DIFFERENCE BETWEEN SOIL AMD SUBSOIL. 235 



Water 3-9 per cent. 



Organic matter (less than) . . 4-1 

 Carbonate of lime (less than) . 3-0 



Clay 19-0 



Sand (very fine) 70.0 



100-0* 

 This soil, therefore, containing 70 per cent, of sand, separable by 

 decantation, is properly a sandy loam. 



§ 4. Of the distinguishing characters of soils and subsoils. 



Beneath the immediate surface soil, through which the plough makes 

 its way, and to which the seed is entrusted, lies what is commonly dis- 

 tinguished by the name of subsoil. This subsoil occasionally consists 

 of a mixture of the general constituents of soils naturally different from 

 that which forms the surface layer — as when clay above has a sandy 

 bed below, or a light soil on the surface rests on a retentive clay beneath. 



This, however, is not always the case. The peculiar characters of 

 the soil and subsoil often result from the slow operation of natural causes. 



In a mass of loose matter of considerable depth, spread over an extent 

 of country, it is easy to understand how — even though originally alike 

 through its whole mass — a few inches at the surface should gradually 

 acquire different physical and chemical characters from the rest, and 

 liow there should thus be gradually established important agricultural 

 distinctions between the first 12 or 15 inches (the soil), the next 15 (the 

 subsoil), and the remaining body of the mass, which, lying still lower, 

 does not come under the observation of the practical agriculturist. 



On the surface, plants grow and die. Through tlie first few inches 

 their roots penetrate, and in the same the dead plants are buried. This 

 portion, therefore, by degrees, assumes a brown colour, more or less dark, 

 according to the quantity of vegetable matter which has been permitted 

 to accumulate in it. Into the subsoil, however, the roots rarely pene- 

 trate, and the dead plants are still more rarely buried at so great a depth. 

 Sill this inferior layer is not wholly destitute of vegetable or other or- 

 ganic matter. However comparatively impervious it may be, still water 

 makes its way through it, more or less, and carries down soluble organic 

 substances^ which are continually in the act of being produced during the 

 decay of the vegetable matter lying above. Thus, though not sensibly 

 discoloured by an admixture of decayed roots and stems, the subsoil in 

 reality contains an appreciable quantity of organic matter which may 

 be distinctly estimated. 



Again, the continual descent of the rains upon th^surface soil washes 

 down the carbonates of lime, iron, and rriagnesia, as well as other soluble 

 earthy substances — it even, by degrees, carries down the fine clay also, 



• Some of these numbers differ by a minute fraction from those in the preceding page : 

 this is because they are calculated from the more correct decimal fractions contained in my 

 own note-book. The organic matter is said to be less than tlie number here given, because 

 by simple drying, as here prescribed, the whole of the water cannot be driven off— a portion 

 being always retained by the clay, which is not entirely expelled, till the soil is raised nearly 

 to a red heat. Hence the loss by this second heating must always be greater than the actual 

 weight of organic matter present. The lime is aho less than the number given, because, as 

 already stated, the acid dissolves a ittle alumina as well as any carbonate of magnesia which 

 may be present. 



