IMPROVEMENT OF THE SOIL BY IRRIGA''nON. 479 



cases a crop may appear to fertilize the land more after it has been eaten 

 and digested, than if it had been ploughed in green, and we can recog- 

 nize the correctness of the opinion at which practical men have arrived. 



But theory does not forsake us here. As in all other cases in which 

 it furnishes a true explanation of known facts, it points to new facts also, 

 which more or less modify our received opinions, and define the limits 

 within which their truth can be rigorously maintained. Thus — 



1°. Theory says that if the animals fed upon the green crop be in a 

 growing state — if they be increasing in muscle or in bone — they will 

 not only dissipate through their lungs and skin a portion of its carbon, 

 but will retain also a part of its nitrogen and saline matter, and will 

 thus return to the soil, in their excretions, a smaller quantity of these 

 substances than the crop would have given to i* if ploughed in green. 

 If, therefore, a maximum fertilizing effect is to ^e produced upon a 

 field by eating off a green crop, it is not altogetner a matter of indif- 

 ference what kind of animals we employ as digesters. 



2°. Again, the practice of green manuring is resorted to chiefly 

 upon soils which are poor in organic matter — to which the carbon of 

 the green crop is of consequence, as well as the nitrogen and saline 

 matter it contains. But when eaten off, much carbon is lost to the 

 soil, and thus the supply of organic matter which it ultimately gets is 

 considerably less than if the crop it bore had been ploughed in in the 

 green state. Such soils, then, cannot be equally enriched by the 

 former as by the latter method. 



This case presents a very interesting illustration, and one which you 

 can readily appreciate, of the kind of useful information which thioreti- 

 cal chemistry is capable of imparting upon almost every branch of prac- 

 tical agriculture. It says to the farmer — yes, you may in some cases, 

 certainly, eat off the crop, with advantage — but if you wish to do most 

 good to your land you must eat it off with fattening, not with growing 

 sheep — and you must do so -upon soils which are not very poor in 

 vegetable matter. And that explains to me also, says the practical 

 man, in reply, why I have always found sheep-folding to be most be- 

 neficial on soils which are rich in vegetable matter* (p. 468.) 



§ 20. Of the improvement of the soil by irrigation. 



Irrigation, as it is practised in our climate, is only a more refined 

 method of manuring the soil. In warm climates, where the parched 

 plant would wither and die unless a constant supply of water were 

 artificially afforded to it, irrigation may act beneficially by merely 

 yielding this supply to the growing crops ; but in our latitudes only 

 a small part of its beneficial effects can be ascribed to this cause. It 

 is to pasture and meadow land almost solely that irrigation is applied 

 by British farmers, and the good effect it produces is to be explained 

 oy a reference to various and natural causes. 



1°. If the water be more or less muddy, bearing with it solid matter 

 which deposites itself in still places, the good effects which follow its 



* Sprengel explains this fact by alleging that the humic acid of the vegetable matter re- 

 tains more effectually the ammonia of the decomposing dung. There may be something 

 in this, but more, in mcst cases, I think, in the fact that digestion separates much of the 

 carbon in which the soils abound, but returns the nitrogen and saline matter almost en- 

 tirely and in a more active state. 



