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The effects of soil organic matter content, rainfall duration and aggregate size on soil detachment
SOIL T E C H N O L O G Y
vol. 4, p. 197-207
Cremlingen 1991 I
THE EFFECTS OF SOIL O R G A N I C MATTER C O N T E N T , RAINFALL D U R A T I O N A N D A G G R E G A T E SIZE O N SOIL D E T A C H M E N T E.I. Ekwue, Maiduguri Summary Soil detachment was measured using a factorial experiment involving five soils (with organic matter contents ranging from 1.23 to 5.64%), three rainfall durations (4, 12 and 20 mins) and two aggregate sizes (<2 mm and 2-5 mm). Detachment was described in terms of the direct effects and the first and second order interactions of these variables above. There were significant differences (P<0.01) in the amount of soil detached between the study soils. The mean values of detachment generally declined with increasing soil organic matter content from 1.15 kg m -2 in the G1 soil to 0.51 kg m -2 in the G5 soil. For each soil, there were significant increases (P<0.01) in detachment with increasing rainfall duration. Soil detachment was smaller for the larger aggregate size. The most significant interactions affecting soil detachment were between rainfall duration and aggregate size, organic matter content and aggregate size, and between organic content and rainfall duration in that order. These inISSN 0933-3630 @1991 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0933-3630/91/5011851/US$ 2.00 + 0.25
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teractions were used to make inferences on the effect of organic matter on soil detachment. For each soil and aggregate size, power relationships were established to relate detachment to rainfall total kinetic energy and organic matter content.
1
Introduction
Soil erosion process comprises two phases: detachment and transport by raindrop and runoff (Ellison 1947). Soil detachment is the first step in the erosion process. Although the potential ability of organic matter to reduce soil erodibility is well known (Luk 1979), few studies have examined its influence specifically on soil detachment. Previous research has concentrated on the effect of organic matter on soil aggregate stability and how stable aggregates affect detachment (Chaney & Swift 1984, Bryan 1968). Verhagen (1984) obtained a low correlation coefficient (r = -0.26) between soil detachment and organic matter content and attributed it to the low range of organic content (1.11 to 4.87%) in the soils he studied. Chandra & De (1978) working with Indian soils observed a significant correlation coefficient (r = -0.68) de-
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spite the narrow range of organic matter content (0.20 to 074%) in their soils. Luk (1979) also noted good relationships between soil detachment and organic matter content for some of his study soils from Canada. Ekwue (1990) observed that detachment reduced exponentially with organic matter content within a wide range (0 to 18%). However, these studies are single factor experiments involving the effect of organic matter on soil detachment. These studies are limited since they are rarely valid outside the observed range of values and prevailing environmental conditions (Quansah 1981) and do not take account of interactions between such factors as rainfall duration, soil slope, aggregate size and soil type. On the other hand, interaction experiments permit a greater number of factors and levels to be studied in the same experiment. This paper investigates the effects of soil organic matter content, aggregate size, rainfall duration and their interactions on soil detachment by rainfall. The aim was to increase the understanding of the effect of organic matter content on soil detachment.
2
Materials and methods
2.1
Soil
The study soils were collected from various locations in the long term organic manuring field plots of Rothamsted Experimental Station at Woburn, Bedfordshire, United Kingdom. The soils were derived from Lower Greensand. They are coded GI to G5 in increasing order of organic matter content (tab. 1). The differences in organic matter depended on the number of years the soils were under grass {tab. 1). Apart from G5 soil which is of sandy clay loam texture, the rest are sandy loams. The details of the soils have been described by Ekwue (1987).
2.2
Aggregate size
The soil samples from the five soils were air-dried and separate portions sieved through 2 mm and 5 mm openings to obtain the test samples. The aggregate size <2 mm was chosen in order to examine the effect of organic matter on soils with small structural elements. The 2-5 mm aggregate size was used to investigate the effect of organic matter on medium-size soil aggregates. Larger aggregate sizes were not considered since they are not very erodible (Luk 1979) and this may have disguised the possible organic matter effects.
The experimental investigation involved a factorial laboratory experiment to assess soil detachment of five soils with or2.3 Simulated rainfall ganic matter contents ranging from 1.23 to 5.64%, each with two aggregate sizes A rotating disc simulator (Morin et al. (<2 mm and 2-5 mm) and exposed to 1967) was used to generate simulated 4, 12, and 20 minutes rainfall durations. rainfall with an intensity of 81 mm hr -1, Each treatment combination was repli- kinetic energy of unit rain of 25.88 J m 2 cated three times. A completely ran- per mm and a median drop diameter of domised design was adopted. 2.60 ram. Natural rainfall of that intensity has, by comparison, a kinetic energy of 28.29 J m -2 per mm (Wischmeier & Smith 1978).
S{)I[~ 'IE('HNOL(~(P£
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Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates
Soil Code
I[
Soil History*
Organic Matter Content
Water Stable Aggregates 0.25 m m
(%)
(%)
199
Particle Size Distribution (%) Silt Sand Clay (2-0.06) (<0.002) (0.06~.002) rnnl mm mm
G1
Intensive cereal up to 1982. Has been under gtrass for at least 2 years.
1.23+0.03"*
7.09__+0.45
70.8+1.2
19.1_+0.4
10.1+0.5
G2
Same as for G1 above.
1.37+__0.06
10.41+1".89
65.8+0.5
22.6__.0.3
11.6+0.3
G3
Continuous arable up to 1971. Has been under grass since.
2.09+0.05
15.59+0.82
72.3+0.9
16.5±0.2
11.2_+0.5
G4
Permanent grass up to 1981. Has been under arable since.
5.04+0.08
23.33+0.99
68.5+0.9
17.3___0.2
14.2___0.6
G5
Permanent grass since living memory.
5.64__+0.21
24.32+1.16
62.3___1.0
12.8±0.4
24.9_+0.9
* Soil samples were collected in September, 1986. • * Mean of 3 replicates + standard deviation.
Tab. 1" History, organic matter content, water stable aggregates and the particle size distribution of the study soils. The samples with <2 mm aggregate size were tested at 7 rainfall durations (2, 4, 6, 8, 12, 16 and 20 mins). Those for the 2-5 mm size were tested at only 3 durations (4, 12 and 20 mins) due to small increases of detachment values with rainfall. The two aggregate sizes were, therefore, compared at the three common rainfall durations of 4, 12 and 20 minutes. 2.4
Measurement of soil detachment and other parameters
Soil detachment was measured using splash cups, 73 mm diameter, 50 mm deep similar to those used by Ellison (1947). Sieved test soil samples were packed into the cups, oven-dried for 48 hours at 105°C and weighed. Ovendrying was done to determine the dry weight of the soil. The samples were then saturated, left overnight on sand ta-
SOIL TECHNOLOGY A cooperating Journal of CATENA
bles at 10 cm suction before exposing them to the design storm. After rainfall, the samples were again oven-dried and weighed. The values of soil detachment reported represent the dry weight of soil thrown out from the splash cups during rainfall. Organic matter content was determined by the Walkley-Black method. Mechanical analysis was performed using the pipette method. The proportions of water stable aggregates (WSA) were measured using the method of Low (1954). The WSA>0.25 mm were retained and sand correction was applied with the formula proposed by Kemper & Koch (1966). Sand correction was done to remove the effect of sand on WSA.
Ekwue
200
Soil Codes
2 (69.62)"
4 (138.98)
Rainfall Duration (mins) 6 8 (208.59) (277.95)
12 (417.19)
16 (556.32)
20 (695.40)
G1
A** B
0.54_+0.01 "*°
0.77+-0.04 0.24+_0.02
1.09+_0.11
1.43 +_0.06
1.84+0.06 0.50_+0.05
2.22_+0.07
2.78__+0.08 0.74+0.05
G2
A B
0.43-+0.01
0.72_+0.02 0.28_+0.03
0.95+0.09
1.17 +0.09
1.5l+0.09 0.48+0.03
1.85+0.21
2.31 +0.26 0.66+0.08
G3
A B
0.32_+0.02
0.63_+0.02 0.26+0.06
0.87_+0.03
1.06+_0.07
1.27+0.06 0.32_+0.02
1.66+0.10
2.08_+0.13 0.52_+0.04
G4
A B
0.29_+0.01
0.48+0.04 0.17___0.01
0.65___0.02
0.81 __+0.04
1.07_+0.09 0.32_+0.02
t.24_+0.08
1.35_+0.09 0.45_+0.05
G5
A B
0.17+_0.01
0.31+0.03 0.16+0.02
0.43___0.05
0.55+_0.03
0.74_+0.02 0.3l+0.01
1.04_+0.07
1.11_+0.06 0.44_+0.02
" Total rainfall kinetic energy (J m -2) values are shown in paranthesis. "" A and B refer to <2 m m a n s 2 5 mm aggregate sizes respectively. "* All values are means of 3 replicates + standard deviation.
T a b . 2"
Values of soil detachment (kg m -2) during simulated rainfall.
Factor level
Mean detachment (kg m -2)
Soil G1 G2 G3 G4 G5
t.15 0.99 0.85 0.64 0.51
Rainfall duration (mins) 4 12 20
0.40 c 0.83 b 1.24 a
Aggregate size (mm) < 2 2 5
1.26 a 0.39 b
" The values followed by dissimilar letters are significantly different at ! %.
a* b c d e
T a b . 3" The mean mass of soil detachment (n = 90). Mean values for each factor were obtained by averaging the experimental values over the levels of the other two factors.
SOIl, TECHNOLOGY
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Soil Detachment, Effects o f Organic Matter, Rain£aH and Aggregates
201
2.0
~LSD(S%) GI
i
1.5
G2 63
l.OG4 G5
"" t.r) 05
/2 Rainfall 3 3.1
Duration (/dins)
Results Factors influencing soil detachment
Tab. 2 details the values of soil detachment during rainfall for the <2 mm and 2-5 mm aggregate sizes. Tab. 3 shows the mean detachment for the main effects of soil organic matter content, rainfall duration and aggregate size. To obtain the values for the main effect of each factor, the mean values of that factor averaged over the levels of the other two factors were calculated from tab. 2. For example, the value of 1.15 kg m -2 for G1 soil represents the detachment value for this soil averaged over the three common durations (4, 12, and 20 mins) and two aggregate sizes. Results showed that mean soil detachment declined with both increasing or-
SOIL T E C H N O L O G Y - - A cooperating Journal of C A T E N A
e'o
Fig. 1: The effect o f the interaction between soil organic content and rainfall duration on detachment. (Soil detachment values are means averaged over the two aggregate sizes).
ganic matter content and aggregate size and increased with increasing rainfall duration. Analysis of variance of the results showed that the main effects of soil organic matter content, rainfall duration, aggregate size and their first and second order interactions significantly influenced the values of detachment. The main effect of aggregate size was highest followed by those of rainfall duration and soil organic matter content. The most significant interactions were between rainfall duration and aggregate size, organic content and aggregate size and between organic content and duration in that order. However, the second order interaction was relatively small compared with the main effects and the first order interactions and only the latter were therefore examined.
202
Ekwue
2-0-
I
1.5-
qa
1.o2 oj c~ ~2 2 m t ~
~, 0.5uq ?-S
I
5o/I Organic
!
hioffer
Confenf(% )
In considering these interactions, the approach of Steel & Torrie (1980) was followed, whereby treatment means were arranged in two-way tables and sufficiently large responses (larger than the computed LSD for each interaction) of the factors to increasing levels of each other were reported. The data from these two-way tables were used to plot fig. 1 to 3. Single LSD values were used for each interaction because according to Gomez & Gomez (1984), unlike the split plot design, for the completely randomised design adopted in the present study, the test criterion for the row factor is the same as that of the column factor in two-way tables. The effect of rainfall duration showed significant (P<0.01) increases in mean detachment for all the soils as rainfall duration increased but the increases were
mrq
Fig. 2: The effect o f the interaction between soil organic content and aggregate size on detachment. (Soil detachment values are means averaged over 4, 12 and 20 rainfall durations).
greater for soils with lower organic content than for soils with higher contents (fig. i). The examination of differences in detachment values between soils at each rainfall duration showed that at 4 mins duration, the differences between values of G1, G2 and G3 soils and between those of G4 and G5 soils were not significant at 5% level. For the 12 mins rainfall duration, all the values were significantly different at 5% level except those of G3 and G4 soils and at 20 mins duration, all values were now significantly different. These results suggest that the effect of organic matter content on detachment increases with rainfall duration. Examination of the interaction between soil organic matter and aggregate size (fig. 2) showed that mean soil detachment reduced significantly (P<0.01) with increase in aggregate size for all
Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates
203
I LSD(5% ) 2.0"
.~2 fnrn
l.s.
1.0 121 flj
2-Smm
~0.5
fe Rainfall
z'o
Duration (Mins)
the soils with different organic contents. The reductions were greater in soils with lower organic matter contents. The effect of organic content at each aggregate size showed that for the <2 mm size, detachment declined significantly (P<0.01) with organic matter contents (from G1 to G5 soils). However, for the 2-5 mm aggregate size, though the detachment values also declined with organic contents, the differences between the values of soils G1 and G2 and between those of G3, G4 and G5 soils were not significantly different at 5% level. The results therefore suggest that the effect of soil organic matter in reducing raindrop detachment decreases with increasing size of soil aggregates. The mean soil detachment values de-
SOIL T E C H N O L O G Y - - A cooperating Journal of CATENA
Fig. 3: The effect of the interaction of rainfall duration and aggregate size on detachment. (Soil detachment values are means averaged over the five soils).
clined significantly (P<0.01) with aggregate size at all levels of rainfall duration (fig. 3). Furthermore, the values increased significantly with rainfall duration for both aggregate sizes. The detachment increases for the 2-5 mm sized samples were less than those for the <2 mm size showing that soils with large aggregates are less susceptible to detachment increases with increasing rainfall duration than those with smaller aggregates.
3.2
Soil detachment relationships
Regression analysis was used to illustrate the relationships between soil detachment and total rainfall kinetic energy for the <2 mm and 2-5 mm aggregate sizes in all the soils (tab. 4). Also, for each aggregate size, combined data for
Ekwue
204
Soil code
i
a
~
C
r
GI
A* B
0.024 0.009
0.70 0.68
G2
A B
0,021 0,017
0.71 0.56
G3
A B
0.013 0.031
0.77 0.41
G4
A B
0.010 0.009
O.78 0.60
G5
A B
0.005 0.008
0.83 0.6l
, --
0.991 0,993
All soils
A B
0,023 0.018
0.74 0.56
-0.49 -0.32
0.983 0,972
---
0.993** 0.964 0.988 0.938
--
0.992 0.870 O,989 0.977
" A a n d B refer to < 2 a n d 2 - 5 mm aggregate sizes respectively. *" All correlation coefficients (r) are significant at 1 per cent.
[I I]
Tab. 4: Values o f constants in power equations relating soil detachment ( D ) to total rainfall kinetic energy ( E ) and organic matter content ( M ) in the form: D = aEbM c.
all the soils were used to derive multiple regression equations relating soil detachment to total rainfall kinetic energy and organic matter content. Total kinetic energy rather than duration of rainfall was used in order to ease comparison of results with previous research. Total rainfall energy values (shown in tab. 2) were obtained by multiplying the reported unit rainfall kinetic energy of 25.88 J m -2 per mm by the total depths of rainfall (ram) corresponding to the various rainfall durations. The multiple regression equations were of the form: D = aEbM ~ where D E
= Soil detachment (kg rn 2)
M
= Organic matter content (%)
= Total rainfall kinetic energy (J m 2)
= Empirically derived constants.
The equations are generally of the power form which best fitted the experimental data. It is also the form usually adopted in soil detachment research (Bubenzer & Jones 1971, Quansah 1981) Highly significant correlation coefficients (P<0.01) were obtained between soil detachment and total rainfall energy, For each soil, and for all the soils combined, the values of the total energy exponents and the intercept values in the <2 mm aggregate size were higher than those for the 2 5 mm size. The absolute value of the organic matter exponent was also higher for the <2 mm aggregate size. For the <2 mm aggregate size, the total exponent increased slightly with organic matter content from 0.70 in G1 to 0.83 in the G5 soil. The intercept values reduced with organic matter content from 0.024 in G1 to 0.005 in the G5 soil. The values suggest that the total energy SOiLI'ECHNOLOGY Acooperating Journal ofCA'I'ENA
Soil Detachment, Effects of Organic Matter, Rainfall and Aggregates
exponents as well as the intercept values may be affected by organic matter content but there were, however, no such trends for the 2-5 mm aggregate size. For each of the two aggregate sizes, the Student's 't' values showed that soil detachment was more affected by the total rainfall kinetic energy than by organic matter content.
4 4.1
Discussion Factors influencing soil detachment
The results of the experiments presented here demonstrate that the effects of soil organic matter content, rainfall duration, aggregate size and their interactions were highly significant.
205
matter content and aggregate size shows that organic matter is most effective in soils with little aggregation and least effective in highly aggregated soils. This is because the large aggregates unlike the small ones resist breakdown and detachment mainly due to their size, rather than as a result of their organic matter contents. Soils may therefore be expected to have low detachment if they have high organic matter content or are well aggregated. These results suggest that the use of organic matter to reduce soil erodibility may be particularly beneficial in tropical areas with high duration rainfalls and poorly aggregated soils.
4.1.2
Rainfall duration
The increase in soil detachment with increasing rainfall duration agree with the Results showed that soil organic mat- observations of Yamamoto & Anderson ter reduced values of soil detachment. (1973) and Jennings et al. (1987) and These reductions may be attributed to are presumably due to increases in agthe ability of soil organic matter (in this gregate breakdown as rainfall duration case originating from grass) to increase increased. The magnitude of the increase the stability of soil aggregates. Tab. 1 in detachment of soils with rainfall durashows that the percentage water stable tion was, however, reduced by increasing aggregates increased with soil organic organic content and soil aggregate size matter content from G1 to G5. Stable as the interactions between soil organic aggregates resist detachment by utilising matter and rainfall duration as well as, part of the rainfall energy, which would between rainfall duration and aggregate otherwise cause soil detachment, in dis- size showed. These findings have not persing the aggregates before apprecia- been shown in earlier studies of soil deble splash can take place (Mazurak & tachment. Mosher 1970). The interaction between soil organic 4.1.3 Aggregate size matter content and rainfall duration shows that as the latter increases, soil The significant reductions of soil detach: detachment increases more rapidly in ment with increasing aggregate size consoils with lower organic matter contents. firms the findings of Ellison (1947) and Quansah (198i) observed similar inter- Mazurak & Mosher (1970) and are preaction between soil and rainfall inten- sumably due to the need for large aggresity. The interaction between soil organic gates to break down before appreciable
4.1.1
Soil organic matter content
SOIL TECHNOLOGY--A cooperating Journal of CATENA
206
Ekwue
detachment takes place.
4.2
Soil detachment relationships
The range of values (0.70 to 0.83) of the experiments for the soil-detachmentrainfall total kinetic energy relatiosnhips for the <2 mm aggregate size are comparable with the range of values of 0.55 to 0.84 recorded by Morgan (1981), Quansah (1981) and Morgan et al. (1987) for soils with high sand contents similar to those adopted in this study. The values obtained in the present study are, however, lower than the 0.9 and 1.13 values obtained by Free (1960) and Bubenzer & Jones (1971) respectively for sandy soils. This may be due to the coarse nature of the study soils as soil coarseness may be expected to lower kinetic energy exponents (Morgan et al. 1987). This is particularly applicable to the soils with 2-5 mm aggregates where the values of the exponent ranged from 0.41 to 0.68. The 0.007 intercept value obtained by Quansah (1981) for a sandy soil in a laboratory experiment falls within the range of 0.005 to 0.024 values obtained in the present study. The 0.021 value recorded by Morgan et al. (1987) for a sandy soil with 2 to 3% organic content in fields tests compared well with the 0.024 and 0.021 intercept values obtained for the G1 and G2 soils (<2 mm agregate size). The values of exponent and the intercept for the <2 mm aggregate size were found to be higher than those for the 2-5 mm aggregate size. This confirms the findings in the analysis of variance that large aggregates are less susceptible to detachment increases with increasing duration or total energy of rainfall. The higher absolute value of the organic matter exponent for the <2 mm aggregate size also confirm that the effect of or-
ganic matter on detachment reduces with increasing soil aggregate size. For the <2 mm aggregate size, higher exponent values for soils with higher organic matter contents indicate that for an increase in rainfall duration, the proportionate increase in detachment is greater in these soils than in those with lower organic contents. This may be due to the greater soil cohesiveness measured during rainfall for the latter soils (Ekwue 1987). Soil cohesiveness reduces the response of soils to rainfall parameters (Meyer 1981). The lower intercept values in the soils with higher organic matter contents, however, ensured that they always had lower detachment values than those with lower contents.
5
Conclusions (i) Soil detachment reduces significantly with increasing soil organic matter content and aggregate size. The effect of soil organic matter in reducing detachment increases with increasing rainfall duration and decreases with increasing aggregate size. (ii) Although raindrop detachment increases with rainfall duration, the magnitude of increase is reduced by increasing organic matter content and aggregate size of soils. (iii) The exponent and intercept values of the equations relating soil detachment to rainfall duration are affected by organic matter content. The effect of organic matter in reducing the intercept values is greater than its effect on the exponent.
Acknowledgements The author is very grateful to Professor R.P.C. Morgan for his advice and reading
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A c~wJpcraltng Journa] of CArl t'~NA
Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates
207
the manuscript. H e is a l s o g r a t e f u l t o Mr. A.J. J o h n s t o n f o r h i s a d v i c e a n d
MEYER, L.D. (1981): How rain intensity affects interrill erosion. Trans. ASAE 24, 1472-1475.
to Rothamsted Experimental a c c e s s to t h e field p l o t s .
MORGAN, R.P.C. (1981): Field measurements of splash erosion. In: Sedimentation and Erosion Transport Measurements. Proceedings of the Florence Symposium, International Association of Scientific Hydrology, Publ. No. 133, 373-382.
Station for
References
BRYAN, R.B. (1968): The development, use and efficiency of indices of soil erodibility. Geoderma 2, 5-26. BUBENZER, G.D. & JONES, B.A. (1971): Drop size and impact velocity effects on the detachment of soils under simulated rainfall. Trans ASAE 14, 625-628. CHANDRA, S. & DE, S.K. (1978): A simple laboratory apparatus to measure relative erodibility of soils. Soil Sci. 125, 115-121. CHANEY, K. & SWIFT, R.S. (1984): The influence of organic matter on aggregate stability in some British soils. J. Soil Sc. 35, 223-230. EKWUE, E.I. (1987): The influence of organic matter on the erodibility of non-cohesive soils. Ph.D. Thesis, Cranfield Institute of Technology, England.
MORGAN, R.P.C., MARTIN, L. & NOBLE, C.A. (1987): Soil erosion in the United Kingdom: a case study from mid-Bedfordshire. Occasional Paper No. 14, Silsoe College, Bedford, England. MORIN, J., GOLDBERG, D. & SEGINER, I. (1967): A rainfall simulator with a rotating disc. Trans ASAE 10, 74-77, 79. QUANSAH, C. (1981): The effect of soil type, slope, rain intensity and their interactions on splash detachment and transport. J. Soil Sci. 32, 215-224. STEEL, R.G.D. & TORRIE, J.H. (1980): Principles and Procedures of Statistics: A Biometrical Approach, 2nd. Ed., McGraw-Hill New York, 341 pp.
EKWUE, E.I. (1990): Effect of organic matter on splash detachment and the processes involved. Earth Surface Processes 15, 175-181.
VERHAEGEN, TH (1984): The influence of properties on the erodibility of Belgian loamy soits. Earth Surface Processes 9, 499-507.
ELLISON, W.D. (1947): Soil erosion studies, Part II. Agric. Eng. 28, 197-201.
WISCHMEIER, W.H. & SMITH, D.D. (1978): Predicting rainfall erosion losses from cropland east of Rocky mountains. Agric. Handbook 537, USDA.
FREE, G.R. (1960): Erosion characteristics of rainfall. Agric, Eng. 41, 447-449, 455. G O M E Z , K.A. & G O M E Z , A.A. (1984): Statistical Procedures for Agricultural Research. 2nd. Ed., Wiley, New York, 607 pp.
YAMAMOTO, T. & ANDERSON, H.W. (1973): Splash erosion related to soil erodibility and other forest soil properties in Hawaii. Water Resources 9, 336--345.
JENNINGS, G.D., JARRETTE, A.R. & HOOVER, J.R. (1987): Simulated rainfall duration and sequencing affect soil loss. Trans. ASAE 30, 158-161, 165. KEMPER, W.D. & KOCH, E.J. (1966): Aggregate stability of soils from Western United States and Canada. ARS, USDA Tech. Bull. 1355. LOW, A.J. (1954): The study of soil structure in the field and the laboratory. J. Soil Sci. 5, 57-74. LUK, S.H. (1979): Effect of soil properties on erosion by wash and splash. Earth Surface Processes 4, 241-255. MAZURAK, A.P. & MOSHER, P.N. (1970): Detachment of soil aggregates by simulated rainfall. Soil Sci. Soc. Am. Proc. 34, 798800.
SOIL TECHNOLOGY
A cooperating Journal of CATENA
Address of author: E.I. Ekwue Department of Agricultural Engineering University of Maiduguri Maiduguri Nigeria
vol. 4, p. 197-207
Cremlingen 1991 I
THE EFFECTS OF SOIL O R G A N I C MATTER C O N T E N T , RAINFALL D U R A T I O N A N D A G G R E G A T E SIZE O N SOIL D E T A C H M E N T E.I. Ekwue, Maiduguri Summary Soil detachment was measured using a factorial experiment involving five soils (with organic matter contents ranging from 1.23 to 5.64%), three rainfall durations (4, 12 and 20 mins) and two aggregate sizes (<2 mm and 2-5 mm). Detachment was described in terms of the direct effects and the first and second order interactions of these variables above. There were significant differences (P<0.01) in the amount of soil detached between the study soils. The mean values of detachment generally declined with increasing soil organic matter content from 1.15 kg m -2 in the G1 soil to 0.51 kg m -2 in the G5 soil. For each soil, there were significant increases (P<0.01) in detachment with increasing rainfall duration. Soil detachment was smaller for the larger aggregate size. The most significant interactions affecting soil detachment were between rainfall duration and aggregate size, organic matter content and aggregate size, and between organic content and rainfall duration in that order. These inISSN 0933-3630 @1991 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0933-3630/91/5011851/US$ 2.00 + 0.25
SOl L T E C H N O L O G Y - - A cooperating Journal o f C A T E N A
teractions were used to make inferences on the effect of organic matter on soil detachment. For each soil and aggregate size, power relationships were established to relate detachment to rainfall total kinetic energy and organic matter content.
1
Introduction
Soil erosion process comprises two phases: detachment and transport by raindrop and runoff (Ellison 1947). Soil detachment is the first step in the erosion process. Although the potential ability of organic matter to reduce soil erodibility is well known (Luk 1979), few studies have examined its influence specifically on soil detachment. Previous research has concentrated on the effect of organic matter on soil aggregate stability and how stable aggregates affect detachment (Chaney & Swift 1984, Bryan 1968). Verhagen (1984) obtained a low correlation coefficient (r = -0.26) between soil detachment and organic matter content and attributed it to the low range of organic content (1.11 to 4.87%) in the soils he studied. Chandra & De (1978) working with Indian soils observed a significant correlation coefficient (r = -0.68) de-
198
Ekwue
spite the narrow range of organic matter content (0.20 to 074%) in their soils. Luk (1979) also noted good relationships between soil detachment and organic matter content for some of his study soils from Canada. Ekwue (1990) observed that detachment reduced exponentially with organic matter content within a wide range (0 to 18%). However, these studies are single factor experiments involving the effect of organic matter on soil detachment. These studies are limited since they are rarely valid outside the observed range of values and prevailing environmental conditions (Quansah 1981) and do not take account of interactions between such factors as rainfall duration, soil slope, aggregate size and soil type. On the other hand, interaction experiments permit a greater number of factors and levels to be studied in the same experiment. This paper investigates the effects of soil organic matter content, aggregate size, rainfall duration and their interactions on soil detachment by rainfall. The aim was to increase the understanding of the effect of organic matter content on soil detachment.
2
Materials and methods
2.1
Soil
The study soils were collected from various locations in the long term organic manuring field plots of Rothamsted Experimental Station at Woburn, Bedfordshire, United Kingdom. The soils were derived from Lower Greensand. They are coded GI to G5 in increasing order of organic matter content (tab. 1). The differences in organic matter depended on the number of years the soils were under grass {tab. 1). Apart from G5 soil which is of sandy clay loam texture, the rest are sandy loams. The details of the soils have been described by Ekwue (1987).
2.2
Aggregate size
The soil samples from the five soils were air-dried and separate portions sieved through 2 mm and 5 mm openings to obtain the test samples. The aggregate size <2 mm was chosen in order to examine the effect of organic matter on soils with small structural elements. The 2-5 mm aggregate size was used to investigate the effect of organic matter on medium-size soil aggregates. Larger aggregate sizes were not considered since they are not very erodible (Luk 1979) and this may have disguised the possible organic matter effects.
The experimental investigation involved a factorial laboratory experiment to assess soil detachment of five soils with or2.3 Simulated rainfall ganic matter contents ranging from 1.23 to 5.64%, each with two aggregate sizes A rotating disc simulator (Morin et al. (<2 mm and 2-5 mm) and exposed to 1967) was used to generate simulated 4, 12, and 20 minutes rainfall durations. rainfall with an intensity of 81 mm hr -1, Each treatment combination was repli- kinetic energy of unit rain of 25.88 J m 2 cated three times. A completely ran- per mm and a median drop diameter of domised design was adopted. 2.60 ram. Natural rainfall of that intensity has, by comparison, a kinetic energy of 28.29 J m -2 per mm (Wischmeier & Smith 1978).
S{)I[~ 'IE('HNOL(~(P£
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Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates
Soil Code
I[
Soil History*
Organic Matter Content
Water Stable Aggregates 0.25 m m
(%)
(%)
199
Particle Size Distribution (%) Silt Sand Clay (2-0.06) (<0.002) (0.06~.002) rnnl mm mm
G1
Intensive cereal up to 1982. Has been under gtrass for at least 2 years.
1.23+0.03"*
7.09__+0.45
70.8+1.2
19.1_+0.4
10.1+0.5
G2
Same as for G1 above.
1.37+__0.06
10.41+1".89
65.8+0.5
22.6__.0.3
11.6+0.3
G3
Continuous arable up to 1971. Has been under grass since.
2.09+0.05
15.59+0.82
72.3+0.9
16.5±0.2
11.2_+0.5
G4
Permanent grass up to 1981. Has been under arable since.
5.04+0.08
23.33+0.99
68.5+0.9
17.3___0.2
14.2___0.6
G5
Permanent grass since living memory.
5.64__+0.21
24.32+1.16
62.3___1.0
12.8±0.4
24.9_+0.9
* Soil samples were collected in September, 1986. • * Mean of 3 replicates + standard deviation.
Tab. 1" History, organic matter content, water stable aggregates and the particle size distribution of the study soils. The samples with <2 mm aggregate size were tested at 7 rainfall durations (2, 4, 6, 8, 12, 16 and 20 mins). Those for the 2-5 mm size were tested at only 3 durations (4, 12 and 20 mins) due to small increases of detachment values with rainfall. The two aggregate sizes were, therefore, compared at the three common rainfall durations of 4, 12 and 20 minutes. 2.4
Measurement of soil detachment and other parameters
Soil detachment was measured using splash cups, 73 mm diameter, 50 mm deep similar to those used by Ellison (1947). Sieved test soil samples were packed into the cups, oven-dried for 48 hours at 105°C and weighed. Ovendrying was done to determine the dry weight of the soil. The samples were then saturated, left overnight on sand ta-
SOIL TECHNOLOGY A cooperating Journal of CATENA
bles at 10 cm suction before exposing them to the design storm. After rainfall, the samples were again oven-dried and weighed. The values of soil detachment reported represent the dry weight of soil thrown out from the splash cups during rainfall. Organic matter content was determined by the Walkley-Black method. Mechanical analysis was performed using the pipette method. The proportions of water stable aggregates (WSA) were measured using the method of Low (1954). The WSA>0.25 mm were retained and sand correction was applied with the formula proposed by Kemper & Koch (1966). Sand correction was done to remove the effect of sand on WSA.
Ekwue
200
Soil Codes
2 (69.62)"
4 (138.98)
Rainfall Duration (mins) 6 8 (208.59) (277.95)
12 (417.19)
16 (556.32)
20 (695.40)
G1
A** B
0.54_+0.01 "*°
0.77+-0.04 0.24+_0.02
1.09+_0.11
1.43 +_0.06
1.84+0.06 0.50_+0.05
2.22_+0.07
2.78__+0.08 0.74+0.05
G2
A B
0.43-+0.01
0.72_+0.02 0.28_+0.03
0.95+0.09
1.17 +0.09
1.5l+0.09 0.48+0.03
1.85+0.21
2.31 +0.26 0.66+0.08
G3
A B
0.32_+0.02
0.63_+0.02 0.26+0.06
0.87_+0.03
1.06+_0.07
1.27+0.06 0.32_+0.02
1.66+0.10
2.08_+0.13 0.52_+0.04
G4
A B
0.29_+0.01
0.48+0.04 0.17___0.01
0.65___0.02
0.81 __+0.04
1.07_+0.09 0.32_+0.02
t.24_+0.08
1.35_+0.09 0.45_+0.05
G5
A B
0.17+_0.01
0.31+0.03 0.16+0.02
0.43___0.05
0.55+_0.03
0.74_+0.02 0.3l+0.01
1.04_+0.07
1.11_+0.06 0.44_+0.02
" Total rainfall kinetic energy (J m -2) values are shown in paranthesis. "" A and B refer to <2 m m a n s 2 5 mm aggregate sizes respectively. "* All values are means of 3 replicates + standard deviation.
T a b . 2"
Values of soil detachment (kg m -2) during simulated rainfall.
Factor level
Mean detachment (kg m -2)
Soil G1 G2 G3 G4 G5
t.15 0.99 0.85 0.64 0.51
Rainfall duration (mins) 4 12 20
0.40 c 0.83 b 1.24 a
Aggregate size (mm) < 2 2 5
1.26 a 0.39 b
" The values followed by dissimilar letters are significantly different at ! %.
a* b c d e
T a b . 3" The mean mass of soil detachment (n = 90). Mean values for each factor were obtained by averaging the experimental values over the levels of the other two factors.
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Soil Detachment, Effects o f Organic Matter, Rain£aH and Aggregates
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2.0
~LSD(S%) GI
i
1.5
G2 63
l.OG4 G5
"" t.r) 05
/2 Rainfall 3 3.1
Duration (/dins)
Results Factors influencing soil detachment
Tab. 2 details the values of soil detachment during rainfall for the <2 mm and 2-5 mm aggregate sizes. Tab. 3 shows the mean detachment for the main effects of soil organic matter content, rainfall duration and aggregate size. To obtain the values for the main effect of each factor, the mean values of that factor averaged over the levels of the other two factors were calculated from tab. 2. For example, the value of 1.15 kg m -2 for G1 soil represents the detachment value for this soil averaged over the three common durations (4, 12, and 20 mins) and two aggregate sizes. Results showed that mean soil detachment declined with both increasing or-
SOIL T E C H N O L O G Y - - A cooperating Journal of C A T E N A
e'o
Fig. 1: The effect o f the interaction between soil organic content and rainfall duration on detachment. (Soil detachment values are means averaged over the two aggregate sizes).
ganic matter content and aggregate size and increased with increasing rainfall duration. Analysis of variance of the results showed that the main effects of soil organic matter content, rainfall duration, aggregate size and their first and second order interactions significantly influenced the values of detachment. The main effect of aggregate size was highest followed by those of rainfall duration and soil organic matter content. The most significant interactions were between rainfall duration and aggregate size, organic content and aggregate size and between organic content and duration in that order. However, the second order interaction was relatively small compared with the main effects and the first order interactions and only the latter were therefore examined.
202
Ekwue
2-0-
I
1.5-
qa
1.o2 oj c~ ~2 2 m t ~
~, 0.5uq ?-S
I
5o/I Organic
!
hioffer
Confenf(% )
In considering these interactions, the approach of Steel & Torrie (1980) was followed, whereby treatment means were arranged in two-way tables and sufficiently large responses (larger than the computed LSD for each interaction) of the factors to increasing levels of each other were reported. The data from these two-way tables were used to plot fig. 1 to 3. Single LSD values were used for each interaction because according to Gomez & Gomez (1984), unlike the split plot design, for the completely randomised design adopted in the present study, the test criterion for the row factor is the same as that of the column factor in two-way tables. The effect of rainfall duration showed significant (P<0.01) increases in mean detachment for all the soils as rainfall duration increased but the increases were
mrq
Fig. 2: The effect o f the interaction between soil organic content and aggregate size on detachment. (Soil detachment values are means averaged over 4, 12 and 20 rainfall durations).
greater for soils with lower organic content than for soils with higher contents (fig. i). The examination of differences in detachment values between soils at each rainfall duration showed that at 4 mins duration, the differences between values of G1, G2 and G3 soils and between those of G4 and G5 soils were not significant at 5% level. For the 12 mins rainfall duration, all the values were significantly different at 5% level except those of G3 and G4 soils and at 20 mins duration, all values were now significantly different. These results suggest that the effect of organic matter content on detachment increases with rainfall duration. Examination of the interaction between soil organic matter and aggregate size (fig. 2) showed that mean soil detachment reduced significantly (P<0.01) with increase in aggregate size for all
Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates
203
I LSD(5% ) 2.0"
.~2 fnrn
l.s.
1.0 121 flj
2-Smm
~0.5
fe Rainfall
z'o
Duration (Mins)
the soils with different organic contents. The reductions were greater in soils with lower organic matter contents. The effect of organic content at each aggregate size showed that for the <2 mm size, detachment declined significantly (P<0.01) with organic matter contents (from G1 to G5 soils). However, for the 2-5 mm aggregate size, though the detachment values also declined with organic contents, the differences between the values of soils G1 and G2 and between those of G3, G4 and G5 soils were not significantly different at 5% level. The results therefore suggest that the effect of soil organic matter in reducing raindrop detachment decreases with increasing size of soil aggregates. The mean soil detachment values de-
SOIL T E C H N O L O G Y - - A cooperating Journal of CATENA
Fig. 3: The effect of the interaction of rainfall duration and aggregate size on detachment. (Soil detachment values are means averaged over the five soils).
clined significantly (P<0.01) with aggregate size at all levels of rainfall duration (fig. 3). Furthermore, the values increased significantly with rainfall duration for both aggregate sizes. The detachment increases for the 2-5 mm sized samples were less than those for the <2 mm size showing that soils with large aggregates are less susceptible to detachment increases with increasing rainfall duration than those with smaller aggregates.
3.2
Soil detachment relationships
Regression analysis was used to illustrate the relationships between soil detachment and total rainfall kinetic energy for the <2 mm and 2-5 mm aggregate sizes in all the soils (tab. 4). Also, for each aggregate size, combined data for
Ekwue
204
Soil code
i
a
~
C
r
GI
A* B
0.024 0.009
0.70 0.68
G2
A B
0,021 0,017
0.71 0.56
G3
A B
0.013 0.031
0.77 0.41
G4
A B
0.010 0.009
O.78 0.60
G5
A B
0.005 0.008
0.83 0.6l
, --
0.991 0,993
All soils
A B
0,023 0.018
0.74 0.56
-0.49 -0.32
0.983 0,972
---
0.993** 0.964 0.988 0.938
--
0.992 0.870 O,989 0.977
" A a n d B refer to < 2 a n d 2 - 5 mm aggregate sizes respectively. *" All correlation coefficients (r) are significant at 1 per cent.
[I I]
Tab. 4: Values o f constants in power equations relating soil detachment ( D ) to total rainfall kinetic energy ( E ) and organic matter content ( M ) in the form: D = aEbM c.
all the soils were used to derive multiple regression equations relating soil detachment to total rainfall kinetic energy and organic matter content. Total kinetic energy rather than duration of rainfall was used in order to ease comparison of results with previous research. Total rainfall energy values (shown in tab. 2) were obtained by multiplying the reported unit rainfall kinetic energy of 25.88 J m -2 per mm by the total depths of rainfall (ram) corresponding to the various rainfall durations. The multiple regression equations were of the form: D = aEbM ~ where D E
= Soil detachment (kg rn 2)
M
= Organic matter content (%)
= Total rainfall kinetic energy (J m 2)
= Empirically derived constants.
The equations are generally of the power form which best fitted the experimental data. It is also the form usually adopted in soil detachment research (Bubenzer & Jones 1971, Quansah 1981) Highly significant correlation coefficients (P<0.01) were obtained between soil detachment and total rainfall energy, For each soil, and for all the soils combined, the values of the total energy exponents and the intercept values in the <2 mm aggregate size were higher than those for the 2 5 mm size. The absolute value of the organic matter exponent was also higher for the <2 mm aggregate size. For the <2 mm aggregate size, the total exponent increased slightly with organic matter content from 0.70 in G1 to 0.83 in the G5 soil. The intercept values reduced with organic matter content from 0.024 in G1 to 0.005 in the G5 soil. The values suggest that the total energy SOiLI'ECHNOLOGY Acooperating Journal ofCA'I'ENA
Soil Detachment, Effects of Organic Matter, Rainfall and Aggregates
exponents as well as the intercept values may be affected by organic matter content but there were, however, no such trends for the 2-5 mm aggregate size. For each of the two aggregate sizes, the Student's 't' values showed that soil detachment was more affected by the total rainfall kinetic energy than by organic matter content.
4 4.1
Discussion Factors influencing soil detachment
The results of the experiments presented here demonstrate that the effects of soil organic matter content, rainfall duration, aggregate size and their interactions were highly significant.
205
matter content and aggregate size shows that organic matter is most effective in soils with little aggregation and least effective in highly aggregated soils. This is because the large aggregates unlike the small ones resist breakdown and detachment mainly due to their size, rather than as a result of their organic matter contents. Soils may therefore be expected to have low detachment if they have high organic matter content or are well aggregated. These results suggest that the use of organic matter to reduce soil erodibility may be particularly beneficial in tropical areas with high duration rainfalls and poorly aggregated soils.
4.1.2
Rainfall duration
The increase in soil detachment with increasing rainfall duration agree with the Results showed that soil organic mat- observations of Yamamoto & Anderson ter reduced values of soil detachment. (1973) and Jennings et al. (1987) and These reductions may be attributed to are presumably due to increases in agthe ability of soil organic matter (in this gregate breakdown as rainfall duration case originating from grass) to increase increased. The magnitude of the increase the stability of soil aggregates. Tab. 1 in detachment of soils with rainfall durashows that the percentage water stable tion was, however, reduced by increasing aggregates increased with soil organic organic content and soil aggregate size matter content from G1 to G5. Stable as the interactions between soil organic aggregates resist detachment by utilising matter and rainfall duration as well as, part of the rainfall energy, which would between rainfall duration and aggregate otherwise cause soil detachment, in dis- size showed. These findings have not persing the aggregates before apprecia- been shown in earlier studies of soil deble splash can take place (Mazurak & tachment. Mosher 1970). The interaction between soil organic 4.1.3 Aggregate size matter content and rainfall duration shows that as the latter increases, soil The significant reductions of soil detach: detachment increases more rapidly in ment with increasing aggregate size consoils with lower organic matter contents. firms the findings of Ellison (1947) and Quansah (198i) observed similar inter- Mazurak & Mosher (1970) and are preaction between soil and rainfall inten- sumably due to the need for large aggresity. The interaction between soil organic gates to break down before appreciable
4.1.1
Soil organic matter content
SOIL TECHNOLOGY--A cooperating Journal of CATENA
206
Ekwue
detachment takes place.
4.2
Soil detachment relationships
The range of values (0.70 to 0.83) of the experiments for the soil-detachmentrainfall total kinetic energy relatiosnhips for the <2 mm aggregate size are comparable with the range of values of 0.55 to 0.84 recorded by Morgan (1981), Quansah (1981) and Morgan et al. (1987) for soils with high sand contents similar to those adopted in this study. The values obtained in the present study are, however, lower than the 0.9 and 1.13 values obtained by Free (1960) and Bubenzer & Jones (1971) respectively for sandy soils. This may be due to the coarse nature of the study soils as soil coarseness may be expected to lower kinetic energy exponents (Morgan et al. 1987). This is particularly applicable to the soils with 2-5 mm aggregates where the values of the exponent ranged from 0.41 to 0.68. The 0.007 intercept value obtained by Quansah (1981) for a sandy soil in a laboratory experiment falls within the range of 0.005 to 0.024 values obtained in the present study. The 0.021 value recorded by Morgan et al. (1987) for a sandy soil with 2 to 3% organic content in fields tests compared well with the 0.024 and 0.021 intercept values obtained for the G1 and G2 soils (<2 mm agregate size). The values of exponent and the intercept for the <2 mm aggregate size were found to be higher than those for the 2-5 mm aggregate size. This confirms the findings in the analysis of variance that large aggregates are less susceptible to detachment increases with increasing duration or total energy of rainfall. The higher absolute value of the organic matter exponent for the <2 mm aggregate size also confirm that the effect of or-
ganic matter on detachment reduces with increasing soil aggregate size. For the <2 mm aggregate size, higher exponent values for soils with higher organic matter contents indicate that for an increase in rainfall duration, the proportionate increase in detachment is greater in these soils than in those with lower organic contents. This may be due to the greater soil cohesiveness measured during rainfall for the latter soils (Ekwue 1987). Soil cohesiveness reduces the response of soils to rainfall parameters (Meyer 1981). The lower intercept values in the soils with higher organic matter contents, however, ensured that they always had lower detachment values than those with lower contents.
5
Conclusions (i) Soil detachment reduces significantly with increasing soil organic matter content and aggregate size. The effect of soil organic matter in reducing detachment increases with increasing rainfall duration and decreases with increasing aggregate size. (ii) Although raindrop detachment increases with rainfall duration, the magnitude of increase is reduced by increasing organic matter content and aggregate size of soils. (iii) The exponent and intercept values of the equations relating soil detachment to rainfall duration are affected by organic matter content. The effect of organic matter in reducing the intercept values is greater than its effect on the exponent.
Acknowledgements The author is very grateful to Professor R.P.C. Morgan for his advice and reading
~,()[L r[ E(?|rINOL(.)f3"I r
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Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates
207
the manuscript. H e is a l s o g r a t e f u l t o Mr. A.J. J o h n s t o n f o r h i s a d v i c e a n d
MEYER, L.D. (1981): How rain intensity affects interrill erosion. Trans. ASAE 24, 1472-1475.
to Rothamsted Experimental a c c e s s to t h e field p l o t s .
MORGAN, R.P.C. (1981): Field measurements of splash erosion. In: Sedimentation and Erosion Transport Measurements. Proceedings of the Florence Symposium, International Association of Scientific Hydrology, Publ. No. 133, 373-382.
Station for
References
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MORGAN, R.P.C., MARTIN, L. & NOBLE, C.A. (1987): Soil erosion in the United Kingdom: a case study from mid-Bedfordshire. Occasional Paper No. 14, Silsoe College, Bedford, England. MORIN, J., GOLDBERG, D. & SEGINER, I. (1967): A rainfall simulator with a rotating disc. Trans ASAE 10, 74-77, 79. QUANSAH, C. (1981): The effect of soil type, slope, rain intensity and their interactions on splash detachment and transport. J. Soil Sci. 32, 215-224. STEEL, R.G.D. & TORRIE, J.H. (1980): Principles and Procedures of Statistics: A Biometrical Approach, 2nd. Ed., McGraw-Hill New York, 341 pp.
EKWUE, E.I. (1990): Effect of organic matter on splash detachment and the processes involved. Earth Surface Processes 15, 175-181.
VERHAEGEN, TH (1984): The influence of properties on the erodibility of Belgian loamy soits. Earth Surface Processes 9, 499-507.
ELLISON, W.D. (1947): Soil erosion studies, Part II. Agric. Eng. 28, 197-201.
WISCHMEIER, W.H. & SMITH, D.D. (1978): Predicting rainfall erosion losses from cropland east of Rocky mountains. Agric. Handbook 537, USDA.
FREE, G.R. (1960): Erosion characteristics of rainfall. Agric, Eng. 41, 447-449, 455. G O M E Z , K.A. & G O M E Z , A.A. (1984): Statistical Procedures for Agricultural Research. 2nd. Ed., Wiley, New York, 607 pp.
YAMAMOTO, T. & ANDERSON, H.W. (1973): Splash erosion related to soil erodibility and other forest soil properties in Hawaii. Water Resources 9, 336--345.
JENNINGS, G.D., JARRETTE, A.R. & HOOVER, J.R. (1987): Simulated rainfall duration and sequencing affect soil loss. Trans. ASAE 30, 158-161, 165. KEMPER, W.D. & KOCH, E.J. (1966): Aggregate stability of soils from Western United States and Canada. ARS, USDA Tech. Bull. 1355. LOW, A.J. (1954): The study of soil structure in the field and the laboratory. J. Soil Sci. 5, 57-74. LUK, S.H. (1979): Effect of soil properties on erosion by wash and splash. Earth Surface Processes 4, 241-255. MAZURAK, A.P. & MOSHER, P.N. (1970): Detachment of soil aggregates by simulated rainfall. Soil Sci. Soc. Am. Proc. 34, 798800.
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Address of author: E.I. Ekwue Department of Agricultural Engineering University of Maiduguri Maiduguri Nigeria