Phosphorus and potassium fertilizer for 2013

Accurate soil test data and crop removal rates enable farmers to adjust phosphorus and potassium fertilizer rates to special situations.

There are two broad approaches for utilizing soil test data to generate phosphorus (P) and potassium (K) recommendations for field crops. Michigan State University Extension uses a build-up, maintenance and drawdown approach (Figure 1). With this approach, a critical soil test level has been established where the optimum yield (95 to 97 percent of maximum yield potential) is attained.

Figure 1. The build-up and maintenance approach
Build-up and maintenance approach

For corn and soybeans, this critical level is 15 ppm for P. The critical level for K may vary from 88 to 150 ppm based on the cation exchange capacity (CEC) of the soil. For soils testing below the critical level, P and K recommendations are made to meet the nutrient requirement of the immediate crop and raise the soil test values to the critical level over a four-year period. For soils testing above the critical level, a maintenance amount is recommended that is equal to the crop removal rate. If nutrient concentrations are even higher than the maintenance range, the soil gets to the drawdown phase where the recommended nutrient applications are less than the crop removal rate. This will allow for a gradual drawdown of nutrients back to the maintenance level. Please refer to MSU Extension publication E2904, Nutrient Recommendations for Field Crops in Michigan, for more information on MSU fertilizer recommendations and crop removal rates.

An alternative approach used by some universities and soil testing laboratories is known as the sufficiency approach. The goal of this approach is to apply just enough fertilizer to maximize profitability (95 to 97 percent maximum yield) in the year of application without considerations to future soil test values.

In certain situations, Michigan famers may take advantage of both approaches to manage P and K fertilizer. For example, rented land having low to average test levels, it may not be economically justified to apply P and K fertilizer at buildup rates. If fertilizer prices are very high and resources are tight, the short-term strategy would be to apply the crop removal amount.

Also, as a result of past build-up, soil test data has shown that nearly 70 percent of Michigan farm fields contain adequate P levels. However, only 20 to 25 percent of fields contain adequate K. Therefore, if a choice has to be made between the two, K may provide a higher return to investment compared to P fertilizer.

Another aspect with K is that the build-up approach may not be always practical, particularly on sandy soils with low CEC. On these soils, considerable K losses may occur due to leaching, prompting a greater need for annual applications of crop removal rates.

In certain areas, the 2013 drought reduced crop yields and nutrient removal. If accurate yield figures are available, then the P rate could be adjusted to account for higher potential carryover.

Other important considerations for P and K are soil pH and tillage. Maintaining the soil pH between 6.5 and 6.8 improves nutrient availability and crop growth. Therefore, liming when needed provides a good return to investment both short and long term. As for tillage practices, MSU recommendations will include the use of a phosphate containing starter fertilizer (25 to 40 lb P205) when planting into heavy residue, unless the soil test P levels are extremely high.

MSU Extension supports an economically and environmentally sound nutrient management system that is based on soil testing and realistic yield goals. Knowing the soil test P and K levels in relationship to the critical level and crop removal rates would enable farmers to adjust and fine-tune their fertilizer rates to suit special situations. The off-season allows plenty of time to formulate nutrient management plans for the farm and visit with your local fertilizer dealership.

If you need assistance with MSU soil test interpretations, please contact .(JavaScript must be enabled to view this email address) or visit the Field Crops Team website to find an expert or a point of contact in your area.

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