SOIL. 



nascent state, and from which th^y 

 obtain the carbon which is so abun- 

 dant in all vef;;etable productions. 

 But organic matter, in every stage of 

 its spontaneous dcconipositiun, keeps 

 the pores of the soil open, and ad- 

 mits, if it does not even attract, air 

 and moisture to the fibres of tlie 

 roots. In all rich soils which have 

 been long cultivated, especially in 

 gardens, there are particles of a dark 

 colour and tibrous te.xture, which, in 

 the microscope, appear like minute 

 logs of charred wood. These keep 

 the soil open, and supply carbonic 

 acid, when the air reaches them, or 

 they are slowly transformed into hu- 

 mus, which remains inert as long as 

 it cannot imbibe oxygen and form 

 carbonic acid by a species of slow 

 combustion. Humus is no doubt one 

 of the chief causes of fertility, but 

 its presence does not appear to be 

 so indispensable as has been ima- 

 gined. A proper texture seems a 

 much more indispensable condition. 

 Humus can undoubtedly be formed 

 from the elements of water and of 

 the atmosphere. Whether it be di- 

 rectly, or by the slow process of ve- 

 getation and subsequent decomposi- 

 tion, does not so readily appear, but 

 it is certain that there are soils which 

 are highly fertile in which scarcely a 

 trace of humus can be discovered, 

 and which, from their igneous forma- 

 tion, cannot well contain organic 

 matter; such are the soils which are 

 produced by the decomposition of 

 the lava which has run in a liquid 

 state from the craters of volcanoes. 

 This is composed of different miner- 

 als, which have been fused by the ac- 

 tion of heat, but in whicli the mi.xture 

 of the earths and salts has not been 

 in such proportions as to form a per- 

 fect glass. When exposed for a time 

 to the influence of the atmosphere, the 

 lava crumbles into an earth, which is 

 neither so loose as silicious sand, nor 

 BO plastic as clay, and which has such 

 a poroeily as suits the growth of the 

 roots of vegetables. By tte effect 

 of a warm climate and frequent rains, 

 vegetation goes on rapidly, and by 

 cultivation humus is soon formed and 



accumulated, so that it is only in the 

 more recently cultivated kivas that it 

 can be said that vegetation goes on 

 wilhuut any supply of organic matter; 

 and the addition of humus greatly in- 

 creases tiie fertility of these soils. 

 It is much easier to supply the defi- 

 ciency of humus, whicli at liest forms 

 but a very small portion of the soil, 

 tlian of silica or alumina, which should 

 enter into its composition in the pro- 

 portion of one half or a third of the 

 whole. It is practicable to carry lime 

 or chalk upon soils which do not con- 

 tain ealeareous matter ; clay may also 

 be carried upon loose, sandy soils, 

 where it can be found below the sur- 

 face, or at a moderate distance ; but 

 if a soil is very deficient in silica, it 

 requires so large a proportion of tliis 

 earth to give porosity to stiff clay, 

 that it very seldom can repay the 

 trouble and expense. Hence the dif- 

 ficulty of bringing poor, wet, clay 

 soils into a fertile state, except where 

 an abundance of chalk and vegetable 

 manures can be easily procured. In 

 this case, the perfect draining of the 

 land, and exposure of the ploughed 

 surface to the frosts of winter, with 

 the addition of chalk and manure, 

 produces such an alteration in the 

 texture of the clay, that, by contin- 

 uing the improving process, it is en- 

 tirely changed into a mellow and fer- 

 tile loam. The burning of a portion 

 of the retentive subsoil into a brick- 

 like earth gives it a porosity which 

 renders it mechanically similar to si- 

 licious sand, and converting the iron 

 which all these clays contain into a 

 peroxide, the soil is thereby greatly 

 improved in fertility ; for it seems 

 that iron, in a state of slight oxida- 

 tion, or combined with any acid, is 

 hurtful to vegetation, whereas the red 

 peroxide is not only innocuous, but 

 seems to have fertilizing properties. 

 " The comparison of the different 

 fertile soils leads, therefore, to the 

 conclusion that the texture or porosity 

 arising from the admixture of parti- 

 cles of various dimensions is the most 

 important object of examination ; and 

 subordinate to this is the chemical 

 constitution of the earths and other 



731 



