SOIL STRUCTURE 185 



for developing a favorable crumb structure is obviously 

 the optimum moisture content for the development of 

 the highest tilth. In fact, it can "be stated with certainty 

 that the optimum moisture condition for plant growth 

 is the optimum for all favorable soil activities, whether 

 physical, chemical, or biological. Granulation, then, 

 becomes the vital factor in placing the soil in a physical 

 condition such that the highest tilth, that physical criterion 

 which every farmer should strive for, may be developed 

 in any soil. Until proper granulation is reached, no soil 

 can be expected to yield maximum paying returns. 



121. Granulation. While it is possible to list the 

 factors that bring about granulation in a soil, it is diffi- 

 cult to state specifically just why this phenomenon takes 

 place. It has been suggested that much of the granule 

 formation in the soil is due to the contraction of the 

 moisture film around the particles when, for any reason, 

 the moisture content is reduced (see Fig. 29) . It is known 



FIG. 29. A puddled and a well-granulated soil. 



that the soil particles tend to be drawn together by this re- 

 duction in the soil moisture, due to the pulling pow r er of the 

 thinned film. If to this condition is added a material which 

 tends to exert not only a drawing power on loss of mois- 

 ture, but also a binding and cementing power when dry, all 



