the decline was 1.5 ppm P per year. Over the years, soil test P (Bray test) the use of 100 lb. phosphate annually had increased to 36 ppm. The decline from this value was 2.7 ppm P per year when the phosphate application ceased.
Results from these two locations illustrate several points. First and most importantly, if no phosphate fertilizer is applied, there is no steep drop-off in soil test P. Decline is gradual and can be corrected with added phosphate when profit return to crop production. If phosphate rates are reduced instead of eliminated, the reduction in soil test values for P will be more gradual. For those growers who have high or very high soil test values for P, the decline in soil test values will not be noticed in measured yields. It is also doubtful if small declines in soil test P will affect yields unless soil test values are in the very low range.
Phosphate rates applied in the study are identified by the symbols on the respective lines. For both locations, the solid diamonds are associated with the control treatment ( no phosphate applied ). The white triangles ( both sites ) represent the treatment of 100 lb. phosphate applied per acre from 1974 through 1985. The dark squares represent the annual application of 50 lb. phosphate per acre during the same time period.
Again, the declines in soil test P shown in Figures 1 and 2 show what might be anticipated if application of phosphate fertilizer was stopped completely. Dramatic declines in soil test P would not be a consequence of reduced rates. A reduction in rate of phosphate would be a good practice in a plan to get profit from crop production in 2015. A switch to a banded application of phosphate is another management practice that is encouraged. When switching to a band, instead of a broadcast application, the phosphate rate can be cut in half without reducing yield.