Field Scale Spatial Analysis of pH on Sodium Soils in Northern Burkina Faso

Abstract

Geografisk Tidsskrift, Danish Journal of Geography 97: 66–75, 1997.

Microscale variations of soil properties may have a pronounced impact on crop yields in the low external input agriculture in the Sahel. Using classical and geostatistical methods, spatial patterns of soil pH at 0.0–0.10 m, 0.30–0.50 m and 0.70–0.90 m depth have been analysed and mapped based on two hundred and eighty-one samples collected at 10 m x 10 m grid nodes on a one hectare test plot on a clayey, sodium affected soil used for millet cultivation in northern Burkina Faso. The mean surface soil pH is 7.39 and semi-variance analysis shows that the nugget effect accounts for nearly 100% of the sample variance, and therefore surface pH exhibits no spatial dependency at the separating distance. The mean soil pH increases to 7.83 and 7.90 at 0.30–0.50 m and 0.70–0.90 m depth, respectively. However, the coefficient of variation also increases, and at 0.70–0.90 m the field has areas of both alkaline and acid soil. The range of influence for soil pH at 0.30–0.50 m was 60 m, increasing to 80 m at 0.70–0.90 m as a result of stronger dependency on geology. On application of the semivariograms, pH values between the grid points were interpolated by point kriging. The study illustrates that pH of sodium soils varies considerably and that millet roots may grow in a diversity of pH conditions. Furthermore, soil characterization depends very much upon the sampling strategies, but the determination of the range allows for choosing the minimum distance required for spacing of non spatially correlated samples. The application of chemical fertilizers are likely to produce very different effects given the chemically variable soil environment.

Keywords:

• Soil microvariability
• pH
• kriging
• Burkina Faso