18 DIFFICULTY OF DEFINING EARTHS AND SOILS. 



of the views which are either new and unsupported, or entirely 

 opposed to all existing authority ; and which wiM. require to be 

 understood and borne in mind by all who desire to study with pro- 

 per advantage the theory of fertilization which will be presented 

 and maintained in this essay. 



Previous to the recent attention of chemists directed to agricul- 

 ture, which may be said to have begun with the publication of 

 Pavy's admirable and very valuable (though necessarily very im- 

 perfect) work on the " Elements of Agricultural Chemistry," 

 agricultural writers had defined and ^described soils by their quali- 

 ties obvious to the senses, and without much, if any, regard to 

 their chemical, or even their physical constitution. Of course 

 they were often in error; as the sensible qualities, or textures of 

 soils, do not always quadrate with, or conform to the proportions 

 or kinds of their materials. For example : an open and light soil, 

 is most generally made so by an excess of silicious sand ; but occa- 

 sionally soils owe their possessing this texture to an excess of 

 humus or vegetable matter, or of chalk ; and which soils may be 

 greatly deficient in sand, and would be rendered even more com- 

 pact by an addition of this earth. Again : the closeness and in- 

 tractability of a soil is generally owing to the excess of clay ; but 

 a soil superabounding in clay, with large intermixture of vegetable 

 and calcareous earths, may be much more friable and light than 

 another with much less clay, and much more of silicious sand in a 

 very finely divided state. 



More recently, when many men of science took their present 

 ground as co-labourers in agricultural investigation, they brought 

 to bear, on this branch of the science, terms and definitions exact 

 and precise enough indeed, they being those recognised in chemis- 

 try; but altogether inapplicable to agriculture, because referring 

 to conditions of purity, and simplicity of composition, having no 

 existence in nature, nor even subject to the observation and senses 

 of the agriculturist. Hence, when chemists, using their scientific 

 nomenclature, attempt to instruct farmers of the composition of 

 soils, and refer to their contents of the chemical earths proper, 

 alumina, lime, magnesia, &c., they are speaking of things which 

 have no existence in nature, nor even in agricultural art; and they 

 might as well go farther back in search of scientific strictness, and 

 treat of the elementary parts of these several earths that is, oxy- 

 gen, with the metals aluminum, calcium, and magnesium, respect- 

 ively ; which elements are rarely produced or preserved separate, 

 and never except in the chemist's laboratory. The substances 

 known in chemistry as earths, are, indeed, defined with precision, 

 and their distinguishing properties are well understood by those 

 who are even slightly acquainted with that science. But of the 

 nine earths known to chemists, one only, silica, exists naturally iu 



