26 PLANT FOOD MUST BE AVAILABLE TO BE USEFUL. 



on these two soils, although the sandy soil contained 

 enough potash for 30 successive crops, and the clayey seal 

 for 117 and more. 



The principle, that potash, phosphoric acid and nitro- 

 gen, to be useful to plants, must be in an available form, 

 refers with equal force to plant food contained in artificial 

 fertilizers, their agricultural value is dependent on and 

 measured solely by the available potash, available phosphoric 

 acid and available nitrogen, which they contain. Thus 

 crude phosphate rock, while very rich in phosphoric acid, 

 is nearly valueless as plant food, and the phosphoric acid it 

 contains must be made soluble (available) by treatment 

 with sulphuric acid (acidulated) and converted into "acid 

 phosphate," so that it may become useful. The same refers 

 to crude bones, which contain their phosphoric acid, for the 

 most part, in an unavailable condition, and must be con- 

 verted into acidulated or " dissolved bone " to become fully 

 effective. The potash in potash salts from the Stassfurt 

 mines is all soluble in water, and, therefore, readily avail 

 able to the crop. The potash contained in feldspar and 

 other natural products is insoluble and practically useless, 

 while that contained in organic matter is slowly available. 

 Nitrogen is also irregular in availability ; in the form of 

 nitrate of soda and sulphate of ammonia, it is readily soluble 

 and available; in dried blood, fish scrap and tankage, it is 

 more slowly available ; while in leather and wool wastes, 

 the nitrogen is so slowly available, that they are well nigh 

 worthless as plant food. 



