50 FIRST PRINCIPLES OF AGRICULTURE. 



vided and too compact, the reverse is the case. The 

 tubes and pores through which the water passes, if un- 

 disturbed, admit of the rapid escape of water ; if dis- 

 turbed, the evaporation is arrested. 



Tillage Conserves Moisture. — Cultivating, harrow- 

 ing, and rolling disturb or break the connection of the 

 pores with the surface, thus reducing the evaporation 

 until the connection of the tubes with the surface is 

 again established. The amount of water transpired by 

 growing plants is enormous — from three to five hun- 

 dred pounds for each pound of dry matter formed ; 

 and its escape, other than through the plant, should be 

 prevented as far as possible. 



Tillage also destroys weeds, which require for their 

 growth quite as much plant-food and moisture as culti- 

 vated plants. 



For cultivated crops frequent tillage is recommended in 

 dry seasons, in order that the greatest possible amount 

 of moisture may be retained where the feeding roots are 

 located ; the dry, pulverized surface soil acts as a mulch 

 or blanket, and diverts more of the moisture to the 

 roots of the plant. Too deep cultivation in dry seasons 

 frequently does more harm than good, unless, in the 

 preparation of the seed-bed, the soil has been thor- 

 oughly and deeply pulverized. 



Chemical Improvement. — Soils are improved chemi- 

 cally by the addition of materials which contain constit- 

 uents that are liable to be lacking, or which have the 

 power of converting dormant into active constituents. In 

 many cases both of these objects are accomplished at 

 the same time. Materials containing nitrogen, phosphoric 



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