average of 1.84 inches per 10 days. This is 18.4 inches per 100 days, and is enough water for a crop of 60 bushels of wheat per acre. It shows that the loss of water from a continuously wet surface, through evaporation, may be very rapid, even in humid climates.

The loss of water from the Rothamsted drain gauge, which was a naked soil surface through which excess drainage water could be collected and measured, and that lost by evaporation determined, has shown a mean evaporation from the soil surface of 4.78 inches per 100 days between April and September. This is the water equivalent of 15 bushels of wheat per acre.

In Wisconsin the loss through ordinary three-inch earth mulches, under field conditions, is not far from 3.8 inches per 100 days, and is the water equivalent for 15 bushels of barley per acre.

EFFICIENCY OF EARTH MULCHES

The efficiency of earth mulches varies under different conditions:

(1) The efficiency increases with the depth of the mulch.

(2) The efficiency increases with the distance of saturated soil below the mulch.

(3 The efficiency decreases with age. (4) The efficiency varies with the character of the soil.

In the case of a humus soil, which has the highest efficiency as an earth mulch of anything we have tried, the loss of water by evaporation from the surface when the soil was saturated 19 inches below was at the rate of 5.48 inches where there was no mulch; and one, two, three and four-inch mulches gave evaporations of 3.24, 2.42, 2.20 and 1.89 inches respectively per 100 days.

In the sandy loam, under the same conditions, the loss of water through the no-mulch condition was 6.39 inches, as compared with the 5.48; but the clay loam, under the same conditions, permitted a loss of 15.71 inches in 100 days, as compared with the 5.48 inches loss from the humus soil.

The relative efficiency of the four-inch

mulches for the three soils stand, 1.89, 2.22 and 7.78 inches of water per 100 days, the smallest being from the humus soil and the largest loss from the clay loam. When the soil was saturated at six feet below the surface, instead of at 19 inches below the surface, the sandy loam lost water at the rate of but 2.3 inches per 100 days, instead of 6.39 inches, and a clay loam lost but 2.99 inches, instead of 15.71 inches. In these cases the surface of the soil became dry quickly, and acted in the capacity of an earth mulch which is firm instead of one which is loose.

Under the three-inch mulch and with the water six feet below the surface, the sandy loam lost water at the rate of 1.14 per 100 days, instead of 2.38 inches where the soil was saturated 19 inches below the surface. The clay loam under the three-inch mulch lost 2.13 inches with the soil saturated six feet below the surface, where it lost 8.6 inches when the soil was saturated 19 inches below the surface.

When a mulch is developed on the sur