An examination of a forest site which has exhibited the ash-bed effect

Abstract

A site which has shown the 'ash-bed effect' to a marked degree was examined 9 years after planting with Pinus radiata. Trees grown in the ash-beds were found to be 1.2 to 1.4 times the height and more than twice the volume of trees grown next to the ashbeds. Chemical analysis of their foliage revealed that phosphorus levels were significantly higher in the ash-bed trees than in the normal trees, while the calcium, magnesium, sodium, potassium, aluminium, nitrogen, and manganese were the same. Soil analyses showed that the ash-bed soils had higher pH and exchangeable calcium contents and lower 'phosphorus adsorption capacity', exchangeable aluminium, and Tamm's oxalate-soluble aluminium than the normal soils, while no difference could be found in the loss on ignition, total nitrogen, ammonium nitrogen, 'total' phosphorus, Tamm's oxalate-soluble iron, exchangeable potassium, sodium, or magnesium. The soil phosphorus was fractionated and it was found that the ratio of the phosphorus associated with aluminium to the 'total' phosphorus was significantly greater in the ash-bed soils than the other soils. In the material examined the 'ash-bed effect' is associated with increased phosphorus availability. This could arise from a change in the composition of the soil phosphorus, a reduction in phosphorus adsorptivity, a reduction in aluminium ion concentration, an increase in phosphorus solubility because of more favourable pH conditions, and increased calcium levels. The possibility of a nitrogen x phosphorus interaction in earlier years is discussed.