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The effect of the physical state of the surface of organic soil material on its wettability
Forest Ecology and Management, 11 (1985) 245--256 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
245
THE E F F E C T OF THE P H Y S I C A L STATE OF THE S U R F A C E OF O R G A N I C SOIL M A T E R I A L ON ITS W E T T A B I L I T Y
A. GRELEWICZ and W. PLICHTA Institute of Biology, Nicolaus Copernicus University, Sienkiewicza 30/32, Toruf~ (Poland)
(Accepted 30 January 1985)
ABSTRACT
Grelewicz, A. and Plichta, W., 1985. The effect of the physical state of the surface of organic soil material on its wettability. For. Ecol. Manage., 11 : 245--256. The water repellency of different forms and types of forest floors was studied, using two methods: (1) measurement of the contact angle 8, and (2) measurement of the rate of the initial phase of water absorption in an Enslin device. Measurements were made on samples with their natural degree of disintegration and roughness, and on the same samples pressed into tablets to give a smooth contact interface between the water and the organic material. The contact angle 8, of organic material from an epihumus subhorizon AoH, with a rough surface, is about 145 °, whereas for the same material with a smooth contact interface it is about 60° . Pressed samples exhibited much higher rates of water absorption than samples of the same materials in their natural state. The present studies demonstrate that, because of its chemical nature, organic material is hydrophilic. Its water repellency is closely related to the degree of dispersion and roughness of its structural elements. A theoretical explanation of the effect of the surface state of organic material on its wettability is attempted.
INTRODUCTION D r y , organic soil material exhibits t h e c h a r a c t e r i s t i c s o f a w a t e r r e p e l l e n t b o d y and, c o n s e q u e n t l y , its w a t e r a b s o r p t i o n is p o o r (Wittich, 1 9 5 4 ; Scholl, 1 9 7 1 ; Singer and Ugolini, 1 9 7 6 ; P r u s i n k i e w i c z et al., 1 9 8 1 ; G r e l e w i c z and Plichta, 1983). Similar characteristics have been f o u n d in mineral soil materials c o n t a i n i n g h u m u s (Van 't W o u d t , 1 9 5 9 ; Scholl, 1 9 7 1 ; Singer and Ugolini, 1 9 7 6 ; McGhie a n d Posner, 1 9 8 1 ; Ellies, 1 9 8 3 ) . Water r e p e l l e n c y is c o m m o n l y f o u n d in softs o f b u r n t - o v e r areas ( K r a m m e s a n d D e B a n o , 1 9 6 5 ; Savage et al., 1 9 6 9 ; D e B a n o et al., 1 9 7 0 ; Savage et al., 1 9 7 2 ; Savage, 1 9 7 4 ; Ellies, 1983). Several papers have dealt with f a c t o r s responsible f o r this p h e n o m e n o n and w i t h the processes in h e a t e d soil. Savage et al. ( 1 9 6 9 ) claimed t h a t w a t e r r e p e l l e n c y can be caused b y intensive d e v e l o p m e n t o f m y c e l i u m a f t e r soil is h e a t e d . D e B a n o et al. ( 1 9 7 0 ) t h o u g h t t h a t w a t e r repellency is d u e t o volatile substances, evolved while t h e soil is being h e a t e d ,
AoL
AoL AoF
AoL AoF AoH
Typical mull
Hygromoder
Typical m o t
a A f t e r T h u n et al., 1955.
Subhorizon
T y p e o f forest floor
1.5 3.0 10.0
2.5 1.5
3.0
Thickness (cm)
96.0 90.4 84.8
94.3 88.2
87.2
Ignition losses (%)
Humificat i o n degree cf. S p r i n g e r a 25 22 27 16 29 55
C/N
19.9 23.2 20.1 38.0 27.4 37.6
S o m e c h e m i c a l and physical p r o p e r t i e s o f t h e samples
TABLE I
58.8 67.4 32.2 44.4 16.9 8.1
2.7 3.7 3.5 3.1 2.4
>2.5
37.3 30.0 9.8
20.2 28.6
26.1
2.5--1.0
15.5 31.9 24.3
7.7 22.1
10.6
1.0--0.5
1.7 12.0 34.0
2.8 12.5
3.1
0.5--0.2
Particle size o f organic debris (%)
3.9
pH KCI
1.1 9.1 22.9
1.0 3.7
0.9
<0.2 mm
O~
247 condensing on soil particles. In a later work, Savage et al. (1972) concluded that the hydrophobic nature of soil is due to hydrocarbon-like aliphatic substances. Most authors in the pedological literature seem convinced of the chemical nature of water repellency in organic soil materials (DeBano et al., 1970; Farmer, 1978; Ellies, 1983). However, little research has been done to relate this phenomenon to the physical characteristics of organic soil material, such as its state of disintegration, porosity and surface roughness. These characteristics of solids affect wettability to a high degree (Moillet and Collie, 1951; Adamson, 1960; Gregg, 1961; Bond and H a m m o n d , 1970; Fink, 1970; Philip, 1971). The present study was undertaken to find o u t if, and to what extent, the surface o f air-dry organic soil material affects its wettability. MATERIALS AND METHODS The experiments used three main types of humus from forest floors, which had developed under different conditions of biocenosis and habitat: (1)typical m o t from an a u t o m o r p h i c podzol (Spodosol, cf. Soil Survey Staff, 1975) in the forest association E m p e t r o nigri--Pinetum, {2)hygromoder from muck soil with shallow ground water table (Mollisol, Soil Survey Staff, 1975) in the forest association Tilio--Carpinetum stachyetosum, (3)typical mull from automorphic rusty brown soil {Inceptisol, cf. Soil Survey Staff, 1975) in t h e forest association Tilio-Carpinetum t y p i c u m . Some of the chemical and physical properties of the samples are presented in Table 1. The basic method used in most studies on surface wettability is c o n t a c t angle 0 measurement, the assesment of which, for polydispersive organic soil material, is extremely difficult. All the methods of contact angle 0 measurement mentioned in Adamson (1960) or proposed by Letey et al. (1962) and Yuan and H a m m o n d (1969) for mineral soils are difficult to apply for organic samples. The methods used to calculate the value of the contact angle in porous mineral soil materials are of no avail in dealing with organic soil material because of the varying dimensions of plant remains and soil pores, which are affected by water content. Krammes and DeBano (1965) p r o p o s e d a method in which the time taken for a water drop to penetrate is used as a wettability index. This m e t h o d is difficult to apply to forest floor samples for the following reasons: (i) the water takes a long time to penetrate the highly humified material, and during this time part of the drop evaporates, (ii) it is difficult to place a water drop on only partially d e c o m p o s e d remains, e.g. from litter subhorizon AoL, (iii) the organic material rises to the surface o f the drop. To avoid these difficulties, the following two methods were used: (1) measurement of the contact angle 0 ; this was used only for the relatively
248 homogenous material from the epihumus subhorizon AoH, (2) the Enslin methods; this was used on all samples to measure the rate of the initial phase of water absorption by various forms of humus. This m e t h o d can be regarded as analogous to the water-drop penetration-time method. To determine the effect of the surface state of the material on its wettability and water absorption rate, measurements were performed on samples in their natural state, i.e. with a rough surface, and on samples pressed into tablets to provide a smooth surface. The tablets were made with a hand press giving about 2000 kg cm -2 pressure. The measurement of the rate of the initial phase o f water absorption in an Enslin device (Fig. 1) was made for samples pressed into tablets and also in their natural (loose) state. The latter were placed in a metal cylinder 12 mm in diameter. Each sample weighed 0.25 g. The measurements were made at 20°C and there were three replicates. To illustrate the character of the surface of the structural elements of the organic material showing various degrees of decomposition and humification, photographs were taken by means of a scanning electron microscope. A photograph was also taken o f the surface of material from the epihumus subhorizon AoH after pressing.
2 ~--~ ~
,
, - , -
- , - , - , - ~ ,
,
,
i
Fig. 1. Enslin device: 1, sample; 2, porous glass plate; 3, calibrated capillary; 4, threeposition tap ; 5, water. RESULTS The difference in value of the contact angle, 0, for the material from the epihumus subhorizon AoH in both its loose and its pressed f o r m are illustrated in Fig. 2 a,b. For loose material, angle 0 is a b o u t 145 °, whereas for the same material after pressing it is a b o u t 60 ° . The same material shows different degrees of affinity to water, depending on the physical state of its surface, which is illustrated in Figs. 3 and 4. The scanning electron micrograph, Fig. 3, shows that the surface of a particle of strongly decomposed a m o r p h o u s humus from the epihumus subhorizon AoH has numerous irregularities, but after pressing it has a relatively s m o o t h surface (Fig. 4). The physical state o f the surface of the organic matter also has a significant effect on the rate o f water absorption in the Enslin device (Fig. 5). The rate of the initial phase of water absorption in samples in the loose form
249
Fig. 2. A drop of water on the surface of material from epihumus subhorizon AoH: (A) in natural -- loose -- form; (B) after pressing. and in those pressed into tablets was f o u n d to differ considerably irrespective of the humus form. T he tablets absorbed in 10 minutes an a m o u n t of water near to saturation unde r the e x p e r i m e n t a l conditions, whereas in the loose material water absorption was much slower. Measurements over 3 days of the rate o f wat er a b s o r p t i o n by samples of different forms of humus have d e m o n s t r a t e d t h a t loose material will eventually b eco me we t t ed (Fig. 6). Moreover, these m easurem ent s have demonstrated significant differences in absorption rate bet w een di fferent forms of humus, depending on their degree of d e c o m p o s i t i o n and humification. The smaller the particle size and t he greater the h u m i f i c a t i o n o f the material in the particular humus forms (Table 1) the slower t he absorption rate. This is particularly evident in the various forms o f the m o r t y p e . In this case the differences in water absorption rate are due to t he character of t he surface o f the tissues making up the organic matter. T h e plant remains from the litter s u b h o r izo n A o L o f t he m o t t ype in t h e initial stage of d e c o m p o s i t i o n are distinguished by their relatively s m o o t h surface (Fig. 7) com pared with the very rough surface of the plant remains f r o m the epihumus subhorizon AoH.
250
Fig. 3. Rough surface of organic remains, of particle size <0.1 mm, from epihumus subhorizon AoH. DISCUSSION AND CONCLUSIONS The wettability of organic soil materials is generally determined by their amphiphilic characteristics. This property depends on the ratio of polar sites to non-polar aliphatic structures occurring in the organic substances (Farmer, 1978). Most authors assume that the main cause of water repeUency in organic soil matter is associated with its chemical nature. According to Farmer (1978), in the process of drying of organic soil matter there m a y be interaction between t h e polar groups. This leads to the formation of hydrogen-type bonds which block the renewed hydratation of these groups. This process is largely irreversible. However, it must be pointed o u t that if water repellency depended solely on the chemical structure of the material, there would be an increase in wettability as the degree of humification increased, since during humification the number of carboxylic groups, which play the main role in hydration, is greatly increased (Broadbent, 1954). However, the reverse p h e n o m e n o n is observed, i.e. there is an increase in water repellency. Without denying the importance of the chemical structure of the humus substances, this factor cannot be regarded as the only one
251
Fig. 4. Comparatively s m o o t h surface o f s a m p l e f r o m e p i h u m u s s u b h o r i z o n A o H after
pressing. o-typical
mor
b - hygromoder
c - typical muir
3O
3O AoL
"25
25
2.0
~
"
20
15
'~0
AoF _
1 2 3 ~ 5 6 7 8 9 ~ 0
2
3
L,
5
[ 6
7
B
9
I0
I
?
3
~
5
6
7
8
9
~0
Time [min] Fig. 5. The process o f water a b s o r p t i o n in an Enslin device in samples o f various h u m u s forms: • • , natural ( l o o s e ) form; c o, pressed form.
252
o- typicat
b-'hfgmrnoder
mot
c- typ~at muir 3.5
3"5t t0
-
30
25
25
,.o
15
1.0
oH
05
i
1 i
2 i
I i
2 i
0
Time [ day ] Fig. 6. The process of water absorption in an Enslin device in various humus samples in a natural (loose) form during three days.
Fig. 7. Fragment o f comparatively s m o o t h surface of remains of a pine needle from litter subhorizon AoL in the initial stage of decomposition.
253
of importance. In the pedological literature, the interaction between water and a solid phase with a rough surface, is shown to affect the wetting process significantly (Moillet and Collie, 1951; Adamson, 1960; Bond and Hammond, 1970). Likewise, Fink (1970) considered that porosity is responsible for the large contact angles, reaching about 150 °, in a mineral material covered with silicon resin. The m a x i m u m angle for water on a s m o o t h glass surface covered with the same resin is 100 °. Bond and H a m m o n d (1970) claimed that the water repellency of sand is due to both porosity and pore shape. The present studies have shown clearly that the wettability of soil organic material is largely dependent on the state of its surface. The pressed and the loose material both had the same chemical structure, but the pressed material with a smooth surface was easily wettable, while the loose material with its discontinuous surface was water repellent. It can be demonstrated on theoretical grounds that the difference in values between the contact angle 0 measured for the same material in different forms depends on the physical state of its surface. Of vital importance in determining the high value o f the wetting angle 0 of unpressed samples, with a very rough and porous surface, is the effect of drop support. This p h e n o m e n o n is similar to the so-called 'effect of reticulum' (Adamson, 1960). A drop of water placed on the surface of rough and porous material comes into contact with the solid phase only at those points f o r m e d by the peaks of the rough surface. The apparently flat contact surface of the drop Sa, seen with the naked eye, is made up of interphase surfaces Ssi(n), water-solid phase, and Svl(n) water--air phase (Fig. 8). The discontinuity of contact between water and material and, in particular, the discontinuity of this contact at the edge of the drop (L) is responsible for the considerable increase in the value of the measured contact angle 0, which in such a case is called the apparent contact angle 0ab (Moillet and Collie, 1951; Adamson, 1960). Its value, even for materials which, because of their chemical nature, are easily wettable, greatly exceeds the value of the real contact angle, 6 r, measured on a truly smooth surface of the same material. Figure 9 illustrates in a simplified way the distribution of the curvature angles of a drop placed on a porous surface. At the points of c o n t a c t water-material Ssl(n) the contact angle 0 a which the water forms with the observation plane x--x' is the real contact angle 0 r of this material. Between the surfaces Ssl(n ) at the contact edge (L) there are the surfaces water--air Svl(n)devoid of contact with the material, where the curvature of the water surface nears 180 °. The apparent contact angle 0ab seen relative to the observation plane x--x' is the geometric sum of angles 0 a and 0 b dependent on the ratios of surface Ssl and Svl to the apparent contact surface Sa. The formula thus obtained for the value of the apparent contact angle 0ab at 0 b = 180° is identical with the formula of Cassie and Baxter (cf. Moillet and Collie, 1951) and has the form:
254 /2
COS0ab =
/2
Y~ Svl(n )
cOS0r-
Sa by substituting
• Svl(n)
(I)
Sa
fsl and fvl f o r , r e s p e c t i v e l y ,
12
t~
E Svl(n)
~ Svl(n)
and
Sa
Sa
we o b t a i n
(2)
COS 0 ab = fsl COS 0 r - fvl / L
/
Sslln+l)-~
Sa
Svlln+'l}~@
%-'\ _esb.__.¢./J..,,
,,,- X , 4f ,
"b,~~
\ \\ \
//
%.
\
\
'\ \
Fig. 8. An illustration of the interphase contact surface water--solid phase in the case of rough surface: Sa, apparently flat interface; Sap interface of real contact water--solid phase; Svl, water--air interface; L, outer circumference of drop contact. Fig. 9. Curvature angles of water drop on rough surface: ea = Sz, real wetting angle at the contact point water--solid phase (Sal); eb, angle of drop curvature between points of support (Svl);#ab,the observed resultant angle of drop curvature. In a c c o r d a n c e w i t h eqns. (1) and ( 2 ) f o r p r e s s e d s a m p l e s o f o r g a n i c m a t e rial w i t h a s m o o t h c o n t a c t s u r f a c e w a t e r - - m a t e r i a l (fsl = 1, fvl = 0, w e o b t a i n COS 0ab = COS 0 r
(3)
T h e m e a s u r e d angle 0 ab in this case is e q u a l to t h e real c o n t a c t a n g l e 0 r. I f t h e r e are b u t f e w s u p p o r t i n g p o i n t s o f t h e w a t e r d r o p o n a v e r y p o r o u s surface (fsl ~ 0, fvl ~ 1), t h e n cos 0 ab ~ - 1 T h e v a l u e o f t h e a p p a r e n t c o n t a c t angle 0ab o n a v e r y p o r o u s s u r f a c e can t h e r e f o r e r e a c h a value n e a r i n g 180 °. A s s u m i n g t h a t a w a t e r d r o p is in real c o n t a c t w i t h t h e p o r o u s s u r f a c e o f t h e o r g a n i c m a t e r i a l o v e r o n l y 10% o f t h e a p p a r e n t c o n t a c t s u r f a c e Sa (fsl = 0.1, fsv = 0.9) w e o b t a i n f r o m e q n . (2) t h e value o f t h e a p p a r e n t c o n t a c t angle
255 0ab = 145 °. The value o b t a i n e d o n this a s s u m p t i o n is similar to t h a t o f the measured w e t t i n g angle o f t h e organic m a t t e r f r o m s u b h o r i z o n A o H . The results o b t a i n e d in t h e Enslin device have c o n f i r m e d t h e e f f e c t o f the surface state o f the organic material o n its w e t t a b i l i t y . Pressed samples with a s m o o t h c o n t a c t surface with w a t e r e x h i b i t g o o d w e t t a b i l i t y a n d b e c o m e w e t t e d within a short time. T h e same material in l o o s e f o r m also b e c o m e s wet, b u t the time n e e d e d for this to o c c u r is m a n y t i m e s longer. This provides clear evidence t h a t t h e main f a c t o r d e t e r m i n i n g b o t h t h e w e t t a b i l i t y o f organic soil materials a n d the w e t t i n g p r o c e s s is t h e p h y s i c a l state o f their surface. This does n o t m e a n t h a t t h e c h e m i c a l p r o p e r t i e s o f the organic material do n o t affect its w e t t a b i l i t y . F r o m the present s t u d y it f o l l o w s t h a t : (1) o r g a n i c soil material b y its chemical n a t u r e is h y d r o p h i l i c ; and (2) w a t e r r e p e l l e n c y o f d r y o r g a n i c soil material is due to the physical state o f its surface, i.e. to t h e degree o f dispersion and t h e r o u g h n e s s o f its m o r p h o l o g i c a l e l e m e n t s .
REFERENCES Adamson, A.W., 1960. Physical Chemistry of Surfaces. Interscience Publishers, New York, NY, 629 pp. Bond, R.D. and Hammond, L.C., 1970. Effect of surface roughness and pore shape on water repellency on sandy soils. Proc. Soil and Crop Sci. Soc. of Florida, 30: 308-315. Broadbent, F.E., 1954. Modification in chemical properties of straw during decomposition. Soil Sci. Soc. Am. Proc., 18: 165--169. DeBano, L.F., Mann, L.D. and Hamilton, P.A., 1970. Translocation of hydrophobic substances into soil burning organic litter. Soil Sci. Soc. Am. Proc., 34: 130--133. Ellies, A., 1983. Die Wirkungen yon Bodenerhitzungen auf die Benetzungseigenschaften einiger Boeden Suedchiles. Geoderma, 29, 2: 129--138. Farmer, V.C., 1978. Water on particle surfaces. In: D.J. Greenland and M.H.B. Hayes (Editors), The Chemistry of Soil Constituents. Wiley and Sons, Chichester, New York, Brisbane, Toronto, pp. 405--448. Fink, D.H., 1970. Water repellency and infiltration resistance of organic-film-coated soils. Soil Sci. Soc. Am. Proc. 34: 189--194. Gregg, S.J., 1961. The Surface Chemistry of Solids. Chapman and Hall, London, 393 pp. Grelewicz, A. and Plichta, W., 1983. Water absorption of xeromor forest floor samples. For. Ecol. Manage., 6: 1--12. Krammes, J.S. and DeBano, L.F., 1965. Soil wettability: a neglected factor in watershed management. Water Resour. Res., 1, 2: 283--286. Letey, J., Osborn, J. and Pelishek, R.E., 1962. Measurement of liquid--solid contact angles in soil and sand. Soil Sci., 93, 3 : 149--153. McGhie, D.A. and Posner, A.M., 1981. The effect of plant top material on the water repellency of fired sands and water repellent soils. Aust. J. Agric. Res., 32: 609--620. Moiilet, J. and Collie, B., 1951. Surface Activity. Van Nostrand, New York, NY, 379 pp. Philip, J.R., 1971. Limitations on scaling by contact angle. Soil Sci. Soc. Am. Proc., 35: 507--509. Prusinkiewicz, Z., Bednarek, R., Deg6rski, M. and Grelewicz, A., 1981. The water regime of sandy soils in dry pine forest (Cladonio--Pinetum) in the northern part of the glacial outwash plains of the Brda and Wda rivers. Ekol. Pol., 29, 2: 283--309.
256 Savage, S.M., 1974. Mechanism of fire-induced water repellency in soil.Soil Sci. Soc. A m . Proc., 33: 149--151. Savage, S.M., Martin, J.P. and Letey, J., 1969. Contribution of some soil fungi to natural and heat induced water repellency in sand. Soil Sci. Soc. A m . Proc., 33: 405--409. Savage, S.M., Osborn, J., Letey, J. and Heaton, C., 1972. Substances contributing to fireinduced water repellency in soil.Soil Sci. Soc. A m . Proc., 36: 674--678. Scholl, D.G., 1971. Soil wettability in Utah juniper stands. Soil Sci. Soc. A m . Proc., 35: 344--345. Singer, M.J. and Ugolini, F.C., 1976. H y d r o p h o b i c i t y in the soils of Findley Lake, Washington. For. Sci., 22, 1: 54--58. Soil Survey Staff, 1975. Soil t a x o n o m y : a basic system o f soil classification for making and interpreting soil surveys. Agric. Handbk. No 436, USDA, U.S. Govt. Printing Office, Washington, DC, 754 pp. Thun, R., Hermann, R. and Knickmann, E., 1955. Methodenbuch. Bd. 1, Die Untersuehung yon Boeden. Neuman, Radebeul and Berlin, 271 pp. Wittich, W., 1954. Die melioration streugenutzter Boeden. Forstwiss. Centralbl., 73: 211--232. Van 't Woudt, B.D., 1959. Particle coatings affecting the wettability of soils. J. Geophys. Res., 64: 263--267. Yuan, T.L. and Hammond, L.C., 1969. Evaluation of available methods for soil wettability measurement with particular reference to soil--water contact angle determination. Proc. Soil Crop Sci. Soc. Florida, 28: 56--63.
245
THE E F F E C T OF THE P H Y S I C A L STATE OF THE S U R F A C E OF O R G A N I C SOIL M A T E R I A L ON ITS W E T T A B I L I T Y
A. GRELEWICZ and W. PLICHTA Institute of Biology, Nicolaus Copernicus University, Sienkiewicza 30/32, Toruf~ (Poland)
(Accepted 30 January 1985)
ABSTRACT
Grelewicz, A. and Plichta, W., 1985. The effect of the physical state of the surface of organic soil material on its wettability. For. Ecol. Manage., 11 : 245--256. The water repellency of different forms and types of forest floors was studied, using two methods: (1) measurement of the contact angle 8, and (2) measurement of the rate of the initial phase of water absorption in an Enslin device. Measurements were made on samples with their natural degree of disintegration and roughness, and on the same samples pressed into tablets to give a smooth contact interface between the water and the organic material. The contact angle 8, of organic material from an epihumus subhorizon AoH, with a rough surface, is about 145 °, whereas for the same material with a smooth contact interface it is about 60° . Pressed samples exhibited much higher rates of water absorption than samples of the same materials in their natural state. The present studies demonstrate that, because of its chemical nature, organic material is hydrophilic. Its water repellency is closely related to the degree of dispersion and roughness of its structural elements. A theoretical explanation of the effect of the surface state of organic material on its wettability is attempted.
INTRODUCTION D r y , organic soil material exhibits t h e c h a r a c t e r i s t i c s o f a w a t e r r e p e l l e n t b o d y and, c o n s e q u e n t l y , its w a t e r a b s o r p t i o n is p o o r (Wittich, 1 9 5 4 ; Scholl, 1 9 7 1 ; Singer and Ugolini, 1 9 7 6 ; P r u s i n k i e w i c z et al., 1 9 8 1 ; G r e l e w i c z and Plichta, 1983). Similar characteristics have been f o u n d in mineral soil materials c o n t a i n i n g h u m u s (Van 't W o u d t , 1 9 5 9 ; Scholl, 1 9 7 1 ; Singer and Ugolini, 1 9 7 6 ; McGhie a n d Posner, 1 9 8 1 ; Ellies, 1 9 8 3 ) . Water r e p e l l e n c y is c o m m o n l y f o u n d in softs o f b u r n t - o v e r areas ( K r a m m e s a n d D e B a n o , 1 9 6 5 ; Savage et al., 1 9 6 9 ; D e B a n o et al., 1 9 7 0 ; Savage et al., 1 9 7 2 ; Savage, 1 9 7 4 ; Ellies, 1983). Several papers have dealt with f a c t o r s responsible f o r this p h e n o m e n o n and w i t h the processes in h e a t e d soil. Savage et al. ( 1 9 6 9 ) claimed t h a t w a t e r r e p e l l e n c y can be caused b y intensive d e v e l o p m e n t o f m y c e l i u m a f t e r soil is h e a t e d . D e B a n o et al. ( 1 9 7 0 ) t h o u g h t t h a t w a t e r repellency is d u e t o volatile substances, evolved while t h e soil is being h e a t e d ,
AoL
AoL AoF
AoL AoF AoH
Typical mull
Hygromoder
Typical m o t
a A f t e r T h u n et al., 1955.
Subhorizon
T y p e o f forest floor
1.5 3.0 10.0
2.5 1.5
3.0
Thickness (cm)
96.0 90.4 84.8
94.3 88.2
87.2
Ignition losses (%)
Humificat i o n degree cf. S p r i n g e r a 25 22 27 16 29 55
C/N
19.9 23.2 20.1 38.0 27.4 37.6
S o m e c h e m i c a l and physical p r o p e r t i e s o f t h e samples
TABLE I
58.8 67.4 32.2 44.4 16.9 8.1
2.7 3.7 3.5 3.1 2.4
>2.5
37.3 30.0 9.8
20.2 28.6
26.1
2.5--1.0
15.5 31.9 24.3
7.7 22.1
10.6
1.0--0.5
1.7 12.0 34.0
2.8 12.5
3.1
0.5--0.2
Particle size o f organic debris (%)
3.9
pH KCI
1.1 9.1 22.9
1.0 3.7
0.9
<0.2 mm
O~
247 condensing on soil particles. In a later work, Savage et al. (1972) concluded that the hydrophobic nature of soil is due to hydrocarbon-like aliphatic substances. Most authors in the pedological literature seem convinced of the chemical nature of water repellency in organic soil materials (DeBano et al., 1970; Farmer, 1978; Ellies, 1983). However, little research has been done to relate this phenomenon to the physical characteristics of organic soil material, such as its state of disintegration, porosity and surface roughness. These characteristics of solids affect wettability to a high degree (Moillet and Collie, 1951; Adamson, 1960; Gregg, 1961; Bond and H a m m o n d , 1970; Fink, 1970; Philip, 1971). The present study was undertaken to find o u t if, and to what extent, the surface o f air-dry organic soil material affects its wettability. MATERIALS AND METHODS The experiments used three main types of humus from forest floors, which had developed under different conditions of biocenosis and habitat: (1)typical m o t from an a u t o m o r p h i c podzol (Spodosol, cf. Soil Survey Staff, 1975) in the forest association E m p e t r o nigri--Pinetum, {2)hygromoder from muck soil with shallow ground water table (Mollisol, Soil Survey Staff, 1975) in the forest association Tilio--Carpinetum stachyetosum, (3)typical mull from automorphic rusty brown soil {Inceptisol, cf. Soil Survey Staff, 1975) in t h e forest association Tilio-Carpinetum t y p i c u m . Some of the chemical and physical properties of the samples are presented in Table 1. The basic method used in most studies on surface wettability is c o n t a c t angle 0 measurement, the assesment of which, for polydispersive organic soil material, is extremely difficult. All the methods of contact angle 0 measurement mentioned in Adamson (1960) or proposed by Letey et al. (1962) and Yuan and H a m m o n d (1969) for mineral soils are difficult to apply for organic samples. The methods used to calculate the value of the contact angle in porous mineral soil materials are of no avail in dealing with organic soil material because of the varying dimensions of plant remains and soil pores, which are affected by water content. Krammes and DeBano (1965) p r o p o s e d a method in which the time taken for a water drop to penetrate is used as a wettability index. This m e t h o d is difficult to apply to forest floor samples for the following reasons: (i) the water takes a long time to penetrate the highly humified material, and during this time part of the drop evaporates, (ii) it is difficult to place a water drop on only partially d e c o m p o s e d remains, e.g. from litter subhorizon AoL, (iii) the organic material rises to the surface o f the drop. To avoid these difficulties, the following two methods were used: (1) measurement of the contact angle 0 ; this was used only for the relatively
248 homogenous material from the epihumus subhorizon AoH, (2) the Enslin methods; this was used on all samples to measure the rate of the initial phase of water absorption by various forms of humus. This m e t h o d can be regarded as analogous to the water-drop penetration-time method. To determine the effect of the surface state of the material on its wettability and water absorption rate, measurements were performed on samples in their natural state, i.e. with a rough surface, and on samples pressed into tablets to provide a smooth surface. The tablets were made with a hand press giving about 2000 kg cm -2 pressure. The measurement of the rate of the initial phase o f water absorption in an Enslin device (Fig. 1) was made for samples pressed into tablets and also in their natural (loose) state. The latter were placed in a metal cylinder 12 mm in diameter. Each sample weighed 0.25 g. The measurements were made at 20°C and there were three replicates. To illustrate the character of the surface of the structural elements of the organic material showing various degrees of decomposition and humification, photographs were taken by means of a scanning electron microscope. A photograph was also taken o f the surface of material from the epihumus subhorizon AoH after pressing.
2 ~--~ ~
,
, - , -
- , - , - , - ~ ,
,
,
i
Fig. 1. Enslin device: 1, sample; 2, porous glass plate; 3, calibrated capillary; 4, threeposition tap ; 5, water. RESULTS The difference in value of the contact angle, 0, for the material from the epihumus subhorizon AoH in both its loose and its pressed f o r m are illustrated in Fig. 2 a,b. For loose material, angle 0 is a b o u t 145 °, whereas for the same material after pressing it is a b o u t 60 ° . The same material shows different degrees of affinity to water, depending on the physical state of its surface, which is illustrated in Figs. 3 and 4. The scanning electron micrograph, Fig. 3, shows that the surface of a particle of strongly decomposed a m o r p h o u s humus from the epihumus subhorizon AoH has numerous irregularities, but after pressing it has a relatively s m o o t h surface (Fig. 4). The physical state o f the surface of the organic matter also has a significant effect on the rate o f water absorption in the Enslin device (Fig. 5). The rate of the initial phase of water absorption in samples in the loose form
249
Fig. 2. A drop of water on the surface of material from epihumus subhorizon AoH: (A) in natural -- loose -- form; (B) after pressing. and in those pressed into tablets was f o u n d to differ considerably irrespective of the humus form. T he tablets absorbed in 10 minutes an a m o u n t of water near to saturation unde r the e x p e r i m e n t a l conditions, whereas in the loose material water absorption was much slower. Measurements over 3 days of the rate o f wat er a b s o r p t i o n by samples of different forms of humus have d e m o n s t r a t e d t h a t loose material will eventually b eco me we t t ed (Fig. 6). Moreover, these m easurem ent s have demonstrated significant differences in absorption rate bet w een di fferent forms of humus, depending on their degree of d e c o m p o s i t i o n and humification. The smaller the particle size and t he greater the h u m i f i c a t i o n o f the material in the particular humus forms (Table 1) the slower t he absorption rate. This is particularly evident in the various forms o f the m o r t y p e . In this case the differences in water absorption rate are due to t he character of t he surface o f the tissues making up the organic matter. T h e plant remains from the litter s u b h o r izo n A o L o f t he m o t t ype in t h e initial stage of d e c o m p o s i t i o n are distinguished by their relatively s m o o t h surface (Fig. 7) com pared with the very rough surface of the plant remains f r o m the epihumus subhorizon AoH.
250
Fig. 3. Rough surface of organic remains, of particle size <0.1 mm, from epihumus subhorizon AoH. DISCUSSION AND CONCLUSIONS The wettability of organic soil materials is generally determined by their amphiphilic characteristics. This property depends on the ratio of polar sites to non-polar aliphatic structures occurring in the organic substances (Farmer, 1978). Most authors assume that the main cause of water repeUency in organic soil matter is associated with its chemical nature. According to Farmer (1978), in the process of drying of organic soil matter there m a y be interaction between t h e polar groups. This leads to the formation of hydrogen-type bonds which block the renewed hydratation of these groups. This process is largely irreversible. However, it must be pointed o u t that if water repellency depended solely on the chemical structure of the material, there would be an increase in wettability as the degree of humification increased, since during humification the number of carboxylic groups, which play the main role in hydration, is greatly increased (Broadbent, 1954). However, the reverse p h e n o m e n o n is observed, i.e. there is an increase in water repellency. Without denying the importance of the chemical structure of the humus substances, this factor cannot be regarded as the only one
251
Fig. 4. Comparatively s m o o t h surface o f s a m p l e f r o m e p i h u m u s s u b h o r i z o n A o H after
pressing. o-typical
mor
b - hygromoder
c - typical muir
3O
3O AoL
"25
25
2.0
~
"
20
15
'~0
AoF _
1 2 3 ~ 5 6 7 8 9 ~ 0
2
3
L,
5
[ 6
7
B
9
I0
I
?
3
~
5
6
7
8
9
~0
Time [min] Fig. 5. The process o f water a b s o r p t i o n in an Enslin device in samples o f various h u m u s forms: • • , natural ( l o o s e ) form; c o, pressed form.
252
o- typicat
b-'hfgmrnoder
mot
c- typ~at muir 3.5
3"5t t0
-
30
25
25
,.o
15
1.0
oH
05
i
1 i
2 i
I i
2 i
0
Time [ day ] Fig. 6. The process of water absorption in an Enslin device in various humus samples in a natural (loose) form during three days.
Fig. 7. Fragment o f comparatively s m o o t h surface of remains of a pine needle from litter subhorizon AoL in the initial stage of decomposition.
253
of importance. In the pedological literature, the interaction between water and a solid phase with a rough surface, is shown to affect the wetting process significantly (Moillet and Collie, 1951; Adamson, 1960; Bond and Hammond, 1970). Likewise, Fink (1970) considered that porosity is responsible for the large contact angles, reaching about 150 °, in a mineral material covered with silicon resin. The m a x i m u m angle for water on a s m o o t h glass surface covered with the same resin is 100 °. Bond and H a m m o n d (1970) claimed that the water repellency of sand is due to both porosity and pore shape. The present studies have shown clearly that the wettability of soil organic material is largely dependent on the state of its surface. The pressed and the loose material both had the same chemical structure, but the pressed material with a smooth surface was easily wettable, while the loose material with its discontinuous surface was water repellent. It can be demonstrated on theoretical grounds that the difference in values between the contact angle 0 measured for the same material in different forms depends on the physical state of its surface. Of vital importance in determining the high value o f the wetting angle 0 of unpressed samples, with a very rough and porous surface, is the effect of drop support. This p h e n o m e n o n is similar to the so-called 'effect of reticulum' (Adamson, 1960). A drop of water placed on the surface of rough and porous material comes into contact with the solid phase only at those points f o r m e d by the peaks of the rough surface. The apparently flat contact surface of the drop Sa, seen with the naked eye, is made up of interphase surfaces Ssi(n), water-solid phase, and Svl(n) water--air phase (Fig. 8). The discontinuity of contact between water and material and, in particular, the discontinuity of this contact at the edge of the drop (L) is responsible for the considerable increase in the value of the measured contact angle 0, which in such a case is called the apparent contact angle 0ab (Moillet and Collie, 1951; Adamson, 1960). Its value, even for materials which, because of their chemical nature, are easily wettable, greatly exceeds the value of the real contact angle, 6 r, measured on a truly smooth surface of the same material. Figure 9 illustrates in a simplified way the distribution of the curvature angles of a drop placed on a porous surface. At the points of c o n t a c t water-material Ssl(n) the contact angle 0 a which the water forms with the observation plane x--x' is the real contact angle 0 r of this material. Between the surfaces Ssl(n ) at the contact edge (L) there are the surfaces water--air Svl(n)devoid of contact with the material, where the curvature of the water surface nears 180 °. The apparent contact angle 0ab seen relative to the observation plane x--x' is the geometric sum of angles 0 a and 0 b dependent on the ratios of surface Ssl and Svl to the apparent contact surface Sa. The formula thus obtained for the value of the apparent contact angle 0ab at 0 b = 180° is identical with the formula of Cassie and Baxter (cf. Moillet and Collie, 1951) and has the form:
254 /2
COS0ab =
/2
Y~ Svl(n )
cOS0r-
Sa by substituting
• Svl(n)
(I)
Sa
fsl and fvl f o r , r e s p e c t i v e l y ,
12
t~
E Svl(n)
~ Svl(n)
and
Sa
Sa
we o b t a i n
(2)
COS 0 ab = fsl COS 0 r - fvl / L
/
Sslln+l)-~
Sa
Svlln+'l}~@
%-'\ _esb.__.¢./J..,,
,,,- X , 4f ,
"b,~~
\ \\ \
//
%.
\
\
'\ \
Fig. 8. An illustration of the interphase contact surface water--solid phase in the case of rough surface: Sa, apparently flat interface; Sap interface of real contact water--solid phase; Svl, water--air interface; L, outer circumference of drop contact. Fig. 9. Curvature angles of water drop on rough surface: ea = Sz, real wetting angle at the contact point water--solid phase (Sal); eb, angle of drop curvature between points of support (Svl);#ab,the observed resultant angle of drop curvature. In a c c o r d a n c e w i t h eqns. (1) and ( 2 ) f o r p r e s s e d s a m p l e s o f o r g a n i c m a t e rial w i t h a s m o o t h c o n t a c t s u r f a c e w a t e r - - m a t e r i a l (fsl = 1, fvl = 0, w e o b t a i n COS 0ab = COS 0 r
(3)
T h e m e a s u r e d angle 0 ab in this case is e q u a l to t h e real c o n t a c t a n g l e 0 r. I f t h e r e are b u t f e w s u p p o r t i n g p o i n t s o f t h e w a t e r d r o p o n a v e r y p o r o u s surface (fsl ~ 0, fvl ~ 1), t h e n cos 0 ab ~ - 1 T h e v a l u e o f t h e a p p a r e n t c o n t a c t angle 0ab o n a v e r y p o r o u s s u r f a c e can t h e r e f o r e r e a c h a value n e a r i n g 180 °. A s s u m i n g t h a t a w a t e r d r o p is in real c o n t a c t w i t h t h e p o r o u s s u r f a c e o f t h e o r g a n i c m a t e r i a l o v e r o n l y 10% o f t h e a p p a r e n t c o n t a c t s u r f a c e Sa (fsl = 0.1, fsv = 0.9) w e o b t a i n f r o m e q n . (2) t h e value o f t h e a p p a r e n t c o n t a c t angle
255 0ab = 145 °. The value o b t a i n e d o n this a s s u m p t i o n is similar to t h a t o f the measured w e t t i n g angle o f t h e organic m a t t e r f r o m s u b h o r i z o n A o H . The results o b t a i n e d in t h e Enslin device have c o n f i r m e d t h e e f f e c t o f the surface state o f the organic material o n its w e t t a b i l i t y . Pressed samples with a s m o o t h c o n t a c t surface with w a t e r e x h i b i t g o o d w e t t a b i l i t y a n d b e c o m e w e t t e d within a short time. T h e same material in l o o s e f o r m also b e c o m e s wet, b u t the time n e e d e d for this to o c c u r is m a n y t i m e s longer. This provides clear evidence t h a t t h e main f a c t o r d e t e r m i n i n g b o t h t h e w e t t a b i l i t y o f organic soil materials a n d the w e t t i n g p r o c e s s is t h e p h y s i c a l state o f their surface. This does n o t m e a n t h a t t h e c h e m i c a l p r o p e r t i e s o f the organic material do n o t affect its w e t t a b i l i t y . F r o m the present s t u d y it f o l l o w s t h a t : (1) o r g a n i c soil material b y its chemical n a t u r e is h y d r o p h i l i c ; and (2) w a t e r r e p e l l e n c y o f d r y o r g a n i c soil material is due to the physical state o f its surface, i.e. to t h e degree o f dispersion and t h e r o u g h n e s s o f its m o r p h o l o g i c a l e l e m e n t s .
REFERENCES Adamson, A.W., 1960. Physical Chemistry of Surfaces. Interscience Publishers, New York, NY, 629 pp. Bond, R.D. and Hammond, L.C., 1970. Effect of surface roughness and pore shape on water repellency on sandy soils. Proc. Soil and Crop Sci. Soc. of Florida, 30: 308-315. Broadbent, F.E., 1954. Modification in chemical properties of straw during decomposition. Soil Sci. Soc. Am. Proc., 18: 165--169. DeBano, L.F., Mann, L.D. and Hamilton, P.A., 1970. Translocation of hydrophobic substances into soil burning organic litter. Soil Sci. Soc. Am. Proc., 34: 130--133. Ellies, A., 1983. Die Wirkungen yon Bodenerhitzungen auf die Benetzungseigenschaften einiger Boeden Suedchiles. Geoderma, 29, 2: 129--138. Farmer, V.C., 1978. Water on particle surfaces. In: D.J. Greenland and M.H.B. Hayes (Editors), The Chemistry of Soil Constituents. Wiley and Sons, Chichester, New York, Brisbane, Toronto, pp. 405--448. Fink, D.H., 1970. Water repellency and infiltration resistance of organic-film-coated soils. Soil Sci. Soc. Am. Proc. 34: 189--194. Gregg, S.J., 1961. The Surface Chemistry of Solids. Chapman and Hall, London, 393 pp. Grelewicz, A. and Plichta, W., 1983. Water absorption of xeromor forest floor samples. For. Ecol. Manage., 6: 1--12. Krammes, J.S. and DeBano, L.F., 1965. Soil wettability: a neglected factor in watershed management. Water Resour. Res., 1, 2: 283--286. Letey, J., Osborn, J. and Pelishek, R.E., 1962. Measurement of liquid--solid contact angles in soil and sand. Soil Sci., 93, 3 : 149--153. McGhie, D.A. and Posner, A.M., 1981. The effect of plant top material on the water repellency of fired sands and water repellent soils. Aust. J. Agric. Res., 32: 609--620. Moiilet, J. and Collie, B., 1951. Surface Activity. Van Nostrand, New York, NY, 379 pp. Philip, J.R., 1971. Limitations on scaling by contact angle. Soil Sci. Soc. Am. Proc., 35: 507--509. Prusinkiewicz, Z., Bednarek, R., Deg6rski, M. and Grelewicz, A., 1981. The water regime of sandy soils in dry pine forest (Cladonio--Pinetum) in the northern part of the glacial outwash plains of the Brda and Wda rivers. Ekol. Pol., 29, 2: 283--309.
256 Savage, S.M., 1974. Mechanism of fire-induced water repellency in soil.Soil Sci. Soc. A m . Proc., 33: 149--151. Savage, S.M., Martin, J.P. and Letey, J., 1969. Contribution of some soil fungi to natural and heat induced water repellency in sand. Soil Sci. Soc. A m . Proc., 33: 405--409. Savage, S.M., Osborn, J., Letey, J. and Heaton, C., 1972. Substances contributing to fireinduced water repellency in soil.Soil Sci. Soc. A m . Proc., 36: 674--678. Scholl, D.G., 1971. Soil wettability in Utah juniper stands. Soil Sci. Soc. A m . Proc., 35: 344--345. Singer, M.J. and Ugolini, F.C., 1976. H y d r o p h o b i c i t y in the soils of Findley Lake, Washington. For. Sci., 22, 1: 54--58. Soil Survey Staff, 1975. Soil t a x o n o m y : a basic system o f soil classification for making and interpreting soil surveys. Agric. Handbk. No 436, USDA, U.S. Govt. Printing Office, Washington, DC, 754 pp. Thun, R., Hermann, R. and Knickmann, E., 1955. Methodenbuch. Bd. 1, Die Untersuehung yon Boeden. Neuman, Radebeul and Berlin, 271 pp. Wittich, W., 1954. Die melioration streugenutzter Boeden. Forstwiss. Centralbl., 73: 211--232. Van 't Woudt, B.D., 1959. Particle coatings affecting the wettability of soils. J. Geophys. Res., 64: 263--267. Yuan, T.L. and Hammond, L.C., 1969. Evaluation of available methods for soil wettability measurement with particular reference to soil--water contact angle determination. Proc. Soil Crop Sci. Soc. Florida, 28: 56--63.