Chapter 7Using soil morphological attributes and soil structure in pedotransfer functions
Introduction
Soil morphological attributes such as pedogenic horizon type, color, texture, structure, consistence, mottles, concretions and clay films are routinely described in the field during the inspection of soil profiles. These descriptions are generally qualitative or semi-quantitative, categorical assessments of the visual and tactile qualities of the soil. Soil structure, along with the abundance and size of plant roots and visible pores, are attributes that are often recorded in these field descriptions, but the quantity, size, shape, and continuity of visible pores are generally less well documented because of the complexity and dynamic nature of soil pores.
As soil morphology is often a key component of soil classification (and of characterization), there is a considerable volume of morphological data available within the databases of many national and regional soil survey organizations. In most cases, the volume of these data is vastly greater than measured soil hydraulic properties despite the necessity of these data for environmental modelling. It has been shown that it is possible to develop rule-based relationships between soil hydraulic properties and soil morphology for a variety of purposes (e.g., McKeague et al., 1982, Boorman et al., 1995) or to improve equation-based PTFs (e.g., Lin et al., 1999b, Pachepsky and Rawls, 1999). Although soil morphological data are mainly suited to the development of class PTFs, continuous PTFs could also be developed using quantified soil morphological attributes as inputs.
Pedotransfer functions that are based on the statistical relationship between soil hydraulic properties and soil texture are not readily transferable to other bioclimatic zones (McKeague et al., 1991, Wösten et al., 2001, O'Connell and Ryan, 2002). This may be due, in a large part, to the fact that soils with similar soil textures, but influenced by different moisture or thermal regimes, will not necessarily develop the same structure and pore architectures. Wagner et al. (1998) reported that predictions of soil hydraulic properties from a number of models based only on soil texture gave poor results in structured soils. However, one of the models that incorporated a measurement of retained moisture content (an additional measure of soil porosity) gave better predictions. In general, PTFs that relate soil hydraulic properties to texture alone cannot correctly predict saturated hydraulic conductivity (Ksat) or other hydraulic values for soils that contain large cracks, worm holes, or root channels. There is evidence that many clayey soils, especially those having strong, fine blocky or granular structure, and with a large number of biopores or cracks, had Ksat values as great as or even greater than coarse-textured soils (e.g., O'Neal, 1949, O'Neal, 1952, McKeague et al., 1982, Coen and Wang, 1989, Bouma, 1991, Lin et al., 1997). Furthermore, shrinking or swelling upon drying or wetting creates dynamic hydraulic behavior in active clayey soils that cannot be accounted for by texture alone (e.g., Lin et al., 1998). As soil structure is a function of the interaction between soil texture and bioclimate, PTFs that predict soil hydraulic properties based on structure should have a greater degree of transferability than regional, texture-based models.
The term soil structure has been used in US soil surveys, and elsewhere, to refer to the natural organization of soil particles into individual units (called peds) separated by planes of weakness. The peds are generally described according to their shape (platy, prismatic, columnar, angular/subangular blocky, or granular), their size range (very fine, fine, medium, coarse, or very coarse) and grade or distinctness (weak, moderate, or strong). Additionally, the internal surface features of the peds are also described, consisting of (1) coats of a variety of substances unlike the adjacent soil material and covering part or all of the surfaces, (2) concentration of material on surfaces caused by the removal of other materials, and (3) stress formations in which thin layers at the surfaces have undergone reorientation or packing by stress or shear (Soil Survey Division Staff, 1993). The kinds of structural surface features include clay films, clay bridges, sand or silt coatings, other coatings, stress surfaces, and slickensides. In general, pores are considered separately from soil structure, hence, in the US, the concept of soil structure is sometimes referred to as “pedality”. Although soil pedality and macroporosity are related to one another, many soils have interpedal, intrapedal, and/or transpedal pores, which are not necessarily well represented by pedality. These pores, along with biopores and structural cracks, are critical in determining the hydraulic properties of field soils.
Soil morphological attributes, including soil structure, have long been used to infer soil hydraulic properties. Soil scientists have been successful in using descriptive morphological information to make qualitative judgments about a number of soil hydraulic properties, notably Ksat (e.g., O'Neal, 1949, King and Franzmeier, 1981, McKeague et al., 1982, Coen and Wang, 1989, Soil Survey Division Staff, 1993). For example, soil structure, as described by soil surveyors, provides clues about the macroporosity of the soil and the dominant pathways of water movement through the soil in saturated and near-saturated conditions. Thus, soil hydraulic properties such as Ksat often lend themselves to prediction from field characterizations of soil structure. National and regional soil surveys have often made use of soil morphological data for semi-quantitative estimates of soil hydraulic properties that are of relevance to land resource evaluation. In addition, soil morphological data are perhaps ideally suited to grouping soils by their hydrological functioning. As demonstrated by Wösten et al. (1990), class PTFs that use well-defined soil horizons as “carriers” of physical information allow efficient use of soil morphological data (Bouma, 1992).
While qualitative soil morphological attributes have been widely applied, quantification of such data is generally lacking. Rawls et al. (1993) noted that a quantitative description of the effects of soil morphological properties on soil water movement is yet to be established. So far, limited studies have demonstrated the potential for quantifying soil macromorphology through field observations or soil micromorphology through thin sections. As illustrated by Bouma and Anderson (1973) and Lin et al., 1999a, Lin et al., 1999b, micro- and macro-morphometric data could be used to quantitatively derive soil hydraulic parameters. The utilization of soil structure descriptors in a quantitative fashion would be a step forward towards incorporating soil structure into PTFs and the modeling of flow and transport in soils. Furthermore, quantification of soil morphology would enhance the understanding of the relationships among different morphological features and permit a better assessment of soil profile descriptions in relation to water movement in soils and over landscapes. Such information could vastly improve the value of existing soil surveys.
In this chapter, we will review the contribution of soil morphology including soil structure to the derivation and improvement of PTFs that predict soil hydraulic properties in various soils around the world. We categorize the approaches into (1) qualitative or semi-quantitative and (2) quantitative methods. We conclude this chapter with some suggestions on how soil morphological data can be further used to improve PTFs.
Section snippets
Predictions of hydraulic conductivity
Some early papers on the use of soil morphology to predict hydraulic properties were by O'Neal, 1949, O'Neal, 1952) who developed a system to estimate soil permeability (equivalent to Ksat) in US soils on the basis of primary and secondary structures as described in the field. Soil permeabilities were grouped into seven classes and ranged from approximately 3 cm day−1 to 610 cm day−1. O'Neal, 1949, O'Neal, 1952 produced a set of guidelines, presented as a table. Initially soils were grouped by
Future improvements
Soil morphological attributes, especially those related to soil structure and macroporosity, provide clues and/or estimates of pore volume that water both flows through and is held in. It would therefore seem logical that these assessments could be used to predict soil hydraulic properties. However, it is clear that the methods for predicting soil hydraulic properties from soil morphology (both tactile and visual) are not as well developed as those that utilize soil texture data in deriving
Summary
This chapter has shown that it is possible to develop both PTFs and PTRs to predict soil hydraulic properties and soil hydrological functioning from soil morphology including soil structure. By doing so, a considerable volume of soil morphological data residing in national and regional databases could be of great use in providing information on soil hydrology. However, while there is scope for further developments, this may require an acceptance of both standardized methodologies to describe
Acknowledgements
H. Lin's contribution to this work was partially supported by the USDA-NRI grant #2002-35102-12547 while the Scottish Executive Environment and Rural Affairs Department funded A. Lilly.
References (88)
- et al.
Relationships between conventional field information and some soil properties measured in the laboratory
Geoderma
(1992) Development of a world data set of soil water retention properties using pedotransfer rules
Geoderma
(1996)- et al.
Estimation of the draining soil moisture characteristic from standard data as recorded in routine soil surveys
Geoderma
(1994) - et al.
The effects of organic matter and particle size on the water retention properties of some soils in the west Midlands of England
Geoderma
(1977) - et al.
A soil wetness class map for Scotland: new assessments of soil and climate data for land evaluation
Geoforum
(1994) - et al.
From pedotransfer functions to soil inference systems
Geoderma
(2002) - et al.
Comparison of different approaches to the development of pedotransfer functions for water-retention curves
Geoderma
(1999) - et al.
Use of soil penetration resistance and group methods of data handling to improve soil water retention estimates
Soil Tillage Res
(1998) - et al.
Prediction of soil water retention using soil physical data and terrain attributes
J. Hydrol
(2002)