soil is depleted there are two methods for restoring soil fertility (1)
The soil can be left idle for several years allowing it to rebuild
naturally or (2) Organic matter together with microbes and nutrients can
be applied from an external source. In the latter case, the microbes
metabolize the organic matter turning it into humus. This process
replenishes and maintains long term soil fertility by providing optimal
conditions for soil biological activity.
soil has been self perpetuating for
millions of years without man's help. It will continue to do so if we
practice sustainable agriculture.