INSTITUTE, PUSA, FOR 1917-18 



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the soil dries under the hot sun, the surface bakes into a 

 hard crust largely impermeable to air. That the crust is 

 impermeable can be seen by immersing in water a portion 

 of the hardened surface soil after irrigation. The air 

 escapes sideways not through the surface skin. Each suc- 

 cessive irrigation destroys the soil texture more and more 

 and the surface crust becomes more and more impermeable 

 to air. The effect of irrigation on alluvial soils, therefore, 

 interferes with its ventilation. The process removes one 

 limiting factor, the want of water, but it introduces 

 another, namely, the need of aeration. That this is so will 

 be clear from Table V which contains the result of a recent 

 experiment at Quetta. 



Table V. 



The introduction of a new limiting factor after irrigation. 



Here the last two irrigations reduced the yield through the 

 introduction of another limiting factor — the need of soil 

 aeration. Similar results 1 were obtained at three stations 

 in the Punjab in 1917. One irrigation gave nearly ten 

 maundsof wheat to the acre, two gave a little over sixteen, 

 while three reduced the yield appreciably. These results 

 prove that successful irrigation involves the working out of 

 a practical compromise between the two conflicting factors 

 — water and air. The aim of the irrigator is not the mere 

 application of water hut the provision of water in such a 



1 A full account of the irrigation results obtained at Quetta and elsewhere will 

 be found in Quetta Bulletins 4 and 7 and in a paper on soil aeration in the Indian 

 Forester of May, 1918. 



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