124 Composition of Waters of Land-Drainafjc and of Rain. 



in the shape of manure has been added to the soil ; the carbonic 

 -acid so formed, assisted by that which naturally exists in the 

 xiir, gradually breaks down and decomposes the minerals of which 

 the soil is composed, rendering them soluble and available for 

 vegetation. In a similar way, of course, it affects such mineral 

 substances as may be added in manure. In this manner, there- 

 fore, drainage produces a more abundant supply of different 

 substances necessary for the growth of plants. But as the water 

 which passes through soils in its way to the drains would carry 

 away with it everything which, under such circumstances, it 

 was capable of dissolving, and as in the absence of any cause 

 operating to prevent it, this water would also remove all the 

 soluble matters of manure, it becomes of great importance to 

 *iscertain whether there really are any preservative causes at work 

 to counteract so very serious a mischief. 



This question has been, to a great extent, answered in the 

 afhrmative, by the discovery of the absorptive property of soils, 

 which enables them to convert into comparatively insoluble com- 

 pounds all, or nearly all, those salts which are valuable as 

 manure. From these experiments we should predicate that 

 ■drainage-water would contain a certain portion of all those sub- 

 stances which are necessary to vegetation, because some degree 

 of solubility is indispensable to everything which is to form the 

 food of plants ; but we should not expect to find them in such 

 <j^uantity as would be the case were there no provision for their 

 retention by the soil itself. 



We shall presently see how far these anticipations are borne 

 out by the result. To obtain a clear notion of the effect of 

 drainage upon the soluble matters of the soil we shall do well to 

 consider separately the following points: — 



First, The quantity of rain that falls, and how much of that 

 <juantity finds its way into the drains. 

 Second, The composition of this water. 



Third, Where the substances (if any) contained in drainage- 

 water are derived from ; and. 



Fourth, What circumstances are likely to increase or diminish 

 the waste from such cause. 



1st. The quantity of rain fallinfi and percolating throur/k soils. — 

 The rain-fall, as every one knows, varies very much in different 

 places. As regards Great Britain, the quantity of rain falling in 

 the west considerably exceeds that which has been observed in 

 the east and centre. Thus I find the following stated as the 

 average annual fall of rain, in inches, in some of the western 

 counties, from north to south, of the kingdowi : — * 



* Morton's 'Cyclopsedia of Agriculture,' article 'Climate,' page 475. I have 

 omitted fractions in the quotations above. 



