Oct. 1, 1921 Effect of Soil Temperature on Development of Nodules 31 



number is reduced, and that peas usually produced greatly increased 

 numbers at 30° C. All these species form nodules in soils at any tem- 

 perature at which the plant can make a growth that is at all vigorous. 



(4) While variation in number was not consistent, size measured by the 

 average dry weight per plant of all those formed on a number of plants 

 was found to differ greatly and consistently within the series, at least so 

 far as the soybean plant was concerned. The maximum weight attained 

 on the soybean plant after a period of two months was found at a soil 

 temperature of 24° C. Examination of nodules on the roots of the other 

 legumes indicated that their maximum production occurred at about the 

 same temperature. 



(5) Weight of nodules produced by soybeans was not found to be corre- 

 lated with the weight of tops or of roots through the series of tempera- 

 tures. Weight of tops was almost or quite as great at 30° to 36° as at 

 24° C, while weight of nodules declined rapidly at the higher tempera- 

 tures. Weight of roots likewise showed no such diminution at the higher 

 temperatures or even at the lower temperatures as did weight of nodules. 

 With the soybean plant, and to a much less marked degree with the other 

 plants, there was a correlation between weight of nodules and color of 

 plant, the largest weight of nodules occurring on plants with the palest 

 green color. 



(6) Generally speaking, plants with large nodules had a higher percent- 

 age of total nitrogen in the tops, though this correlation is not exact. 



(7) Factors of soil environment that are regarded as having an influence 

 upon nodule formation have been taken into account. Soil moisture has 

 been controlled within narrow limits. Concentration of nitrates and the 

 hydrogen-ion concentration of the soil solution have been recorded. It 

 is not believed that variations in any of these factors are to be regarded 

 as having produced the variations in nodule development recorded at the 

 different temperatures in these series. 



