100 SOILS AND FERTILIZERS 



in many soils, and that this element should be included in the list of 

 limiting factors in crop production. 



Fertilizers. A fertile soil contains in the requisite 

 amount all the elements not directly derived from the 

 air that are necessary for the nutrition of plants in a 

 form easily available by them. Crop-growing, among 

 other causes, tends to reduce this fertility. The break- 

 ing up and decay of rock and the deposits from the 

 air brought down by rain and snow will restore some- 

 thing of this fertility, so that crops may still be grown 

 upon the soil ; but if the farmer wishes the maximum 

 yield from his land, he must supplement nature's work 

 by himself, supplying in one form or another, directly 

 or indirectly, the exhausted elements to the soil, that 

 is, by adding fertilizers. 



Classes of Fertilizers according to Elements. When 

 a soil has sufficient moisture, there is generally little 

 or no need of supplying any of the elements of plant 

 tissue except nitrogen, phosphorus, potassium, and 

 occasionally calcium, for the others will be present 

 in the soil in soluble form in sufficient quantities 

 for the plant. But the four named are often lack- 

 ing or, being present in the soil, are not in available 

 form, in which case they must be supplied or made 

 available if profitable crops are to be grown. The 

 fertilizers that supply the first three are called ni- 

 trogenous, phosphoric, potassic, according as nitrogen, 

 phosphorus, or potassium predominates in the com- 

 pound. Calcium is found in every part of the plant, 

 more lime being found in the leaves than in the 

 other parts. Its artificial application will be discussed 

 later. 



Conservation of Soil Fertility. It is possible by 



