CHANGES IN PHOSPHATE STATUS OF SITKA-SPRUCE PLANTATIONS OF INCREASING AGE, AS DETERMINED BY ROOT BIOASSAY

Using a root bioassay, a peak of phosphorus demand of a developing series of Sitka spruce (Picea sitchensis (Bong.) Carr) plantations, growing on a peaty-gley soil in upland Great Britain, has been shown to occur at canopy closure. Demonstration of the increased demand by the forest at this stage of development agrees well with theoretical considerations of nutrient immobilization but does not correlate to changes in foliar P concentration nor to soil chemical analysis. It does, however, correlate with litter depth, indicating a relationship between peak demand by the crop due to maximal growth rate, and reduced soil supply due to nutrient accumulation in recalcitrant organic matter prior to the development of efficient retranslocation in the canopy. The bioassay detected true nutrient demand; this was alleviated by phosphorus supply at 100 kg ha-1 which resulted in significant increase in girth. The potential of the bioassay as a management tool, allowing detection and subsequent alleviation of phosphate deficiency by fertilization at a critical stage in the development of the crop, is discussed. © 1990.