On Irrigation. 465 



ascribe the fertilizing value to the carbonic acid cTissolved in the 

 water. Others even maintain that it is the carbonic acid in the water 

 which does the mischief which is occasionally observed. Further the 

 warmth of the water is by some considered the sole fertilising agent, 

 whilst others do not hesitate to say that temperature has nothing to do 

 with the beneficial effects. I might have alluded to other matters, as 

 showing that there is a great diversity of opinion prevalent upon this 

 subject, but it is not necessary. Let us then inquire which of these 

 views are right, and which are evidently founded on misconceptions or 

 prejudice. 



To render my subject more perspicuous, it may perhaps be well 

 to glance for a moment at the action of waters on soils, for I have 

 noticed that in all the papers which treat on the causes of the efficiency 

 of water for irrigation, the soil is left altogether out of consideration ; 

 yet a very few illustrations will show you at once that it is in vain 

 to discuss merely the qiialities of v/ater, -without reference to the 

 quality of the soil. We know that in this immediate neighbourhood, on 

 the London clay, heavy showers of rain, which in a measure irrigate 

 the land — for sometimes the water remains for days upon our meadow 

 lands — have more effect than even London manure. It is a well- 

 known fact that in bad seasons — that is, when there is little rain in the 

 spring — manures applied to the land have no effect ; and in the favour- 

 able seasons, when heavy rains fall during the spring months, manures 

 are not required, because the rain does more good than all the manures 

 the farmers can possibly apply to land so tenacious as the London 

 clay. Here evidently the water dissolves the fertilising matter out of 

 the soil. It must render soluble a very large quantity, for we obtain a 

 large produce of grass. On the other hand, we find that even on well- 

 manured soils of a purely sandy character, the water, when it falls 

 copiously in the spring, has the effect of washing-in such soluble 

 matters as nitrate of soda, and even guano. Whilst, then, on sandy 

 land pure water does produce a decidedly beneficial effect, on other 

 land it may have the contrary effect ; and further, what pure rain-water 

 does will also in a measure be done by natural spring-waters. Let 

 us then glance for a moment in a general way at the action of water 

 on the soil, to assist us in arriving at a satisfactory conclusion with 

 regard to the water which is most useful for irrigation. 



In the first place, I notice that water carries air into the soil. I say 

 into the soil, for I take it to be a well-recognised principle that on 

 irrigated meadows the water should not merely flow over the soil, but 

 . also percolate through the soil. The soil, for this reason, must be 

 porous ; not only its surface drained, but its imder-drainage must be 

 either naturally good, or rendered perfect by art. In bringing down 

 air then into the soil, and with it fertilising matter — ammonia and 

 carbonic acid — from the atmosphere, water also carries along with it 

 chemical agents, v»^hich render both organic and mineral fertilisers 

 soluble and fit to become food for plants. First, the organic matters 

 are rapidly destroyed by the oxygen of the air; the nitrogenous 

 substances are converted into nitrates, which we know have a most 

 powerful stimulating effect on the growth of aU vegetable produce. 



2 H 2 



