Book I. 



FOOD OF THE VEGETATING PLANT. 



163 



not deliquescent ; and if its action consist merely in accelerating putrefaction, why is its 

 beneficial effect confined but to a small number of plants ? Grisenthwaite [New Theory 

 of Agriculturet 1819, p. 111.) answers this question by stating, that as in the principal 

 grain-crops which interest the agriculturist, there exists a particular saline substance, pe- 

 culiar to each, so, if we turn our attention to the clovers, and turnips, we shall still find 

 the same discrimination. Saintfoin, clover, and lucerne, have long been known to con- 

 tain a notable quantity of gypsum (sulphate of lime) ; but such knowledge, very strange 

 to relate, never led to the adoption of gypsum as a manure for those crops, any more than 

 tliat of phosphate of lime for wheat, or nitrate of soda, or potassa for barley. It is true 

 that gypsum has been long, and in various places, recommended as a manure, but its uses 

 not being understood, it was recommended without any reference to crop, or indeed to the 

 accomplishment of any fixed object. It is very well known that some particular ingre- 

 dient may be essential to the composition of a body, and yet constitute but a very small 

 proportion of its mass. Atmospheric air contains only about one part in the 100 of 

 carbonic acid ; and yet no one will venture to affirm that carbonic acid gas is merely an 

 adventitious and accidental element existing by chance in the air of the atmosphere, and 

 not an essential ingredient in its composition. Phosphate of lime constitutes but a very 

 small proportion of animal bodies, perhaps not one part in 500 ; and yet no one doubts 

 that it is essential to the composition of the bones. But the same salt is found in the 

 ashes of all vegetables ; and who will say that is not essential to their perfection ? 



734. Earths. As most plants have been found by analysis to contain a portion of 

 alkaline or earthy salts, so most plants have been found to contain also a portion of 

 earths : and as the two substances are so nearly related, and so foreign in their character 

 to vegetable substances in general, the same enquiry has consequently been made with 

 regard to their origin. Whence are the earths derived that have been found to exist in 

 plants ? Chiefly from the soil. But in what peculiar state of combination do they enter 

 the vessels of the plant ? The state most likely to facilitate their absorption is that of their 

 solution in water, in which all the earths hitherto found in plants are known to be in a 

 slight degree soluble. If it be said that the proportion in which they are soluble is so 

 very small that it scarcely deserves to be taken into the account, it is to be recollected that 

 the quantity of water absorbed by the plant is great, while that of the eartli necessary to 

 its health is but little, so that it may easily be acquired in the progress of vegetation. 

 Such is the manner in winch their absorption seems practicable : and Woodward's expe- 

 riments afford a presumption that they are actually absorbed by the root. The proportion 

 of earths contained in the ashes of vegetables depends upon the nature of the soil in which 

 they grow. The ashes of the leaves of the rhododendron ferrugineum, grov^ang on 

 Mount Jura, a calcareous mountain, yielded 43*25 parts of earthy carbonate, and only 

 0*75 of silica. But the ashes of leaves of the same plant, growing on Mount Breven, a 

 granitic mountain, yielded two parts of silica, and only 16*75 of earthy carbonate. It is 

 probable, however, that plants are not indebted merely to the soil for the earthy particles 

 which they may contain. They may acquire them partly from the atmosphere. Margray 

 has shown that rain-water contains silica in the proportion of a grain to a pound ; which, 

 if it should not reach the root, may possibly be absorbed along with the water that adheres 

 to the leaves. But although the earths are thus to be regarded as constituting a small 

 proportion of vegetable food, they are not of themselves sufficient to support the plant, 

 even with the assistance of water. Giobert mixed together lime, alumine, silica, and 

 magnesia, in such proportions as are generally to be met with in fertile soils, and moistened 

 them with water. Several different grains were then sown in this artificial soil, wliich 

 germinated indeed, but did not thrive ; and perished when the nourishment of the cotyle- 

 dons was exhausted. It is plain, therefore, that the earths, though beneficial to the growth 

 of some vegetables, and perhaps necessary to the health of others, are by no means capable 

 of aflTording any considerable degree of nourishment to the plant. 



735. Swpiily of food by manures and culture. With regard to the food of plants derived 

 frojn the atmosphere, the supply is pretty regular, at least, in as far as the gases are con- 

 cerned ; 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 con- 

 tinually 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 atmosphere, 

 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. Tlie supply of food may be increased by alter- 

 ing the mechanical or chemical constitution of soils ; and by the addition of food in the 

 form of manures. Tlie mechanical constitution of soils may be altered by pulverisation, 

 consolidation, draining, and watering ; their chemical properties by aeration and torrifica- 

 tion ; both mechanical and chemical properties, by the addition of earths or other sub- 



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