182 A. F. GUSTAFSON 



Wilson (105) found that heating at 60 to ISO'C. for 2 hours increased the amount of soluble 

 matter and changed the physical condition so that its water-holding capacity was affected. 

 lie accounts for increased productivity on these grounds. 



Huck (7) reports results of a study on the effect of heat on soils, by Mann who found the 

 water-soluble constituents increased with the rise in temperature to which the soil was 

 heated. He notes greater growth of rice seedlings immediately after heating, quite the reverse 

 of liis experience with other plants, which may be due to the ability of the rice seedling to 

 withstand any harmful effects of, or to use in growth, the ammonia which many hold to be 

 a result of heating. 



The work of Kelley and Thompson (41) shows that nitrates undergo decomposition, 

 gradually disappearing as the temperature is raised. Only slight decomposition took place 

 at 100°C. Steam heating at 2 atmospheres produced effects similar to those resulting from 

 heating at 150°C. without pressure. 



Houyoucos (5) heated sandy loam, loam, clay and peat at 15 atmospheres pressure in an 

 autoclave for three hours, thereby increasing the water-soluble material, as shown by the 

 depression of the freezing point, respertivelj% 75, vSO, 190 and 333 per cent. 



In e.Kplanation of the effect of heat, he points out that water films in intimate contact 

 with the soil particles are more concentrated than capillary or interstitial water, due to the 

 slowness of diffusion. If only the capillary water is extracted, the quantity of soluble matter 

 recovered would be less than the total actually in solution in the soil moisture. He suggests 

 that adsorption may account for a higher concentration at the immediate surface of the par- 

 ticles than in the bulk of the solution. Furthermore, there is wide variation in the solubility 

 of the minerals composing the soil and because of the extremely slow rate of diffusion, different 

 mineral particles would be enveloped by films of varied concentration. This, too, would 

 interfere with recovering from moist soil all of the soluble material. 



Allen and Bonazzi (2) quote Stevens and Withers showing "only about 40 per cent of the 

 citrates were recovered by 1:3 extraction when small quantities were added and more than 

 twice this amount when larger quantities were added." Allen and Bonazzi recovered in the 

 ■first extraction (1:5 with 100 gm. of soil) from 65.9 to 83.9 per cent of the nitrate added, or 

 as an average of ten results reported, 77.4 per cent. 



Potter and Snyder (78) report a recovery of 93 to 97 per cent of the nitrate added at the 

 rate of 3 parts per million of soil (phenoldisulfonic acid method). 



These authors (79) report complete extraction of nitrates when 1 part of soil is shaken with 

 2 parts of water for 30 minutes (aluminum reduction method). 



Johnson (38) reports preliminary results, showing that heating increased the solubility of 

 minerals and the growth of plants. Heating to 250°C. produced more water-extractable 

 substance than lower temperatures. 



Skalskij (95) in studying methods of sterilization heated soil in an autoclave for 1 hour 

 at 2.5 atmospheres. Plants in this soil grew as well as those in soil receiving complete ferti- 

 lization, the number of bacteria was greater than in the untreated soil, the inoculation coming 

 from the air. The improved fertility was due to a large increase in the soluble phosphorus, 

 from 47 to 121 per cent, and while the soluble nitrogen content was not affected by heating, 

 the dark green color clearly showed an increase in the available nitrogen. 

 Connor (15) reports a reduction in acidity as a result of heating. 



Coleman, Lint and Kopeloff (14) found tliat the soluble solids recovered by 1 :4 extraction 

 •of a moist Penn clay loam soil (25 per cent water on the dry basis) after intermittent partial 

 sterilization at 82°C. for 1 hour on each of 5 consecutive days, was increased 46 per cent, but 

 the amount of soluble salts was not appreciably increased after the first day's heating. 

 The air-dry soil (4.5 per cent moisture) when similarly treated showed no increase in water- 

 soluble solids. Sterilization by moist heat at 120°C. for 15 minutes at 15 pounds pressure 

 increased the water-soluble solids recovered 0.0220 to 0.1805 gm., an amount 8.2 times as 

 great as that recovered from the original soil. It should be noted also that volatile antiseptics 

 .applied as vapor in a partial vacuum, increased the water-soluble solids in air-dry soil 22 per 



