Effects of plant spacing and soil conditions on the growth of five Atriplex species

Agriculture, Ecosystems and Environment, 21 (1988) 265-279

265

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

E f f e c t s of P l a n t S p a c i n g and Soil C o n d i t i o n s on the G r o w t h of F i v e Atriplex S p e c i e s C.V. MALCOLM 1, A.J. CLARKE 2, M.F. D'ANTUONO 1and T.C. SWAAN1

1Department o[ Agriculture, South Perth, Western Australia, 6151 (Australia) 254 BedweU Crescent, Booragoon, Western Australia, 6154 (Australia) (Accepted for publication 19 April 1988)

ABSTRACT Malcolm, C.V., Clarke, A.J., D'Antuono, M.F. and Swaan, T.C., 1988. Effects of plant spacing and soil conditions on the growth of five Atriplex species. Agric. Ecosystems Environ., 21: 265-279. Five Atriplex species were planted as seedlings on salt-affected agricultural soil at spacings of 1 X 1, 1 X 2, 2 X 2, 2 X 3 and 3 X 3 m. After 20 months there were highly significant differences in yield between the 5 species at the wider spacing but there was no significant difference at 1 X 1 m. The species with the largest bushes, A. amnicola Paul G. Wilson, gave 1.6 t ha- ' dry matter yield at spacings of 2 X 1, 2 X 2, 2 X 3 and 3 X 3 m. The species with the smallest bushes, A. vesicaria Heward ex Benth., gave progressively higher yields per hectare as spacing was decreased. Other species were intermediate in yield. The soil was sampled to 2 m depth and the depth and salinity of the groundwater were monitored. Yield was correlated poorly with soil and groundwater salinity and groundwater depth, although these characteristics varied greatly over the site. There were highly significant differences in the salt content of the leaves and twigs of the different species. A. vesicaria contained the most salt and A. amnicola the least. The leaf to twig ratios were also different for the different species. One severe hand stripping resulted in the death of 78% of bushes of A. ves icaria and 30% of A. paludosa R. Br., the other species survived well. Salinity levels increased in the top 2 m of soil beneath most plots during the experiment (by up to 21.7 t h a - ' C1- ) although up to 0.66 t ha -1 of ash was removed from plots during yield measurement. The experiment indicates that it may be possible to reduce establishment costs, without sacrificing yield, by planting less bushes per hectare if larger species are used. Differences in salt content and leafiness between species must be considered in selecting species. From these results it may be inferred that Atriplex spp. may assist in lowering saline groundwater levels.

INTRODUCTION T h e n a t u r a l p e r e n n i a l v e g e t a t i o n h a s b e e n r e m o v e d f r o m a b o u t 15 m i l l i o n h e c t a r e s o f l a n d i n s o u t h - w e s t e r n A u s t r a l i a t o m a k e w a y for c e r e a l c r o p s a n d a n n u a l p a s t u r e s p e c i e s . T h e c e r e a l a n d p a s t u r e s p e c i e s u s e less w a t e r t h a n t h e n a t u r a l v e g e t a t i o n . C o n s e q u e n t l y , g r o u n d w a t e r r e c h a r g e is i n c r e a s e d . W a t e r

0167-8809/88/$03.50

© 1988 Elsevier Science Publishers B.V.

266 percolating through deep cla:~ subsoils carries salt to discharge areas in hillside seeps or valley floors. As a result of the disturbance of the hydrological balance (Nulsen and Henschke, 1981 ), 264 000 hectares of previously arable land has become salt affected (Henschke, 1980 ). Attempts are being made to return the land to cereal and pasture production by reversing the hydrological changes (Sedgley et al., 1981; Nulsen and Baxter, 1982) or by drainage (George and Lenane, 1982 ). On areas which remain salt-affected, salt-tolerant forage plants may be grown to provide feed for sheep; this has been the subject of research in Western Australia for many years (Malcolm, 1969). A collection of salt-tolerant forage shrubs (Malcolm et al., 1984) is being evaluated for adaptation to salt-affected soils in Western Australia. There are major differences in size between the species which could lead to different spacing requirements. For example, bushes of A. vesicaria are commonly less than a metre in diameter and less than 0.7 m high, whereas bushes of A. amnicola are commonly 2-4 m in diameter and may exceed 0.7 m in height. In this paper we report the results of an experiment to determine the effects of spacing on the yield of 5 Atriplex species and how the yield of these species is related to soil and groundwater conditions. MATERIALSAND METHODS

The study site The experiment was conducted at Kellerberrin, Western Australia (Fig. 1) (approximately 31 ° 30'S, 117 ° 45'E ). The mean annual rainfall for the site is about 330 mm, the mean maximum temperature for the hottest month (January) is about 34°C and the mean minimum for the coldest month (July) about 5 ° C. The summer is hot and dry and the rainfall is received mainly in winter (June-August). The experimental area was cleared for agriculture about 50 years ago and has since become salt-affected. Prior to clearing for agriculture the site carried a stand of salmon gum (Eucalyptus salmonophloia F. Muell.) and york gum (E. loxophleba Benth.). The soil consists of a gritty sandy loam ranging from 0.1 to 0.3 m depth overlying a gritty clay with variable amounts of iron mottling and concretion encountered at depths ranging from 0.4 to 2.0 m. (Principal Profile Form Dy 3.83, Northcote, 1971). Lime was encountered at a depth of 0.2-0.8 m in most profiles. The site carried a patchy cover of annual pasture including Wimmera ryegrass (Lolium rigidum Gaud. ), Mediterranean barley grass (Hordeum geniculatum All.), sand spurry (Spergularia rubra (L) J. and C Presl. ), woolly clover (Trifolium tomentosum L. ) and slender ice plant (Mesembryanthemum nodiflorum L. ). During the experimental period the small saltbush Atriplex pumilio R. Br. volunteered on the site.

267

I 0

I 100

,. J 200

kilometres

Fig. 1. Map of south-westernAustraliashowingKellerberrintownship.

Planting Five Atriplex species selected for study were accession numbers (Malcolm et al., 1984), 471, A. undulata D. Dietr. (wavy leaf saltbush); 440, A. amnicola Paul G. Wilson (river saltbush); 466, A. vesicaria Heward ex Benth. (bladder saltbush); 701, A. paludosa R. Br. (marsh saltbush); 528, A. bunburyana F. Muell. (silver saltbush). Each species was planted at 5 spacings 1 × 1, 2 × 1, 2 × 2, 2 × 3 and 3 × 3 m, making a total of 25 treatments. Within each treatment there were 5 rows of 5 plants (total 25) on a square or rectangular grid. The central 9 plants were sampled and the remaining 16 comprised a surrounding buffer. There were 3 replications, each consisting of a randomised block. Plants were raised in plastic bags of sand/peat mix in a glasshouse. When the plants were about 6 months old the site was tyne-cultivated to 0.1-m depth to kill weeds and loosen the soil for hand planting. The experiment was planted on 25 and 26 August 1976, the plot fenced and stock excluded.

Soil and groundwater sampling In November 1976, the soil was sampled to 2-m depth at 0.2-m intervals between the centre plant and the surrounding row of treatment plants in each plot. Galvanised°iron observation wells were installed in each hole to 1.5 m, sealed into the ground with clay, covered and used to monitor the depth and salinity of groundwater. On 13-14 December 1978, a second hole was sampled

268

in each plot. The soil samples were analysed for field moisture content, pH (1:5 soil:water), C1- and conductivity of the saturation extract (ECe). Depth to groundwater was measured initially, then at 3, 6 and 8 months and monthly thereafter. The groundwater was analysed for chloride and electrical conductivity at 3-monthly intervals.

Plant measurements The central 9 bushes in each plot were stripped once by hand between 1827 April 1978 to simulate a severe grazing. This treatment involved removal of all leaf material and all twigs up to about 5-mm diameter. All removed material was weighed immediately in the field. Subsamples from each plot were returned for determination of moisture content. The samples were then separated into leaves and twigs and analysed for chloride. Samples from the 2 X 2m spacing plots were analysed for ash, crude protein, crude fibre, chloride, oxalate and in vitro digestibility. In view of the high ash content of Atriplex species (Table 1 ) the dry matter yields were calculated on an ash-free basis.

Statistical analyses Analysis of variance and regression analyses were performed on the data using the statistical program package GENSTAT version 4.03. TABLE 1 Chloride and ash contents of material stripped from five Atriplex spp. Plant part

Species

A. undulata Mean % chloride (all spacings) Leaves (5% LSD 1.17) 8.95a Twigs (5% LSD 0.40) 1.35 Mean % ash (2 × 2-m plots only) Leaves 30.7 Twigs 6.5

A. aranicola

A. vesicaria

A. paludosa

A. bunburyana

7.56

12.55

10.30

9.35

1.18

2.96

1.48

1.22

26.3 7.0

39.3 11.4

36.5 9.4

30.2 6.1

46

72

64

59

Mean yield of ash (1 × 1-m plots, kg h a - 1) 354 532

663

626

360

Leaves (as % total DM)

"On oven-dry basis.

54

269 RESULTS

Plant data The differences in ash-free yield (Fig. 2a,b) between species and between spacing treatments were highly significant ( P < 0.001 ). The yield of ash-free dry matter per bush for all species except A. amnicola is approximately 0.15 kg at the 1 X 1-m spacing. The resultsfor A. amnicola at this spacing are about twice as high as the others and are suspect because of possible contributions to the yield from extended branches from large corner bushes in the buffers. Extrapolation from wider spacings suggests it should come close to 0.15 kg per bush. At wider spacings bush yields for the different species were significantly different. Yields from A. vesicaria bushes, (the smallest shrub), were consistent over the spacings 2 X 2, 2 X 3 and 3 × 3 m and the other species showed a reduction in yield per bush, as spacing was reduced from 3 X 3 m. The ash-free yields ofA. vesicaria expressed on an area basis (Fig. 2b) increased steadily from 0.40 t h a - 1 at 3 X 3-m spacing, to 1.51 t h a - 1 at 1 X 1 m. By contrast, A. amnicola yields were about 1.6 t h a - ~ at spacings from 2 X 1 to 3 X 3 m. The other species were intermediate in their responses between these two. The maximum yield for all species, (excluding the aberrant result for A. amnicola) 1.5-2 t ha -~, was achieved for A. amnicola at 3X3 m spacing but with the other species at closer spacings. The differences between species in the chloride contents of both leaves and twigs (Table 1 ) were highly significant ( P < 0.001 ). There was no significant linear relationship between spacing and plant chloride. A. vesicaria had the highest chloride and ash levels of all the species, while A. amnicola had the lowest. The percentage of the sample of stripped material composed of leaves was highest for A. vesicaria (72%) and lowest for A. amnicola (46%). Expressing the yields on an ash-free basis changes the relativity of the yield differences between species in favour of those with lower ash contents. For example the use of ash-free yield favours A. bunburyana relative to A. vesicaria. The mean yield of ash for 1 X 1 m plots ranged from 354 kg h a - ~ for A. undulata to 663 kg h a - ~ for A. vesicaria. Mean oxalate levels ranged from 0.8% in the twigs ofA. bunburyana to 6.6% in the leaves of A. undulata (Table 2). These levels of oxalate are unlikely to cause problems in grazing animals (Seawright, 1982 ). The mean crude protein levels of the leaves were from between 9.3 and 10.3%. The twigs contained about half as much protein as the leaves and 3-4 times as much crude fibre. In-vitro digestibility of the leaves ranged from a mean of 66% for A. vesicaria to 74% for A. paludosa; the twigs were about half as digestible as the leaves. Survival of the treatment bushes following hand stripping is recorded in

270 1.5

LSD ,.¢ 1.0 m nIO.

w ul

N o.6 X m IE

I

I

I

I

I,

1

2

4

6

9

I 6

I 8

AREA PER PLANT (m 2)

I@ J~

2

a w w Ilc h

0 0

I I

I 2

I 4 AREA PER P L A N T

(m 2)

Fig. 2. Yields from 5 Atriplex species at 5 p l a n t spacings (a) a s h free yield per b u s h (kg); (b) a s h free yield ( t h a - 1); (/k ) Atriplex undulata, (C) ) A. amnicola, ( V ) A.vesicaria, ( • ) A. paludosa,

( • ) A. bunburyana.

271 TABLE 2 Analyses of leaves and twigs removed from plots planted at 2 × 2-m spacing (each figure is the mean of 3 replications expressed as per cent of oven dry weight) Criterion

Plant part

Species

A. undulata

A. arnnicola

A. vesicaria

A. paludosa

A. bunburyana

Oxalate

Leaves Twigs Whole

6.6 1.3 4.1

5.9 1.4 3.5

3.7 1.2 3.0

5.4 1.8 4.1

5.5 0.8 3.6

Crude protein

Leaves Twigs Whole

9.8 4.9 7.5

9.3 4.2 6.6

10.1 6.8 9.2

9.5 5.3 8.0

10.3 4.3 7.8

Crude fibre

Leaves Twigs Whole

12.2 41.1 25.5

13.1 42.8 29.1

11.4 43.7 20.5

10.3 44.4 22.5

11.1 47.5 26.0

In-vitro digestibility

Leaves Twigs

69 35

72 38

66 39

74 36

72 32

TABLE 3 Number ofAtriplex spp. bushes surviving hand stripping (out of 135) Time

17 April 1978 (before stripping) 30 Oct. 1978 (after stripping 18-27 April 1978)

Species

A. undulata A. arnnicola A. vesicaria A. paludosa

A. bunburyana

135

135

123

135

135

133

135

27

94

128

T a b l e 3. T h e m o s t s e n s i t i v e species, A. vesicaria, d r o p p e d f r o m 123 b u s h e s to 27 following h a n d s t r i p p i n g . B e s t s u r v i v a l w a s e x h i b i t e d b y A. amnicola a n d A. undulata.

E n v i r o n m e n t a l data Water quality T h e d e p t h s o f t h e g r o u n d w a t e r in e a c h p l o t o n 9 D e c e m b e r 1976, 3.5 m o n t h s a f t e r t h e e x p e r i m e n t w a s p l a n t e d , r a n g e d f r o m 0.67 t o 0.99 m; w i t h i n t h e first o r d e r o f w a t e r - t a b l e d e p t h h a z a r d (0-1.1 m ) o f V o l o b u e v (1946). D u r i n g t h e e x p e r i m e n t t h e d e p t h s to g r o u n d w a t e r w e r e f o u n d t o f l u c t u a t e w i t h s e a s o n

272

(Fig. 3 ), the rises in groundwater levels occurring soon after significant rainfall events. Data for only two treatments are shown in Fig. 3, because all treatments showed similar trends. The two treatments, for the largest bushes (A. amnicola) at the closest spacing (1 × 1 m) and the smallest bushes (A. vesicaria ) at the widest spacing (3 × 3 m), gave similar seasonal responses. Significant linear correlations between depth to water table and plant spacing for this experiment were reported by Greenwood and Beresford (1980) and ranged from r2=0.79 ( P < 0 . 0 5 ) on 20 January 1978 to r2=0.95 ( P < 0 . 0 1 ) on 17 March 1978 for A. vesicaria. The electrical conductivities of the groundwaters in the 75 test wells, 3.5 m o n t h s after planting, ranged between 1620 and 3810 mS m -1. Only two test wells gave conductivities below 2000 mS m-1 and only three gave conductivities above 3700 mS m - 1. The distribution of groundwater salinities across the site is shown in Fig. 4. Changes in groundwater salinity (both up and down) occurred in the course of the experiment, but there were no significant differences in water salinity owing to species or spacing treatments at any sampling time (data not reported). More detailed chemical analyses of groundwaters, representing the range of ~0

"~

it 100 ~.

0.5

/

/

//

// \

1.0

1.5

I 1976

J

\\\~Z~I//// /

\

I

I

I

//

I

i

l

i

I

l

~977

i

,

D

I

l

5

l

i

i

'1978'

DATE

Fig. 3. Mean groundwater levels in test wells in experiment plots planted to Atriplex vesicaria at 3 × 3-m spacing (C)) andA. amnicola at 1 X 1-m spacing (/x ) and rainfall recorded at KeUerberrin townsite. Arrow length indicates the period over which the rainfall was summed (same as between groundwater measurements) and the vertical position of the arrow shows the amount. Asterisk: bushes stripped, 18-27 April. J =January.

273

,

TDS m@ L4 )26100 23100 - 26000 [<23000 ~

REP1~

REP 2

REP 3

Fig. 4. Plan of the experiment showing the distribution of groundwater salinities in December 1976. TABLE 4 Chemical composition of groundwater samples selected to represent the range of salinities occurring on the site, arranged in order of increasing salinity Sample no. 7-1 7-2 7-3 8-5 7-6 6-5 7-8 6-10 8-15 8-20 9-19 6-19 6-17 6-8

CI (mgl -I ) 5433 5493 7891 8498 9762 10319 10956 11583 12261 13536 13961 15448 16389 17006

TDS" (mgl -I ) 11118 10797 17886 15828 22404 19823 20195 20944 22587 25494 25539 29392 29434 29931

Cations (mg I-I) Na

K

Ca

Mg

3400 3300 4000 4200 5000 5600 5500 5500 6150 7050 6900 7500 7800 7500

85 82 82 120 94 180 209 156 150 212 163 228 244 190

14 25 320 160 120 160 100 210 230 200 420 240 370 390

160 200 390 510 530 510 620 690 710 750 780 930 1040 1080

SAR

Boron (mg I-I)

56.2 48.2 35.5 36.6 43.7 48.7 45.2 41.3 45.2 51.2 46.0 49.0 47.0 44.4

4.7 5.4 2.8 5.1 3.9 6.1 5.2 5.9 5.3 6.9 4.7 7.2 5.4 7.0

"TDS = total dissolved solids. s a l i n i t i e s o n t h e s i t e , a r e s h o w n i n T a b l e 4. T h e m a j o r a n i o n is c h l o r i d e a n d the major cation sodium, associated with high levels of sodium adsorption ratio ( S A R ) . M a g n e s i u m is s u b s t a n t i a l l y h i g h e r t h a n c a l c i u m i n a l l c a s e s a n d b o r o n levels are sufficiently high to be toxic to many crop plants. The waters would

274

be unsuitable for irrigation because of high total salinity, chloridity, SAR, magnesium and boron, (Richards, 1954).

Soil chloride Chloride levels in the soil ranged from 0.1 to 0.3% C1- on the oven-dry basis (or 0.2-0.5% NaCI). They place the soil in or close to the high salinity class (0-30 cm > 0.15% NaC1 and 30-60 cm > 0-30% NaC1) used to define areas as unsuitable for agricultural crops in Western Australian soil surveys (Teakle et al., 1940). The soil would be classified as a Solonchak by the standards used for the Soil Map of the World (F.A.O./U.N.E.S.C.O., 1974). The soil chloride contents ( % dry weight) 3 months after planting have been subtracted from those obtained 28 months after planting. The species and spacing means of these differences are shown in Table 5 together with the species means for soil chloride after 28 months (1978). Chloride levels in the soil at 0.2-1.2 m (Table 5a) increased slightly in plots ofA. vesicaria and were significantly lower in 1978 than in A. amnicola plots ( P < 0.05-0.001). The largest and most significant differences occurred in the depth range 0.4-0.8 m. Chloride levels in A. paludosa plots at 0.2-1.0-m depth were also significantly higher than in A. vesicaria plots. Lesser differences occurred between the other species. There was a tendency for chloride levels in the surface and at 1.6-2.0 m to be lowered and at 0.2-1.4-m depth to increase. The final (1978) soil chloride levels (Table 5c) were significantly higher in the A. amnicola plots at 0.81.2 m than all other species ( P < 0.05). Spacing treatments had less effect than species on soil chloride levels (Table 5b). However, there was a significant, positive, linear relationship between reduced spacing and increasing soil chloride at 0.4-0.8 m ( P < 0.01) and 1.61.8 m (P<0.05). The mean chloride contents of the soil profiles in the plots of A. amnicola at 1 X 1 m spacing in 1976 and 1978 are 0.21 and 0.28% Cl-, respectively. If the soil is assumed to have a bulk density of 1.5 g cm -3 the increase in salinity is equivalent to 21.7 t ha -1 of C1-. Plant/environment relationships No significant correlations were found ( P > 0.05) between yield and any of the following: twig chloride, leaf chloride, soil salinity (0-2 m) 1976 or 1978, groundwater salinity 1976, groundwater EC on 3 occasions, and groundwater level on 5 occasions. Correlation coefficients were determined for the relationships, fresh or ovendry yield vs. soil EC 1:5, ECe, chloride or pH in 1976, at each of 10 depths. As there were no consistent trends the data are not reported. The gravimetric soil-water contents were determined on the same samples as the soil-salinity data. As there were no significant differences between treatments the data are not reported.

275 TABLE 5 Soil chloride (%C1- o v e n - d r y basis) a t t e n d e p t h s

Sampling depth (m)

A. undulata A. amnicola A. vesicaria A. paludosa A. bunburyana Significance level for m e a n s at a n y one d e p t h

Species means for differences between 1976 and 1978 samplings 0 -0.2 0.030 0.012 -0.124 -0.037 0.2-0.4 0.012 ~'b 0.149 ¢ -0.024 ~ 0.123 c 0.4-0.6 0.122 ~'b 0.214 ¢ -0.000 d 0.178 b'~ 0.6-0.8 0.188 "b 0.192 ~ 0.012 d 0.161 b'~ 0.8-1.0 0.106 b'c 0.141 c 0.030" 0.128 b 1.0-1.2 0.052 "'b 0.122 c 0.015" 0.084 b'~ 1.2-1.4 0.033 a 0.110 b 0.008" 0.037" 1.4-1.6 0.016 0.048 0.005 0.027 1.6-1.8 0.020 0.050 -0.034 0.009 1.8-2.0 -0.036 --0.006 0.009 --0.005 Species m e a n s for 1978 s a m p l i n g 0 -0.2 0.407 0.283 0.2-0.4 0.325 0.401 0.4-0.6 0.438 a'b'¢ 0.487 c 0.6-0.8 0.401 b'¢ 0.447 ~ 0.8-1.0 0.370 ~ 0.407 b 1.0-1.2 0.333" 0.389 b 1.2-1.4 0.338 0.361 1.4-1.6 0.325 0.345 1.6-1.8 0.335 0.365 1.8-2.0 0.280 0.327

0.298 0.350 0.364 ~'b 0.313 ~ 0.316" 0.307 ~ 0.316 0.316 0.299 0.294

0.299 0.369 0.451 b'c 0.419 b'c 0.372" 0.339 a 0.299 0.293 0.293 0.280

0.057 0.076 b'c 0.085" 0.098" 0.075 a'b 0.067 "'h'¢ 0.025 a -0.004 0.003 -0.027

n.s. 0.001 0.001 0.001 0.01 0.01 0.05 n.s. n.s. n.s.

0.277 0.295 0.346 "b 0.349 "'b 0.317 a 0.304 a 0.281 0.280 0.318 0.280

n.s. n.s. 0.05 0.01 0.05 0.05 n.s. n.s. n.s. n.s.

Spacing means for differences between 1976 and 1978 samplings spacing (m) 3)<3 3)<2 2)<2 2)<1 1)<1 0 -0.2 -0.152 0.113 -0.115 0.034 0.058 0.2-0.4 0.043 0.032 0.051 O. 121 0.089 0.4-0.6 0.089 a 0.091 a'b 0.109 a'b'c 0.137 a'b'c 0.173 c 0.6-0.8 0.086" 0.090" 0.121" 0.119 a 0.166 b 0.8-1.0 0.080 0.090 0.105 0.079 0.127 1.0-1.2 0.057 0.049 0.090 0.063 0.082 1.2-1.4 --0.005 0.051 0.059 0.037 0.071 1.4-1.6 --0.035 0.061 --0.001 0.031 0.037 1.6-1.8 --0.024" --0.028" 0.004" 0.033 a 0.063 b 1.8-2.0 --0.034 --0.011 --0.011 --0.027 0.017

n.s. n.s. O.01(linear) 0.01 (linear) n.s. n.s. n.s. n.s. 0.05 (linear) n.s.

M e a n s at one depth are not significantly different, P < 0.05, ifthey occur beside the same superscript. n.s. = not significant.

276 DISCUSSION The experiment has shown that the spacing between plants in a stand of

Atriplex spp. is an important determinant of yield. Species respond differently to spacing but the maximum yields obtainable from the 5 Atriplex spp. were similar. The effects of spacing are probably the result of interbush competition. The species least affected by close spacing was the one with the smallest bushes (A. vesicaria). Atriplex amnicola has the largest bushes and showed the greatest effect of spacing. To obain comparable yields per hectare from A. vesicaria and A. amnicola, at least 9 times as many bushes of the former are required. By planting A. amnicola, costs for seeds, mulch and sprayed coatings (Malcolm and Allen, 1981 ) can be reduced to one ninth that for A. vesicaria. As a result, expensive treatments may be used as spot placements without incurring an unacceptable cost per hectare. In Western Australia vermiculite is applied as a mulch on Atriplex spp. seeds in commercial sowing of forage on farms using the Mallen Niche Seeding technique to deliver spot placements of seeds and mulch. Attempts to establish relationships between yields and other factors were unsuccessful. The growth of Atriplex halimus L. has been shown to benefit from the presence of low levels of salinity (120 mM NaCl) (Gale et al., 1970) and higher levels reduce growth progressively. The salinity level estimated to produce a 50% growth reduction in A. halimus is 290-430 mM NaC1 depending on relative humidity (Gale et al., 1970 ) and in A. amnicola approximately 400 mM NaCl (Z. Aslam, personal communication, 1984). These levels may be compared with the groundwater salinities in the experimental area which ranged from 1620 to 3810 mS m -1 (or about 160-400 mM) soon after the experiment commenced. If the bushes were relying on groundwater for growth, a correlation would have been expected between growth and groundwater salinity. The data on groundwater depth (Fig. 3 ) indicate that for long periods the capillary fringe above the water table must have coincided with at least a proportion of the shrub root zone. The shrubs would be expected to utilise the most easily obtained water in the root zone first and this would have been rainfall stored in the surface soil. Data on water use (Greenwood and Beresford, 1980 ) indicate that areas planted to A. vesicaria would have lost from 0.7 to 1.3 mm of water per day depending on plant spacing. Available water storage in the top 0.6 m could be estimated at about 60 mm (Malcolm and Swaan, 1989). Therefore the shrubs would exhaust the stored rainfall in spring within 1.5-3 months. Growth would be determined, firstly by the quantity of rain water stored, secondly by the depth and salinity of the groundwater and soil and finally by the growth characteristics of the plant. Soil salinity ranged from around 0.1-0.4% Cl- (dry-weight basis ). The field capacity of subsoil material in similar duplex soils is of the order of 20% on a dry-weight basis. The salt content of the soil solution at field capacity would therefore range from about

277 0.5 to 2% C1- ( 172-517 raM). As with the groundwater data this may be compared with available data on the growth response of Atriplex spp. to salinity. This salinity range would be expected to correspond to a 60% reduction in growth ofA. halimus (Gale et al., 1970) and similar reductions in A. amnicola, but such reductions were not observed in this experiment. The increase in salinity observed in the root zone of the shrubs occurred over 25 months, a relatively short period. The duration of the present experiment has been too short to allow an assessment of the ultimate salinity level that could be reached beneath a long-term shrub stand. A natural mixed stand of Atriplex bunburyana and A. vesicaria, with groundwater of 60 000 mg l- 1 total dissolved solids at 1.65-m depth, gave a mean annual yield of 0.54 t ha-1 of shrub material under light annual hand stripping over 5 years (Malcolm, 1963). This is less than the lowest yields of A. bunburyana and A. vesicaria in the present experiment (at 3 X 3-m spacing), but is for only 12 months and indicates that long-term forage production is possible. The value of Atriplex spp. plantings may be determined by the factors which limit the yield of forage in the long term. It has been suggested that the build-up of salinity in the top soil beneath Atriplex spp. is caused by accumulation of salts from leaf litter (Sharma and Tongway, 1973). In the present experiment, soil salinity increased by up to 21.7 t ha -1 C1- (for A. amnicola at 1 X 1-m spacing), while the highest yield of ash recorded was 0.66 t h a - 1 for A. vesicaria at 1 X 1-m spacing. The increases cover a much greater depth of soil (2 m) occurring mainly at a depth of 0.21.4 m and could not be accounted for by leaf fall. The data on soil and plant salinity also indicate that under the conditions of this experiment the suggestion that soil salinity may be reduced by growing, harvesting and discarding halophytes (Chaudhri et al., 1964) is untenable. The data demonstrate the importance of sampling to a sufficient depth to represent the whole root zone of the plants. Differences in ash content of the Atriplex spp. are of considerable relevance for grazing. The intake of salt-rich forage by sheep may be limited by the amount of salt ingested, either in the forage or the water supply (Wilson, 1966). Atriplex vesicaria leaves in the present study had a higher chloride content (12.55%) than any reported by Wilson (1966) for A. nummularia Lindl. and approached the highest he reported for A. vesicaria (13.4%). The chloride content for A. amnicola leaves (7.56%) is higher than the lowest values reported by Wilson (1966) for A. vesicaria and A. nummularia. In the present study A. amnicola contained much less salt than A. vesicaria, despite significantly higher levels of salt in the root zone of the former by the end of the experiment. Greater competition for water between bushes of A. amnicola than A. vesicaria may have been expected to raise tissue salt levels higher in the former. Differences in leafiness must also be considered in assessing feed value. The twigs are less digestible but also contain far less salt than the leaves. Studies

278 are n e e d e d to d e t e r m i n e t h e feeding value of t h e m a t e r i a l a c t u a l l y e a t e n b y t h e sheep as t h e y graze Atriplex spp., a n d t h e m a n n e r in w h i c h t h e feed value declines as t h e s h r u b s are p r o g r e s s i v e l y stripped. In no case was t h e r e a closed c a n o p y in t h e plots. I n t e r b u s h c o m p e t i t i o n m u s t t h e r e f o r e h a v e b e e n below g r o u n d r a t h e r t h a n above. T h e s t r o n g e s t comp e t i t i o n o c c u r r e d w i t h A. amnicola, for w h i c h t h e r e was t h e m o s t increase in salt in t h e r o o t zone. It would be of i n t e r e s t to e x a m i n e t h e w a t e r e x t r a c t i o n p a t t e r n s a n d r o o t s y s t e m s b e n e a t h t h e d i f f e r e n t species, a n d to a t t e m p t to relate t h e m to t h e salinity profiles t h a t h a v e developed. T h e soil b e n e a t h A. amnicola plots t e n d e d to increase in salinity to a g r e a t e r d e p t h t h a n u n d e r A. paludosa. P e r h a p s this o b s e r v a t i o n relates to genetic differences in d e p t h o f rooting; A. amnicola c o m e s f r o m a m o r e arid e n v i r o n m e n t a n d A. paludosa f r o m s a l t - m a r s h s i t u a t i o n s w h e r e r o o t i n g is likely to be r e s t r i c t e d to shallow depths. ACKNOWLEDGEMENTS T h e c o n s i d e r a b l e a s s i s t a n c e o f J.R. F e l d m a n in m e a s u r i n g a n d s a m p l i n g t h e t e s t wells is g r a t e f u l l y acknowledged. M a n y p e o p l e assisted w i t h t h e field a n d l a b o r a t o r y w o r k a n d a p p r e c i a t i o n is e x p r e s s e d to N . L . B . R i c h a r d s , T . J . D o n e y , J.A. B e s s e l l - B r o w n e , M. G r e e n , B.A. W r e n a n d E.J. Solin. T h e figures were d r a f t e d b y L. M o r e y .

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