SCALE-DEPENDENT CORRELATIONS AMONG SOIL PROPERTIES IN 2 TROPICAL LOWLAND RICE FIELDS

Long-term irrigated rice (Oryza sativa L.) cultivation is an important yet unstudied source of soil variation in tropical lowland rice field s, A field study was conducted to quantify spatial soil variability an d to analyze correlations among soil properties at different spatial s cales, Soil samples from the 0- to 20-cm depth were collected from adj acent rice fields consisting of Typic Tropaquepts and Typic Tropaqualf s, Structural variogram analysis and factorial kriging (FKA) were used to describe the coregionalization of 14 soil properties, Coregionaliz ation was described by a model comprising (i) a nugget effect, (ii) a spherical model with a range of 12 m, and (iii) a Gaussian model with a range of 36 m, Correlations among soil properties varied depending o n spatial scale. Short-range soil variation was caused by irrigation a nd land leveling, but results were not consistent for both fields, Sim ilar autovariograms and high correlations between depth to the Cg hori zon and extractable K in the Gaussian structure (rho(u nu)(2)=-0.95 to -0.99) revealed a strong influence of weathering of volcanic tuff on long-range trends in K, Electrical conductivity (EC), Na, and B were s patially trended and correlated with depth to the Cg horizon (rho(u nu )(2)=0.59-0.97) in the Gaussian structure, Subsurface water flow throu gh cracks above an impermeable layer and surface water dow along the g entle field slope were sources of long-range variation in EC, Na, and B caused by irrigation, The FKA provides quantitative measures of comp lex spatial process results and offers new ways for generating hypothe ses about the phenomenon itself.