PHOSPHORIC ACID FERTILIZERS 175 



acid phosphate, which dates back more than half a century, 

 and has been accepted during that time as an improvement 

 over the use of untreated bone, which was largely super- 

 seded when the process of making acid phosphate was in- 

 vented. 



On most soils acid phosphate apparently gives the more 

 profitable immediate returns. On some of the rich soils of 

 the Middle West, however, there is an indication that 

 ground rock is a more economical source of phosphoric acid. 

 Except in those regions where the superiority of floats has 

 been demonstrated it is probably safer to use acid phosphate. 



229. Effect of phosphoric acid on plant growth. — As has 

 been previously stated, phosphoric acid is essential to the 

 growth of plants. It is absorbed by plants at a fairly uniform 

 rate throughout the period of their active growth, while nitro- 

 gen is largely taken up during the early stages of growth. 

 Nitrogen and phosphoric acid are closely associated in plant 

 development. 



One very apparent effect of phosphoric acid is to hasten 

 ripening. Cereal plants that receive an ample supply of 

 avail able phosphoric acid reach the heading stage and final 

 maturity sooner than do plants having an insufficient supply. 

 This may be an advantage in a climate having a cool short 

 season as it may help the crop to avoid frost in the fall. On 

 the other hand this rapid ripening may limit the yield 

 in a dry season, when there is a tendency for the crop to 

 shorten its growing periods sufficiently to curtail the quan- 

 tity of nutrients it absorbs and the food it elaborates. 



Root development is always stimulated by available phos- 

 phoric acid. Young plants send their roots more deeply 

 into the soil, which is an advantage in dry regions, where the 

 top soil dries out quickly. Under any circumstances it in- 

 creases the absorbing surfaces and benefits growth. 



The quality of many crops, particularly of pastures, is 

 improved by phosphoric acid. Animals reared on pastures 



