EXTENSION COURSE IN SOILS. 25 
part of the soil can hold. In clay soils humus also has a considerable 
indirect influence on water-holding capacity through its power to 
affect granulation. 
The total quantity of water held by different soils when saturated 
has been found to vary from about 40 per cent of their dry weight in 
coarse sand to about 55 per cent in well-granulated clay, and up to 
over 300 per cent, or three times its dry weight, in muck. The quan- 
tity of capillary water which these same soils have been found to hold 
varies from about one-fourth of the amount held upon saturation in 
coarse sand to over one-half in well-granulated clay, and up to nearly 
the total amount in muck. The larger capillary capacity of the muck 
is due largely to its high absorptive power. (See tables, Ref. No. 3, 
pp. 154-162.) 
Water available to plants (Ref. No. 3, pp. 200-202). — Crops growing 
in soil are unable to take all the water which it holds. If soil in which 
plants have died for lack of water is thoroughly dried in an oven it will 
be found that there is expelled a small quantity of moisture which the 
plants were unable to secure. Coarse-textured or sandy soils retain 
very much less of such water than do the fine-textured clay loams or 
clays. This is because the plants are able to withdraw the water only 
to a given thinness of water film around the soil grains, and the larger 
total exposed surface of the fine-textured soils causes them to retain 
the larger quantity of water. It is evident, therefore, that only a 
part of the capillary water can be considered as available for growing 
crops. When the ground-water level is 10 feet below the surface the 
upper 4 feet of a very sandy soil can hold available water equal to a 
layer of about 3 inches in depth, a sandy loam 4£ inches, a silt loam 
6 inches, and a well-granulated clay soil 7£ inches. 
Water required by growing crops (Ref. Nos. 1, pp. 12-16; 10, pp. 12- 
17). — It was stated in Lesson II that water is used by plants directly 
as a plant food, and further, that water dissolves mineral substances 
in the soil and carries them to all parts of growing plants, where the 
mineral elements are utilized so as to perform their special function. 
In fact, all movements of substances within the plant take place 
largely through the medium of water. The larger portion of the cell 
sap of growing plants is composed of water. An average of 80 per 
cent or more of the green weight of staple farm crops is water. 
When the water supply from the soil is insufficient, the plant cells 
become shrunken, causing wilting. The temperature of growing 
plants is also regulated to some degree by the transpiration of water 
in the form of vapor from the leaves and stems. The quantity of 
water transpired, that is, given off as vapor to the surrounding air by 
growing plants far exceeds the quantity directly utilized to form plant 
substance. Experiments have shown that for every pound of dry 
