Abstract Knowledge of where roots are active is crucial for efficient management of nutrients in tree crops but measurement of root activity is problematic. Measurement using soil water depletion is an approach that has not been tested in a humid climate. We hypothesised that the three dimensional distribution of root activity of a tree crop in the humid tropics (a) can be deter- mined by measuring soil water depletion during rain-free periods, and (b) is influenced by envi- ronment (soil type and climate) and surface management. A field study was carried out in which soil water content was measured and water uptake calculated (by difference between soil water content at beginning and end of rain-free periods) for different surface management zones and depths (0.1 m intervals to 1.6 m depth) under oil palm (Elaeis guineensis Jacq.) at a loam–clay site and a sandy site. Significant differences were measured between sites and between surface management zones at each site. At both sites water uptake was highest under the weeded zone close to the palm stem, slightly lower under the zone where pruned fronds are placed, and lowest under the path used for removing harvested fruit. Vertical distribution of root activity differed be- tween the sites, with higher activity near the surface at the finer textured site. Total water uptake values were lower than estimates of evapotranspiration made using climate data. The difference was probably largely due to water up- take from deeper than 1.6 m. This study showed that the spatial distribution of tree root activity in a humid climate could be quantified using a rel- atively simple method. Keywords Evapotranspiration Humid tropics Root biomass Root distribution Tree crops Water uptake Introduction In order to understand, manage and model uptake of water and nutrients by plants we need to know where roots are active. There have been few studies of root systems of perennial crops in P. N. Nelson (&) James Cook University & NRMW, PO Box 6811, Cairns Qld 4870, Australia e-mail: paul.nelson@jcu.edu.au M. Banabas Papua New Guinea Oil Palm Research Association, PO Box 28, Popondetta, Papua New Guinea D. R. Scotter Massey University, Private Bag 11 222, Palmerston North, New Zealand M. J. Webb Papua New Guinea Oil Palm Research Association, PO Box 36, Alotau, Papua New Guinea Plant Soil (2006) 286:109–121 DOI 10.1007/s11104-006-9030-6 123 ORIGINAL PAPER Using soil water depletion to measure spatial distribution of root activity in oil palm (Elaeis guineensis Jacq.) plantations Paul N. Nelson Murom Banabas David R. Scotter Michael J. Webb Received: 26 February 2006 / Accepted: 7 May 2006 / Published online: 15 July 2006 Ó Springer Science+Business Media B.V. 2006