Two weeks ago, we discussed the benefits of soil sampling during the fall. This week, we will try and answer the question, will I need to fertilize for next year? The answer will likely come down to the classic extension educator response, “it depends” or “it varies.” What we might be able to tell you however is if you will need to fertilize based on the results of your soil test.
In an age of tight profit margins, many farmers are wondering what inputs they can skimp out on without hurting yield. According the 2018 Purdue Crop Cost & Return Guide, farmers are spending about $103/acre on fertilizer for rotational corn and $43/acre on rotation beans. This accounts for approximately 25% and 17% of variable costs for the two crops, respectively. The budget assumes that the farmer is putting on about 200 lbs of nitrogen (N), 50 lbs of phosphorous (P), 57 lbs of potassium (K), and 600 lbs of lime per acre for rotation corn. For rotation beans, we assume 0 N, 34 lbs P, 79 lbs K, and 0 lime per acre. Assuming N in corn goes unchanged, how much leeway do we have with P, K and lime? Well, it depends and it varies.
Soil can be broken down into 3 categories in regards to P & K: below maintenance limit, above the critical range, and somewhere between the two. If a soil is above the critical range, little to no response to fertilizer can be expected. Soils below the maintenance limit will of course see a major response to fertilizer. For soils between the critical level and maintenance limit, only the amount of P & K removed by the crop each year will be needed. According to the Tri-State Fertilizer Guide, which is available for free at: https://www.extension.purdue.edu/extmedia/AY/AY-9-32.pdf, corn will remove about 0.37 lbs of P (P2O5) and 0.27 lbs of K (K2O) per bushel of yield. Beans remove about 0.8 lbs of P and 1.4 lbs of K per bushel of yield. Thus, 180 bushel corn will remove 66.6 lbs of P2O5 and 48.6 lbs of K2O in a season. You can expect P levels to drop about 1 part per million for every 20 lbs of P2O5 removed. K levels will vary based on the soils Cation Exchange Capacity (CEC), but you can expect a drop of about 1 ppm per 5 to 8 lbs of K2O removed, depending on the CEC (see your soil test for this number).
Nutrients in the soil will not matter much if the soil pH is off. Both corn and beans have an optimum pH between 6.0 and 6.5. The pH should be corrected through lime once it falls 0.2 to 0.3 pH levels below the recommended levels. In summary, soils above the maintenance limit of P & K are in a position to let those levels drop. Soils below the critical level should be fertilized, as the loss in yield will likely be greater than the cost of fertilizer. Soils between the maintenance and critical range can be determined based on a soil test and recommendations from the Tri-State Fertilizer Guide mentioned above.