1 8 Journal of Agricultural Research voi. xxii. No. x 



ENVIRONMENTAL FACTORS WHICH HAVE PREVIOUSLY BEEN FOUND 

 TO MODIFY THE FORMATION AND DEVELOPMENT OF NODULES 



In the extensive studies which have been made of the conditions 

 which may favor or hinder the development of nodules, no one, so far 

 as the writers are aware, has concerned himself with the factor which 

 is considered here — ^namely, the temperature of the soil. There are a 

 number of factors, however, which are known to have very much influence 

 upon nodule development, and two of these which may have become 

 modified by our experimental methods for controlling soil tem- 

 perature must be considered. These are soil moisture and the concen- 

 tration of nitrates. 



With regard to soil moisture, there appears to be both observational 

 and experimental data which indicate that high soil moisture tends to 

 increase nodule formation. Gain * notes that peas grown in wet soil 

 have far more nodules than those grown in drier soil close by. Wilson ^ 

 in his experimental work reports that wet soil induces the formation of 

 a greater number of nodules on soybeans. Fortunately, in experimen- 

 tal work with controlled temperatures, it is comparatively easy to main- 

 tain soil moisture at a predetermined point with very slight fluctuation. 

 In the preliminary experiments, although no attempt was made to 

 control soil moisture exactly, it is not believed to have fluctuated suffi- 

 ciently to affect results appreciably. In the later work, soil moisture 

 was maintained in each series at one-half the moisture-holding capacity 

 of the soil used (14 per cent of the wet weight) by weighing the pots 

 each day, if necessary, and restoring the water lost by evaporation and 

 transpiration. It is believed that this method kept variation in soil 

 moisture within such very narrow limits that this factor could not have 

 produced appreciable variation in nodule formation. 



That the amount of nitrate present in the soil affects nodule develop- 

 ment, completely inhibiting it when high concentrations have been 

 reached, has been demonstrated by several investigators. Wilson ^ has 

 added a considerable number of nitrates to soils in different amounts to 

 determine the concentration at which nodule formation is inhibited by 

 each of the compounds. Although complete inhibition is effected only 

 at concenti"ations which are not likely to occur in normal soils, the 

 marked effect of variations is so well attested that any differences 

 arising unavoidably during an experimental series must be taken into 

 account in the consideration of results. 



The control of the concentration of nitrates in the soil solution in a 

 soil held at different temperatures offers difficulties which can be over- 



1 Gain, Edmond. influence de i.'humidite sur le developpement des NODosixfes des l^gumineu- 

 SES. In Compt. Rend. Acad. Sci. [Paris], t. ii6, no. 24, p. 1394-1397. 1S93. 



2 Wilson, J. K. physiological studies of baollus radiocola of soy bean (soja max piper) and 

 OF factors influencing nodule production. N. Y. Cornell Agr. Exp. Sta. Bui. 386, p. 363-413, fig. 

 80-94. 1917. 



