70 THE CHEMISTRY OP THE FARM. 



The quantities of plant food required by different crops 

 are given in the table on pp. 48 49; these also furnish 

 reasons for the alternation of crops. It will be seen, for 

 instance, that the cereals require but little potash and 

 lime, while root crops, beans, and clover demand a large 

 supply ; it is obvious, therefore, that the resources of the 

 soil are husbanded by growing these two classes of crops 

 in alternation, the greater demand for. potash and lime 

 thus falling every alternate year. 



The net result of a judicious alternation of crops, in 

 which the special characteristics of each are turned to good 

 account, is the production of a maximum total yield of 

 produce with a minimum amount of manure. 



Losses to the Land during Rotation. The table 

 showing the composition of ordinary farm crops will 

 supply the requisite information as to the loss which a 

 farm may suffer by the sale of individual crops. We will 

 now consider briefly the losses during a rotation. 



The conservation of plant food on a farm is generally 

 effected by confining the exports to corn and meat, the 

 rest of the produce being consumed on the farm, and the 

 manure returned to the land. Let us assume that a farm 

 is managed on the four-course system, and that the 

 average crops obtained per acre are swedes, 14 tons; 

 barley, 40 bushels ; seeds (half clover, half grass), 3 tons 

 of hay, and wheat, 30 bushels. Further, that nothing is 

 sold save corn and meat ; that 2 bushels both of wheat 

 and barley are returned to the land as seed ; that 700 Ibs. 

 of linseed cake are fed with each acre of swedes ; that 

 1 10 Ibs. of oats are purchased per acre per annum for the 

 horses. Finally, that half a ton of straw is fed per acre 



