water capacity of the surface four feet, for available moisture. In each succeeding two weeks the crop used, on the average, a little more than six inches, so that one full irrigation would be required at least as often as once in 15 days.

In the humid soils of the eastern United States it is very rare that a crop does not suffer for lack of water, after it is well started in growth, if the intervals of rain are longer than 10 days. It is seldom, too, that our soils can receive more than two or three inches of rain in as many consecutive days without lowering the yield as a result of over-saturating the soil. We get our best crops and heaviest yields when about two inches of water falls every 10 days and when the rainfall is all in one period covering one or two days.

In our experimental studies regarding the duty of water in crop production, conducted both under field conditions and laboratory methods, the best results were obtained when about two inches of water were applied to loamy soils once in seven to 10 days, after

the crop had reached the stage of active growth.

In the case of a coarse, sandy soil, upon which were grown corn, potatoes, melons and strawberries, critical soil moisture studies, to a depth of four feet, showed that an application of irrigation water, amounting to more than 1.5 inches at one time, produced percolation below the four-foot depth and that in midsummer, if we delayed irrigation more than seven to 10 days, the growth of the crop was retarded.

AMOUNT OF WATER APPLIED IN DIFFERENT COUNTRIES

In Italy, for different crops, the amounts of water used are such that if applied once every 10 days the average depth of each irrigation is 3.39 inches.

In southern France, in the valley of the upper Garonne, water is used at the rate of 2.5 inches, and in the Department of Vaucluse at a rate as low as 1.36 inches per 10 days. In Spain, where the rainfall is less

than in Italy and where greater economy of water is practiced, the average of 19 allotments of water gives 2.35 inches for every 10 days of the growing season.

In Egypt, in winter, the irrigations are at the rate of 3.94 inches once in 40 days, but in summer cotton receives 3.4 inches in 20 days.

An average of 100 cases from all parts of the world gives the duty of water in irrigation at the mean rate of 2.02 inches once in 10 days for a period of 100 days.

CHAPTER X

CONSERVATION AND APPLICATION OF WATER IN CROP PRODUCTION

A

S we have said before, the ideal application of water to land is fully realized when it falls as a gentle rain at just the rate which permits each drop to sink at once into the soil where it falls. When this is the case very important results are secured: (1) The soil is uniformly wet.

(2) The soil texture is improved rather than injured.

(3) The more soluble plant food, which tends to accumulate at the surface through upward capillary sweeping, as the result of evaporation from the soil, is returned to the roots of growing crops. It is practically almost impossible to secure these results by irrigation methods. This makes it all the more important to thoroughly understand

the principles which govern the practices which must be followed.

THE FORCES WHICH DISTRIBUTE WATER IN THE SOIL

When water reaches the soil as rain or by irrigation its distribution through it is effected by two forces:

(1) Surface tension which produces capillary movement such as that causing the ink to enter the blotter, or the rise of oil in the lamp wick.

(2) Gravity which causes the direct descent of water into the soil, always along the line of shortest distance available.

ACTION OF SURFACE TENSION

The surface tension force may move water in any direction, moving it horizontally, upward or downward, sweeping the soil moisture to the active roots, whatever their position, and, with it, the plant food carried. It acts with gravity whenever it