Boron retention within a catena of rhyolitic soils and Its effect on radiata pine growth and nutrition

Abstract

Boron deficiency in Pinus radiata (D. Don) plantations in N.S.W. has been particularly evident on soils derived from acid igneous parent materials. A slope sequence (catena) of soils on a rhyolitic parent material was selected to study the amount of boron retention and its relationship to soil development. The soils at three positions, a hillcrest, mid-slope and lower slope, were described and various soil chemical and physical properties were determined for each horizon. Tree height, deformity, survival and foliar chemistry of 6 year old P. radiata were measured in plots adjacent to the three soil profiles studied. Boron adsorption isotherms varied within individual soil profiles by horizon and also between the different soil profiles in the catena. Boron adsorption was highest in the hillcrest soil B horizons and lowest in the bleached A2 horizons of the lower slope soil. The Freundlich isotherm constant k was found to be significantly correlated with clay content (r = 0.88), exchangeable aluminium (r = 0.79, exchangeable potassium (r = 0.68), and dithionite-citrate extractable iron (r = 0.66). Leaching and illuviation of iron oxides and clay has been accentuated in the two soil profiles on the hillslope sites of the catena. As clay, aluminium and iron oxides have been removed from the surface horizons of the lower slope soil, the ability of these horizons to adsorb and therefore retain boron has been greatly diminished. Position on the catena also affected P. radiata growth survival and foliar chemistry. Tree height decreased while the incidence of deformity increased down the slope. It is suggested that the progressive deterioration in tree growth down the slope is mainly caused by the trees being increasingly affected by boron stress as a result of decreasing capacity of the soils to retain boron by adsorption against leaching towards the lower slope.