SOIL 103 



soil die upon it and return to it that which they took from 

 it. But from cultivated soil, crops are removed year after 

 year, and the materials which the crop-plants took from the 

 soil are not returned to it as they are in nature. 



The elements essential to plant life which are obtained 

 from the soil are phosphorus, potassium, magnesium, cal- 

 cium, iron, sulphur, and nitrogen. Iron, magnesium, and 

 sulphur are present in such great amounts beyond what 

 the plant needs that there is no problem so far as these 

 are concerned. It is about the supply of phosphorus, 

 calcium, potassium, and nitrogen that the farmer has 

 to concern himself. 



C. Fertilizers and Cultivation. Very early in the his- 

 tory of agriculture it was found that the crop-producing 

 power of soil is increased by the addition of various kinds 

 of substances. These substances, when applied to soil, 

 are called fertilizers. Manure is the most widely used of 

 artificial fertilizers. Humus may be regarded as a sort of 

 natural fertilizer. Bone-dust, rock-phosphate, lime, and 

 various specially prepared fertilizers are also widely used. 

 The original idea in the use of these fertilizers was, of 

 course, to enrich the soil in those substances which had 

 been removed from it in the form of crops. It was be- 

 lieved that plants used the fertilizers as " food." It has 

 been found, however, that fertilizers may produce their 

 good effects without acting as sources of food-material. 

 Thus they may cause the destruction of poisonous sub- 

 stances given off by the root-hairs of previous generations 

 of plants. (See page 79.) Or they may stimulate the 

 growth in the soil of certain microscopic forms of life which 

 are essential to its fertility. Or they may change the gen- 



