PRINCIPLES OF MANURING 105 



which it dissolves from the soil in a definite 

 time, and under definite conditions, being 

 taken as an indication of the percentage of 

 these substances that is immediately available 

 as the food of crops. It cannot be denied 

 that the results thus obtained are of con- 

 siderable value as a guide to manuring, 

 especially under extreme conditions ; that 

 is to say, such a method of analysis enables 

 one to conclude with considerable confidence 

 whether a soil undoubtedly requires or does 

 not require the addition, let us say, of 

 phosphoric acid or potash. But in the case 

 of a soil which is neither very rich nor very 

 poor in these substances, it is doubtful 

 whether such determination of " availability " 

 is of much value as a guide in the practice of 

 manuring. 



While it cannot be said that we apply 

 manure to land in order to grow a crop, or 

 to furnish something previously non-existent, 

 it can with confidence be said that manuring 

 is undertaken in order to obtain a full crop, 

 in other words, a crop that is economically 

 profitable. It has always to be remembered 

 that a large part of the yield of any crop 

 goes to meet the standing expenses of the 

 farm ; and if the yield is only sufficient 

 to meet such expenses it is evident that 



