Phosphorous and potassium fertilizer recommendation variability for two mid-Atlantic Coastal Plain fields

Fertilizer recommendations for variable rate treatments developed from grid soil sampling protocols are unproven for mid-Atlantic Coastal Plain soils. The objectives of this study were to compare soil test results for P and K fertilizer recommendations for two fields utilizing two grid sampling size s (0.33 ha and 0.83 ha), sampling by soil type, and standard composite samp ling. The study location contained alluvial soils ranging from a loamy sand to a silt loam. The two fields totaled approximately 21 ha and were sample d on grids 18.5 by 30.4 m, Samples consisted of composites of eight cores t o a 20-cm depth that were analyzed for Mehlich I extractable P and K, Two s tatistical models were developed for comparing the extractable P and K data and the resulting fertilizer recommendations. The first model, following a precision farming approach, implies sources of variation are systematic an d attributable to narrow geographic locations. The second model, associated with composite sampling, utilizes less specific patterns of variability. C omparisons showed that the smaller grid (0.33 ha) produced more precise est imates of extractable K in only one field (with 67% of tested locations rec eiving appropriate fertilizer rates), with no improvement for extractable P in either field. Both grid-sampling systems improved estimate precision fo r extractable P and K (with a smaller average misapplication rate) compared with a whole-field composite. The composite-by-soil-type approach was supe rior to the whole-field composite for estimating extractable P and K with a lower average misapplication and higher percentage receiving appropriate f ertilizer rates. The composite-by-soil approach produced the most precise f ertilizer recommendations for small systematic variation and required fewer laboratory measurements, It approached the grids-sampling system precision of fertilizer recommendations for large in-field variation. Only when stro ng trends in extractable P and K exist mould grid sampling be recommended o ver the composite-by-soil-type sampling approach.