80 THE rKINCIl'LES 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 Avhich are needed to 

 unite chemically with other sul^stances already con- 

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

 serviceable. 



When roots of growing plants arc present, these nutri- 

 tive suljstances are immediately aI)Sorbed, so far as they 

 are needed. If formed faster than needed, tlic surplus 

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

 rains and wasted. Some varieties of soil, particularly 

 those containing clay or vegetable mohl, 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 nifroj/cn, plioftphoric arid, and 

 pofasiJt. 



Some peculiar soils may he wanting in soiue other 

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



