These soils agree in having a sufficient amount of clay (17 to 19%) to characterize them as clayey loams, associated with a very large proportion 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 ingredient 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 productiveness, 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 deflocculation 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 particles previously held together by the surface tension of the capillary water, or otherwise flocculated or cemented. Freezing of the soil is therefore of material assistance in _disintegrating_ cloddy, ill-conditioned soils, leaving them in loose, crumbly condition after the ice has melted and the surplus water drained off; so as to materially facilitate tillage and root penetration. When, however, soils thus circumstanced are tilled or trodden while too wet, they quickly become puddled, being practically reduced to single-grain structure. (See this chapt. p. 110). Hence the injury caused by allowing cattle to range in winter on cultivated land subject to freezing and thawing, which it sometimes takes years to correct. A disagreeable effect often produced by the freezing and thawing of wet lands is the “heaving-cut” of grain, resulting from the upward expansion of the surface soil in freezing, that may readily rupture the roots; while on thawing, the soil surrounding the upheaved stool is apt to settle down, especially in case of a rain, leaving the stool and roots exposed either to drying or freezing, as the case may be. Hence the desire of grain farmers in northern climates, for a sufficient covering of snow to protect the fall-sown grain, rather than an “open winter,” during which the grain is exposed to alternate freezes and thaws, or extreme cold.