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Healthy soil is an essential condition to obtaining water that is drinkable and clean. It acts as a natural filter, capable of regenerating itself.
It works for free, constantly, and does so very discreetly. Few people think about it when they open their taps, but soil is the cornerstone of the entire water purification process, from rainfall through to the storage of the precious liquid within groundwater tables.
If soil didn’t perform this essential filtering role, groundwater reserves would not be as clean. Human societies would then be unable to use it for drinking water, which is however vital for their survival – contaminated soil affects the quality of water. This is equally true with degraded soil through plant protection agents, for example, which would have wiped out the majority of micro-organisms living under ground. For the natural filtration process to regenerate, the soil’s structure therefore has to remain undamaged, and the factors which threaten to degrade it don’t’ exponentially increase.
The soil has several functions regarding water purification. It is firstly a filter – its composition allows it to mechanically clean seepage. Like a sieve, it catches hold of pollutants. Depending upon the type of soil – whether light and sandy or rich and silty – impurities that are very small in size (up to 0.2 micrometres) can even be trapped, which stops them from then ending up within groundwater tables.
However, this filter function can be ruined by soil erosion or compaction – also called soil compacting or settling. Indeed these are some of the most serious environmental problems caused by conventional agriculture.
The process of microbial degradation
The soil equally plays a role as a chemical filter. Having caught hold of the pollutants, it dissolves some of these through absorption. They are then bonded to the earth’s clay or to the humus layer. Some of them will even be recycled and will be used to feed micro-organisms or plants present at the surface. But again, the chemical filter role requires stable soil. As a result, pollution of the air, rainfall that is too sudden or heavy, dirt deposits, mineral fertilisers, pesticides, or any substance that is not originally present in the soil represents a potential threat to its health.
Finally, soil plays a role as a biological filter, and one which is especially crucial as it enables pollutants to be both caught and destroyed. This process is made possible by microbial degradation, which is very effective against all types of organic pollutants. These can be micro-organisms, fungi, or smaller living organisms present in the soil which perform a decontamination and soil recovery role. Here again, there is no substitute for living soil for gaining access to clean water. Soil which has been made fertile by conventional agriculture – with a gradual loss of biodiversity, particularly as far as micro-organisms are concerned – will play practically no role as a biological filter.