Chemical vulnerability of red soils in La Mancha (Central Spain)

Abstract

Red soils are widely distributed in Mediterranean regions. They constitute one of the most important soil resources for crop production and other uses. The main objective of this paper is to estimate the inherent chemical susceptibility of red soils of La Mancha to chemical degradation. Selected 47 soil profiles, were analyzed in the area by means of multivariate analysis; we identified five soil attributes that are well represented in the local soils database, which are most likely to control the chemical vulnerability. These attributes are the calcium carbonate, organic matter and clays together with soil depth and drainage. The attributes were transformed and a simple vulnerability index (VI) was deviced. We concluded that the index may be used as a first approximation rating of La Mancha's red soils vulnerability against an eventual chemical degradation, or to aid resource management and prevent anthropogenic potential impacts.

Introduction

Soil constitutes a key system as it works as a complex reactor, capable of undertaking the functions of filtering, decomposing, neutralizing, inactivating, storing, etc. Therefore depending on the fragility and susceptibility level of each specific profile, soil will be degraded more or less drastically, after undergoing eventual impacts and environmental unbalance due to unsuitable use (Keeney, 1986, Rodvang and Simpkins, 2001). Red soils represent perhaps one of the most important soil resources for the agricultural production of the Mediterranean areas. The term red soil as used in this article covers all red soils sequences found in La Mancha (central region of Spain, Fig. 1), including the Terra Rossas. The great majority of these soils are classified as Alfisols under the US System of Soil Taxonomy, and are rarely related to the orders of Mollisols or Ultisols. The La Mancha red soils are similar to the red soils in the other Mediterranean areas by having:

• Munsell colour of 2.5 YR or redder in the Bt horizon with high clay content,

• basic or acidic pH,

• low organic matter contents,

• medium to high cation exchange capacities,

• textures varying from clayey to sandy loam.

The dominant clay minerals are illite, kaolinite and smectite along with high contents of sesquioxides, hematite and/or amorphous iron. They are moderately weathered soils in good physical and chemical condition, and sustain arable cropping systems with careful management, but are susceptible to erosion and contamination. In fact, nutrients are added to many of these soils by fertilizers and residues, which are not entirely absorbed by the plants allowing excess percolation and contamination to underground and superficial waters, as it is in many developed countries (McLaughlin et al., 1985, Zhang et al., 1996). A reduced supply of organic C and nutrients caused by deforestation, soil erosion and intensive agriculture, and especially by environmental stress conditions (heavy metal contamination, for example) reduces biodiversity in the soils (Brown et al., 1994). So, being aware of soil vulnerability, a term used to express the intrinsic properties which determine the capacity of soil to buffer contamination or allowing it to pass on to another environmental echelon, is of prime importance.

Our primary objective is to suggest a series of intrinsic properties that can help control the red soils vulnerability to chemical degradation. The second objective is to combine these variables, and suggest an index to predict the chemical vulnerability of La Mancha's red soils.