104 THE PRINCIPLES OF SOIL MANAGEMENT 



and the internal strains and pressures unequal, a ten- 

 dency arises for the mass to divide along the lines of 

 weakness into groups of particles, as the soil moisture 

 is much reduced below a certain optimum condition. 

 Tillage operations, development of roots, burrowing of 

 animals and insects, the presence of humus, and the 

 development of frost crystals, may assist in further 

 developing these lines of weakness in the soil mass, 

 upon which the tension of the moisture films around 

 the soil particles is brought to bear. The flocculation 

 of soil particles may also develop lines of cleavage by 

 the aggregation of particles around certain centers. 

 The movement of the soil particles is, in every case, 

 facilitated by the presence of a moderate amount of 

 moisture. 



On the other hand, conditions opposite from the 

 above, including tillage at inopportune times, the 

 operation of some natural agencies, as the beating of 

 rain, erosion, and bad drainage, may not only destroy 

 the tendency to the granular condition, which is always 

 strongest in the finest soil, but may induce the opposite 

 or separate grain structure. 



43. Means of modifying structure. It is apparent 

 that some of the means of modifying the soil structure 

 are natural, others are within the control of man. The 

 following are among the better-known of these factors: 

 (1) Variation in the water content. (2) Development 

 of frost crystals. (3) Tillage. (4) Growth of plant 

 roots. (5) Organic matter. (6) Certain soluble salts. 

 (7) Earth-worms and other forms of animal life. (8) 

 Heavy rain storms. Whether a. desirable or an undesir- 



