HUMUS, OR VEGETABLE MOULD. 413 



soils, its depth is usually very small. It constitutes the rich and 

 fertile upper stratum of a soil which is usually cultivated by the 

 plough ; and it becomes gradually deepened as the land is culti 

 vated and manured. The depth of this loam or mould may be 

 considered, in general, as the best test of the goodness of the soil, 

 or its productive character. I know that this is sometimes 

 denied. The dark-colored condition of the upper stratum is not 

 always an indication of mould, for occasionally there is met with 

 an upper stratum of deep sand, colored with some mineral sub 

 stance, which is almost utterly barren, and very difficult of im 

 provement; but ordinarily, other circumstances being equal, the 

 surest test of the fertility of a soil is the depth of the vegetable 

 mould or loam on the surface. 



Loamy soils receive their particular designation from the 

 mineral substance with which they abound ; thus we speak of 

 sandy loams, or clayey loams, from the predominance of either 

 of these substances in the soil ; and undoubtedly the richest of 

 all soils is that in which there is an intermixture of various 

 elements some one says, where lime, clay, and sand, are mixed 

 in equal proportions with mould, or decayed vegetable matter ; 

 but it is not certain that the exact proportions are ascertained. 



LXXII. HUMUS, OR VEGETABLE MOULD. 



The substance designated as vegetable mould, or humus, in 

 its pure or unmixed state, is not an infallible indication of the 

 fertility of a soil, as I have already stated in respect to peat 

 formations. Liebig refers to the soils in the neighborhood of 

 Mount Vesuvius, composed wholly of matter thrown from the 

 crater, as highly fertile. &quot; The land in the vicinity of Vesuvius 

 may be considered as the type of a fertile soil, and its fertility is 

 greater or less, in different parts, according to the proportion of 

 clay or sand which it contains. The soil which is formed by 

 the disintegration of lava cannot possibly, on account of its 

 origin, contain the smallest trace of vegetable matter ; and yet 

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