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DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

Phosphorous and Potassium Fertilizer Recommendation Variability for Two Mid-Atlantic Coastal Plain Fields

^a Dep. of Statistics (0439), Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061 USA
^b Dep. of Crop and Soil Environmental Sciences (0404), Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061 USA

candcook{at}vt.edu

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 sizes (0.33 ha and 0.83 ha), sampling by soil type, and standard composite sampling. The study location contained alluvial soils ranging from a loamy sand to a silt loam. The two fields totaled approximately 21 ha and were sampled on grids 18.5 by 30.4 m. Samples consisted of composites of eight cores to a 20-cm depth that were analyzed for Mehlich I extractable P and K. Two statistical 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 and attributable to narrow geographic locations. The second model, associated with composite sampling, utilizes less specific patterns of variability. Comparisons showed that the smaller grid (0.33 ha) produced more precise estimates of extractable K in only one field (with 67% of tested locations receiving appropriate fertilizer rates), with no improvement for extractable P in either field. Both grid-sampling systems improved estimate precision for extractable P and K (with a smaller average misapplication rate) compared with a whole-field composite. The composite-by-soil-type approach was superior to the whole-field composite for estimating extractable P and K with a lower average misapplication and higher percentage receiving appropriate fertilizer rates. The composite-by-soil approach produced the most precise fertilizer 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 strong trends in extractable P and K exist would grid sampling be recommended over the composite-by-soil-type sampling approach.

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