sustainability of soil structure quality in rice paddy— soya-bean cropping systems in south...

~ ' ~ ? ~ : ' ~"' TillageS°il & Hesearcn

[~LSI:.V I ER Soil & Tillage Research 31 (1994) 339-352

Sustainability of soil structure quality in rice paddy soya-bean cropping systems in South Sulawesi,

Indonesia

A. Cass *'a, S. Gusli a, D.A. MacLeod b aCooperative Research Centre for Soil and Land Management, Private Bag 2,

Glen Osmond, S.A. 5064, Australia bDepartment of Agronomy and Soil Science, University of New England,

Armidale, N.S. W. 2351, Australia

(Accepted 9 February 1994)

Abstract

In South Sulawesi, paddy rice (Oryza sativa L. ) is generally double cropped with soya- bean (Glycine max L. ) in the dry season. Decline in soya-bean yield and response to fertilisers after some years indicates that these cropping systems may not be sustainable in terms of soil structural properties. We investigated the deterioration of soil structure by measuring penetration resistance, air-filled porosity, bulk density and plant available water content in rice paddy-soya-bean rotations of 10, 16 and 21 years duration. These results were compared with those from a 21 year rotation in which rice was followed by grass fallow. The non-limiting available water range was used to assess cropping system differences.

Rice-soya-bean cropping systems were detrimental to soil physical conditions in the long term. Bulk density and soil strength increased and air-filled porosity decreased significantly over a period of 21 years. At this time, aeration porosity at the upper drained limit was below the critical value for adequate aeration, soil penetration resistance exceeded the limits for root growth within the plant available water content range, and the non-limiting range of plant available water was narrowed. These results indicate that double cropping is unsustainable in terms of soil structure quality. Shorter periods ( 10 years) of double cropping did not cause the same level of deterioration and the grass fallow system had no discernible limitation to available water as a result of poor aeration or high soil strength. We conclude that grass fallow cropping systems appear to be more sustainable and in long- term double cropped systems, a period of grass fallow is probably required to restore soil structure to a favourable state.

Keywords: Crop rotation; Non-limiting available water; Porosity; Soil strength

*Corresponding author.

0167-1987/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDIOI67-1987(94)OO392-R

340 A. Cass et al. / Soil & Tillage Research 31 (I 994) 339-352

I. Introduction

In South Sulawesi, Indonesia, a large proportion of the rice (Oryza sativa L. ) crop is produced under rainfed paddy conditions during the wet season. During the dry season, a second crop such as soya-bean (Glycine m a x L. ) or maize (Zea mays L. ) is often sown to utilise soil water stored from rain falling late in the monsoon season. Paddy fields are prepared by puddling the surface soil after flooding to several centimetres depth. After the rice harvest, soil is tilled by water buffalo with a single tined implement prior to sowing the second crop. At this time, the soil is usually wet and tillage often takes place with the soil moisture content above the plastic limit.

Prior to 1964, land in the Balocci District (4°51 'S, 119°27'E), 70 km north of Ujung Pandang, was used for rainfed paddy rice production in the wet season with either a grazed natural pasture fallow or a second crop of maize or vegetables in the dry season, depending on the decision of individual farmers. In 1964, the South Sulawesi Department of Agriculture introduced soya-bean for cropping after rice. Growth periods for rice and soya-bean are 4 and 3.5 months, respectively. In favourable years, an additional crop of maize may be sown after the soya-bean harvest to utilise the land during the remaining 4 months of the year. Combina- tions of rice-soya-bean or rice-soya-bean-maize or rice-maize are common crop sequences. These intensive systems were initially successful in raising crop production and almost every paddy field in the Balocci District was cultivated in this way by the late 1960s and early 1970s.

Indications of unsustainability began to appear in intensively cropped fields in the late 1970s and became widespread in the 1980s. Farmers complained that soya-bean response to applied fertilisers was declining and yields decreasing. There was a marked decline in soil structural quality, manifested as an increase in soil compaction and strength. As a result, the soil became more difficult to till in preparation for the dryland cropping phase and more susceptible to waterlogging after rain.

Soya-beans deplete soil water reserves more than most other crops (Burch et al., 1978 ) and they frequently suffer from water stress because of restricted available soil water (Brown ct al., 1985 ). The sensitivity of soya-bean to water stress suggests that the yield reduction observed in intensive cropping systems in South Sulawesi may be due to reduced availability of soil water associated with decline in soil structure. This assertion is supported by results reported by Meelu et al. ( 1979 ) who found that grain yield of wheat following paddy rice progressively decreased in the second and third years of their experiment, in part owing to the deleterious effects of paddy rice on soil structure and water storage.

The top 100-200 mm of paddy rice soil is puddled to create the conditions necessary for rice growth. Puddling produces changes such as reduced drainage, lower air-filled porosity and higher soil strength (Prihar et al., 1985 ). In partic- ular, Sharma and De Datta ( 1985 ) showed that, although total porosity changes were not large, puddling eliminated most pores larger than 30 pm in favour of

A. Cass et al. ~Soil & Tillage Research 31 (1994.) 339-352 341

pores less than 0.6/~m. Although puddling may increase water retention (Sharma and De Datta, 1985 ), other structural effects may reduce plant available water. Letey ( 1985 ) defined the non-limiting water range as the range of water content within which there is no restriction in either aeration or soil strength between the upper drained limit and wilting point which will limit root activity and growth. Reduction of air-filled porosity or an increase of the slope of the strength characteristic (strength-water content relationship) as a result of puddling may reduce the non-limiting available water range.

Tillage of the soil after the rice harvest may recreate pores larger than 30 #m. However, the structure of paddy soil is likely to be unstable as a result of mechan- ical shearing of aggregates during puddling. Furthermore, intensive cropping decreases soil organic matter and hence the organic bonding of soil particles. Con- sequently the large pores formed during tillage will tend to collapse during subsequent rainfall and the soil will tend to revert to a less favourable structural condition.

The long-term effects on sustainability of rice-dryland crop rotations arising from these adverse physical changes are not known and little information on the effects of these systems on soil physical condition has been reported. The aim of the work reported here was to measure progressive changes in soil structure due to rice cropping without a period of structure restoration under grass. We wished to compare the soil physical properties of continuous rice-soya-bean crop rotations with a less intensive rice-grass fallow system. Information of this nature can assist in identifying practices that cause reduced crop productivity and which limit the sustainability of the soil resource. This information can be used to for- mulate management systems that sustain both production and soil quality over time.

2. Methods and materials

2.1. Site selection

Four unreplicated experimental sites, each about 0.5 ha in area, were selected in the Balocci District of South Sulawesi, on the basis of similarity in soil type and properties. All sites lay within a radius of less than 1 km of each other. The cropping rotation of three sites was similar: rice followed by soya-bean (and possibly maize during favourable years). These three sites differed in the time that this rotation had been practised: one was of 10 years duration (RS 10), the second 16 years (RS 16), and the third 21 years (RS21 ). The fourth site, which acted as a control, had a rice-fallow rotation of 21 years duration (RF21 ). All sites re- ceived similar management during the rice cropping phase, i.e. puddled for rice cultivation in the wet season, subjected to a fertiliser program organised by the Indonesian Department of Agriculture with no organic matter or mulch applica-

342 A. Cass et al. /Soil & Tillage Research 31 (1994) 339-352

tions. The double cropped sites were tilled to a depth of approximately 150 mm (using buffalo or bullocks) after rice harvesting was complete. The rice-fallow site (RF21 ) was allowed to revert to native grass, which was grazed by a limited number of livestock through the dry season.

A soil survey was carried out just after the rice was harvested in 1985, prior to soil preparation for soya-bean cropping, to assess the homogeneity between and within the four experimental sites. One profile pit and eight auger holes were used to characterise the soil at each site. The survey showed the soils had formed on the same alluvial parent material and the same soil type was present at each site. This similarity enables valid comparisons to be made between sites to assess the effect of cropping history on soil physical conditions.

The soils were classified as Aquic Ustifluvents. They have a clay content of 30- 34% in the uppermost 100 mm, increasing to 40-44% from 100 to 300 mm. Ka- olinite is the dominant clay mineral. Organic carbon content was low, typically 20 g, 11 g and 9 g kg -~ at 0-100 mm, 100-200 mm, and 200-300 mm, respectively. Visible structure in the topsoil at the end of the rice harvest was classified as weak to moderate, coarse, sub-angular blocky.

2.2. Phys ical propert ies

The upper drained limit of available water (0ud~) (field capacity) was measured at three locations (replicates) in the field (Peters, 1965 ) and at three times during the soya-bean growing season: May 1985 (after the rice harvest, before tillage for soya-bean establishment), July 1985 (mid-point of the soya-bean season ), and September 1985 (after the soya-bean harvest). Permanent wilting point water content of disturbed soil samples was determined once during the season using sunflower seedlings as the indicator species (Peters, 1965 ).

Bulk density (Pb) was measured at three locations in the field at the same times as 0ud~, using the excavation'method of Blake (1965a), at depth intervals of 0- 100, 100-200 and 200-300 mm. Particle density (pp) was determined once during the season by the method of Blake (1965b). These relationships were used to calculate air-filled porosity ( 0 a) at the upper drained limit as [ 1 - (Pb/Pp) ] - - 0 u d l .

Soil penetration resistance was measured on 11 occasions through the soya- bean growing season, at 7-14 day intervals, using a cone penetrometer (Rimik, Australia) fitted with a cone of 12.7 mm diameter and 30 ° included angle (American Society of Agricultural Engineering, 1983 ). Three insertions (aver- aged by the penetrometer software) were made at five randomly chosen locations (replicates) at each time of measurement on each site. The penetrometer was inserted to a depth of 300 mm, and both force on the cone and depth of insertion were recorded electronically at 15 mm intervals. Average penetration resistance over depth intervals of 0-50, 50-100, 100-200, and 200-300 mm were calculated from the values at 15 mm intervals. Soil samples from the same depth intervals were taken for water content determination at the five locations where soil penetration resistance was measured.

A. Cass et al. /Soil & Tillage Research 31 (1994) 339-352 343

The relationship between penetration resistance (P) and degree of saturation (0/0s), the penetration resistance characteristic, for the rice-fallow and rice-soya- bean rotations fitted a power function

e=Po(O/Os) b ( 1 )

where 0 is volumetric water content, 0~ is saturated water content and Po and b are curve fitting coefficients. Penetration resistance differences between treatments were evaluated by a statistical analysis of the magnitude of the b exponent of the penetration resistance characteristic. A pooled standard error (SEp) was calculated from the standard error of a regression of P on 0/0~ as

SEp = [(SE~ +SE~ + ... +SE~) /n ] ,/2 (2)

where n is the number of regression lines being tested and SE1, SE2...SEn are standard errors of the b coefficients being tested. A t-test (Snedecor and Cochran, 1980) was used to evaluate the significance of the difference between coefficients using n - 1 degrees of freedom. The t value was calculated for each comparison of the b exponents from any two of the regressions as follows

t= ( b , - bj) / (x/~SEp) (3)

The non-limiting available water content (AO) (Letey, 1985 ) was calculated as the difference between either the upper drained limit or the water content at an air-filled porosity of 0.1 m 3 m - 3 ( 0,fp ) and wilting point (0wp) or the water content at which penetration resistance reached a value of 2.5 MPa (0pr). Non-limiting available water contents can be calculated in at least four ways in order to reveal the most limiting condition and identify and highlight those soil properties which determine actual plant available water. The four equations used were:

(a) if air-filled porosity (0a) is greater than 0.1 m 3 m - 3 and 0pr < 0wp, then

AO1 -~ Oud I -- Owp ( 4 )

(b) if air-filled porosity (0.) is greater than 0.1 m 3 m - 3 and Opt is greater than Owp, then

=0ud,-0p (5)

(c) if0, is less than 0.1 m 3 m -3 and 0pr< 0wp, then

A03 = 0alp - 0wp (6)

( d ) or, if 0a < 0.1 m 3 m - 3 and 0p~ > 0wp, then

AO4=Oafp--Opr (7)

Mean values of the quantities used in these calculations are listed in Table 1. These quantities and their inter-relationships define a soil physical parameter set that can be used to evaluate soil structure quality in relation to root growth requirements.

Actual available water capacities (mm) to a depth of 300 mm (AW) were de-

344 A. Cass et al. / Soil & Tillage Research 31 (1994) 339-352

termined from water contents measured prior to sowing, midway through and after harvest of the soya-bean as well as routine monitoring of water content at 7-14 day intervals, over depth intervals of 100 mm, from the surface to 300 mm. The actual available water capacity values were calculated as the difference between measured water contents (0") and a lower limiting water content (0:,), which was the greater value of either the water content at wilting point (0wo) or at a penetration resistance of 2.5 MPa (0pr), multiplied by depth interval (Az=z~ -Zo, z2-z~, etc.), summed to 300 mm depth:

aw=s(o*-Ox) Z (8) Values o f f Ware plotted in Fig. 1 for all cropping systems as a function of time

after sowing soya-beans.

3. Results and discussion

3.1. Bulk density

Throughout the monitoring periods, May through to July and September 1985, the bulk density of soil on the rice-soya-bean cropping system of 21 years dura-

Table 1 Average limiting water content values determined at three times (May, July and September) during a soya-bean cropping season, for four cropping systems: rice-fallow for 21 years (RF21), and rice- soya-bean for 21 years (RS21 ), 16 years (RS 16 ) and 10 years (RS 10). Each depth interval has been analysed separately for the Duncan Multiple Range Test. For each depth interval, values followed by different letters, within any column, are significantly different at P< 0.05

Cropping Depth system interval

Water contents (m m-~ )

0 s Oaf p Oud I Owp Opr

RF21 0-100 0.523a 0.423a 0.413a 0.173a 0.152a 100-200 0.490c 0.390c 0.430c 0.192b 0.137c 200-300 0.493e 0.393e 0.434d 0.191c 0.148e

RS10 0-100 0.510a 0.410a 0.416a 0.183a 0.250b 100-200 0.457d 0.357d 0.422c 0.203b 0.281d 200-300 0.487ef 0.387ef 0.427d 0.187c 0.254f

RS16 0-100 0.477b 0.377b 0.430b 0.189a 0.257b 100-200 0.460d 0.360d 0.439c 0.195b 0.275d 200-300 0.470f 0.377f 0.431d 0.184c 0.252f

RS21 0-100 0.457b 0.357b 0.425b 0.187a 0.285b 100-200 0.437d 0.337d 0.416c 0.204b 0.306d 200-300 0.440g 0.340g 0.408d 0.199c 0.313g

0s, saturated water content (or total porosity), described by the equation 1 - (BD/PD) (mean, n = 9 ); BD, dry bulk density (n=9) ; PD, particle density (2.65 Mg m -3) (n=3) ; 0afv, water content at an air-filled porosity of 0.1 m m- t (n=9) ; 0u,u, upper drained limit water content (Peters, 1965) (n=9) ; 0,~, wilting point water content for sunflower seedlings (Peters, 1965 ) (n = 3 ); 0pr, water content at which penetration resistance reached 2.5 MPa (calculated from mean penetration resistance characteristics, n = 5 ).

A. Cass et al. ~Soil & Tillage Research 31 (1994) 339-352 345

80 g >, 70 (..) ~. 60

50

• 40

30'

20

~ 10

~ 0 ~ 0

I e - - o RF21 ,o~. & - - A RS10

,' - - o ~ • n - - I RS16

,,' * ' ~ . o / ~ *--* RS21

" -

, , , , , ~ ,

2 4 6 8 10 12 14

Time after sowing soybeans (weeks)

I

16

Fig. 1. Actual available water capacity, LJ W (mm, Eq. (8)) to a depth of 300 mm, calculated from water content measured before sowing, midway in the season and after harvest as well as at 7-14 day intervals throughout the soya-bean growing season for four cropping systems, rice-fallow for 21 years (RF21), and rice-soya-bean rotations for 21 (RS21), 16 (RS16) and 10 years (RS10). Vertical bar indicates a pooled LSD (P=0.05). The broken line bridges a missing value in the second week.

tion, RS21, was higher than that of the shorter duration rice-soya-bean sites RS 10, RS 16 and the rice-grass fallow system, RF21 (Table 2 ), except for the 100-200 mm depth in the RS 16 and RS21 treatments at harvest. Generally, the longer the duration of the rice-soya-bean rotation, the greater was the soil bulk density, and therefore the lower the quality of soil structure (Letey, 1985). The bulk density

Table 2 Effect of rice-fallow (RF) and rice-soya-bean (RS) cropping systems for different durations ( l 0, 16 and 21 years) on soil bulk density (n = 3 ). Each depth interval has been analysed separately for the Duncan Multiple Range Test. For each depth interval, values followed by different letters within col- umns, and by different digits within rows, are significantly different at P< 0.05

Cropping Depth system interval

(mm)

Bulk density ( Mg m - 3 )

Sowing (May) Mid-season (July) Harvest (Sept.)

RF21 0-100 1.26al 1.25al 1.19a2 100-200 1.37el 1.32el 1.30e2 200-300 1.34h I 1.33h I 1.27h2

RSI0 0-100 1.34bl 1.30bl 1.25b2 100-200 1.46fl 1.43fl 1.36f2 200-300 1.40il 1.36il 1.30i2

RSI 6 0-100 1.39cl 1.38cl 1.32c2 100-200 1.42fl 1.39fl 1.45f2 200-300 1.40j I 1.38il 1.42i2

RS21 0-100 1.48dl 1.41dl 1.38d2 100-200 1.55gl 1.54gl 1.4292 200-300 1.52k 1 1.5 lj 1 1.44j2

346 A. Cass et al. ~Soil& Tillage Research 31 (1994) 339-352

of the 100-200 mm depth interval was generally significantly greater than that of the 0-100 mm and the 200-300 mm depth intervals, suggesting the development of a tillage pan (Table 2 ).

During drying, development of surface cracks up to 5 mm wide and 200 mm long was observed. At all sites, bulk density tended to decrease with time after sowing soya-beans in May through to September (Table 2), possibly owing to root growth of the crop regenerating structure.

3.2. Air-filled porosity at upper drainage limit

Air-filled porosity at the upper drainage limit was significantly affected by the rice-soya-bean rotation, especially in the 0-100 mm depth interval (Table 3). The rice-fallow and the rice-soya-bean (10 years duration) cropping systems had the highest air-filled porosity at this depth, values being greater than the critical value of 0.1 m 3 m -3 (Dexter, 1988). The longer duration rice-soya-bean systems had lower values (less than 0.1 m 3 m - 3 ) . Air-filled porosity declined with depth in the soil and with duration of the soya-bean rotation, especially in the sub-tillage zone, 100-200 mm. At this depth and at 200-300 mm, air-filled porosity values of the rice-soya-bean systems did not maintain a consistent pat- tern of decline with duration of cropping (Table 3) for reasons which we are unable to explain.

Air-filled porosity increased slightly during the soya-bean growing season (data not presented here) because of the small decreases in bulk density (Table 2 ). The cracks that developed during this time probably improved soil aeration and acted as preferential pathways for root growth when soil strength became high.

3.3. Penetration resistance

The rice-fallow cropping system had lower penetration resistance than the rice- soya-bean systems over the available water content range at all depths between 0 and 300 mm. Data shown in Fig. 2 are typical of all the strength characteristics

Table 3 Mean air-filled porosity (m 3 m-3) at the upper drained limit measured after soya-bean harvest (Sep- tember) as affected by rice-fallow (RF) or rice-soya-bean cropping systems (RS) for different durations ( I0, 16 or 21 years)

Cropping system Soil depth ( mm )

0-100 100-200 200-300

RF21 0.138a 0.082bc 0.092b RSI0 0.127a 0.055d 0.094b RSI6 0.080bcd 0.023e 0.030e RS21 0.062cd 0.078bcd 0.063cd

Values followed by the same letters are not statistically different according to Duncan Multiple Range Test (P< 0.05 ).


a_

~D o

m

m

== ~D

n

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0 0.15

\,x \ - fT, N ~ Wilting point x & ~ •

Willing po,nt ~ " " : ~ ' I b

• R i c e - F a l l o w 21 years - -

• Rice - Soybean 10 years • Rice - Soybean 16 years • R ice- Soybean 21 years

i I I I i

0.2 0.25 0.3 0.35 0.4 0.45

Water content (m 3 m -3)

Fig. 2. The relationship between water content and penetration resistance (penetration resistance characteristic) at 0-50 mm depth as influenced by rice-fallow and rice-soya-bean rotations of different duration.

Table 4 Relationship between penetration resistance (P) and degree of saturation (0/02) using a power function P=Po (0/0~)b- Relationships are significant at P< 0.001. Cropping systems are rice-fallow 21 years (RF21) and rice-soya-bean for 10 (RS10), 16 (RS16) and 21 years (RS21)

Cropping Soil depth Power function coefficients Regression system (mm) coefficient

Po (MPa) b r 2

RF21 0-50 0.5513 -0.7295 0.9560 50-100 0.5001 -0.8528 0.9360

100-200 0.6146 -0.7062 0.9390 200-300 0.4978 -0 .8464 0.9070

RS10 0-50 0.3657 - 1.2593 0.9160 50-100 0.4123 - 1.3023 0.9700

100-200 0.2838 - 1.7214 0.9530 200-300 0.3034 - 1.5406 0.9800

RSI6 0-50 0.3758 - 1.2756 0.9810 50-100 0.4159 - 1.3238 0.9620

100-200 0.3942 - 1.4305 0.9440 200-300 0.4001 -1.3291 0.9400

RS21 0-50 0.4180 - 1.2892 0.9880 50-100 0.3100 -1.6566 0.9810

100-200 0.2150 -2.0713 0.9750 200-300 0.1471 -2.4378 0.9540

obtained. The t-test applied to the b coefficient of the penetration resistance characteristic (Table 4) showed that the fallow system had a significantly lower slope than the soya-bean systems (Table 5 ).

The fallow system was the only cropping system that had a penetration resistance below the critical value of 2.5 MPa (Taylor et al., 1966; Gerard et al., 1982 )

348 A. Cass et al. / Soil & Tillage Research 31 (1994) 339-352

Table 5 Significance of differences between strength characteristics (Table 4) of rice-fallow (RF) and rice- soya-bean cropping systems (RS) of 10, 16 and 21 year durations

Soil depth Pooled SE Cropping system Calculated t (mm) comparison value

(Eq. (3 ) )

0-50 0.0780 RF21 vs RSI0 4.84* RS16 5.02* RS21 5.02*

RS21vs. RS10 0.18NS 50-100 0.0802 RF21 vs, RS10 3.94*

RS16 4.12" RS21 7.11"*

RS21 vs. RS 10 3.17NS 100-200 0.1079 RF21 vs. RSI0 6.64**

RSI6 4.74* RS21 8.94**

RS21 vs. RSI0 2.29NS 200-300 0.0813 RF21 vs. RS10 6.04**

RS16 4.21" RS21 14.21 ***

RS21 vs. RS10 8.17"** RS 16 10.00"*

RS16 vs. RS10 1.83NS

Asterisks indicate significance at: *P< 0.05; **P< 0.01; ***P< 0.001. NS, not significant.

within the available water content range. All other cropping systems showed penetration resistances that exceeded this critical value at water contents in excess of the permanent wilting point water content. This means that intensive cropping systems may suffer strength-induced limitations to plant available water during the dry season when soya-beans are dependent on stored soil water.

The b coefficients of the penetration resistance characteristics (Table 4, Fig. 2) suggest that the longer the rice-soya-bean rotation, the stronger the soil. How- ever, a t-test applied to these data (Table 5 ) shows that the slopes of the penetration resistance characteristic were not significantly different. Only at the depth of 200-300 mm did the 21 year duration have significantly greater strength than the shorter durations.

3.4. Non-limiting available water

Fig. 3 shows the most restrictive non-limiting available water range calculated from one of Eqs. ( 4 ) - ( 7 ) for three times during the soya-bean growing season and for four depth intervals. The rice-fallow system had more available water than rice-soya-bean systems at all times and at all depth intervals to 300 mm. Limitations to available water were particularly severe in the sub-tillage depth interval of 100-200 mm, especially for the longest rice-soya-bean system. The

A. Cass et al. / Soil & Tillage Research 31 (1994) 339-352 349

Crop rotation and duration RS21 R 8 1 6 R S 1 0 RF21

0.30 8oii deplh (ram) (a) At sowing o.2s [ ] o-so I

[ ] 50-100 [ ] loo.~

0.15

0.10

0.05

0

0.30 - - - -

0.25

O 0

0.15

0.10 L 0.05

0

0.30 t

0.25

0.20

0.15

0.10

0.05

0 RS21 RS16 RSIO RF21

Crop rotation and duration

Fig. 3. Effect of rice-fallow for 21 years (RF21) and rice-soya-bean rotations for 10 years, 16 years and 21 years ( RS 10, RS 16, and RS21, respectively ) on the non-limiting available water content range of soil at four depths at different times during the soya-bean season: (a) at sowing; (b) mid-season; ( c ) after harvest. Vertical bars indicate LSD ( P = 0.05 ).

duration of rice-soya-bean rotations markedly affected water availability. In the depth interval 0-50 mm, available water decreased from over 0.2 m 3 m - 3 for RF21 to less than 0.2 m 3 m - 3 for RS10 and to less than 0.1 m 3 m - 3 for RS21 at sowing. This disparity was maintained throughout the soya-bean growing period except for RS 10 and RF21 which had similar non-limiting available water contents in the 0 to 50 mm depth interval after harvest.

Actual available water capacity (A W, Eq. (8) ) of the soil to a depth of 300 mm (Fig. 1 ) in the rice-fallow cropping system was relatively lower at the start of the

350 A. Cass et al. ~Soil & Tillage Research 31 (1994) 339-352

season owing to the early establishment of volunteer pasture. The rice-soya-bean cropping systems had relatively more water available at this time owing to the immaturity of the soya-bean crop. Overall, during the soya-bean season, rice- fallow particularly and the shorter duration rice-soya-bean cropping systems had considerably more water available than the 21-year rice-soya-bean system. These data conform to the non-limiting available water content ranges established for the various cropping systems from soil structural measurements (Figs. 3 (a ) - 3(c)).

The overall consequence of increased bulk density, reduced air-filled porosity and increased strength under continuous long-term rice-soya-bean systems is a reduction in the non-limiting available water content. At the wet end, soil water may be limiting because of aeration restriction (air-filled porosity less than 0.1 m 3 m-3). At the dry end, high penetration resistance may restrict root extension and hence available water (Table 1 ). These observations provide at least a partial explanation for the observed decline in soya-bean production and response to fertiliser application with duration of the double cropping system.

Fig. 4 shows soil structure quality in terms of the classification system of Hall et al. (1977) for two depth intervals, 0-50 and 200-300 mm. These examples show that long-term flee-fallow and short-term rice-soya-bean systems have good to moderate structure quality. However, as the duration of double cropping pro- gresses, a decline towards poor and very poor soil structure occurs. Strength and aeration limitations were particularly severe when continuous, intensive crop-

0.20 - - , ,.,. , , I t", I

E 3 ' , 0 < '81 28 % o ,, ,_ , , . , ~ >- , ~ , ,.,, ~o

= 0. s o o

~, ~. 0 " - ~, .&B ,ee O

" ~ ~,R810(50) (..9 " 0 "~

010 .s 0(3001 D . . . .

~_ I • RF21(300) • I LU :~ ] • RS16(50)

"~ RS21 (300) a A ~ o , o . ~ e o

" ~ 0 , 0 5 R821(50) . . . . . . . . . 0 O a E o ~ o ~ O

Rs15(3oo) ~ I ~.

_ 0 ' ' ' ' J '

¢-._ 0 0.1 0.2 0.3 0.4

Plant available water (m 3 m -3)

Fig. 4. Classification of soil structure quality in terms of available water content and air-filled porosity at the upper drained limit (Hall et al., 1977) for 0-50 mm (solid symbols) and 200-300 mm depths (open symbols ) under four cropping systems. Data were obtained after soya-bean harvest. Available water was determined for non-limiting conditions with aeration and strength not taken into consid- eration (circles), aeration limiting (diamonds), strength limiting (squares) and both aeration and strength limiting (triangles).


ping had been practiced for long periods of time (21 years) and less severe for shorter periods. These results suggest that continuous double or triple cropping is not sustainable in terms of soil structural quality and as a result soya-bean productivity will continue to decline. The system with a grass fallow appeared to be more sustainable. The short duration rice-soya-bean rotation (RS 10 ) was not as damaging as RS 16 and RS21 (Fig. 4). Our results indicate that, although double cropping may be important in order to increase productivity, it should not be continued indefinitely. Restricted periods of less than 10 years of double cropping followed by a return to grazed fallowing might increase the sustainability of these cropping systems. A period of grass fallow before reaching 10 years of double cropping may restore soil structure to a favourable state, but the number of annual grass fallow rotations necessary to achieve satisfactory soil structure quality in rice-soya-bean cropping systems is unknown.

4. Conclusions

Cropping systems involving rice-soya-bean rotations in South Sulawesi were found to be detrimental to soil physical conditions in the long term. Increases in bulk density and soil strength and decreases in air-filled porosity were observed the longer the duration of the rice-soya-bean system. This caused air-filled porosity at the upper drained limit to drop below the critical value for adequate aeration, and an increasing tendency for soil penetration resistance to reach root- growth limiting values within the plant available water content range. Conse- quently the non-limiting range of plant available water was reduced. The reduced available water content range and measured water contents through the season suggest that the reported decline in soya-bean production with duration of double cropping at Bollocci was due to increasingly limited plant available water. Con- tinuous double cropping appeared to be unsustainable in terms of soil structure quality.

A rice-fallow system that was not tilled at the end of the paddy rice season, but left to regenerate grass and grazed, maintained better soil structure with more available water, lower strength and more aeration capacity than rice-soya-bean cropping systems. This suggests that a rice-fallow cropping system is able to sustain soil structural quality at a satisfactory level. However, continuous double cropping of paddy rice fields damaged soil structure in the long term (21 years) but less damage occurred in the shorter term ( 10 years).

Acknowledgements

We acknowledge field and laboratory assistance from Amir Tjoneng, Mr. Far- chan, and Anthony Balaka and financial support from The International Devel- opment Program of the Australian Universities and Colleges.

352 A. Cass et al. ~Soil & Tillage Research 31 (1994) 339-352

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