Plants in Relation to Man 459 



Amount of soil water. The quantity of water contained 

 in any soil is greater than one may imagine. The surface 

 soil may be very dry ; but a few inches below there is much 

 moisture. If you reach clay, you will find it actually wet. 

 This is not free water, but what adheres to the particles of soil 

 and subsoil. It is estimated that a cubic foot of clay, which 

 when dry weighs about eighty pounds, has a moisture capac- 

 ity of forty per cent or about thirty-two pounds. The moisture 

 capacity of soils varies approximately from forty per cent to 

 as low as five per cent of their dry weight. A hundred pounds 

 of sandy loam may hold twenty per cent or twenty pounds of 

 water. Humus or decayed organic material has still greater 

 water capacity and greater retaining power. 



Character of soil water. Soil water remains after the soil 

 has been drained of free or rain water and is quite different in 

 character from this. Water, one of the most active natural 

 solvents, begins to dissolve soil particles immediately when it 

 comes into contact with them. It takes up something of 

 practically all substances in the soil and subsoil. The finer 

 the texture of the soil the greater is the total surface area ex- 

 posed to the action of water, and the greater, therefore, is the 

 yield of substances to its solvent power. The longer water 

 remains in contact with soil materials, the more concentrated 

 will be the resulting solution. Even ordinary free or well 

 water shows under chemical examination that, although it 

 has moved rapidly through soil and rock strata, it has dissolved 

 away some substances, such as lime. But the quantity of 

 materials contained in free water is very small in comparison 

 with the amount held in solution by soil water. 



Substances in soil water. Because it is in contact with the 

 soil particles for a longer time, soil water dissolves away and 

 retains larger quantities of more substances than does free 

 water. Among the substances which it thus obtains are com- 

 pounds of nitrogen, potassium, calcium, magnesium, iron, and 

 silicon. All of these are necessary as food materials for plants 



