ENKinilNG SUIL— COMMnRClAL HESOrUCES 93 



3. Phosphoric Arid 



140, IMiosplioru' acid is, noxt tt) nitropfon, tlie 

 most important plaiit-fnod to lit> applitMl to 

 land, and of tlio niintTal const ilucnts it is the 

 most important. It is a coiistitncnt of all soils, 

 thonLch the amount may he variahlc It is par- 

 ticularly needed to insure hardiness and fruit- 

 fulness. Consequently tlie dilTerent grain crops 

 are larire us»'rs of phos]>hori(' acid. A liheral 

 supply of available phosj)horic acid is necessary 

 to young plants to give them strength and 

 vigor. 



141. As humus decays or decomposes in the 

 soil it n«>t only siijiplics nitrogen, but it also 

 makes some of the phosphoric acid available. 

 Hence when the hunuis diminishes in the soil, 

 there is often a correspouding lack i>f available 

 phosphoric acid. Ram niamires 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 su(di soils phosphoric acid may be suj)plied 

 in an available form in acid phosphates. 



14J. Pure phosj)horic acid (P2O5), however, is 

 not use<l 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 



