Using Precision Agricultural Technologies to Assess Within-Field Variation of Grain Yield and Soil-Test Potassium Responses to Potassium Fertilization.

Using precision agriculture technologies are useful tools when studying the effect of fertilization on yield and soil-test values across the landscape.  The use of dense soil sampling along with yield monitors on replicated farm strip trials allow data to be collected and analyzed using Global Positioning Systems (GPS) and Geographic Information Systems (GIS) methods in meaningful ways.  A study based on 60 site/year replicated strip-trials was conducted from 2001 to 2007 in Iowa to evaluate the effects of K fertilization on corn and soybean grain yield.  Initial soil-test K (STK) was measured with the ammonium acetate (AA) and Mehlich 3 (M3) procedures.  Two K treatments (0 and 168 kg K ha^-1) were broadcast before planting. Soil samples were collected before K application (one composite sample every 0.06 to 0.08 ha depending on the field).  Grain yield was recorded with combine yield monitors.  When averaged across strips, a response to K was observed in most years and locations when initial average AA or M3 STK was at levels at which K fertilization is recommended in Iowa (< 171 mg K kg^-1).  The yield data were further analyzed with GIS and statistical methods to study yield responses for field areas with different initial STK levels and soil series.  Most fields included two or three soils series typical of major Iowa crop production areas.  This analysis showed much clearer responses to K in field areas with low STK.  The study also showed that the soil series seldom was an important or consistent factor at explaining yield response to K fertilizer or its relationship with STK.