LECTURE XI. 



Nature and origin of soils.— Organic matter in the soil.— General constitution of the earthy 

 part of the soil. — Classification of soils from their chemical constituents. — Method of ap- 

 proximate analysis for the purposes of classification.— General origin of soils and subsoils. 

 —Structure of the earth's crust.— Stratified and unstratified rocks.— Crumbling or degra- 

 dation of rocks. — Diversity of soils produced.— Superficial accumulations.— Tabular view 

 of the character and agricultural capabilities of the soils of the different pans of Great 

 Britain. 



Such are the inorganic compounds which minister to the growth of 

 plants, and such the proportions in which they severally occur in the 

 living vegetable. Whence are these inorganic constituents all derived 1 



We have seen that the atmosphere, when pure, contains no inorganic 

 matter, and that if dust, spray, or vapours occasionally float in the air, 

 and are carried by the winds to great distances — yet that they. are 

 only accidentally present, and cannot be regarded as a source from 

 which the general vegetation of the globe derives a constant supply of 

 those mineral substances which are necessary to its healthy existence. 



The soil on which they grow is the only natural source from which 

 their inorganic food can be derived. We are led, therefore, as the next 

 subject of our study, to inquire into the nature and origin of soils.* 



§ 1. Of the organic matter in the soil. 



Soils differ much as regards their immediate origin, their physical 

 properties, their chemical constitution, and their agricultural capabili- 

 ties ; yet all soils which in their existing state are capable of bearing a 

 profitable crop, possess one common character — they all contain organic 

 matter in a greater or a less proportion. 



This organic matter consists in part of decayed animal, but chiefly of 

 decayed vegetable substances, sometimes in brown or black fibrous por- 

 tions, exhibiting still, on a careful examination, something of the origi- 

 nal structure ofthfe organized substances from which they have beende- 

 ri,ved — sometimes forming only a fine brown powder intimately inter- 

 mixed with the mineral matters of the soil — sometimes scarcely percep- 

 tible in either of those forms, and existing only in the state of organic 

 compounds more or less void of colour and at times entirely soluble in 

 water. In soils which appear to consist only of pure sand, or clay, or 

 chalk, organic matter in this latter form may often be detected in con- 

 siderable quantity. 



The proportion of organic matter in soils which are naturally produc- 

 tive of any useful crops, varies from one-half to 70 per cent, of their 

 whole weight. With less than the former proportion they will scarcely 

 support vegetation — with more than the latter, they require much ad- 

 mixture before they can be brought into profitable cultivation. It is 



• On the subject of this and the following lecture, the reader will consult with advantage 

 an excellent little work, " On the nature and property of soils," by Mr. John Morton. 



