xr.] PLANT FOOD REMOVED FROM A FARM 303 



fertilisers into crops, and steadily increases the fertility 

 of his soil by putting on more plant food every year 

 than he removes in his crops. 



We can begin by considering what is necessary to 

 maintain the condition of the land under a conservative 

 system of farming, and we may take the case of a farm 

 under a four-course rotation, where nothing but corn 

 and meat are sold and all the dung goes back to 

 the land. Under such conditions, as we have already 

 learnt, the feeding animals only retain about 10 per 

 cent, of the fertilising constituents of the food they 

 consume ; the other 90 per cent, comes back in the 

 manure and wholly or in part reaches the land again. 



Table LXXXVIII. — Fertilising Constituents removed from 

 Farm in Corn and Meat Sold. 



In this way the land loses 226 lb. of nitrogen per 

 acre per annum ; but this estimate fails to take into 

 account the very considerable losses that occur during 

 the making of the farmyard manure, which may be 

 estimated at 50 lb. in the four years, and those due to 

 drainage and bacterial action. On the other hand, the 

 nitrogen contained in the clover crop has been obtained 



