Pedotransfer functions for estimating plant available water and bulk density in Swedish agricultural soils

Pedotransfer functions (PTFs) to estimate plant available water were developed from a database of arable soils in Sweden. The PTFs were developed to fulfil the minimum requirements of any agro-hydrological application, i.e., soil water content at wilting point (theta(wp)) and field capacity (theta(fc)),from information that frequently is available from soil surveys such as texture and soil organic carbon content (SOC). From the same variables we also estimated bulk density (rho) and porosity (epsilon), which seldom are included in surveys, but are needed for calculating element mass balances. The seven particle-size classes given in the data set were aggregated in different ways to match information commonly gained from surveys. Analysis of covariance and stepwise multiple linear regression were used for quantifying the influence of depth, particle size class, textural class and soil organic carbon on the characteristic variables. PTFs developed from other data sets were also tested and their goodness-of-fit and bias was evaluated. These functions and those developed for the Swedish database were also tested on an independent data set and finally ranked according to their goodness of fit. Among single independent variables, clay was the best predictor for theta(wp), sand ( or the sum of clay and silt) for theta(fc) and SOC for rho and epsilon. A large fraction of the variation in theta(wp) and theta(fc) is explained by soil texture and SOC ( up to 90%) and root mean square errors (RMSEs) were as small as 0.03 m(3) water m(-3) soil in the best models. For the prediction of rho and epsilon in the test data set, the best PTF could only explain 40-43% of the total variance with corresponding RMSEs of 0.14 g cm(-3) and 5.3% by volume, respectively. Recently presented PTFs derived from a North American database performed very well for estimating theta(wp) ( low error and bias) and could be recommended for Swedish soils if measurements of clay, sand and SOC were available. Although somewhat less accurately, also theta(fc) could be estimated satisfactorily. This indicates that the determination of plant available water by texture and SOC is rather independent of soil genesis and that certain PTFs are transferable between continents.