ENRICHING SOIL COMMERCIAL RESOURCES 93 



3. Phosphoric Acid 



140. Phosphoric acid is, next to nitrogen, the 

 most important plant -food to be applied to 

 land, and of the mineral constituents it is the 

 most important. It is a constituent of all soils, 

 though the amount may be variable. It is par- 

 ticularly needed to insure hardiness and fruit- 

 fulness. Consequently the different grain crops 

 are large users of phosphoric acid. A liberal 

 supply of available phosphoric acid is necessary 

 to young plants to give them strength and 

 vigor. 



141. As humus decays or decomposes in the 

 soil it not only supplies nitrogen, but it also 

 makes some of the phosphoric acid available. 

 Hence when the humus diminishes in the soil, 

 there is often a corresponding lack of available 

 phosphoric acid. Barn manures make available 

 a considerable quantity of phosphoric acid. Soils 

 which contain a fair supply of humus do not 

 necessarily have enough of phosphoric acid. 

 To such soils phosphoric acid may be supplied 

 in an available form in acid phosphates. 



142. Pure phosphoric acid (P 2 O 5 ), however, is 

 not used directly as a plant -food, but only when 

 it is combined with some other substance, as 

 lime. One of the chief sources of phosphoric 

 acid is bone, in which it is found combined 



