138 SOIL PHYSICS AND MANAGEMENT 



regions where limestone is absent and where the sediment of streams 

 comes from acid soils the water is rarely clear. Even stock ponds 

 in regions of acid soils where the water is seldom disturbed never 

 become clear. 



While clay soils are difficult to manage, due to the danger of 

 puddling when too wet and from clods when too dry, yet with 

 proper care, drainage, incorporating organic matter and maintain- 

 ing the supply of limestone, the condition of these soils may be im- 

 proved so they work fairly well. In addition to the flocculation 

 produced by the substances mentioned above, natural causes hasten 

 it. Wetting and drying, and freezing and thawing, will change the 

 character of the soil from a cloddy to a granular condition, or cause 

 it to "slake." The alternate expansion and contraction of the col- 

 loidal material, whether of mineral or organic origin, tend to break 

 the soil into granules. Fall plowing is especially desirable on 

 "heavy" soils that are well drained, because of the good tilth 

 developed during winter by these natural agencies. If a clay soil 

 becomes cloddy it is practically impossible to reduce it to a condition 

 of good tilth by any mechanical means, but if freezing and thawing 

 occur, or a shower falls, working it under the right moisture con- 

 ditions will break the clods easily into masses of granules. 



3. Silt and Silt Loams. Silt is divided into three classes, 

 fine, medium, and coarse, ranging in size from 0.001 to 0.032 

 mm. in diameter (Hopkins), 0.005 to 0.05 m. (Bureau of Soils) or 

 0.01 to 0.07 mm. (Hilgard) . The particles of fine silt are sufficiently 

 small to give to soils properties somewhat similar to those of clay, 

 but without so much danger of puddling. Silt enables soils to 

 retain much moisture and gives great capillary power, and hence 

 forms some of the best soils for resisting drouth. They are suf- 

 ficiently coarse, however, <to' permit of fair aeration, but not to an 

 excessive degree, as in tlae case of sands. The silt loam soils cover 

 extensive areas in the middle west of the United States and owe 

 their origin to the loess. 



They possess sufficient tenacity to give the necessary stability, 

 but not enough to cause $h jm to work with any great difficulty. The 

 shrinkage, however, is not usually sufficient to produce very in- 

 jurious effects. Since granulation depends upon the amount of col- 

 loids present, and since organic matter as well as clay may furnish 

 this constituent, the silt loams containing the largest amount of 

 organic matter granulate best. Silt soils deficient in organic matter, 

 such as gray or yellow timber soils, show little or no granulation 



