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Towards a reference environment for topsoil: The fourth dimension of Thanatia

Abstract:

Thanatia represents a model of degraded Earth where all resources would have been extracted and dispersed throughout the crust. It is composed by a degraded atmosphere, hydrosphere and upper continental crust, in terms of the inorganic species of each of the aforementioned layers of the Earth. The model of Thanatia is the baseline to assess the exergy of natural resources. Particularly, for the upper continental crust, it represents the starting point for assessing the exergy of the mineral capital on Earth. Apart from minerals, an important resource of the Earth is soil. Soils are living ecosystems composed of biotic and abiotic components key for human nutrition. The degradation of soils is an important issue currently faced by humanity that can be accounted for using exergy. In order to do that, an adequate reference baseline needs to be established. In this paper we propose to incorporate to the Thanatia model a fourth layer, namely that of a “degraded topsoil”, serving as a starting point to assess the fertility of soils. The degraded topsoil would represent a state of maximum degradation. Through the “Liebig Law of the minimum” the parameters describing the degraded topsoil will be preliminary set. This law states that yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be. Thanatia's topsoil would be one in which all key chemical, physical and biological components necessary for a soil to be fertile (i.e. microelements, nutrients, microbiota, water storage properties, etc.) are below the fertility limit. From that reference, the value of a fertile soil can be assessed with an exergy point of view, as difference with the degraded conditions. Concepts of fertility, infertility and fertilization of soils can be physically assessed with a single and rigorous unit of measure through exergy. This paper is a first attempt in establishing the chemical composition of Thanatia's topsoil.