Continuing Response of Pinus radiata to Phosphatic Fertilizers Over Two Rotations

Abstract

Abstract Considerable growth responses to two differing phosphatic fertilizers (rock phosphate and superphosphate) applied to Pinus radiata (D. Don) were previously reported by the authors (Forest Science 11, 1965). At that time the experimental site had carried two rotations of trees, the first unfertilized. The same site has now been grown through a similar period of time in the third rotation (to first thinning) with no further fertilizer applications. Growth responses, foliage and soil chemical analyses have been compared with those obtained over the first fertilized rotation. Results showed that tree growth responses have been maintained significantly through two fertilized rotations to the present time by the original 1947 fertilization. The bulk of the phosphorus from both sources is still present in the soil surface horizon, that from rock phosphate mainly as calcium phosphate, and from superphosphate mostly in aluminum- and iron-bound forms. Implications for forest management are that where adequate growth responses can be obtained from a single long-lived fertilizer source, the investment in fertilizer becomes a long-term addition to forest site quality. Cost effectiveness has been compared for both rotations in terms of 1983 dollars. Forest Sci. 32:822-829.