supplement. SCIENCE AND ART OF AGRICULTURE. 1305 



with nitrate of strontian. In each case they absorbed the substance thus presented to them, for it was 

 detected by analysis in their structure, but they expelled it again. These experiments offered strong 

 confirmation of the above views, and afforded a satisfactory explanation of the fact, that a plant like 

 wheat, which contains much potash, will not flourish after another crop likewise requiring potash, and 

 that it thrives after clover, beans, peas, or other leguminous plants, which scarcely contain any alkalies ; 

 but they did not explain how land becomes fertilised by fallowing, nor how leguminous plants cause an 

 increa-e of carbonaceous matter in the soil. Other experiments, however, made by him have esta- 

 blished the fact, that most if not all plants expel whatever their organs are unable to convert into woody 

 fibre, starch, gluten, &c, and that these substances are of two kinds, inorganic matters derived from the 

 soil, which are incapable of assimilation, and organic compounds formed in the plants In the vital pro- 

 cess. He found, for instance, that when leguminous plants were grown in water, it acquired a brown 

 colour ; that other plants of the same kind would not grow in this water, but that the plants of corn 

 throve in it and removed some of the colouring matter. He ascertained, too, that of the organic matters 

 thus expelled, some were of an acrid resinous nature, some mild, like gums ; the former of which he 

 regarded as poisonous, the latter as nutritious to succeeding crops. These organic matters, thus ex- 

 pelled by the roots, and deposited in the soil, restore to it the carbon, w hich in the early stages of their 

 growth the plants had extracted from it in the shape of carbonic acid. Before they can be converted 

 into nutriment for other plants, they must undergo decomposition : their putrefaction must be converted 

 into decay by the access of air, which tillage produces, and thus they become capable of performing the 

 functions of humus, by affording a supply of carbonic acid. In calcareous soils, the process of decay is 

 accelerated by the presence of lime, while argillaceous soils are those in which the longest time is re- 

 quired for its completion. The excrements of a given crop must be thoroughly decomposed before the 

 land will produce another of the same species ; and on those soils on which the longest intervals are re- 

 quired between crops of the same kind, it is found that the time cannot he shortened by the most power- 

 ful manures. Now calcareous soils are those on w hich peas, clover. &c. w ill bear to be repeated at the 

 shortest intervals, and argillaceous soils are those on which the longest periods are required between 

 them. But though these excrements, undecomposed, either wholly or in part, are injurious to plants of 

 the same species as those which expelled them, they are not so, nay, are even capable of affording nu- 

 triment, to those of other species ; and therefore the introduction* into cultivation of every new plant 

 which can supply the place of another, such as clover or turnips, which will not bear frequent repetition, 

 confers a great benefit on the farmer, by furnishing him with the means of varying and extending his 

 rotation." (Trimmer's Chemistry for Farmers, Ijc, p. 1%.) 



Ch a p. II. — Manures. 



8135 — 2224. The use of all manures is to increase the natural fertility of the soli, or to restore that 

 which has been diminished by vegetation. The idea of a universal pabulum for all plants is nearly ex- 

 ploded ; and all the attempts to discover it are, by many considered to be on a par with the finding of 

 the philosopher's stone or the universal medicine. The improvements in chemistry have discovered 

 various and different substances in every different family of plants ; not only such as are peculiar to or- 

 ganised matter, and are the result of the decomposition of vegetable and animal substances, but others 

 likewise, which belong to the mineral kingdom. These can be exhibited unaltered in the residue of 

 chemical decomposition, whether in the dry way, by means of heat, or in the humid way, by means of 

 the action of other substances, which destroy the cohesion of the parts, or change their affinities. Thus 

 the earths, silica, lime, magnesia, alumina, and several of the metals, especially iron, are found in the 

 ashes of plants which have been burned ; and from the regular proportion- of these in plants of the 

 same kind, whatever be the nature of the soil in which they are raised, we must conclude that they are 

 in some measure essential to their formation. However involved in darkness and doubt the growth and 

 nourishment of plants is in the present state of science, there are certain principles which may be con- 

 sidered to be fully established by experiment : of these one is, that whatever enters the body of a plant, 

 w hether by the roots or the pores, which are distributed along its surface, especially in the leaves, when they 

 are developed, must be so minutely divided, that its particles are invisible, not only to our naked eyes, but 

 even assisted by the high magnifying powers of the microscope ; that is, they must be fluid, w hether in 

 a liquid or aeriform state. It is useless, therefore, to present to the pores, or mouths, if we may so call 

 them, of plants, substances which cannot enter into them, however well adapted they maybe to serve 

 as nourishment or increase. Mineral substances must, therefore, be dissolved in suitable menstrua be- 

 fore the plants can imbibe them. Organic substances naturally decompose in the state of gas. and these 

 gases may contain various matters in solution. It is more than probable that water and atmospheric air 

 are the chief menstrua in which the food of plants is dissolved ; as we well know that, without the pre- 

 sence of both, plants soon become diseased, and die. (G. C. 1843, p. 67.) 



8136. The modern theury of manures, as founded by Sprengel and lately established by Liebig, is thus 

 ably and concisely stated by Mr. Pusey. " Plants consist in the main of several vegetable substances, 

 which are, however, all composed of four kinds of air variously combined ; these gases are named oxygen, 

 hydrogen, carbon, and nitrogen. Dr. Liebig supposes that the two first are derived by the plant from 

 water ; the third, which is charcoal, from the air ; and the fourth, nitrogen, which constitutes the most 

 nutritious part of our food, from ammonia ; which substance he has found not merely in the dung of 

 animals, but in the water of rain, — a new and remarkable fact. But there exists also in crops a consider- 

 able quantity of earthy matter ; in every ton of oat-straw, for instance, nearly one cwt. of flint ; whence, 

 if a hayrick be burnt, lumps of a substance like glass are often found in the ashes. These mineral 

 substances vary in different plants as to quantity, but eight are generally to be found in their ashes, four 

 of the eight being acids, namely, that of flint, which is silica ; of bones, phosphorus ; of brimstone, 

 sulphuric acid ; of common salt, muriatic acid : and also four alkalies, potash, soda, lime, and magnesia. 

 A very small quantity of alumine. or the earth of clay, is also usually detected in the ashes of plants. 

 These, Dr. Liebig says, cannot of course be formed in the plant, but must be derived from the soil ; and 

 accordingly there they are generally to be found when the soil is examined by chemists, but in limited 

 quantity, so that the soil may become exhausted of one or more of them. But further, all these eight 

 mineral subtances are to be found in farm-yard dung, besides ammonia, the source of nitrogen ; hence the 

 excellence of dung for ail crops indifferently. Some crops, however, require more of one ingredient than 

 of another : hence the good effect of bones upon turnips, which contain a great deal of phosphorus ; and 

 of gypsum or peat-ashes, which contain sulphate of lime, upon clover ; of Epsom salts also. Dr. Liebig 

 states, which contains magnesia, upon potatoes. Some soils, again, may contain so much of one of these 

 eight minerals, that it may be useless to add any more. Thus gypsum is found to be useful in one part 

 of a field and not in another, and bones are useless in Mecklenburg, where the fields are dressed with 

 a marl full of phosphorus ; or, on the other hand, a tract of country may be deficient altogether in 

 some one of the eight ingredients which is necessary for all crops, as in lime : in such a district lime will 

 be a standing manure. This new theory of agriculture, though but a theory at present, certainly pro- 

 mises important results. In order to test it first, and. if it hold good, to apply it afterwards, two courses 

 of inquiry are requisite : 1. as Dr. Liebig informed Mr. Pusey, " a more minute examination of the ashes 

 of plants' in which these mineral substances are found, and further a more accurate analysis of our 



