32 The Colorado Experiment Station. 
too much rather than too little water is used. The number of irri¬ 
gations required cannot be foretold for any soil or season, and is a 
matter of experienced judgment based on observing the weather 
and the actual moisture conditions prevailing under the plant and 
about the roots, and will vary from two to seven times.* Irrigation 
must cease before the growth of the tubers ceases, or the potatoes 
will be coated with dirt, which would have been loosened by the 
surface motion of growth. 
The Per Cent, of Slope affects the methods and amount of 
water used in irrigating. On the plains, in many places, there is 
not more than eight or ten feet fall to the mile, while narrow val¬ 
leys usually have several times this amount. With plenty of slope, 
*Some measurements taken on the E. R. Bliss ranch show the 
amount of water actually used in growing a crop of potatoes, both on 
alfalfa land and on old potato land. The applications on the potato field, 
preceded by three years of alfalfa, were as follows: July 25th-26th the 
water ran 17 hours with a delivery of 4.05 feet per second. August 1 and 
2, 27 hours with 1.9 6 feet per second. August 8 and 9, 24 hours at 2.31 
feet per second, and August 15 and 16, 30 hours at 2.37 feet per second. 
This field contained 17.88 acres, and the depth of water used in irriga¬ 
tion was 13.76 inches. The rainfall by months from April till October 
was: April, 3.04 inches; May, 1.73; June, 1.10; July, 2.24; August, .64, 
and September, 2.31, or 11.05 inches. The September rain was mostly 
in the latter part of the month, and probably did little if any good to 
the potato crop. If the September rainfall is left out, the precipitation 
that should be counted as contributing to the growth of the crop will be 
8.75 inches. The rainfall plus the irrigation gives us 22.51 inches as 
the total water used on the crop. The field is Billings clay loam with 
quite a large per cent, of sharp granitic gravel. The soil is about two 
and one-half feet deep, underlaid with gravel, so it has good drainage. 
The field was plowed in early May, eight inches deep, harrowed imme¬ 
diately and planted to Pearl and Snowflake potatoes June 1st. The yield 
of Pearls on this field was above 150 sacks per acre, which is near the 
maximum for the season. 
The field adjacent to this one had grown potatoes the year before, 
and was watered just previous to the alfalfa potato field. The first run 
was 14 hours at a discharge of 4.05 feet per second and the second 18 
hours at 1.96 feet per second, the third 16 hours at 2.31 feet per second 
and the fourth 24 hours at 2.37 feet per second. This field had an area 
of 19.74 acres, and an average depth of water was used over the field 
for the season of 9.3 5 inches. The difference in the irrigating water 
between the old potato land and the alfalfa land was 4.41 inches. This 
field was planted just previously to the alfalfa field and the potatoes 
ripened (or the vines died from fungus troubles) about two weeks earlier. 
The yield was about 130 sacks per acre, as against something over 150 
sacks for the alfalfa land. Frequently a greater difference than this 
results between alfalfa land for potatoes and land preceded by other 
crops, but it seems that the difference comes not from the amount of 
plant food in the soil, but from disease: for, after potatoes have been 
grown on soil even three years, the cereals grown on it will produce heavy 
crops. 
The difference in the amount of water can be attributed to the 
mulched condition of the old field and to the physical condition of the 
soil in the two fields. The decaying alfalfa stems and roots make the 
newly broken land more porous, and the first irrigation particularly takes 
more water to fill the soil. 
