48 ORIGIN AND KINDS OF SOILS. 



chemically combined, in clay they are never found chemically separate. 

 Hence, though clay when prepared by the chemist, and kept apart from 

 water, appears as a light dry powder, scarcely different to the eye from pure 

 sand or pure lime, yet in soils it forma an adhesive mass, the particles of 

 which cannot be permanently separated excepting by burning to expel the 

 water held in fixation. When clay is burnt and reduced to powder, it be- 

 comes for all practical purposes sand, and in that state it may be employed 

 to great advantage for reducing the cohesive properties of stiff clay. Rela- 

 tively to heat, clays do not admit the atmosphere between their particles, 

 and an unimproved clayey soil is generally a cold one partly because the 

 heat penetrates with difficulty into it, and partly from the evaporation which 

 during great part of the year is going on from its moist surface. The obvious 

 improvement of clays is by the addition of sand or gravel ; and when the 

 clay does not contain lime, by the addition of that material, either in a caustic 

 or mild state, or as chalk. 



156. Lime, or the basis of chalk and limestone rock, is much less common 

 as a soil than either clay or sand ; though there are scarcely any soils which 

 are naturally fertile that are absolutely without it. Lime is found in a state 

 of carbonate in white or statuary marble, and more or less so in chalk-rock ; 

 and in some limestone-roclis. Lime is never found pure in a state of nature, 

 but always combined with carbonic acid and water, which are driven off from 

 it by burning, leaving the earth in the caustic state called quicklime. In this 

 state lime rapidly reabsorbs water and carbonic acid from the atmosphere, 

 or from any other material which comes in contact with it containing these 

 elements. Hence its use in a caustic state in promoting the putrefaction of 

 imperfectly decomposed organic matter in soils, and in attracting carbonic 

 acid and moisture from the atmosphere. Relatively to the retention of water, 

 a limey or chalky soil may be considered as intermediate between a sandy 

 and a clayey soil, without becoming so tenacious as clay on the one hand, or 

 parting with water so readily as sand on the other. Hence the use of lime 

 or chalk in reducing the tenacity of stiff clays, and increasing the absorbent 

 powers of sandy soils, and improving their texture. A calcareous soil is im- 

 proved by sand and clay, especially if laid on in sufficient quantity to destroy 

 the tenacity and compactness of its texture. 



157. Magnesia, for all practical purposes, may be considered as lime ; 

 it is not very common in soils, and though it is said to be inimical to vege- 

 tation under some circumstances, yet this appears very doubtful. 



158. The iron of soils is mostly found in a state of rust, or oxide. There 

 is scarcely any soil without it ; but it is never very abundant in soils naturally 

 fertile. In a dry state the oxide of iron is insoluble in water, and not inju- 

 rious to vegetation ; but when in consequence of saline substances in the soil, 

 or applied to it, a salt of iron is produced, the iron becomes soluble in water, 

 is taken up by the roots of plants, and is very injurious to them. Iron in this 

 state is termed hydrate, and its evil effects are to be counteracted by caustic 

 lime, with W 7 hich it forms an insoluble compound. 



159. Alluvial soils have been already described as composed of very fine 

 particles of the debris of several kinds of rocks, which have been held in sus- 

 pension by water, and deposited in plains, or along the banks of rivers, along 

 with organic matter also held in suspension. The earthy character of this 

 soil must necessarily always partake of the character of the rocks of the 

 country in which it is found. 



