ABSORPTION OF HUMIS. 



13 



seemed to afford incontestable proof of its 

 correctness.* 



Yet, this position, when submitted to a 

 strict examination, is found to be untenable, 

 and it becomes evident from most conclusive 

 proofs that humus in the form in which it 

 exists in the soil, does not yield the smallest 

 nourishment to plants. 



The adherence to the above incorrect 

 opinion has hitherto rendered it impossible 

 or the true theory of the nutritive process 

 /n vegetables to become known, and has thus 

 deprived us of our best guide to a rational 

 practice in agriculture. Any great improve- 

 ment in that most important of all arts is in- 

 conceivable without a deeper and more per- 

 fect acquaintance with the substances which 

 nourish plants, and with the sources whence 

 they are derived ; and no other cause can 

 be discovered to account for the fluctuating 

 and uncertain state of our knowledge on 

 this subject up to the present time, than 

 that modern physiology has not kept pace 

 with the rapid progress of chemistry. 



In the following inquiry we shall suppose 

 the humus of vegetable physiologists to be 

 really endowed with the properties recog- 

 nised by chemists in the brownish black de- 

 posits which they obtain by precipitating an 

 alkaline decoction of mould or peat by 

 means of acids, and which they name humic 

 acid. 



Humic acid, when first precipitated, is a 

 flocculent substance, is soluble in 2500 

 times its weight of water, and combines 

 with alkalies, lime and magnesia, forming 

 compounds of the same degree of solubility. 

 (Sprengel.) 



Vegetable physiologists agree in the sup- 

 position that by the aid of water humus is 

 rendered capable of being absorbed by the 

 roots of plants. But according to the ob- 

 servation of chemists, humic acid is soluble 

 only when newly precipitated, and becomes 

 completely insoluble when dried in the air, 

 or when exposed in the moist state to the 

 freezing temperature. (Sprengel.) 



Both the cold of winter and the heat of 

 summer, therefore, are destructive of the solu- 

 bility of hurnic acid, and at the same time of 

 its capability of being assimilated by plants. 

 So that, if it is absorbed by plants, it must 

 be in some altered form. 



The correctness of these observations is 

 easily demonstrated by treating a portion of 

 good mould with cold water. The fluid re- 

 mains colourless, and is found to have dis- 

 solved less than 100,000 part of its weight 

 of organic matters, and to contain merely 

 the salts which are present in rainwater. 



Decayed oak wood, likewise, of which 

 humic acid is the principal constituent, was 

 found by Berzelius to yield to cold water 



* This remark applies more to German than 

 to English botanists and physiologists. In Eng- 

 land, the idea that humus, as such, affords nour- 

 ishment to plants is by no means general ; but on 

 the Continent, the views of Berzelius on this sub- 

 ject have been almost universally adopted. ED. 



only slight traces of soluble materials ; and 

 I have myself verified this observation on 

 the decayed wood of beech and fir. 



These facts, whicn show that humic, in 

 its unaltered condition, cannot serve for the 

 nourishment of plants, have not escaped the 

 notice of physiologists ; and hence they have 

 assumed that the lime or the different alka- 

 lies found in the ashes of vegetables render 

 soluble the humic acid and fit it for the pro- 

 cess of assimulation. 



Alkalies and alkaline earths do exist in 

 the different kinds of soil in sufficient quan- 

 tity to form such soluble compounds with 

 the humic acid. 



Now, let us suppose that humic acid is 

 absorbed by plants in the form of that salt 

 which contains the largest proportion of 

 humic acid, namely, in the form of humate 

 of lime, and then from the known quantity 

 of the alkaline bases contained in the ashes 

 of plants, let us calculate the amount ol 

 humic acid which might be assimulaied in 

 this manner. Let us admit, likewise, that 

 potash, soda, and the oxides of iron and 

 manganese have the same capacity of satu- 

 ration as lime with respect to humic acid, 

 and then we may take as the basis of our 

 calculation the analysis of M. Berthier, who 

 I found that 1000 Ibs. of dry fir wood yielded 

 ; 4 Ibs. of ashes, and that in every 100 Ibs. of 

 these ashes, after the chloride of potassium 

 and sulphate of potash were extracted, 53 

 j Ibs. consisted of the basic metallic oxides, 

 ' potash, soda, lime, magnesia, iron, and 

 manganese. 



One Hessian acre* of woodland yields 

 annually, according to Dr. Heyer, on an 

 average, 2920 Ibs. of dry fir wood, which 

 | contain 6.17 Ibs. of metallic oxides. 



Now, according to the estimates of Mala- 

 guti and Sprengel, 1 Ib. of lime combines 

 chemically with 12 Ibs. of humic acid ; 6.17 

 Ibs. of the metallic oxides would accordingly 

 introduce into the trees 67 Ibs. of humic 

 acid, which, admitting humic acid to con- 

 tain 58 per cent, of carbon, would corres- 

 pond to 100 Ibs. of dry wood. But we have 

 | seen that 2920 Ibs. of fir wood are really- 

 produced. 



Again, if the quantity of humic acid 

 which might be introduced into wheat in 

 the form of humates is calculated from the 

 known proportion of metallic oxides exist- 

 ing in wheat straw, (the sulphates and 

 chlorides also contained in the ashes of the 

 straw not being included, it will be found 

 that the wheat growing on 1 Hessian acre 

 would receive in that way 63 Ibs. of humic 

 acid, corresponding to 93.6 Ibs. of woody 

 fibre. But the extent of land just mentioned 

 produces, independently of the roots and 

 grain, 1961 Ibs. of straw, the composition 

 of which is the same as that of woody fibre. 



It has been taken for granted in these cal- 



* One Hessian acre is equal to 40,000 square 

 feet, Hessian, or 26,910 square feet, English mea- 



B1U*. 



