434 



SCIENCE 



[N. S. Vol. XLIII. No. 1108 



PRELIMINARY STUDIES ON HEATED SOILS 



A FAIRLY extensive amount of data lias been 

 accumulated upon the immediate changes in- 

 duced in soils heated to temperatures between 

 50° C. and 500° C, with reference to the effect 

 of such treatment upon seed germination and 

 plant growth. The results appear to have some 

 value in explaining the striking effects, in- 

 jurious and beneficial, observed on sterilized 

 or partially sterilized soils. The work of Eus- 

 sell^ and his associates, Pickering- and 

 Schreiner and Lathrop^ have led in general to 

 quite different conclusions as regards the 

 nature of the injurious action. These and 

 other workers have also maintained consider- 

 ably different views regarding the nature of 

 the beneficial action of sterilized soils. The 

 difference in opinion is perhaps due in a large 

 measiire to the point of view from which the 

 investigation has been undertaken, as well as 

 to the manner in which the sterilization of the 

 soil has been accomplished. The conclusions 

 drawn here are considered to apply partic- 

 ularly to soils heated above 100° C, although 

 it is believed that the same principles apply 

 in soils heated to lower temperatures. An 

 endeavor has been made to give due considera- 

 tion to the several phases of the subject since 

 these involve not only chemical, biological and 

 physical changes in the soil, but also the physi- 

 ological and pathological conditions of the 

 seeds and plants grown in these soils. 



The method of investigation by which the 

 results presented in this paper were obtained 

 has been largely that of attempting to corre- 

 late the chemical changes produced in the 

 heated soils with their effect upon seed germi- 

 nation and plant growth. The amount of 

 water-soluble material formed by heating has 

 been measured by the lowering of the freezing 

 point. For this the Beckmann thermometer 

 was used. Ammonia was determined by the 

 ordinary method of distilling in the presence 

 of magnesium oxide. The nitrate was deter- 



1 Eussell and Petherbridge, Jour. Agr. Sci., 5 : 

 248-287, 1913. 



i Pickering, Jour. Agr. Sd., 3: 277-284, 1910. 

 Soils Bui. 89, pp. 7-37, 1912. 



3 Schreiner and Lathrop., tJ. S. Dept. Agr., Bur. 



mined colorimetrieally by the phenoldis\il- 

 phonic acid method. Seed germination tests 

 were made on the soil in Petri dishes. The 

 seeds were placed on the surface of the soil, 

 which was almost saturated with distilled 

 water. Various kinds of seeds were employed, 

 but especial use was made of cabbage. 



The results in general were similar for the 

 different seeds, though they varied much in 

 their susceptibility to the injurious action. 

 Lettuce and clover seeds, for instance, were very 

 susceptible to the injurious action of highly 

 heated soils, whereas rye and buckwheat were 

 very resistant. Plant growth is affected in 

 much the same manner, wheat, for example, 

 recovering rapidly from the deleterious action 

 of certain heated soils where tomatoes ap- 

 peared to be permanently injured. Different 

 soils give markedly different results upon heat- 

 ing to the same temperatures. The action ap- 

 pears to be dependent particularly upon the 

 content of organic matter in the heated soil, 

 as this influences both the amount of decom- 

 position and the absorptive power of the soil 

 for the substances produced upon heating. 

 These results are in general confirmatory of 

 the work of others upon this subject. 



The temperature to which the soil is heated 

 is seemingly the most important factor in 

 determining the extent of the injurious or 

 beneficial action. Approximately 250° C. was 

 found to be the most critical temperature in 

 all the soils used. At this temperature seed 

 germination was most strikingly retarded. 

 Early plant growth was usually checked for 

 the longest period of time on soils heated to 

 250° C, although late plant growth, in the 

 case of some crops at least, was most vigorous 

 on these soils. Heating to temperatures of 

 300° C, or above, in all the soils used, again 

 reduced the injurious action to seed germina- 

 tion and early plant growth, as well as the 

 beneficial action to late plant growth. 



Heating soils to 250° C. produced greater 

 amounts of material extractable with water 

 than heating to higher or lower temperatures. 

 The ammonia content of the soil increased 

 proportionally to the temperature of heating 

 up to about 250° C, after which it rapidly 



