?32 SCIENCE OF AGRICULTURE. Part II. 



marsliy, as is frequently the case, it is often sufficient to prepare it merely by means of 

 draining off the superfluous and stagnant water, and of paring and burning the turf upon 

 the surface. If the soil has been exhausted by too frequent a repetition of the same 

 crop, it often happens that a change of crop will answer the purpose of the cultivator ; 

 for although a soil may be exhausted for one sort of grain, it does not necessarily follow 

 that it is also exhausted for another. And accordingly, the practice of the farmer is to 

 sow his crops in rotation, having in the same field a crop, perhaps, of wheat, barley, 

 beans, and tares in succession ; each species selecting in its turn some peculiar nutriment, 

 or requiring, perhaps,, a smaller supply than the crop that has preceded it. But even 

 upon the plan of rotation, the soil becomes at length exhausted, and the cultivator obliged 

 to have recourse to other means of restoring its fertility. In this case, an interval of re- 

 pose is considerably efficacious, as may be seen from the encreased fertility of fields that 

 have not been ploughed up for many years, such as those used for pasture ; or even from 

 that of the walks and paths in gardens when they are again broken up. Hence also the 

 practice of fallowing, and of trenching or deep ploughing, which in some cases has nearly 

 the same effect. 



1509. The fertility of a soil is restored, in the case of draining, by means of its 

 carrying off all such superfluous moisture as may be lodged in the soil, which is well 

 known to be prejudicial to plants not naturally aquatics, as well as by rendering the 

 soil more firm and compact. In the case of burning, the amelioration is effected by 

 means of the decomposition of the vegetable substances contained in the turf, and sub- 

 jected to the action of the fire, which disperses part also of the superfluous moisture, but 

 leaves a residue of ashes favorable to future vegetation. In the case of the rotation of 

 crops, the fertility is not so much restored as more completely developed and brought into 

 action ; because the soil, though exhausted for one species of grain, is yet found to be 

 sufficiently fertile for another, the food necessary to each being different, or required in 

 less abundance. In the case of the repose of the soil, the restored fertility may be owing to 

 the decay of vegetable substances that are not now carried off in the annual crop, but left to 

 augment the proportion of vegetable mould ; or to the accumulation of fertilising particles 

 conveyed to the soil by rains ; or to the continued abstraction of oxygen from the atmo- 

 sphere. In the case of fallows, it is owing undoubtedly to the action of the atmospheric 

 air upon the soil, whether in rendering it more friable, or in hastening the putrefaction 

 of noxious plants ; or it is owing to the abstraction and accumulation of oxygen. In 

 the case of trenching, or deep ploughing, it is owing to the increased facility with which 

 the roots can now penetrate to the proper depth, and thus their sphere of nourishment is 

 increased. But it often happens that the soil can no longer be ameliorated by any of the 

 foregoing means, or not at least with sufficient rapidity for the purposes of the cultivator ; 

 and in this case there must be a direct and actual application made to it of such sub- 

 stances as are fitted to restore its fertility. Hence the indispensable necessity of manures, 

 which consists chiefly of animal and vegetable remains that are buried and finally decom- 

 posed in the soil, from which they are afterwards absorbed by the root of the plant, in a 

 state of solution. 



1510. But as carbon is the principal ingredient furnished hy manures, as contributing to 

 the nourishment of the plant, and is not itself soluble in water, nor even disengaged by 

 fermentation in a state of purity ; under what state of chemical combination is its solu- 

 tion effected ? Is it effected in the state of charcoal ? It has been thought, indeed, that 

 carbon in the state of charcoal is soluble in water; because water from a dunghill, when 

 evaporated, constantly leaves a residuum of charcoal, as was first ascertained by the ex- 

 periments of Hassenfratz. But there seem to be reasons for doubting the legitimacy of the 

 Conclusion that has been drawn from it ; for Senebier found that plants whose roots were 

 immersed in water took up less of the fluid in proportion as it was mixed with water from 

 a dunghill. Perhaps then the charcoal of water from a dunghill is held merely in sus- 

 pension, and enters the plant under some other modification. But if carbon is not 

 soluble in water in the state of charcoal, in what other state is it soluble ? It is soluble 

 in the state of carbonic acid gas. But is this the state in which it actually enters the 

 root? On this subject phytologists have been somewhat divided in opinion. Senebier 

 endeavors to prove that carbonic acid gas, dissolved in water, supplies the roots of plants 

 with almost all their carbon, and founds his arguments upon the following facts : In the 

 first place, it is known that carbonic acid gas is soluble in water ; in the second place, it 

 is known to be contained in the soil, and generated by the fermentation of the materials 

 composing manures ; and, in the next place, it is known to be beneficial to vegetation 

 when applied artificially to the roots, at least in a certain degree. This is evident from 

 the following experiment of Ruckert, as well as from several experiments of Saussure's, 

 previously related. Ruckert planted two beans in pots of equal dimensions, filled with 

 garden-mould ; the one was moistened with distilled water, and the other with water im- 

 pregnated with carbonic acid gas. But the latter appeared above ground nine days 

 sooner than the former, and produced twenty-five beans; while the former produced only 



