Scientific Agriculture. 



40 



[Jan, 1908. 



expands as it is heated, and thus some of it is driven into the atmosphere. If the 

 rise in temperature amounts to 10° when the temperature of the soil stands at 45°, 

 then one-fifth 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 fall half an inch, the air expands and about one-sixtieth of it 

 escapes ; if the pressure falls one inch, one-thirtieth escapes. Rain is a very potent 

 factor. As the water sinks into the ground, an equal volume of air must be dis- 

 placed. 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 laud, the imprisoned air must escape \ipward 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 thi ough 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 supe- 

 riority 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 to the depletion of the stock of plant food 

 as to the lack of proper sanitary condition. 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 important part, certain fertilizer ingredients are effective under certain 

 conditions, 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 rotations 

 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, without any apparent exhaustion, and indeed by 

 wise rotation even increasing the yield.— Ontario Department of Agriculture, 

 Bulletin 156, March, 1907. 



