IMPORTANCE AND PROPERTIES OF THE SOIL 163 



of the vapour-tension is not merely nullified, but undergoes a rise of about 0-24 mm. 

 Hence, this trifling difference of temperature will suffice to cause a i per cent, 

 solution of KNO 3 to give off water-vapour in a saturated atmosphere. For the 

 same reason a very slight warming is sufficient to render transpiration possible 

 in saturated air, while a trifling difference of temperature will cause a formation of 

 dew upon the colder body. Movements of the air will be of material assistance 

 in accelerating such accumulative processes. 



SECTION 28. importance and Properties of the Soil. 



Substances are taken in by the plant in essentially the same manner 

 whether the absorptive organs are immersed in water or in moist soil, 

 for only water and dissolved substances are capable of absorption, and 

 in all cases the quantitative selective power is regulated by the diosmotic 

 properties of the plant and the character of its metabolism. The poverty 

 of the vegetation of sandy soils is an indication that an admixture with 

 organic material, as in loam or humus, is of marked advantage for the 

 growth and nourishment even of those plants which may be cultivated 

 in sand or water, and which do not extract any organic nutriment from 

 the soil, but only water and inorganic salts. 



The properties of humus soils are of the utmost biological importance, 

 but it is the task of text-books on agriculture to describe them fully l . 

 We are at present concerned only with the importance of the soil as the 

 source of inorganic nutriment, and the question whether certain plants 

 may obtain a portion or the whole of their organic nutriment from 

 humus will be discussed later (Sects. 64 and 67). 



The soil is of use and advantage to the plant not only as the source 

 from which its necessary nutriment is drawn, but also because (i) it 

 enables the roots to obtain a firm hold and thus to support the erect 

 aerial organs (cf. Sect. 26) ; (2) all the organs buried in the soil come 

 into contact with air, as well as water and the substances dissolved in 

 the latter ; (3) useful nutritive materials, obtained from the rain or by 

 manuring, or rendered soluble by the weathering of the soil, are retained 

 owing to the absorptive power of the latter, and are presented to the 

 plant in the most suitable form, namely in an exceedingly dilute solution. 



The absorptive power of the soil has also a marked effect upon the 

 form in which the nutritive materials are presented to the plant, for by 

 this means particular salts may be decomposed, retained, or removed in 

 some way or other, and thus many injurious effects prevented, including 

 those due to over-concentration. When plants are grown in water-cultures, 



1 A. Meyer, Lehrb. d. Agriculturchemie, 1895, Bd. ll; R. Sachsse, ibid., 1888; Detmer, Die 

 naturwiss. Grundlagen d. landw. Bodenkunde, 1876, &c. 



M 2 



