T.Ishida and H.Ando
Soil Sci Plant Nutr. 40-3, 391-402 (1994)
Key words: interpolation, kriging, paddy soils, soil mapping, spline
Nine chemical properties of soils in the Aizu Basin in
the northern part of Japan, were predicted from sample data using two different
methods. The predictors were the universal kriging estimator and a simplified
smoothing spline, referred to as nonlinear optimization method, The performance
of the methods was evaluated and compared using cross-validation.
The nonlinear optimization method is based on the concept
that optimal grid data satisfy two requirements: smooth interpolation,
and minimum differences between predicted and observed values at the sampling
points. This method contrasts with kriging in that it is a global estimate,
whereas kriging is a local one.
The kriging estimator provided a better estimate than
the other method for most of the nine attributes: contents of total carbon
and nitrogen, free Fe2O3, and exchangeable Ca, Mg. K. and Na. However,
it was not as effective for the phosphate adsorption coefficient, the low
estimate of kriging was affected by the low estimate of the semivariogram
due to a sawtooth-like pattern in the sample semivariogram. This pattern
was ascribed to the presence of Ando soils in the vicinity of the Aizu
Basin, For the CEC, the low estimate of kriging was due to a pure nugget
effect in the semivarogram. The application of fertilizers may widen the
range of semivariograms for exchangeable cations.