The soil : a natural resource to be preserved both for production and the environment

摘要

The aim of this paper is to elucidate the importance of soil in both the northern and southern regions of the globe. In the southern regions of the planet, arable soil represents nearly 2 154 millions hectares, although these are mainly oxisols and ultisols. The main causes of recent degradation (physical, chemical and biological) in these soils are reviewed. In large tropical forests, the main result of deforestation, especially when performed mechanically, is a compaction of the soil which leads to a disappearance of the surface layers’porosity, and to erosion. Other factors such as over-grazing and over-farming lead to a disturbing acceleration of wind-borne, and especially rain-borne erosion. The first stage is the destruction of aggregates and the formation of impermeable crusts. Secondary salination due to irrigation and poor drainage is also in the increase. The degradation is chemical due to the salts and physical due to the dispersion effect on the clays. Water availability is also a major, and ever-increasing, problem. The essential limiting factors of southern soils are their low chemical fertility (P and K), their acidity and aluminium-based phytotoxicity related to the degree of alteration. 20 % of the world’s soils are affected by aluminium-based phytotoxicity ; in the tropics, this figure reaches 40 to 50 %. This is very hard to remedy in the soil, the future lies in plant adaptation. Biological degradation is often associated with other types of degradation but it is clear that intensive farming, its methods (ploughing) and agricultural inputs (pesticides) are a main reason for biological impoverishment, and this itself is linked to that of organic matter. In the second part, the role of soil in the environment is considered ; this aspect is of more importance in the northern regions of the globe. The soil formed by the effect of the biosphere’s atmosphere and the climate on the lithosphere is at the interface of these various components of the environment. Acid rain and its influence on the soil, the soil’s role in the greenhouse effect (C and N cycles), and the fixation of radioactive elements (137Cs) from Chernobyl in the first few centimetres of soil are discussed in terms of soil-atmosphere exchange, farming-practice and fertilisation especially have a direct influence on the quality of water. Even with assessments being made and input being monitored, the nitrate level in the water table will increase, as it will for pesticides. If well managed, the soil can be a good biological reactor for all C- and N- based compounds (liquid manure, compost, industrial and urban waste). The same cannot be said for heavy metals and very strict legislation must be enforced. In the northern regions, intensive farming has caused a substantial increase in soils’ tendency to erosion ; the mechanism is the same as in the south. To conclude, arable soil is limited (22 % of the surface) and all the abovementioned degradations will increase even further. The soil must be considered as a non-renewable resource, and one which needs to be protected. The problems facing the north and south have been dichotomised, yet in fact the two are converging more and more. Erosion due to rain is becoming more significant in the northern hemisphere and environmental problems are spreading even wider in the south (pesticides, acid rain, greenhouse effect, waste, water quality...). Sustainable agriculture using little agricultural inputs must be put into practice. This type of farming should make the methods fit the agroclimatic potential. Soil must be taken more seriously into consideration, but the way it is thought about (importance of various levels of organisation, biological...) should change too.