232 CLASSIFICATION OF SOILS. 



lish agricultural books,* and as it is really desirable to possess a word to 

 which the above meaning can be attached, I shall venture in future to 

 employ it always strictly in this agricultural sensed 



By alumina, then, I shall in all cases express the pure earth of alum, 

 which exists in clays, and to which they owe their tenacity — by clay, a 

 finely divided chemical compound^ consisting very nearly of QQ of silica 

 and 40 of alumina, ivith a little oxide of iron, and from which no siliceous 

 or sandy matter can be separated mechanically or by decantati&n. 



Of this clay the earthy part of all known soils is made up by mere 

 mechanical admixture with the other earthy constituents (sand and 

 lime), in variable proportions. On a knowledge of these proportions the 

 following general classification and nomenclature are founded. 



§ 3. Of the classification of soils from their chemical constituents. 



Upon the principles above described soils may be classified as fol- 

 lows : — 



1°. Pure clay (pipe-clay) consisting of about 60 of silica and 40 of 

 alumina and oxide of iron, for the most part chemically combined. It 

 allows no siliceous sand to subside when diffused through water, and 

 rarely forms any extent of soil. 



2°. Strongest clay soil (tile-clay, unctuous clay) consists of pure clay 

 mixed with 5 to 15 per cent, of a siliceous sand, which can be separated 

 from it by boiling and decantation. 



3°. Clay loam differs from a clay soil, in allowing from 15 to 30 per 

 cent, of fine sand to be separated from it by washing, as above described. 

 By this admixture of sand, its parts are mechanically separated, and 

 hence its freer and more friable nature. 



4°. A loamy soil deposits from 30 to 60 per cent, of sand by mechani- 

 cal washing. 



5°. A sandy loam leaves from 60 to 90 per cent, of sand, and 



6°. A sandy soil contains no more than 10 per cent, of pure clay. 



The mode of examining with the view of naming soils, as above, is 

 very simple. It is only necessary to spread a weighed quantity of the 

 soil in a thin layer upon writing paper, and to dry it for an hour or two in 

 an oven or upon a hot plate, the heat of which is not sufficient to dis- 

 colour the paper — the loss of weight gives the water it contained. While 

 this is drying, a second weighed portion may be boiled or otherwise 

 thoroughly incorporated with water, and the whole then poured into a 

 vessel, in which the heavy sandy parts are allowed to subside until the 

 fine clay is beginning to settle also. This point must be carefully 

 watched, the liquid then poured off, the sand collected, dried as before 

 upon paper, and again weighed. This weight is the quantity of sand 

 in the known weight oi moist soil, which by the previous experiment has 

 been found to contain a certain quantity of water. 



Thus, suppose two portions, each 2'"J grs., are weighed, and the one 

 in the oven loses 50 grs. of water, a ^d the other leaves 60 grs. of sand, 

 — then, the 200 grs. of moist are equal to 150 of dry, and this 150 of dry 



• As in British Husbandry, p. 113, and in Loudon's EncydopcRdia of Agriculture, p. 315, 

 where classifications of soils are given chiefly from Von Thaer, though neither work, ex- 

 hibits with sufficient prominence the meaning to be attached to af^ricultural clay, as distia> 

 guished from alumina, sometimes called pure clay by the chem:s; 



