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Sowing wheat or field pea as rotation crops after irrigated cotton in a grey Vertosol.

Abstract

The effects of green manured field pea (Pisum sativum L.), low-input (LI) wheat (Triticum aestivum L.) (seeding rate of 40 kg/ha and 85 kg/ha of diammonium phosphate), and high-input (HI) wheat (seeding rate of 100 kg/ha, 85 kg/ha of diammonium phosphate, and 180 kg/ha of urea) sown as rotation crops after cotton on soil quality; cotton growth, yield and nutrient uptake; and gross margins ($AU/ha and $AU/ML of irrigation water) were evaluated from 1993 to 1998 in an irrigated Vertosol in the central-west of New South Wales. Soil quality indicators monitored were aggregate stability (dispersion index), compaction (air-filled porosity), soil resilience to structural destruction (as geometric mean diameter of soil aggregates formed after puddling and drying of soil), exchangeable cations, calcium carbonate, nitrate-N, pH, organic C, development of arbuscular mycorrhiza (AM), and incidence of cotton root diseases (black root rot). In comparison with wheat, field pea increased soil nitrate-N levels during the early stages of the experiment and formed smaller aggregates after puddling and drying, but it was ineffective in ameliorating soil compaction. In contrast wheat was very effective in ameliorating soil compaction. Nitrate-N values under wheat-cotton rotations increased with time such that after 4 years they were similar to that under the field pea-cotton rotation. Soil chemical fertility indicators such as organic C, pH, EC, and exchangeable cations were not affected consistently by either wheat or field pea, whereas minimum tillage, retention of crop residues, and cropping phase (i.e. rotation crop or cotton) affected them more. A net decrease in organic C and an increase in EC was observed with time in all treatments. By sowing either field pea or wheat, the mycorrhizal colonisation of cotton roots was improved. Black root rot incidence was increased 3-fold by sowing field pea, but was not significantly affected by wheat. Cotton lint yield was unaffected by rotation crop, although profitability shown as gross margins/ha and gross margins/ML irrigation water were greater with wheat compared with field pea. Gross margins/ha were in the order HI wheat [is greater than] LI wheat [is greater than] field pea, and gross margins/ML irrigation water were in the order LI wheat [is greater than] HI wheat [is greater than] field pea. In terms of ameliorating soil compaction, minimising black root incidence, and maximising returns to the cotton grower, wheat is a better rotation crop than field pea. The decision to apply fertiliser and sow wheat at a higher seeding rate will depend on whether land or water is the major limiting factor.

Additional keywords: cracking clay, minimum tillage, organic C, farming system, mycorrhiza, black root rot, gross margin, Haplustert, compaction.

Introduction

In eastern Australian Vertosols, irrigated cotton (Cossypium hirsutum L.) is grown as a monoculture, in rotation with cereal or leguminous crops, or alternating with fallow (Cooper 1993). A survey conducted in December 1992 (Cooper 1993) indicated that 53% and 80% of cotton growers surveyed in the central-west and north-west, respectively, of New South Wales sowed rotation crops after cotton on a regular basis. Benefits of rotation crops were claimed to include amelioration and maintenance of soil quality, reduced incidence of disease and weeds in the following cotton crop, and increased yield of cotton (Cooper 1993). Wheat (Triticum aestivum L.) was the favoured rotation crop with 71% of cotton growers who sowed rotation crops in the central-west and 74% in the north-west of New South Wales. Other cereals such as barley (Hordeum vulgate L.) and maize (Zea mays L.) were used by 3% and 11% of cotton growers who used rotations in the central-west and north-west, respectively, of New South Wales. Grain legumes such as field pea (Pisum sativum L.) and soybean (Glycine max L.) were preferred by 13% of cotton growers in the central-west who sowed rotations, whereas 6% in the north-west preferred legumes such as soybean, field pea, faba bean (Vicia Faba L.), chickpea (Cicer arietinum L.), and dolichos (Lablab purpureus L.).

The effects of different rotation crops on soil quality and cotton yield in Australian Vertosols have been addressed in only a few studies with the major focus being soil structure and N balance (Hodgson and Chan 1984; Hulme et al. 1991; Hulugalle and Cooper 1994; Hulugalle and Entwistle 1996; Rochester et al. 1998). Depth and width of soil cracks, depth of water extraction, and, consequently, soil structural amelioration were better with safflower (Carthamus tinctorius L.) than with wheat (Hodgson and Chan 1984), although in a soil with a lower proportion of smectite no major differences were detected between the two crops (Hulme et al. 1991). In comparison with non-leguminous rotation crops, aggregate stability and soil N, but not other soil quality indices, were improved by leguminous rotation crops (Hulugalle and Cooper 1994; Hulugalle and Entwistle 1996; Rochester et al. 1998). Faba bean, in particular, was reported to increase soil N more than field pea, chickpea, or dolichos (Rochester et al. 1998). At the same time leguminous crops such as field pea and green bean (Phaseoulus vulgate L.) are known to be alternative hosts of Thielaviopsis basicola, the causal agent of bacterial root rot of cotton (Allen 1990). By sowing field pea or bean, therefore, the spread of bacterial black root of cotton may be facilitated. Vetch (Vicia sativa L.), however, can reduce the intensity of bacterial black root rot of cotton but increases infestation by Rhizoctonia solani (Rothrock and Kirkpatrick 1990). In some situations, residues of leguminous rotation crops can have allelopathic effects on the following cotton crop (Hulugalle et al. 1998a). Arbuscular mycorrhizal (AM) symbiosis is an important factor in cotton growth. Slow AM colonisation of cotton roots due to rotation with non-host crops such as Brassica spp. is reported to cause poor growth in some cotton crops (McGee et al. 1997). However, there is little published information on the effects of other rotation crops on subsequent AM colonisation of cotton in Vertosols. No economic evaluations appear to have been published on the relative costs and benefits of different rotation crops in cotton-based farming systems.

Concurrent evaluation of issues such as soil quality, soil-borne diseases, soil microbiology, and economic effects of different cropping systems on cotton production are sparse (Hulugalle et al. 1998b). The objective of this study, therefore, was to quantify the effects of wheat at 2 levels of management intensity, and green manured field pea sown as rotation crops on temporal changes in soil quality (including incidence of soil-borne diseases and arbuscular mycorrhiza); cotton growth, yield and nutrient uptake; and gross margins in an irrigated Vertosol in the central-west of New South Wales.

Materials and methods

Experimental site and treatments

The experiment was located at `Auscott-Warren', a commercial cotton farm near Warren in central-western New South Wales (147 [degrees] 46'E, 31 [degrees] 47'S) which has a semi-arid climate. The experimental site experiences a mild winter and a hot summer. The hottest month is January (mean daily maxima and minima of 33 [degrees] C and 18 [degrees] C, respectively) and the coldest month is July (mean daily maxima and minima of 15 [degrees] C and 3 [degrees] C, respectively). In central-western NSW, cotton is usually sown in early October and harvested in late April of the following year. Mean annual rainfall is 479 mm. The soil at the experimental site is a deep uniform grey clay [Ug5.25 (Northcote 1979); fine, thermic, montmorillonitic Entic Haplustert (Soil Survey Staff 1996); grey, self-mulching, calcareous Vertosol (Isbell 1996)] belonging to the Mulla Grey Phase soil profile class (McKenzie 1992). It is commonly referred to as a cracking clay as it shrinks and forms deep cracks on drying. Clay mineralogy is dominated by kaolinite, illite, and smectite in a ratio of 3:2:3 (McKenzie 1992). Soil physical and chemical properties in the 0-0.6 in depth of the experimental site are given in Table 1.

Table 1. Soil properties at 0-15, 0.15-0.30, 0.30-0.45, and 0.45-0.60 m depths in June 1993 at `Auscott-Warren'

Values of organic carbon and Ca[CO.sub.3] without parantheses are expressed on a volumetric basis (kg/[m.sup.2]), and those within parantheses are expressed on a gravimetric basis (g/100 g). Values of exchangeable cations without parantheses are expressed on a volumetric basis (kg/[m.sup.2]), and those within parantheses are expressed on a gravimetric basis (cmol (+)/kg).

EC, electrical conductivity
Soil property 0-0.15 m 0.15-0.30 m

Sand (g/100 g) 30 34
Silt (g/100 g) 18 14
Clay (g/100 g) 52 52
Organic C 1.4 (0.53) 2.1 (0.60)
pH (1:5 soil:0.01 M
 Ca[Cl.sub.2]) 7.7 7.6
EC (1:5 soil:water) (dS/m) 0.02 0.06
Air-filled porosity
 ([m.sup.3]/[m.sup.3]) 0.34 0.13
Exchangeable cations:
 Ca 2.2 (20.6) 2.9 (21.0)
 Mg 0.8 (12.6) 1.1 (12.6)
 K 0.1 (1.0) 0.1 (0.9)
 Na 0.1 (0.7) 0.1 (1.0)
Ca[CO.sub.3] 11.3 (4.3) 14.6 (4.2)

Soil property 0.30-0.45 m 0.45-0.60 m

Sand (g/100 g) 34 32
Silt (g/100 g) 12 15
Clay (g/100 g) 54 53
Organic C 2.0 (0.57) 1.8 (0.52)
pH (1:5 soil:0.01 M
 Ca[Cl.sub.2]) 7.6 7.6
EC (1:5 soil:water) (dS/m) 0.09 0.07
Air-filled porosity
 ([m.sup.3]/[m.sup.3]) 0.13 0.13
Exchangeable cations:
 Ca 3.1 (22.4) 2.7 (19.2)
 Mg 1.2 (14.2) 1.2 (13.6)
 K 0.1 (1.0) 0.1 (0.7)
 Na 0.1 (1.5) 0.2 (2.1)
Ca[CO.sub.3] 15.6 (4.5) 14.9 (4-3)


The experimental treatments (rotation crops) were first sown in June 1993 following 3 years of continuous cotton which had been subjected to intensive tillage (disc and chisel ploughing, and ridging every year; deep ripping every 2-5 years) and burning of crop residues. The rotation crops were (1) green manured field pea (Pisum sativum L.); (2) low-input (LI) wheat (Triticum aestivum L.) where seeding rate was 40 kg/ha and fertiliser was applied as diammonium phosphate at a rate of 85 kg/ha; and (3) high-input (HI) wheat where seeding rate was 100 kg/ha, and fertiliser was applied as diammonium phosphate at a rate of 85 kg/ha and urea at a rate of 180 kg/ha. The rotation crops were managed as rainfed crops except in 1993 when the high-input wheat received a single irrigation in September. The field peas were sprayed with glyphosate (Roundup[TM]) in October 1993 and incorporated into the ridges, and the wheat was harvested in December 1993. The plots were fallowed during the following summer and winter (December 1993-September 1994) and sown with cotton in October 1994 and picked in May 1995. Prior to sowing cotton, fertiliser was applied in July and August 1994 as anhydrous ammonia at a rate of 116 kg N/ha and mono-ammonium phosphate at a rate of 90 kg P/ha, and in December 1994 as water-run urea at a rate of 84 kg N/ha. From 1994 onwards, cotton was sown after minimum tillage. This consisted of re-forming old ridges (1 m spacing, 0.15 m high) and deepening furrows by direct listing with disc-hillers without any prior tillage immediately after the rotation crop phase (Table 2), followed by ridge cultivation to a depth of 75 mm with a Lilliston cultivator. A secondary working of the ridges occurred about 6 months later and involved stalk-pulling and mulching of crop residues, middle-busting of the ridges and re-formation of ridges (`hilling-up') (Table 2).

Table 2. Timetable of soil sampling times and cropping activities at `Auscott-Warren' from June 1993 to March 1998

X, time of soil sampling. Activities shown for any one date were completed during the month preceding that date
Date Months Soil
 after start sampling

30/6/93 0 X
31/7/93 1
31/10/93 4
31/12/93 6

31/1/94 7
31/3/94 9 X
30/6/94 12

31/8/94 14
30/9/94 15

31/10/94 16
31/12/94 18

31/1/95 19
28/2/95 20
31/3/95 21

30/4/95 22
31/5/95 23

30/6/95 24
31/7/95 25
31/10/95 28
31/12/95 30

31/1/96 31
30/6/96 36

31/7/96 37
31/8/96 38
30/9/96 39

31/10/96 40
31/12/96 42

31/1/97 43
28/2/97 44
31/3/97 45
30/4/97 46
31/5/97 47

30/6/97 48
31/7/97 49
31/10/97 52
31/12/97 54

31/1/98 55
31/3/98 57

Date Cropping activities

 Rotation crop phase

30/6/93 Wheat and field pea sown; fertiliser applied
31/7/93 Cotton stubble slashed
31/10/93 Field pea sprayed out
31/12/93 Wheat harvested; disc-hilling completed

 Post-rotation crop fallow phase

31/1/94
31/3/94
30/6/94 Stalk-pulling, mulching, middle-busting, and hilling
 up completed
31/8/94 Fertiliser and herbicide applied to soil
30/9/94

 Cotton phase

31/10/94 Cotton sown
31/12/94 Cotton irrigated; fertiliser applied in irrigation water;
 weeds controlled by mechanical cultivation and
 manual weeding
31/1/95 Cotton irrigated
28/2/95 Cotton irrigated
31/3/95 Cotton irrigated
30/4/95 Cotton picked
31/5/95

 Rotation crop phase

30/6/95 Wheat and field pea sown; fertiliser applied
31/7/95 Cotton stubble slashed
31/10/95 Field pea sprayed out
31/12/95 Wheat harvested; disc-hilling completed

 Post-rotation crop fallow phase

31/1/96
30/6/96 Stalk-pulling mulching, middle-busting, and hilling up
 completed
31/7/96
31/8/96 Fertiliser and herbicide applied to soil
30/9/96

 Cotton phase

31/10/96 Cotton sown
31/12/96 Cotton irrigated; fertiliser applied in irrigation water;
 weeds controlled by mechanical cultivation and
 manual weeding
31/1/97 Cotton irrigated
28/2/97 Cotton irrigated
31/3/97 Cotton irrigated
30/4/97 Cotton picked
31/5/97

 Rotation crop phase

30/6/97 Wheat and field pea sown; fertiliser applied
31/7/97 Cotton stubble slashed
31/10/97 Field pea sprayed out
31/12/97 Wheat harvested; disc-hilling completed

 Post-rotation crop fallow phase
31/1/98
31/3/98


Traffic was controlled by restricting it to specific furrows and avoiding trafficking on ridges. However, some trafficking did occur on ridges during cotton-picking as axle widths of cotton-pickers (hired from external sources) differed from other farm machinery, which was provided by the co-operating farmer. The plots, when sown with cotton, were furrow-irrigated with approximately 100 mm of water (1 ML/ha) during each irrigation event (4-6 times per growing season). After cotton harvest (in May 1995), the previously mentioned rotation crop-cotton sequences were repeated in June 1995 and 1997 by sowing into the cotton stubble. The cotton stubble was slashed after rotation crop emergence and the residues were retained in situ (n.b. residue retention replaced burning from 1993 onwards). Rotation crop management was the same as in 1993.

A summary of the cropping activities in this experiment is given in Table 2. The experimental design used was a randomised complete block with 3 replications. Individual plots consisted of 40 rows (ridges), 700 m long, spaced at 1-m intervals.

Soil physical and chemical properties

Soil was sampled with a spade from the 0-0.15, 0.15-0.30, 0.30-0.45, and 0.45-0.60 m depths of 4 sampling sites (soil pits) in each plot in rows adjacent to non-wheel-tracked furrows by using a stratified random sampling pattern (Webster and Oliver 1990). Soil from the 4 sampling sites was then bulked to give a composite sample for each depth in every plot. Sampling was done in June 1993, March 1994 and 1995, July 1996, and March 1997 and 1998 (Table 2), approximately I week after either rainfall or irrigation when profile soil water content approximated that of field capacity and the soil was traffickable. The sampling zone was restricted to a 100-m-wide zone that centred on the spot which was 300 m from the head-ditch. Measurements based on pre-trial grid sampling and analyses of particle size distribution to a depth of 1.5 m and soil strength to a depth of 0.45 m had indicated that soil variability was least in this zone.

Air-dried soil ([is less than] 2 mm diameter) was used to determine nitrate-N (by steam distillation after extraction with 1 M KCl); pH (1:5 soil: 0.01 M Ca[Cl.sub.2]); electrical conductivity (EC, 1:5 soil:water suspension); Ca[CO.sub.3] equivalent and exchangeable Ca, Mg, K, and Na (after washing with aqueous alcohol and aqueous glycerol to remove soluble salts and extraction with alcoholic 1 M [NH.sub.4]Cl at a pH of 8.5) (Rayment and Higginson 1992). Electrical conductivity was determined only in 1996, 1997, and 1998, and Ca[CO.sub.3] equivalent only in 1996 and 1998. Total soil organic carbon was determined by the wet oxidation method of Walkley and Black on soil which had been passed through a sieve with aperture diameters of 0.5 mm (Rayment and Higginson 1992). Oven-dried bulk density of the samples taken from the 0-0.15 m depth was determined on air-dried soil aggregates (1-10 mm diameter) with the kerosene saturation method of McIntyre and Stirk (1954). The data were converted to an oven-dried equivalent using an air-dry water content determined on subsamples (n.b. soil shrinkage curves from this site had indicated that volume change between air-dried and oven-dried soil was insignificant), and on oven-dried soil clods with the `saran' resin method (Blake and Hartge 1986). The results were expressed as a weighted mean of the 2 methods, with a one-third weighting allocated to values derived from the clods and two-thirds to those from the aggregates. This was done on the basis of a visual assessment of the distribution of clods and loose aggregates at 0-0.15 m depth (which included soil from the ridges). Bulk densities of soil from the 0.15-0.60 m depth were determined with the `saran' resin method only on oven-dried soil clods (mean oven-dried volume of 178x[10.sup.-6] [m.sup.3]). Coating of soil clods from all depths with the resin was done when they were air-dry, after which they were oven-dried at 110 [degrees] C for 96 h and weighed in air and water to evaluate oven-dried weight and volume (Blake and Hartge 1986). [n.b. When compared with soil cores, bulk density measured with the clod method in irrigated cotton fields tends to overestimate values by 10-20% (N. R. Hulugalle, unpublished data).] The data were converted to air-filled porosity [1-(bulk density/particle density)], where particle density is taken to be equal to 2.65 Mg/[m.sup.3]. Exchangeable Ca, Mg, K, and Na, and organic C were expressed as kg/[m.sup.2], and nitrate-N as g/[m.sup.2] by multiplying the values for each depth interval by its bulk density and the depth increment, followed by summing up all depth intervals. Soil reactivity, a measure of the self-mulching ability of the soil, was determined by puddling and oven-drying at 40 [degrees] C for 72 h a sample of air-dried soil which had been previously passed through a sieve with aperture diameters of 2 mm. The size distribution of the aggregates formed (determined by dry-sieving on a mechanical shaker at 1440 vibrations per min for 5 min) was expressed as the geometric mean diameter of the soil aggregates (Hulugalle and Cooper 1994). Dispersion (after immersion in water of EC 0.4 dS/m) of the samples taken in July 1996 and March 1998 was determined with a sediment density-specific gravity meter on soil aggregates of 1-4 mm diameter previously wetted by evaporation in a humidifier, at soil water contents ranging from 30 to 4 g/100 g (Entwistle et al. 1997). The dispersion index (g/100g) was expressed as
Dispersion = Mass of soil particles [is less than] 20 [micro]m
 index released into the suspension due to immersion in
 water/Mass of soil particles [is less than] 20 [micro]m
 released into suspension after complete dispersion of
 sample x 100


The dispersion index was plotted against soil water content, and the data were fitted to equations of the form

Y = a + b lnX

where Y is the dispersion index and X is the soil water content, and the effects of rotation crops and depths on dispersion were evaluated by comparing best-fit dispersion curves (Entwistle et al. 1997).

Disease and arbuscular mycorrhizal incidence

The tap roots of 5 seedlings were sampled from each of 10 randomly selected sites per plot in late November 1994 and 1996 (approximately 6 weeks after sowing). Sites were selected using a step-point method while traversing plots at an angle to the rows. Roots were washed free of soil and the severity of black root rot caused by Thielaviopsis basicola (Berk. & Br.) was assessed using the scale of Tabachnik et al. (1979) where 0 is healthy; 1 is [is greater than] 0-25% of tap root blackened; 2 is [is greater than] 25-50% of tap root blackened; 3 is [is greater than] 50-75% of tap root and few lateral roots blackened; and 4 is [is greater than] 75-100% of tap root blackened and no lateral roots remaining.

Soil and roots from 5 randomly selected seedlings were also collected from each plot in late November 1994 and 1996 to assess mycorrhizal development. Roots were immediately washed from the soil and stored in 50% alcohol before being cleared and stained (Koske and Gemma 1989) and assessed for the percentage of mycorrhizal colonisation (Giovanetti and Mosse 1980).

Crop growth, nutrient uptake, and yield

Cotton growth was monitored by measuring dry matter in 3 randomly selected sites of 2 [m.sup.2] in each plot at intervals of 4-6 weeks between 2 and 5 months and 2 and 6 months after sowing during the 1994-95 and 1996-97 growing seasons, respectively. Dry matter sampled from each plot was bulked to give a single sample. Total boll production was monitored in 3 randomly selected sites of 2 [m.sup.2] in each plot at intervals of 4-6 weeks between 2 and 5 months after sowing during the 1994-95 growing season, and at intervals of 1-2 weeks between 2 and 6 months after sowing during the 1996-97 growing season. Following Constable and Gleeson (1977) the dry matter data (DM, g/[m.sup.2]) were fitted to an equation of the form:

[log.sup.e] DM = a + b exp([T.sup.-1])

and total boll production (TB, bolls/[m.sup.2]) was fitted to an equation of the form

TB = a + b/(1 + exp T)

where T is time (months after sowing). The resulting slopes, intercepts, and total regressions for the cropping systems were compared within growing seasons using analysis of variance. Cotton lint yield was evaluated by mechanical harvesting at crop maturity in late April.

Cotton plants sampled in early March 1997 were used to evaluate nutrient uptake. Nitrogen in plant tissues was measured with a near-infrared protein analyser which had been pre-calibrated with the Kjeldahl method for tissue N (Handson and Shelley 1993). Plant uptake of S, Mo, Zn, B, Mn, Cu, Mg, Ca, K, and Na was evaluated by determining nutrient concentration in plant dry matter with an inductively coupled plasma-atomic emission spectrometer after microwave digestion with concentrated nitric acid (Handson and Shelley 1993). All data were analysed using the MSTAT-C data analysis software package (Michigan State University 1993).

Gross margins

Gross margins per ha and per ML of irrigation water supplied were used to show the profitability of each treatment. A gross margin is the gross income from an enterprise less the variable costs (costs directly attributable to the enterprise). Fixed costs such as depreciation, permanent labour, and overhead costs are not included. Gross margins were calculated using the input operations and yield results for each treatment. Some input prices (such as for harvesting and irrigation) were drawn from Scott (1997). The commodity and individual input prices used were the same for each season (e.g. $495/t for cotton lint). This was to prevent fluctuations in commodity prices and input prices concealing rotation effects on the gross margins.

Results and discussion

Soil reactivity and dispersion index

Greatest soil reactivity, indicated by the smallest geometric mean diameter of soil aggregates formed after puddling, occurred in the 0.15-0.30 and 0.30-0.45 m depths with field pea compared with either of the wheat crops (Table 3). Rotation crops did not have any consistent significant effects on soil reactivity in other depths. Aggregate size formed after puddling and drying of a self-mulching soil is reported to be inversely related to the degree of slaking which occurs (Grant and Blackmore 1991). Greater soil slaking may, therefore, be a feature of leguminous rotation crops in comparison with non-legumes.

Table 3. Effect of rotation crop, depth, and time on geometric mean diameter of aggregates (mm) formed after puddling and drying of soil

HI, high-input; LI, low-input; n.s., non-significant
Depth Rotation March March July
(m) crop 1994 1995 1996

 0-0.15 Field pea 0.8 4.5 3.6
 LI wheat 1.4 3.9 2.9
 HI wheat 1.7 3.3 3.5
 Mean 1.3 3.9 3.3
0.15-0.30 Field pea 4.9 5.4 3.4
 LI wheat 9.2 5.5 5.7
 HI wheat 8.7 5.4 5.3
 Mean 7.6 5.4 4.8
0.30-0.45 Field pea 5.1 5.3 5.6
 LI wheat 6.7 5.3 5.6
 HI wheat 7.1 5.5 5.4
 Mean 6.3 5.4 5.5
0.45-0.60 Field pea 7.9 4.7 6.1
 LI wheat 7.4 5.0 5.8
 HI wheat 6.4 5.0 4.9
 Mean 7.2 4.9 5.6
 Year mean 5.6 4.9 4.8

Depth Rotation March March Mean
(m) crop 1997 1998

 0-0.15 Field pea 5.5 2.5 3.4
 LI wheat 4.6 1.0 2.8
 HI wheat 5.3 1.3 3.0
 Mean 5.2 1.6 3.1
0.15-0.30 Field pea 5.1 4.7 4.7
 LI wheat 5.3 6.0 6.3
 HI wheat 5.5 5.1 6.0
 Mean 5.3 5.3 5.7
0.30-0.45 Field pea 5.3 4.9 5.2
 LI wheat 7.1 5.6 6.1
 HI wheat 6.1 5.2 5-9
 Mean 6.2 5.2 5.7
0.45-0.60 Field pea 4.1 5.9 5.8
 LI wheat 5.4 5.4 5.8
 HI wheat 4.7 5.5 5.3
 Mean 4.7 5.6 5.6
 Year mean 5.3 4.4

 ANOVA

 P s.e.

 Year (Y) 0.01 0.19
 Depth (D) 0.00 0.12
 Rotation crop (R) n.s. 0.15
 Y x D 0.00 0.28
 R x D 0.00 0.21
 Y x R n.s. 0.33
 Y x R x D 0.05 0.48


Dispersion was least with field pea at 0.15-0.30 m depth in 1996. Fitted dispersion curves for the 3 rotation crops at this time were

Field pea: DI= 6.29 - 1.18 ln SWC

r = -0.71** (s.e.(slope) = 0.38, n = 12)

LI wheat: DI = 8.87-1.81 ln SWC

r = -0.78** (s.e.(slope) = 0.46, n = 12)

HI wheat: DI= 9.27 - 1.74 In SWC

r= - + 0.83*** (s.e.(slope) = 0.37, n = 12)

where DI is the dispersion index (g/100 g) and SWC is soil water content (g/100 g). Significant differences occurred with respect to the total regression (P [is less than] 0.05) and the intercepts (P [is less than] 0.05) of the dispersion curves. However, dispersion did not differ significantly between rotation crops at other depths during 1996. Rotation crops did not have any significant effect on dispersion at all depths during 1998. Significant differences also occurred between depths (P [is less than] 0.001) during 1996 and 1998. Furthermore, between 1996 and 1998, dispersion increased at 0-0.15 m depth at soil water contents [is less than] 10 g/100 g and decreased at soil water contents [is greater than] 10 g/100 g (Table 4). Over the same period, dispersion decreased significantly at other depths. The fall in dispersion at the 0.15-0.60 m depth may be due to the increase in electrical conductivity (Table 5) (Rengasamy and Olsson 1991), whereas the changes at 0-0.15 m depth may be caused by an interaction between the cultivation associated with residue incorporation into the ridges and increased electrical conductivity.

Table 4. Regression analysis of dispersion curves for the 0-0.15, 0.15-0.30, 0.30-0.45, and 0.45-0.60 m depths in 1996 and 1998

DI, dispersion index (g/100 g); SWC, soil water content (g/100 g); n.s., non-significant
 Depth 0-0.15 m

Year Dispersion curve

 Depth 0-0.15 m
1996 DI = 6.27 -1.24 In SWC
 r = -0.63***, s.e.(slope) = 0.27, n = 36
1998 DI = 14.35 - 4.64 in SWC
 r = -0.88***, s.e.(slope) = 0.44, n = 36

 Depth 0.15-0.30 m
1996 DI = 7.94 -1.50 In SWC
 r = -0.71***, s.e.(slope) = 0.26, n = 36
1998 DI = 6.31 -1.66 In SWC
 r = -0.66***, s.e.(slope) = 0.33, n = 36

 Depth 0.30-0.45 m
1996 DI = 9.94 -2.17 In SWC
 r = -0-71***, s.e.(slope) = 0.37, n = 36
1998 DI = 3.76 -1.01 In SWC
 r = -0.55***, s.e.(slope) = 0.27, n = 36

 Depth 0.45-0.60 m
1996 DI = 18.73 -5.29 In SWC
 r = -0.89***, s.e.(slope) = 0.48, n = 36
1998 DI = 6.06 -2-14 In SWC
 r = -0.79***, s.e.(slope) = 0.29, n = 36

Year ANOVA

1996 Total regression: P < 0.001
 Intercept: n.s.
1998 Slope: P < 0.001

1996 Total regression: P < 0.001
 Intercept: P < 0.001
1998 Slope: n.s.

1996 Total regression: P < 0-001
 Intercept: P < 0.001
1998 Slope: P < 0.05

1996 Total regression: P < 0.001
 Intercept: P < 0.001
1998 Slope: P < 0.001


Table 5. Effect of rotation crop, depth, and time on electrical conductivity (dS/m) of soil

HI, high-input; LI, low-input; n.s., non-significant
Depth Rotation July March
(m) crop 1996 1997

 0-0.15 Field pea 0.08 0.26
 LI wheat 0.06 0.26
 HI wheat 0.07 0.26
 Mean 0.07 0.26
0.15-0.30 Field pea 0.11 0.20
 LI wheat 0.11 0.20
 HI wheat 0.13 0.22
 Mean 0.12 0.21
0.30-0.45 Field pea 0.16 0.20
 LI wheat 0.17 0.20
 HI wheat 0.18 0.23
 Mean 0.17 0.21
0.45-0- 60 Field pea 0.20 0.61
 LI wheat 0.22 0.63
 HI wheat 0.22 0.81
 Mean 0.21 0.68
 Year mean 0.14 0.34

Depth Rotation March Mean
(m) crop 1998

 0-0.15 Field pea 0.26 0.20
 LI wheat 0.18 0.17
 HI wheat 0.19 0.18
 Mean 0.21 0.18
0.15-0.30 Field pea 0.27 0.19
 LI wheat 0.24 0.18
 HI wheat 0.26 0.20
 Mean 0.26 0.19
0.30-0.45 Field pea 0.25 0.20
 LI wheat 0.25 0.21
 HI wheat 0.27 0.23
 Mean 0.26 0.21
0.45-0- 60 Field pea 0.27 0.36
 LI wheat 0.24 0.36
 HI wheat 0.28 0.43
 Mean 0.26 0.39
 Year mean 0.25

 ANOVA

 P s.e.
 Year (Y) 0.001 0.011
 Depth (D) 0.001 0.014
 Rotation crop (R) n.s. 0.011
 YxD 0.001 0.024
 RxD n.s. 0.024
 YxR n.s. 0.020
 YxRxD n.s. 0.041


Air-filled porosity

A significant interaction in air-filled porosity of oven-dried soil occurred between years, depths, and rotation crops (P [is less than] 0.05) (Fig. 1). Air-filled porosities at 0-0.15 m depth for both wheat treatments were greater than that for field pea in March 1997, and that for LI wheat plots was greater than those for the other treatments in March 1998 (Fig. 1). At 0.15-0.30 m depth, air-filled porosities for both wheat treatments were greater (P [is less than] 0.01) than that for field pea in July 1996, March 1997, and March 1998. Field pea also had a lower air-filled porosity at 0.30-0-45 m depth (P [is less than] 0.05) than either of the wheat treatments in March 1998. Reductions in air-filled porosities occurred with all 3 rotation crops between March 1997 and March 1998, with the greatest reduction occurring for field pea at 0.15-0.30 m depth. The reduction in air-porosity with all 3 rotation crops may be due to compaction caused by cotton-pickers trafficking on ridges during cotton-picking in April 1997. The differences in air-filled porosity between the 3 rotation crops were probably caused by their differing efficacies with respect to subsequent soil structural amelioration. The 3 rotation crop systems differ in their patterns and intensities of water extraction, with the wheat crops extracting more water and drying the soil profile to a greater depth than field pea (Cooper 1994). This is probably due to a combination of differing crop durations (green manured field pea sprayed out in October compared with wheat harvested in December) and root systems (tap root system of field pea compared with fine, fibrous root system of wheat; lower root length densities of field pea compared with higher densities of wheat). In addition, EC values measured at this site from 1996 to 1998 suggest that field pea may have suffered from salinity stress (see later discussion). These differences in water extraction are likely to have affected the intensities and rates of soil wetting and drying under the 3 rotation crops, and hence, their individual abilities to ameliorate soil structural damage (Pillai-McGarry and McGarry 1996).

[Figure 1 ILLUSTRATION OMITTED]

Air-filled porosity was significantly affected by year (P [is less than] 0.01) and depth (P [is less than] 0.001). A significant interaction also occurred between years and depths (P [is less than] 0.01). Increases in air-filled porosity occurred with all rotation crops from March 1994 to March 1997 at 0-0.15, 0.30-0.45, and 0.45-0.60 m depths. This may be a consequence of sowing rotation crops, minimum tillage, and controlled traffic which was practiced at this site since 1993 (Yule and Tullberg 1995). At 0.15-0.30 m depth, however, mean air-filled porosity was higher in March 1994 and March 1997 than in July 1996. The lower air-filled porosity in 1996 may be due to stalk-pulling, middle-busting, and ridging of wet soil in June 1996 (n.b. 130 mm of rainfall was received at the site during the 8 weeks prior to the aforementioned land preparation). The subsequent increase between July 1996 and March 1997 is probably due to sowing irrigated cotton in all plots in October 1996. The 8 wetting/drying cycles which occurred during the 1996-97 summer with irrigated cotton would have increased air-filled porosity (Pillai-McGarry and McGarry 1996). Between March 1997 and March 1998, mean air-filled porosity decreased at all depths with the greatest fall occurring at the 0.15-0.30 m depth (Fig. 1; see also discussion in previous paragraph).

Soil organic carbon

Soil organic C did not differ significantly between rotation crops at any time during the experiment. Mean soil organic C at 0-0.60 m depth averaged over times of sampling in field pea-cotton, LI wheat-cotton, and HI wheat-cotton plots was 6.9, 6-7, and 7.0 kg/[m.sup.2], respectively. Probable causal factors for the absence of significant differences in soil organic C between cotton-based cropping systems were discussed in an earlier report (Hulugalle et al. 1998b) and can be summarised in the context of the present results as follows:
 (i) The relatively high C/N ratios of cotton residues (40-80) means that
 their decomposition is very slow, and that there is a store of partially
 decomposed residues from previous cotton crops remaining in the soil.
 Organic C from these residues is added continuously to the soil by their
 slow decomposition and mineralisation. Consequently, the differential
 effects of cereals and legumes on soil organic C reported from monocultures
 of cereals and legumes (Ghidey and Alberts 1993; Holford 1990) are
 `muffled' in these soils, and the manifestation of any significant
 differences may require much longer periods of time than in this study (56
 months).

 (ii) Minimum tillage, controlled traffic, and residue retention replaced
 intensive tillage and residue burning in 1993. Reduction of tillage
 intensity and retention of crop residues reduce carbon losses from soil
 (Loch and Coughlan 1984; Dalal 1989; Lal 1997), and may, therefore, have
 dominated decomposition processes at this site such that it minimised any
 effect of rotation crop on soil organic C.

 (iii) Movement of crop residues, and soil organic C with dry soil and as
 dissolved organic C in irrigation and rainwater from the topsoil to the
 subsoil via soil cracks, and differential swelling and shrinking in
 different soil horizons may have caused redistribution of soil organic
 carbon from surface to subsoil horizons; i.e. `soil inversion' (Dalal 1989;
 Skjemstad et al. 1997).


Significant differences (P [is less than] 0.01) did occur, however, between times of sampling. Mean values of soil organic C in the 0-0.60 m depth in June 1993, March 1994, March 1995, July 1996, March 1997, and March 1998 were 7.2, 8.5, 6-3, 7.8, 5.2, and 6.3kg/[m.sup.2], respectively (s.e. [+ or -] 0.42 P [is less than] 0.01). High values of soil organic C occurred in soil sampled during the post-rotation crop fallow phase, whereas low values occurred during the cotton phase. The highest value of organic C, which was observed during March 1994, may be due partly to charcoal resulting from burning cotton stubble in 1993. Linear regression analysis indicated that there had been a significant decline in soil organic C with time such that:

Organic C (kg/[m.sup.2]) = 7.72 - 0.031t

r = -0.48*** (s.e.(slope) = 0.008, n = 54)

where t is time (months) after commencement of the experiment. Sequestration of organic carbon in soil by using minimum or zero tillage as management systems to reduce carbon-based greenhouse gas emissions has been suggested by some authors (Lal 1997). These results suggest, however, that substitution of minimum tillage and residue retention for intensive tillage and residue burning did not increase soil organic C. The decline in soil organic C is probably related to the intensive cropping and irrigation practiced at this site (McKenzie et al. 1991).

EC, pH, exchangeable cations, and calcium carbonate

EC was not affected significantly by rotation crop, but was affected by year and depth (Table 5). Highest values occurred during years when cotton was sown and at 0.45-0.60 m depth. In addition, compared with values of June 1993 at the commencement of the study (Table 1), EC has increased considerably. This suggests that salinity has increased with time, and that the salts are entering the site in the irrigation water (average EC 0.4 dS/m). Furthermore, the changes in EC with time and depth also suggest that much of this salt is leaching into the subsoil below the root-zone. These comments are supported by measurements of soil chloride at this site which indicate that a sharp increase occurs in chloride concentrations at depths between 1.0 and 1.95 m (J. Friend, pers. comm. 1998). These changes in EC and chloride concentration are unlikely to be caused by a rise in the watertable as depth to the watertable at this site is about 9 m (J. Friend, pers. comm. 1998). Conversion of EC to [EC.sub.e] by multiplying by 7.5 (the conversion factor for medium clays) (Slavich and Patterson 1993) suggests that field pea would have been subject to salinity stress at 0-45-0.60 m depth during 1996 and in the entire measured soil profile during 1998, whereas wheat would not have been affected (Ayers and Westcot 1976). These differences may partly explain the differing ability of the two crops to ameliorate soil compaction at this site (see earlier discussion).

Soil pH at any depth was not significantly affected by the individual rotation crops, although both depth and time of sampling did affect soil pH (Table 6). A significant interaction was also observed between depth and time of sampling. These changes were such that, from 1993 to 1997, pH in soil sampled during the rotation crop-summer fallow phase (1994, 1996) was less than that in soil sampled during the cotton phase or shortly thereafter (1993, 1995, 1997) (Tables 1 and 6), and may be associated with aerobic decomposition of rotation crop residues (Chorom and Rengasamy 1997). Furthermore, the intensity of these pH changes was greater in the surface 0.30 m than at 0.30-0.60 m depth. During this period, a decrease in pH also occurred, and is probably due to the substitution of minimum tillage and residue retention for intensive tillage and residue burning from 1993 onwards (Dalal 1989). In 1998 (rotation crop-summer fallow phase), however, the pattern of pH changes altered with an increase rather than a decrease occurring (Table 6). This may be caused by the fall in air-filled porosity due to an increase in soil compaction (see earlier discussion, Fig. 1) and probable associated increases in waterlogging, and reductions in microbial activity and aerobic soil processes (Torbert and Wood 1992).
Table 6. Effect of rotation crop, depth, and time on soil pH

HI, high-input; LI, low-input; n.s., non-significant

Depth Rotation March March July March
(m) crop 1994 1995 1996 1997

 0-0.15 Field pea 7.2 7.6 6.7 7.5
 LI wheat 7.2 7.5 6.6 7.5
 HI wheat 7.1 7.5 6.7 7.6
 Mean 7.2 7.5 6.7 7.5
0.15-0.30 Field pea 7.0 7.6 7.3 7.4
 LI wheat 7.0 7.6 7.3 7.4
 HI wheat 6.9 7.6 7.3 7.5
 Mean 7.0 7.6 7.3 7.4
0.30-0.45 Field pea 6.7 7.5 7.5 7.5
 LI wheat 6.8 7.5 7.5 7.4
 HI wheat 6.6 7.5 7.5 7.4
 Mean 6.7 7.5 7.5 7.4
0.45-0.60 Field pea 6.9 7.7 7.5 7.6
 LI wheat 7.0 7.7 7.5 7.6
 HI wheat 7.0 7.6 7.6 7.6
 Mean 7.0 7.7 7.5 7.6
 Year mean 7.0 7.6 7.2 7.5

Depth Rotation March Mean
(m) crop 1998

 0-0.15 Field pea 7.7 7.3
 LI wheat 7.6 7.3
 HI wheat 7.7 7.3
 Mean 7.7 7.3
0.15-0.30 Field pea 7.7 7.4
 LI wheat 7.7 7.4
 HI wheat 7.7 7.4
 Mean 7.7 7.4
0.30-0.45 Field pea 7.7 7.4
 LI wheat 7.7 7.4
 HI wheat 7.7 7.4
 Mean 7.7 7.4
0.45-0.60 Field pea 7.6 7.5
 LI wheat 7.5 7.5
 HI wheat 7.7 7.5
 Mean 7.7 7.5
 Year mean 7.7

 ANOVA

 P s.e.
 Year (Y) 0.001 0.02
 Depth (D) 0.001 0.02
 Rotation crop (R) n.s. 0.01
 YxD 0.001 0.04
 RxD n.s. 0.03
 YxR n.s. 0.03
 YxRxD n.s. 0.08


Exchangeable cations were not significantly affected by the rotation crops except during March 1995 when both wheat treatments had more Ca than field pea (P [is less than] 0.05); March 1997 when both wheat treatments had more Mg than field pea (P [is less than] 0.05), and March 1998 when LI wheat had the least Mg (P [is less than] 0.05); and March 1994 when field pea had more K (38%) than either of the wheat treatments (P [is less than] 0.01) (Fig. 2). As these treatment differences with respect to exchangeable Ca and Mg are small (8%) and inconsistent between years we suggest that they be ignored. With respect to exchangeable K, however, the observed differences in 1994 may be caused by a combination of residual effects of residue burning which can lead to higher exchangeable K (Loch and Coughlan 1984; Hulugalle and Cooper 1994) and K removal in grain by the harvested wheat. Significant differences also occurred in values of exchangeable Ca, Mg, and K between years. This was such that, from 1993 to 1997, exchangeable Ca, Mg, and K in soil sampled during the rotation crop-summer fallow phase (1994, 1996) was more than that in soil sampled during the cotton phase or shortly thereafter (1993, 1995, 1997). In 1998 (rotation crop-summer fallow phase), the pattern of exchangeable Ca and Mg changes altered with no significant changes occurring between 1997 and 1998. Exchangeable K values, however, increased between 1997 and 1998. The variations in time with respect to exchangeable Mg and Ca parallel the changes in soil pH. Values of exchangeable Na were not significantly affected by time or rotation crop. Residual effects of burning may be partly responsible for the values of exchangeable Ca, Mg, and K observed in March 1994 (Loch and Coughlan 1984; Hulugalle and Cooper 1994), which were the highest observed during this experiment.

[Figure 2 ILLUSTRATION OMITTED]

Soil carbonates, measured as the Ca[CO.sub.3] equivalent, were not significantly affected by rotation crop either in July 1996 or in March 1998. The Ca[CO.sub.3] equivalents in field pea, LI wheat, and HI wheat plots were 33.2, 35.5, and 37.1kg/[m.sup.2], respectively, in July 1996, and 58.1, 51.1, and 57.7kg/[m.sup.2], respectively, in March 1998. Significant differences occurred, however, between times of sampling, with a decrease occurring between 1993 and 1996, followed by an increase in March 1998. Mean values of soil carbonates at 0-0.6 m depth were 55.1, 35.2, and 55.6 kg/[m.sup.2] in 1993, 1996, and 1998, respectively (s.e. [+ or -] 2.16, P [is less than] 0.001). The changes in carbonates may be related to the changes in pH observed with time at this site (Table 6).

The relationship between soil organic C, exchangeable cations, and soil carbonates (primarily Ca[CO.sub.3]) in alkaline soils has been described previously (Chorom and Rengasamy 1997). To summarise, the process involves solubilisation of native carbonates by H+ released by (a) crop residue and soil organic matter mineralisation, and (b) ionisation of carbonic acid formed by dissolution of carbon dioxide, a product of microbial respiration; and release of [Ca.sup.2+] and [Mg.sup.2+], which may either become adsorbed onto the clay minerals or remain in soil solution. Regression analyses indicated that organic C and soil carbonates in the 0-0.6 m depth were related to exchangeable Ca and Mg (kg/[m.sup.2]):

Exch. Ca = 9.77 - (7.36 x [10.sup.-2])X + 0.72Y

[R.sup.2] = 0.70.** (s.e.(est.) = 0.80, n = 27)

Exch. Mg = 2.23 - (1.03 x [10.sup.-2])X + 0.27Y

[R.sup.2] = 0.43.** (s.e.(est.) = 0.40, n = 27)

where X is the Ca[CO.sub.3] equivalent (kg/[m.sup.2]), and Y is soil organic C (kg/[m.sup.2]).

Nitrate-N

Nitrate-N at 0-0.60 m depth was significantly (P [is less than] 0.01) greater with field pea than with either of the wheat treatments in 1994 and 1995; and significantly (P [is less than] 0.05) greater with either HI wheat or field pea than with LI wheat in 1996 (Fig. 3). Significant differences did not occur between rotation crops in 1997 and 1998. During the early stages of this experiment (1993-95), compared with wheat, values of nitrate-N were higher in plots where field pea was sown as a rotation crop due to its ability to fix atmospheric N, with highest values occurring during the rotation crop-summer fallow phase. The comparatively higher values of nitrate-N in field pea plots during the cotton phase also indicate that the benefits of N fixed during the rotation phase carry over to the cotton phase. Similar findings have also been reported by Rochester et al. (1998). The increase in nitrate-N in the HI wheat treatment between 1994 and 1996 suggests that the nitrogen fertiliser taken up by the wheat rotation crop is being returned to the soil due to mineralisation of wheat crop residues and has been observed with fertilised wheat-cotton rotations in north-western NSW (Hulugalle et al. 1996). The higher values of nitrate-N during March 1997 (cotton phase) in comparison with the preceding cotton phase (March 1995) suggest that this process occurs with all 3 rotation crop systems. The increase in the field pea plots may also be due partly to the increased incidence of black root rot (see later discussion) causing a reduction in root activity and a reduced N uptake by cotton (Hulugalle et al. 1998b). The very sharp increase in nitrate-N between 1997 and 1998 is probably caused by the combination of nitrates returning to the soil due to mineralisation of crop residues and a reduction of leaching losses due to the higher soil compaction (Fig. 3).

[Figure 3 ILLUSTRATION OMITTED]

Disease and mycorrhizal incidence

Black root rot incidence in cotton did not differ significantly between rotation crops in 1994 but did in 1996. Severity scores in cotton sown after field pea, LI wheat, and HI wheat were 0.6, 1.3, and 1.0, respectively in 1994 (s.e. 0.53, P [is greater than] 0.05), and 2.5, 1.5, and 1.4, respectively, in 1996 (s.e. 0.19, P [is less than] 0.05). The greatest increase (3-fold) in black root rot incidence between times of sampling occurred with field pea (s.e. 0.48, P [is less than] 0.05), whereas significant increases did not occur with either LI wheat or HI wheat. This is because leguminous crops such as field pea are alternative hosts for the black root rot pathogen, whereas cereal crops such as wheat are not alternative hosts (Allen 1990).

Mycorrhizal colonisation of cotton did not differ significantly between rotation crops and was comparable with that previously reported for healthy cotton in irrigated Vertosols (McGee et al. 1997; Hulugalle et al. 1998b). Mycorrhizal colonisation of cotton sown after field pea, LI wheat, and HI wheat was 46.7%, 48.0%, and 49.0%, respectively, in 1994, and 66.0%, 58.7%, and 73.0%, respectively, in 1996. Significant increases in mycorrhizal colonisation occurred with all rotation crops. Mean mycorrhizal colonisation in 1994 was 47.9% and in 1996 it was 65.9% (s.e. 2.23, P [is less than] 0.01). The AM colonisation of cotton roots follows a logistic growth pattern in which the rate and plateau level of colonisation vary according to the density of fungal propagules in the soil (McGee et al. 1997). Hence, the single observations made here may not give a quantitative measure of the density of AM propagules in the soil. The apparent increase in AM colonisation between 1994 and 1996 may reflect differences in crop growth at sampling or seasonal conditions. Nevertheless, rotation with either cereal or leguminous crops can provide sufficient propagules for adequate AM development of cotton in irrigated Vertosols.

Nutrient uptake, crop growth, yield, and gross margins

Cotton sown after LI wheat had lower concentrations of Ca, Mg, and B in dry matter at crop maturity in March 1997. This was such that cotton sown after field pea, LI wheat, and HI wheat had 27.0, 21.0 and 26.0 g Ca/kg, respectively (s.e. 1.20, P [is less than] 0.05); 5.2, 4.1, and 5.2 g Mg/kg, respectively (s.e. 0.20, P [is less than] 0.05); and 55.0, 40.7, and 56.0 mg B/kg, respectively (s.e. 2.51, P [is less than] 0.05). Concentrations of all other nutrients were not significantly affected by the rotation crops, although trends similar to the above significant differences did occur. Mean values of N, S, P, K, Na, Cu, Mn, and Zn were 17.6, 5.8, 1.3, 14.4, and 4.9 g/kg; and 8.5, 61.6, and 14.3 mg/kg, respectively. Nutrient uptake of cotton seems to be improved wherever an external supply of N, either as atmospheric N or fertiliser N, was available to the rotation crops. Improved uptake of all nutrients by cotton sown after a wheat crop which had been fertilised with N has been reported for a Vertosol from north-western NSW (Hulugalle et al. 1996). The additional N resulted in greater root growth by wheat, which in turn was thought to improve its `nutrient scavenging ability' in the deeper soil horizons. Decomposition of the nutrient-rich wheat residues with time resulted in these nutrients becoming easily available to the succeeding cotton crop (Hulugalle et al. 1996).

Dry matter production of cotton was not significantly affected by rotation crops during the 1994-95 and 1996-97 cotton growing season. Mean plant dry matter at 2, 3.5, and 4.7 months after sowing during 1994-95 was 15.1, 444.0, and 713.7 g/[m.sup.2], respectively, and at 1.5, 2.3, 3.3, and 5.5 months after sowing during 1996-97 was 9.6, 61.3, 276.3 and 869.0 7 g/[m.sup.2], respectively. By using pooled data for the 1994-95 and 1996-97 growing seasons, variation of dry matter production, DM (g/[m.sup.2]), with time, T (months after sowing), was described by:

[log.sub.e] DM = 18.76 - 9.70 exp ([T.sup.-1])

(s.e. (slope) = 0.27, r = -0.99***, n = 27)

[log.sub.e] DM = 13.75 - 6.00 exp ([T.sup.-1])

(s.e. (slope) = 0.18, r = -0.98***, n = 36)

Boll production was significantly greater during the 1994-95 season when cotton followed rotation crops which had N inputs either in the form of atmospheric N (field pea) or fertiliser (HI wheat), but did not differ significantly between rotation crops during the 1996-97 growing season (Table 7; Fig. 4). Cotton lint yield did not, however, differ significantly between rotation crops. Lint yield from cotton sown after field pea, LI wheat, and HI wheat was 219.3, 209.1, and 219.3 g/[m.sup.2], respectively, during 1994-95, and 201.9, 188.0, and 207.2 g/[m.sup.2], respectively, during 1996-97.

Table 7. Regressions of boll production with time for cotton sown after field pea and low-input (LI) and high-input (HI) wheat and during the 1994-95 and 1996-97 growing seasons

TB, total boll production/[m.sup.2]; T, time (months) after sowing; n.s., non-significant
Rotation crop 1994-95

Field pea TB = 77.50 -[642.86/(1+exp T)]
 r = -0.89***, s.e.(slope) = 68.96,
 n = 24

LI-wheat TB = 68.34-[558.47/(1+exp T)]
 r = -0.92***, s.e.(slope) = 49.58,
 n = 24

HI-wheat TB = 77.46-[648.88/(1+exp T)]
 r = -0.91***, s.e.(slope) = 65-18,
 n = 24

 Total regression Slope Intercept
 n.s. n.s. p < 0.05

Rotation crop 1996-97

Field pea TB = 82.48 -[859-62/(1+exp T)]
 r = -0.80***, s.e.(slope) = 117.55,
 n = 33

LI-wheat TB = 70.95-[806.14/(1+exp T)]
 r = -0.80***, s.e.(slope) = 109.39,
 n = 33

HI-wheat TB = 78.01-[791.67/(1+exp T)]
 r = -0.80***, s.e.(slope) = 105.80,
 n = 33

 Total regression Slope Intercept
 n.s. n.s. n.s.


[Figure 4 ILLUSTRATION OMITTED]

Gross margin/ha was greater with wheat than with field pea and was in the order HI wheat [is greater than] LI wheat [is greater than] field pea (Table 8). Cumulative lint yield was, however, in the order HI wheat = field pea [is greater than] LI wheat. The difference in total gross margin/ha between the highest (HI wheat) and lowest (field pea) was $1058. This was due to the gain of income from the wheat crops compared with incurring costs (and no income) from the green manured field pea crops. The field pea treatment did not result in higher cumulative lint yield compared with HI wheat, and so could not make up for the costs incurred in green manuring. The gross margin/ML of irrigation water gave different rankings than the gross margin/ha (Table 8).

Table 8. Effect of rotation crop on gross margins in $AU/ha and $/ML of irrigation water, 1993-1997
HI, high-input; LI, low-input

 Field pea LI wheat

Cumulative cotton lint yield (t/ha) 4.1 3.9
Gross margins/ha ($AU) 5128 5878
Gross margins/ML irrigation water ($AU) 546 625
Cumulative irrigation water applied (ML/ha) 9.4 9.4

 HI wheat

Cumulative cotton lint yield (t/ha) 4.1
Gross margins/ha ($AU) 6186
Gross margins/ML irrigation water ($AU) 584
Cumulative irrigation water applied (ML/ha) 10.6


When the gross margins/ML (derived from total cumulative gross margin/ha divided by cumulative irrigation water applied/ha) were compared, the results were LI wheat [is greater than] HI wheat [is greater than] field pea. The LI wheat had the highest gross margin/ML of irrigation water, even though it returned the lowest cotton lint yield of the 3 treatments and used the same amount of water as the field pea treatment. The difference was due to the positive gross margins from LI wheat crops compared with no income from the green manured field pea crops. With the irrigation water resource likely to become more costly in future years, LI wheat offers the most efficient use of irrigation water. If water was the most limiting resource, a LI wheat rotation would be the most suitable, whereas if land was the most limiting resource, an HI wheat rotation would be preferable.

Conclusions

In comparison with wheat, field pea sown in rotation with cotton increased soil nitrate-N levels during the early stages of the experiment and formed smaller aggregates after puddling and drying, but was ineffective in ameliorating soil compaction. In contrast, wheat was very effective in ameliorating soil compaction. Nitrate-N values under wheat-cotton rotations also increased with time such that after 4 years they were similar to that under the field pea-cotton rotation. Neither wheat nor field pea had any consistent differential effects on soil chemical fertility indicators such as organic C, pH, EC, exchangeable cations, and calcium carbonate. Factors such as the commencement of minimum tillage, elimination of residue burning, cropping phase (i.e. rotation crop or cotton), and compaction events appear to be the major determinants of the above indicators. By sowing either field pea or wheat, mycorrhizal colonisation of cotton roots was improved. Black root rot incidence was increased by sowing field pea, but was not significantly affected by wheat. Cotton lint yield was unaffected by rotation crop, although profitability shown as gross margins/ha and gross margins/ML irrigation water were greater with wheat than field pea. We conclude that in terms of ameliorating soil compaction, minimising black root rot incidence, and maximising returns to the cotton grower, wheat is a better rotation crop than field pea. The decision to sow wheat at a higher seeding rate and apply fertiliser will depend on whether land or water is the major limiting factor.

Acknowledgments

Funding for this project was provided by the Co-operative Research Centre for Sustainable Cotton Production and Cotton Research and Development Corporation as CRDC Project nos. DAN 83C, 100C, DAN 95C, and DAN 108C. The manager of `Auscott-Warren', Mr Chris Hogendyke, and his staff are thanked for their support in maintenance and operation of the field site, and their continuing support and interest in the project.

References

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Manuscript received 8 November 1998, accepted 24 May 1999

N. R. Hulugalle(A)(B), P. C. Entwistle(A)(B)(E), J. L. Cooper(A)(C), F. Scott(A)(D), D. B. Nehl(A)(B), S. J. Allen(A)(B), and L. A. Finlay(A)(B)

(A) Co-operative Research Centre for Sustainable Cotton Production, Narrabri, NSW 2390, Australia.

(B) Australian Cotton Research Institute, NSW Agriculture, Myall Vale Mail Run, Narrabri, NSW 2390, Australia.

(C) Agricultural Research Centre, NSW Agriculture, Trangie, NSW 2823, Australia.

(D) Tamworth Centre for Crop Improvement, NSW Agriculture, Calala Lane, Tamworth, NSW 2340, Australia.

(E) Present address: Melaleuca Plantations of Bungawalbyn, Coraki, NSW 2471, Australia.3
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Author:Hulugalle, N. R.; Entwistle, P. C.; Cooper, J. L.; Scott, F.; Nehl, D. B.; Allen, S. J.; Finlay, L.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Geographic Code:8AUNS
Date:Sep 1, 1999
Words:11646
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