30 Journal of Agricultural Research voi. xxii. No. i 



temperatures at one-half the moisture-holding capacity of the soil, 

 previously determined to be 14 per cent of its dry weight. In order to 

 get further evidence as to the extent of the effect of high moisture con- 

 tent, a single can containing three plants was placed at each temperature 

 in the last series with moisture content of 18 per cent of the dry weight — 

 a distinctly wet soil. Accidents which befell several plants in the series 

 produce irregularities in the figures which would require long explana- 

 tion. Suffice it to say here that though the tops were increased in size 

 there is no evidence that the nodules were increased either in number or 

 size. Apparently moderate differences in moisture content of the soil 

 were not large factors influencing results in the previous experimental 

 work where exact control of soil moisture was not accomplished. 



HYDROGEN-ION CONCENTRATION OF THE SODv SOL,UTlON 



Inasmuch as it was considered possible that the extreme temperatures 

 at which the soil was held might produce changes which would alter the 

 hydrogen-ion concentration of the soil solution, and hence the formation 

 and perhaps development of nodules, a determination of this environ- 

 mental factor was made toward the close of the last series described. 

 Samples of soil were taken from the unplanted pots at 15°, 24°, and 

 36° C. A determination of the hydrogen-ion concentration of the soil 

 solution of the three samples by the colorometric method gave identical 

 results, the Ph value being 6.3 in all three cases. Thus no evidence 

 was obtained that temperature had altered this important factor in this 



series. 



SUMMARY 



(i) Preliminary studies have been made upon the effect of soil tem- 

 perature on the development of four legumes, alfalfa, red clover, field 

 peas, and soybeans, with special reference to its effect upon the infec- 

 tion of these plants by Bacillus radicicola and the subsequent develop- 

 ment of nodules. The larger part of the data were obtained by growing 

 plants in soil held at a series of temperatures 3° apart from 12° to 36° C. 

 The air temperature was uniform for all plants, ranging from 14° to 20°. 



(2) As was anticipated, the four plants differed in their ability to 

 tolerate soil temperatures at the ends of the series. Peas were dwarfed 

 at 30° C, clover developed poorly at 36°, while alfalfa and soybeans 

 still grew very well at 36°. Soybean plants grown in the soils held at 

 12°, 15°, 33°, and 36*^ showed very dark green color of leaves, whereas 

 those toward the center of the series became progressively lighter, those 

 at 24° being lightest. 



(3) With regard to the number of nodules formed on plants grown in 

 soil held at this series of temperatures, irregularities were found in each 

 series; but no large consistent differences were discovered, except that 

 at the extreme upper and lower limits at which a plant will survive the 



