A soil technical classification system for Pinus radiata (D. Don) plantations. II. A basis for estimation of crop yield.

The Technical Classification is a system for encoding soil attributes according to parent rock; texture profile; depth to and type of impeding layer; texture, condition, and weathering of surface horizons; and nature and colour of the subsoil. The encoded data may then be used as a basis for soil-based silvicultural management decisions, such as prediction of wood yield, fertilizer prescription, thinning schedules, trafficking for off-road equipment, and placement of roads. The objective of this paper is to examine the suitability of the classification as a basis from which to estimate the yield of wood from Pinus radiata plantations. Data were collected from trees and soil profiles on 181 sites in New South Wales, South Australia, Victoria and Tasmania. All but one of the attributes (texture profile) of the Technical Classification were significantly related to wood volume as single factors at the 5% level of significance. The combined effect of soil attributes in explaining the variation in wood volume across the sites was analysed by backward stepwise elimination of variables from a multiple analysis of variance model. When summed, the parameter estimates for each soil attribute gave an estimate of wood volume produced at age 11 years. The model included the attributes: parent rock, soil depth, impeding layer, texture of surface soil, surface condition, and an interaction between texture profile and weathering of surface soil horizons. The attributes subsoil texture and subsoil colour were not significant in explaining the variation in wood volume in the data set under test. The model accounted for 75% of the variation in wood volume. The addition of rainfall did not improve the variation accounted for by the model. By comparison, the Great Soil Group classification accounted for 32% of variation in wood volume. The Factual Key classification at the level of principal profile form accounted for 53% of the variation in wood volume. We conclude that the Technical Classification is suitable as a basis from which to estimate the yield of wood from Pinus radiata plantations.