51 
degree of fertility. The four earths, silica, alumina, 
lime, and magnesia, a small proportion of the muriate 
of soda, sulphate of mag-nesia, muriate and sulphate of 
potash, nitrate and sulphate of lime, oxide of iron, the 
mild alkalies, potass, and soda, are all the ingredients 
which are found in soils, and their permanent pro- 
ductiveness must therefore depend upon a certain 
admixture and combination of several of these. Now if 
Von Thaer's classification of soils* be examined, it will 
be seen that they receive their value in proportion to 
the quantity of clay which they contain, and decrease 
in value as they possess silica. But if, on the other 
hand, we compare the soils of Sinclair, no estimate of 
their value can be deduced from the quantity of either 
silica or alumina : e. g. Sinclair's " best rich alluvial" 
soil contains 17 per cent, of alumina. Von Thaer's 74 per 
cent, of clay. Sinclair's " rich black clayey" contains 
only 5 per cent, of alumina, and Von Thaer's 2nd soil" 
81 per cent, of clay. Sinclair's " tenacious clay" contains 
only 13.7 of alumina. Neither will the proportions of 
sand serve as a criterion of infertility, even in Thaer's 
own list, his soils, Nos. 5 and 6, containing more than 
Nos. 7, 8, 9, 10, and 11, all inferior to the former. 
Again, the best loam in France analysed by Tillet 
would approach very near to Thaer's " rich barley land 
No. 6," except that it contains more lime than Thaer's. 
But the quantity of lime will not serve for a better test 
of fertility, as may be seen by the inspection of Thaer's 
list; indeed, it is well known that the Flemish soils 
scarcely contain any carbonate of lime. The Polder 
of Orderen, an alluvial soil, (which Polders are 
cropped many years in succession without the benefit 
of manures,) contains 81 per cent, alumina, 18 per 
cent, sand, but no lime. A soil in the Waes district 
• See the Table of Soils at the end of the paper. 
