Environmental Management of Soil Phosphorus: Modeling Spatial Variability
in Small Fields
Brian A. Needelman,* William J. Gburek, Andrew N. Sharpley, and Gary W. Petersen
ABSTRACT
management (Lemunyon and Gilbert, 1993). The P in-
dex accounts for source (soil P and rate, method, and
The mapping of soil P concentration is necessary to assess the
timing of applied P) and transport (surface runoff, ero-
risk of P loss in runoff. We modeled the distribution of Mehlich-3
sion, leaching, and landscape position) factors control-
extractable soil P (M3P) in an east-central Pennsylvania 39.5-ha water-
shed (FD-36) with an average field size of 1.0 ha. Three interpolation
ling P loss in surface runoff and ranks sites for their
models were used: (i) the field classification model—simple field
potential risk of P loss.
means, (ii) the global model—ordinary kriging across the watershed,
In areas with large fields, the mean or median soil
and (iii) the within-field model—ordinary kriging within fields with
test value is generally used as the best estimate of P
a pooled within-stratum variogram. Soils were sampled on a 30-m
concentration in a field, except in cases where precision
grid, resulting in an average of 14 samples per field. Multiple validation
sampling and fertilizer application are used. In areas
runs were used to compare the models. Overall, the mean absolute
with small fields, such as Pennsylvania, a single bulk
errors (MAEs) of the models were 76, 71, and 66 mg kg1 M3P for
composite or the mean or median soil test value is tradi-
the field classification, global, and within-field models, respectively.
tionally used as the best estimate of P concentration.
The field classification model performed substantially worse than did
Under these models, information on farm- and field-
the kriging models in five fields; these fields exhibited strong spatial
scale variability is not used for the estimation of P distri-
autocorrelation. The within-field model performed substantially bet-
ter than did the global model in thr