168 SOILS AND MANUEES 



rally disposed of in this way. If they are sold off the farm 

 the whole of the ingredients are lost, and the soil is im- 

 poverished to that extent. In the case of corn crops the 

 grain alone is usually sold ; the straw is retained, and either 

 fed to the stock or used as litter, and so ultimately returned 

 to the soil. 



The introduction of animals enlarges the cycle of changes 

 thus : 



Crops - > Stock 



Soil < - Muck 



Starting from the soil the arrows show the course of the 

 plant food. It is absorbed first by the crops and then by 

 the stock, or the plants may undergo decomposition 

 directly, i.e., without passing through the bodies of 

 animals. Generally some goes in both ways, but, in any 

 case, it is all finally returned to the soil. A certain 

 amount of plant food may be withdrawn from the cycle 

 in the form both of vegetable and of animal products, and 

 the fertility of the soil will not be diminished unless the 

 amount withdrawn exceeds what becomes available in the 

 same time. 



Quantity of Available Plant Food required by Crops. 

 The following table shows the average weight per acre 

 of the ordinary farm crops and the quantities of nitro- 

 gen, phosphoric acid, potash, soda, lime, magnesia, and 

 silica contained in them. 



