142 Mineral Nutrition of Plants 



pacted soils. The soils were compacted while moist to densities ranging 

 from 1.46 to 2.06 g. per cc. At the beginning of the experiment they con- 

 tained amounts of water calculated in all but one case to be in excess of 

 their permanent wilting percentages, and in all cases to be well below 

 saturation at the final, increased density sought; but no roots penetrated 

 any of these soils and there was negligible loss of water from them. 

 These authors have also observed little or no water extraction by pine 

 trees, grapevines, fig trees, and chaparral shrubs from subsoils having 

 the densities stated. No one density represented the limiting density for 

 all soils examined, but results suggested that somewhat lower densities 

 prevented root penetration more in fine than in coarse-textured soils. 



SOIL AERATION 



Soil aeration is the interchange of the soil atmosphere with the free 

 atmosphere above the soil. Continued soil aeration is essential to re- 

 move the carbon dioxide produced by plants and microorganisms and 

 to supply the oxygen needed by plant roots and the microorganisms 

 in the soil. Obviously this interchange must occur in pores large enough 

 that capillary forces do not keep them filled with water, and which 

 have access to the atmosphere through a network of interconnected, 

 unobstructed pore space channels. It has long been recognized that 

 good soil aeration is needed for normal plant growth, and most farmers 

 or gardeners are of the opinion that one of the chief purposes of tillage 

 or drainage is to assure good soil aeration. In spite of this, comparatively 

 little work has been done on the actual role of soil aeration in plant 

 growth and little is known about the mechanics of the interchange of 

 gases in the soil. Two facts have, however, been quite definitely estab- 

 lished: oxygen is essential for normal root growth and extension, 

 and oxygen is essential to the root if it is to carry out its normal func- 

 tion of absorbing nutrients and water. 



Much of the work which has been done on the role of oxygen in root 

 growth has been done in solution cultures. Admittedly, solution cul- 

 tures and soil offer quite different environments for the growth of roots. 

 Roots of different plants may develop and function differently in solu- 

 tion cultures than they do in soils. If, however, it is demonstrated that 

 roots require oxygen for their normal functioning in solution culture, 



