1 1 6 Journal of A gricultural Research voi. x. No. 3 



a greater loss of soil moisture than evaporation from a bare soil. Small 

 grains were found to dry out the soil more completely than cultivated 

 crops. He found straw mulches, deep cultivation, and shallow culti- 

 vation to rank in efficiency in the order named. Rain water penetrated 

 more rapidly into a wet than into a dry soil, the latter being wetted to a 

 depth of 6 inches by i inch of rain. 



Effect of Irmgation 



The distribution of soil moisture following an irrigation or rainfall 

 has-been studied by many investigators. King (14) observ'ed a very 

 slow rate of penetration in a gravelly clay after 1.4 inches of rain, and 

 the lateral movement did not exceed 3 feet. Loughridge (24), working 

 with sandy loam, found the downward movement of irrigation water 

 very irregular in its rate and in the amount retained at various depths. 

 The lateral movement of water from the furrows did not exceed 2 feet, 

 and the relative proportion of dry soil to that wetted decreased with 

 the depth at first, then increased. 



Elaborate experiments by Widtsoe and McLaughlin (32) brought out 

 the following: Irrigation water penetrates very rapidly to a depth of 

 6 feet; with any amount of irrigation the percentage distribution is 

 always the same for each foot shortly after irrigation; it is believed that 

 in an unsaturated soil where / is the percentage of water at the depth 

 indicated by d (c?i = i foot deep), and K is the percentage of water 

 which must be satisfied before rapid movement will take place, the fol- 

 lowing law holds for the distribution of moisture in a uniform soil : 



(/ -K) d = (/t - K) d^ = (Z. - K) dn = constant . 



The lateral movement of water increases with depth; in Greenville 

 loam the movement of water is slow, with less than 12.75 P^r cent of mois- 

 ture. Experiments by Allen (i) indicate that with 2.5- , 5.0- , and lo-inch 

 irrigations the same amount of water is retained in the upper 4 feet of soil 



after 24 hours. 



Effect of Soil Type 



Briggs (4) has pointed out that coarse and fine soils having the same 

 percentage of water will not be in moisture equilibrium. Such soils when 

 brought together would experience a movement from the coarse into the 

 fine soil. King's percolation experiments (19, 20, 21) with long columns 

 of sand showed that sand will retain very little capillary moisture — 

 sometimes less than 2 per cent. Reynolds (27), Hilgard (11), Tulaykov 

 (29), Willard and Humbert (33), and WoUny (34) agree that the capillary 

 rise is always least in the coarse soil and greatest in fine soils, but the 

 rate of rise at first is directly proportional to the coarseness of the parti- 

 cles. Later the reverse is true. 



