76 SOILS AND FERTILIZERS 



the roots to come in contact with a greater number of 

 new soil particles, and thus a much larger surface is 

 open to the action of the root hairs. As a consequence of 

 this, a greater supply of plant food is rendered available. 



(3) Forming a dirt mulch by cultivation and thus 

 decreasing the evaporation of soil water has already 

 been discussed. It is equally true that evaporation at 

 the surface will be increased by tillage, but then little 

 of this water would be available to the roots, and the 

 formation of the dust mulch will conserve the moisture 

 that is deeper in the soil, that is, near the roots. 



(4) Weeds are harmful because they take for their 

 own sustenance what more useful plants require. They 

 are greedy eaters and hard drinkers. Cultivation at 

 the right times uproots and destroys these, and thus 

 saves the nourishment and moisture in the soil for the 

 growing plants. 



(5) The beneficial effects of aeration, or admitting 

 air to the soil, have been referred to before, but not 

 explained. The oxygen of the air combines with the 

 carbon of decaying vegetable matter in the soil forming 

 carbonic acid, which is a powerful solvent of some of 

 the compounds of the soil. The breaking up of some 

 of these compounds makes possible the formation of 

 other more soluble compounds, and thus contributes 

 directly to plant growth. 



Note. Microscopic plants abound in the soil. They are called 

 bacteria. They need air for growth. These bacteria are beneficial in 

 that they cause decay of plant and animal bodies in the soil and thus 

 increase the supply of humus. They are useful in forming soluble 

 nitrates from the various insoluble nitrogen compounds in the soil, and 

 thus render available an important plant food. Tillage, by supplying 

 more air to the soil, promotes the growth and work of these organisms. 



