REVERSIBILITY OF THE COLLOIDAL CONDITION OF SOILS A485 
The beneficial effects on fertility from burning the soil have long been 
known. In Europe and England the practice of burning, and of paring 
and burning, the soils was very common at one time. It has been thought 
by many that the beneficial effects of this practice were. due to a physical 
bettering of the soil (King, 1906). 
It has been observed in India (Howard and Howard, 1910) that the 
drying of the alluvial soils in direct sunlight increases their productivity, 
and some of the natives make a practice of exposing their soils to this 
ameliorating influence. 
According to Alway and Vail (1909), the moistening and drying-out 
of soils under some conditions causes a natural fertilization of the deeper 
layers due to the falling of the organic matter into the cracks. 
Ehrenberg (1915) states that many investigators have observed a 
bettering of the quality of the soil as a result of its drying out, and quotes 
Vanha as saying that the energetic drying-out of soils rich in humus favors 
the quality of such soils especially. 
In connection with the benefits of drying scils, Ehrenberg cites the old 
custom of using garden walls made of loam as a fertilizer; and King (1911) 
tells of the practice among the Chinese of tearing out ‘“ kangs”’ that 
have been made of clay subsoil and thoroly dried, and using them as 
fertilizer. 
Ehrenberg is of the opinion that only soils containing considerable 
organic matter are affected by drying. The effects of drying upon other 
soils are very slight indeed, and are quickly lost on the soil’s being wetted, 
according to that author. 
The effects of drying the soil are sometimes injurious. Hilgard (1911) 
reports injuries to plants caused by the drying and cracking of soils and 
the consequent mechanical tearing of the root systems. Sometimes the 
shrinking of the surface crust of soil around the stems of grain causes a 
constriction that is injurious. Ehrenberg has reported that a disease of 
sugar beets is traceable to such mechanical injury. 
Mathews (1916) has pointed out the importance of moisture conditions 
in the irrigation of certain heavy soils. If the surface of these soils has 
been dried and then wetted, the irrigation water enters slowly, due to the 
swelling of the soil and the closing of the cracks. 
The extensive literature bearing on the specific physical, chemical, and 
biological effects of moisture changes in soils has been rather fully sum- 
