20 Market Gardening. 



being porous, the air forces its way into the crevices, 

 and the water (of rains) passes through it from above. 

 We will suppose that the water comes to a stratum that 

 is impervious to its onward course. What happens ? 

 Simply this : it dams up slowly, inch by inch, forcing 

 out the air as it rises. All motion and circulation is 

 stopped. Fermentation and decomposition soon begin. 

 The earth is drowned out — suffocated — dead for 

 want of air. Water is good for the ground ? Yes ; but 

 not in this way. The water must be moving constantly. 

 There must be a current of air and water, and not too 

 much or too little of the latter." 



The processes of pulverization, which will be de- 

 scribed under the title " Preparation of Soil," are such 

 as to provide for the admission of the air. We shall 

 now consider, in the remainder of the present chapter, 

 by what means we may supply the needful amount of 

 the other indispensable element, water. By drainage 

 we provide for the removal of a surplus, whether from 

 rains or springs ; our next care must be for supply and 

 distribution of a quantity adequate, in the longest and 

 severest droughts, to the exacting needs of the grow- 

 ing crops. This supply and distribution constitute 

 what is intended by the term " Irrigation." 



Irrigation of Crops. 



Artificial watering, especially as now conducted, 

 forms perhaps one of the most important subjects that 

 we could write upon. All vegetables are composed 

 largely of water, some containing more than 75 per 

 cent. A single hill of cucumbers, as has been said, 



