Soil
Some species move and live in the upper soil strata and feed primarily on soil and associated organic matter (geophages). They do not have permanent burrows, and their temporary channels become filled with cast material as they move through the soil, progressively passing it through their intestines. These species live in or near surface plant litter. They are typically small and are adapted to the highly variable moisture and temperature conditions at the soil surface. The worms found in compost piles are epigeic and are unlikely to survive in the low organic matter environment of soil.
As they consume organic matter and mineral particles, earthworms excrete wastes in the form of casts, a type of soil aggregate. Charles Darwin calculated that earthworms can move large amounts of soil from the lower strata to the surface and also carry organic matter down into deeper soil layers. A large proportion of soil passes through the guts of earthworms, and they can turn over the top six inches (15 cm) of soil in ten to twenty years.
Water can be saved by reducing evaporation direct from the soil. Water evaporates faster in high temperatures and wind. So if wind speed can be reduced at soil level (and above it) while the soil can be kept cool, much water loss can be avoided. Covering the soil with mulch and erecting wind break hedges is one solution. In Spain and around the Mediterranean Sea, where the climate is too dry in summer, farmers till the soil under their olive trees to prevent weeds drawing water and mulch the soil with tilled, dry soil. However, this method leaves the soil wide open to erosion when sudden rains appear.
The water collecting in a reservoir is the run-off from rain falling on the upper-catchment area. In its journey to the lake, it has dissolved valuable nutrients but also the not so valuable salts that have been discarded by living soil. If this water were applied to soils that experience a good soaking several times per year, the salts would be washed further down the slopes, eventually ending in the sea.