40 



AGRICULTURAL CHEMISTRY. 



yield to the latter all the salt of lead which 

 ihey had previously absorbed. They return, 

 therefore, to the soil all matters which are 

 unnecessary to their existence. Again, when 

 a plant, freely exposed to the atmosphere, 

 rain, and sunshine, is sprinkled with a solu- 

 tion of nitrate of strontia, the salt is ab- 

 sorbed, but k is again separated by the roots 

 and removed farther from them by every 

 shower of rain, which moistens the soil, so 

 that at last not a trace of it is to be found in 

 the plant. 



Let us consider the composition of the 

 ashes of two lir-trees as analysed by an acute 

 and most accurate chemist. One of these 

 grew in Norway, on a soil the constituents 

 of which never changed, but to which solu- 

 ble salts, and particularly common salt, were 

 conveyed in great quantity by rain-water. 

 How did it happen that its ashes contained 

 no appreciable trace of salt, although we are 

 certain that its roots must have absorbed it 

 after every shower? 



We can explain the absence of salt in 

 this case by means of the direct and positive 

 observations referred to, which have shown 

 that plants have the power of returning to 

 the soil all substances unnecessary to their 

 existence; and the conclusion to which all 

 the foregoing facts lead us, when their real 

 value and bearing are apprehended, is that 

 the alkaline bases existing in the ashes of 

 plants must be necessary to their growth, 

 sinoe if this were not the case they would 

 not be retained. 



The perfect developement of a plant, ac- 

 cording to this view, is dependent on the 

 presence of alkalies or alkaline earths ; for 

 when these substances are totally wanting 

 its growth will be arrested, and when they 

 are only deficient it must be impeded. 



In order to apply these remarks, let us 

 compare two kinds of trees, the wood of 

 which contains unequal quantities of alka- 

 line bases, and we shall find that one of 

 these grows luxuriantly in several soils upon 

 which the others are scarcely able to vege- 

 tate. For example, 10,000 parts of oak 

 wood yield 250 parts of ashes, the same 

 quantity of fir wood only 83, of linden wood 

 500, of rye 440, and of the herb of the po- 

 tato plant 1500 parts.* 



Firs and pines find a sufficient quantity 

 of alkalies in granitic and barren sandy soils 

 in which oaks will not grow ; and wheat 

 thrives in soils favourable for the linden 

 tree, because the bases which are necessary 

 to bring it to complete maturity, exist there 

 in sufficient quantity. The accuracy of 

 these conclusions, so highly important to 

 agriculture and to the cultivation of forests, 

 can be proved by the most evident facts. 



All kinds of grasses, the Equisetacece, for 

 example, contain in the outer parts of their 

 leaves and stalk a large quantity of silicic 

 acid and potash in the form of acid silicate 



* Berthier, Annalos de Chimie et de Physique, 

 f. xxx. p. 248. 



of potash. The proportion of this salt does 

 not vary perceptibly in the soil of corn-fields, 

 because it is again conveyed to them as ma- 

 nure in the form of putrefying straw. But 

 this is not the case in a meadow, and hence 

 we never find a luxuriant crop of grass* on 

 sandy and calcareous soils, which contain 

 little potash, evidently because one of the 

 constituents indispensable to the growth of 

 the plants is wanting. Soils formed from 

 basalt, gfauwacke, and porphyry are, cceteri* 

 paribus, the best for meadow land, on ac- 

 count of the quantity of potash which enters 

 into their composition. The potash ab- 

 stracted by the plants is restored during the 

 annual irrigation. The potash contained in 

 the soil itself is inexhaustible in comparison 

 with the quantity removed by plants. But 

 when we increase the crop of grass in a 

 meadow by means of gypsum, we remove 

 a greater quantity of potash with the hay 

 than can under the same circumstances be 

 restored. Hence it happens that, after the 

 lapse of several years, the crops of grass on 

 the meadows manured with gypsum dimi- 

 nish, owing to the deficiency of potash. But 

 if the meadow be strewed from time to time 

 with wood-ashes, even with the lixiviated 

 ashes which have been used by soap-boilers, 

 (in Germany much soap is made from the 

 ashes of wood,^ then the grass thrives as 

 luxuriantly as before. The ashes are only 

 a means of restoring the potash. 



A harvest of grain is obtained every thirty 

 or forty years from the soil of the Luneburg 

 heath, by strewing it with the ashes of the 

 heath plants (.Erica vul^aris} which grow 

 on it. These plants during the long period 

 just mentioned collect the potash and soua, 

 which are conveyed to them by rain-water; 

 and it is by means of these alkalies that oats, 

 barley, and rye, to which they are indis- 

 pensable, are enabled to grow on this sandy 

 heath. 



The woodcutters in the vicinity of Heidel 

 berg have the privilege of cultivating the 

 soil for their own use, after felling the trees 

 used for making tan. Before sowing the 

 land thus obtained, the branches, roots, and 

 leaves, are in every case burned, and the 

 ashes used as a manure, which is found to 

 be quite indispensable for the growth of the 

 grain. The soil itself upon which the oats 

 grow in this district consists of sandstone ; 

 and although the trees find in it a quantity 

 of alkaline earths sufficient for their own 

 sustenance, yet in its ordinary condition it is 

 incapable of producing grain. 



The most decisive proof of the use of strong 

 manure was obtained at Bingen (a town on 

 the Rhine,) where the produce and deve- 

 lopement of vines were highly increased by 



* It would be of importance to examine what 

 alkalies are contained in the ashes of the sea-shore 

 plants which grow in the humid hollows of downs, 

 and especially in those of the millet-grass. If 

 potash is not found in them, it must certainly bf> 

 replaced by soda as in the Salsola, or by lime aa 

 in the PlumbaginecB. 



