A bioassay for determining P-deficiency, based on the rate of uptake of 32P-labelled phosphorus by roots from a standard solution and developed on pot grown tree seedlings, has been applied to roots from 10-18 year old lodgepole pine and Sitka spruce stands in field fertilizer trials on peat. Rates of 32-phosphorus uptake by roots were negatively related to quantities of P fertilizer previously given and to tree heights. The P content of first whorl needle samples were generally high but were not significantly different between fertilizer treatments nor related to tree heights.There was, however, a general relationship between rates of 32P uptake by roots and P content of needles over all sites and tree species. The bioassy, however, rapidly detects changes in internal P status of the tree which takes time to manifest itself in a change of needle P content.Preliminary work suggests that the bioassay detects a P-status in trees, not identifiable from needle analyses. A number of factors which might influence the results of the bioassay and therefore the field applicability of the method are discussed.
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