What is the level of uncertainty: percent clay, percent silt, percent sand, polygon model (topsoil and subsoil)?
The scale of the various soil maps used in deriving this map is shown in Figures A2 and A3. The Northcote Factual Key (a soil classification – Northcote 1979) uses soil texture as a differentiating characteristic. Estimation of texture for soils with a uniform primary profile form is straightforward, but it is more difficult to be definite about other soil types (e.g. duplex soils can have a range of surface textures). The reliability of estimates of texture for Principal Profile Forms that do not have texture as a diagnostic varies. The main sources of uncertainty in deriving particle size distributions from texture are: • is in the choice of representative values of percent clay, percent silt and percent sand for each texture class-use of Budiman’s methods has improved these estimates significantly; and • because field texture classes and particle size distribution are not completely equivalent (see McKenzie et al. [2000] for a discussion of the ways in which field texture may differ
