EFFECT OF FALL IRRIGATION ON" CROP YIELDS. 3 
The table shows the light precipitation which occurs during the 
winter months, from November to March, inclusive. During two of 
the three winters included hi the period of experiment there was 
somewhat less than the mean winter rainfall for the period of record, 
2.15 inches, and during the other there was slightly more than the 
mean. Likewise, two of the three fall periods affecting the experi- 
ments were drier and one was wetter than the mean for this fall 
period, 3.75 inches. From this it would appear that if fall irrigation 
were desirable at Belle Fourche, its benefits would have been empha- 
sized during the period of these experiments. 
SOIL. 
The soil on which these experiments were conducted is an extremely 
heavy clay of the class popularly known as u gumbo." Soils of this 
general character cover wide areas in the northern Great Plains. This 
particular soil has been classified by the Bureau of Soils as Pierre 
clay, of which the average mechanical composition is reported 1 as 
follows : 
Percent. 
Fine gravel 0. 2 
Coarse sand 1.1 
Medium sand 1. 4 
Fine sand 5. 5 
Very fine sand 13. 
Silt 43.2 
Clay 35. 
According to the same authority, the Pierre clay covers about one- 
third of the area of the Belle Fourche Reclamation Project and 
occurs on this area to a uniform depth of about 6 feet. It has been 
formed through the weathering of the underlying beds of Pierre 
shale. According to a report 2 of a reconnoissance soil survey made 
by the Bureau of Soils, the Pierre clay covers an area of nearly 
eight million acres in the western half of South Dakota. 
Important characteristics of this soil are its great water-carrying 
capacity, its high water retentivity, and the extreme changes of 
volume which accompany changes in its moisture content. Mathews 3 
found that this soil will carry about 30 per cent of water and that 
about hah of this is available to crops. The latter do not begin to 
suffer from lack of water unless the soil moisture content approaches 
17 per cent, which is approximately the wilting coefficient of this 
soil. The importance in fall irrigation of the extreme changes of 
volume accompanying changes in moisture content of this soil will 
be discussed later. 
1 Strahorn, A. T., and Mann, C. W. Soil survey of the Belle Fourche area, South Dakota. In V. S. 
Dept. Agr., Bur. Soils, Field Oper. 1907 [9th Rpt.], p. 898. 1909. 
2 Coffey, G. N., and others. Reconnoissance soil survey of western South Dakota. In XJ. S. Dept. Agr., 
Bur. Soils, Field Oper. 1909 [11th Rpt.], p. 1401-1476, illus., pi. 9-15. 1912. 
3 Mathews, O. R. Water penetration in the gumbo soils of the Belle Fourche Reclamation Project. 
U. S. Dept. Agr. Bui. 447, 12 p., 4 fig. 1916. 
