‘ REVERSIBILITY OF THE COLLOIDAL CONDITION OF SOILS 489 
of the humate. Humus gel made by evaporating the sol, as ordinarily 
prepared, is reversible. 
Rohland (1914) reports that the adsorptive power of soil colloids for 
water decreases after repeated drying, but that new colloids are formed 
by clays standing in an excess of water. 
According to Ehrenberg (1915), Schubler reported that a humus colloid 
prepared by him could not take up as much water after drying out as 
before. Wiegmann confirmed this statement. A similar observation was 
made by Schubler on a strongly humus-containing soil. Lasius reported 
‘that peat once dried does not soften again, and Ruhlmann stated that 
peat when once air-dried cannot be brought back to its original slimy 
condition thru working. 
Tacke and Immendorff (1898) report that peaty soils not only shrink 
strongly by natural or artificial drying, but that they lose their swelling 
capacity, take on a peculiar sandy condition, and never again become 
strongly colloidal. Further, these investigators report that the solu- 
bility of phosphoric acid in moor soil is increased by drying the soil at 
about 80° C. They also found an increase in the water-soluble potassium 
and calcium, due to drying out. This effect was masked by using 0.5-per- 
cent hydrochloric acid as a solvent. 
Mitscherlich (1902), working with moor soil, found that moistening 
and then drying at 100° C. affects the soil structure so that it holds less 
water.. He concludes that One and drying a soil is not a simple 
reversible process. 
The destruction of the crumb structure of soil in good tilth by the 
beating of rains has been recorded by several writers, including Warington, 
Hilgard, and Ehrenberg. ‘This is a case of deflocculation brought about 
by physical agencies. 
Harrison (1917) reports that wet methods of cultivation when first 
applied to paddy soils tend to bring about a physical deflocculation fol- 
lowed by a weathering of particles, thus causing the soils to become 
heavier. This heaviness is probably due to a production of the colloidal 
condition. 
Tempany (1917), in studying the shrinkage of the soils of the West 
Indies, assumes that the gel condition of the colloidal matter in these 
soils is restored by moistening and kneading the air-dry soil —a case of 
physical reversibility, largely. 
