SALINE MATTERS IN SOILS.
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viously mentioned (210). A very minute quantity ofthis salt of iron confers upon the soil peculiar propertieswhich render it appropriate for particular plants; but aslight increase of its amount is attended with bad results,for, unless in very minute quantity, it is poisonous toplants; and hence such soils as contain much sulphate ofiron are invariably barren, or incapable of supportinghealthy vegetation.
310. The saline matters in the soil are principallythe sulphates, muriates, nitrates, and phosphates, of thealkaline and earthy bases. The nature and quantity ofthese substances vary considerably, but they never con-stitute more than a very small portion of the soil; theirinfluence upon vegetation is, however, considerable, forcertain plants seem to require particular saline sub-stances, and do not grow well in soils not containingthem *. Thus all kinds of grass and corn contain silicaand’phosphoric acid: the former substance requires thepresence of an alkali, either potash or soda, to render itsoluble, and to enable it to enter the roots when it isdissolved in water ; whilst the latter is also alwaysassociated with a base, which is usually either lime ormagnesia. In the same way all plants are found tocontain small quantities of similar saline and earthymatters.
311. The nature and quantity of the organic sub-stances in the soil have great influence upon its charac-ter and fertility. They act in several ways. By slowly