The farmer develops the most superficial part of the earth's crust, the soil, that is to say the portion of our globe which interests geology. However, the farmer should not lose interest in geology, because this science helps him in more than one circumstance. The arable land serves as a support for the plant; it is also in its bosom that the roots seek the mineral elements which serve to build the plant cell. Each harvest therefore removes a certain amount of mineral elements from the soil, which the farmer must return to it in various ways, either in the form of manure or in the form of mineral fertilizers or amendments. These mineral elements can be found either in the subsoil at depth (marl, carbonate of lime), or in nearby or distant quarries (gypsum stone, carbonate of lime, lime phosphate, sodium nitrate, potash and magnesia salts). The basis of any rational cultivation is nowadays the preliminary study of the physico-chemical and biological properties of the soil. This study of the soil is one of the most important approaches applied to agronomy.