70 IRRIGATION PRACTICE 



pack the top soil, and that cultivation must be performed 

 after each irrigation, if the top soil is to be kept in a thor- 

 oughly loose condition. This is probably due, chiefly, to 

 the excessive wetting after each irrigation, which breaks 

 down the soil crumbs into a single-grain structure. The 

 effect of the successive thorough wetting and drying 

 characteristic of irrigation is of interest to the farmer. 



47. Successive wetting and drying. When irriga- 

 tion water is applied, the soil mass expands, only to con- 

 tract gradually as the water is lost by evaporation or 

 transpiration. The effect of this successive expansion and 

 contraction was also investigated by Cameron and 

 Gallagher, with rather definite results. At the first irri- 

 gation the soil expands, and then contracts to a certain 

 definite degree; at the second irrigation the soil does not 

 expand quite so much, but contracts a little more than at 

 the first irrigation; at the third irrigation the expansion 

 is yet smaller and the contraction proportionally larger; 

 at each successive irrigation, the soil becomes more and 

 more compacted, until a condition of natural packing is 

 reached at which the expansion and the contraction, 

 after each irrigation, are so nearly the same as to result 

 in no practical volume change. If too much or too little 

 water is applied at each irrigation, so that the soil is per- 

 manently kept too dry or too wet, the condition of natural 

 packing is prevented. 



48. Natural packing of soil. The condition of natural 

 packing is, however, far from being the closest possible 

 packing; it is rather the packing of highest advantage 

 to plant-growth. If the soil has become too tightly 

 packed, then the expansions and contractions of succes- 

 sive irrigations will tend to loosen the soil, until the con- 

 dition of natural packing is reached; if the soil has become 



