HUMUS, Oil VEGETABLE EARTH. 25 



becomes the carbonate of magnesia, which is the ordinary mild 

 substance used as medicine. This is a combination of 48 parts of 

 magnesia with 52 of carbonic acid. It is to this compound only, 

 the carbonate of magnesia, I affix the term of magnesian earth, 

 and not to any other form of combination with other earths or with 

 acids, nor to the pure chemical earth magnesia, whiclj has no exist- 

 ence in nature, and, of course, can have no natural influence on 

 soils or on agriculture. 



5. Humus is the partially decomposed remains of dead vegetable 

 growth, reduced by time to nearly an earthy texture, pulverulent 

 when dry, and soft and slimy, and almost semi-fluid when full of 

 water. This vegetable earth, as peat, and in its purest state, is 

 very abundant in Great Britain and other cool and moist countries. 

 But in Eastern Virginia, it has scarcely any existence, separate or 

 alone, except in the Great Dismal Swamp, and in marshes covered 

 by the tides. In these places, and also in the still larger swamps 

 of North Carolina, the continual wetness and dense shade serve to 

 prevent the complete decomposition of vegetable matter, as is done 

 in Europe by the prevalence of cloudy and damp air, r.nd low 

 average temperature; and under such coiiclhionsonly, in cur hotter 

 and dryer climates, does humus occur alone, or even as forming 

 the principal material of any soiJ. The peat soil of EMI ^ e is com-j^ 

 T;os3d of pare vegetable matte v, for 60 per cent, or moro of its dry 

 weight. (Johnston.) The peat r.sed for fuel is probably still more 

 of vegetable constitution. Of four specimens of soil of the Dismal 

 Swamp, selected and examined by myself, the vegetable parts were, 

 respectively, 75, 90, and, in the other two, 96 per cent, of the 

 bulk of the soil. Different specimens of soils, from both salt and 

 fresh-water tide marshes, bordering on Powhatan (or James) 

 Biver, lost full 50 per cent, of their dry weight by being burnt 

 thoroughly ; showing that half their weight, and probably five-sixths 

 of their bulk, is pure vegetable matter. These soils are, per- 

 haps, as near to pure humus as any in our climate. 



As a small, or chemical ingredient of soil, intermixed or com- 

 bined with other earths and far more abundant materials, humus 

 is present universally, serving as aliment to be drawn up by the 

 roots of growing plants, and without which no healthy or luxuriant 

 growth could be produced. Humus gives colour and value to the 

 black rich mould of old garden ground, and to the richest forest or 

 alluvial soils, before they are reduced in fertility by tillage. 



Soils and /Sub-soils in General. 



All the agricultural earths, including humus as one, when sepa- 

 rated pure, or as nearly pure as ever presented by nature, are 

 nearly or entirely barren. This might be inferred from the mere 

 3 



