230 PROPORTION OF ORGANIC MATTER IN SOILS. 



only in bdagy and peaty soils that the latter large proportion is evci 

 found — in tfie best soils the organic matter does not average five per cent., 

 and rarely exceeds ten or twelve. Oats and rye will grow upon land 

 containing only one or one and a half per cent. — barley where two or 

 three per cent, are present — but good wheat soils contain in general from 

 4 to 8 per cent., and, if very stiff and clayey, from 10 to 12 per cent, 

 may occasionally be detected. 



Though, however, a certain proportion of organic matter is always 

 found in a soil distinguished for its fertility, yet the presence of such sub- 

 stances is not alone sufficient to impart fertility to the land. I do not 

 allude merely to such as, like peaty soils, contain a very large excess of 

 vegetable matter, but to such also as contain only an average proportion. 

 Thus of two soils in the same neighbourhood — the one contained 4-05 

 per cent, of organic matter, and was very fruitful — the other 4-19 per 

 cent., and was almost barren. This fact is consistent with what has been 

 stated in the two preceding lectures, in regard to the influence exercised 

 by the dead inorganic matter of the soil, on the general health and luxu- 

 riance of vegetation. 



§ 2. General constitution of the earthy part of the soil. 



From what is above stated, it appears that, on a general average, the 

 earthy part of the soil in our climate does not constitute less than 96 pei 

 cent, of its whole weight, when free from water. This earthy part con- 

 sists principally of three ingredients: — 



1°. Oi' Silica., siliceous sand, or siliceous gravel— of various degrees 

 of fineness, from that of an imi>alpable powder as it occurs in clay soils, 

 to the large and more or less rounded sandstones of the gravel beds. 



2°. Alumina — generally in the form of clay, but occasionally occur- 

 ring in shaly or slaty masses more or less hard, intermingled with the 

 soil. 



3°. Lime, or carbonate of lime — in the form of chalk, or of fragments 

 more or less large of the various limestones that are met with near the 

 surface in different countries. Where cultivation prevails it often hap- 

 pens that all the lime which the soil contains has been added to it for 

 agricultural purposes — in the form of (juick-lime, of chalk, of shell-sand, 

 or of one or other of the numerous varieties of marl which different dis- 

 tricts are known to produce. 



It is rare that a superficial covering is anywhere met with on the 

 surface of the earth, which consists solely of any one of these three sub- 

 stances — a soil, however, is called sandy in which the siliceous sand 

 greatly predominates, and calcareous, where, as in some of our chalk 

 and limestone districts, carbonate of lime is present in considerable abun- 

 dance. When alumina forms a large proportion of the soil, it constitutes 

 a clay of greater or less tenacity. 



The term clay, however, or pure clay, is never used by writers on 

 agriculture to denote a soil consisting of alumina only, for none such ever 

 occurs in nature. The pure porcelain clays are the richest in alumina, 

 but even when free from water they contain only from 42 to 48 per cent, 

 of this earth, with from 52 to 58 of silica. These occur, however, only 

 in isolated patches, and never alone form the soil of any considerable 



