184 A. F. GUSTAFSON 



Robinson (84) states that the lime requirement of soils, as shown by the Veitch method, 

 is afifected by (a) "the temperature at which evaporation is made," (b) continued heating 

 after soil is dehydrated, (c) length of time during which treated and dried soil is in contact 

 with water, and (J) the source of heat, such as steam or sand bath or hot plate. 



An enormous amount of interesting work on the soluble-solid content of soils under many 

 different conditions of cropping is reported by King (42, 43, 44, 46), King and Jeffery (49), 

 King and Whitson (50, 51, 52) and Whitson (104). 



The literature of drying, heating and sterilizing soil has been quite extensively reviewed 

 by Lyon and Bizzell (62) and (63), Schreiner and Lathrop (90), Kelley and McGeorge (40), 

 Klein (53), Hall (27), Kopeloff and Coleman (57), Stewart (96), Beaumont (3) and Johnson 

 (39). 



SUMMARY OF LITERATURE 



1. Heating soil in various ways for its beneficial effect on crops is an ancient 

 practice. 



2. For several decades past, commercial greenhouse men have sterilized the 

 soil used, to kill detrimental organisms, and have noted beneficial results 

 other than from sterilization, particularly increased growth of leaves and stems. 



3. Much careful experimental work on heating and drying soils has been 

 reported, both before soil organisms were recognized, and in connection with 

 soil-biology studies, which shows that drying and heating soil at 100°C., or 

 higher, increases its productiveness, even though germination may be retarded 

 and early growth depressed. 



4. The literature shows that the quantity of soluble mineral and organic 

 constituents recovered by extraction with distilled water is increased by 

 heating. The increase bears some relationship to the temperature of heating, 

 the maximum of soluble constituents being found at about 250°C., above 

 which the total salts recovered decreases. 



5. Investigators are not in general agreement as to the effect on nitrates of 

 heating at 100°C. Many workers note a decrease as the temperature is 

 further raised and almost total disappearance of nitrates at 250°C. 



6. Soil workers are not a unit as to the cause of the increase in soluble 

 material due to drying and heating. Some hold the effect of heating to be 

 largely physical; others that it is mainly chemical, and still others lay most 

 stress on the biological phase. Nearly all admit that the physical is usually 

 a factor and others add colloids as a physio-chemical factor. 



7. There is wide variance of opinion as to the degree to which nitrates are 

 recovered by one or more extractions. 



EXPERIMENTAL WORK 



Introduction 



While the literature shows that an enormous amount of work has been 

 done on the effect of heating at a wide range of temperatures and under varied 

 moisture contents on the amount of soluble constituents of soils, it was con- 

 sidered desirable to study the effect of drying at 105°C. for 8 hours, the ordinary 

 method of driving of so-called hygroscopic moisture, on the total soluble solids 

 that may be recovered by 1 : 5 extraction with distilled water. 



