34 The Plant World. 



of texture is also the best for the growth of nearly all useful 

 plants, the critical moisture content assumes a large importance 

 in practical agriculture. * 



It is apparent, further, that with a definite mechanical com- 

 position there will be for each water content a corresponding 

 structure or arrangement of the particles which may be con- 

 sidered normal therefor. It does not follow, however, that soil 

 in the field always possesses the structure and texture which are 

 normal to it and for the water content which it happens at that 

 moment to hold. Were soil particles perfectly free to move 

 among themselves the soil mass would no doubt always possess 

 the structure theoretically required by its water content, and 

 changes in water content would be followed immediately by 

 the corresponding changes in structure. But soil particles are 

 not free to move. Gravity (in the form of the weight of super- 

 posed soil), the adhesion of the giains, the friction between them, 

 and similar factors furnish an inertia which strongly resists any 

 change of structure, especially one which would increase the size 

 or amount of the interspaces. In general, therefore, the actual 

 structure of a soil at an)' particular moment is not that corres- 

 ponding to the water content it then holds, but rather that which 

 has resulted from the actions of its various past water contents 

 working against the inertial resistances and complicated bv many 

 extraneous factors of which the main are the operations of agri- 

 culture. But although a soil does not of itself respond readily 

 to changing amounts of water, if it be stirred artificially, as e. g., 

 by plowing, the structure which it will take on is largely deter- 

 mined by its water content. Hence the importance of conducting 

 cultural operations when the soil is neither too dry nor too wet — 

 when its water content is at or near the "critical" value. The 

 presence of this critical (or optimum) amount of water in a soil 

 does not necessarily mean that the soil then possesses its optimum 

 structure, but does mean that it has then and only then the ca- 

 pacity to take on and retain that structure when stirred. 



The water-film forces are by far the most important of the 

 factors influencing soil structure, but there is one other which 

 can not be neglected. In some soils, especially those contain- 



♦In many cases the critical moisture content seems to be practically identical with the so- 

 called "optimum water content" as determined by direct experiments with plants. 

 This "optimum water content" is, however, by no means a constant, and depends 

 upon many factors most of which ire biological and do not concern the soil at all. The 

 author hopes to treat these matters more fully in a later paper. 



