U8K OF MANURES 145 



CHAPTKIJ XXVH.— Use of Manures 



137. Purpose of Manures. — The purpose of applying 

 manures is always to increase the yield of cultivated 

 crops. The manures may be applied to furnish food 

 for some one particular crop, or they may be used to 

 increase the fertility of the soil, but in each case the 

 object is the same. Manures that are used to furnish 

 plant food for some one particular crop are called 

 sii< fal manures. Manures that are used to improve 

 the condition of the soil are called GENERAL manures. 

 For special manures only such compounds as furnish 

 easily available plant food are used. For general 

 manures substances sre used which increase the store 

 of plant food in the soil, either by being gradually 

 changed into plant food themselves, or by forming 

 available compounds with the unavailable plant food 

 already in the Boil. The process of increasing the fer- 

 tility of worn soils is called sou. RESTORATION. 



138. Restoration of Worn Soils. — A worn soil is one 

 that for some reason fails to produce profitable < 

 Thil failure to produce crops may triee from poor culti- 

 vation, or it may be that one or all of the different 

 compounds supplying available plant food are lacking; 

 but whatever the cause, the trouble should l>e known 

 before a cure is attempted. The successful physician 

 always learns, if possible, the nature of his patients 

 illness before attempting a cure, and so should those 



10 



