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Soil erosion on Alfisols in Western Nigeria
Geoderma, 16 (1976) 377--387 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
S O I L E R O S I O N O N A L F I S O L S I N W E S T E R N N I G E R I A , II. E F F E C T S O F MULCH RATES
R. LAL
International Institute of Tropical Agriculture, Ibadan (Nigeria) (Received December 18, 1975; accepted August 2, 1976)
ABSTRACT
Lal, R., 1976. Soil erosion on Alfisols in Western Nigeria, II. Effects of mulch rates. Geoderma, 16: 377--387. The effects of four rates of straw mulching on runoff and soil loss were compared with those of no-tillage treatments under natural rainfall conditions using field runoff plots of 25 x 4 m established at 1, 5, 10 and 15% slopes on the International Institute of Tropical Agriculture (IITA) research site near Ibadan, Nigeria. The four rates of straw mulching were 0, 2, 4 and 6 t/ha. The mean annual runoff was 50, 10, 4 and 2% of the total annual rainfall for mulch rates of 0, 2, 4 and 6 t/ha, respectively. Runoff from unmulched treatments was not related to slope. Runoff loss from no-till treatments was only 2% of the rain received. The mean soil losses for the rainstorms greater than 25 mm were 143, 16, 2 and 0.4 kg/ha per mm of rain received for mulched rates of 0, 2, 4 and 6 t/ha, respectively. The soil loss declined exponentially with increasing mulch rate with exponents ranging from approximately 0.3 to --0.7. The soil losses from the no-till plots were equal to those from plots that received mulch at the rate of 6 t/ha. Soil erodibility was significantly influenced by time after clearing, with maximum K reached two to three years after forest removal. The nutrient loss in runoff and eroded soil was significant only for unmulched treatments. The maximum annual loss of NO3--N in runoff was about 15 kg/ha. The maximum annual loss of total N in eroded soil from unmulched plots was about 180 kg/ha, that of P, 5 kg/ha, and that of K, about 14 kg/ha.
INTRODUCTION Soil e r o s i o n , a w o r k f u n c t i o n i n v o l v i n g d e t a c h m e n t a n d t r a n s p o r t a t i o n o f soil p a r t i c l e s , is s i g n i f i c a n t l y i n f l u e n c e d b y r a i n f a l l c h a r a c t e r i s t i c s a n d s u r f a c e soil c o n d i t i o n s . T h e e n e r g y t o p e r f o r m t h i s w o r k f u n c t i o n is s u p p l i e d b y r a i n d r o p i m p a c t a n d t h e s h e a r stress e x e r t e d b y c o n c e n t r a t e d r u n o f f . E f f e c t i v e control of erosion, therefore, requires reducing the direct impact of raindrops; m a i n t a i n i n g t h e soil i n f i l t r a t i o n r a t e ; a n d d e c r e a s i n g t h e q u a n t i t y , v e l o c i t y a n d transport capacity of runoff water. These control measures can be achieved t h r o u g h r e s i d u e m u l c h e s o n soil s u r f a c e s . E a r l y w o r k o n t h e r o l e o f m u l c h e s o n soil a n d w a t e r c o n s e r v a t i o n has b e e n s u m m a r i z e d b y J a c k s e t al. ( 1 9 5 5 ) a n d M c C a l l a e t al. ( 1 9 6 2 ) . D u l e y a n d K e l l y
37S (1939) reported that covering the soil surface with straw increased mfiltratiot: and prevented crust formation. Borst and Woodburn (1942b) attributed the decrease in r u n o f f from mulching to an increase in surface detention capacity. Subsequent experiments by Swanson et al. (1965), Adams (1966), Barnett et al. (1967), and Meyer et al. {1970) have indicated that although mulching can prevent runoff and soil loss its effectiveness depends on the quantity of crop residue and slope gradient. The effect of stone mulches in preventing r u n o f f and soil loss has been reported by various workers (Chapman, 1937; Tsiang, 1948; Hide, 1954; Jung, 1960; Meyer et al., 1972). These workers reported that stone cover increases surface roughness, prevents surface sealing and therefore, decreases r u n o f f and soil loss. One practical means of mulching is by crop residue management associated with no-tillage systems. The effectiveness of the no-tillage system in conserving soil and water conservation has been reported by various workers (Motdenhauer and Amemiya, 1969; Greb et al., 1970; Unger et al., 1971). This report describes the use of crop residue in controlling r u n o f f and soil loss from some soils of western Nigeria. MATERIALS AND METHODS Experiments were conducted during 1974 at the International Institute of Tropical Agriculture (IITA) research site near Ibadan, Nigeria using 25 × 4 m field r u n o f f plots established on natural slopes of 1, 5, 10 and 15%. The design and construction of these r u n o f f plots has been described in an earlier report (Lal, 1976). The effects of four rates of rice straw mulching, viz. 0, 2, 4 and 6 t/ha, on r u n o f f and soil loss were compared with a no-tillage system of soil management. The r u n o f f plots were plowed, harrowed and mulched in April and August, beginnings of the first and second growing seasons, respectively. The no-till plots were sprayed with paraquat at the rate of 2.5 kg/ha. The crop residue from the previous season's crop and dead weeds were left on the surface as insitu mulch. No crops were grown to eliminate the effects of the cover. R u n o f f and soil were collected from each plot after every rainstorm. The exponential relationships between slope and soil or water loss and mulch rate and soil or water loss were calculated. For each plot and each date, the relation of r u n o f f and soil loss to slope was determined by equations of the form y = as b, where y is the r u n o f f or soil loss and s is the slope. The relationship between r u n o f f and soil loss and mulch rate was determined by equations of the form y = a M -b, where M is the mulch rate. To find the values of a and b, straight lines of the form log y = log a +- b log x were fitted to the data. The values of a and b were printed for each analysis, and the value of r, the coefficient of linear correlation, was tested for significance. The calculations were separately made for rainstorms above 25 mm and below 25 mm. Nitrogen was applied at the rate of 120 kg/ha as urea, phosphorus at the rate of 30 kg/ha of
379 P as single superphosphate, and potassium at the rate of 30 kg/ha of K as muriate of potash. R u n o f f samples of each rainstorm were analysed for NO3--nitrogen, PO4--phosphorus, K, Ca and Mg. Concentrations of Ca and K were determined by flame p h o t o m e t r y . Mg concentration was determined by atomic absorption spectrophotometry. Phosphorus was measured colorimetrically, using the molybdic acid-blue method, and NO3--nitrogen was measured colorimetrically, using brucine. A composite sample of the eroded sediment from all r u n o f f plots was collected separately at the end of each season. These samples were ground and passed through a 2-mm sieve. The samples were analysed for organic carbon by wet combustion, for total nitrogen by Kjeldahl, for Bray-1 available phosphorus, and for a m m o n i u m acetate extractable cations, viz. Ca 2+, Mg2+ and K ÷ RESULTS AND DISCUSSION R u n o f f losses from the unmulched plots during the first season were 48, 67, 42 and 57% of the total annual rainfall, respectively, for the 1, 5, 10 and 15% slopes. During the second season the r u n o f f losses were 52, 55, 34 and 32% for the 1, 5, 10 and 15% slopes, respectively. The mean annual r u n o f f was 54% from the 1% slopes, 63% from the 5% slope, 39% from 10% slope and 49% from the 15% slope. The annual soil losses on unmulched plots were 9, 134, 138 and 96 t / h a for the 1, 5, 10 and 15% slopes, respectively. Soil losses on the 10 and 15% slopes were affected considerably by a high concentration of gravel on the soil surface as a result of prior erosion. A similar effect of gravel mulch on r u n o f f and erosion has been reported (Chapman, 1937; Tsiang, 1948; Hide, 1954).
Mulch rate and runoff R u n o f f was significantly influenced by rate of straw mulch (Table I). Mean runoff losses during the first season were 55, 8, 3 and 2% of the total rainfall for the mulch rates of 0, 2, 4 and 6 t/ha, respectively. The r u n o f f from the no-till plot was 2% of the total rainfall. During the second season, the mean r u n o f f losses were 43, 16, 7 and 2% for mulch rates of 0, 2, 4 and 6 t/ha, respectively. R u n o f f from the no-till plot during the second season was only 4%. The data in Table I do not indicate any relationship between r u n o f f and slope. The average r u n o f f losses for all mulch rates during the first season were 12, 17, 12 and 14% for slope gradients of 1, 5, 10 and 15%, respectively. During the second season, the average r u n o f f losses were 16, 22, 14 and 16% of total rainfall for 1, 5, 10 and 15% slopes, respectively. For individual rainstorms the relationships between r u n o f f and slope for different mulch rates are shown in Table II. The correlation coefficients
380
TABLE I Effect of m u l c h rate on r u n o f f (ram) on different slopes* Slope (%)
Mulch rate (t/ha) 0
2
4
6
No-till
1 5 10 15
(a) F i r s t - s e a s o n r u n o f f ( m m ) ( A p r i l - - J u l y ) 283 6 4 0 6 346 61 10 7 9 219 46 21 12 15 294 47 20 12 14
1 5 10 15
(b) S e c o n d - s e a s o n r u n o f f ( m m ) ( A u g . - - N o v . ) 129 30 3 0 5 137 65 18 4 6 84 28 14 9 9 80 40 31 8 9
* R a i n f a l l first s e a s o n 5 1 0 r a m , s e c o n d s e a s o n 249 r a m .
relating runoff with slope are significant only for the high mulch rates. For storms exceeding 25 mm of rainfall, the slope exponents increased from 0.04 to 3.3 and the coefficients decreased from 59.2 to 0.001 with mulch rates increasing from 0 to 6 t/ha. T A B L E II S l o p e - - r u n o f f r e l a t i o n s h i p s for d i f f e r e n t m u l c h rates M u l c h rate (t/ha)
r
Equation
M e a n r u n o f f (%)
0 2 4 6 No-till
(a) R a i n s t o r m s > 25 m m --0.1 W = 5 9 . 2 S -°'°~ 0.4 W = 2 . 1 S °'66 0.76 W = 0 . 8 S ''°5 0.89 W = 0 . 0 0 1 S 3-3 0.57 W = 1 . 2 0 S °'29
55.7 6.7 2.7 0.6 1.9
0 2 4 6 No-till
(b) R a i n s t o r m s < 25 m m --0.27 W = 4 7 . 5 S -°'2~ 0.49 W = 1 . 5 S °'4~ 0.73 W = 0 . 8 S °'66 0.90 W = 0 . 0 0 2 S ~'°7 0.48 W = 1 . 6 2 S °'3°
33.0 3.4 2.3 0.4 2.6
W = ( R u n o f f / r a i n f a l l ) . 100.
381
Mulch rate and soil loss
The effects of mulch rate on soil loss during the first and second seasons of 1974 are shown in Figs. 1 and 2. Maximum soil losses during the first season were 110, 3.5, 0.5 and 0.3 t/ha for mulch rates of 0, 2, 4 and 6 t/ha, respectively (Fig. 1). Similar results were obtained during the second season (Fig. 2). Mean soil losses for rainstorms greater than 25 mm were 143, 16, 2 and 0.4 kg/ha per mm of rain for mulch rates of 0, 2, 4 and 6 t/ha, respectively. For rainstorms of less than 25 mm, the mean soil losses were 110, 2, 0.7 and 0.4 kg/ha per mm o f rain. The low mulch rate of 2 t/ha effectively prevented soil loss even from strong slopes. The analysis of soil loss from individual rainstorms based on the exponential relationships between slope and soil loss is given in Table III. For the rainstorms exceeding 25 mm, the most significant correlation for slope and soil loss was obtained for unmulched plots. Although the slope exponents did not
lOOO!
100 100 O
on'ha
Ton/ha 2 Tons/ha I0
10
o
xlX~X 2 Tons/ha
/
1.0
o -
x
0.1C
~
£u
-
NO-TiLLAGE
0
~ To~/ha
o
~
4 Tons/ha
6 Tons/ha
6 Tons/ha /n / t
0.1C /
0 001
~1,0
1~0 Slope(%)
J flO0
0.0"01
# 10
Slope(°/o)
i~ ~.NO -TILLAGE
, flO
Fig. 1. Effects of mulch rate on soil losses for different slopes during the first season, 1974 (log --log scale). Fig. 2. Effects o f mulch rate on soil losses for different slopes during the second season, 1974 (log--log scale).
382 TABLE III Slope--soil loss relationships for different mulch rates Mulch rate (t/ha)
r
Equation
Mean soil loss (kg ha -j mm -~ of rain)
0 2 4 6 No-till
(a) Rainstorms > 25 mm 0.81 E = 11.8S '-':~ 0.35 E = 0.5S °'~7 0.57 E = 0.07S ''°~ 0.46 E = 0.01 S ''° 0.35 E = 0.01 S '~'~
143.1 16.4 2.0 0.4 0.6
0 2 4 6 No-till
(b) Rainstorms < 25 mm 0.38 E = 11.2 S °'~ 0.32 E = 0.3S °'~ 0.41 E = 0.1S ~'-~' 0.49 E = 0.04 S °-~'2 0.12 E = 0.13S "-j4
100.5 2.2 0.7 0.4 0.4
E = Soil loss in kg ha- ' ram-1 of rain. c h a n g e significantly with m u l c h rate, the c o e f f i c i e n t s did. F o r r a i n s t o r m e x c e e d i n g 25 m m the c o e f f i c i e n t s were 11.8, 0.5, 0.07 and 0.01 for 0, 2, 4, and 6 t / h a o f m u l c h , respectively. Mulch
factor
Wischmeier ( 1 9 7 3 ) p r o p o s e d a m u l c h f a c t o r , the ratio o f soil and w a t e r loss with m u l c h to the c o r r e s p o n d i n g loss with n o m u l c h , as a f u n c t i o n o f the p e r c e n t a g e o f the surface c o v e r e d b y m u l c h . T h e m u l c h f a c t o r can be used to p r e d i c t soil loss in t h e universal soil-loss e q u a t i o n . The average m u l c h f a c t o r s for slopes ranging f r o m 1 t o 15% axe s h o w n in Fig. 3. The m u l c h factors decreased e x p o n e n t i a l l y with increasing m u l c h rate. Mulch rate--soil loss and m u l c h r a t e - - r u n o f f relationships f o r d i f f e r e n t slopes are s h o w n in Table IV. T h e e x p o n e n t s o f the m u l c h rates are negative, ranging b e t w e e n - - 0 . 3 and - - 0 . 7 d e p e n d i n g on slope a n d o t h e r characteristics. Changes in soil-erodibility
factor (K)
T h e time d e p e n d e n c e o f the soil-erodibitity f a c t o r (K), f r o m 1 9 7 2 t o 1 9 7 4 is s h o w n in Fig. 4. I m m e d i a t e l y a f t e r f o r e s t clearing the K f a c t o r was extremely low, n o t o n l y because o f the presence o f r o o t s and o t h e r organic m a t t e r , b u t also because o f the e x c e l l e n t soil s t r u c t u r e and p e r m e a b i l i t y c r e a t e d b y the a c t i o n o f e a r t h w o r m s and termites. T h e soil-erodibility f a c t o r r e a c h e d its m a x i m u m value t w o or three years a f t e r f o r e s t clearing. T h e s u b s e q u e n t decline o f the K f a c t o r m a y have been due t o preferential erosion o f fine
383
10
nr
First Season ] X---X Second Seas0 Runoff
(>---o Frost Season Soil LOSS o---o Second Season \\ \\
\ \
\\\
NNN
\\
X\\
O.olh
0.001
I
0
t
"r
2 4 Mulch r'ate (Tons/ha)
6
Fig. 3. M u l c h f a c t o r as a f f e c t e d b y m u l c h r a t e .
TABLE IV E f f e c t s o f m u l c h r a t e (M) o n r u n o f f (%) a n d s o i l l o s s ( k g h a -~ m m slopes S l o p e (%)
r
Equation
1 5 10 15
0.78 0.80 0.86 0.75
(a) Runoff W = 0.39 M W = 1.16 M W = 5.53 M W = 5.26 M
1 5 10 15
0.85 0.85 0.86 0.72
-°'7~ -°'3~ -°'27 -°'s~
(b) S o i l l o s s E E E E
M = M u l c h r a t e in t / h a .
= = = =
0.19 1.25 1.09 0.98
M -°'$4 M -°'7~ M -°'67 M -°'24
1 of rain) on different
384
t: 5 r
r !
c~-~o I "o Slope pk:t 5°/~ SIoDe pl(t A - ~ A 10% Slope r) ot D~---~j 1~-~°/~Slope Dlot
i
X----x
i
i'
o3I
/
i
~o.2 /
Xs
o °~l
x x
I 2 3 Time o f f e r forest cleQring ( y e o r s )
4
Fig. 4. Changes in soil-erodibility f a c t o r (K) w i t h t i m e a f t e r f o r e s t removal.
particles from the surface layer in the previous years. Consequently, higher concentrations of gravel on the surface acting as a mulch also contributed towards a lower K factor. A "gravel factor" may be necessary for making appropriate corrections. TABLE V Annual nutrient losses (kg/ha) in runoff water for different mulch r a t e s Mulch
(t/ha)
Slope gradient 1%: . . . . NO,--N PO,--P
K
Ca
0 2 4 6 No-till
13.0 0.6 0.2 T 0.3
16.2 0.7 0.2 T 0.3
20.7 0.3 0.2 T 0.4
rate
3.3 0.3 0.1 T 0.1
T = Less than 0.1 kg/ha.
.
.
.
Slope gradient 5%: . . . . . . Mg NO3--N PO4--P 2.4 0.2 0,1 T 0.1
12.1 3.1 0.7 0.3 0.5
3.6 1.0 0.3 0.1 0.1
. K 24.6 2.5 1.2 0.3 0.5
. Ca
Mg
25.0 5.5 1.4 0.3 0.6
3.3 1.3 0.2 0.1 0.2
385
Mulch rate and nutrient loss in runoff and eroded soil materials Annual n u t r i e n t losses in r u n o f f water are shown in Table V. The mean annual total n u t r i e n t losses for all slopes, obtained by adding the N, P, K, Ca and Mg losses, were 55, 10 and 4 and 2 kg/ha for mulch rates of 0, 2, 4 and 6 t/ha, respectively. The mean annual n u t r i e n t loss from the no-till t r e a t m e n t s was a b o u t 4 kg/ha. The m a x i m u m annual losses of plant nutrients from unmulched plots were a p p r o x i m a t e l y 15, 4, 25, 25 and 3 kg/ha of NO3--N, PO4--P, K, Ca and Mg, respectively. The total n u t r i e n t loss in r u n o f f decreased exponentially with increasing mulch rate. The loss of organic carbon in eroded soil materials during the first season ranged f r o m 200 to 1000 kg/ha for no mulch, from 16 to 100 kg/ha for the mulch rate of 2 t/ha, and f r o m 10 to 15 kg/ha for the mulch rate of 4 t/ha. There were negligible organic carbon losses f r o m the no-till plots and those that received 6 t / h a of mulch. The losses of different plant nutrients in eroded sediment are shown in Table VI. For the u n m u l c h e d t r e a t m e n t , the annual nut ri ent losses were 3 9 - 1 7 6 kg of N, 0.6--5.0 kg of inorganic P, 1.4--14 kg of K, 18--90 kg of Ca and 1--13 kg/ha of Mg. The n u t r i e n t losses in eroded soil materials for the mulch rates of 4 and 6 t / ha and for the no-till t r e a t m e n t were negligible. CONCLUSIONS (1) R u n o f f and soil loss decreased exponentially with increasing mulch rates, with e x p o n e n t s ranging f r om --0.3 to --0.7. A mulch rate of 2--4 t / ha effectively controlled erosion. (2) The exponential soil loss--slope relationship does n o t apply to mulched plots. (3) No-till tr eatm e nt s had minimal r u n o f f and soil loss and were as effective as a mulch rate of 6 t/ha.
Slope gradient 10%:
Slope gradient 15%:
NO3--N
PO4--P
K
Ca
Mg
NO3--N
PO4--P
K
Ca
Mg
8.5 1.5 0.9 0.5 0.6
2.2 0.9 0.3 0.1 0.2
13.9 3.3 0.9 0.9 1.3
14.3 3.4 2.1 0.7 2.1
2.4 0.6 0.6 0.2 0.3
15.1 1.8 1.1 0.6 0.7
3.2 0.8 0.4 0.2 0.2
17.7 4.2 2.7 1.1 2.5
14.1 5.2 3.0 1.1 1.2
3.1 0.9 0.5 0.3 0.4
T = Less t h a n 0.1 kg/ha.
0.6 T T T T
1.4 0.2 0.1 T T
17.6 1.8 0.1 T T
1.2 0.1 T T T
117.9 27.3 5.4 T T
4.2 0.4 0.1 T T
P 14.2 1.0 0.3 T T
K
38.5 4.5 0.3 T T
Mg
0 2 4 6 No-till
Ca
N
K
N
P
S l o p e g r a d i e n t 5%:
S l o p e g r a d i e n t 1%:
Mulch rate (t/ha) 90.2 10.9 2.5 T T
Ca 5.8 0.4 0.2 T T
Mg
A n n u a l n u t r i e n t losses ( k g / h a ) in e r o d e d soil m a t e r i a l s f o r d i f f e r e n t m u l c h r a t e s
T A B L E VI
76.3 16.3 12.1 T T
N 4.9 0.5 0.1 T T
P 13.6 1.3 0.6 T T
K
S l o p e g r a d i e n t 10%:
87.0 9.1 5.1 T T
Ca
8.5 0.9 0.4 T T
Mg
176.1 63.9 8.7 2.7 T
N
3.0 0.5 0.1 T T
P
14.4 3.3 0.5 0.2 T
K
S l o p e g r a d i e n t 15%:
112.1 36.3 5,5 1,9 T
Ca
12.9 2.8 0A 0,2 T
Mg
387
(4) Nutrient losses in r u n o f f water were significant only for the unmulched treatments. A considerable loss of both applied and native plant nutrients occurs through r u n o f f and soil loss. REFERENCES Adams, J.E., 1966. Influence of mulches on runoff, erosion and soil moisture depletion. Proc. Soil Sci. Soc. Am., 30: 110--114. Barnett, A.P., Disker, E.G. and Richardson, E.C., 1967. Evaluation of mulching methods for erosion control on newly prepared and seeded highway back slope. Agron. J., 59: 83--85. Borst, H.L. and Woodburn, R., 1942a. The effect of mulching and methods of cultivation on runoff and erosion from Muskingwan silt loam. Agric. Eng., 23: 19--22. Borst, H.L. and Woodburn, R., 1942b. Effect of mulches and surface conditions on the water relations and erosion of Muskingum soil. USDA Tech. Bull., 825, Washington, D.C., 76 pp. Chapman, B.B., 1937. Climate. In: J.L. Buce (Editor), Land Utilization in China. University of Chicago Press, Chicago, Ill. Duley, F.L. and Kelly, L.L., 1939. Effect of soil type, slope and surface conditions on intake of water. Nebraska Agric. Stn. Res. Bull., 112, 16 pp. Greb, B.W., Smika, D.E. and Black, A.L., 1970. Water conservation with stubble mulch fallow. J. Soil Water Conserv., 25: 58--62. Harrold, L.L., 1972. Soil erosion by water as affected by reduced tillage systems. Proc. No-tillage Systems Symp., Feb. 21--22, 1972, Center for Tomorrow, Columbus, Ohio. Harrold, L.L., Triplett, G.B. and Edwards, W.H., 1974. No-tillage corn, characteristics of the system. Trans. Am. Soc. Agric. Eng., 51(3): 128--131. Harrold, L.L., Triplett, G.B. and Youker, R.E., 1967. Watershed tests of no-tillage corn. J. Soil Water Conserv., 22: 98--100. Hide, J.C., 1954. Observations on factors influencing the evaporation of soil moisture. Proc. Soil Sci. Soc. Am., 18: 234--239. Jacks, G.V., Baird, V.D. and Smith, R., 1955. Mulching Commonwealth Bur. Soil Sci. Tech. Comm., 49. Jung, L., 1960. The influence of the stone cover on runoff and erosion on slate soil. Int. Ass. Sci. Hydrol. Publ., 53: 143--153. Lal, R., 1976. Soil erosion on Alfisols in Western Nigeria. 1. Effects of slope, crop rotation and residue management. Geoderma, 16: 363--375. McCalla, T.M., Army, T.J. and Whitfield, C.J., 1962. Stubble--mulch farming. J. Soil Water Conserv., 17 : 204--208. Meyer, L.D. and Mannering, J.V., 1967. Tillage and land modification for erosion control. Conf. Proc. Tillage for Greater Crop Production: ASA, 87: 58--62. Meyer, L.D., Wischmeier, W.H. and Forster, G.R., 1970. Mulch rates required for erosion control on steep slopes. Proc. Soil Sci. Soc. Am., 34: 928--931. Meyer, L.D., Johnson, C.B. and Foster, G.R., 1972. Stone and woodchip mulches for erosion control on construction sites. J. Soil Water Conserv., 27(6): 264--272. Moldenhauer, W.C. and Amemiya, M., 1969. Tillage practices for controlling crop land erosion. J. Soil Water Conserv., 24: 1 9 - 2 1 . Swanson, N.P., Dedrick, A.R., Weekly, H.E. and Haise, H.R., 1965. Comparing mulches -Scientists check effects of four mulching materials on 6% slope. Agric. Res., 13(8): 15. Tsiang, T.C., 1948. Soil conservation, an international study. Ford. Agric. Organ., U.N.P.: 83--84. Unger, P.W., Allen, R.R. and Wiesc, A.F., 1971. Tillage and herbicide for surface residue maintenance, weed control, and water conservation. J. Soil Water Conserv., 26: 147--150. Wischmeier, W.H., 1973. Conservation tillage to control water erosion. Proc. Nat. Conf. Conservation Tillage. Soil Conservation Soc. Am., Ames, Iowa, 1973: 133--140.
S O I L E R O S I O N O N A L F I S O L S I N W E S T E R N N I G E R I A , II. E F F E C T S O F MULCH RATES
R. LAL
International Institute of Tropical Agriculture, Ibadan (Nigeria) (Received December 18, 1975; accepted August 2, 1976)
ABSTRACT
Lal, R., 1976. Soil erosion on Alfisols in Western Nigeria, II. Effects of mulch rates. Geoderma, 16: 377--387. The effects of four rates of straw mulching on runoff and soil loss were compared with those of no-tillage treatments under natural rainfall conditions using field runoff plots of 25 x 4 m established at 1, 5, 10 and 15% slopes on the International Institute of Tropical Agriculture (IITA) research site near Ibadan, Nigeria. The four rates of straw mulching were 0, 2, 4 and 6 t/ha. The mean annual runoff was 50, 10, 4 and 2% of the total annual rainfall for mulch rates of 0, 2, 4 and 6 t/ha, respectively. Runoff from unmulched treatments was not related to slope. Runoff loss from no-till treatments was only 2% of the rain received. The mean soil losses for the rainstorms greater than 25 mm were 143, 16, 2 and 0.4 kg/ha per mm of rain received for mulched rates of 0, 2, 4 and 6 t/ha, respectively. The soil loss declined exponentially with increasing mulch rate with exponents ranging from approximately 0.3 to --0.7. The soil losses from the no-till plots were equal to those from plots that received mulch at the rate of 6 t/ha. Soil erodibility was significantly influenced by time after clearing, with maximum K reached two to three years after forest removal. The nutrient loss in runoff and eroded soil was significant only for unmulched treatments. The maximum annual loss of NO3--N in runoff was about 15 kg/ha. The maximum annual loss of total N in eroded soil from unmulched plots was about 180 kg/ha, that of P, 5 kg/ha, and that of K, about 14 kg/ha.
INTRODUCTION Soil e r o s i o n , a w o r k f u n c t i o n i n v o l v i n g d e t a c h m e n t a n d t r a n s p o r t a t i o n o f soil p a r t i c l e s , is s i g n i f i c a n t l y i n f l u e n c e d b y r a i n f a l l c h a r a c t e r i s t i c s a n d s u r f a c e soil c o n d i t i o n s . T h e e n e r g y t o p e r f o r m t h i s w o r k f u n c t i o n is s u p p l i e d b y r a i n d r o p i m p a c t a n d t h e s h e a r stress e x e r t e d b y c o n c e n t r a t e d r u n o f f . E f f e c t i v e control of erosion, therefore, requires reducing the direct impact of raindrops; m a i n t a i n i n g t h e soil i n f i l t r a t i o n r a t e ; a n d d e c r e a s i n g t h e q u a n t i t y , v e l o c i t y a n d transport capacity of runoff water. These control measures can be achieved t h r o u g h r e s i d u e m u l c h e s o n soil s u r f a c e s . E a r l y w o r k o n t h e r o l e o f m u l c h e s o n soil a n d w a t e r c o n s e r v a t i o n has b e e n s u m m a r i z e d b y J a c k s e t al. ( 1 9 5 5 ) a n d M c C a l l a e t al. ( 1 9 6 2 ) . D u l e y a n d K e l l y
37S (1939) reported that covering the soil surface with straw increased mfiltratiot: and prevented crust formation. Borst and Woodburn (1942b) attributed the decrease in r u n o f f from mulching to an increase in surface detention capacity. Subsequent experiments by Swanson et al. (1965), Adams (1966), Barnett et al. (1967), and Meyer et al. {1970) have indicated that although mulching can prevent runoff and soil loss its effectiveness depends on the quantity of crop residue and slope gradient. The effect of stone mulches in preventing r u n o f f and soil loss has been reported by various workers (Chapman, 1937; Tsiang, 1948; Hide, 1954; Jung, 1960; Meyer et al., 1972). These workers reported that stone cover increases surface roughness, prevents surface sealing and therefore, decreases r u n o f f and soil loss. One practical means of mulching is by crop residue management associated with no-tillage systems. The effectiveness of the no-tillage system in conserving soil and water conservation has been reported by various workers (Motdenhauer and Amemiya, 1969; Greb et al., 1970; Unger et al., 1971). This report describes the use of crop residue in controlling r u n o f f and soil loss from some soils of western Nigeria. MATERIALS AND METHODS Experiments were conducted during 1974 at the International Institute of Tropical Agriculture (IITA) research site near Ibadan, Nigeria using 25 × 4 m field r u n o f f plots established on natural slopes of 1, 5, 10 and 15%. The design and construction of these r u n o f f plots has been described in an earlier report (Lal, 1976). The effects of four rates of rice straw mulching, viz. 0, 2, 4 and 6 t/ha, on r u n o f f and soil loss were compared with a no-tillage system of soil management. The r u n o f f plots were plowed, harrowed and mulched in April and August, beginnings of the first and second growing seasons, respectively. The no-till plots were sprayed with paraquat at the rate of 2.5 kg/ha. The crop residue from the previous season's crop and dead weeds were left on the surface as insitu mulch. No crops were grown to eliminate the effects of the cover. R u n o f f and soil were collected from each plot after every rainstorm. The exponential relationships between slope and soil or water loss and mulch rate and soil or water loss were calculated. For each plot and each date, the relation of r u n o f f and soil loss to slope was determined by equations of the form y = as b, where y is the r u n o f f or soil loss and s is the slope. The relationship between r u n o f f and soil loss and mulch rate was determined by equations of the form y = a M -b, where M is the mulch rate. To find the values of a and b, straight lines of the form log y = log a +- b log x were fitted to the data. The values of a and b were printed for each analysis, and the value of r, the coefficient of linear correlation, was tested for significance. The calculations were separately made for rainstorms above 25 mm and below 25 mm. Nitrogen was applied at the rate of 120 kg/ha as urea, phosphorus at the rate of 30 kg/ha of
379 P as single superphosphate, and potassium at the rate of 30 kg/ha of K as muriate of potash. R u n o f f samples of each rainstorm were analysed for NO3--nitrogen, PO4--phosphorus, K, Ca and Mg. Concentrations of Ca and K were determined by flame p h o t o m e t r y . Mg concentration was determined by atomic absorption spectrophotometry. Phosphorus was measured colorimetrically, using the molybdic acid-blue method, and NO3--nitrogen was measured colorimetrically, using brucine. A composite sample of the eroded sediment from all r u n o f f plots was collected separately at the end of each season. These samples were ground and passed through a 2-mm sieve. The samples were analysed for organic carbon by wet combustion, for total nitrogen by Kjeldahl, for Bray-1 available phosphorus, and for a m m o n i u m acetate extractable cations, viz. Ca 2+, Mg2+ and K ÷ RESULTS AND DISCUSSION R u n o f f losses from the unmulched plots during the first season were 48, 67, 42 and 57% of the total annual rainfall, respectively, for the 1, 5, 10 and 15% slopes. During the second season the r u n o f f losses were 52, 55, 34 and 32% for the 1, 5, 10 and 15% slopes, respectively. The mean annual r u n o f f was 54% from the 1% slopes, 63% from the 5% slope, 39% from 10% slope and 49% from the 15% slope. The annual soil losses on unmulched plots were 9, 134, 138 and 96 t / h a for the 1, 5, 10 and 15% slopes, respectively. Soil losses on the 10 and 15% slopes were affected considerably by a high concentration of gravel on the soil surface as a result of prior erosion. A similar effect of gravel mulch on r u n o f f and erosion has been reported (Chapman, 1937; Tsiang, 1948; Hide, 1954).
Mulch rate and runoff R u n o f f was significantly influenced by rate of straw mulch (Table I). Mean runoff losses during the first season were 55, 8, 3 and 2% of the total rainfall for the mulch rates of 0, 2, 4 and 6 t/ha, respectively. The r u n o f f from the no-till plot was 2% of the total rainfall. During the second season, the mean r u n o f f losses were 43, 16, 7 and 2% for mulch rates of 0, 2, 4 and 6 t/ha, respectively. R u n o f f from the no-till plot during the second season was only 4%. The data in Table I do not indicate any relationship between r u n o f f and slope. The average r u n o f f losses for all mulch rates during the first season were 12, 17, 12 and 14% for slope gradients of 1, 5, 10 and 15%, respectively. During the second season, the average r u n o f f losses were 16, 22, 14 and 16% of total rainfall for 1, 5, 10 and 15% slopes, respectively. For individual rainstorms the relationships between r u n o f f and slope for different mulch rates are shown in Table II. The correlation coefficients
380
TABLE I Effect of m u l c h rate on r u n o f f (ram) on different slopes* Slope (%)
Mulch rate (t/ha) 0
2
4
6
No-till
1 5 10 15
(a) F i r s t - s e a s o n r u n o f f ( m m ) ( A p r i l - - J u l y ) 283 6 4 0 6 346 61 10 7 9 219 46 21 12 15 294 47 20 12 14
1 5 10 15
(b) S e c o n d - s e a s o n r u n o f f ( m m ) ( A u g . - - N o v . ) 129 30 3 0 5 137 65 18 4 6 84 28 14 9 9 80 40 31 8 9
* R a i n f a l l first s e a s o n 5 1 0 r a m , s e c o n d s e a s o n 249 r a m .
relating runoff with slope are significant only for the high mulch rates. For storms exceeding 25 mm of rainfall, the slope exponents increased from 0.04 to 3.3 and the coefficients decreased from 59.2 to 0.001 with mulch rates increasing from 0 to 6 t/ha. T A B L E II S l o p e - - r u n o f f r e l a t i o n s h i p s for d i f f e r e n t m u l c h rates M u l c h rate (t/ha)
r
Equation
M e a n r u n o f f (%)
0 2 4 6 No-till
(a) R a i n s t o r m s > 25 m m --0.1 W = 5 9 . 2 S -°'°~ 0.4 W = 2 . 1 S °'66 0.76 W = 0 . 8 S ''°5 0.89 W = 0 . 0 0 1 S 3-3 0.57 W = 1 . 2 0 S °'29
55.7 6.7 2.7 0.6 1.9
0 2 4 6 No-till
(b) R a i n s t o r m s < 25 m m --0.27 W = 4 7 . 5 S -°'2~ 0.49 W = 1 . 5 S °'4~ 0.73 W = 0 . 8 S °'66 0.90 W = 0 . 0 0 2 S ~'°7 0.48 W = 1 . 6 2 S °'3°
33.0 3.4 2.3 0.4 2.6
W = ( R u n o f f / r a i n f a l l ) . 100.
381
Mulch rate and soil loss
The effects of mulch rate on soil loss during the first and second seasons of 1974 are shown in Figs. 1 and 2. Maximum soil losses during the first season were 110, 3.5, 0.5 and 0.3 t/ha for mulch rates of 0, 2, 4 and 6 t/ha, respectively (Fig. 1). Similar results were obtained during the second season (Fig. 2). Mean soil losses for rainstorms greater than 25 mm were 143, 16, 2 and 0.4 kg/ha per mm of rain for mulch rates of 0, 2, 4 and 6 t/ha, respectively. For rainstorms of less than 25 mm, the mean soil losses were 110, 2, 0.7 and 0.4 kg/ha per mm o f rain. The low mulch rate of 2 t/ha effectively prevented soil loss even from strong slopes. The analysis of soil loss from individual rainstorms based on the exponential relationships between slope and soil loss is given in Table III. For the rainstorms exceeding 25 mm, the most significant correlation for slope and soil loss was obtained for unmulched plots. Although the slope exponents did not
lOOO!
100 100 O
on'ha
Ton/ha 2 Tons/ha I0
10
o
xlX~X 2 Tons/ha
/
1.0
o -
x
0.1C
~
£u
-
NO-TiLLAGE
0
~ To~/ha
o
~
4 Tons/ha
6 Tons/ha
6 Tons/ha /n / t
0.1C /
0 001
~1,0
1~0 Slope(%)
J flO0
0.0"01
# 10
Slope(°/o)
i~ ~.NO -TILLAGE
, flO
Fig. 1. Effects of mulch rate on soil losses for different slopes during the first season, 1974 (log --log scale). Fig. 2. Effects o f mulch rate on soil losses for different slopes during the second season, 1974 (log--log scale).
382 TABLE III Slope--soil loss relationships for different mulch rates Mulch rate (t/ha)
r
Equation
Mean soil loss (kg ha -j mm -~ of rain)
0 2 4 6 No-till
(a) Rainstorms > 25 mm 0.81 E = 11.8S '-':~ 0.35 E = 0.5S °'~7 0.57 E = 0.07S ''°~ 0.46 E = 0.01 S ''° 0.35 E = 0.01 S '~'~
143.1 16.4 2.0 0.4 0.6
0 2 4 6 No-till
(b) Rainstorms < 25 mm 0.38 E = 11.2 S °'~ 0.32 E = 0.3S °'~ 0.41 E = 0.1S ~'-~' 0.49 E = 0.04 S °-~'2 0.12 E = 0.13S "-j4
100.5 2.2 0.7 0.4 0.4
E = Soil loss in kg ha- ' ram-1 of rain. c h a n g e significantly with m u l c h rate, the c o e f f i c i e n t s did. F o r r a i n s t o r m e x c e e d i n g 25 m m the c o e f f i c i e n t s were 11.8, 0.5, 0.07 and 0.01 for 0, 2, 4, and 6 t / h a o f m u l c h , respectively. Mulch
factor
Wischmeier ( 1 9 7 3 ) p r o p o s e d a m u l c h f a c t o r , the ratio o f soil and w a t e r loss with m u l c h to the c o r r e s p o n d i n g loss with n o m u l c h , as a f u n c t i o n o f the p e r c e n t a g e o f the surface c o v e r e d b y m u l c h . T h e m u l c h f a c t o r can be used to p r e d i c t soil loss in t h e universal soil-loss e q u a t i o n . The average m u l c h f a c t o r s for slopes ranging f r o m 1 t o 15% axe s h o w n in Fig. 3. The m u l c h factors decreased e x p o n e n t i a l l y with increasing m u l c h rate. Mulch rate--soil loss and m u l c h r a t e - - r u n o f f relationships f o r d i f f e r e n t slopes are s h o w n in Table IV. T h e e x p o n e n t s o f the m u l c h rates are negative, ranging b e t w e e n - - 0 . 3 and - - 0 . 7 d e p e n d i n g on slope a n d o t h e r characteristics. Changes in soil-erodibility
factor (K)
T h e time d e p e n d e n c e o f the soil-erodibitity f a c t o r (K), f r o m 1 9 7 2 t o 1 9 7 4 is s h o w n in Fig. 4. I m m e d i a t e l y a f t e r f o r e s t clearing the K f a c t o r was extremely low, n o t o n l y because o f the presence o f r o o t s and o t h e r organic m a t t e r , b u t also because o f the e x c e l l e n t soil s t r u c t u r e and p e r m e a b i l i t y c r e a t e d b y the a c t i o n o f e a r t h w o r m s and termites. T h e soil-erodibility f a c t o r r e a c h e d its m a x i m u m value t w o or three years a f t e r f o r e s t clearing. T h e s u b s e q u e n t decline o f the K f a c t o r m a y have been due t o preferential erosion o f fine
383
10
nr
First Season ] X---X Second Seas0 Runoff
(>---o Frost Season Soil LOSS o---o Second Season \\ \\
\ \
\\\
NNN
\\
X\\
O.olh
0.001
I
0
t
"r
2 4 Mulch r'ate (Tons/ha)
6
Fig. 3. M u l c h f a c t o r as a f f e c t e d b y m u l c h r a t e .
TABLE IV E f f e c t s o f m u l c h r a t e (M) o n r u n o f f (%) a n d s o i l l o s s ( k g h a -~ m m slopes S l o p e (%)
r
Equation
1 5 10 15
0.78 0.80 0.86 0.75
(a) Runoff W = 0.39 M W = 1.16 M W = 5.53 M W = 5.26 M
1 5 10 15
0.85 0.85 0.86 0.72
-°'7~ -°'3~ -°'27 -°'s~
(b) S o i l l o s s E E E E
M = M u l c h r a t e in t / h a .
= = = =
0.19 1.25 1.09 0.98
M -°'$4 M -°'7~ M -°'67 M -°'24
1 of rain) on different
384
t: 5 r
r !
c~-~o I "o Slope pk:t 5°/~ SIoDe pl(t A - ~ A 10% Slope r) ot D~---~j 1~-~°/~Slope Dlot
i
X----x
i
i'
o3I
/
i
~o.2 /
Xs
o °~l
x x
I 2 3 Time o f f e r forest cleQring ( y e o r s )
4
Fig. 4. Changes in soil-erodibility f a c t o r (K) w i t h t i m e a f t e r f o r e s t removal.
particles from the surface layer in the previous years. Consequently, higher concentrations of gravel on the surface acting as a mulch also contributed towards a lower K factor. A "gravel factor" may be necessary for making appropriate corrections. TABLE V Annual nutrient losses (kg/ha) in runoff water for different mulch r a t e s Mulch
(t/ha)
Slope gradient 1%: . . . . NO,--N PO,--P
K
Ca
0 2 4 6 No-till
13.0 0.6 0.2 T 0.3
16.2 0.7 0.2 T 0.3
20.7 0.3 0.2 T 0.4
rate
3.3 0.3 0.1 T 0.1
T = Less than 0.1 kg/ha.
.
.
.
Slope gradient 5%: . . . . . . Mg NO3--N PO4--P 2.4 0.2 0,1 T 0.1
12.1 3.1 0.7 0.3 0.5
3.6 1.0 0.3 0.1 0.1
. K 24.6 2.5 1.2 0.3 0.5
. Ca
Mg
25.0 5.5 1.4 0.3 0.6
3.3 1.3 0.2 0.1 0.2
385
Mulch rate and nutrient loss in runoff and eroded soil materials Annual n u t r i e n t losses in r u n o f f water are shown in Table V. The mean annual total n u t r i e n t losses for all slopes, obtained by adding the N, P, K, Ca and Mg losses, were 55, 10 and 4 and 2 kg/ha for mulch rates of 0, 2, 4 and 6 t/ha, respectively. The mean annual n u t r i e n t loss from the no-till t r e a t m e n t s was a b o u t 4 kg/ha. The m a x i m u m annual losses of plant nutrients from unmulched plots were a p p r o x i m a t e l y 15, 4, 25, 25 and 3 kg/ha of NO3--N, PO4--P, K, Ca and Mg, respectively. The total n u t r i e n t loss in r u n o f f decreased exponentially with increasing mulch rate. The loss of organic carbon in eroded soil materials during the first season ranged f r o m 200 to 1000 kg/ha for no mulch, from 16 to 100 kg/ha for the mulch rate of 2 t/ha, and f r o m 10 to 15 kg/ha for the mulch rate of 4 t/ha. There were negligible organic carbon losses f r o m the no-till plots and those that received 6 t / h a of mulch. The losses of different plant nutrients in eroded sediment are shown in Table VI. For the u n m u l c h e d t r e a t m e n t , the annual nut ri ent losses were 3 9 - 1 7 6 kg of N, 0.6--5.0 kg of inorganic P, 1.4--14 kg of K, 18--90 kg of Ca and 1--13 kg/ha of Mg. The n u t r i e n t losses in eroded soil materials for the mulch rates of 4 and 6 t / ha and for the no-till t r e a t m e n t were negligible. CONCLUSIONS (1) R u n o f f and soil loss decreased exponentially with increasing mulch rates, with e x p o n e n t s ranging f r om --0.3 to --0.7. A mulch rate of 2--4 t / ha effectively controlled erosion. (2) The exponential soil loss--slope relationship does n o t apply to mulched plots. (3) No-till tr eatm e nt s had minimal r u n o f f and soil loss and were as effective as a mulch rate of 6 t/ha.
Slope gradient 10%:
Slope gradient 15%:
NO3--N
PO4--P
K
Ca
Mg
NO3--N
PO4--P
K
Ca
Mg
8.5 1.5 0.9 0.5 0.6
2.2 0.9 0.3 0.1 0.2
13.9 3.3 0.9 0.9 1.3
14.3 3.4 2.1 0.7 2.1
2.4 0.6 0.6 0.2 0.3
15.1 1.8 1.1 0.6 0.7
3.2 0.8 0.4 0.2 0.2
17.7 4.2 2.7 1.1 2.5
14.1 5.2 3.0 1.1 1.2
3.1 0.9 0.5 0.3 0.4
T = Less t h a n 0.1 kg/ha.
0.6 T T T T
1.4 0.2 0.1 T T
17.6 1.8 0.1 T T
1.2 0.1 T T T
117.9 27.3 5.4 T T
4.2 0.4 0.1 T T
P 14.2 1.0 0.3 T T
K
38.5 4.5 0.3 T T
Mg
0 2 4 6 No-till
Ca
N
K
N
P
S l o p e g r a d i e n t 5%:
S l o p e g r a d i e n t 1%:
Mulch rate (t/ha) 90.2 10.9 2.5 T T
Ca 5.8 0.4 0.2 T T
Mg
A n n u a l n u t r i e n t losses ( k g / h a ) in e r o d e d soil m a t e r i a l s f o r d i f f e r e n t m u l c h r a t e s
T A B L E VI
76.3 16.3 12.1 T T
N 4.9 0.5 0.1 T T
P 13.6 1.3 0.6 T T
K
S l o p e g r a d i e n t 10%:
87.0 9.1 5.1 T T
Ca
8.5 0.9 0.4 T T
Mg
176.1 63.9 8.7 2.7 T
N
3.0 0.5 0.1 T T
P
14.4 3.3 0.5 0.2 T
K
S l o p e g r a d i e n t 15%:
112.1 36.3 5,5 1,9 T
Ca
12.9 2.8 0A 0,2 T
Mg
387
(4) Nutrient losses in r u n o f f water were significant only for the unmulched treatments. A considerable loss of both applied and native plant nutrients occurs through r u n o f f and soil loss. REFERENCES Adams, J.E., 1966. Influence of mulches on runoff, erosion and soil moisture depletion. Proc. Soil Sci. Soc. Am., 30: 110--114. Barnett, A.P., Disker, E.G. and Richardson, E.C., 1967. Evaluation of mulching methods for erosion control on newly prepared and seeded highway back slope. Agron. J., 59: 83--85. Borst, H.L. and Woodburn, R., 1942a. The effect of mulching and methods of cultivation on runoff and erosion from Muskingwan silt loam. Agric. Eng., 23: 19--22. Borst, H.L. and Woodburn, R., 1942b. Effect of mulches and surface conditions on the water relations and erosion of Muskingum soil. USDA Tech. Bull., 825, Washington, D.C., 76 pp. Chapman, B.B., 1937. Climate. In: J.L. Buce (Editor), Land Utilization in China. University of Chicago Press, Chicago, Ill. Duley, F.L. and Kelly, L.L., 1939. Effect of soil type, slope and surface conditions on intake of water. Nebraska Agric. Stn. Res. Bull., 112, 16 pp. Greb, B.W., Smika, D.E. and Black, A.L., 1970. Water conservation with stubble mulch fallow. J. Soil Water Conserv., 25: 58--62. Harrold, L.L., 1972. Soil erosion by water as affected by reduced tillage systems. Proc. No-tillage Systems Symp., Feb. 21--22, 1972, Center for Tomorrow, Columbus, Ohio. Harrold, L.L., Triplett, G.B. and Edwards, W.H., 1974. No-tillage corn, characteristics of the system. Trans. Am. Soc. Agric. Eng., 51(3): 128--131. Harrold, L.L., Triplett, G.B. and Youker, R.E., 1967. Watershed tests of no-tillage corn. J. Soil Water Conserv., 22: 98--100. Hide, J.C., 1954. Observations on factors influencing the evaporation of soil moisture. Proc. Soil Sci. Soc. Am., 18: 234--239. Jacks, G.V., Baird, V.D. and Smith, R., 1955. Mulching Commonwealth Bur. Soil Sci. Tech. Comm., 49. Jung, L., 1960. The influence of the stone cover on runoff and erosion on slate soil. Int. Ass. Sci. Hydrol. Publ., 53: 143--153. Lal, R., 1976. Soil erosion on Alfisols in Western Nigeria. 1. Effects of slope, crop rotation and residue management. Geoderma, 16: 363--375. McCalla, T.M., Army, T.J. and Whitfield, C.J., 1962. Stubble--mulch farming. J. Soil Water Conserv., 17 : 204--208. Meyer, L.D. and Mannering, J.V., 1967. Tillage and land modification for erosion control. Conf. Proc. Tillage for Greater Crop Production: ASA, 87: 58--62. Meyer, L.D., Wischmeier, W.H. and Forster, G.R., 1970. Mulch rates required for erosion control on steep slopes. Proc. Soil Sci. Soc. Am., 34: 928--931. Meyer, L.D., Johnson, C.B. and Foster, G.R., 1972. Stone and woodchip mulches for erosion control on construction sites. J. Soil Water Conserv., 27(6): 264--272. Moldenhauer, W.C. and Amemiya, M., 1969. Tillage practices for controlling crop land erosion. J. Soil Water Conserv., 24: 1 9 - 2 1 . Swanson, N.P., Dedrick, A.R., Weekly, H.E. and Haise, H.R., 1965. Comparing mulches -Scientists check effects of four mulching materials on 6% slope. Agric. Res., 13(8): 15. Tsiang, T.C., 1948. Soil conservation, an international study. Ford. Agric. Organ., U.N.P.: 83--84. Unger, P.W., Allen, R.R. and Wiesc, A.F., 1971. Tillage and herbicide for surface residue maintenance, weed control, and water conservation. J. Soil Water Conserv., 26: 147--150. Wischmeier, W.H., 1973. Conservation tillage to control water erosion. Proc. Nat. Conf. Conservation Tillage. Soil Conservation Soc. Am., Ames, Iowa, 1973: 133--140.