Cm. Anderson-cook et al., Phosphorous and potassium fertilizer recommendation variability for two mid-Atlantic Coastal Plain fields, SOIL SCI SO, 63(6), 1999, pp. 1740-1747
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.