254 Bacteria in Relation to Country Life 



Fig. 47. 



Tubercles on the loots of an old 

 alfalfa plant. 



success. While on 

 light soils, green- 

 manures serve the 

 threefold purpose of 

 supplying nitrogen, 

 fermentable, organic 

 material as an aid in 

 dissolving the mineral 

 plant-food in the soil, 

 and finally humus as 

 an aid in increasing 

 the water - holding 

 power of the soil. 

 The last-named func- 

 tion of green-manures 

 is not as important 

 on the heavier soils. 

 Furthermore, because 

 of the greater quanti- 

 ties of moisture con- 

 tained in them, the 

 heavier soils do not 

 warm up as rapidly 

 in the spring, nor does 

 the air circulate in 

 them as freely. 



The decay bacteria 

 are not stimulated in 

 their activities to such 

 an extent as in the 

 more open sandy soils, 



