EFFECT OF FALL IRRIGATION ON CROP YIELDS. 11 



Table IX. — Rainfall and soil conditions of the Scottsbluff and Belle Fourche Experiment 



Farms. ' 



Items of comparison. 



Scottsbluff. 



BelleFourche. 



Mean precipitation (inches): 



Annual 



Fall period (August to October, inclusive) 



Winter period (November to March, inclusive) 



Total (August to March, inclusive) 



Soil: 



Type 



Field moisture-carrying capacity per cent 1 . . 



Wilting coefficient do i 



14.47 

 4.96 

 1.35 



14. 05 

 3.75 

 2.15 



Sandy loam. 

 13 

 6 



5.90 



Pierre clay. 

 27 

 17 



1 These figures, which represent average conditions at the two 'field stations, were obtained from the 

 Office of Dry-Land Agriculture. 



It will be noted that the precipitation at Scottsbluff does not differ 

 materially from that at Belle Fourche, either as to the total quantity 

 or as to the quantities received during the fall and winter periods. 

 The soils represent the essential difference between the two places in 

 relation to fall irrigation. The figures showing moisture-carrying 

 capacity and wilting coefficient indicate something of the wide 

 dissimilarity of the two types of soil. The sandy loam, being coarse 

 grained, has a low moisture-carrying capacity and a low wilting co- 

 efficient. On the other hand, the extremely fine-grained Pierre clay 

 has more than double the moisture-carrying capacity of the sandy 

 loam and its wilting coefficient is nearly three times as high. 



The soil at Scottsbluff is readily pervious to water to a depth of at 

 least 6 feet, and its permeability is higher when the soil is moist than 

 when it is dry. Knorr 1 found with this soil that water applied 

 during the regular irrigation season percolated deeper on the plats 

 on which the moisture content was relatively high as a result of fall 

 irrigation than on the check plats where the soil moisture content 

 was comparatively low. This fact, together with the greater quan- 

 tity of moisture available in the fall-irrigated plats at planting time, 

 is believed to explain the beneficial effects of fall irrigation at Scotts- 

 bluff. 



The moisture relationships of the soil at Belle Fourche are very 

 different from those at Scottsbluff. At Belle Fourche the soil is 

 practically impervious except when it is dry, and the depth of water 

 percolation depends chiefly upon the extent to which the soil is 

 cracked as a result of dryness. The addition of moisture causes the 

 soil to expand rapidly, and as expansion increases permeability de- 

 creases. After making an extensive study of water penetration in 

 these soils, Mathews 2 states that ' ■ On a dry soil, penetration takes 

 place rapidly to a depth of about 2 feet because of the cracked con- 



1 Knorr, Fritz. Experiments with crops under fall irrigation at the Scottsbluff Reclamation Project 

 Experiment Farm. U. S. Dept. Agr. Bui. 133, 17 p., 5 fig. 1914. 



2 Mathews, O. R. Water penetration in the gumbo soils of the Belle Fourche Reclamation Project. 

 U. S. Dept. Agr. Bui. 447, p. 11. 1916. 



