Early Growth of Picea sitchensis (Bong.) Carr. on Deep Oligotrophic Peat in Northern Ireland

Abstract

Relationships are examined between growth of young Sitka spruce, planted on deep oligotrophic blanket peat, and uptake of N, P, and K by the trees under various fertilizer and herbicide regimes. Despite the provision of adequate P, tree growth eventually becomes limited by a shortage of available N. This can be overcome at present only by repeated applications of fertilizer N. The use of herbicides to kill heather in checked crops increases the availability of N temporarily, and fertilizer K in all situations improves growth, though often only slightly. It is suggested that following the initial application of phosphate a finite fraction of the total N content of the peat becomes available to the trees. Once this has been used the release of mineral nitrogen is so slow that crops become N-deficient and growth is limited. There is no evidence to suggest that once canopy closes the situation will change, so that on oligotrophic peat sites managers are faced with the possibility of having to apply nitrogenous fertilizers every 3 or 4 years to maintain growth at a satisfactory level. Although there is as yet no direct evidence on oligotrophic peat that there will not be an increase in the rate of N mineralization once crops reach 5–7 m in height, work on other poorly drained soils in Northern Ireland indicates that it is unlikely to happen.