4 ORIGIN AND KINDS OF SOILS, CONSIDERED 



eye from pure sand or pure lime, yet in soils it forms an adhesive mass, 

 the particles of which cannot be permanently separated except by 

 burning to expel the water held in fixation. When clay is burnt and 

 reduced to powder, it becomes for all practical purposes sand, and in 

 that state it may be employed to great advantage for reducing the 

 cohesive properties of stiff clay. Relatively 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 

 way to improve 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 a mild state, or as chalk. 



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 whfte or statuary marble, and more or 

 less so in chalk-rock and in some limestone-rocks. Lime is never found 

 pure in a state of nature, but always combined with carbonic or sulphu- 

 ric 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 putre- 

 faction of imperfectly decomposed organic matter in soils, and in 

 attracting carbonic acid and moisture from the atmosphere. Relatively 

 to the retention of water, a limy 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 improved by 

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

 tenacity and compactness of its texture. 



Magnesia is not very common in soils, and is said to be inimical 

 to vegetation, under some circumstances. Magnesian limestone, when 

 burned as lime, should not be used for manuring purposes. 



The iron of soils is mostly 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 injurious 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 which 

 it forms an insoluble compound. 



Alluvial soils have been already described as composed of very fine 

 particles of the debris of several kinds of rocks, which have been 



