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PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Sand and gravel separately have an air space corresponding to 

 about 40 per cent, of their total volume. When mixed together, 

 the small particles of sand enter the free space of the gravel, and 

 diminish the volume of the free space. In ordinary soils, the vol- 

 ume of the free space is somewhat greater than in sand or gravel, 

 owing to the presence of porous or compound particles. 

 A soil abounding in porous compound particles decreases in water 

 capacity when reduced to a powder. Zenger found that the soil 

 from a peaty meadow held 178 parts water to 100 parts soil, 

 but it held only 103 parts water when finely powdered. Colloidal 

 bodies take up water and make the soil swell when wet. 



A soil is seldom completely saturated under natural conditions. 

 The soil cannot become fully saturated unless the air which fills 

 the interstices of the soil is allowed to escape. Rain covers the 

 surface of the soil and closes the path of the air so that it gets 

 out with difficulty. Only after long continued rains do soils be- 

 come nearly saturated. 



The following table shows the quantity of water in naturally 

 saturated soils, the samples being collected after continued rains : 



The first four analyses were made by King. 1 The last 

 three samples, from the Rothamsted 2 Experimental fields, 

 show the effect of humus upon the water capacity. The 

 accumulation of humus in the soil manured with barnyard manure 

 increases the water capacity of the soil decidedly. On account 



1 Wisconsin Report, 1890. 



' 2 Jour. Roy. Agr. Soc., 1871, p. no. 



