IX. SOIL MOISTURE 



WE have just seen that the soil is richly stored with all kinds of 

 food for plants, but none of it can be absorbed so long as it remains 

 in a dry state. The minerals of the soils are dissolved by the water 

 which conveys them to the roots and from there through the stem 

 and branches to the leaves, where they are prepared for food and 

 taken wherever needed. The cells in all parts of the plant, 

 even to the remotest root tips, are fed by this leaf-formed food. 

 It is estimated that more than four fifths of the weight of a growing 

 plant is water. Besides the water found in the plant a large por- 

 tion is being constantly exhaled by the leaves. This may be read- 

 ily proved in two ways. If we put a drinking glass or a fruit jar 

 over a small growing plant, in a short time the inner surface of the 

 vessel will be covered with moisture. Again, if we place a freshly 

 cut leaf or twig with its stem in water, it will live for some time ; but 

 if we place it in the vessel without adding the water, it soon withers 

 and dries to a crisp. It is estimated that an acre of grass land 

 will exhale in a day's time more than thirty hogsheads of water. 

 As a general rule we can say that more than three hundred pounds 

 of water pass through a plant and are exhaled or transpired through 

 its leaves for every pound of dry matter held by the plant. Young 

 plants generally give off more water in this way than those which 

 are older and more mature. 



Kinds of Moisture. Soils may be regarded as made up of a 

 number of particles of different shapes and sizes thrown loosely 

 together, with spaces between them. In dry soils these spaces 

 are generally filled with air; but as the soil becomes moist or wet, 

 the water drives the air out. Soil moisture may be classified 

 as follows: (1) ground water; (2) capillary water; (3) hygro- 

 scopic water. 



Ground Water. The rain as it falls to the ground usually 

 sinks through the soil until it reaches an impervious layer of hard 

 pan or rock. At such depth it collects and forms what we call 



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