A study of Sitka spruce plantations of varying age on stagnopodsols in Wales has shown that relatively little nitrate leaches from forests of under 30 years, but it does so to a statistically significant and increasing extent from older forests. It has been hypothesised that P and K deficiencies develop in later stages of the forest rotation which effectively limit N demand by the trees resulting in nitrate leaching. To test this hypothesis, bioassays to determine tree nutrient demands relative to supply, based on rates of uptake of isotopically labelled ions (P-32, Rb-86 and N-15) by live roots were applied to four stands on stagnopodsol soils in each of five Welsh forests (Beddgelert, Dyfi, Dyfnant, Hafren and Tywi). Older forest sites where stream nitrate concentrations were generally over 0.16 mg N 1(-1) tended to have a higher demand for P and K, but a low demand for N, indicating that nitrate leaching may be associated with deficiencies in supply of P and K. Multivariate analysis of root bioassay data showed that sites leaching nitrate could be distinguished at P < 0.01 from those not leaching nitrate. The critical load for pollutant N inputs to the Welsh forests will therefore need to be calculated using the mass balance equation modified to take account of P and K deficiencies on certain soil types.