|Abstract||In its broad meaning soil is that friable upper layer of the earth composed for the most part of mineral matter resulting from the breaking up and decay of rocks. It is thus the product of rock weathering, brought about by the action of the destructive forces of nature. These forces include the stresses set up in the rock mass due to alternate heating and cooling and the action of running water as an abrasive agent, assisted by the sand and gravel which it carries along. In cooler regions the water which enters the cracks of the rock may become frozen, and the force exerted in this way tends further to open up the cracks and hasten the break-down of the rock. Under humid tropical conditions the rock decay is effected chiefly by the chemical action of water, aided by gases such as oxygen and carbonic acid which it carries in solution. The products of decay of vegetation are frequently acid substances which also exert a solvent influence.