12 



AGRICULTURAL CHEMISTRY. 



has naturally been recognised as the cause 

 of their fertility. To this matter, the terra 

 " vegetable mould" or humus has been ap- 

 plied. Indeed, this peculiar substance ap- 

 pears to play such an important part in the 

 phenomena of vegetation, that vegetable 

 physiologists have been induced to ascribe 

 the fertility of every soil to its presence. It 

 is believed by many to be the principal nu- 

 triment of plants, and is supposed to be ex- 

 tracted by them from the soil in which they 

 grow. It is itself the product of the decay 

 of vegetable matter, and must, therefore, con- 

 tain many of the constituents which are 

 found in plants during life. Its action will, 

 therefore, be examined in considering whence 

 these constituents are derived. 



CHAPTER II. 



OF THE ASSIMILATION OF CARBON. 

 COMPOSITION OF HUMUS. 



THE humus, to which allusion has been 

 made, is described by chemists as a brown 

 substance easily soluble in alkalies, but only 

 slightly so in water, and produced during 

 the decomposition of vegetable matters by 

 the action of acids or alkalies, k has, how- 

 ever, received various names according to 

 the different external characters and chemi- 

 cal properties which it presents. Thus, 

 ulmin, humic acid, coal of humus, and humin, 

 are names applied to modifications of humus. 

 They are obtained by treating peat, woody 

 fibre, soot, or brown coal with alkalies ; by 

 decomposing sugar, starch, or sugar of milk: 

 by means of acids; or by exposing alkaline 

 solutions of tannic and gallic acids to the 

 action of the air. 



The modifications of humus which are 

 soluble in alkalies, are called humic acid; 

 while those which are insoluble have re- 

 ceived the designations of humin and coal of 

 humas. 



The names given to these substances 

 might cause it to be supposed that their 

 composition is identical. But a more erro- 

 neous notion could not be entertainad ; since 

 even sugar, acetic acid, and resin do not 

 differ more widely in the proportions of their 

 constituent elements, than do the various 

 modifications of humus. 



Humic acid formed by the action of hy- 

 drate of potash upon sawdust contains, ac- 

 cording to the accurate analysis of Peligot, 

 72 per cent, of carbon, while the humic acid 

 obtained from turf and brown coal contains, 

 according to Sprengel, only 58 per cent.; 

 that prod ced by the action of dilute sul- 

 phuric acid upon sugar, 57 per cent, accord- 

 ing to Malaguti; and that, lastly, which is 

 obtained from sugar or from starch, by means 

 of muriatic acid, according to the analysis 

 of Stein, 64 per cent. All these analyses 

 have been repeated with care and accuracy, 



and the proportion of carbon in \he respective 

 cases has been found to agree with the esti- 

 mates of the differen chemists above men- 

 tioned ; so that there is no reason to ascribe 

 the difference in this respect between the 

 varieties of humus to the mere difference in 

 the methods of analysis or degrees of ex- 

 pertness of the operators. Malaguti states, 

 moreover, that humic acid contains an equal 

 number of equivalents of oxygen and hy- 

 drogen, that is to say, that these elements 

 exist* in it in the proportions for forming 

 water; while, according to Sprengel, the 

 oxygen is in excess, and Peligot even esti- 

 mates the quantity of oxygen at 14 equiva- 

 lents, and the hydrogen at only 6 equiva- 

 lents, making the deficiency of hydrogen as 

 great as 8 equivalents. And although Mul- 

 der* has very recently explained many of 

 these conflicting results, by showing that 

 there are several kinds of humus and humic 

 acids essentially distinct in their characters, 

 and fixed in their composition, yet he has 

 afforded no proof that the definite compounds 

 obtained by him really exist, as such, in the 

 soil. On the contrary, they appear to have 

 been formed by the action of the potash and 

 ammonia, which he employed in their pre- 

 paration. 



It is quite evident, therefore, that chemists 

 have been in the habit of designating all 

 products of the decomposition of organic 

 bodies which had a brown or brownish 

 black colour, by the names of humic acid or 

 humin, according as they were soluble or 

 insoluble in alkalies ; although in their 

 composition and mode of origin, the sub- 

 stances thus confounded might be in no 

 way allied. 



Not the slightest ground exists for the be- 

 lief that one or other of these artificial pro- 

 ducts of the decomposition of vegetable 

 matters exists in nature in the form and en- 

 dowed with the properties of the vegetable 

 constituents of mould ; there is not the 

 shadow of a proof that one of them exerts 

 any influence on the growth of plants eithei 

 in the way of nourishment or otherwise. 



Vegetable physiologists have, without any 

 apparent reason, imputed the known pro- 

 perties of the humus and humic acids of 

 chemists to that constituent of mould which 

 has received the same name, and in this 

 way have been led to their theoretical no- 

 tions respecting the functions of the latter 

 substance in vegetation. 



The opinion that the substance called 

 humus is extracted from the soil by the roots 

 of plants, and that the carbon entering into 

 its composition serves in some form or 

 other to nourish their tissues, is considered 

 by many as so firmly established that any 

 new argument in its favour has been deemed 

 unnecessary ; the obvious difference in the 

 growth of plants according to the known 

 abundance or scarcity of humus in the soil. 



* Bulletin des Scienc. Phys. et Natur. de Neerl. 

 1840, p. 1102. 



