the effects of soil organic matter content, rainfall duration and aggregate size on soil detachment

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SOIL TECHNOLOGY vol. 4, p. 197-207 Cremlingen 1991 I THE EFFECTS OF SOIL ORGANIC MATTER CONTENT, RAINFALL DURATION AND AGGREGATE SIZE ON SOIL DETACHMENT 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 du- rations (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 sec- ond 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 in- creasing 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 de- tachment with increasing rainfall dura- tion. Soil detachment was smaller for the larger aggregate size. The most significant interactions af- fecting soil detachment were between rainfall duration and aggregate size, or- ganic matter content and aggregate size, and between organic content and rain- fall duration in that order. These in- ISSN 0933-3630 @1991 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0933-3630/91/5011851/US$ 2.00 + 0.25 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 en- ergy 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 or- ganic matter to reduce soil erodibility is well known (Luk 1979), few studies have examined its influence specifically on soil detachment. Previous research has con- centrated 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 corre- lation coefficient (r = -0.26) between soil detachment and organic matter content and attributed it to the low range of or- ganic content (1.11 to 4.87%) in the soils he studied. Chandra & De (1978) work- ing with Indian soils observed a signifi- cant correlation coefficient (r = -0.68) de- SOl L TECHNOLOGY--A cooperating Journal of CATENA

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Page 1: 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 ORGANIC MATTER CONTENT,

RAINFALL D U R A T I O N A N D AGGREGATE SIZE ON 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 du- rations (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 sec- ond 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 in- creasing 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 de- tachment with increasing rainfall dura- tion. Soil detachment was smaller for the larger aggregate size.

The most significant interactions af- fecting soil detachment were between rainfall duration and aggregate size, or- ganic matter content and aggregate size, and between organic content and rain- fall duration in that order. These in-

ISSN 0933-3630 @1991 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0933-3630/91/5011851/US$ 2.00 + 0.25

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 en- ergy 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 or- ganic matter to reduce soil erodibility is well known (Luk 1979), few studies have examined its influence specifically on soil detachment. Previous research has con- centrated 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 corre- lation coefficient (r = -0.26) between soil detachment and organic matter content and attributed it to the low range of or- ganic content (1.11 to 4.87%) in the soils he studied. Chandra & De (1978) work- ing with Indian soils observed a signifi- cant correlation coefficient (r = -0.68) de-

SOl L T E C H N O L O G Y - - A cooperating Journal o f C A T E N A

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198 Ekwue

spite the narrow range of organic matter content (0.20 to 074%) in their soils. Luk (1979) also noted good relationships be- tween soil detachment and organic mat- ter 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 fac- tor experiments involving the effect of or- ganic matter on soil detachment. These studies are limited since they are rarely valid outside the observed range of val- ues and prevailing environmental con- ditions (Quansah 1981) and do not take account of interactions between such fac- tors as rainfall duration, soil slope, ag- gregate 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 interac- tions 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

The experimental investigation involved a factorial laboratory experiment to as- sess soil detachment of five soils with or- ganic matter contents ranging from 1.23 to 5.64%, each with two aggregate sizes (<2 mm and 2-5 mm) and exposed to 4, 12, and 20 minutes rainfall durations. Each treatment combination was repli- cated three times. A completely ran- domised design was adopted.

2.1 Soil

The study soils were collected from var- ious locations in the long term organic manuring field plots of Rothamsted Ex- perimental Station at Woburn, Bedford- shire, 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 ex- amine the effect of organic matter on soils with small structural elements. The 2-5 mm aggregate size was used to in- vestigate the effect of organic matter on medium-size soil aggregates. Larger ag- gregate sizes were not considered since they are not very erodible (Luk 1979) and this may have disguised the possible organic matter effects.

2.3 Simulated rainfall

A rotating disc simulator (Morin et al. 1967) was used to generate simulated rainfall with an intensity of 81 mm hr -1, kinetic energy of unit rain of 25.88 J m 2 per mm and a median drop diameter of 2.60 ram. Natural rainfall of that inten- sity has, by comparison, a kinetic energy of 28.29 J m -2 per mm (Wischmeier & Smith 1978).

S{)I[~ ' IE( 'HNOL(~(P£ A ~¢*~!uc;atmg J~tlr[la/ o I ' C A [ E N A

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Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates 199

Soil Code

G1

G2

G3

G4

Soil History*

Intensive cereal up to 1982. Has been under gtrass for at least 2 years.

Same as for G1 above.

Continuous arable up to 1971. Has been under grass since.

Permanent grass up to 1981. Has been under arable since.

G5 Permanent grass since living memory.

Organic Matter Content

(%)

1.23+0.03"*

Water Stable Aggregates

0.25 mm (%)

7.09__+0.45

Sand (2-0.06)

r n n l

Particle Size Distribution (%) Clay

(<0.002) mm

70.8+1.2

Silt (0.06~.002)

m m

19.1_+0.4

22.6__.0.3

16.5±0.2

1.37+__0.06

2.09+0.05

5.04+0.08

10.41+1".89

15.59+0.82

23.33+0.99

65.8+0.5

72.3+0.9

68.5+0.9 17.3___0.2

12.8±0.4 5.64__+0.21 24.32+1.16 62.3___1.0

10.1+0.5

11.6+0.3

11.2_+0.5

14.2___0.6

24.9_+0.9

I[ * 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. Oven- drying was done to determine the dry weight of the soil. The samples were then saturated, left overnight on sand ta-

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 deter- mined by the Walkley-Black method. Mechanical analysis was performed us- ing the pipette method. The propor- tions of water stable aggregates (WSA) were measured using the method of Low (1954). The WSA>0.25 mm were re- tained 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.

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200 Ekwue

Soil Rainfall Duration (mins) Codes 2 4 6 8 12 16 20

(69.62)" (138.98) (208.59) (277.95) (417.19) (556.32) (695.40)

G1 A** 0.54_+0.01 "*° B

G2 A 0.43-+0.01 B

G3 A 0.32_+0.02 B

G4 A 0.29_+0.01 B

G5 A 0.17+_0.01 B

0.77+-0.04 0.24+_0.02

0.72_+0.02 0.28_+0.03

0.63_+0.02 0.26+0.06

0.48+0.04 0.17___0.01

0.31+0.03 0.16+0.02

1.09+_0.11

0.95+0.09

0.87_+0.03

0.65___0.02

0.43___0.05

1.43 +_0.06

1.17 +0.09

1.06+_0.07

0.81 __+0.04

0.55+_0.03

1.84+0.06 0.50_+0.05

1.5l+0.09 0.48+0.03

1.27+0.06 0.32_+0.02

1.07_+0.09 0.32_+0.02

0.74_+0.02 0.3l+0.01

2.22_+0.07

1.85+0.21

1.66+0.10

t.24_+0.08

1.04_+0.07

2.78__+0.08 0.74+0.05

2.31 +0.26 0.66+0.08

2.08_+0.13 0.52_+0.04

1.35_+0.09 0.45_+0.05

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 t.15 a* G2 0.99 b G3 0.85 c G4 0.64 d G5 0.51 e

Rainfall duration (mins) 4 0.40 c 12 0.83 b 20 1.24 a

Aggregate size (mm) < 2 1.26 a 2 5 0.39 b

" The values followed by dissimilar letters are significantly different at ! %.

T a b . 3" The mean mass of soil detachment (n = 90).

Mean values for each factor were obtained by averaging the experi- mental values over the levels of the other two factors.

SOIl, TECHNOLOGY A c~operatmg Journal of CATENA

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Soil Detachment, Effects o f Organic Matter, Rain£aH and Aggregates 201

2.0

i 1.5

l.O-

"" 05 t.r)

~LSD(S%) GI

G2

63

G4

G5

/2 e'o Rainfall Duration (/dins)

Fig. 1: The effect o f the in- teraction between soil organic content and rainfall duration on detachment. (Soil detach- ment values are means aver- aged over the two aggregate sizes).

3 Resul t s

3.1 Factors influencing soil detachment

Tab. 2 details the values of soil detach- ment 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 du- ration 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 exam- ple, 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 detach- ment declined with both increasing or-

ganic matter content and aggregate size and increased with increasing rainfall du- ration. Analysis of variance of the results showed that the main effects of soil or- ganic matter content, rainfall duration, aggregate size and their first and sec- ond order interactions significantly in- fluenced the values of detachment. The main effect of aggregate size was high- est followed by those of rainfall dura- tion 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 dura- tion 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.

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202 Ekwue

2-0-

I

1.5-

qa

1.o- 2 oj c~

~, 0.5- uq

~2 2 m t ~

? - S m r q

I !

5o/I Organic hioffer Con fen f (% )

Fig. 2: The effect o f the interaction between soil or- ganic content and aggregate size on detachment. (Soil detachment values are means averaged over 4, 12 and 20 rainfall durations).

In considering these interactions, the approach of Steel & Torrie (1980) was followed, whereby treatment means were arranged in two-way tables and suffi- ciently 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

greater for soils with lower organic con- tent 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 signifi- cant 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 differ- ent. These results suggest that the effect of organic matter content on detachment increases with rainfall duration.

Examination of the interaction be- tween soil organic matter and aggregate size (fig. 2) showed that mean soil de- tachment reduced significantly (P<0.01) with increase in aggregate size for all

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Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates 203

2.0"

l .s.

1.0 121

flj

~ 0 . 5

I LSD(5% )

fe z'o Rainfall Duration (Mins)

.~2 fnrn

2 - S m m

Fig. 3: The effect of the interaction of rainfall dura- tion and aggregate size on detachment. (Soil detach- ment values are means av- eraged over the five soils).

the soils with different organic contents. The reductions were greater in soils with lower organic matter contents. The ef- fect of organic content at each aggregate size showed that for the <2 mm size, de- tachment 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 con- tents, the differences between the values of soils G1 and G2 and between those of G3, G4 and G5 soils were not signif- icantly different at 5% level. The results therefore suggest that the effect of soil organic matter in reducing raindrop de- tachment decreases with increasing size of soil aggregates.

The mean soil detachment values de-

clined significantly (P<0.01) with aggre- gate size at all levels of rainfall dura- tion (fig. 3). Furthermore, the values increased significantly with rainfall dura- tion for both aggregate sizes. The detach- ment increases for the 2-5 mm sized sam- ples were less than those for the <2 mm size showing that soils with large aggre- gates are less susceptible to detachment increases with increasing rainfall dura- tion than those with smaller aggregates.

3.2 Soil detachment relationships

Regression analysis was used to illus- trate the relationships between soil de- tachment 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

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204 Ekwue

Soil code a

G I

G 2

G 3

G 4

G 5

All soils

A* 0.024 B 0.009

A 0,021 B 0,017

A 0.013 B 0.031

A 0.010 B 0.009

A 0.005 B 0.008

A 0,023 B 0.018

~ C r

0.70 - - 0.993** 0.68 - - 0.964

0.71 0.988 0.56 0.938

0.77 -- 0.992 0.41 - 0.870

O.78 O,989 0.60 0.977

0.83 , 0.991 0.6l -- 0,993

0.74 -0.49 0.983 0.56 -0.32 0,972

i " A a n d B refer to < 2 and 2 -5 mm aggregate sizes respectively. I] [I

*" All correlation coefficients (r) are significant at 1 per cent.

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 mul- tiple regression equations relating soil detachment to total rainfall kinetic en- ergy and organic matter content. Total kinetic energy rather than duration of rainfall was used in order to ease com- parison 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

M

= Soil detachment (kg rn 2)

= Total rainfall kinetic energy (J m 2)

= Organic matter content (%)

= Empirically derived constants.

The equations are generally of the power form which best fitted the exper- imental data. It is also the form usu- ally adopted in soil detachment research (Bubenzer & Jones 1971, Quansah 1981)

Highly significant correlation coeffi- cients (P<0.01) were obtained between soil detachment and total rainfall energy, For each soil, and for all the soils com- bined, the values of the total energy ex- ponents 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 to- tal exponent increased slightly with or- ganic matter content from 0.70 in G1 to 0.83 in the G5 soil. The intercept val- ues reduced with organic matter content from 0.024 in G1 to 0.005 in the G5 soil. The values suggest that the total energy

SOiL I'IECHNOLOGY A cooperating Journal of CA'I'ENA

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Soil Detachment, Effects of Organic Matter, Rainfall and Aggregates 205

exponents as well as the intercept values may be affected by organic matter con- tent 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 de- tachment was more affected by the total rainfall kinetic energy than by organic matter content.

4 D i s c u s s i o n

4.1 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.

4.1.1 Soil organic matter content

Results showed that soil organic mat- ter reduced values of soil detachment. These reductions may be attributed to the ability of soil organic matter (in this case originating from grass) to increase the stability of soil aggregates. Tab. 1 shows that the percentage water stable aggregates increased with soil organic matter content from G1 to G5. Stable aggregates resist detachment by utilising part of the rainfall energy, which would otherwise cause soil detachment, in dis- persing the aggregates before apprecia- ble splash can take place (Mazurak & Mosher 1970).

The interaction between soil organic matter content and rainfall duration shows that as the latter increases, soil detachment increases more rapidly in soils with lower organic matter contents. Quansah (198i) observed similar inter- action between soil and rainfall inten- sity. The interaction between soil organic

matter content and aggregate size shows that organic matter is most effective in soils with little aggregation and least ef- fective in highly aggregated soils. This is because the large aggregates unlike the small ones resist breakdown and detach- ment mainly due to their size, rather than as a result of their organic matter con- tents. Soils may therefore be expected to have low detachment if they have high organic matter content or are well aggre- gated.

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 in- creasing rainfall duration agree with the observations of Yamamoto & Anderson (1973) and Jennings et al. (1987) and are presumably due to increases in ag- gregate breakdown as rainfall duration increased. The magnitude of the increase in detachment of soils with rainfall dura- tion was, however, reduced by increasing organic content and soil aggregate size as the interactions between soil organic matter and rainfall duration as well as, between rainfall duration and aggregate size showed. These findings have not been shown in earlier studies of soil de- tachment.

4.1.3 Aggregate size

The significant reductions of soil detach: ment with increasing aggregate size con- firms the findings of Ellison (1947) and Mazurak & Mosher (1970) and are pre- sumably due to the need for large aggre- gates to break down before appreciable

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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-detachment- rainfall total kinetic energy relatiosnhips for the <2 mm aggregate size are com- parable 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, how- ever, 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 ex- ponents (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 lab- oratory 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 inter- cept 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 mat- ter 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 or- ganic matter contents indicate that for an increase in rainfall duration, the propor- tionate 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 (Ek- wue 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 al- ways 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 ef- fect of soil organic matter in reducing detachment increases with increasing rainfall duration and decreases with increasing aggregate size.

(ii) Although raindrop detachment in- creases 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 detach- ment to rainfall duration are affected by organic matter content. The effect of organic matter in reducing the in- tercept 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 A c~wJpcraltng Journa] of CArl t'~NA

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Soil Detachment, Effects o f Organic Matter, Rainfall and Aggregates 207

t h e m a n u s c r i p t . H e is a l so g r a t e f u l to

M r . A.J . J o h n s t o n fo r h is a d v i c e a n d

to R o t h a m s t e d E x p e r i m e n t a l S t a t i o n fo r

a cce s s to t h e f ie ld p lo t s .

R e f e r e n c e s

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 de- tachment of soils under simulated rainfall. Trans ASAE 14, 625-628.

CHANDRA, S. & DE, S.K. (1978): A simple lab- oratory apparatus to measure relative erodibility of soils. Soil Sci. 125, 115-121.

CHANEY, K. & SWIFT, R.S. (1984): The influ- ence 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.

EKWUE, E.I. (1990): Effect of organic matter on splash detachment and the processes involved. Earth Surface Processes 15, 175-181.

ELLISON, W.D. (1947): Soil erosion studies, Part II. Agric. Eng. 28, 197-201.

FREE, G.R. (1960): Erosion characteristics of rainfall. Agric, Eng. 41, 447-449, 455.

GOMEZ, K.A. & GOMEZ, A.A. (1984): Statis- tical Procedures for Agricultural Research. 2nd. Ed., Wiley, New York, 607 pp.

JENNINGS, G.D., JARRETTE, A.R. & HOOVER, J.R. (1987): Simulated rainfall du- ration and sequencing affect soil loss. Trans. ASAE 30, 158-161, 165.

KEMPER, W.D. & KOCH, E.J. (1966): Ag- gregate 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 Pro- cesses 4, 241-255.

MAZURAK, A.P. & MOSHER, P.N. (1970): Detachment of soil aggregates by simulated rainfall. Soil Sci. Soc. Am. Proc. 34, 798- 800.

MEYER, L.D. (1981): How rain intensity affects interrill erosion. Trans. ASAE 24, 1472-1475.

MORGAN, R.P.C. (1981): Field measurements of splash erosion. In: Sedimentation and Ero- sion Transport Measurements. Proceedings of the Florence Symposium, International Associ- ation of Scientific Hydrology, Publ. No. 133, 373-382.

MORGAN, R.P.C., MARTIN, L. & NOBLE, C.A. (1987): Soil erosion in the United King- dom: a case study from mid-Bedfordshire. Oc- casional 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): Princi- ples and Procedures of Statistics: A Biometrical Approach, 2nd. Ed., McGraw-Hill New York, 341 pp.

VERHAEGEN, TH (1984): The influence of properties on the erodibility of Belgian loamy soits. Earth Surface Processes 9, 499-507.

WISCHMEIER, W.H. & SMITH, D.D. (1978): Predicting rainfall erosion losses from cropland east of Rocky mountains. Agric. Handbook 537, USDA.

YAMAMOTO, T. & ANDERSON, H.W. (1973): Splash erosion related to soil erodibility and other forest soil properties in Hawaii. Water Resources 9, 336--345.

Address of author: E.I. Ekwue Department of Agricultural Engineering University of Maiduguri Maiduguri Nigeria

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