OSMOSIS I N SOILS. SOILS ACTPAS SEMI-PERMEABLE MEMBRANES.' I~ BY
c. J.
LYNDE,
Ph.D.,!Professor of Physics, Macdonald College, P. Q., Canada
It is usually stated that the water in the soil is subject to three types of movement, namely : gravitational, capillary and thermal. The experiments described below go to show that soil water is subject to a fourth type of movement, namely, an osmotic movement. These experiments show for the first time : (I) that soil acts as a semi-permeable membrane; (2) that water is moved through the soil by osmotic pressure. Before describing the experiments it may be well, for the sake of clearness, to review briefly the epoch-making work of Pfeffer, van't Hoff and Arrhenius.
with membrane Fig. I.-Pfeffer's
apparatus.
Read before the American Society of Agronomy, Lansing,July
II,
1912.
%. J . Lynde
760
Concentration Percent I 2
Osmotic pressure Cm 1
I I
4
53.5 101.6 208.2
1
Pressure Concentration
55.5 50.8 52 . o
pressure are nearly conconcentration stant, which goes t o show that the osmotic pressure varies directly as the concentration of the solution. Pfeffer measured the osmotic pressure of given solutions at different temperatures. The results with' a I percent cane sugar solution were as follows : TABLE2 The figures obtained for
I
Temp. C
6.8' 13.2' 14.2' 22.0'
36.0' Osmotische Untersuchungen, Leipzig, 1877.
Osmotic pressure Cm
50.5 52. I 53.I 54.8 56.7
Osmosis i n Soils
761
These results show that the osmotic pressure of the solution increases with increase of temperature. In 1887 van’t Hoff‘ reviewed Pfeffer’s results, and pointed out the remarkable parallelism between osmotic pressure and gas pressure. He showed from Pfeffer’s results that : I . At a given temperature, the osmotic pressure of a sugar solution is equal to the gas pressure of a gas which has the same number of molecules in a given volume as there are sugar molecules in the same volume. 2. The osmotic pressure and gas pressure change a t the same ra4e for equal changes of temperature, namely, 1/273 of the pressure a t o o C for each change of I O C. This relation has been found to hold for a large number of substances. Van’t Hoff pointed out, moreover, that there are many substances such as acids, bases and salts which give higher osmotic pressures than should be expected from the relations stated above. To explain these exceptions, Arrhenius2 in I 887 brought forward the theory of electrolytic dissociation which had been previously stated by C l a ~ s i u s . ~This theory states that acids, bases and salts in aqueous solution are more or less dissociated into positively and negatively charged particles or ions. Arrhenius assumed that each ion has the s a v e effect as a molecule in producing osmotic pressure, and tqat the increase in the number of particles by dissociation dccounts for the increase in osmotic pressure. Arrhenius further pointed out that the percentage of the molecules dissociated could be determined by measurements of the electrical conductivity, and of the lowering of the freezing point of the solution. Zeit. phys. Chem., a
I, 481 (1887). Ibid., I, 631 (1887). Pogg. Ann., 101, 338 (1857).
762
C. J . Lynde
Experiments In introducing the experiments, I may say that, in studying the movement of moisture in soils, I was gradually led t o the conclusion, that, in producing the movement of moisture in soils, there must be an agency at work more powerful than either surface tension or heat. In considering what this agency might be, I was led t o the following theory: Theory that Soils Aot as Semi-permeable Membranes It is possible that : (I) Soils act as semi-permeable membranes. (2) The greater the depth of the soil the greater its efficiency as a semi-permeable membrane, up t o the point at which i t becomes a perfect semi-permeable membrane. (3) A soil solution moves through the soil by osmotic pressure from points where the solution is less concentrated t o points where it is more concentrated. To test the validity of this theory I made the experiments described below. Modillcation of Pfeffer’s Experiment Object. ( I ) To determine whether a soil acts as a semi-permeable membrane, by making observations on the rate of osmotic flow of water through the soil, if any. ( 2 ) To observe the effect, if any, of change of temperature upon the rate of osmotic flow. The apparatus used is illustrated in Fig. 2 . The tube A was 1.1 cm inside diameter, and 15 cm long. The lower end was closed with one layer of cotton cloth covered with brass wire gauze 40 mesh t o the inch. The upper end of the tube was fitted with a rubber stopper in which was inserted the bent tube B. The bent tube B was approximately 1.5 mm inside diameter. Water, sugar solution and potassium sulphate solution rose in this tube 1 . 3 cm by surface tension. Four tubes were prepared in this way. The soil used was a heavy clay subsoil (a physical analysis of this soil appears in paper 2 below). The air-dried subsoil was allowed t o stand in water for
Osmosis in Soils
763
one week, and was then disintegrated and stcrilized by boiling. The four tubes A were sterilized and filled with the hot mixture of subsoil and water. They were placed a t once in the centrifuge, and the cups of the centrifuge were filled with boiling water to the level of the liquid in the tubes. The centrifuge was then run at top speed for 15 minutes. The middle of each soil column, when settled, was 2 5 cm from the center of the axis of the centrifuge, and the centrifuge made 1300 revolutions per minute.
t--
Fig. 2.-Modification of Pfeffer’s apparatus. Clay subsoil acts as the semi-permeable membrane.
The solutions used were I O percent sugar solution, and percent potassium sulphate solution. The sugar solution was made by dissolving IOO grams of sugar in 1000 cc of solution. The potassium sulphate solution was made by dissolving IOO grams of potassium sulphate in 1000 cc of solution. Both solutions were sterilized by boiling for onehalf hour. As soon as the soils werc settled, the liquid above them was emptied out, and the tubes were filled with hot sugar solution or hot potassium sulphate solution. The tubes were then fitted with the rubber stoppers and bent tub( s B, which had been sterilized. The tubes were then washed on the outside and placed in wide-mouth bottles. The bottles were dilled with distilled water t o a level 2 cm below the lower side IO
,
C. J . Lynde
764
.
of the horizontal tube. The tops of the bottles were covered with paper, fastened around thc neck with rubber bands. A centimeter scale was placed under each horizontal tube to measure the osmotic flow, if any. The tubes were numbered I , 2 , 3, and 4. I and 2 were filled with sugar solution, 3 and 4 with potassium sulphate solution. The results were as follows : Sugar solution Scale readings
Date Started May 29, 1911
May 29, 9.30 A.M. 30, 9 . 3 0 A.M. 31, 1 1 .oo A.M. June I , 9.30 A.M. 2, 9 . 0 0 A.M. 2 , 9.00A.M.
June
5.00 3, 5.40 2,
P.M. P.M.
Tube
I
j
I
K,SO, s o h . Celltimeters I
Tube
2
1 Tube 3 1 Tube i1 4
I
26.0 1 2 6 . 8 23.3 26.2 27.25 2 2 . 2 2 6 . 5 27.65 21.75 2 6 . 8 27.95 1 2 1 . 7 27.1 2 8 . 1 5 , 21.75
28.5 28.7 1 29.25 I 29.8 1 30.35 1
24.5' c 21.5'c
ing 27.52 27.83
21.76 21.63
28.62 29.7
24.2' 24.4O
28. I
22.3
-
-
28.5 28.6 29.4 31.6
24.0 24.3 25.5 27.2
21'
20.3' 18.5'
7.02
4, 4 . 3 0 P.M. 5, - 6, I O . IO .4.M. 6, 5.15 P.M. 7, 4 . 4 5 P . M . 8, 7.15 P.M.
8.05
25' C'
11.55 12.0
24' C 24.5' 24' C
-
14.0 16.3
--
2.5'
c
c
These results are represented graphically in Fig. 3 below.
Conclusions These results indicate : (I) That clay subsoil acts as a semi-permeable membrane. ( 2 ) That water moves through clay subsoil towards a solution. Liquid reached end of horizontal tube; emptied the horizontal tube and started it again a t 5.40 P.M., June 3rd.
Osmosis
in Soils
765
(3) That the rate a t which the water moves increases with the temperature.
39
35
jg
Fig. 3.-Graphs
/
showing the moveLent of the solutions in the tubes B.
It will be noticed that in tube 3 the liquid fell back for t h e first six days, and then moved forward rapidly. It seems probable that in all cases there are two movements: the solution moving out of the tube by gravity, and the water moving towards the solution by osmotic pressure. If the osmotic movement of the water towards the solution is more rapid than the movement of the solution out of the tube, the liquid in the horizontal tube moves forward; if not, it moves back. It would seem that, in the case of tube 3, the movement of the solution out of the tube was the more rapid in the beginning, but that when the temperature was raised the osmotic movement of the water became the more rapid.
