330 SCIENCE OF AGRICULTURE. Part II. 



Our conclusion from various experiments is, that hydrogen Is unfavorable' to vegetation, and does not 

 serve as the food of plants. But hydrogen is contained in plants as is evident from their analysis ; and if 

 they refuse it when i)resented to them in a gaseous state, in what state do they then acquire it ? To this 

 question it is sufficient for the present to reply, that if plants do not acquire their hydrogen in the state of 

 gas, they may at least acquire it in the state of water, which is indisputably a vegetable food, and of 

 which hydrogen constitutes one of the component parts. 



1502. Vegetable extract. When it was found that atmospheric air and water are not, 

 even conjointly, capable of furnishing the whole of the aliment necessary to the de- 

 velopement of the plant, it was then alleged that, with the exception of water, all sub- 

 stances constituting a vegetable food must at least be administered to the plant in a 

 gaseous state. But this also is a conjecture unsupported by proof; for even with 

 regard to such plants as grow upon a barren rock, or in pure sand, it cannot be said that 

 they receive no nourishment whatever besides water, except in a gaseous state. Many of 

 the particles of decayed animal and vegetable substances, which float in the atmosphere 

 and attach themselves to the leaves, must be supposed to enter the plant in solution with 

 the moisture which the leaves imbibe ; and so also similar substances contained in the 

 soil must be supposed to enter it by the root : but these substances may certainly con- 

 tain vegetable nourishment; and they will perhaps be found to be taken up by the 

 plant in proportion to their degree of solubility in water, and to the quantity in which 

 they exist in the soil. Now one of the most important of these substances is vegetable 

 extract. When plants have attained to the maturity of their species, the principles of 

 decay begin gradually to operate upon them, till they at length die and are converted 

 into dust or vegetable mould, Which, as might be expected, constitutes a considerable 

 proportion of the soil. The chance then is, that it is again converted into vegetable 

 nourishment, and again enters the plant. But it cannot wholly enter the plant, because 

 it is not wholly soluble in water. Part of it, however, is soluble, and consequently 

 capable of being absorbed by the root, and that is the substance which has been denomi- 

 nated extract. 



1503. Saussure filled a large vessel with pure mould of turf, and moistened it with distilled or rain 

 water till it was saturated. At the end of five days, when it was subjected to the action of the press^, 

 10 OOO' parts in weight of the expressed and filtered fluid yielded, by evaporation to dryness, 26 parts of ex- 

 tract. In a similar experiment upon the mould of a kitchen-garden which had been manured with dung, 

 10 000 parts of a fluid yielded 10 or extract. And in a similar experiment upon mould taken from a well- 

 Cu'ltivated corn-field, 10,000 parts of fluid yielded four parts of fextract. Such was the result in these par- 

 ticular cases. But the quantity of extract that may be separated from the common soil is not in general 

 very considerable. After twelve decoctions, all that could be separated was about one eleventh of its 

 weight ; and yet this seems to be more than suflHcient for the purposes of vegetation : for a soil containing 

 this quantity was found by experiment to be less fertile, at least for peas and beans, than a soil that con- 

 tained only one half or two thirds the quantity. But if the quantity of extract must ))ot be too much, 

 neither must it be too little. Plants that were put to vegetate in soil deprived of its extract, as far as re- 

 peated decoctions could deprive it, were found to be much less vigorous and luxuriant than plants vegetating 

 HI soil not deprived of its extract ; and yet the only perceptible difference between them is, that the former 

 can imbibe and retain a much greater quantity of water than the latter. From this last experiment, as 

 well as from the great proportion in which it exists in the living plant, it evidently follows that extract 

 constitutes a vegetable food. But extract contains nitrogen ; for it yields by distillation a fluid impregnated 

 with ammonia. The diflficulty, therefore, of accounting for the introduction of nitrogen into the vegeta- 

 ting plant, as well as for its existence in the mature vegetable substance, is done away ; for, although the 

 plant refuses it when presented in a gaseous state, it is plain that it must admit it along with the extract. 

 It seems also probable that a small quantity of carbonic acid gas enters the plant along with the extractive 

 principle, as it is known to contain this gas also. 



1504. Salts, in a certain proportion, are found in most plants, such as nitrate, muriate, 

 and sulphate of potass or soda, as has been already shown. These salts are known to 

 exist in the soil, and the root is supposed to absorb them in solution with the water by 

 which the plant is nourished. It is at least certain that plants may be made to take up 

 by the roots a considerable proportion of salts in a state of artificial solution. But if 

 salts are thus taken up by the root of the vegetating plant, does it appear that they are 

 taken up as a food ? Some plants, it must be confessed, are injured by the application of 

 salts, as is evident from the experiments of Saussure ; but others are as evidently benefited 

 by it. Trefoil and lucerne have their growth much accelerated by the application of sul- 

 phate of lime, though many other plants are not at all influenced by its action. The 

 parieftaria, nettle, and borage will not thrive, except in such soils as contain nitrate of lime, 

 or nitrate of potass ; and plants inhabiting the sea-coast, as was observed by Du Hamel, 

 " will not thrive in a soil that does not contain muriate of soda. It has been thought, how- 

 ever, that the salts are not actually taken up by the root, though converted to purposes of 

 utility by acting as astringents or corrosives in stopping up the oritices of the vessels of the 

 plant, and preventing the admission of too much water : but it is to be recollected that 

 the salts in question are found ])y analysis in the very substance of the plant, and must 

 consequently have entered in solution. It has been also thought that salts are favorable 

 to vegetation only in proportion as they hasten the putrefaction of vegetable substances 

 contained in the soil, or attract the humidity of the atmosphere. But sulphate of lime is 

 not deliquescent ; and if its action consist merely in accelerating putrefaction, why is its 

 beneficial eifect confined but to a small number of plants? Grisenthwaite [Neiv Theory 

 of Agriculture, 1819, p. 111.) answers this question by stating, that as in the principal 

 grain-crops which interest the agriculturist, there exists a particular saline substance, pe- 



