•30 The Pi^ant World. 



In considering these significant mechanical properties the 

 soil may be regarded as composed of mineral grains, organic 

 substances and water. * All the various mineral grains may be 

 classed together, for, although they are of the most diverse 

 minerals and widely different in chemical composition, yet in 

 those of their physical properties which affect the soil they are 

 practically alike. It is these mineral grains which form the frame- 

 work, the skeleton of the soil. In the laboratory of their inter- 

 spaces occur the complex chemical and biological reactions 

 which we are just beginning to recognize and to understand. 

 They are the chief materials of soil physics, and the physical 

 effects of water and organic matter are produced by actions 

 upon them. Furthermore, the amount and character of the 

 soil water and the organic matter are variable and more or less 

 under the control of the cultivator. Fundamentally, then, 

 the ultimate physical character of any soil, — -its possible 

 physical character, — is determined by the character of its min- 

 eral grains, and, as already implied, the specific characters of 

 these mineral grains, are, so far as we know, of little importance. 

 Their important properties are the purely superficial ones of 

 size and shape. The ultimate physical property of soils is there- 

 fore the mechanical composition which may be 

 defined as the size and shape of the constituent particles. Ob- 

 viously the particles of ordinary soils are so widely varied in 

 size and shape that a complete and direct expression of the 

 mechanical composition is impracticable. It is necessary to 

 establish groups of soil particles divided by arbitrary size 

 limits and separated from each other by more or less arbitrary 

 methods. The percentages of the particles of a soil which fall 

 into each of these groups can then be stated as the mechan- 

 ical analysis. J This is far from a complete statement 

 of mechanical composition Perhaps it is not even the best 

 possible statement. || But it is the one in general use and it 

 does give some idea of this fundamental property — the property 

 in accordance with which soils are sands, clays, loams, etc. 



*The soil air is excluded from consideration since it represents, so far as physical condition 

 is concerned, simply the empty space of the soil. 



JThe limits of these groups are purely arbitrary. The system of the U. S. Bureau of Soils 

 (now coming into general use) is as follows: Fine gravel, 2-1 mm. diameter; coarse 

 sand, 1-.05 mm.; medium sand, 0.S-0.2S mm.; fine sand, 0.2 5-0.1 mm.; very fine sand, 

 0.1-0.05 mm., silt, 0.05-0.005 mm.; and clay, below 005 mm. For methods of sep- 

 aration see Bulletin No. 24, Bureau of Soils, U. S. Dept. of Agriculture. 



1 1 In a later paper the author hopes to discuss the errors of the mechanical analysis and the 

 possibility of obtaining a substitute therefor. 



