122 The Potato 



PHOSPHORUS 



Phosphorus, usually referred to under the name of its 

 form as phosphoric acid, or P2O5, is often in very small 

 quantities in American soils, even in those abundantly 

 supplied with other elements of plant-food.^ Its actions 

 on growth are not fully understood, but are very im- 

 portant and are becoming more and more appreciated. 

 A deficiency in soluble phosphoric acid quickly reduces a 

 crop yield. Unless a sufficient amount is present, plants 

 do not mature properly. Its use tends to produce early 

 maturity and is marked by dark green foliage. 



The principal supply for use in fertilizers comes from 

 deposits in the South which are supposed to be derived 

 from fossil bones. When ground finely, this material is 

 sold under the name of "floats" or "raw rock phosphate." 

 It is much used in the corn belt states and gives good 

 results there on general farm crops. It is too slow- 

 acting for the more delicate potato crop, and has not given 

 very good results in most experiments in the colder north- 

 eastern states. Acid phosphate is made by treating this 

 raw rock phosphate with sulphuric acid. This unites with 

 part of the lime in natural coml)inations with phosphoric 

 acid. Instead of the insoluble jjhosphate in which three 

 parts of lime are united to each part of phosphoric acid, 

 there is formed the water-soluble form in which only one 

 part of phosphoric acid is united to each part of lime, to- 

 gether with a small proportion of the reverted form soluble 

 in weak soil acids in which two parts of lime are united 

 to each part of phosphoric acid. The sum of these makes 

 up the available r205. Acid phosphate is also made from 

 animal bones, but the supply is only enough for a small 



' See Bulletins by Hopkins, 111. Agri. Expt. Sta. 



