ridge tillage and contour natural grass barrier strips...

Ridge tillage and contour natural grass barrier strips reduce

tillage erosion$

B.B. Thapaa, D.K. Cassela,*, D.P. Garrityb

aDepartment of Soil Science, NCSU, Raleigh, NC 27695-7619, USAbSystems Agronomist and Coordinator, Southeast Asian Regional Research Programme, ICRAF, P.O. Box 161, Bogor, 16001, Indonesia

Abstract

Large amounts of soil are eroded annually from tilled, hilly upland soils in the humid tropics. Awareness has been increasing that

much of this erosion may be due to tillage operations rather than water-induced soil movement. This ®eld study estimated soil

translocation and tillage erosion for four tillage systems on Oxisols with slope gradients of 16±22% at Claveria, Misamis Oriental,

Philippines. Soil movement was estimated using `soil movement tracers' (SMT) which consisted of painted 12-mm hexagonal steel

nuts. The SMT were buried in three replicate plots of the following tillage treatments: (1) contour moldboard plowing in the open

®eld (MP-open); (2) contour ridge tillage in the open ®eld (RT-open); (3) contour moldboard plowing plus contour natural grass

barrier strips (MP-strip); and (4) contour natural grass barrier strips plus ridge tillage (RT-strip). Two hundred SMT were placed at

the 5-cm depth at 5-cm spacings on 10 rows and 20 columns in two microplots within each plot. The microplots were oriented with

the boundaries running downslope and along the contour of each 8-m-wide � 38-m-long (downslope) tillage plot. After tilling the

land for four successive corn (Zea mays L.) crops (20 tillage operations), the SMT were manually excavated and their positions

recorded. Recovery of SMTranged from 82% to 85%. Displacement of SMTwas directly related to slope length, percent slope, and

tillage method. Mean displacement distance of SMT during the four corn growing seasons was 3.3 m for MP-open, 1.8 m for RT-

open, 1.5 m for the RT-strip, and 2.2 m for MP-strip. Based on tillage operations associated with two corn crops per year, mean

annual soil ¯ux was estimated to be 241, 131, 158 and 112 kg mÿ1 for MP-open, RT-open MP-strip, and RT-strip, respectively.

Compared to the mean annual soil loss for MP-open of 63 Mg haÿ1, soil loss was reduced by 30%, 45%, and 53% for the MP-strip,

RT-open, and RT-strip systems, respectively. Both ridge tillage and natural grass barrier strips reduced soil displacement, soil

translocation ¯ux, and tillage erosion rates. # 1999 Elsevier Science B.V. All rights reserved.

Keywords: Tillage erosion; Ridge tillage; Grass barrier strips; Soil erosion; Land degradation; Animal powered-tillage; Humid tropics

1. Introduction

Over the last few decades, many researchers have

studied rainfall and runoff water as agents for soil

detachment and transport to lower positions on the

landscape. This research led to the development and

re®nement of empirical models such as USLE (Wisch-

meier and Smith, 1958, 1978), RUSLE (Renard et al.,

1991), and process-based models such as GUEST

(Rose and Freebairn, 1985; Rose, 1988), WEPP (Fos-

ter et al., 1989; Nearing et al., 1989), EUROSEM

(Morgan et al., 1990), and MEDALUS (Kirkby et al.,

1993). Even though these models use climatic data,

the condition of the soil surface and its variability

Soil & Tillage Research 51 (1999) 341±356

*Corresponding author.$ Paper presented at International Symposium on Tillage

Translocation and Tillage Erosion held in conjunction with the

52nd Annual Conference of the Soil and Water Conservation

Society, Toronto, Canada, July 24±25, 1997.

0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 0 4 7 - 1

create uncertainties in the soil loss estimates. The

various soil erosion models have not considered the

effects of tillage on soil erosion.

Tillage is a dynamic process that alters the nature of

the soil surface, detaches and displaces soil aggregates

and clods, and moves or translocates soil to lower

elevations (Powell and Herndon, 1987). This translo-

cation of soil to lower elevations (tillage erosion) has

not been considered to be a major factor contributing

to the overall soil erosion process even though Mech

and Free (1942) reported tillage-induced downslope

soil movement over ®ve decades ago. The new chal-

lenge is to model soil erosion by considering water,

tillage, soil creep, wind, and fauna-induced soil trans-

port. In most situations, however, water and tillage-

induced soil erosion dominate all other forces of soil

erosion.

Animal-pulled tillage implements and human-pow-

ered hoes (Turkelboom et al., 1997; Lewis and Nya-

mulinda, 1996) are common in present-day farming

systems in the humid tropics. For many soils on

convex slopes, tillage erosion may be responsible

for a signi®cant reduction of topsoil depth and expo-

sure of subsoil (Veseth, 1986). Current knowledge of

tillage erosion is limited, and most of the information

was developed for tillage operations using large trac-

tor-pulled tillage implements and farm machinery on

gently sloping land in North America and Europe

(Lindstrom et al., 1990, 1992; Lobb et al., 1995;

Govers et al., 1994).

On sloping land in the humid tropics, reduction of

net downhill soil movement due to animal or human-

powered tillage implements is essential to sustain crop

production. This is true even for contour alley crop-

ping systems where a reduction in tillage erosion

would reduce the rate of soil scouring in the upper

part of an alley. Alley cropping is a system for which

annual crops are grown in the area or `alleys' between

rows of perennial legume shrubs or trees. This practice

emerged in the early 1970s as a conservation farming

technique on acid, steepland Oxisols and Utisols in the

humid tropics (Benge, 1987; Kang et al., 1990; Nair et

al., 1995). Contour strips of legume shrubs or trees on

sloping land proved to be an effective barrier to water-

induced soil erosion (O'Sullivan, 1985; Lal, 1987;

Young, 1989; Agus, 1993). In the Philippines, Thapa

(1991) found that a 1-m-wide contour strip of the

legume shrub, Desmanthus virgatus, in 12-m-long

plots on a 14±19% slope reduced water-induced soil

loss from 200 to <10 Mg haÿ1 yearÿ1 for land tilled up

and downslope.

On moderately sloping and steep lands, many til-

lage operations up and downslope still occur, but

plowing across the slope is gaining in popularity

worldwide. Ridge tillage is a conservation tillage

practice in which ridges are reformed atop the planted

row by cultivation, and the ensuing row crop is planted

into ridges formed the previous growing season. The

soil ridges are left from year to year. Crop residue is

left on the soil surface between adjacent ridges and

weeds are controlled with herbicides, interrow culti-

vation, or a combination of the two. As the crop grows,

soil from between the ridges is thrown up on the ridges

to maintain them. Thapa (1997) evaluated ridge tillage

for the ®rst time on sloping uplands in the humid

tropics and reported signi®cantly higher corn grain

yield for ridge tillage systems at reduced operating

cost compared to moldboard plow tillage systems.

The use of contour grass barrier strips is an alter-

native for the perennial shrubs or trees in some alley

cropping systems (Thapa, 1997). Contour ridge tillage

in combination with contour natural grass barrier

strips is a simple, practical and effective alternative

conservation method for intensively tilled sloping

uplands in the humid tropics. Information on the

effects of various tillage systems on tillage-induced

soil erosion will provide useful information to further

evaluate their ef®cacy. The objectives of this study

were to evaluate the effects of four tillage systems on

tillage-induced soil translocation downslope (tillage

erosion) and compare the effectiveness of contour

ridge tillage and natural grass barrier strips in reducing

tillage erosion for corn production on highly erodible

steepland soils in the humid tropics.

2. Materials and methods

2.1. Site description

The study began in July 1994 and was superim-

posed on ongoing experimental plots established in

1992 at Claveria municipality, Misamis Oriental, the

Philippines (88380 N, 1248550 E). The experiment was

located on hillsides ranging from 16% to 22% slope

gradients at ca. 500 m above mean sea level. The soil

342 B.B. Thapa et al. / Soil & Tillage Research 51 (1999) 341±356

is a very ®ne, kaolinitic, isohyperthemic, Lithic Hap-

ludox (Soil Survey Staff, 1992) of volcanic origin

(Table 1). Bulk density was <1.0 Mg mÿ3 throughout

the soil horizons. Pan evaporation in this region

typically exceeds rainfall from January through

May and rainfall exceeds pan evaporation from June

through December. Monthly rainfall and pan evapora-

tion for 1994 and 1995 are presented in Fig. 1. Total

rainfall in 1994 and 1995 was 2020 and 1901 mm,

respectively.

2.2. Tillage and cultural practices

The four tillage systems, each consisting of a

combination of one tillage method and one ®eld

condition, were: (1) contour moldboard plowing in

the open ®eld, the control (MP-open); (2) contour soil

barriers formed by ridge tillage in the open ®eld (RT-

open); (3) contour barriers formed by natural grass

strips plus moldboard plowing (MP-strip); and (4)

contour barriers formed by a combination of ridge

tillage and natural grass strips (RT-strip). The 8-m-

wide � 38-m-long plots were arranged in a rando-

mized complete block design with the four tillage

systems replicated three times. The continuity of the

slope length (Fig. 2(A)) was disrupted by contour

natural grass barrier strips in the MP-strip and RT-

strip systems (Fig. 2(B)). Five contour natural grass

strips formed four alleyways in each MP-strip and RT-

strip experimental plot. Five 50-cm-wide unplowed

contour natural grass strips were established at inter-

vals based on a 1.5 m vertical drop in elevation.

Because these narrow strips were not tilled, natural

grasses were soon established. The grass strip was

dominated by bahia (Paspalum notatum), guinea

(Panicum maximum), and Rottboellia (Rottboellia

cochinchinensis) grasses. Alley width in plots with

grass barrier strips was 9±10 m depending on slope.

By September 1995, distinct terraces had formed

between adjacent grass strips.

Tillage operations to a depth of 20 cm were per-

formed alternately along the contour for all four

Table 1

Selected soil properties at the Ane-i site, Claveria, Philippines, 1992

Soil

depth

(cm)

Clay

(%)

Bulk

density

(Mg mÿ3)

Base

saturation

(%)

CEC

(cmol�

kgÿ1)

Exchangeable

aluminum

(cmol� kgÿ1)

Soil pH

in H2O

(1 : 1)

Total N

(g kgÿ1)

Bray P

(mg kgÿ1)

Organic

C (g kgÿ1)

0±15 78 0.73 33 9.1 1.0 4.1 2.0 5.7 16.0

15±37 85 0.85 21 8.2 1.6 3.7 1.0 2.0 9.0

37±77 88 0.94 21 8.0 2.1 4.1 1.0 2.3 5.0

Fig. 1. Monthly rainfall and pan evaporation at Claveria, Misamis Oriental, Philippines (IRRI, 1994 to IRRI, 1995).

B.B. Thapa et al. / Soil & Tillage Research 51 (1999) 341±356 343

systems. For the MP-open system, a single animal

(ox)-pulled, single bottom moldboard plow and har-

row (in separate operations) were used to prepare the

land prior to corn seeding. In accordance with local

custom the moldboard plow direction was such to

displace soil downslope. The moldboard plow was

also used to create 15-cm-wide furrows at seeding and

for two `hilling up' operations of the MP-open plots.

Hilling operations, performed primarily to control

weeds and incorporate side-dressed fertilizers, moved

soil both upslope and downslope.

The RT-open system was managed analogous to the

MP-open system for planting and the ®rst hilling

operations. During the second hilling operation, 20-

cm-high ridges spaced 60 cm apart (distance between

adjacent corn rows) were constructed on the contour

using an ox-pulled, locally-fabricated ridger. The

ridger was a double-blade moldboard plow, which

displaced soil both uphill and downhill from the

furrow to both corn rows. Once formed initially in

1992, the ridges were maintained permanently in the

same position for all four crops. The MP-strip system

was identical to the MP-open system except that

natural grass strips in the former were left in place

permanently after they became established. The RT-

strip system was managed using the combination of

practices described for the RT-open and MP-strip

systems.

Corn (Pioneer 3274) was planted within one or two

weeks following harrowing (Table 2). The corn was

hand seeded 2-cm deep and 25-cm apart in each

furrow and covered by foot. Plant population was

�67 000 plants haÿ1. Nitrogen, P and K fertilizers

were applied at rates of 80, 30, and 30 kg haÿ1 on

elemental bases, respectively. Hand weeding by

machete in all four systems was performed 40 days

after corn emergence. For RT-open and RT-strip, an

additional hand weeding was necessary two weeks

before crop harvest. The dates and types of imple-

ments used for the cultural operations for the four

tillage systems from August 1994 to October 1996 are

given in Table 2.

Fig. 2. Schematic diagram for (A) open field (MP-open and RT-open) and (B) contour natural grass barrier strip (MP-strip and RT-strip) plots.


2.3. Soil movement tracers

Prior to inserting soil movement tracers (SMT), the

soil surface elevation was measured at 240 points in

each plot and the average percent slope of each plot

was computed. The SMT were installed on July 1994

at the shoulder slope position in the 8-m-wide � 38-

m-long erosion plots (Fig. 3). The cross slope was

minimal. The SMTwere metal 12-mm hexagonal steel

nuts painted with a red or white anti-rust coating. The

SMT were buried in two 90- � 95-cm demarcated

microplots. Each microplot was divided into cells

of 10 rows down the hill by 20 columns along the

contour. One SMT was placed at the 5 cm depth at the

nodes of the grid by pressing an index ®nger into the

soil. Red SMT were placed in the left-hand microplot

and white SMT were placed in the right-hand micro-

plot of each plot. The location of all buried tracers was

recorded.

2.4. Excavation of tracers and distance measurement

The SMT were excavated in May 1996 after all

cultural operations and harvests for four successive

corn crops were completed. The SMT were recovered

by manually excavating the entire 8-m-wide � 11-m-

long plot section. Labor to excavate the tracers for 12

erosion plots required the equivalent of 538 eight-hour

days. Each plot was excavated by systematically

removing 10 cm � 10 cm � 20-cm deep soil blocks.

The excavation began 0.5 m uphill from row 1 of the

buried SMT. A 20-cm-high metal framework was

pushed into the soil and a metal soil dividing strip

was pushed into the soil to cut the soil 20-cm deep.

Then the soil was cut using a locally fabricated 10-

cm-wide � 20-cm-long knife and removed in 10 cm

� 10 cm � 20 cm deep blocks. Each soil block was

crushed manually and forced through a screen with

10-mm openings. Upslope±downslope and horizontal

coordinates for the recovered SMT were recorded

relative to the references shown in Fig. 3.

Using the center of the original 90- � 95-cm tracer

placement area as the origin, the horizontal and

upslope±downslope displacement distance for each

SMT was computed by counting rows and columns.

The actual distance, which is the total distance of each

SMT translocation was calculated using the Pythagor-

ean theorem.

2.5. Statistical analysis and nomogram development

The horizontal, downslope, and actual displacement

distances of all recovered tracers for each erosion plot

were analyzed using the SAS univariate procedure

Table 2

Dates and types of implements used for four tillage systems for four successive corn crops at Claveria, Misamis Oriental, Philippines

Tillage systema Plowing Harrowing Planting Ist Hilling 2nd Hilling

Crop 1, 1994

20 August 25 August 9 September 30 September 14 October

MP-open, MP-strip Moldboard Harrow Moldboard Moldboard Moldboard

RT-open, RT-strip None None Moldboard Moldboard Ridger

Crop 2, 1995

28 December 3 January 5 January 27 January 12 February



Crop 3, 1995

2 May 7 May 12 May 5 June 20 June



Crop 4, 1995

5 September 10 September 22 September 14 October 29 October



a MP � contour moldboard plowing; RT � contour ridge tillage.


(SAS Institute Inc., 1988). Analysis of variance was

performed using the generalized linear model (GLM)

procedure (SAS Institute Inc., 1988). Treatment

means were compared by Duncan's multiple range

test. The mean and Q3 displacement distances were

regressed on percent slope for contour moldboard

plowing and ridge tillage.

Nomogram development for each tillage system

involved measurement of the displaced distance of

SMT (discussed above), determination of soil ¯ux,

and computation of tillage erosion rates. The soil ¯ux

(k) was computed as:

k � MD� TD� BD (1)

where MD is the mean actual displaced distance

(m yearÿ1), TD the tillage depth of 0.2 m, and BD

is soil bulk density (kg mÿ3). Eq. (1) estimates the soil

¯ux caused by tillage operations based on either the

actual or total displacement distance of SMTassuming

that runoff water, wind, faunal activity, and soil creep

were minimal during the two-year period. Later, in

Fig. 8(A) and (C), we will see that MD can be

expressed in terms of slope gradient using a regression

equation. This soil ¯ux computation is similar to those

of Turkelboom et al. (1997) and Govers et al. (1994),

except that our unit of soil ¯ux, kg mÿ1, represents the

total soil ¯ux for all tillage operations on an annual

rather than a per tillage event basis. Soil ¯ux

(kg mÿ1 yearÿ1) in our case is de®ned as the mass

of soil that moves from a speci®ed location in the ®eld

in response to the summation of an unit length of one

pass of all tillage operations performed annually to

Fig. 3. Schematic of area where soil movement tracers were inserted in open field (MP-open, RT-open) and contour natural grass strip (MP-

strip, RT-strip) plots. Two hundred red and 200 white 12-mm hexagonal painted stainless-steel nuts were inserted in each plot. Plot dimensions

not to scale.


grow crops. Mean annual soil loss due to a given

tillage practice refers to the mean annual soil ¯ux that

occurs due to tilling an unit land area with a given

slope length or contour interval.

Mean annual tillage erosion rate (TER) (Mg haÿ1

yearÿ1) was calculated by dividing the soil ¯ux value

by the plot or slope length (L) (downslope for MP-

open and RT-open, and alley width or contour interval

distance for the MP-strip and RT-strip systems) and

converting the value to a per ha basis as follows:

TER � MD�cm� � TD�cm� � BD�kg=m3�L�m�

� �1 m�2�100 cm�2 �

1 Mg

103 kg� 1

2� 104 m2

ha(2)

where TD is 20 cm and 1/2 represents the conversion

of data collected over a two-year period to a one-year

period. Simplifying, the nomogram becomes

TER � ��19:2� Sÿ 72:8�TD� BD�=20� L (3)

for contour moldboard plowing and

TER � ��8:8� Sÿ 5:7�TD� BD�=20� L (4)

for contour ridge tillage. The function in parentheses

in Eqs. (3) and (4) replaces MD and was derived by

regressing mean displacement distance on percent

slope (S) (presented later in Fig. 8(A) and (C)). Like-

wise MD in Eq. (2) can be replaced by regression

equations (presented later in Fig. 8(B) and (D)) to

estimate soil erosion rates based on the Q3 distance of

SMT movement.

3. Results and discussions

3.1. Tracer recovery and movement

Percent recovery of the 400 SMT buried per plot

was 82±85% and was not affected by tillage system

(Table 3). However, tillage system did affect the

percent of SMT displaced downslope (p < 0.0001).

Only 54% of the SMT recovered in the RT-strip

system were displaced downslope, but for the other

systems, >80% of recovered SMT were displaced

downslope.

In reality, each SMT likely was displaced from its

original location. Since each SMT was not numbered,

and therefore could not be assigned an exact original

location, it was necessary to measure displacement

distance using the center of the SMT microplot as a

reference (Fig. 3). Consequently, a few SMT for the

ridge tillage system were recovered in a location

having an upslope±downslope component identical

to the original row in which it was placed, but moved

laterally from the original column in which it was

placed. In this situation, calculation of SMT displace-

ment distance relying only on row counts (down-

slope), such as reported by Turkelboom et al.

(1997), would ignore lateral soil movement and, thus,

underestimate actual SMT displacement. Measure-

ment of downslope displacement distance by Turkel-

boom et al. (1997) was appropriate because hand

digging on a 60% slope is systematically done by

hoeing down hill. Our situation is different in that

animal-powered, contour plowing can create both

lateral and upslope±downslope soil movement. Plot-

ting of x and y coordinates of each recovered SMT in

our study, in reference to the original position, allows

comparison of both upslope±downslope and horizon-

tal displacement of SMT for the different tillage

systems.

The displaced position of each recovered SMT after

growing four corn crops, in reference to the center of

the 90- � 95-cm microplots is shown in Fig. 4 for the

four tillage systems. The greater spread of tracers

downslope for the MP-open compared to the RT-open

system was expected because the soil in the ridges in

the latter tend to reduce the rate of downward move-

ment. Likewise, downhill tracer movement was

greater for the MP-strip than for RT-strip system.

Table 3

Percent recovery of soil movement tracers (SMT) and percentage

that moved downslope from original position after growing four

corn crops at Claveria, Philippines

Tillage

systema

Recovery

of SMT (%)

Displacedc

(%)

MP-open 85.2ab 90.3a

RT-open 83.7a 80.0a

MP-strip 81.5a 80.6a

RT-strip 82.7a 53.9b

Standard error �1.2 �6.1

a MP � contour moldboard plowing; RT � contour ridge tillage.b Means in a given column followed by common letters are not

significantly different at the p � 0.05 level.c Percent displaced downslope based on row counts only.


Fig. 4. Displacement of SMT for four tillage systems on 16% slope after cultivating the land for four corn crops. Total number of recovered

SMTs out of 200 for each color are given in parentheses. The reference point marked by the large dot represents the center of the 90- � 95 cm

area of SMT installed microplot. One row � 10 cm, one column � 10 cm.


Fig. 5. Displacement of red SMT for four tillage systems on 19% and 22% slopes after cultivating the land for four corn crops. Total number

of recovered SMTs out of 200 for each treatment are given in parentheses. The reference point marked by the large dot represents the center of

the 90 � 95 cm area of SMT installed microplot. One row � 10 cm, one column � 10 cm.


For each tillage method, SMT movement was less

under grass strips (MP- and RT-strip) compared to the

open ®eld. Tracers for MP-open and MP-strip moved

further downslope than those for RT-open and RT-

strip. An increase in percent slope exacerbates down-

hill movement of SMT for the MP systems (Fig. 5).

Slope steepness had less effect on tracer movement for

the RT systems.

The reference point for calculation of downslope

movement of tracers is between rows 5 and 6 (Fig. 3).

This coincides with row zero in Figs. 4 and 5. The

reference line in the microplot used to calculate

horizontal tracer movement is between columns 10

and 11 (Fig. 3). When we ®rst began to excavate the

SMT, we excavated the border area between tillage

plots in addition to the entire plot area because we did

not know how far the tracers would move horizontally.

After excavation of three plots (RT-open, MP-strip,

and RT-strip in Fig. 4), we discovered that few tracers

moved into the border area, and we discontinued

excavation of the border area.

The relative frequencies of actual SMT displace-

ment distance for the four tillage systems and three

slopes are presented in Fig. 6. Incorporation of ridge

tillage into a tillage system reduces actual SMT move-

ment. Most SMT under the grass strip systems moved

<6 m from the original position, whereas some of the

SMT under open ®eld systems moved 8 m or more.

Observation of relative frequency data in Fig. 6(A, C,

and E) shows that the greatest actual displaced dis-

tance for the open system increased with percent

slope. The open system has no barriers to restrict

downslope SMT transport as exists for the grass strip

systems.

Fig. 6. Relative frequency distribution of actual displaced tracers for four tillage systems at three slopes.


Lindstrom et al. (1990) reported a strong correlation

between tillage-induced soil movement and percent

slope. Other factors reported to affect tillage-induced

soil movement include slope length (Turkelboom et

al., 1997), speed of tillage operation, size of tillage

implement, and direction of tillage (Lobb et al., 1995).

Factors such as tilling fallow land during high weed

incidence, or ®rst plowing for seed bed preparation,

might contribute to the dispersion of SMT in the MP

systems. Roots and large clods accumulating in front

of the moldboard plow often cause soil to be dragged

along with the plow. In addition, when turning a

moldboard plow at the end of a row, a plowman

applies extra force to free the plow of any accumulated

soil so as to lighten the plow before lifting it from the

furrow. Lifting the plow takes less physical effort

when the plow is turned downhill.

3.2. Displacement distance of tracers

Various statistical parameters based on actual dis-

placement distance for the four tillage systems at 16%,

19%, and 22% slope gradients are shown in Table 4.

Mean displacement distance increased with percent

slope for each tillage system, but the rate of increase in

displacement distance with respect to slope was higher

for the MP systems compared to the RT systems. The

highest standard deviation occurred for MP-open,

which had the greatest tracer dispersion. The standard

deviation of displacement distance is useful as an

indicator of random dispersion of tracers due to

different tillage systems. Standard deviation for each

treatment is expected to be smallest in the ®rst year of

tillage and to increase as the number of tillage opera-

tions increases.

The analysis of variance for mean, Q3, and max-

imum actual and downslope displacement distances

are shown in Table 5. Highly signi®cant differences in

these statistical parameters as affected by tillage sys-

tem were observed. In Fig. 7 we see that the greatest

mean and maximum actual displacement distances

occurred for MP-open.

The observed difference in mean SMT displace-

ment distance between MP-open and MP-strip was

due to the contour natural grass barrier strips. The

grass strips reduced the amount of soil moving down-

Table 4

Summary of actual displacement distances of soil movement tracers for four tillage systems on three slopes

Tillage systema nb Mean Maximum Q3c SDd (cm2) p < We

cm

16% Slope

MP-open 341 288 780 361 78 0.0001

RT-open 329 135 462 177 40 0.0880

MP-strip 327 195 512 239 48 0.0010

RT-strip 334 125 371 170 42 0.0001

19% Slope

MP-open 322 319 789 418 90 0.0001

RT-open 327 192 517 230 45 0.0251

MP-strip 325 213 508 255 46 0.0001

RT-strip 320 165 463 201 44 0.2994

22% Slope

MP-open 359 383 1013 470 92 0.0025

RT-open 350 211 542 255 47 0.2678

MP-strip 326 241 622 309 53 0.0068

RT-strip 338 170 510 211 45 0.4513

a MP � contour moldboard plowing; RT � contour ridge tillage.b Number of tracers recovered out of 400 SMT buried per plot.c The distance beyond which 25% of the tracers were displaced.d Standard deviation.e Probability test in univariate procedure which is similar to `T' in ANOVA.


slope and gradually reduced the percent slope in the

alley between adjacent grass strips. Failure to observe

differences in mean SMT displacement distance

between RT-open and RT-strip was due to the large

reduction in SMT movement under ridge tillage alone.

Both ridge tillage systems reduced SMT displacement

distance compared to the MP systems.

3.3. Models for displacement distance of tracers

Linear regression models relating the mean and Q3

parameters for actual SMT displacement distance as a

function of percent slope for two tillage systems are

presented in Fig. 8. The high r2 value for moldboard

plowing in Fig. 8(A) indicates that soil displacement

is highly related to percent slope. The slope of the

curves for contour moldboard plowing (Fig. 8(A) and

(B)) was two- to three-fold higher compared to the

slopes of contour ridge tillage (Fig. 8(C) and (D)). The

low r2 value and gentle slope for the ridge tillage

curves indicate that soil movement for this practice is

not as closely related to percent slope as for moldboard

plowing.

3.4. Soil flux and tillage erosion rates

Estimation of soil ¯ux and tillage erosion rates was

based on actual displacement distance of tracers

assuming that the tracers and soil material move at

the same rate. The mean and Q3 for soil ¯ux and

tillage erosion rate were highest for MP-open and

lowest for RT-strip (Table 6). Annual soil ¯ux and

tillage erosion rate were greater for the MP-open

Table 5

Mean squares from randomized complete block analysis of variance for various statistical parameters associated with SMT downslope and

actual displacement distances (cm) as a function of tillage system

Source df Mean Q3a Maximum

Downslope displaced distance of tracers

Block 2 17658.8 5293.8 9300.0

Tillage system 3 17257.0b 101913.9b 181355.6b

Pooled error 6 334.0 2257.6 2257.6

CV (%) 16.5 16.0 16.0

R2 0.97 0.96 0.96

Actual displaced distance of tracers

Block 2 4318.2 5581.4 5581.4

Tillage system 3 18224.8b 29569.0b 29569.0b

Pooled error 6 244.2 264.8 264.8

CV (%) 7.1 5.9 5.9

R2 0.98 0.98 0.98

a The distance beyond which 25% of tracers displaced.b Indicates significance at the p � 0.001 level.

Fig. 7. Mean (A) and maximum (B) distances of displaced tracers

based on actual and row (downhill) distances for four tillage

systems (MP � moldboard plowing; RT � ridge tillage; O � open

field; S � grass strip). Each value is the mean of three observations.

Means based on actual distance followed by common letters above

the bars are not significantly different at the p � 0.05 level. Vertical

lines at the top of each bar indicate the standard error.


compared to the MP-strip, but no differences in soil

¯ux and erosion rates were observed between RT-open

and RT-strip. The relation of soil ¯ux to tillage opera-

tions performed on an annual basis are compatible

with water erosion rates reported in a companion study

(Thapa, 1997). The soundness of using Q3 to estimate

Fig. 8. Relationship between percent slope and mean and Q3 actual distance of displaced tracers for moldboard plowing and ridge tillage

systems at Claveria, Philippines. The symbols * and ** indicate significance at the p � 0.05 and 0.01 levels, respectively.

Table 6

Annual soil flux and tillage erosion rates for four tillage systems at Claveria, Philippines

Tillage systema Annual soil flux rate Annual tillage erosion rate

Mean Q3b Mean Q3b

Kg mÿ1 yearÿ1 Mg haÿ1 yearÿ1

MP-open 241ac 304 a 63.4 a 80.0 a

RT-open 131 c 161 c 34.4 c 42.4 c

MP-strip 158 b 195 b 41.5 b 51.4 b

RT-strip 112 c 141 c 29.5 c 37.2 c

Standard error �6.6 �6.9 �1.7 �1.6

Source df Mean squares from ANOVA

Blocks 2 2301.6 2975.5 159.5 205.4

Treatmentsd 3 9714.5 15760.2 672.9 1091.6

Residuals 6 130.1 141.0 9.0 9.7

CV % 7.1 5.6 7.1 5.9

aMP � contour moldboard plowing; RT � contour ridge tillage.b Q3 soil flux and erosion rate are computed based on the distance beyond which 25% of the tracers were displaced.cMeans in a given column followed by common letters are not significantly different at the p � 0.05 level.d Significant at the p � 0.0001 for all soil flux and tillage erosion variables.


soil ¯ux and tillage erosion rates as proposed by Thapa

et al. (1999) needs further investigation but the con-

cept appears to be valid.

The 63.4 Mg haÿ1 yearÿ1 of tillage erosion in the

MP-open system is very high and much soil eventually

will be lost from the ®eld if soil continues to move

downslope unimpeded. Installation of contour grass

strip barriers reduced the annual tillage erosion rate

by one-third. The higher tillage erosion rate in MP-

strip (42 Mg haÿ1 yearÿ1) compared to RT-strip

(30 Mg haÿ1 yearÿ1) indicates a more rapid rate of

terrace formation for MP-strip. Soil moved from the

upper alley area in the MP-strip system accumulates

on the upslope side of contour grass strips and forms a

terrace more quickly. This process accelerates topsoil

loss in the upper alleyway, which may cause serious

soil fertility degradation in the zone just below the

contour grass strip. Thus, from the ®eld or mass

balance point of view, redistributed soil is not lost

from the ®eld because the soil material remains on the

terrace. However, from a nutrient management point

of view, breaking the continuum of slope length with

contour grass strips creates zones of land with scoured

and nutrient-poor surface soil, and a zone in the lower

alleyway with accumulation of a nutrient rich and

deeper topsoil (Govers et al., 1994; Poesen, 1995;

Garrity, 1996; Turkelboom et al., 1997). We estimate

that as much as 40% of the cropped alleys might

eventually be degraded physically, chemically, or

biologically if the alleys are moldboard plowed fre-

quently. In this study, 42 Mg haÿ1 yearÿ1 of soil in the

MP-strip system moved downhill from the upper alley,

leaving an exposed acidic subsoil with high Al satura-

tion. Although the reduced yields in the upper alley-

ways may be compensated partially or wholly by

increased yields in the lower alleyways, and the soil

fertility of the upper alleyways may recover gradually

over a number of years, careful management of these

spatially variable areas within an individual alley must

be provided to avoid short- and long-term negative

effects on crop production.

3.5. Nomogram for tillage erosion rates

Nomograms to determine soil losses for contour

moldboard plowing and ridge tillage as a function of

percent slope and plot length (distance between adja-

cent contour grass strips or length of open plot) are

shown in Fig. 9. Regression equations relating dis-

placement distance to percent slope (Fig. 8(A) and

(B)) were used for the MD term in Eq. (1). Soil

redistribution to lower alleyway zones under mold-

board plowing can be as high as 500 Mg haÿ1 yearÿ1

if a farmer cultivates land with a slope of 20% and a

grass strip contour interval or plot length of 5 m.

About 53% of this soil movement can be reduced

by adopting ridge tillage.

4. Conclusions

Intensive moldboard plowing, where practiced, is a

primary factor of land degradation in humid tropical

uplands because the more fertile topsoil is physically

Fig. 9. Nomograms to determine tillage erosion rate (TER) for contour moldboard plowing and contour ridge tillage systems as a function of

percent slope and countour interval distance or downhill plot length at Claveria, Philippines. S � percent slope, TD � tillage depth (m),

BD � bulk density (kg mÿ3), and L � slope length or contour interval distance (m).


displaced downhill exposing less fertile, acidic sub-

soil. Soil degradation by moldboard plowing and its

long-term consequences on soil physical, chemical,

biological, and hydrological properties has been

omitted from alley cropping research and soil erosion

modeling. This omission may help explain the failure

of alley cropping to meet farmers' expectations to

have higher crop production. The present challenge is

to model soil erosion by considering all soil transport

agents-tillage, water, wind, soil creep, macro-fauna

and human activities. In this study, we assumed neg-

ligible rates of soil erosion by wind, soil creep, and

faunal activity. This limitation should be taken into

consideration when extrapolating our results to other

®eld conditions.

Reduction of tillage erosion is indispensable to

maintain soil fertility and sustain crop production

on sloping lands in the humid tropics, especially under

contour hedgerow alley cropping systems. Contour

ridge tillage in conjunction with natural grass barrier

strips gradually reduces the degree of slope, breaks the

continuum of downhill slope length, and reduces the

amount of soil removed from the ®eld.

5. List of symbols

SMT soil movement tracersÐdevices, natural or

manmade, placed in the soil to assess the mass

transport of soil

MP moldboard plow

RT ridge tillage

ks soil flux, i.e. the mass of soil that moves

through a vertical cross-section of the plough

layer of unit width due to the combined effects

of all tillage passes performed annually to

grow crops

MD displaced distanceÐthe distance a SMT is

displaced in one year

TD tillage depth

BD bulk density

TER mean soil loss due to contour moldboard

plowing on ridge tillage contour (Mg haÿ1

yearÿ1)

Q3 the distance beyond which 25% of all

recovered SMT moved

Q1 the distance beyond which 75% of all

recovered SMT moved

S percent slope

L downslope length or contour interval distance

(m)

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