No. 2. 



Improvement of the Soil by Irrigation. 



57 



Improvement of the soil by Irrigation. 



We take pleasure in transferring the following arti- 

 cle to our pages. We find it in the third part of John- 

 ston's Lectures on Agricultural Chemistry and Geolo 

 gy, which a friend has placed in our hands. It treats 

 briefly, but understandingly, of a subject to which 

 we have several times latterly called the attention of 

 our readers; and which, we apprehend, is not suffi- 

 ciently attended to. Many situations through the 

 country, admit of this mode of enriching the soil, 

 which are yet not appreciated, and their advantages 

 are suffered to remain unused. Irrigation is one of 

 the cheapest and most efficient modes of keeping up 

 and enriching pasture lands. It is also, where it can 

 be practised, the neatest, and as our lecturer observes 

 the most "refined" of all modes of manuring.— Ed. 



Irrigation, as it is practised in our cli- 

 mate, is only a more refined method of ma- 

 nuring the soil. In warm climates, where 

 the parched plant would wither and die un- 

 less a constant supply of water were artifi- 

 cially afforded to it, irrigation may act bene- 

 ficially by merely yielding this supply to the 

 growing crops ; but in our latitudes only a 

 small part of its beneficial effects can be as- 

 cribed to this cause. It is to pasture and 

 meadow lands almost solely, that irrigation 

 is applied by British farmers, and the good 

 effect it produces is to be explained by a 

 reference to various natural causes. 



1°. If the water be more or less muddy, 

 bearing with it solid matter which deposits 

 itself in still places, the good effects which 

 follow its diffusion over the soil, may be as- 

 cribed to the layer of visible manure which 

 it leaves everywhere behind it. Thus the 

 Nile and the Ganges fertilize the lands over 

 which their annual floods extend, and partly 

 in this way do some of our smaller streams 

 improve the fields over which they either 

 naturally flow or are artificially led. 



2°. Or if the water hold in solution, as 

 the liquid manures of the farm-yard do, sub- 

 stances on which plants are known to feed, 

 then to diffuse them over the surface is a 

 simple act of liquid manuring, from which 

 the usual benefits follow. Such is the irri- 

 gation which is practised in the neighbour- 

 hood of our large towns, where the contents 

 of the common sewers are discharg-ed into 

 the waters which subsequently spread them- 

 selves over the fields. In so far also as any 

 streams can be supposed to hold in solution 

 the washings of towns or of higher lands — 

 and there are few which are not more or 

 less impregnated in this manner — so far may 

 their beneficial action, when employed for 

 purposes of irrigation, be ascribed to the 

 same cause. 



3°. But spring waters which have run 

 only a short way from their source, are oc- 



casionally found to be valuable irrigators. 

 In such cases also, the good effect may be 

 due in whole or in part, to substances held 

 in solution by the water. Thus, in lime- 

 stone districts, and especially those of the 

 mountain limestone formation — in which 

 copious springs are not unfrequently met 

 with — the watrt- is generally impregnated 

 with much carbonate of lime, which it 

 slowly deposits as it flows away from its 

 source. To irrigate with such water is, in 

 a refined sense, to lime the land, and at the 

 same time to place within the reach of the 

 growing plants, an abundant supply of this 

 substance, in a form in which it can readily 

 enter into their roots. 



In other districts, again, the springs con- 

 tain gypsum and common salt, and sulphate 

 of soda and sulphate of magnesia, and thus 

 are capable of imparting to plants many of 

 those inorganic forms of matter, withoiit 

 which, as we have seen, they cannot exhibit 

 a healthy growth. 



4°. Again, it is observed that the good 

 eflfects of irrigation are produced only by 

 running water — coarse grasses and marsh 

 plants springing up when the water is al- 

 lowed to stagnate. This is explained in 

 part by the fact, that a given quantity of 

 water will soon be deprived of that portion 

 of matter held in solution, of which the 

 plants can readily avail themselves, and 

 that when this is "the case it can no longer 

 contribute to their growth in an equal de- 

 gree. 



But there is another virtue in running 

 water, which makes it more wholesome to 

 the living plant. It comes upon the field 

 charged with gaseous matter, with oxygen 

 and nitrogen, and carbonic acid, in propor- 

 tions very different from those in which 

 these gases are mixed together in the air. 

 To the root, and to the leaf also, it car- 

 ries these gaseous substances. The oxy- 

 gen is worked up in aiding the decompo- 

 sition of decaying vegetable matter. The 

 carbonic acid is absorbed by and feeds the 

 plant. Let the same water remain on the 

 same spot, and its supply of these gaseous 

 substances is soon exhausted. In its state 

 of rest it re-absorbs new portions from the 

 air with comparative slowness. But let it 

 flow along the surface of the field, exposing 

 every moment new particles to the moving 

 air, and it takes in the carbonic acid espe- 

 cially with much rapidity— and as it takes 

 it from the air, almost as readily again gives 

 it up to the leaf or root with which it first 

 comes into contact. This is no doubt one 

 of the more important of the several pur- 

 poses which we can understand running 

 water to serve when used for irrigation. 



