Potential for utilization of wood ash on coastal arenosols with limited buffer capacity in KwaZulu-Natal and its effect on eucalypt stand nutrition and growth

Abstract: A field trial was established to test the effects of various wood ash and fertilizer application rates on the nutrition and early growth of a clonal Eucalyptus grandis × urophylla stand near Richards Bay, KwaZulu-Natal province, South Africa. The trial consisted of wood ash treatments of 0, 0.3, 0.6 and 1.2 t ha-1, combined with fertilizer treatments of no fertilizer (control), 150 g tree-1 of conventional ammonium sulphate fertilizer or 320 g tree-1 of controlled release fertilizer mixture containing N, P and a balanced suite of several plant nutrients. The experiment was conducted on a young sandy soil of aeolian origin with a very low buffer capacity. Ash application rates were chosen after a pilot study was conducted to test the effect of CaCO3 on the soil reaction. At 4 and 8 months after treatment, soil heavy metal concentrations for cadmium (Cd), mercury (Hg), chromium (Cr) and lead (Pb) levels were substantially lower than toxic levels. Foliar heavy metal concentrations (for the same elements) were less than 1mg kg-1 at both time intervals. The wood ash induced a temporary liming effect up to 8 months after application. Foliar nutrient assessments revealed sub-optimal nutrient concentrations for phosphorous (P), potassium (K) and zinc (Zn) at 4 months and K at 8 months of age. The positive growth responses (expressed as a biomass index) at 8 months, ranged between 13% and 683% relative to the untreated control. At 21 months, the growth response to ash and fertilizer combinations ranged from -0.5% to 50% relative to the control. This research demonstrated that 1.2 t ha-1 of wood ash can safely be disposed of on a typical, poorly buffered Zululand coastal sand with little environmental risk and minimal growth suppression, provided that it is balanced with an appropriate NPS plus trace element fertilizer mixture.