80 THE PRINCIPLES OF AGRICULTURE. 



tent in nearly all varieties of soil, is largely composed of 

 the elements of plant food, which are gradually rendered 

 available, year after year. 



Chemical action, or the formation of plant food in these 

 substances, is checked by cold weather, but goes on con- 

 tinuously in the summer season. It is aided by a proper 

 supply of moisture in the soil, by the oxygen and car- 

 bonic acid of the atmosphere, and by the small quan- 

 tities of ammonia and nitric acid which are brought to 

 the soil in rain. 



An application of fertilizers to the soil not only di- 

 rectly supplies the elements of plant food, but is also 

 useful in furnishing substances which are needed to 

 unite chemically with other substances already con- 

 tained in the soil, in order that the latter may become 

 serviceable. 



When roots of growing plants are present, these nutri- 

 tive substances are immediately absorbed, so far as they 

 are needed. If formed faster than needed, the surplus 

 is either retained for future use, or is washed away by 

 rains and wasted. Some varieties of soil, particularly 

 those containing clay or vegetable mold, are able to 

 retain large quantities of these elements for a long time, 

 but from loose, gravelly, or sandy soil they are easily 

 washed away. 



The Elements Needed in Fertilizers. A majority of the 

 elements of fertility are contained in most soils in 

 sufficient quantities to last many years. Those which 

 generally fail the soonest, and which we must aim to 

 supply in fertilizers, are nitrogen, phosphoric acid, and 

 potash. 



Some peculiar soils may be wanting in some other 

 substance, as sulphur or lime ; but when soils begin to 



