MANURES. 56 



and silica 28.70 lbs., the bean contains potash 16.56, and silica 

 2.20 lbs. in 1,000. 



Healthy and perfect plants of the same kind, will always be 

 found to contain the same number of these substances, and in the 

 same or very near the same proportions. This constancy of com- 

 position, whether the plants are grown upon the same or different 

 soils, is indisputable evidence that these substances are indispen- 

 sable. 



These portions of the vegetable structure are all derived from 

 the earth, and a fertile soil always contains them. Not always, 

 it is true, in the same proportions, but they must be there in some 

 quantity, or it is unproductive. The amount of some one may be 

 very small in comparison with the mass of soil, and yet in con- 

 nection with others give it a character of high fertility, as every 

 farmer has seen in the application of a minute quantity of gypsum 

 to a field of red clover. 



It will now be readily understood, to what this wearing out, or 

 degeneration of the soil is attributable. In successive crops these 

 substances are taken from the soil, and have not been restored in 

 equal quantity in the manures that have been applied. The 

 grain — hay — milk — butter — cheese — beef — wool, «&c., have been 

 taken to the market, and all that was contained in them of a mine- 

 ral nature, was so much robbed from the soil. The process of 

 deterioration may not have been apparent, for originally the supply 

 was large. But such a drain kept up for years has had the effect 

 of impoverishing the soil and leaving it at last, in a measure at 

 least, unproductive. The amount taken off in any one year was 

 small, but continued for a number of years makes a large quantity, 

 and if the whole has not been restored in the shape of manures 

 containing the very substances carried away, the land must even- 

 tually cease to be productive. At the same time another cause has 

 been operating to bring about the same result. By the processes 

 of cultivation some of these materials, which formerly existed in 

 the earth in an insoluble state, are rendered soluble. They are 

 taken up by the rain which falls, and carried down with it in its 

 passage to the subsoil. In this way lime is often entirely exhaus- 

 ted from a soil. 



It will be understood from what has been said, that all crops do 

 not exhaust the soil of the same substances in the same proportion. 



