Assessing spatial variability in an agricultural experiment station field:Opportunities arising from spatial dependence

Spatio-temporal field soil and crop processes are important for site-specif ic farming. The objectives of this study were to spatially evaluate selecte d soil physical and chemical properties and their relationship to wheat (Tr iticum aestivum L.) yield, and to discuss stochastic approaches to help ide ntify processes underlying yield variability in heterogeneous field sites. Modified grid sampling included 330 sites including a primary transect. Soi l properties measured for the Ap, E if present, and upper B horizons at eac h site included pH, P, Zn, Cu, exchangeable rations, percentage base satura tion, ration exchange capacity, bulk density, soil water contents at -10, - 33, and -1500 kPa, texture, and humic matter content. Wheat grain and straw were hand-harvested on 1- by 2-m plots centered on each site. Soil water c ontent on the primary transect was determined by neutron attenuation on nin e dates. Field and primary transect means and semivariograms for a given so il or plant parameter were similar. The range of spatial dependence or auto correlation of soil parameters ranged from 10 m for Ap horizon depth to 100 m for -1500 kPa water content of the Ap. Base saturation and available wat er storage capacity were cross-correlated with grain yield to a distance of +/-15 and 12.5 m, respectively. State-spare analysis was used to develop a grain yield model using these two variables, Spearman rank correlation of the soil water content data suggests that the temporal stability of soil wa ter storage is less for shallow than for deeper soil layers.