THE PHYSICAL PROPERTIES OF THE SOIL. 767 



tides will be either so far separated as to cause the formation of the 

 "separate graiu" structure, or maybe more closely consolidated by the 

 action of frost. 



Destruction of the crumbly structure is promoted by violent shaking 

 with water, kneading, and boiling of the mass. These means, as is 

 well known, are used in pottery work to increase the plasticity of the 

 clay, the object being to break up aggregates of particles as completely 

 as possible. 



Atmospheric water exerts a decided intluence on the structure of the 

 soil by altering tlie position of the soil particles and also by carrying 

 the smaller grains downward. The crumbly structure of the soil is 

 destroyed more or less by the percolating water, which loosens particles 

 from the fragments and washes them into the larger spaces, and this 

 takes places in proportion to the quantity of the percolating water and 

 the lack of tendency toward tlocculation in the soil. The greater the 

 force of the tailing water tlie sooner the soil assumes the separate grain 

 structure, and therefore violent thunderstorms may entirely change the 

 mechanical condition of the soil in a short time. From snch facts we 

 may infer that the frequency of atmospheric precipitation, other things 

 being equal, exercises a great influence on the structure of the soil. 

 Frequent light rainfalls have much less effect than violent storms at 

 longer intervals. 



Covering the soil with living or dead plants lessens the mechanical 

 effect of atmospheric precipitation by breaking the force of the falling 

 water and retarding its penetration into the soil, and this influence of 

 plant cover varies with its compactness and state of development. If 

 the ground is covered with straw, the compacting of the soil by rain 

 will diminish as the thickness of the covering increases.^ 



Structural changes in the soil may also be caused by the washing 

 away of the surftice particles on sloping land by the water that does 

 not sink into the soil (erosion). In the case of leaching, the wash- 

 ing of the smaller particles into the subsoil increases with the differ- 

 ences in the size of the particles; in tlie case of erosion the washing 

 away of the flner-grained constituents is greater the less compact the 

 soil, the greater tlie slope, and the heavier the rainfall. In both cases 

 change in position of the soil ])articles is prevented to a great extent 

 by a covering of living plants whose roots hold the soil together. 

 Forest growth has a stronger retarding action than that of perennial 

 herbs. 



The wind transports the finer particles of dusty soils in proportion 

 to its strength. The places from which the material has been blown 

 necessarily have a coarser soil structure than those composed of the 

 deposited dust.- 



Finally, the animal life of the soil exerts an imjjortant influence on 



IE. Wollny, Forsch. Geb. agr. Phys., 12, p. 31. 

 «T. F. Hensele, Ibid., 16, p. 31L 



