626 
Journal of Agricultural Research 
Vol. XXVII, No. 9 
limited to the surface foot. The soil represented by the second column 
in the figure was located in the same plat as that shown in column i 
and only 15 feet from it. In this location there was a good stand and 
a good growth of alfalfa nearly ready to cut. The same irrigation that 
added 1.2 inches of water to the soil shown in column 1 added 6.5 inches 
of water to the soil shown in column 2. Both these soils were rich in clay. 
The soils of columns 3 and 4 were of a sandy type and were taken 
from adjacent fields on the Prosser Experiment Farm, Wash. Both 
fields were carrying a good growth of alfalfa about ready to cut. Irriga¬ 
tion added 3.9 inches of water to the soil shown in column 3 and 5 inches 
of water to the soil shown in column 4. 
It is altogether probable that when these soils were sampled before 
irrigation the available water supply in the first 3 feet had not been 
entirely exhausted. But the growth conditions were such that it was 
deemed advisable to irrigate. It is not always safe nor is it good 
farming to force the plants to use the last drop of available water. 
It is also probable that in the sandy soil of the Prosser Experiment 
Farm the roots of alfalfa extend below the third foot. Notwithstanding 
these reservations with respect to the conditions which these figures 
represent, they afford a comparison as to the relative quantities of sub- 
available and available water in each location. This comparison is 
shown in Table V. 
The results of a large number of observations covering a wide range 
of soil types indicate that the capacity of the irrigated soils for holding 
available water ranges from about 1 inch per foot up to 2 inches per 
foot, or possibly a little more in exceptional cases. 
Table) V .—Quantities of water held in the first j feet of soil in four locations on two 
different types of soil 
Quantity of water expressed in inches of depth. 
Classification of water. 
Newlands Experiment 
Farm, clay soil. 
Prosser Experiment 
Farm, sandy soil. 
Location i. 
Location 2. 
Location 1. 
Location 2, 
Subavailable. 
9.4 
I. 2 
7-9 
6-5 
I. 0 
2. 4 
Available. 
7 
V 0 
<. 0 
v/ 7 
j 
THE PENETRATION OF IRRIGATION WATER 
It is the aim in applying irrigation water to moisten the soil to the 
depth of several feet and in doing so to store enough water in this layer 
of soil to meet the needs of the crop plants for two or three weeks, or 
until the next irrigation. If this aim is to be realized, it is essential that 
the water penetrate the soil readily, for it is often not practicable to 
keep water on the land more than a few hours at a time. 
When irrigation water is applied to the land, it penetrates in part by 
flowing into such cracks in the soil as have been formed by shrinkage 
and in part by percolating into the minute spaces between the soil parti¬ 
cles. Where the soil conditions are favorable, most crop plants develop 
their roots to the depth of 4 or 5 feet and are thus able to use the water 
