A GOOD DRAINAGE NECESSARY. 48* 



absorbed by and feeds the plant. Let the same water remain on the 

 same spot, and its supply of these gaseous substances is soon ex- 

 hausted. 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 especially 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 in contact. This is no doubt one of the 

 more important of the several purposes which we can understand 

 running water to serve when used for irrigation. 



But further, if water be allowed to stagnate over the finer grasses, 

 they soon find themselves ih circumstances in which it is not consist- 

 ent with their nature to exhibit a healthy growth. They droop, 

 therefore, and die, and are succeeded by new races, to which the wet 

 land is more congenial. 



5^. It is known also, that even running water, if kept flowing with- 

 out intermission for too long a period, will injure the pasture. This 

 is because a long immersion in water induces a decay of vegetable 

 matter in the soil which is unfavorable to the growth of the grasses — 

 producing chemical compounds which are not naturally formed in 

 those situations in which the grasses delight to grow, and which are 

 unwholesome .to them. Although, therefore, the water continues to 

 support those various kinds of food by which the grasses are benefit- 

 ted, yet it becomes necessary to withdraw it for a time, in order that 

 other injurious consequences may be avoided. 



6°. Lastly. — Irrigation is most beneficial where the land is well 

 drained beneath — where the water, after the irrigation is stopped, can 

 sink and find a ready outlet. The same benefits indeed flow from the 

 draining of irrigated as from that of arable lands. The soil and sub- 

 soil are at once washed free of any noxious substances they may 

 naturally contain, or may have derived from the crops they have 

 grown, and are manured and opened by the water which passes 

 through them. As the water descends also, the air follows it, to 

 change and mellow the under-soil itself 



Such are the main principles upon which the beneficial action of 

 irrigation depends, and they appear to me satisfactorily to account 

 for all the facts upon the subject with which I am acquainted. I 

 pass over the alleged beneficial action of water in keeping the tem- 

 perature of irrigated fields from sinking too low. As irrigation is 

 practised in our islands, little of the good done to watered meadows 

 can be properly attributed to this cause. 



1 have now drawn your attention to the most important and readily 

 available means, mechanical and chemical, for improving the soil. 

 Let us next study the products of the soil — their composition, their 

 diflferences, and the purposes they are intended to serve in the feed 

 ing and nourishment of animals. 



