36^ RATE OF EXHAUSTION. 



to see that a crop of 25 tons may in truth take more 

 inorganic matter out of the land than one of 30 tons; 

 and that the mere weight of the crop is no proof of 

 the extent to which it has exhausted the soil. 



987. This effect is quite independent of the mere 

 quantity of water in the crop, because the same result 

 is shown when we ascertain the relation which exists 

 between the inorganic matter and the dry organic 

 matter which the plants contain; in different crops of 

 mangel-wurzel it is found that the quantity of dry 

 organic matter which is formed for every 100 parts of 

 inorganic matter absorbed from the soil, varies from 

 568 to 1045. 



988. It is consequently evident that, in order to 

 raise the most profitable crops, we must endeavor to 

 obtain the largest possible quantity of useful veget- 

 able matter, with the smallest relative proportion of 

 inorganic matter. This is a subject of the greatest 

 practical importance, and one well worthy of the 

 attention of scientific agriculturists. 



989. Having now briefly stated the average com- 

 position of some of the most important plants culti- 

 vated as crops, it may be useful to enter shortly into 

 the composition of the most common substances em- 

 ployed as manure, in order more fully to show the 

 relation which exists between plants and the manures 

 employed in their cultivation. Of the simple animal 

 manures the most important are dung, urine, and 

 bones. 



990. The dung of sheep (according to Zierl) con- 



