THE SOIL AND SOIL FORMATION 71 



put in near together in heavy, close soils, accelerates 

 the circulation of air in the soil to a considerable degree. 

 Drains remove the water, allowing the air more room 

 among the particles at a greater depth. When rain 

 causes the surface of the free water, or ground water, in 

 the soil to rise, the air is, of course, crowded out of this 

 saturated part, but when the surface of the zone saturated 

 with water, is again lowered by the drains, the air again 

 settles into the interstices among the soil particles. 

 Drainage allows the roots of plants to penetrate deeper, 

 and these, on decaying, leave deeper holes through which 

 the air circulates more freely in dense soils. It also in- 

 creases organic matter, bacterial action and plant feeding 

 deeper down in the subsoil. 



Plowing and subsoiling give aid to all these forces 

 which tend to make air circulate in the soil. In very dry 

 weather, clay soils sometimes become so much shrunken 

 that large cracks are produced and through these the 

 air can circulate in the soil. In the northern states 

 freezing does a great deal to prevent the soil from be- 

 coming too compact. By the freezing and the resultant 

 expanding of the soil, the particles are pushed apart and 

 left more open for the following summer season. Veg- 

 etable manures make dense soils looser, and open soils 

 closer; in both cases adding to the power of the soil to 

 retain water and soluble plant food. 



Soil water and the plant. The plant secures its min- 

 eral and nitrogenous food supply through the water in 

 the soil, absorbing it through the membranous surface 

 of the new roots and root hairs. These root hairs, which 

 are elongations of the outer cells of the new root near 

 the growing point, increase the root surface, extend 

 thin membranous branches out into contact with the 

 soil and enable them to absorb more of the soil moisture 

 and secure more food. Only a small part of the water 

 taken up by the plant is used in forming new tissue, but 



