148 Mhieral Nutrition of Plants 



system is established. Such root systems are only established in soils 

 when soil aeration is good. 



Loehwing (48) stated: "Improper composition of soil air manifests 

 itself in reduced, slow-growing root systems, inadequate absorption, 

 short-lived, discolored foliage and delay or failure of reproductive pro- 

 cesses." Albert and Armstrong (/) found that fruit bud shedding and 

 poor plant growth in cotton definitely resulted from poor soil aeration. 

 Cannon (24) stated: ". . . it can be seen therefore, that there comes a 

 point in the diminution of the oxygen content of the soil atmosphere 

 when the growth of the root ceases because it is no longer sufficient to 

 supply demands for energy correlated with physiological activities of 

 higher temperatures." He stated further that ". . . in puddled soils with 

 consequent poor aeration, and in summer, the matter of oxygen supply 

 to the roots must be acute." He concluded that for corn to attain a fair 

 rate of root growth at high temperature, aeration must be good indeed. 



PHYSICAL PROPERTIES AND NUTRIENTS 



The role played by soil clays in exchange reactions, and replenish- 

 ment of the soil solution by replaceable metallic cations subsequent on 

 their depletion by plants, is prima facie evidence of the significance of 

 the soil particle-size distribution upon nutrient availability. This state- 

 ment is not intended to minimize the fundamental importance of the 

 mineralogical species present in the clay fraction, the possibility of non- 

 replaceable fixation of ions, or the degree of accessibility to solutions 

 and living roots of the sites of exchange reactions. There is, however, 

 much evidence of significant positive correlation between amounts of 

 exchangeable cations and fineness of soil texture. 



For illustration, reference may be made to analyses by Hosking (43) 

 of the cation exchange capacity at pH 9 of a number of soils and their 

 mechanical fractions from Australia and New Guinea. The soils repre- 

 sent a wide variety of genetic groups and lithologic origins. The ex- 

 change capacities per unit mass are by far the greatest for the material 

 finer than 1 micron and, where observed in the coarser fractions, are, 

 with apparent justification, attributed to incomplete dispersion. The 

 exchange capacities of the whole soils, corrected for organic matter, 

 increase with content of clay finer than 1 micron. This is particularly 

 marked for those soils reported to contain some form of montmorillo- 



