Uncertainty of Soil Test P and K Across Tillage Systems of the Midwest.

Soil testing is the foundation for determination of nutrient recommendations, the reliability of which is fundamentally dependent on the collection of a representative soil sample. Previous, survey based research of whole field composite data, based on 20-35 cores, for 112 fields across the corn belt indicate that for soil test P (STP) and K (STK) precision averages 42% and 24% respectively. Across the region, to characterize spatial variability in soils, fields are increasingly being grids sampled at a grid density of 0.25 - 2.0 ha, compositing 2-18 cores per grid point, from which nutrient maps are developed. However, limited information has been published on grid point soil test uncertainty. The a study objective was to assess STP and STK grid point uncertainty across tillage systems.  For the study, 12 individual cores were collected within 3 m radius at a single field grid point for fifty-six sites across eight midwest states to assess STP and STK grid point uncertainty. Additional information was collected on field history and tillage systems. Results indicate substantial improvement in STP grid point precision going from 2 to 12 cores and lesser extent for STK.  Although optimum precision was obtained with 12 cores, generally for manure, conventional and reduced tillage sites, 6 - 8 cores resulted in STP RSD values of 5% - 26%. For no-till sites RSD values ranged from 17% to 132%.  Increasing the number of soil cores composited for low STP and no-till fields, improved grid point precision for STP, especially on fields that had a history of fertilizer banding. Overall soil test STP grid point uncertainty increased with decreasing soil tillage and declining STP levels.