484 A. B. BEAUMONT 
great number that have been evolved from time to time and used with 
varying success by different investigators. 
There is little doubt that the amounts of colloids in soils vary consid- 
erably with the kind of soil and the conditions. Schloesing (quoted by 
Ehrenberg, 1915) estimated that the colloidal clay of normal agricultural 
soils seldom exceeds 1.5 per cent. However, he found as much as 35 per 
cent of colloidal matter in extremely heavy soils. 
Hilgard (1911) estimated the amounts of colloidal clay in soils as varying 
from 0.5 per cent for very sandy soils to 45 per cent and over for heavy 
clay soils. 
Tempany (1917) calculated the amounts of colloidal matter in soils 
from their linear shrinkage on drying, and gives figures varying from 
9 to 64 per cent. 
REVIEW OF LITERATURE 
It has been observed by many investigators that for some soils previous 
drying is beneficial from the standpoint of plant production. This benefit 
has been ascribed by some writers to the effect on the physical condition 
of the soil. Alternate wetting and drying has been considered particularly 
efficacious in this respect. Of late the effect on the colloidal matter in 
particular has become a favorite assumption of some writers. In other 
cases the benefits of drying or of alternate drying and wetting are attri- 
buted to chemical or biological effects, or to a combination of these with 
physical effects. On this point Lyon, Fippin, and Buckman (1915 :188) 
say: “Just what may be the effects of wetting and drying on the colloidal 
matter of soil is a question.” 
The increased productiveness of soils due to fallowing is also a matter 
of common experience. The stirring of the soil is accompanied by rapid 
drying. To what extent the benefits are due to physical, chemical, or 
biological effects, has not been worked out. 
Buckman (1911) concluded from the work of others that under condi- 
tions of extreme dryness an increase in moisture means an increase in 
nitrates. Lyon and Bizzell (1913) found that an increase in moisture 
after a dry period was sometimes accompanied by an increase in nitrates 
in an unplanted soil, and Lyon (1907) expressed the opinion that a dry 
period previous to the time of heading causes wheat to be harder and 
higher in protein. 
