This is the fifth column in a series on phosphorus (P) management proficiency. Last week I wrote the about (Part 2) the study evaluating the differences in P management approaches in continuous corn production. In that same Nebraska study, they did soil sample analysis in 2-inch increments for soil P concentration. I think we will find the results interesting, and it help us realize the importance of controlling soil sample depth.
For simplicity, let us look at the soil test data after several years in the P management approach of maintaining the soil test P at 25 parts per million (ppm) Bray-1 P in the top 8 inches at Concord and Mead. University of Nebraska-Lincoln (UNL) soil test P interpretations and recommendations are based on a 0-8 inch sample depth. Phosphorus fertilizer was broadcast applied in both no-till and tillage treatments. Soil test P was 15% higher at the 0-2 inch depth, but 25% less at the 2-4 inch depth in no-till compared to disked. However, I want to focus on the overall composite average across both tillage systems.
Near Mead in southeast Nebraska, the soil test P in the 0-2, 2-4, 4-6, and 6-8 inch soil depths were 48.5, 16.2, 11.6, and 10.5 ppm, respectively. The soil test P would be 25 ppm at 0-6 inches and 22 ppm at 0-8 inches. Shallow sampling would result in the recommended 70 pounds of Monoammonium phosphate (MAP) per acre not being applied. Math on the approach to maintain soil test P at 25 ppm is 25 ppm minus 22 ppm that equals 3 ppm, then multiply by 12 lbs P205 per 1-ppm increase in soil test P and divide by the 52% concentration of P205 in MAP fertilizer that results in 70 pound of MAP per acre. The U.S. fertilizer standard uses the percentage of P205 (oxide form of P) of the fertilizer weight. What if we sampled down to 10 inches? Since we do not have the soil test P concentration from the 8-10 inch depth, let us assume it is 1 ppm less than the 6-8 inch depth (9.5 ppm). The soil test value would be 19 ppm for the 0-10 inch sample depth. Therefore, the recommended fertilizer rate would be 139 pounds of MAP per acre of MAP or $31 per acre more fertilizer cost at this field because of sampling too deep.
Near Concord in northeast Nebraska, the soil test P in the 0-2, 2-4, 4-6, and 6-8 inch soil depths were 68.4, 18.4, 6.0, and 5.3 ppm, respectively. What would be the difference in soil test P between a composite sample taken at 0-6 inches versus 0-8 inches? The soil test P concentration in ppm would be 31 ppm at 0-6 inches and 25 ppm at 0-8 inches. If we sampled too shallow (0-6 inches) at this field, the recommended fertilizer rates would still be 0 lbs of MAP fertilizer per acre since at or above 25 ppm soil test P. What if we sampled down to 10 inches? Since we do not have the soil test P concentration from the 8-10 inch depth, let us again assume it is 1 ppm less than the 6-8 inch depth (4.3 ppm). The soil test P would be 21 ppm for the 0-10 inch sample depth. Therefore, the recommended fertilizer rate would be 92 pounds per acre of MAP or $42 per acre more cost at this field because of sampling too deep.
Controlling and consistently pushing the soil probe to the appropriate depth that the calibration and correlation work was done at UNL (8 inches) is important. Contact me with questions or suggest topics for me to write about in regards to phosphorus management at or 402-821-1722. Know your crop, know your tech, know your bottom line at croptechcafe.org.
