Volume 14 Number 2 



The Plant World 



A Magazine of General Botany 

 FE:BRUARY, 1911 



STUDIES IN SOIL PHYSICS. 

 I. The Physical Condition of Soils. * 



E. E. FREE. 



In the study of the soil certain properties have come to be 

 called "physical" and certain others "chemical," following 

 roughly in their division the fast vanishing line of cleavage be- 

 tween the formal "sciences" of physics and chemistry. It is 

 true that in soil studies these distinctions are even more difficult 

 to define and maintain than in the parent sciences. Soil phys- 

 ics and soil chemistry are but little apart. Pretty nearly any 

 soil problem will have both "physical" and "chemical" aspects 

 and the modern soil investigator must apply to his problems all 

 the "pertinent methods of modern physics, chemistry and 

 biology." t Nevertheless a formal distinction is convenient; 

 and it is useful, although perhaps not necessary, to consider as 

 physical those properties which concern the mutual and exter- 

 nal relations of the soil constituents, leaving for separate dis- 

 cussion (under the head of Soil Chemistry) the proximate or 

 ultimate composition of these constituents and the changes 

 therein. Of these physical properties the mechanical are easily 

 the most important. Pore space, mechanical resistance (as to 

 plowing, or to plant roots), the absorption and storage of water 

 and its capillary movement — these and their analogues are the 

 chief physical criteria of soil character. Heat relations have 

 much less importance || — optical, electrical and magnetic prop- 

 erties practically none. 



♦ Published bv permission of the Secretary of Agriculture. 



tCameron, — jour. Ind. Eng. Chem., 1: 810 (1909). 



llThermal relations are, of course, very important in detenuining the b i o 1 o g i c a 

 properties of a soil, and temperature has sometimes a considerable effect upon soi 

 water, but these matters must be left for later disciission 



