136 Mineral Nutrition of Plants 



SOIL PHYSICAL PROPERTIES 



The physical properties of a soil play a significant role in plant growth 

 by controlling the air, water, and, to a certain extent, nutrient supply 

 to the roots of growing plants. In considering these properties we are 

 primarily concerned with the pore spaces between the soil particles 

 because it is here that roots exist, and the necessary water and air move- 

 ments occur. The proportion of the total soil volume made up of pore 

 space, the size, and shape of the pores are all extremely important. 

 These characteristics in turn are governed by two properties: the par- 

 ticle-size distribution and the arrangement of the soil particles, particu- 

 larly as they may be clustered into larger aggregates or crumbs. 



Particle-size distribution and dependent properties 



That quality of a given sample of soil taken from the soil profile, 

 which may be expressed as a particle-size distribution curve, probably 

 represents the closest approach to an intrinsic physical soil property. 

 But it must be understood that in order to obtain reproducible results 

 particle-size measurements must be made according to standardized 

 arbitrary methods of soil pretreatment and dispersion. 



For purposes of obtaining expressions of "ultimate" particle-size dis- 

 tribution, dispersion methods commonly in use aim at the highest pos- 

 sible dispersion short of actual solution. Expressions having this basis 

 are now used in the choice of soil texture names. However, C. L. Clark 

 (29) pointed out in 1933 that at that time there was no proof in support 

 of the correctness of the assumption that a unique particle-size distri- 

 bution exists for a given soil sample and, to the authors' knowledge, 

 none has been presented in the subsequent fifteen years. 



From the published experiences of investigators in soil physics it 

 must be concluded that most, if not all, other physical properties that 

 have been examined are remarkably susceptible to modification by 

 external factors, and to external forces of one kind or another which 

 have been brought to bear upon the soil. The physical factors which 

 affect plant-soil interrelations, therefore, are— with the qualified excep- 

 tion of particle size of the solid phase— notably subject to quantitative 

 change, brought about both by natural events and by the practice of 

 soil management. Under field conditions, alterations associated with the 



