22 Journal of Agricultural Research voi. xxii. no. i 



DISCUSSION OF THE LEGUMES USED AND THEIR BEHAVIOR UNDER 

 THESE EXPERIMENTAL CONDITIONS 



Four legumes were selected for tlie soil temperature series requiring 

 four different strains of Bacillus radicicola for their inoculation. One of 

 these, the soybean, flourishes well at high soil temperatures; one, the 

 Canada field pea, requires a low soil temperature for good growth; and 

 red clover and alfalfa occupy intermediate positions. 



A few of the more striking reactions of the plants in these series will be 

 noted. The Canada field pea does not flourish vigorously at a soil tem- 

 perature as high as 30° C. (Table I) and is intolerant of temperatures 

 above this point, maintaining roots only very close to the surface of the 

 soil. It is perhaps misleading to infer that the lower surface temperature 

 is alone responsible for the position of the roots, since in field plots in hot, 

 exposed positions the death of deeper roots and the formation of surface 

 roots has been noted in hot weather. 



Perhaps the more striking effect of the series of soil temperatures upon 

 the soybean plants, aside from the fact of the wide range through which 

 it grows vigorously, is the effect upon the color of the foliage. After 

 the plants had become 5 or 6 inches tall, in both series the leaf color was 

 much darker at the two ends of the series, especially at 30° C. and above, 

 than at 2 1° and 24°. This difference persisted, tending rather to increase 

 as long as the plants were grown. 



One striking difference in behavior between peas and soybeans on the 

 one hand and clover and alfalfa on the other was noted in this series. 

 The annuals formed a rather regular series of plants as judged by appear- 

 ance (PI. 1) and also by dry weights (Table I). But with alfalfa and 

 red clover the seedlings at the lower temperatures, 12° and 15° C, though 

 little delayed in starting, remained small Alpine plants with thick dark 

 green leaves and \vith much red color in the petioles; whereas at 18° the 

 plants were more nearly what may be termed "normal" plants, larger, 

 with fairly long petioles containing less red color. ^ 



EXPERIMENTAL DATA 



EFFECT OF SOIL TEMPERATURE UPON THE NUMBER OF NODULES FORMED 



As previously noted, in the fiirst two preliminary trials the seeds were 

 planted in soil in the metal cans and grown for about 10 days at green- 

 house temperature (about 22° to 23° C.) before they were inoculated 

 with the suitable strains of Bacillus radicicola and placed in the tanks 

 adjusted at the temperatures designated. When it was believed that 

 sufficient time had elapsed for infection at all temperatiu-es, the tops 

 were cut from the plants, dried, and weighed. The roots were carefully 



1 Since this was written clover and alfalfa have been grown under similar conditions at controlled tem- 

 peratures. The marked dwarfing of plants at 15° and 12° C. was found to disappear when the plants became 

 older, and especially later in the spring when light intensity became greater. 



