328 



MANURES. 



mode of action, or they may partake of both of these cha- 

 racters. For instance, barn-yard manure is both mechanical 

 and chemical in its effect. 



By reason of its bulk and its coarseness it loosens the soil 

 and makes it more porous when mixed with it ; when it is 

 used as a top-dressing it shades the ground, and protects it 

 in a measure against the effect of frost and of too great heat ; 

 being a very active absorbent of moisture, it modifies the 

 effect of drought; its decomposition produces heat, and 

 raises the temperature of the soil. 



All of these are mechanical effects. 



On the other hand, it affords to the roots of plants sub- 

 stances which enter directly into their structures, as chemical 

 constituents ; it also yields various acids, alkalies, and salts 

 which enter into combination with the constituent parts of 

 the soil, and in one way or another make them more 

 available as plant food. 



These are chemical effects. 



The use of Manures. 



In the use of manures the farmer should be guided not 

 only by the effect that will be produced on the immediate 

 crop although this is, of course, the first consideration 

 but quite as much by the condition in which the soil will be 

 left for the production of future crops. Unless he does this 

 he may find that, while he has reaped a temporary benefit, 

 he has inflicted a lasting injury on his fields. 



It will be remembered that in our account of the soil it 



