138 THE PRINCIPLES OF SOIL MANAGEMENT 



based on the dry weight of the soil. (2) In terms of 

 per cent based on the wet weight of the soil. (3) In 

 terms of the per cent of volume based on the total 

 volume occupied by the soil. (4) In cubic inches per 

 cubic foot, or in cubic centimeters per liter or per cubic 

 meter. (5) In inches in depth of water over the surface 

 of the soil. 



Of these methods the first is most largely used, 

 because it gives the most definite and constant basis 

 from which to derive any other quantities. The dry 

 weight of a soil remains constant, and percentages 

 referred to that base are always comparable. But it 

 has several disadvantages which lead to inconsistent 

 results in practical work. For example, 10 per cent of 

 water in a cubic foot of clay soil represents a very 

 different quantity of water from the same percentage 

 in a sand or a muck soil, because of the very different 

 volume weights of these materials. In the clay it would 

 mean about 7 pounds, or 3.5 liters; in the sand soil 

 10 pounds or 4.5 liters; and in the muck soil 3.5 pounds, 

 or 1.6 liters, manifestly very different quantities of 

 water. Or, to state the matter in a different way, 30 

 per cent of water in a clay, 12 per cent in sand, and 

 150 per cent in muck, do not represent as different 

 volumes of water as is indicated by the figures, because 

 of the relative weights of the soils. But, because almost 

 any other figure can be readily derived from the moisture 

 percentage expressed in terms of dry weight of soil, 

 it has been very generally used, especially in laboratory 

 studies. In field practice, a volume method is more 

 convenient. 



