u8 



SOILS. 



PHYSICAL ANALYSES OF CRUST-FORMING SOILS. 



These soils agree in having a sufficient amount of clay (17 to 19 

 to characterize them as clayey loams, associated with a very large pro- 

 portion of the grain-sizes of less than .025 mm., or .5 mm. hydraulic 

 value. A higher proportion of clay, even though associated with a 

 similarly high or even larger proportion of these fine sediments, seems 

 to prevent crusting, probably because the swelling of the clayey ingre- 

 dient on wetting and its extravagant contraction in drying breaks up 

 the continuity of the surface. The heaviest clay soils, such as those 

 shown on a preceding page, neither crust nor crumble on drying after 

 wetting, but contract into lumps of stony hardness, as a whole. 



The burning-out of the humus from well-tilled surface soils 

 during the extended heat and dryness of rainless summers, 

 brings about such a contraction or packing of the surface soil 

 of orchards in California as to greatly reduce their productive- 

 ness, and to render necessary diligent green-manuring as the 

 only practical remedy. In many cases, liming of the surface 

 also serves well to prevent this injurious effect, which to some 

 extent of course follows surface irrigation as well as rains. 



In most soils, repeated alternate wetting and drying in place 

 produces a loose, flocculated texture, so long as no defloccula- 

 tion is brought about by mechanical causes, such as beating 

 rains or running water. 



Effects of Frost on the Soil. The expansion suffered by 

 water in freezing necessarily tends to separate the soil par- 

 ticles previously held together by the surface tension of the 



