16 



CALCAREOUS MANURES— THEORY. 



earths, and which it serves to cure in them (as will hereafter be shown) 

 when used as a manure. 



Most of those who have applied chemistry to agriculture consider mag- 

 nesia as one of the important earths.* Magnesia, like lime, is never found 

 pure, but always combined with some acid, and its most general form is 

 the carbonate of magnesia. But even in this, its usual and natural state, 

 it exists in such very small quantities in soils, and is found so rarely, that 

 its name seems a useless addition to the lists of the earths of agriculture. 

 For all practical purposes, gypsum (though only another combination of 

 lime,) would more properly be arranged as a distinct earth, or element of 

 soils, as it is found in far greater abundance and purity, and certainly affects 

 some soils and plants in a far more important manner than has yet been 

 attributed to magnesia in its natural form. 



Magnesia (or, more properly speaking, the carbonate of magnesia) is 

 here treated as comparatively unimportant as an agricultural earth, be- 

 cause of its being rarely found, and in but inconsiderable quantities in 

 natural soils, and being unattainable as a manure. It was not thereby in- 

 tended to be asserted that magnesia would not be important as an ingre- 

 dient of soil and as a manure, if it were abundant as the one, or at command 

 for the other. From the great similarity in chemical properties of mag- 

 nesia to lime, it is most probable -that their action in soils is also similar. 

 The alluvial soil of the Red-river bottom, in Arkansas, which is of the 

 highest grade of fertility, I have found by analysis to contain between one 

 and two per cent, of carbonate of magnesia, and two to three per cent, of 

 carbonate of lime. The mud deposited by the floods of the Nile, and which 

 forms the celebrated rich soil of Egypt, also contains carbonate of mag- 

 nesia. {Lyell, p. 223, vol. i.) Yet most writers have deemed magnesia in 

 soils a cause of sterility. 



All the earths, when as pure as they are ever furnished by nature, are 

 entirely barren, as might be inferred from the description of their qualities; 

 nor would any addition of putrescent manuresf enable either of the earths 

 to support healthy vegetable life. 



The mixture of the three important earths in due proportions will correct 

 the defects of all; and with a sufficiency of putrescent animal or vegeta- 

 ble matter, soluble in water, a soil is formed, in which plants can extend 

 their roots freely, yet be firmly supported, and derive all their needful sup- 

 plies of air, water and warmth, without being oppressed by too much of 

 either. Such is the natural surface of almost all the habitable world; and 

 though the qualities and values of soils are as various as the proportions of 

 their ingredients are innumerable, yet they are mostly so constituted that 

 no one earthy ingredient is so abundant but that the texturej of the soil 

 is mechanically suited to some one valuable crop ; as some plants require a 

 degree of closeness, and others of openness in tlie soil, which would cause 

 other plants to decline or perish. 



Soil seldom extends more than a few inches below the surface, as on the 

 surface only are received those natural supplies of vegetable and animal 

 matters, which are necessary to constitute soil. Valleys subject to inun- 

 dation have soils brought from higher lands, and deposited by the water, 



* Davy's Agricultural Chemistry, p. 110. Phiia. ed. 1821. 



f Putrescent or enriching manures, are those formed of vegetable and animal matters, 

 capable of putrefying, and thereby furnishing soluble food to plants. Farmyard and 

 stable manure, and the weeds and other growth of the fields left to die and rot on them, 

 are almost the only enriching manures that have been used as yet in this country. 



X The texture of a soil means the disposition of its parts, which produces such sensi- 

 ble qualities as being close, adhesive, open, friable, &c. 



