Maintenance Application of Phosphorus and Potassium

Maintenance application is a common strategy to keep soil nutrient test levels from decreasing appreciably.

In order to calculate maintenance rates, farmers need to know the total amount of phosphorus (P) and potassium (K) removed since the last application or fertility evaluation.

Maintenance applications are used when target soil test levels have been reached or on fields without recent soil test information.

In the latter case, they keep a field in a holding pattern while the farmer awaits proper fertility assessments. Because maintenance is such an important part of managing nutrients, knowing what these applications can and can’t do helps growers make informed decisions. Incorporate these steps in your P and K maintenance application plan:

Look backward to improve accuracy.

Often, farmers conduct maintenance applications according to the yield expected from succeeding crops, that is, by looking forward. The problem with this practice is that future yields are unknown. If those yields come in below expectations, you’ll have applied too much. If they come in higher, you’ll have applied too little. To improve accuracy, look back at the known yields of crops grown since the last applications of P and K. Adding up past crop removals and applying those fertilizer amounts is the best way to keep from over- or under-applying.

Determine your target soil test levels. 

Since maintenance applications are meant to keep soil test levels about the same, make sure you have recent soil tests to assess present nutrient levels. Low testing areas will be kept low and higher testing areas will be kept high. Do you want to draw down levels? If so, apply less than the previous removal. If you want to build them up, apply more.

Know your profit risks.

Maintenance applications may or may not be the most profitable ones in the short term. If soil test levels are already high, only a small probability exists for crop response to nutrient applications, so there’s also little chance of short-term revenue gains. However, at lower soil test levels, very profitable yield gains from maintenance applications are very likely. In fact, at very low soil test levels, profitable fertilizer rates exceed maintenance rates, and under-applications can leave substantial profits unrealized.

Know your risks of managing soil test variability.

Maintenance applications made at higher soil test levels to catch any possible lingering responses from lower testing areas could have been missed in an overall fertility assessment of a given area. Be sure to understand test variability to prevent over-correction of fertility issues.

Keeping these aspects of maintenance applications in mind can help farmers and advisers improve their rate selection decisions.

Source: Dr. T. Scott Murrell, Northcentral Director, IPNI, 2422 Edison Dr., West Lafayette, IN 47906. Email: