136 



SOIL PHYSICS AND MANAGEMENT 



Highly plastic soils become very hard upon drying. Plasticity may 

 be diminished by organic matter, granulation or change of texture. 

 Plasticity may be increased by the breaking down of soil gran- 

 ules into their individual soil particles. While this is detrimental to 

 soils, it is of decided advantage to the ceramist. 



(d) Puddling. Clays and clay loams are usually made up of 

 crumbs or granules, composed of many soil particles united by a 

 weak cementing substance, such as humus or some other colloid. If 



the soil is worked or trampled 

 by stock when wet these granules 

 are broken down, the colloids be- 

 come somewhat uniformly dis- 

 tributed throughout the mass 

 and an impervious condition re- 

 sults. The soil is puddled. 

 Water or air will not penetrate 

 it and a worse condition could 

 not well be produced. The pres- 

 ence of sodium carbonate or 

 black alkali, or the long-con- 

 tinued application of certain 

 fertilizers, such as ammonium 

 sulfate or sodium nitrate, brings 

 about a puddled condition. Some 

 clay and clay loam soils are pud- 

 dled naturally. This is likely 

 to be the case if they are strongly 

 acid and low in organic matter. 

 Water in a soil acts as a lubri- 

 Fio.73.-crack 8 in black clay loam after cant and movement takes place 

 i9i6 ng dry pcriod ' photo ra P hed August, more readily between the par- 

 ticles. It also softens the 



cementing material so that the granules are easily broken down. 

 When the soil is turned by the plow a shearing, slipping movement 

 is produced as it curves over the mold board. This will pulverize it 

 if in good condition, but puddle it more or less if wet. When a 

 heavy animal steps on the dry soil it is compacted, but if wet the 

 foot sinks into the soil, causing a movement which breaks down 

 many granules, thus puddling the soil. 



When puddling is produced in a heavy soil it may be almost 

 worthless for a time, but the natural agencies of freezing and thaw- 



