Data from 1,219 soil sample sites, collected from 38 to 42 center-pivot systems annually, in a 4-year on-farm irrigation study show that potassium nutrient levels decreased from the topsoil (O"-6") to the deeper subsoil level (12"-18"). Deep applications of phosphorous and/or potassium fertilizer increased lint yields where low levels of these nutrients occurred in the subsoil. In other related studies, when mid to late April applications of deep fertilizer were made, some stand problems were experienced in the dry conditions of 1988.

Sampling to deeper soil depths is necessary to determine the potential success of the dry materials applicator. At least three questions should be answered: (1) What nutrient is deficient? (2) Where in the soil profile is it deficient? (3) Does the subsoil have adequate pH levels and physical properties to support good root growth?