242 SOILS. 



must, on the contrary, vary in the opposite sense, since if these 

 are too far apart, the water near the head ditch will in sandy 

 lands be wasting into the subdrainage before the end of the 

 furrows is reached ; so that the distribution will be very un- 

 even. The great differences observed between crops, and es- 

 pecially trees, below and above the head ditches, are mainly due 

 to this unevenness in water distribution, caused by too great 

 distance between successive head ditches. Each farmer must 

 himself, however, determine by actual trial the proper dis- 

 tances between ditches as well as furrows, for his particular 

 case; since everything depends upon the rapidity with which 

 water will penetrate the soil and subsoil. Actual tests to de- 

 termine this point 1 should be the first step, before laying off 

 the system of ditches as well as furrows. It not uncom- 

 monly happens that the failure to do this at first, compels a 

 subsequent total change of arrangements in this respect. (See 

 page 253 below). 



Thus while in some very pervious land furrows may be six or even 

 eight feet apart, in other cases, in certain finely pulverulent or silty 

 soils such as the " dust soils " described in a former chapter ; (see chapter 

 6, p. 104), furrows drawn three feet apart may fail to allow the water to 

 penetrate so as to prevent grain on the middle foot from suffering from 

 drought after the water has run for twenty-four hours. 



Irrigation by lateral Seepage. Is in reality a mere modifi- 

 cation of furrow irrigation, practiced in the case of lands very 

 readily permeable, and where water is abundant. The fields 

 are laid off in " lands " twelve to twenty-five feet wide, with 

 a deep furrow or narrow ditch between, from which water 

 percolates in a short time so as to overlap from the two sides. 

 In this case sometimes the water does not reach the surface 

 visibly at all ; a very great advantage where alkali exists, as 

 surface evaporation, and the consequent accumulation of 

 alkali, is thus effectually prevented; while deep rooting is 

 favored to the utmost. 



1 Such tests can be readily made by any one, by dipping a P'* to f tir or ^ ve f eet 

 depth, and supplying water to a shallow basin dug into the surface 8 to 12 inches 

 distant from the vertical wall of the pit. The descent of the water is then readily 

 observed on the vertical side of the pit nearest to the water basin. Preliminary 

 tests with soil probe (see chap. 10, p. )I77- 



