TILLAGE 206 



soil may cause disease because it interferes mechanically 

 with the growth of roots. If the soil is excessively wet, 

 its proper aeration is prevented. If it is too dry, the plant 

 suffers from lack of water. These produce abnormal or 

 diseased conditions in the plant, disturbing normal 

 physiological activities, resulting in unusual and un- 

 healthy appearance and lessened crop production. All 

 of these difficulties — alkalinity, acidity, as well as me- 

 chanical defects — may be alleviated by tillage. 



Tillage. — In many parts of our country the American 

 farmer, while liberal in the use of water and fertilizer, 

 giVBS insufficient attention to the physical condition of 

 the soil. Deep plowing, for example, which keeps down 

 weeds an^d exposes fresh surfaces to the action of air and 

 light, also brings toward the surface fresh parts of the 

 soil from which food materials may be the more readily 

 extracted. 



Polluted Air. — Certain air pollutions may cause 

 chemical injury and disease in plants. In the air of 

 cities, or in the vicinity of industrial plants in which 

 coal or ore containing sulphur is burned or roasted, 

 sulphurous vapors are invariably present. These cause 

 disease by interfering with the normal functions of 

 the plant, especially those connected with food manu- 

 facture (photosynthesis) (Fig. 52). Pollution of the air 

 by solid particles, whether of soot or dust from roads 

 or such works as cement manufacturing plants, may 

 cause such impairment of the clarity of the air as greatly 

 to reduce the amount of light reaching the surface of the 

 earth, or the solid particles may form an opaque coating 

 over the leaves and so prevent the absorption of light by 

 the chlorophyll-containing cells. In either event, the 

 amount of food manufactured may be greatly reduced. 



Moist and Dry Air. — The humidity of the air may 

 sometimes by sudden change cause disease, not always 

 recognized as to its origin. Sudden dryness of the air, 

 such as sometimes accompanies high wind, may " burn " 



