Crop Removal and Fertilizer Recommendations

2/20/2008 Kurt Lawton
Nearly all farmers have reported excellent yields in the past several years and this improved production has raised some very real questions, says George Rehm, University of Minnesota agronomist. The major one, of course is: should phosphate and/or potash rates be increased because of the higher yields?
Trials at Waseca, Lamberton and Morris, Minnesota provided some information that helps to answer this question. I will focus on three of the treatments in that study and will concentrate on the data collected at Waseca. The study was started in 1999 and completed in 2003.
The control treatment received adequate N, but no phosphate in those years. The initial P test (Bray) 12.8ppm. In the spring of 2004, the P test for this treatment was 9.5 ppm. With yields in the range of 160 to 180 bushels per acre, a reduction in soil test P would be expected.
In a second treatment, an estimated removal of 60 lb. phosphate per acre was applied each year. With no increase in yield in 2003 when compared to the control, the Bray test in the spring of 2004 was 30.8 ppm. This was a substantial increase in soil test P not accompanied by an increase in yield. The 60 lb. phosphate per acre was applied when corn was grown. This rate was reduced to 45 lb./acre soybean was the intended crop.
In a third treatment, University 0f Minnesota banded suggestion for corn and soybeans were used. That rate was 30 lb. phosphate per acre for corn and 15 lb. phosphate per acre for soybeans. In the spring of 2004, the soil test P for this treatment was 11.8 ppm. That’s not much different from the initial soil test value for P (12.8 ppm). The yield from this treatment was almost identical to the yield from the control in 2003 (174.8 bu./acre vs. 172.7 bu./acre) In 2003, the yield from the crop removal treatment was 176.7 bu./acre.
With current prices for phosphate fertilizer, the difference in cost when removal use is compared to banded use is substantial. With no difference in either corn or soybean yield, how can anyone justify using crop removal as a basis for making fertilizer recommendations? The answer is: YOU CAN’T. Using crop removal in fertilizer recommendations does nothing more than increase soil test levels without increasing yields. Since land value is not adjusted for soil test levels, the increase in soil test for P means very little except money spent with no return. I didn’t major in economics, but, I don’t believe that use of that strategy makes any money.
I realize that replacing what the crop moves makes sense. However, we must give the soil credit for supplying some of the needed nutrients. The soil test procedures currently used are designed to predict the amount that a crop might obtain from the soil. If the soil does not supply any plant nutrients to a growing crop, the crop removal concept is valid. However, the soil is an excellent reservoir for some nutrients. So, use of crop removal in arriving at fertilizer recommendations is not a good thing.