54 GENEEAL CONSIDERATIONS 



All of the above are forms of what is called sur- 

 face irrigation. They are all open to the objection 

 that they pack the surface of the soil, leaving it 

 hard and in a condition to bake badly unless each 

 irrigation is followed by tillage. Excepting where 

 the land is very nearly level there will also be loss 

 from surface washing. These troubles can be obviated 

 and better results can be obtained by what is known as 

 subirrigation. In this system the water is distrib- 

 uted under the surface by means of porous pipes. It 

 permeates the subsoil and keeps it moist without in- 

 terfering with the condition of tilth of the surface. 

 Lines of draintile are sometimes so arranged that 

 by closing the outlets and flooding them they can 

 serve for subirrigation during dry weather, and by 

 opening them again their action as underdrains can be 

 restored in case of heavy rains. Subirrigation gives 

 very fine results in greenhouses and on ornamental 

 grounds where there is no objection to expense, but 

 it is too costly for ordinary farm operations, and we 

 must rely on frequent tillage to obviate the bad 

 effects of surface ajDplications of water. With all 

 crops that permit it tillage should invariably follow 

 an irrigation as soon as the land becomes dry enough. 

 This prevents baking and the formation of a hard 

 crust; it aerates the soil and, by the formation of a 

 dust mulch, preserves the moisture and renders future 

 irrigations necessary at much less frequent intervals. 

 The greatest temptation to the irrigation farmer is 

 to rely too much on water and too little on tillage. 



The overabundant use of water should always be 

 avoided, especially in arid regions. Where there is 



