SOURCES OF SOIL VARIATION IN AN ACID ULTISOL OF THE PHILIPPINES

Field-scale variation in soil fertility can be a major source of uneve n crop growth on acid soils of the humid tropics, but this has not rec eived much research attention. We used factorial kriging (FKA) to anal yze processes causing spatial variation in soil chemical properties (p H, 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 coregionaliza tion comprising (i) a nugget effect, (ii) a spherical structure with a range of 20 m, and (iii) a linear structure was fitted to the experim ental auto- and cross-variograms of the topsoiI properties. Variograms for subsoil variables were modeled using a nugget effect plus a linea r structure. Regionalized factors associated with short- and long-rang e variation were then mapped and formed the basis for a conceptual mod el of soil formation and soil variation at the study site. Patchy inpu ts of nutrients from buffalo excrement have led to pronounced short-ra nge variation in P, Ca, Mg, and K concentrations in the topsoil. The e ffects of these are superimposed on long-range processes operating alo ng the topographic slope. The latter include vertical and lateral wate r how causing translocation of basic and acidic cations. Leaching of a cidity along the major directions of the field slope seems to be a pro cess of general importance in sloping lands of the humid tropics. Amel ioration of soil acidity as well as design of field experiments have t o account for this kind of slope-dependent systematic soil variation. Combining pedological expert knowledge with techniques such as FKA hel ps to explore complex processes in a quantitative way.