EFFECT OF DRYING SOILS ON WATER-SOLUBLE CONSTITUENTS 181 



the subsequent relation of heat to the destruction of this colloid are two of the most im- 

 portant of these factors." When soil contains some capillar}' or film water this moisture is 

 distributed about the particles as a thin film varjang in thickness with the quantity of water 

 present in any given soil. It is stated that tlie moisture film in air-dry soils is held with a 

 force equal to 10,000 atmospheres and that under such conditions "the concentration of film 

 water with reference to the mineral matter should be much greater than that of the free 

 or capillary water in tlie soil." They hold that air-dried soils should, and their results are 

 claimed to, show least solubility. 



The films with organic and inorganic matter in solution may be looked upon as colloidal 

 in nature. Upon heating to 100°C. alteration in the film occurs through evaporation and by 

 partial dehj-dration of the colloids, destroying the pressure by which the film was i)reviously 

 held around the particles. During evaporation the concentration of the soil moisture would 

 increase to the saturation point, after which mineral matter would be deposited with further 

 evaporation. 



The solution obtained upon adding water to oven-dried soil should be of greater con- 

 centration than that from air-dried soil. With water films absent and the colloids altered, 

 the water has more ready access to the soil particles. They found some mineral constituents 

 more soluble at 250°C. than at 100°C. and think it due to "more complete elimination of 

 soil moisture and especially the water of chemical combination." 



Hulett and Allen (37) showed that the concentration of the solution in equilibrium with 

 a curved surface is greater than that in equilibrium with a plane surface and that gypsum is 

 most soluble in water at 40°C. Above 80°C. it is less soluble than at 0°C. 



McGeorge (66) reports further results of heating soils in sunlight, in an oven for 2 hours 

 at 80, 110 and 165°C., and in an autoclave for 1 hour at 10 pounds pressure. Onions and 

 cowpeas showed detrimental effects while millet show^ed increased vigor with the higher 

 temperature of sterilization. Heating gave better results than volatile antiseptics. 



Ehrenberg (21) speaks of the old custom of using as fertilizer old garden walls made of 

 soil and says that many soil workers have noted an improvement as the result of a soil drying 

 out. He thinks soils rich in organic matter, only, are affected materially by drying. 



Hall (27) allowed eight pots of similarly treated Dunkirk clay loam to dry from October 19 

 to March 1. At this time the moisture content of four pots was brought up to 20 per cent 

 and held there until April 12. The other four continued to dry until JSIarch 19, at which 

 time the moisture content was 1.8 per cent. The first four pots had an average of 847 parts 

 per million (of dry soil) of total soluble solids and 5.18 parts per million of nitrates, while the 

 average for the four air-dry soils was 1303 parts per million of total salts and 324 parts per 

 million of nitrates, a marked increase due to drying. In October, a sample of the soil was 

 dried. In IMarch it had 1628 parts per million of total soluble-matter and 397 parts per 

 million of nitrates. A sample of the original soil bottled at 12.2 per cent moisture in October 

 had in March 1459 parts per million total salts and 495 parts per million of nitrates. This 

 shows that nitrification had been active as in the soil which dried to March 1 . 



Klein (53) conducted experiments with Dunkirk clay loam (a) low in organic matter and 

 (b) well supplied with organic matter, this being timothy sod which had been piled up and 

 allowed to decay. Keeping soil a at 15, 20, 25 and 30 per cent moisture and b at these mois- 

 ture contents with an additional sample at 40 per cent, gave an increase in growth of wheat 

 on a with a decrease in moisture, and the same general relation held for b except that the soil 

 with 40 per cent moisture gave practically the same yield as that with 15 per cent. There is 

 no important difference in the yield of buckwheat following the wheat. Soil a, unplanted, 

 contained more total soluble solids with the lower moisture contents, while soil b showed an 

 increase in total soluble solids with an increase in the water content. Nitrates decreased 

 wath the lowering of the water content. Difference in water content had no effect on solu- 

 bility of potassium, calcium and phosphorus in this soil. Air-drj-ing reduced the nitrates, 

 but when later brought up to and kept at optimum moisture content for various periods 

 greater than 16 days, the nitrates increased materially. The nitrifying power and power to 

 produce carbon dioxide is, in general, affected in the same way. 



