26 SOILS AND THEIR COMPOSITION. 



description of their respective qualities. Further the too large 

 proportion of any one earth, in the mixture of several, is injurious 

 to fertility in proportion to such excess. But the quantity which 

 would thus be hurtful by excess would be very different in the dif- 

 ferent earths, and also as to each one, as modified by attendant 

 circumstances Thus, as a supposition, or, at best, a mere ap- 

 proximation to truth, we may suppose the following named pro- 

 portions to be as large as can be present, respectively, in different 

 soils, and under ordinary circumstances, without being injurious 

 to production : 



Silicious earth (as pure sand), in a particular soil, 

 will be injurious by its excess, if more in propor- 

 tion to the soil than - 85 per cent. 



Or aluminous earth (argil, or purest clay), in ano- 

 ther soil, ' - 25 " 



Or calcareous earth (carbonate of lime, or calx), in 



another, - 5 ? t( 



Or magnesian earth (carbonate of magnesia), in 



another, - - - 2? " 



Or humus (nearly pure vegetable matter), in another, 12 ? " 



In such large proportion as indicated by the above quantities, 

 the greater part of each earth could act only physically or mechan- 

 ically. If considered merely as chemical or manuring constituents, 

 and embraced in one soil, perhaps one per cent, of calx, a mere 

 trace of magnesian earth, and five per cent, of humus, would be 

 enough ; while nearly all the remainder of the hundred parts would 

 be of silicious earth mainly, and aluminous earth, serving merely as 

 physical constituents, for nearly their whole quantities. 



But whatever may be the most suitable proportions, and however 

 much the action, and power of each one may be in some cases 

 modified by other ingredients, or by attendant circumstances, still 

 the admixture, in due proportions, of the different earths will serve 

 to correct the defects of all, and thus to form soils of every charac- 

 ter and variety. And various as are the soils naturally formed by 

 mixtures of some or all of the different earths, and greatly defective 

 as most of them are, there are but few which do not more or less 

 fulfil their purpose of serving to sustain the growth of useful plants ; 

 in which they may extend their roots freely, yet be firmly sustained 

 in their erect position ; and obtain the necessary supplies of air, 

 moisture, warmth, and food, without being too much oppressed. by 

 the excess of either. Such are the soils, though of various pro- 

 portions and values, on all the surface of the globe wherever fit 

 for culture. 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 



