Generalisation of continuous models to estimate soil characteristics into similar delineations of a detailed soil map

The aim of soil mapping is to partition soil bodies using map units, which are more homogenous for specific soil properties than are the soil bodies as a whole. Soil properties are expected to be similar at delineations of a specified soil map unit. Therefore, it is supposed that a model developed to estimate a soil property for one of these delineations could be generalised for the others. This study was conducted to determine the possibility of generalisation (extrapolation) of continuous models of spatial variability to estimate soil physical and chemical properties in similar delineations of a soil map unit. A consociation soil map unit in two different locations of a detailed soil map (1 : 20 000 scale), as similar delineations, was selected in the north-west of Faradonbeh region, Iran. Sixty topsoil samples (0–20 cm) were randomly collected in each delineation (totally 120 samples) with 30-m intervals and the samples were GPS-recorded. Laboratory studies consisted of bulk density, pH, calcium-carbonate equivalent, organic matter content, percentage of coarse fragments, and particle-size distribution. First, variography was done according to the soil data of each delineation (named areas A and B) and kriged maps were generated based on their own semivariogram parameters. Then, the kriged map of the soil properties for the second similar delineation (area B) was regenerated based on the corresponding models and their parameters obtained from the first similar delineation (area A). Finally, the regenerated kriged map of each variable was compared with its original kriged map. Visual comparison of the kriged maps of area B obtained from two steps of variography showed very high accordance for all of the soil properties. Quantitative comparison of the kriged maps suggests that the accuracy expected by the users of the soil information should be considered before generalisation of the data for similar units. Lower values of accordance obtained by the Kappa index and, especially, the classification success index than overall accuracy indicate that model generalisation should not be used where high precision of soil information is expected. Discrepancies observed for the kriged maps of the same variables in similar delineations could be due to different soil management practices in the past as a result of different historical developments.

Spatial and temporal trends in soil properties at a soil quality benchmark site in central Nova Scotia

In response to concerns over the widespread soil degradation occurring on Canada's agricultural lands, Agriculture and Agri-Food Canada established a network of benchmark sites to assess soil quality change by monitoring agronomically important soil properties. The Nova Scotia site was established in 1989 on Queens (Gleyed Brunisolic Gray Luvisols) and Debert (Gleyed Dystric Brunisols) soil series under a corn–forage rotation within the Annapolis–Minas Lowlands ecoregion. The objectives of the project were to quantify temporal changes in agronomically important soil properties and determine how the property values vary with the landscape and components of the soil map unit. A 20 × 25 m grid was used in 1990 and again in 1995 to sample soil from the Ap horizon and to locate saturated hydraulic conductivity and penetration resistance measurements. The samples were analyzed for pH, organic C, total N, and available P and K. The results indicate that over 5 yr, organic C, C:N ratio, available P, and saturated hydraulic conductivity declined by 7.9, 4.7, 12.5, and 53%, respectively. Significant differences in pH, organic C, total N, available K, penetration resistance and saturated hydraulic conductivity were associated with selected components of the soil map unit. Slope position had a minimal effect on soil properties except for available K where the highest levels were located on middle slope positions. Interactions between particle size and slope position were found, with soils with coarse-loamy sola on upper slope positions having the lowest pH. The results of this study also demonstrated the utility of the method for monitoring soil quality change and the importance of the soil map unit in interpreting the spatial and temporal differences in soil properties. Key words: Soil variability, soil monitoring, soil properties, soil map unit, spatial correlation

Physico-empirical approach for mapping soil hydraulic behaviour

Abstract: Pedo-transfer functions are largely used in soil hydraulic characterisation of large areas. The use of physico-empirical approaches for the derivation of soil hydraulic parameters from disturbed samples data can be greatly enhanced if a characterisation performed on undisturbed cores of the same type of soil is available. In this study, an experimental procedure for deriving maps of soil hydraulic behaviour is discussed with reference to its application in an irrigation district (30 km2) in southern Italy. The main steps of the proposed procedure are: i) the precise identification of soil hydraulic functions from undisturbed sampling of main horizons in representative profiles for each soil map unit; ii) the determination of pore-size distribution curves from larger disturbed sampling data sets within the same soil map unit. iii) the calibration of physical-empirical methods for retrieving soil hydraulic parameters from particle-size data and undisturbed soil sample analysis; iv) the definition of functional hydraulic properties from water balance output; and v) the delimitation of soil hydraulic map units based on functional properties.