The: Potato Industry or Colorado. 33 
deep ditches are not required; and in fact, where the land is steep, 
shallow ditches are desirable. Neither is such deep cultivation 
necessary although thorough cultivation gives the best results in 
all parts of the State. The length of time required to wet the soil 
by irrigating varies with the per cent of slope. That is, the greater 
the fall the greater length of time the water must be run. 
Sub-Irrigation. —In the San Luis Valley, where the land is 
level enough and the subsoil will permit, a system known as sub¬ 
irrigation is practiced. For this purpose lateral ditches are run at 
uniform intervals of from 30 to 150 feet across the field. The 
water from these ditches in a short time fills the subsoil from the 
ditches to the center of the spaces, then gradually rises to the surface 
and moistens the soil. This system has an advantage in ease of 
operation but is open to the objection that usually more water is 
used than is good for the plants. The valley has a great deal of 
soil disease, in consequence, among her potatoes. 
The Advantage of Sufficient Slope. —Where there is fall 
enough so the vines or uneven grades do not back up the water in 
the row at any time, we have the condition shown on the right. 
The hill space and the dirt about the tubers is not puddled and set, 
but is moistened like a wick from below as indicated at (a). Ideal 
slope for all crops including potatoes is about 1 per cent., or 50 
feet to the mile.* On much steeper lands, great care is required to 
prevent heavy washing. However, with good subsoil, record crops 
of potatoes and grain are grown where the fall is at least 200 feet 
to the mile. 
Flat Lands Not so Good. —Where the fall is slight, very short 
runs and much labor and skill are required, or conditions will ob¬ 
tain like those shown to the left. With the water level at x, the 
time must be brief if the soaked portion be confined to the line a', 
and it is pretty apt to rise to b and b', and the hill surface is sure 
to be puddled to the level x; while with the water level at x', the 
whole side hill surface and much of the tuber bed is sure to be 
damaged. 
Accurate Grading of Flat Lands is essential that the existing 
*Such slope does not permit of the turning out of large heads at one 
spot, but requires the substitution of corrugation for flooding, and row 
irrigation for all fields, to the great benefit of every crop. 
