Chapter 21
Pedotransfer functions for tropical soils

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This chapter compares the performance of well-documented pedotransfer functions (PTFs) developed in tropical soils. The performance is examined in terms of their accuracy, reliability, and utility. The potential effects of bulk density, the selection of independent variables, and the methodology used for deriving the PTFs on their performance are discussed in the chapter. The overall accuracy of PTFs for tropical soils seems to be affected by the presence of soils having low bulk density. Those soils should be treated in a separate group for which PTFs should be derived separately. Considering the wide range of soils to which the PTFs were applied in the study discussed in the chapter, the general performance of PTFs developed for tropical soils is quite acceptable and certainly comparable to that of the PTFs for the soils of temperate regions.

Introduction

Most of the pedotransfer functions (PTFs) available in the literature have been derived and tested using extensive databases of soils of temperate regions. The lack of data for tropical soils has been pointed out as a major constraint for the development of PTFs for tropical soils (Hodnett and Tomasella, 2002). More recently, the increased interest of understanding the effect of land use and land cover change in the tropics on global climate has raised the need for an improved knowledge of the hydrological functioning of tropical soils. The impacts of such changes are usually assessed using general circulation models, or GCMs, which require detailed soil information on a global scale. Since tropical soils have been surveyed from a pedological perspective, with very little information of the hydraulic characteristics, PTFs are the only tool that can provide the necessary hydraulic information on the spatial scale needed by GCMs.

Kaolinitic tropical soils have usually clay contents ranging from 60 to 90%. In temperate climates, soils with more than 60% of clay are considered as low permeability heavy clays and are regarded as “non-agricultural soils” (Carsel and Parrish, 1988). Measurements from Correa, 1984, Tomasella and Hodnett, 1996, among others, clearly showed that kaolinitic tropical soils show “unusual” properties when compared with typical temperate clayey soils: low bulk density (0.7–1.2 Mg m−3), high permeability (Ksat usually 10–1000 mm h−1), have low available water capacity (AWC) (70 mm m−1), and almost 80% of the plant available water between −10 and −100 kPa (Demattê, 1988).

The pronounced differences between temperate and tropical clayey soils are usually explained by the micro-aggregated structure of oxisols. In kaolinitic tropical soils, major cations such as Ca2+, Mg2+, K+ and silica are eliminated from the soil profile as a result of the high rainfall and continuous leaching (Vieira and Santos, 1987). The increasing concentration of hydrogen relative to basic cations results in low pH. The removal of alkali elements, added to the transport of oxides from the upper horizon, increase the concentration of sesquioxides (compounds of Fe3+ and Al3+) in the B horizon (Sanchez, 1976, Vieira and Santos, 1987). In oxisols and nitosols, Fe and Al oxides play an important role as binding agents of negatively charged clay minerals, creating stable micro-aggregates within the size range of silt to fine sand. This explains why their field texture is loamy rather than clayey as determined by laboratory analysis (Cassel and Lal, 1992). Since oxisols are very permeable, most of the soil water is released between saturation and water potentials above −10 kPa. This behavior resembles that of sandy soils, although the water contents are comparatively higher because of the clayey character of oxisols. (Sharma and Uehara, 1986). At lower potentials a significant proportion of water is held within the micro-aggregates. Therefore, the water retention curve is almost flat from −100 kPa up to water potentials as low as −4000 kPa (Chauvel et al., 1991), where a sudden drop of soil water content occurs, which probably coincides with the air-entry value of the aggregates. Since water below 1500 kPa is no longer available for the majority of plants, the plant available water in oxisols is lower compared to “temperate” clays.

Although the mineralogical composition and the characteristic chemical processes cause oxisols and related soils to be characterized by uniform texture, high friability, and the presence of extremely stable micro-aggregates (Demattê, 1988), this observation cannot be generalized to other soils of the tropics, particularly those under intensive land use. Surface horizons of many soils of the tropics, with relatively less clay and organic matter, are less aggregated than soils of temperate zones (Cassel and Lal, 1992).

Physical and chemical differences between temperate and tropical soils might explain why the PTFs derived for soils of temperate climate appeared to be inadequate for oxisols and related soils (van den Berg et al., 1997). It may also be argued that the poor performance of “temperate soil” PTFs arises because the clay content of oxisols frequently exceeds 60%, while PTFs developed for temperate soils often do not cover that range: as an example, the PTF by Rawls and Brakensiek (1985) is only valid for soils with clay contents between 5 and 60%. Tomasella et al. (2000) compared the performance of a PTF derived for Brazilian soils with a range of temperate soil PTFs and concluded that the former performed substantially better, even within the range of validity of latter. This result implies that there must be a marked difference in the hydraulic properties of tropical and temperate soils (Hodnett and Tomasella, 2002). It is not surprising then that the PTF proposed by Tomasella et al. (2000) performed better in Cuban oxisols (Medina et al., 2002) compared to temperate soil PTFs.

More recently, Sobieraj et al. (2001) argued that the hydrological behavior of tropical soils cannot be attributed to the mineralogical factors as it had been suggested by Tomasella and Hodnett (1996). Based on data from kaolinitic soils with hydrated vermiculite interlayers from western Amazonia (which are similar to ultisols found everywhere in the world, according to the authors), they concluded that the poor performance of temperate PTFs in predicting saturated hydraulic conductivity resulted from the lack of ability to reproduce the effects of macroporosity in tropical soils. This may indeed be the case, but it is likely that tropical soils typically have a greater macroporosity than temperate soils, and there is no reason why this should not be linked to their different mineralogy.

The aim of this work is to compare the performance of well-documented PTFs developed in tropical soils. As suggested by Pachepsky et al. (1999), the performance was examined in terms of their accuracy, reliability and utility. The potential effects of bulk density, selection of independent variables, and the methodology used for deriving the PTFs on their performance is discussed.

Section snippets

Materials and methods

The limited availability of detailed soil information in the tropics has generally precluded the development of PTFs able to provide hydraulic parameters in great detail. Most of the PTFs developed for tropical soils are limited to the prediction of the water content at a few water potentials, mostly at −10, −33 and −1500 kPa. Therefore, to compare the performance of PTFs developed for tropical soils on a common basis, it is necessary to constrain comparisons to a few number of measurements of

Results

Table 2 presents the applicability index and statistics R2, ME and RMSE resulting from the estimation of water content at −10, −33 and −1500 kPa. As expected, the PTF of Hodnett and Tomasella (2002) was applicable in all the samples, followed by the PTF proposed by Tomasella et al. (2000), which was applicable in 85% of the cases. The PTFs of Aina and Perisawamy, 1985, Lal, 1979 were applicable to less than 20% of the dataset, while for the remainder of the PTFs the applicability varied from 60

Discussion

Table 2 demonstrated that the PTF proposed by Tomasella et al. (2003) was more accurate than the PTF of Hodnett and Tomasella (2002), at least for the water content at −10, −33 and −1500 kPa. This result was somewhat surprising, since Hodnett and Tomasella (2002) used IGBP data from 22 countries in the derivation, and the PTF was applicable to all of the soils in the dataset to which it was applied in this test. The PTF of Tomasella et al. (2003) was derived using only Brazilian soil data and

Conclusions

In general, the PTF of Tomasella et al. (2003) showed the best performance for estimating water content at −10, −33 and −1500 kPa. The reason why this PTF produced the best result might be related to the development based on the prediction of individual water-retention points rather the than the parameters of an analytical retention curve.

With regard to the estimation of AWC10–1500 kPa and AWC33–1500 kPa, the PTF of Lal (1979) exhibited a good performance, although the range of applicability was

Acknowledgements

The first author received financial support form the Fundação de Amparo a Pesquisa do Estado de São Paulo – Fapesp.

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