VARIABILITY OF SOIL CHEMICAL-PROPERTIES IN 2 SANDY, DUNAL SOILS OF NIGER

Variability in crop stands over very short distances is a widespread problem on the sandy soils of the Sahelian zone of Africa, and this phenomenon may be related to soil variability. The objectives of this study were to: (i) elucidate patterns in the variability of some soil chemical properties using classical statistics and geostatistics, (ii) compare the chemical properties and their variation in two dunal soils, and (iii) identify the implications of variability patterns in dunal soils for sampling technique. Two soils (Psammentic Paleustalfs) in Niger, West Africa were sampled extensively on a macro-scale (3500 m(2)) and on a micro-scale (0.2 m(2)). Soil pH, exchangeable acidity, available P, exchangeable bases, and organic matter were determined. Coefficients of variation (CVs) and sample number requirements were calculated, and semivariograms were constructed. On the macro-scale, the CVs for exchangeable Mg (74-114%) were highest. Exchangeable Ca (32-77%) and Al (20-100%) and available P (25-55%) also had high CVs on both soils. Semi-variograms revealed that the sill, range, and nugget variance of available P were greater than those for soil pH, exchangeable Al, and exchangeable H. Variance was slightly greater in the North-South direction than in the East-West direction for soil pH, exchangeable Al, and exchangeable H. The anisotropy is probably related to the predominant East-West direction of the axis of sand dunes in this region. Available P was significantly positively correlated with micro-elevation, which may be related to the spatial distribution of bushes and trees in the previous fallow period. Low P contents are widespread in the soils of Niger, and large responses to fertilizer P have been observed. Depth to acid subsoil was more variable in these soils than in those described in the literature, and variation in rooting depth could be reflected in aboveground plant growth. However, the sill for available P was considerably higher than for pH or exchangeable Al at all sampling depths, and spatial variability in crop stands is likely to be related to soil P levels as they vary with micro-elevation.