120 



HISTORY OF THE VEGETABLE KINGDOM. 



and perislieil when the nourishment of tlie coty- 

 ledons was exhausted. It is plain, therefore, 

 that the earths, though bexieficial to the growth 

 of some vegetables, and perhaps necessary to the 

 health of others, are by no means capable of 

 afiFording any considerable degree of nourishment 

 to the plant. 



Manures. In the preceding pages has been 

 given a brief view of the different species of 

 vegetable food, whether it be regarded as derived 

 from the soil or the atmosphere. It now remains 

 to show how the food necessary to the support of 

 the vegetating plant may be supplied when defec- 

 tive, or restored when exhaxisted : but this un- 

 avoidably involves the subject of manures, or 

 artificial preparations of vegetable food, so im- 

 portant to the advancement of agriculture, and 

 consequent interest of mankind. 



With regaid to the food of plants derived 

 from the atmosphere, the supply is pretty regu- 

 lar; at least, in as far as the gases are concerned ; 

 for they are not found to vary materially in their 

 proportions on any part of the surface of the 

 globe : but the quantity of moisture contained 

 in the atmosphere is continually varying, so that 

 in the same season you have not always the same 

 quantity, though, in the course of the year, the 

 deficiency is perhaps made up. From the at- 

 mosphere, therefore, there is a regular supply 

 of vegetable food kept up by nature for the sup- 

 port 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 com- 

 position, they are at least more within the reach 

 of human management. We have already seen 

 the materials of which soils are composed : but 

 what are the proportions of the materials in soils 

 best suited for culture ? According to the analy- 

 sis of Bergman, the soil best suited for culture 

 contains four parts of clay, three of sand, two 

 of calcareous earth, and one of magnesia: and, 

 according to the analysis of Fourcroy and Has- 

 senbratz, 9216 parts of fertile soil contained 305 

 parts of carbon, together with 279 parts of oil ; 

 of which, according to the calculations of La- 

 voisier, 220 parts may be regarded as carbon : 

 80 that the whole of the carbon contained in the 

 soil in question, may be estimated at about 625 

 parts, exclusive of the roots of vegetable, or to 

 about iV of its weight. 



According to Mr Young, equal weights of 

 different soils, when dried and reduced to powder, 

 yielded by distillation quantities of air some- 

 what corresponding to the ratio of their values. 

 The air was a mixture of fixed and inflammable 

 nirs, proceeding probably from decomposition of 

 the water ; but partly, perhaps, from its capacity 

 of abstracting a portion of air from the atmos- 

 phere, which the soil at least is capable of doing. 



The following is the analysis of a fertile soil, 

 as occuning in the neighboui-hood of Bristol. 

 In 400 grains, there were of 



Water 52 



Sillcious 8and 240 



Vegetable fibre ; 5 



extract 3 



Alumine 48 



Slagnesia 2 



Oxide of iron 14 



Cale;ircous earth 30 



Loss 6 



Total 400 



But Mr Kirwin has shown in his Geological 

 Essays, that the fertility of a soil depends in a 

 great measure upon its capacity for retaining 

 water J and if so, soils containing the same in- 

 gredients must bo also equally fertile, all other 

 circumstances being the same; though it is plain 

 that their actual fertility will depend ultimately 

 upon the quantity of rain that falls, because the 

 quantity suited to a wet soil cannot be the same 

 that is suited to a dry soil. And hence it often 

 happens that the ingredients of the soil do not 

 correspond to the character of the climate. Silica 

 exists in the soil under the modification of sand, 

 and alumine under the modification of clay. 

 But the one or the other is often to be met with 

 in excess or defect. Soils in which the sand 

 preponderates retain the least moisture; and soils 

 in which the clay preponderates retain the most; 

 the fonner are dry soils ; the latter are wet soils. 

 But it may happen that neither of them is suffi- 

 ciently favourable to culture; in which case, 

 their peculiar defect or excess must be supplied 

 or retrenched, before they can be brought to a 

 state of fertility. 



But soils in a state of culture, though consist- 

 ing originally of the due proportion of ingre- 

 dients, may yet become exhausted of the prin- 

 ciple of fertility by means of too frequent crop- 

 ping, whether by repetition or rotation of tht 

 same, or of different crops. And 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 ex- 

 hausted soil ; or communicated 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 supei-fluous and stag- 

 nant water, and of paring and burning the turf 

 upon the surface. This mode of preparation is 

 at present much practised throughout England, 

 but particularly in Yorkshire and Lincolnshire, 

 as being the best suited to the character of the 

 soil of these counties that remains to be taken 

 into cultivation. 



If the soil has been exhausted by too frequent 

 a repetition of the same crop, it often happens 



