334 Irrigation and Drainage 



water a certain piece of land when the surface soil 

 was yet quite moist, and found it impossible to do so 

 with the available head, because the water would sink 

 into the ground faster than it could be supplied ; but 

 by letting the soil become dryer the same head spread 

 the water easily over the whole area, wetting it 

 evenly, though there was greater hindrance from the 

 clover having become thicker and larger. 



In furrow irrigation, the same principle may be 

 taken advantage of in cases where the rows are long 

 and the head of water too small, though not to the 

 same extent ; but the difference is sufficiently pro- 

 nounced to be sometimes quite helpful in open soils. 



PRINCIPLES GOVERNING THE PUDDLING OF SOILS 



A puddled soil is one in which the compound soil 

 kernels or crumbs have been broken down more or 

 less completely into separate grains and run together 

 into a closely compacted mass. Such a soil may hold 

 its pores between the grains so completely filled with 

 water until lost by evaporation that little free air 

 is present except that absorbed in the water itself. In 

 such a soil roots quickly suffer for lack of air, the 

 process of nitrification cannot go on, and, what is 

 even worse, the nitrates already present in the soil 

 when the puddling occurred may be rapidly lost by 

 the process of denitrification. 



The water -logging of a soil has the same dis- 

 astrous effects regarding the roots of plants and on 

 the processes of nitrification and denitrification. Both 



