CHAPTER VII 

 SOIL STRUCTURE AND ITS MODIFICATION 



The structural condition of the soil is very important to 

 plant growth, since the circulation of air and water so nec- 

 essary to normal development is controlled thereby. The struc- 

 tural condition may be loose or compact, hard or friable, gran- 

 ulated or non-granulated, as the case may be. Of these con- 

 ditions granulation, especially in heavy soils, is of vital im- 

 portance, since it is really a summation of all favorable struc- 

 tural conditions. By granulation is meant the drawing to- 

 gether of the small particles around suitable nucleii, so that 

 a crumb structure is produced. The grains thus cease to 

 function singly. The importance of such a structural condi- 

 tion on a heavy soil is obvious. The soil becomes loose because 

 of the larger units, air moves more freely, and water not only 

 drains away readily when in excess, but responds with celerity 

 to the osmotic pull of the plant. 



76. Soil structure types. — The structural condition of 

 a soil can generally be attributed directly to its textural nature 

 as can readily be seen by comparing sandy and clayey soils. 

 For convenience of discussion two general structural groups 

 may be established: (1) single-grained, and (2) compound- 

 grained. In the former the particles function more or less 

 separately and the soil is, as a consequence, rather open and 

 friable. In the latter group the particles, being small, tend 

 to stick together and the units instead of being solid are aggre- 

 gates, their size and character as well as their relations to each 

 other being a determining factor in the physical condition of 

 the soil. As most soils are mixtures of large, medium, and 



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