effect of compaction on the growth of pigeonpea on clay soils. iii. effect of soil type and water...

Soil & Tillage Research, 26 ( 1993 ) 163-178 163 Elsevier Science Publishers B.V., Amsterdam

Effect of compaction on the growth of pigeonpea on clay soils. III. Effect of soil type and water

regime on plant response

J .A . K i r k e g a a r d , H . B . So a n d R . J . T r o e d s o n I

Department of Agriculture, University of Queensland, St. Lucia, Qld 4072, Australia

(Accepted 12 October 1992 )

ABSTRACT

Kirkegaard, J.A., So, H.B. and Troedson, R.J., 1993. Effect of compaction on the growth of pigeonpea on clay soils. III. Effect of soil type and water regime on plant response. Soil Tillage Res., 26:163- 178.

Field studies reported in previous papers in this series showed that rainfall distribution critically determined the response of pigeonpea (Cajanus cajan [L.] Millsp.) to soil compaction. This paper reports experiments conducted under controlled conditions to further investigate the influence of water regime and soil type on the response of pigeonpea seedlings to compaction.

Undisturbed cores, 23.5 cm in diameter and 60 cm deep were recovered from compaction treatments (control, moderate, severe) at field sites on an oxisol (Krasnozem, Uf 6.31, Rhodic Paleus- talf ) and a vertisol (Black earth, Ug 5.16, Entic Pellusturt ). Pigeonpea seedlings were grown for 30 days in two experiments under either drying or well-watered conditions.

Under drying conditions, the vertisol retained more water in the surface than the oxisol. This reduced soil strength in the vertisol and root and shoot growth were unaffected by compaction. The water applied to the oxisol drained to lower depths and the surface dried rapidly, increasing soil strength and reducing root and shoot growth.

Under well-watered conditions, compaction had no effect on plant growth in the vertisol, but in the oxisol growth in both the control and severe compaction treatments was significantly lower than under the moderate compaction treatment. Reduced volumetric water and nutrient content in the control and low air-filled porosity in the severe compaction treatment are thought to be responsible for these effects.

Our results indicate the potential influence of rainfall distribution and soil hydraulic properties on plant response to compaction. Predicting yield losses resulting from compaction will require modelling approaches that incorporate the effects of compaction on root growth and crop water use.

Correspondence to: J.A. Kirkegaard, CSIRO, Division of Plant Industry, PO Box 1600, Can- berra A.C.T. 2601, Australia. ~Present address: Parliament House Library, George St., Brisbane 4000, Australia.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0167-1987/93/$06.00

164 J.A. KIRKEGAARD ET AL.

I N T R O D U C T I O N

Pigeonpea is a perennial food legume which has demonstrated considerable potential as a new field crop in Australian agriculture (Meekin et al., 1987 ). In the summer rainfed cropping areas of Australia, where pigeonpea offers promise as a profitable rotation crop, compaction of clay soils causes significant yield reductions in a number of crops (McGarry, 1990), and was thought responsible for poor growth and yield of pigeonpea at several sites (Kirke- gaard, 1990).

In the previous papers of this series, the effect of compaction on the growth of pigeonpea was investigated at two sites in south-east Queensland, Australia, with contrasting soils and climatic conditions (Kirkegaard et al., 1992a,b). At both sites, the distribution of rainfall was critical in determining plant response. In dry conditions, the high soil strength of the compacted layer restricted root penetration which reduced water uptake and shoot growth. Under wet conditions, soil strength was low and plants did not rely on stored subsoil water. As a result, the impact of compaction on plant growth was small. In contrast, Voorhees (1987) reported greater compaction related yield reductions in soybean during wet seasons in northern USA. However in the cool wet conditions in that area, waterlogging and low temperatures were the main mechanisms of growth restriction and these were exacerbated in wet seasons.

In addition to rainfall distribution, soil hydraulic properties may influence plant response to compaction. The effect of compaction on infiltration and storage of water was a major limitation on a compacted vertisol (Kirkegaard et al., 1992a) but was mitigated on an oxisol owing to its higher hydraulic conductivity (Kirkegaard et al., 1992b ). Similarly, deeper penetration of light rain on the oxisol temporarily alleviated the high strength in the compacted zone and enabled root development, although the surface soil dried rapidly.

These interactions between soil and climatic factors determine the temporal variability in soil water and strength profiles to which plants continually respond. Such interactions are difficult to interpret from the results of trials at different sites since many climatic factors vary simultaneously.

This paper describes two glasshouse experiments using undisturbed cores taken from the field sites described previously (Kirkegaard et al., 1992a,b). Plants were grown in a glasshouse with controlled water regimes to investigate the interactions of soil type and water regime on plant response to compaction without the confounding effects of different climates.

MATERIALS A N D M E T H O D S

Collection of undisturbed cores

Deep-ripping and roller compaction were used to create compaction treatments for field trials on a vertisol at Dalby Agricultural College ( 151 ° 17' E,

EFFECT OF COMPACTION ON PIGEONPEA GROWTH. IlL 165

27°9'S) and an oxisol at Redland Bay Research Farm ( 153 ° 19'E, 27°37'S) Queensland, Australia (Kirkegaard et al., 1992a,b). Each site was ripped to a depth of 25 cm using rigid vertical tines spaced 50 cm apart. Secondary tillage was carried out using tined implements on a tractor with 2 m wheel spacing to avoid recompaction of the plots. Compaction treatments were applied by removing 5 cm of loose topsoil with a rake, and rolling the moist subsoil with either two (moderate) or six (severe) passes of a road roller weighing 2200 kg and roller width of 1.2 m. The loose soil was replaced to provide a seedbed. Plots ( 10 m × 2 m) of each of the three treatments were prepared adjacent to the field trial sites for the collection of undisturbed cores viz: ( 1 ) C1, Deep ripped 25 cm; (2) C2, Deep ripped 25 c m + moderate compaction (two passes of roller) at 5 cm depth, (3) C3, Deep ripped 25 cm + severe compaction (six passes of roller) at 5 cm depth.

Large undisturbed cores, 23.5 cm in diameter, were collected from both sites I week after compaction using the method described by Marchant et al. (1987). In this method polyvinylchloride (PVC) pipe, cut to the desired length, was fitted with a steel cutting tip and forced to the desired depth using a truck-mounted hydraulic drilling rig. After extraction, the cutting tip was removed and a moulded PVC cap fitted to the bottom of the core to produce a pot of undisturbed soil for experimental use.

Four short pots (60 cm) and four long pots ( 120 cm ) were collected from each treatment at both sites. The length of the four long pots varied from 80 to 120 cm owing to variation in subsoil conditions from site to site which limited the depth to which the pipe could be inserted.

Treatments and experimental design

The effect of the three field imposed compaction treatments on the early growth of pigeonpea on the two clay soils was investigated in two consecutive glasshouse experiments. All pots were initially intended for use in Experiment 1, the four 60 cm pots of each treatment to be harvested 23 days after sowing and the four 80-120 cm pots 40 days after sowing. The second stage of Experiment l was abandoned owing to difficulties in the interpretation of water use data resulting from the variation in the length of the longer pots. It was also clear that growth differences which arose prior to the first harvest in the short pots were simply being maintained in the longer pots. The plant tops were removed from the long pots on Day 40 and the pots cut to 60 cm length, resealed, and used in Experiment 2. In both experiments the 24 pots (factorial arrangement of four replicates of two soil types and three compaction treatments ) were arranged across the glasshouse in four rows in a completely randomized design with guard plants placed in extra pots at the ends of each r o w .

166

Cultural conditions and watering regime

J.A. KIRKEGAARD ET AL.

Experiment I--Growth into drying soil All pots were initially wet up to estimated field capacity (EFC) based on

the bulk density profiles measured at the field site and the water desorption curve (EFC calculated at -0 .1 MPa) for each soil (Kirkegaard et al., 1992a,b ). The vertisol was wet to EFC 10 days prior to planting and the oxisol 4 days prior to planting. All pots were covered from the time of wetting until 2 days prior to planting when they were uncovered to allow surface drying. Approximately 40-50 mm of water were required to wet the pots to EFC, and on the vertisol much of this flowed directly into soil cracks with little surface ponding.

Three pregerminated and inoculated seeds of pigeonpea (cv 'Quantum') selected for uniformity of size were planted in each pot on 7 January 1988. Each pot was fertilized 4 h prior to planting with the fertilizer mix shown in Table 1. The fertilizer was added to the surface in 200 ml of water (4.5 ram) and watered in with a further 200 ml. These rates were equivalent to those used in the field trials at each site.

During the first 3 days after planting the surface soil was kept moist to allow even emergence. On Day 7 each pot received a final 200 ml and plants were left to grow in a drying soil profile. By Day 15, plants in some treatments had ceased to grow owing to extremely dry surface soil and an additional 1000 ml (23 mm ) of water was added to each pot. The pots received no further water for the remainder of the experiment. Each pot therefore experienced two drying cycles; the first between Days 7 and 14, and the second between Days 15 and 26.

Water loss from the pots throughout the experiment was measured by weighing the pots periodically. A mobile frame with a pot harness attached to an electronic spring balance allowed pots to be weighed in situ with minimum disturbance. The pots were weighed on Days 7, 14, 15, 16, 18, 20, 23 and 26.

TABLE 1

The forms and equivalent rates of elements applied to pots in Experiments 1 and 2

Element Form Equivalent rate (kg ha- l )

Vertisol Oxisol

P P205 9 18 K KCI 0 50 Cu CuSO4 2 2 Zn ZnSO4 1.8 1.8 Mo Trace in P205 0.02 0.04

EFFECT OF COMPACTION ON PIGEONPEA GROWTH. IlL 167

Experiment 2--Growth into soil maintained at EFC After cutting and resealing the longer pots from Experiment 1, all were rewet

to EFC, covered, and left to equilibrate for 4 days. Five pregerminated seeds of pigeonpea (cv 'Quantum' ) were planted in each pot on 17 March 1988 and pots were left covered prior to emergence to prevent surface evaporation. Three days after planting, when seedlings were beginning to emerge, the cover was removed and the soil surface kept moist until Day 6 when plants were thinned to three per pot. After thinning, a 3 cm layer of white plastic beads was added to the surface to reduce direct evaporative loss. The pots were weighed and rewet to EFC on Days 6, 8, 12, 14, 18, 21, 25, 28 and 33 for calculation of water use.

The glasshouse conditions during each experiment are given in Table 2. The range of temperatures was similar to those experienced at the trial sites (Kirkegaard et al., 1992a,b ), although the higher humidity reduced the evaporation rate. Temperature, humidity and evaporation rates were lower for Experiment 2 owing to the later planting although the heaters maintained the air temperature above 20 ° C.

Soil measurements

Soil measurements were taken at the end of each experiment. Two replicates of each treatment were measured in the driest state at harvest and the remaining two were rewet to EFC and measured after 5 days of equilibration.

Pots were cut longitudinally to expose the soil core which was cut into measured sections of 0-5, 5-10, 10-15, 15-20, 20-25, 25-35, 35-45 and 45-55 cm. The soil sections were weighed to determine bulk density and subsampled

TABLE2

Glasshouse conditions during Experiments 1 and 2

Experiment 1 Experiment 2

Temperature ( ° C) Average maximum 30.7 28.0 Average minimum 23.0 20.7 Range 20-32 20-30

Humidi ty (%) Average 09 : 00 h 71 68 Range 65-95 65-94

Evaporation ( m m day- 1 ) Average daily 5.5 3.3 Range 2.1-6.7 2.0-4.2


for water content. The remaining soil was stored at 4 °C for root length determination. Penetrometer readings were taken on each exposed surface as the soil was cut using a Geotester hand-held penetrometer (Effegi, 43011, Alfon- sine, Italy) with a blunt 6.2 m m diameter trip. Air-filled porosity (AFP) was calculated from bulk density and water content data assuming a particle density of 2.65 Mg m -3 for both soils.

Plant measurements

Experiment 1 Plant height was measured on Days 11, 13, 15, 17, 19, 23 and 26. The length

of the terminal leaflet on the third trifoliolate leaf was measured on Days 17, 19 and 23 for estimation of leaf elongation rate. On Day 26, leaf water potential was measured using a pressure bomb on the youngest fully expanded leaf on two plants in two replicates of each treatment. Plants were then cut at the soil surface and separated into leaf and stem components for leaf area (plan- imeter) and dry weight measurement.

Observations of root distribution were made on the exposed soil cores after they were cut longitudinally. After weighing for bulk density determination, the soil sections were hand washed through a 1 m m sieve and root length was measured using computer assisted image analysis.

Experiment 2 Plant height was measured on Days 6, 8, 12, 14, 18, 21, 25, 29 and 33. The

length of the terminal leaflet on the third trifoliolate leaf was measured on Days 18, 21 and 25 for estimation of leaf elongation rates. The stomatal conductance of the youngest fully expanded leaf was measured using a steady state porometer in each pot at 12:00 and 16:00 h on Day 28 (after 3 days drying) and again the following day, 24 h after rewetting the pots to EFC.

On Day 33, after 5 days of drying, leaf water potential was measured on the youngest fully expanded leaf in two replicates of each treatment. Plants were then cut at the soil surface and separated into leaf and stem for leaf area and dry weight measurements. No root measurements were taken owing to the roots remaining in the pots from plants grown in Experiment 1.

Data analysis

Data from soil measurements at EFC and final harvest were analysed sep- arately for each soil using ANOVA with three compaction treatments and four replications. Least significant differences were calculated to separate treatment means where significant differences existed at 5% probability (P= 0.05 ).

Data from water loss and plant measurements were analysed using ANOVA of the completely randomized design with factorial combinations of two soil

EFFECT OF C O M P A C T I O N ON PIGEONPEA G R O W T H . Ill. 169

types, three compaction treatments and four replications. LSD's were calculated to separate treatment means where significant differences existed at a probability level of 5%.

RESULTS

Soil measurements

The drying cycle, and the presence of plant roots in the longer pots cut for use in Experiment 2, had no significant effect on the bulk density profiles. Bulk density values were similar in both experiments. Mean bulk density profiles for the two soils are shown in Fig. 1. The bulk density for the vertisol is expressed at a water content of 0.35 g g- 1 assuming that normal soil shrinkage occurs over the range of water contents measured. In addition to the similar- ity between experiments, the bulk density profiles compare well with those measured on the field plots (Kirkegaard et al., 1992, a,b). Compaction increased bulk density in the 5-10 cm layer in both soils. The C3 treatment had significantly higher bulk density than the C2 treatment in the vertisol, but not in the oxisol.

Gravimetric water and soil strength profiles at EFC and in the driest profile measured at the end of Experiment 1 are shown in Fig. 2. The difference in these profiles represents the change which occurred during Experiment 1, while the conditions at EFC represent those which existed throughout Experiment

0

10

~ 20

..c: 30

~ 4o r---i

5o

6o

080 o

lO

E 2o

.c: 30 N_ ~ 40

5O

60

Bulk density (Mg m -3) 0.80 1.00 1.20 1.40 ~.60

(a) vertisol (at eg = 0.35g g 1)

1.00 1.20 1 40 1.60 • ~ I

b) oxisol

Fig. 1. Bulk density profiles for (a) vertisol and (b) oxisol undisturbed cores used in Experi- ments 1 and 2. (O Deep ripped (CI); X, moderate compaction (C2); • severe compaction (C3) ). LSD's ( P = 0.05 ) are indicated by horizontal bars where significant differences exist.


~ 20

~= 30

c~ 40

60

(a) vertisol Gravimetric Water (gg-1)

0.10 0.20 0.30 0.40 0.50 . I i i I

DRIEST ~.q~ ~ EFC PROFILE ~ - -

0.10

10

~ 20

30

4O

50

Penetrometer resistance (MPa) 1.0 2.0 3.0 4.0 5.0

i i i i i ~ o , , _ \ t -

~ , ~ ' ~ D RIEsT

(b) oxisol Gravimetric Water ( g g - 1 ) Penetrometer resistance (MPa) 0 . 2 0 0 . 3 0 0.40 0 . 5 0 0 1 . 0 2 . 0 3 . 0 4 . 0 5 . 0

'

DRIEST -~,\'~[. EFC I ~ ~(~ EFC ~'~,.~.~DRIEST

Fig. 2. Gravimetric water and soil strength profiles at EFC ( - - ) and the driest state measured in Experiment l ( .. . . ) in undisturbed cores of (a) vertisol and (b) oxisol. (© Deep ripped (C 1 ); X, moderate compaction (C2); • severe compaction (C3) ). LSD's ( P = 0.05 ) for the main effect of compaction are shown by horizontal bars to the right of relevant treatments where significant differences exist.

2. The gravimetric water content for surface soil (0-10 cm) at field capacity ( - 0 . 1 MPa) is 0.36 g g - i for the vertisol and 0.33 g g - l for the oxisol and this does not change significantly to 60 cm depth. Neither soil was wet to field capacity throughout the profile at EFC.

At EFC the vertisol held water above field capacity near the surface which led to low levels of soil strength in all treatments (Fig. 2 ( a ) ). The lower water content and higher soil strength at depth in the C2 treatment may have resulted from drier soil conditions at the time of sampling (owing to an underestimation of water required to wet these pots to EFC). This is consistent with difficulties that were encountered in the penetration of the cores into the soil in this treatment during collection of the pots. At final harvest the water content had declined at all depths in the vertisol except in the 20- 30 cm zone of the C2 treatment. In this drier state, significant differences in soil strength were present among treatments although only the C3 treatment exceeded 3 MPa (considered the critical limit for root growth) in the surface zone.

On the oxisol, rapid drainage and evaporation from the surface resulted in high soil strengths (Fig. 2 (b) ). Compaction increased soil strengths to a depth

E F F E C T O F C O M P A C T I O N O N P I G E O N P E A G R O W T H . I l l . 1 71

of 20 cm. At final harvest, the strengths in the surface of C2 and C3 treatments exceeded 5 MPa (limit for penetrometer) in the top 20 cm while no differences existed below that depth. Little change in water content occurred below 20 cm for C3 or below 40 cm for C2 while the water content in C1 was reduced at all depths.

An AFP of 10% is considered to be critical for adequate oxygen availability to roots (Dexter, 1988). Air filled porosity at EFC remained above 10% in the surface of the vertisol and between 8 and 10% at depth (data not shown). In the oxisol, AFP remained above 10% at all depths except in the 5-10 cm layer of C2 and C3 treatments where it was 6.8% and 3.5%, respectively.

Shoot growth

Plant heights measured on the two soils throughout the experiments are shown in Fig. 3. Overall, growth was better in the vertisol than the oxisol and there was no significant effect of compaction on plant growth in the vertisol in either experiment.

(a) EXPERIMENT 1 56 [ 56 48 vertisol 48

Eo 40 40 3 2 32

f "~ 24 24

I I I I o s 1'o ,'52o2 3; 3s o

56 48

E 4o

32 "~ 24 J~ "E 16 ¢13 E" 8

DRYING SOIL)

oxisol

, i i l l I I

S I I I I I 5 10 15 20 25

I ' 30 35

Days after sowing

(b) EXPERIMENT 2 SOIL at EFC) 56

oxi o

4O

32

24

16

8

; ,'o 2; ; 1'o 25 3() 35

Days after sowing

Fig. 3. Effect of compaction on plant height of pigeonpea in undisturbed cores of two clay soils under (a) dry soil conditions (Experiment 1 ) and (b) soil maintained at EFC (Experiment 2 ). ( ( O ) Deep ripped (C 1 ); X, moderate compaction (C2), ( Q ) severe compaction (C3) ). LSD's (P= 0.05 ) for the interaction of soil type X compaction treatments are indicated by vertical bars where significant differences exist.


The response to compaction on the oxisol differed between the experiments. Significant differences in height were apparent on Day 11 in Experi- ment 1 and on Day 12 in Experiment 2, and these early differences persisted throughout the experiments. In Experiment 1, early growth was greatest for the C 1 treatment while for C2 and C3 treatments, growth had ceased prior to the addition of water on Day 15 (Fig. 3 (a)) . Despite an immediate response to the applied water, the growth of plants in C3 pots had again virtually ceased by Day 20. The trends in plant height were reflected in other plant parameters at harvest (Table 3). Reduced growth with increasing compaction on the oxisol was associated with decreased leaf elongation (Table 4). The trend toward increased leaf water potential on the oxisol with increasing compaction was not significant primarily owing to large variation in the C2 treatment (Table 4 ).

Under the wetter conditions of Experiment 2, growth was greatest in C2 pots on the oxisol while growth on C 1 and C3 was restricted (Fig. 3 (b), Table 3 ). There was no significant effect of compaction treatments on the vertisol although there was a trend toward reduced growth on the C2 treatment. Leaf development and elongation and stomatal conductance followed similar trends to overall growth (Table 4). The lower leaf water potential on C2 vertisol pots was consistent with the trend toward reduced growth and presumably resulted from the drier subsoil conditions on this treatment (Fig. 2 (a)) . The high levels of leaf water potential for all treatments on the oxisol suggest that either growth differences on the oxisol were not the result of low soil water availability, or that leaf water potential is insensitive to these effects. Reduced stomatal conductance and leaf elongation may occur without changes

TABLE3

Effect of compaction on shoot dry weight, leaf area and specific leaf area (SLA) of pigeonpea in undisturbed cores of two clay soils under dry (Experiment 1 ) and well-watered conditions (Experi- ment 2 ). LSD's ( P = 0.05 ) are shown for the soil type X compaction treatment interaction

Vertisol Oxisol LSD (P=0 ,05 )

C1 C2 C31 C1 C2 C3

Experiment 1 (Day 26) Shoot dry wt (g per plant) 0.53 0.39 0.53 0.36 0.17 0.10 0.18 Leaf area (cm 2 per plant) 92 61 86 61 29 14 35 SLA (cm2g -~ ) 264 240 245 253 237 194 35

Experiment 2 (Day 33) Shoot dry wt (g per plant) 1.54 1.28 1.44 0.48 1.25 0.49 0.38 Leaf area (cm 2 per plant) 250 184 234 84 220 86 68 SLA (cm2g - l ) 275 227 269 285 296 258 45

~C 1, deep ripped; C2, moderate compaction; C3, severe compaction.

EFFECT OF COMPACTION ON PIGEONPEA GROWTH. Ill. 173

TABLE4

Effect of compaction on midday leaf water potential (MLWP), leaf elongation rate (LER) and stomatal conductance (SC) of pigeonpea on two clay soils under dry (Experiment 1 ) and well watered conditions (Experiment 2). LSD's (P=0 .05 ) are shown for soil typeXcompaction treatment interaction

Vertisol Oxisol LSD (P=0 .05 )

CI C2 C3 j CI C2 C3

Experiment 1 (Day 26) MLWP ( - M P a ) 1.88 2.08 2.01 1.30 2.50 3.30 ns LER (mm day -~ ) 6.4 5.7 6.6 6.6 3.1 0.8 2.9 Days 17-19

Experiment 2 (Day 33) MLWP ( - M P a ) 1.12 1.59 1.32 1.00 0.92 0.98 0.15 LER (mm day -~ ) 12.0 9.7 11.2 5.5 9.7 4.2 3.0 Days 18-21 SC ( c m s - I ) 12 :00h 1.91 1.22 1.81 2.48 2.84 2.14 0.65 Day 28 16:00 h 2.05 0.89 1.62 2.52 2.92 1.65 0.89

IC1, deep ripped; C2, moderate compaction; C3, severe compaction.

in leaf water potential in response to poor aeration (Jackson and Hall, 1987 ) or increased soil strength (Masle and Passioura, 1987 ). This may explain the reduced growth of plants in C3 oxisol pots in the absence of reductions in leaf water potential (Tables 3 and 4). Lower leaf water potential and lower stomatal conductance on the vertisol on Days 28 and 33 presumably reflect the greater extent of drying by the larger plants in those pots (after 4 days of drying).

Root growth and water use

The root length density (Lv) measured at the end of Experiment 1 is shown in Fig. 4. Greater root growth occurred in the vertisol pots, particularly in the surface 0-20 cm zone, and there was no effect of compaction on root growth on this soil. On the oxisol, root length decreased with increasing compaction at all depths. A zone of high root length density was present at 10-30 cm depth in C l pots while no roots were found below 30 cm in C3 pots. This pattern of root growth compares well with the change in water content during Experiment 1 described earlier (Fig. 2 ). Many roots were observed growing down the side of the pots in all treatments of the vertisol, but this was not evident on the oxisol. In most cases on the vertisol these roots re-entered the soil mass through structural cracks lower down and many roots were concen- trated in these cracks.


0

10

~ 20 ..~ 30

~ 4O

5O

6O

L(v ) (cm cm 3) 0.10 0.20

Fig. 4. Effect of compaction on root length density (Lv) of pigeonpea seedlings in undisturbed cores of two clay soils under drying conditions in Experiment 1. (( .... ) oxisol; ( - - ) vertisol; ( O ) deep ripped (C 1 ); X, moderate compaction (C2), ( • ) severe compaction (C3) ). LSD's (P= 0.05) for the interaction of soil type × compaction treatment are indicated by horizontal bars where significant differences exist.

(a) EXPERIMENT (1) - DRYING SOIL 3°f 20 .~ / ' j -

E ts E 0 Emergenc~" I Rewe,t// I v

10 20 31 o Days after sowing

(b) EXPERIMENT (2) - SOIL AT EFC i i

3o

~ 20 ,//

0

10

Emergence ~ . , ~ u ~ = ~ - ~ : : :&:'- - - - - ~ 0 ~ I

10 210 30 Days after sowing

Fig. 5. Effect of compaction on cumulative water loss during early vegetative growth of pigeonpea in undisturbed cores of two clay soils under (a) dry soil conditions in Experiment 1 and (b) soil maintained at EFC in Experiment 2. (( .... ) oxisol; ( - - ) vertisol, ( O ) deep ripped (C 1 ); X, moderate compaction (C2); ( • ) severe compaction (C3) ). LSD's (P= 0.05 ) for the interaction of soil type × compaction treatments are indicated by vertical bars where significant differences exist.

The pattern of cumulative water loss in each experiment is shown in Fig. 5. The addition of beads to the soil surface reduced direct evaporative loss from 2.2 mm day-~ in Experiment 1 to 0.7 mm day-~ in Experiment 2 (Days 6- 14) although some of this reduction is also presumably owing to lower evaporative demand during Experiment 2 (Table 2 ).

After re-wetting the pots in Experiment 1 on Day 15 the initial water loss from the pots was similar presumably owing to the large proportion lost

EFFECT OF COMPACTION ON PIGEONPEA GROWTH. Ill. 175

TABLE 5

Effect of compaction on water loss from pots during various stages in Experiments 1 and 2. LSD's ( P = 0.05 ) are shown for soil type × compaction treatment interaction

Water loss (mm day- ~ )

Vertisol Oxisol ( P = 0.05 )

C1 C2 C31 CI C2 C3

Experimentl Days 6-14 2.2 1.9 2.4 2.2 1.9 1.9 ns

20-26 1.8 1.4 1.6 1.8 1.4 0.9 0.4 7-26 2.6 2.2 2.6 2.6 2.1 2.0 0.5

Experiment2 Days 6-14 0.8 0.7 0.8 0.6 0.8 0.6 ns

28-33 4.2 2.7 2.9 1.8 4.2 1.7 1.5 6-33 2.0 1.4 1.7 0.9 1.9 1.0 0.3

IC1, deep ripped; C2, moderate compaction; C3, severe compaction.

through direct evaporation. By Day 18 the vertisol pots had lost more water than the oxisol pots, and by Day 23 significant differences between compaction treatments were evident and were consistent with the differences in plant growth at that stage (Fig. 3 ). Table 5 summarizes water loss from each treatment during the early stages, when direct evaporation dominated water loss, and the latter stages when a greater proportion of water loss would presumably have been due to transpiration. In Experiment 2 significant differences in water loss were also evident by Day 18 which persisted until harvest and reflected the differences in plant growth (Fig. 3 ).

DISCUSSION

These experiments demonstrate the significant effect of water regime during early growth on pigeonpea response to compaction. In addition soil hydraulic properties influenced retention and redistribution of water in the pots resulting in different responses between soils.

Under the dry conditions in Experiment l, the vertisol retained more of the 45 mm of water initially applied to the pots in the compacted zone (Fig. 2 ), and despite similar levels of initial evaporative loss (Table 5 ), the soil strength in the surface remained lower than that on the oxisol (Fig. 2 ). Root growth, water uptake and shoot growth were unaffected by the level of surface compaction, similar to results in field trials on the vertisol, when wet conditions at sowing reduced the strength of the compacted layer (Kirkegaard et al., 1992a).


Reduced water retention in the surface of the oxisol resulted in higher soil strengths (Fig. 2) which reduced the depth and proliferation of roots (Fig. 4), reduced water uptake (Table 5 ) and increased the level of plant water- stress (Table 4). This response was similar to that observed on C3 plots in dry conditions under a rainout shelter at Redland Bay (Kirkegaard et al., 1992b) although in that trial, the C2 treatment increased early growth. The conditions in the glasshouse trial were drier than in the field trial since sup- plementary irrigation was applied on Day 6 in the field, but not until Day 15 in the glasshouse. These results indicate the potential limitation to plant growth caused by moderate compaction (C2) when conditions remain dry after sowing.

Under well-watered conditions in Experiment 2, the reduction of surface soil strength and increased availability of water compared with Experiment 1 reduced the level of plant water stress and increased plant growth in all treatments. The response to compaction on the vertisol was similar to that observed in Experiment 1 and under well watered conditions in the field trials.

On the oxisol, the absence of high evaporative water loss changed the pattern of plant response to compaction to one which was consistent with the response observed in the field trials when rainfall was frequent during early growth (Kirkegaard et al., 1992b). The low bulk density in the C1 pots reduced the volumetric water content and water uptake. In C2 pots the main- tenance of high water content in the surface resulted in soil strengths near 1.0 MPa which presumably allowed adequate root growth while the increased bulk density increased volumetric water (and presumably nutrient) availability in the surface. In C3 pots, soil strength of 2.0 MPa existed at EFC despite the high water content. The roots of pigeonpea seedlings are able to grow at 60% of their maximum rate at this level of strength (Kirkegaard et al., 1992c) which explained the lack of response to severe compaction in the field when conditions remained wet during early seedling growth. However, in this experiment the 5-10 cm layer was kept permanently at AFP of 3.8% which is likely to have impaired root growth and function as a result of oxygen defi- ciency (Dexter, 1988).

Empirical models based on the 'degree of compactness' (existing bulk density as a fraction of maximum bulk density achievable under high compres- sion) have been used successfully in northern Europe to estimate crop yield losses resulting from compaction (Arvidsson and Hakansson, 1991 ). In those environments, rainfall during the growing season is high and reliable and a consistent relationship between crop yield and degree of compactness has been achieved over several sites and seasons. The results presented in this series of papers indicate that such an approach is unlikely to be successful in sub-humid dryland cropping regions where variability in rainfall is a major problem. For example, Table 6 summarizes the influence of water regime during early growth on plant response to compaction on the oxisol in the field and glasshouse studies. At the same 'degree of compactness' (e.g. C3 ) plant growth in

EFFECT OF COMPACTION ON P1GEONPEA GROWTH. IlI. 177

TABLE 6

The effect of water regime during Days 0-26 on the response of pigeonpea to soil compaction in field and glasshouse experiments on an oxisol. The numbers in parenthesis represent growth relative to the C I treatment in each experiment

Experiment Total rainfall Absolute (g per plant ) and relative LSD (mm) growth (%) (P= 0.05 ) ( Days 0-26 )

CI C2 C31

Glasshouse Maintained at

E2 (EFC) EFC 0.48(100) 1.25(260) 0.49(82) 0.38 Field 1987 224 1.03(100) 1.18(115) 1.23(119) 0.13 Field 1988 174 1.08(100) 1.06(98) 0.88(82) ns Irrigated Field 1988 55 0.78(100) 1.12(143) 0.46(59) 0.22 Dryland Glasshouse E1 (Dry) 23 0.36(100) 0.17(47) 0.10(28) 0.18

'C1, deep ripped; C2, moderate compaction; C3, severe compaction.

the field was increased by 19% in the wet year of 1987 but decreased by 41% under dry conditions in 1988. Even larger differences have been reported under controlled glasshouse conditions in this paper. Plant response cannot be predicted by bulk density alone, but is related to the temporal variability in soil water content which influence soil strength, aeration and root growth.

The results of this paper indicate that the interaction of both soil and climatic factors will ultimately determine plant response to compaction. Pre- dicting yield losses resulting from compaction will require modelling approaches that incorporate the effects of compaction on root growth and crop water use. The availability of improved water balance models which can account for changes in hydraulic properties resulting from compaction (e.g., SWIM, Ross, 1990) allow more accurate prediction of temporal changes in the soil water profile. The prediction of temporal changes in soil strength profiles based on bulk density data and soil water data generated from such models may be possible if general equations relating these parameters such as those reported by Mirreh and Ketcheson ( 1972 ) and Kirkegaard (1990) could be established for particular soils. The linkage of such models with crop growth models accounting for the effect of soil strength on root growth would provide a powerful tool for predicting yield reductions resulting from compaction.

A C K N O W L E D G E M E N T S

This research was supported by funds from the ACIAR Pigeonpea Pro- gramme. The use of soil coring equipment and technical support from CSIRO


Division of Tropical Crops and Pastures and Division of Soils (Brisbane) is gratefully acknowledged.

REFERENCES

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effect of compaction on the growth of pigeonpea on clay soils. iii. effect of soil type and water...

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