172 MANURES. v 



The next point is to cause the water of rains to 

 pass through the soil. If it lies on the surface, or 

 runs off without entering the soil, it is not probable 

 that the fertilizing matters which it contains will all be 

 abstracted. Some of them will undoubtedly return 

 to the atmosphere on the evaporation of the water ; 

 but, if the soil contains a sufficient supply of absorb- 

 ents, and will allow all rain water to pass through it, 

 the fertilizing gases will all be retained. They will 

 be filtered out of the water, which will pass out ot 

 the drains almost pure. 



This subject will be more fully treated in Section 

 IV., in connection with under-draining. 



Besides the properties just described, the soil 

 ought to possess the power of admitting a free cir- 

 culation of air. To effect this, the soil should be 

 well pulverized to a great depth. If, in addition to 

 this, it be of such a character as to allow water to 

 pass through it, it will facilitate such a circulation of 

 air as is best calculated to give the greatest supply 

 of ammonia. 



CARBONIC ACID . 



Carbonic acid is received from the atmosphere, 

 both by the leaves and by the roots of plants. 



It is absorbed by the water in the soil, and greatly 

 increases its power of dissolving earthy plant food. 

 This use is one of very great importance, as it is 

 equivalent to making the minerals themselves more 

 soluble. Water dissolves carbonate of lime, etc., 



