IMPORTANCE AND PROPERTIES OF THE SOIL 169 



Meister 1 found that 1000 grammes of a sandy soil absorbed 304 c.c. of water, while 

 the same weight of a turfy soil took up 1052 c.c., hence containing 45-4 vol. per cent, 

 in one case, and 63-7 in the other. Other soils showed intermediate values between 

 these two extremes. The amount of water which a soil can retain bears no relation 

 to the amount which plants are able to withdraw from it, for when air-dry, different 

 soils retain very varied amounts of water, which the plant is in general unable to 

 absorb to any appreciable extent. A rough generalization of the quantities of water 

 available for the plant's use in different soils when saturated with water is given by 

 Sachs' 2 experiments. In these the amounts of water still present when plants begin 

 to wither, were compared with the quantity necessary to saturate the soil. A young 

 tobacco plant began to wither when the water was reduced to 12-3 per cent., and the 

 soil, when saturated, contained 46 per cent, by weight of water, so that in such 

 a saturated soil 33-7 per cent, of water was available for use. In a clay soil this 

 amount was 52-1 8 = 44-1 per cent., in coarse quartz sand 20-8 1-5 = 19-3 

 per cent., according to corresponding experiments performed with similar tobacco 

 plants, the amounts 12-3, 8, and 1-5 per cent, being the approximate percentages of 

 water held in the air-dry condition by the soils under examination. These results 

 also show that a plant cannot reduce the amount of water to so low a percentage 

 in a soil rich in humus as in a sandy one, but nevertheless when the soils are 

 saturated, more water is available for the plant's use in the former, owing to its 

 power of retaining water being very much more marked than it is in a sandy soil. 



Empirical results show that the soil is unable to supply the plant with water by 

 the condensation of water vapour (cf. Sect. 27), and therefore plants slowly die if the 

 leaves are freely exposed while the roots and soil are kept in saturated air. The 

 stunted growth which Sachs 3 observed during his experiments under such conditions 

 was probably rendered possible by the formation of dew upon the soil and roots. 



Absorption by the soil. The power of soils to retain dissolved substances was 

 originally discovered by Gazzeri. Amplified and extended observations were then 

 made by Th. Way, and at a later date Liebig called attention to the importance of 

 this property in the economy of nature 4 . Since then various researches have been 

 performed to determine the actual processes and causes of absorption 6 . 



The question as to the mode of absorption can only be touched upon. Liebig 

 and others were inclined to regard the process as a physical one, whereas Rauten- 

 berg, A. Beyer and others supposed it to be chemical in nature. Discussion on 

 this point is largely purposeless, since the boundary between chemistry and physics 

 is an arbitrary one, and moreover varies according to the standpoint from which the 

 subject under discussion is regarded. Chemical processes do certainly come into 

 play, when, for example, potassium chloride is added to a soil, and the potassium is 

 retained while calcium chloride passes away in solution. Other substances, however, 



1 Meister, Jahresb. d. Agriculturchemie, 1859-60, p. 40. Cf. Sachsse, 1. c., p. 199. 



2 Sachs, Versuchsst., 1859, Bd. I, p. 234. 



3 Sachs, 1. c., p. 236. Cf. A. Mayer, 1. c., Bd. n, p. 131 ; Sachsse, 1. c., p. 209. 



4 See Versuchsst., 1873, Bd. xvi, p. 56; Way, Jour, of the Royal Agric. Soc., 1850, vol. xi, 

 p. 313, and vol. xv, p. 91 ; Liebig, Ann. d. Chem. u. Pharm., 1858, Bd. CV, p. 109. 



5 Details in the quoted works of A. Mayer, Sachsse, &c. See also van Bemmelen, Versuchsst., 

 1879, Bd. xxin, p. 265 ; 1888, Bd. xxxv, p. 69 ; and Die Absorption, 1897. 



