lilPROVEHENT OF SOIL BY MANURING. 63 



needed by plants, they are called complete mineral 

 manures. Wood ashes are a good specimen of such 

 a manure. If the supply of nitrogen is the chief pur- 

 pose of the manure, it may be called a nitrogenous 

 manure. 



The commercial value of artificial manures is esti- 

 mated from the quantity of phosphoric acid, potash 

 and nitrogen they contain. A sample of such a ma- 

 nure may be said to contain a given percentage of 

 potash, of "soluble phosphoric acid," or of "nitrogen, 

 equivalent to ammonia." The number of pounds of 

 each of these substances contained in a ton can then 

 be determined. By multiplying this by the prices at 

 which they can be bought from manufacturing chem- 

 ists, the estimated value per ton of the manure is 

 obtained. 



This expression is often misunderstood and its com- 

 mon use is unfortunate. 



It is not meant that if a farmer uses an artificial 

 manure his crops will be increased in value to the 

 amount the manure is said to be worth. This may or 

 may not be true. On soils already well supplied with 

 plant food it may not perceptibly increase the 

 yield. Thus in experiments with several kinds of arti- 

 ficial manures at the Agricultural Experiment Sta- 

 tion in Illinois, od the fertile prairie soils of that 

 State, and at the Station in Ohio, on equally fertile 

 river bottom soil, little or no increase of yield was 

 found by liberal use of manures which produce re- 

 markable results on many soils. Even if the crop is 

 increased by the use of the manure, the money value 

 of the increase varies greatly in different parts of the 

 country. 



