164 SCIENCE OF GARDENING. Part II. 



stances ; and manures, either liquid or solid, are supplied by irrigation and distribution 

 of dun«-s and other nourisbing matters, with or without their interment. (See Book II.) 

 736.° Soils in a state of culture, though consisting originally of the due proportion of 

 ingredients, may yet become exhausted of the principle of fertility by means of too frequent 

 cropping ; whether by repetition or rotation of the same, or of different crops. In this 

 case, it° should be the object of the phytologist, as well as of the practical cultivator, 

 to ascertain by what means fertility is to be restored to an exhausted soil, or commu- 

 nicated to a new one. In the breaking up of new soils, if the ground has been wet or 

 marshy, 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 al- 

 though 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 requir- 

 ing;, 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 repose is 

 considerably efficacious, as may be seen from the increased 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. 



737. 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 in- 

 creased. 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 substances 

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

 consist chiefly of animal and vegetable remains that are buried and finally decomposed in 

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

 solution. 



738. But as carbon is the principal ingredient furnished by manures, as contributing to the 

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

 mentation in a state of purity ; under what state of chemical combination is its solution 

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

 bon 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 experi- 

 ments 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 duno-hill. 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 endea- 

 vours 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 



