CHARACTERISTICS OF SOILS 19 



SOIL STRUCTURE 



Soil texture has reference to the sizes of the soil particles 

 and the proportions in which the various sizes of particles 

 exist in the soil. Texture is not greatly subject to modi- 

 fication or control, and is rather stable. 



Soil structure has reference to the manner of arrange- 

 ment of the soil particles. The particles in a soil mass 

 may lie in very close contact with each other. Under 

 some conditions the soil particles may be cemented to- 

 gether by certain salts existing in the soil. Except possi- 

 bly in coarse soils, the former condition is very undesir- 

 able. The latter condition is undesirable in any soil. 

 It is best illustrated in puddled or " baked " soil, and in 

 the hard-pans. 



25. Ideal condition of structure. When a soil proper 

 crumbles readily into a soft mass under the pressure of 

 the hands, and readily becomes loose mellow from 

 the action of tilling and stirring tools, it may be said to 

 approximate the ideal condition of structure, or tilth. 



While the subsoil will seldom break down in the same 

 way, it should crumble also, but usually into somewhat 

 larger masses, cuboid in shape. This would indicate 

 that the whole mass of the subsoil, as it lies in position, 

 is already separated into these small cubes (cuboids) by 

 a very great number of cleavage planes. More will be 

 said of these planes in another paragraph, (47) . 



26. Over-mellowness. Some soils, and especially 

 those in arid or semi-arid climates, possess a structure 

 such that the plow renders them over-mellow for immedi- 

 ate planting. Such soils must be plowed some time 

 before planting the crops upon them, frequently in the 

 fall when the planting is to be done the following spring. 



