Sources of soil variation in an acid Ultisol of the Philippines

Abstract

Field-scale variation in soil fertility can be a major source of uneven crop growth on acid soils of the humid tropics, but this has not received much research attention. We used factorial kriging (FKA) to analyze processes causing spatial variation in soil chemical properties (pH, P, K, Na, Ca, Mg, Al) measured at two depths within a single field on an acid Ultisol in the Philippines. A linear model of coregionalization comprising (i) a nugget effect, (ii) a spherical structure with a range of 20 m, and (iii) a linear structure was fitted to the experimental auto- and cross-variograms of the topsoil properties. Variograms for subsoil variables were modeled using a nugget effect plus a linear structure. Regionalized factors associated with short- and long-range variation were then mapped and formed the basis for a conceptual model of soil formation and soil variation at the study site. Patchy inputs of nutrients from buffalo excrement have led to pronounced short-range variation in P, Ca, Mg, and K concentrations in the topsoil. The effects of these are superimposed on long-range processes operating along the topographic slope. The latter include vertical and lateral water flow causing translocation of basic and acidic cations. Leaching of acidity along the major directions of the field slope seems to be a process of general importance in sloping lands of the humid tropics. Amelioration of soil acidity as well as design of field experiments have to account for this kind of slope-dependent systematic soil variation. Combining pedological expert knowledge with techniques such as FKA helps to explore complex processes in a quantitative way.