238 SOILS. 



the dikes is objectionable, especially in large-scale culture. 

 For the case of alfalfa fields, which remain permanently set 

 for a number of years, it is however the largely preferred 

 method. In the case of field cultures, the consolidation of the 

 surface that follows flooding on the heavier soils renders sub- 

 sequent tillage necessary in all but very sandy soils ; and hence 

 it should always precede broadcast sowing. 



One disadvantage of the surface-flooding system is the slow 

 penetration of the water caused by the resistance of the air in 

 the soil to downward displacement; its buoyancy acting di- 

 rectly contrary to the percolation of the water. In close- 

 grained, heavy soils this objection is very serious, on account 

 of the loss of time involved when the irrigator's time is limited. 

 On sandy lands the air bubbles up quite livelily at first, but this 

 soon ceases and the air is compelled to escape sideways as best 

 it can. 



Furrow Irrigation. By this method it is intended to soak 

 the land uniformly by allowing the water to flow through fur- 

 rows drawn 3 to 8 feet apart, with a gentle slope from the 

 supply or head ditch ; the flow being continued until the water 

 has reached the far end of the furrows, or longer according to 

 the nature of the soil, especially if another ditch to receive the 

 surplus flow lies below. The furrows should subsequently be 

 closed by means of the plow or cultivator; but even if left 

 open they are much less a source of waste by evaporation than 

 would be a flooded surface. The water thus, in the main, soaks 

 downward and only reaches the surface by capillary rise, so 

 that the land between the furrows is not sensibly compacted 

 when the furrows have been made deep enough. Evidently 

 this is a much more rational procedure than surface flooding, 

 as it tends to leave most of the surface in loose tilth, while 

 penetrating to much greater advantage, because -of the ready 

 escape of the air from the soil. It is the system naturally and 

 almost exclusively used in truck gardens and orchards, and 

 generally where crops are grown in drills or rows sufficiently 

 far apart to permit of cultivation. 



The figure annexed l shows the manner in which water 

 sinks and spreads from furrows of various depths and widths, 



1 Published by permission of the Department. 



