292 DRY-FARMING 
come from without. The nitrogen question will 
undoubtedly soon be the leading one before the 
students of dry-farm fertility. A liberal supply of 
organic matter in the soil with cultural methods 
favoring the growth of the nitrogen-gathering bac- 
teria appears at present to be the first solution of the 
nitrogen question. Meanwhile, the activity of the 
nitrogen-gathering bacteria, like azotobacter, is one 
of our best explanations of the large presence of 
nitrogen in cultivated dry-farm soils. 
To summarize, the apparent increase in produc- 
tivity and plant-food content of dry-farm soils can 
best be explained by a consideration of these factors: 
(1) The intrinsically high fertility of the arid soils; 
(2) the deep feeding ground for the deep root systems 
of dry-farm crops; (3) the concentration of the plant 
food distributed throughout the soil by the upward 
movement of the natural precipitation stored in the 
soil; (4) the cultural methods of dry-farming which 
enable the weathering agencies to liberate freely and 
vigorously the plant-food of the soil grains; (5) the 
small annual crops; (6) the plowing under of the 
header straw, and (7) the activity of bacteria that 
gather nitrogen directly from the air. 
Methods of conserving soil-fertility 
In view of the comparatively small annual crops 
that characterize dry-farming it is not wholly im- 
