24 THE CHEMISTRY OF THE FARM. 



that has reared them. Provision must therefore be 

 made, sooner or later, to return to the land a part at 

 least of the plant food removed from it, if permanent 

 fertility is to be maintained. Hence the necessity for 

 manuring. 



The most complete return to the land would be ac- 

 complished by manuring it with the excrements of the 

 men and animals consuming the crops. This is partially 

 done by the application of farmyard manure ; but the 

 congregation of men in cities, and the difficulty of employ- 

 ing sewage with profit, prevent this plan being thoroughly 

 carried out. The farmer is thus generally obliged to 

 purchase manures for the land in exchange for the crops 

 and stock sold off it. 



On very poor soils it is necessary to make a very com- 

 plete return of all the elements of plant food removed by 

 the crops, but in most soils there is an abundance of some 

 one or more of these elements, and a partial manuring will 

 consequently suffice. With high farming the contribu- 

 tions to the soil may be in excess of the exports, and the 

 land consequently increase in fertility. The nature of the 

 exhaustion resulting from the growth of particular crops, 

 and the economic application of manure to meet their 

 special requirements, will be considered in Chapter IV. 

 The losses which a farm sustains by the sale of animal 

 products will be treated of in the section on " The Con- 

 stituents of Animals," page 61. 



Parmyard Manure consists of the liquid and solid 

 excrements of the farm stock, plus the straw employed as 

 litter. Its composition will vary according to the charac- 

 ter of the animals contributing to it, the quality of their 



