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: 
14.05 
3.75 
2.15 
Type 
Field moisture-carrying capacity per cent l 
Wilting coefficient do ». . . 
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 x 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 "Ona dry soil, penetration takes 
place rapidly to a depth of about 2 feet because of the cracked con- 
i 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. 
