8 



air in the soil expands as it is heated, and thus some of it is driven into 

 the atmosphere. If the rise in temperature amounts to io° when 

 the temperature of the soil stands at 45 , then one-fiftieth of the 

 air in the heated zone is expelled ; and if it amounts to 20 , 

 then one-twenty-fifth is expelled, and so on. The change of atmospheric 

 pressure also aids. If the pressure falls half an inch, the air expands and 

 about one-sixtieth of it escapes ; if the pressure falls one inch, one-thir- 

 tieth escapes. Rain is a very potent factor. As the water sinks into the 

 ground, an equal volume of air must be displaced. As it passes away, 

 by drainage, by evaporation, or by absorption into the plant, the air is 

 drawn into the soil again. Drainage aids very materially. When rain 

 falls on undrained land, the imprisoned air must escape upward through 

 the water as the water sinks down; the two actions thus opposing one 

 another, the air escapes very slowly, often so slowly that large quantities 

 of water, being unable to make their way into the soil, run off the surface 

 and are lost. But if the soil is well drained some of this run-off may be 

 prevented, the imprisoned air escaping downward through the drains as 

 the weight of water above increases, fresh air following the rain into the 

 soil. This gives us another reason for the great superiority of the drained 

 soil over the undrained. Proper tillage increases the efficiency of all these 

 agencies of aeration. 



Another factor, and one that is gaining some prominence at the 

 present time, is a proper sanitary environment for the roots. The latest 

 investigations of the Bureau of Agriculture at Washington arouse the 

 suspicion that the apparent "exhaustion" of soils is not due so much lo 

 the depletion of the stock of plant food as to the lack of proper sanitary 

 conditions. Animals forced to exist in an atmosphere rendered foul by 

 their own poisonous exhalations soon cease to thrive; the plant above 

 ground likewise gives up waste products, which if not removed, become 

 a menace to its safety ; is it not therefore natural to expect that from the 

 roots of the plant also there are excreta that, if allowed to accumulate, 

 threaten its very existence? As proper ventilation is necessary to insure 

 the health of the animal, as diffusion, drafts and winds must bring fresh 

 air to the leaves, so must tillage or other treatment purge the soil of the 

 injurious substances cast off by the roots. In this purifying process it is 

 believed that air, and therefore cultivation and drainage, plays an im- 

 portant part, certain fertilizer ingredients are effective under certain con- 

 ditions, but more potent still is organic matter in the form of humus. 

 There is another method, however, of eliminating the toxic or poisonous 

 effects of these excreta. Whatever they may be, it appears that those 

 cast off by one variety of plant are not, as a rule, injurious to another 

 variety, hence the possibility of rotation of crops. By the time the first 

 crop comes round again, the intervening cultivations having stirred up 

 the soil, exposed it to the weathering processes, allowed the air to enter 

 in and permitted the humus to do its work, all the excretions injurious 

 to that crop have been removed or neutralized and we secure a yield 

 equal to the last one. Hence it is that by proper rotation we may go on 

 cropping our fields from year to year, cropping them indefinitely, with- 

 out any apparent exhaustion, and indeed by wise rotation even increasing: 

 the yield. 



