Book II. USES OF SOIL TO VEGETABLES. 223 



1045. The presence of clay and sand in any soil is known, the first by its tenacity } the 

 other by its roughness to the touch, and by scratching glass when rubbed onjrt. 



1046. The presence of calcareous matter in soil may be ascertained by simply pouring 

 any acid on it, and observing if it effervesces freely. Calcareous soils are also softer to 

 the touch than any other. 



1047. The presence of organised matter in any soil may be ascertained very satisfactorily 

 by weighing it after being thoroughly dried ; 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. 



1048. The 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. 



1049. The presence of salts, 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. 



1050. The capacity of a soil for retaining U'ater may be thus ascertained. An equal portion 

 of two soils, perfectly dry, may be introduced into two tall glass cylindrical vessels, 

 (fig. 74.) in the middle of each of which a glass tube is pre- 

 viously placed. The soils should be put into each in the same 

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

 lidity 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. 



1051. Soils afford to jylants a fixed abode and medium of nouris/nnent. 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, afford at once 

 support and food. 



1052. The pure earths merely act as meclianical and indirect chemical agents in the soil. 

 The earths consist of metals united to oxygen, and these metals have not been decomposed ; 

 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 portions 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 mag- 

 nesia, 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 substances, 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 ascertained 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 them, and defend them from the attacks of insects and parasitical 

 plants. 



1053. Tlie true nourishment of plants is water, 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 efficacious 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. 



1054. 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 systems of roots, branches, and leaves, are very different in different vegetables, so 

 they flourish 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 



