ARABLE LAND. 



improving the natural soil, so as to 

 increase its produce. 



3. The most advantageous succes- 

 sion of crops, so as to obtain the 

 greatest returns with the least dimi- 

 nution of fertility. 



1. Soils. — When the surface of the 

 earth is penetrated, we generally find 

 that the appearance, texture, and col- 

 our vary at different depths. There 

 is a layer of earth nearest the surface, 

 of greater or less thickness, which 

 covers the more solid and uniform 

 materials which lie below it. This 

 may be particularly observed wherev- 

 er there are natural or artificial exca- 

 vations or pits. A distinct line, near- 

 ly parallel to the surface, generally 

 marks the depth of the upper soil, and 

 separates it from the subsoil. The 

 soil is more or less composed of mi- 

 nute parts of various kinds of earth, 

 mixed with animal and vegetable sub- 

 stances in different states of decom- 

 position ; and to these, in a great 

 measure, it owes its colour, which is 

 generally darker than that of the sub- 

 soil. Except where iron, peat, coal, 

 or slate abound in the soil, a dark 

 colour is an indication of correspond- 

 ing fertility. The rich soil of gardens 

 long cultivated and highly manured is 

 nearly black. As the soil is the bed 

 in which all vegetable productions are 

 to be reared, and in which they are to 

 find their proper nourishment, its tex- 

 ture and composition become objects 

 of great importance to the cultivator ; 

 and, without a competent knowledge 

 of these, no practical rules can be laid 

 down or depended upon. 



All soils are composed of earthy 

 and metallic oxides, saline substan- 

 ces, vegetable and animal matter, and 

 water. The earths are chiefly clay 

 or alumina, flint or silica, and lime. 



Magnesia, barytes, and other earths 

 are occasionally met with, but in so 

 few instances that they may be omit- 

 ted in the list. 



Of the metals, the most abundant 

 is iron in the state of peroxide. The 

 other metals are rarely found near the 

 surface. 



Saline substances form a small part 

 of a soil, but an important one. 

 83 



Potassa exists in almost every ve- 

 getable, soda in a few, and ammonia 

 is produced by the decomposition of 

 animal matter ; but, from its volatile 

 nature, it is not long retamed in the 

 soil, except when it forms a fixed 

 compound with other substances. 



The vegetable acids, as a general 

 rule, are perhaps limited to small por- 

 tions of acetic acid in combination 

 with some base, as lime or potash. 



The mineral acids arc found united 

 with earths and alkalies, in the state 

 of neutral compounds. 



These saline substances have a 

 powerful effect on vegetation ; and a 

 knowledge of their proportions in the 

 soil, and of their various qualities, is 

 indispensable in order to modify or 

 correct their action by other substan- 

 ces for which they have an affinity. 



Water, in a state of combination, 

 or of mere mechanical diffusion, is 

 essential to the growth of all plants : 

 without it and atmospheric air there 

 is no lile, either animal or vegetable. 



The Earths. — Clay is called alumina 

 because it is obtained in its purest 

 state from alum, in which it is com- 

 bined with the sulphuric acid : it is 

 the basis of all strong and heavy soils. 

 When clay is minutely divided, it is 

 easily suspended in water ; when 

 dried slowly, and stirred while dry- 

 ing, it becomes a fine powder, soft 

 to the feel ; and when kneaded with 

 water, a tough, ductile mass, easily 

 moulded into hollow vessels which 

 retain liquids. This property of being 

 impervious to water gives the specific 

 character to clay as an ingredient of 

 the soil. In a pure and unmixed state 

 it is absolutely barren, ^^'hen clay is 

 heated to a great degree it parts with 

 the water combined with it ; it is then 

 said to be baked, as we see in bricks. 

 It is no longer diffusible in water. 



Silica, or the earth of flints, suffers 

 no change in water. It consists of 

 crystals, or fragments of very hard 

 stone, forming gravel or sand accord- 

 ing to their size ; and the finest sili- 

 cious sand, when examined with a 

 magnifying glass, has the appearance 

 of irregular fragments of sione wjlh- 

 out any cohesion between theiQ. 



