232 SCIENCE OF AGRICULTURE. Part II. 



though hi the course of the year the deficiency is perhaps made up. From the atmo- 

 sphere, therefore, there is a regular supply of vegetable food kept up by nature for the 

 support of vegetable life, independent of the aid of man : and if human aid were even 

 wanted, it does not appear that it could be of much avail. But this is by no means the 

 case with regard to soils ; for if soils are less regular in their composition, they are at 

 least more within the reach of human management. The supply of food may be in- 

 creased by altering the mechanical or chemical constitution of soils ; and by the addi- 

 tion of food in the form of manures. The mechanical constitution of soils may be 

 altered by pulverisation, consolidation, draining, and watering ; their chemical properties 

 by aeration and torrification ; both mechanical and chemical properties, by the addition 

 of earths or other substances ; and manures, either liquid or solid, are supplied by the 

 distiibution of prepared fluids, dungs, and other nourishing matters, with or without 

 their interment. (See Book III.) 



1534. 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 of the same, or rotation 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 suflScient 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 onei sort of grain, it does not necessarily follow 

 that it is also exhausted for another. 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 which has preceded it. But even 

 upon the plan of rotation, the soil becomes at length exhausted, and the cultivator is 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 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 as trenching. 



1535. 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 its 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 favourable 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 which 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 soU, 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, by which 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 consist 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. 



1536. 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 

 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- 



