EXTENSION COURSE IN SOILS. 25 



part of tlie 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 availaUe to plants (E,ef. 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 smaU 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 4J inches, a silt loam 

 6 inches, and a well-granulated clay soil 7^ inches. 



Water required hy 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 fimction. 

 In fact, all movements of substances within the plant take place 

 largely through the medium of water. The larger portion of the ceU 

 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 ceUs 

 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 



