Fallowing Crops 267 



soil could be stirred frequently with implements of 

 tillage so as to hasten the formation of plant-food and 

 the conservation of moisture. In the case of the green- 

 manuring crops, the weeds could be also suppressed, 

 while the subsequent turning under of a mass of green 

 material induced certain fermentations in the soil and 

 hastened thereby the weathering of the rock particles. 

 The succeeding crops were thus placed in possession of 

 larger quantities of available plant-food, the mechan- 

 ical condition of the soil was improved, and, as modern 

 investigations have shown, the store of total plant-food 

 in the soil was increased by the addition to it of nitrogen 

 from the air. 



All of these considerations tended to eliminate bare 

 fallows, and the fallowed areas decreased accordingly. 

 At the same time, some farmers in Europe, not a few of 

 them progressive in their farm methods, have persistently 

 retained the bare fallow as a part of their system of 

 soil management, notably on certain types of soil. More 

 recently, bare fallows have again been given a greater 

 degree of attention on account of various theoretical 

 considerations, among them that of the addition of 

 nitrogen fo fallow soils by non-symbiotic nitrogen- 

 fixing bacteria. 



Fallow and cropped lands. — In comparing fallow and 

 cropped soils of the same origin, there are noticeable 

 differences of temperature, moisture, and aeration. 

 The investigations of Wollny have shown that fallow 

 soils are warmer in summer and colder in winter than 

 corresponding cropped soils. The summer temperatures 

 may show differences between the two kinds of soil of 



