38 The Plant Wori.d. 



SUMMARY. 



1. The physical condition of a soil is determined 

 by its mechanical composition and its structure and is made 

 manifest or evidenced by various physical properties 

 which may be divided into those included by texture and by 

 mechanical resistance. 



2. The word "texture," as here used, covers those prop- 

 erties related to the interspaces of the soil. 



3. The mechanical composition of the soil determines the 

 limits of variation of texture. The actual texture within 

 these limits is determined by structure. 



4. Structure (and therefore texture) in any particular 

 soil is controlled by the surface tension forces in the water-films 

 about the soil grains, and by flocculations analogous to those oc- 

 curring in colloidal solutions and suspensions. The relative 

 influence of these two factors on structure varies with the me- 

 chanical composition; the former decreasing, the latter increas- 

 ing, as the soil becomes finer. 



5. In ordinary soils when well stirred the structure and 

 texture depend largely upon the water content and are most 

 loose and open at an intermediate percentage which is called the 

 "critical moisture content." This value depends upon mechani- 

 cal composition, being higher with increasing fineness. 



6. The properties classed as mechanical resistance depend 

 directly and almost exclusively upon the number of contacts 

 between particle and particle in the soil. They are greater the 

 finer the soil. 



Of cojrse the importance of these relations is derived from 

 their effects on the conditions of plant growth. These effects 

 may be classified as: first, the indirect effect upon the supply 

 of soil solution and air to the plant roots; and, second, the direct 

 effects of the physical condition of the soil upon root growth. 

 As to these latter it can be said that anything which makes for 

 opener texture and for less mechanical resistance favors plant 

 growth, the importance of texture being probably much greater 

 than that of mechanical resistance. The indirect effects of 



