Irrigation. 155 



broken down into clay (hydiated aUiminium silicate), lime, and 

 sodium carbonate (especially where the soils are of doleritic origin). 

 The action of irrigation water is partly then to fine down the soil, 

 making it more clayey, and frequently to cause the production of 

 carbonate of soda, a substance that tends to deflocculate the soil 

 granules, making the soil more or less impervious to both air and 

 water. 



(b) On soil granules or aggregates of soil particles : A normal soil 

 has its particles aggregated into little crumbs composed of fine and 

 coarse particles. These crumbs are porous, and capable of holding 

 a large amount of water in the capillary (useful to plants) state. 

 When water is allowed to stand on such granulated soils, the tendency 

 is for the crumbs to break down into separate particles, so that when 

 the water dries up the soil no longer has its desirable crumb structure 

 and, as a result, tends to become cloddy and difficult to work. This 

 condition will be made worse if sodium carbonate he present, for this 

 substance also tends to break down the crumbs. It will be noticed, 

 then, that the application of a large quantity of water tends to bring- 

 about a condition of the soil in which the soil particles become more 

 closely packed together, not in aggregates or crumbs, but as 

 individuals. The soil having the granular or crumb structure will 

 have more space (pore space) than the soil in which the particles 

 function as individuals, for in the former not only is there space 

 between the crumbs, but the crumbs themselves are porous. This 

 means that water will penetrate more readily into such a soil, for the 

 pores in this case are large between the crumbs, and every one knows 

 that water will enter and replace the air in a bottle more quickly 

 when the bottle has a wider mouth. This fact is of importance in 

 irrigation practice, as water standing on the surface for some time 

 tends to bring about the injurious effects on the soil in the manner 

 previously described. A method that permits of water standing for 

 some time on the land during irrigation is very far removed from the 

 ideal principle laid dowu by Professor King. One should not over- 

 look the fact, too, that just as it takes longer for water to penetrate 

 into soils having their physical make-up injured by excessive applica- 

 tions of water, so will the entrance of air into such a soil be slower. 



The air in the soil becomes charged with carbon-dioxide, due 

 largely to the action of soil organisms. This accumulation of carbon- 

 dioxide is at the expense of the oxygen in the soil, so that unless a 

 rapid exchange can take place between the soil and the atmospheric 

 air, the plant is likely to become unthrifty owang to the lack of the 

 essential oxygen. Virgin soils, as a rule, have a fairly good structure, 

 so that the irrigator must see to it that he does not ruin the physical 

 condition of the soil by the improper use of water and thereby render 

 the soil impervious, and so cut off the oxygen supply of the soil. This 

 brings us to the methods of applying water to the soil and the kinds 

 of natural waters. 



Kinds of Irrigation Water. — The waters available for irrigation 

 purposes may be classified roughly into (a) non-brak or non-alkaline, 

 and (6) alkaline or brak waters. Non-alkaline waters may be further 

 divided as follows : (1) Those carrying matter in suspension, (2) those 

 free from suspended matter; while the alkaline or brak waters may be 

 divided up thus : (1) Those containing sodium carbonate, (2) those 



