F0RMATIOX OF SOILS.

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mixture of the three earths, clay, sand, andlime, is absolutely necessary. The oxyd ofiron appears also to be a requisite ingredi-ent. The proportion necessary for the for-mation of good soil depends much on thenature of the climate, but more on the qualityof the sub-soil, and its power of retaining orabsorbing moisture. This alone may makeone soil barren, which upon another sub-soilwould be exceedingly productive. When thisis the case, drainage or irrigation oilers theonly means of permanent improvement.

Different vegetables also require differentadmixtures of earth. They require it, first,because it is necessary to their growth thatthe soil should be sufficiently stiff and deepto keep them firm in their place; and alsothat it should not be too stiff to permit theexpansion and growth of their roots; and,lastly, that it should supply them with a con-stant quantity of water, neither too abundantnor deficient. Hence we may learn why dif-ferent degrees of tenacity, depth, and powerof retaining or absorbing moisture, are re-quired in soils for different kinds of plants.

0 Thus,