. Soil physics and management. decomposition of rocks and thefurther decomposition of soil material, many soluble substances areformed which may not be leached out by the small rainfall of ther(>gion but may be brought to the surface by capillary of the stratified rocks contained much salt, due to the factthat they were formed in salt or l)rackish waters. When these be-came dry land the salt was leached out later and carried into tem-porary lakes. This accumulation continued and ultimately thelake became dry and a deposit of alkali was left (Fig. 127). Saltsprings sometimes oc
. Soil physics and management. decomposition of rocks and thefurther decomposition of soil material, many soluble substances areformed which may not be leached out by the small rainfall of ther(>gion but may be brought to the surface by capillary of the stratified rocks contained much salt, due to the factthat they were formed in salt or l)rackish waters. When these be-came dry land the salt was leached out later and carried into tem-porary lakes. This accumulation continued and ultimately thelake became dry and a deposit of alkali was left (Fig. 127). Saltsprings sometimes occur, the waters of which carry considerableamounts of alkali into depressions, where they may accumulate inlarge quantities. Whatever the source of the alkali, its existence278 ALKALI LANDS AND THEIR RECLAMATION 279 is usually duo to cliinatic conditions. It naturally results from arainfall insufficient to carry soluble material out of the soil, whichultimately becomes so impregnated with it as to be unproduc-tive (Fig. 128).. ^ ..>i2i _ _ _ ^-.•_^^^ ,.2.^:r:m^ Fig 127—Beginning of an alkali spot (U S. Dept. of Agriculture.) 1 : ^;=-^.....~-«**»«-«*^. ?HMMpHI ^^HNB iut-Jpik.,-, 5P^ ? •»? -^BBP fH^^-r .. ~...s- .^L^lh^^ jJI ?p. -?...?•: ja IHk \JiH i * • ^#^<v » ,V 1 ? *.« ?m,- ?**.;. amr «f- j^%:^4- ?.,... ?:? ?4«mm *,. ikL^ V .:-:-.^: Fig. i2S,—Alkali area showing the absence of vegetation. (IT. S. Dcpt. of Agriculture.) 280 SOIL PHYSICS AND :^IANAGEMEXT Kinds of Alkali.—The alkalies of arid regions are commonlyclassilied as black, white, and b^o^^u. The black consists of forms ofsodium carbonate, which owe their name to the color produced bythe solution of organic matter and its deposition on soil particlesduring evaporation. There are at least two forms of sodium car-bonate included in the black alkali, the bicarbonate (HXaCOg) andthe normal carbonate (XaoCOg). The white alkalies are composed mainly of common salt (XaCl)and sodium sulfate (XaoSO^
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