Book III. USES OF SOIL TO VEGETABLES. 315 



2091 . The presence of organised matter in any soil may be ascertained very satisfactorily 

 by weighing it after being thoroughly dried j then subjecting it to a red heat, and weigh- 

 ing it again, the weight last found will be the proportion of organic matter. The same 

 object may also be attained by ascertaining the specific gravity of the soil, but with less 

 accuracy. 



2092. T'he presence of metallic oxides in a soil may generally be known by their color. 

 Ferrugineous soils are red or yellow ; cupreous soils, interspersed with greenish 

 streaks, &c. 



2093. Tlie presence of salt, sulphur, coal, &c. may be known by the absence or 

 peculiarity of vegetation, as well as by color, and the appearance of the water of such 

 soils. 



2094. The cajmcity of a soil for retaining water may be thus ascertained. An equal 

 portion of two soils, perfectly dry, may be introduced into two tall glass cylindrical ves- 

 sels {fig. 247.), in the middle of each of which a glass tube is 

 previously placed. The soils should be put into each in the ^^^ 

 same manner, not compressed very hard, but so as to receive a 

 solidity approaching to that which they possessed when first ob- 

 tained for trial. If, after this preparation, a quantity of water 

 be poured into the glass tubes, it will subside; and the capillary 

 attraction of the soils will conduct it up the cylinders towards 

 the tops of the vessels. That which conducts it most rapidly, provided it does not rise 

 from the weight of the incumbent column of water in the tube, may be pronounced to 

 be the better soil. ( Grisenthwaite. ) 



Sect. IV. Of the Uses of the Soil to Vegetables. 



2095. Soils afford to plants a fixed abode and medium of nourishment. Earths, exclu- 

 sively of organised matter and water, are allowed by most physiologists to be of no other 

 use to plants than that of supporting them, or furnishing a medium by which they may 

 fix themselves to the globe. But earths and organic matter, that is, soils, afibrd at once 

 support and food. 



2096. The pure earths merely act as Tnechanical and indirect chemical agents in the soil. 

 The earths consist of metals united to oxygen, and these metals have not been decom- 

 posed ; there is consequently no reason to suppose that the earths are convertible into the 

 elements of organised compounds, that is, into carbon, hydrogen, and azote. Plants 

 have been made to grow in given quantities of earth. They consume very small por- 

 tions only ; and what is lost may be accounted for by the quantities found in their ashes ; 

 that is to say, it has not been converted into any new products. The carbonic acid 

 united to lime or magnesia, if any stronger acid happens to be formed in the soil during 

 the fermentation of vegetable matter, which will disengage it from the earths, may be 

 decomposed ; but the earths themselves cannot be supposed convertible into other sub- 

 stances, by any process taking place in the soil. In all cases the ashes of plants contain 

 some of the earths of the soil in which they grow ; but these earths, as has been ascer- 

 tained from the ashes afforded by different plants, never equal more than one fiftieth of 

 the weight of the plant consumed. If they be considered as necessary to the vegetable, 

 it is as giving hardness and firmness to its organisation. Thus, it has been mentioned 

 that wheat, oats, and many of the hollow-stalked grasses, have an epidermis principally 

 of siliceous earth ; the use of which seems to be to strengthen tliem, and defend them 

 from the attacks of insects and parasitical plants. 



2097. The true nourishment of plants is xvater, and decomposing organic matter; 

 both these exist only in soils, not in pure earths ; but the earthy parts of the soils are 

 useful in retaining water, so as to supply it in the proper proportions to the roots of 

 the vegetables, and they are likewise eflicacious in producing the proper distribution of 

 the animal or vegetable matter. "When equally mixed with it they prevent it from 

 decomposing too rapidly ; and by their means the soluble parts are supplied in proper 

 proportions. 



2098. The soil is necessary to the existence of plants, both as affording them nourishment, 

 and enabling them to fix themselves in such a manner as to obey those laws by which 

 their radicles are kept below the surface, and their leaves exposed to the free atmosphere. 

 As the system of roots, branches, and leaves, are very different in different vegetables, so 

 they florish most in different soils ; the plants that have bulbous roots require a looser 

 and a lighter soil than such as have fibrous roots ; and the plants possessing only short 

 fibrous radicles demand a firmer soil than such as have tap-roots or extensive lateral 

 roots. 



2099. Tlie constituent parts of the soil which give tenacity and coherence are the finely 

 divided matters ; and thfey possess the power of giving those qualities in the highest 

 degree when they contain much alumina. A small quantity of finely divided matter is 

 sufficient to fit a soil for the production of turnips and barley j and a tolerable crop of 



