The phosphate status of the soils of Riverhead forest in relation to growth of Radiata pine : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Soil Science at Massey University
Extensive plantations of exotic softwood species in New Zealand have for the major part been restricted to land considered marginal for agricultural and pastoral pursuits. On these marginal lands of relatively low fertility and difficult terrain, forestry can compete economically with agriculture. Radiata pine (Pinus radiata D. Don), the most important of the exotic softwoods, occupies some 600,000 acres in New Zealand, but, despite its versatility and adaptability, has in some instances been established on sites which are outside the limits of its tolerance. Such is the case at the Riverhead State Forest in the Auckland conservancy. In many sectors of this forest, radiata pine, planted during the period 1926-33 on podzolized gumland clays, manifested symptoms of ill-health and unthriftiness within a short time of establishment. These symptoms gave rise to concern and stimulated a programme of research into the possible causal factors. The fertilizer trial work conducted at Riverhead up till 1958 has been reviewed by Weston (1956, 1958). Conway (1962) discussed aerial application of phosphatic fertilizers at Riverhead, while Will (1965) reported the more recent nutritional work. This nutritional work, which has included tissue analyses and the study of growth responses to phosphatic and zinc fertilizer treatments, has shown quite conclusively that unthrifty trees in the trial areas manifest a considerable response to phosphate, but not to zinc. The degree of response to the application of superphosphate has been shown to be dependent upon the prior condition of the stand and the rate of application. Topdressing at a rate of 5cwt/acre of superphosphate as a standard practice has been tentatively adopted following these trials. Will (1965) has shown that foliar analysis can be a most useful aid in assessing the need for fertilizer and in predicting the likely response to fertilizer treatment.