Most phosphorus and potassium fertilizer recommendations are based on both the buildup value of the soil (the amount of nutrient required to raise soil test one unit) and the critical value (the target soil test level where no economic response to added fertilizer is anticipated). Research has shown soils can differ in the buffer capacity for phosphorus. Our objective was to evaluate the need for and feasibility of soil-specific buildup and critical values for Missouri soil test recommendations.

The buildup value for phosphorus and potassium was determined in the laboratory for 40 Missouri soils selected to have a wide range of regional and mineralogical differences. Known rates of nutrient were added to each soil and the soils were cycled through wet-dry cycles until testing indicated soil test levels had stabilized. Greenhouse experiments were used to study phosphorus and potassium critical values. For each nutrient, 12 soils predicted to have different buildup values were used.

Nutrient additions resulted in a linear increase in the stabilized soil test level in most cases where multiple rates of phosphorus or potassium were added to a soil. There were significant differences in buildup values for phosphorus and potassium among soils. Regional differences were significant indicating that fertilizer recommendations could potentially be adjusted based on the location of the soil sample.