21 



seems to be due to free humic acid, rather than to salts of this 

 acid. This point, however, has not been determined. 



VII. Humus is believed to possess great absorptive prop- 

 erties though experimental evidence supporting this view is 

 exceedingly limited and not very conclusive. The relation be- 

 tween humus and phosphoric acid is not well defined. 



VIII. Ferrous sulphate in conjunction with phosphates, 

 applied to some soils is beneficial to crops. It seems to increase 

 the availability of the phosphoric acid. In large quantities on 

 acid soils, it acts as a poison to plants. The injurious effect is 

 due in part to the formation of sulphuric acid. The same effect 

 may be produced in neutral soils, and soils not sufficiently basic 

 to neutralize all the acid formed. 



IX. Sand and orthoclase offer no resistance to the extrac- 

 tion of soluble phosphoric acid by pure water. 



