Peaty soils, however, and all those containing a relatively large amount of humus, are also liable to visible shrinkage when passing from the wet to the dry condition. But on account of their looseness and porosity such shrinkage does not usually result in the formation of cracks or rupture of the roots, as is the case in heavy clay lands. The entire mass of the soil then shrinks downwards, but rarely forms cracks on the surface. Hence the introduction of humus into “heavy” soils is among the best means of improving their tilling qualities. _Formation of Surface Crusts._—Some soils, especially those of a clay-loam character, are very liable to the formation of hard surface crusts from the beating of rains, and from surface irrigation; owing, doubtless, to the ready deflocculation of their clay substance. It is not easy to define the precise physical composition conducive to this crust formation; but the subjoined physical analyses show examples of soils in which this tendency is very prominent and is frequently annoying, in that when they occur in the regions of frequent summer rains, it becomes necessary after each one to till the surface in hoed crops (_e. g._, in cotton-fields) in order to prevent the injurious effects of such consolidation of the surface. It may, of course, be prevented by mulching, or on the large scale by green-manuring, to such extent as to prevent contraction. The subjoined physical analyses of two soils from the Brown-Loam region of Northern Mississippi (see chap. 24), shows the composition of lands excellent in every respect other than the tendency to crust after each rain: PHYSICAL ANALYSES OF CRUST-FORMING SOILS. =========================================================== |Diameter.|Hydr. Value.|No. 219.|No. 197. ------------------+---------+------------+--------+-------- Coarse materials | 1-3 mm. | | } | | | | } .23 | { |.5-1 “ | | } | { | .50 | 64 mm. | 1.47 | Sand { | .30 | 32 “ | 2.33 | .79 { | .16 | 16 “ | 1.17 | { | .12 | 8 “ | .78 | .18 | | | | { | .072 | 4 “ | .76 | .78 { | .047 | 2 “ | 9.79 | 3.56 Silt { | .036 | 1 “ | 7.20 | 13.12 { | .025 | .50 | 13.11 | 16.64 { | .016 | .25 | 15.07 | 27.28 { | .010 | <.25 | 26.36 | 18.87 | | | | Clay | ? | <.0023 | 19.10 | 17.23 ------------------+---------+------------+--------+--------