Soil sampling in oil palm plantations: a practical design that accounts for lateral variability at the tree scale

The aim was to devise a practical soil sampling design for oil palm plantations that takes into account tree-scale variability, thus facilitating detection of trends in soil properties over time. We geometrically evaluated the ability of linear sampling transects to represent the distribution of typical management zones and radial patterns known to influence soil properties. The effect of sampling point density was tested using interpolated surfaces of soil biological, chemical and physical properties derived from values measured on a 35-point sampling grid covering the repeating tree unit in plantations with 15-25-year old palms. The ability of sampling transects to represent the proportion of the plantation in various zones improved with increasing transect length and sampling density. Increasing the number of sampling points from 10 to 50 (using an acceptably long transect with length 5.57 x palm spacing) decreased the maximum deviation between the overall mean and the transect-derived mean from 15.9 to 5.6 % for the most variable parameter, respiration, and 3.2 to 0.6 % for the least variable parameter, bulk density. Transect sampling provides an efficient means of obtaining a composite soil sample that accounts for tree-scale variability in oil palm plantations. The method is readily adaptable for other tree crops.