Revised Iowa P and K Recommendations: New Soil-test K Interpretations and Support for a New Soil P Test

The Iowa soil-test interpretations and fertilizer recommendations for phosphorus (P) and potassium (K) were last updated in 1996 and 1999. The 1996 update involved the most important changes compared with previous recommendations. New soil-test P and K interpretation classes (very low, low, optimum, high, and very high) were developed to classify soil-test values into two sets corresponding to Iowa soils with either low or high subsoil P and K levels. This classification recognized research results suggesting that less topsoil P or K was needed when subsoil nutrient levels were comparatively higher. The probability of yield response to fertilization in the optimum class was considered small, and only maintenance fertilization based on nutrient removal was recommended for this class. No fertilization was recommended for soils testing in the high or very high classes, except for starter fertilization for corn under specific conditions.