Australian Journal of Soil Research

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1 Publishing Australian Journal of Soil Research Volume 39, 2001 CSIRO 2001 An international journal for the publication of original research into all aspects of soil science All enquiries and manuscripts should be directed to: Australian Journal of Soil Research CSIRO Publishing PO Box 1139 (150 Oxford St) Collingwood, Vic. 3066, Australia Telephone: Fax: sr@publish.csiro.au Published by CSIRO Publishing for CSIRO and the Australian Academy of Science

2 etal. M. Aust. J. Soil Res., 2001, 39, Effect of application of bauxite residue (red mud) to very sandy soils on subterranean clover yield and P response R. N. Summers AC, M. D. A. Bolland B, and M. F. Clarke A AAgriculture Western Australia, PO Box 376, Pinjarra, WA 6208, Australia. B Agriculture Western Australia, PO Box 1231, Bunbury, WA 6231, Australia; and Plant Science, Faculty of Agriculture, University of Western Australia, Nedlands, WA 6907, Australia. CCorresponding author; rsummers@agric.wa.gov.au Abstract Bauxite residue (red mud) is the byproduct from treatment of crushed bauxite with caustic soda to produce alumina. When dried the residue is alkaline and has a high capacity to retain phosphorus (P). The residue is added to pastures on acidic sandy soils to increase the capacity of the soils to retain P so as to reduce leaching of P into waterways and so reduce eutrophication of the waterways. This paper examines how red mud influences the effectiveness of P from single superphosphate for producing subterranean clover (Trifolium subterraneum) dry herbage, in the year of application and in the years after application (residual value). Red mud was applied at 0, 2, 5, 10, 20, and 40 t/ha and the P was applied at 0, 5, 10, 20, 40, 80, and 160 kg P/ha. In the year of application and the year after application of red mud, dry matter yields were doubled on the soil treated with 20 t/ha of red mud compared with the untreated control. Improvements in production were initially greater in the red mud treatments than in the lime treatment (2 t lime/ha). Red mud increased the maximum yield plateau for P applied in current and previous years. When P was applied to freshly applied red mud, more P needed to be applied to produce the same yield as the amount of red mud applied increased. Red mud increased soil ph, and the increases in yield are attributed to removing low soil ph as a constraint to pasture production. This initial need for higher amounts of fertiliser P when increasing amounts of red mud were applied may be due to increased P sorption caused by increased precipitation of applied P when the fertiliser was in close contact with the freshly alkaline red mud. When P was freshly applied to red mud that had been applied to the soil 12 months ago, yield response and P content increased. This was attributed to the reduction in sorption of P due to red mud being neutralised by the soil and because sorption of P already present in the soil reduced the capacity of the red mud to sorb freshly applied fertiliser P. Residues of P in the soil and ph were also increased with application of red mud. In the years after application of red mud and lime, relative to P applied to nil red mud and nil lime treatment, the effectiveness of fertiliser P applied to the red mud and lime treatments increased. This was so as determined using plant yield, P concentration in plant tissue, and soil P test. Additional keywords: pasture production, soil amendment, eutrophication, lime alternative, heavy metals. R. SR97095 N. Summ er sm. D. A.Bol land, F. Clarke R. Ba N. uxi Summers te residue and yi eld ofsubt erranean cl over Introduction Red mud is the material that remains after crushed bauxite is treated with caustic soda to remove the alumina. The residue is pumped onto drying beds and the alkaline solution is drained away and reused to extract more alumina from bauxite. After the residue dries, it is broken down with a rotary hoe so it can be handled in a fertiliser spreader. Exposure to air converts the caustic soda in the residue to sodium carbonate. The alkalinity of the red mud is equivalent to only 10% of pure calcium carbonate despite the ph (1 : 5 red mud : water) being initially >10. The main components of red mud are: 36% Fe 2 O 3, 23% SiO 2, 17% Al 2 O 3, and 3.9% CaO (Summers et al. 1996). CSIRO /SR /01/050979

3 980 R. N. Summers et al. Red mud applied at 80 t/ha reduces by about 70% the amount of P lost in water drainage from sandy sites (Summers et al. 1994). Applications of 20 t/ha of red mud to sandy soil increased production of subterranean clover based pasture by 25% (Summers et al. 1996). Early studies used very large amounts of red mud, from 200 to 2000 t/ha (Barrow 1982; Ward 1983; Vlahos et al. 1989) and required the incorporation of gypsum (5 10% by weight) when the red mud was mixed into the soil. The gypsum was used to reduce the alkalinity of the red mud (ph > 10) by combining with the sodium carbonate in the red mud to produce a precipitate of calcium carbonate (ph 8.5). Subsequent experiments (Summers et al. 1996) used much lower levels of application (up to 80 t/ha) and found that neither gypsum nor mixing the red mud into the soil affected the pasture yields. It is not known if the effectiveness of superphosphate is altered by amendment of soil with red mud or if the residual value of superphosphate is enhanced by red mud. This would alter the amount of P fertiliser required for optimum pasture production. Soil testing is used to estimate the current P status of the soil when providing fertiliser advice. This requires a knowledge of the relationship between soil test P, measured on soil samples collected in January to March, and pasture yields measured later in the next growing season (i.e. the soil P test calibration, Fixen and Grove 1991). It is not known whether red mud affects the calibration. Consequently, soil P testing was undertaken in the experiment discussed in this paper to test whether the calibration was affected by red mud applications. The alkalinity of red mud in the amounts we used is likely to raise soil ph as well as increase P retention by the soil. A lime treatment at 2 t/ha was therefore included to assess how lime applied at a typical rate used for pastures in the area affected soil ph and pasture production. Materials and methods The experiment was conducted 2 km west of Pinjarra, which is 90 km SSE of Perth, Western Australia, on grey Bassendean sand known locally as Joel sand (McArthur and Bettenay 1974). It is a humus podzol, classified as Uc3.33 in the Northcote (1979) system, and as an Arenic Haplohumod in the US system (Soil Survey Staff 1975). The site was subject to intermittent waterlogging between June and August and had a subterranean clover (Trifolium subterraneum cv. Dalkeith) dominant pasture. Some properties of soil samples collected from the site before the experiment began are described in Table 1. The design of the experiment was a split-plot randomised block replicated twice. The main treatments were 6 levels of red mud (0, 2, 5, 10, 20, 40 t/ha) and one level of lime (2 t/ha). Each main plot was 7 m by 105 m and there was 3 m of untreated area between each. Red mud and lime were applied in February The red mud was applied with a fertiliser spreader that is widely used on farms to spread lime. Rubber skirts Table 1. The characteristics of the soil before the experiment was conducted P (H 2 SO 4 ) is determined from soil digested in concentrated sulfuric acid; P (HCO 3 ) is determined from a bicarbonate extraction of the soil as Colwell (1963); PRI, the P retention index of Allen and Jeffery (1990), which is the ratio of the P sorbed by the soil from a 10 µg P/mL solution to the P concentration in equilibrium; P Buff, the buffer capacity of Ozanne and Shaw (1967), which is the difference in the amount of P adsorbed by the soil between the equilibrium (16 h) concentration of P in the solution from 0.25 to 0.35 µg P/mL; Fe (AmOx), ammonium oxalate extractable iron (Schwertmann 1964) Depth ph Sand Silt Clay P Fe (cm) 1:5 H 2 O 1:5 CaCl 2 (%) H 2 SO 4 HCO 3 PRI Buff (AmOx) (mg/kg) (mg/kg) (mg/kg) (mg/kg) < <

4 Bauxite residue and yield of subterranean clover 981 were fixed around the spinners to retain the red mud inside the plots. The red mud was applied to the soil surface and was not mixed into the soil. Single superphosphate was used as the P fertiliser. It was granulated (0.5 5 mm diam.) and contained 9.1% total P (7.3% water-soluble P, 1.3% neutral ammonium citrate soluble P, 0.5% acid-soluble P, 10.5% sulfur, and 21% calcium). The subplots were the levels of single superphosphate applied down the length of the main plots in a randomised block design repeated twice on each main plot. Each subplot was 2 m by 7 m. The superphosphate treatments were applied once only to each subplot in 1992, 1993, and 1994, using a new section of each main plot in each year. The levels of superphosphate were 0, 5, 10, 20, 40, 80, and 160 kg P/ha. That is, 2 replicates of superphosphate were applied to each main plot in Another 2 replicates were applied in 1993 to sites not previously treated with superphosphate in Finally, another 2 replicates of the superphosphate treatments were applied on sections of the main plots not previously treated with superphosphate in 1992 and The areas treated with the 2 replicates of superphosphate in each year were randomly allocated to the red mud plots. Due to seasonal conditions and poor growth in 1994, assessment was not possible and the design was modified. The 1992 P-applied plots were split in half and treated with P in 1995 using the same randomisation as The 1992 P-applied plots were chosen because they had the greatest time since P application, i.e. the red mud treatments were applied once at the beginning of the experiment (1992) and the superphosphate was applied only once to the plots (except where the 1992 plots had to be split to make room for the 1995 plots). Basal fertilisers were applied to ensure that P was the only element to limit yield (Table 2). Table 2. Basal fertilisers applied (kg/ha) Compound Element (%) April 1992 April annually August annually CaSO 4 18% S 23% Ca KCl 50% K Na 2 BO 4 11% B MgSO 4 10% Mg 50 MnSO 4 32% Mn 30 CuSO 4 25% Cu 10 ZnSO 4 80% Zn 2 Na 2 MoO 4 67% Mo 0.2 FeSO 4 26% Fe 50 Measurements The yield of the pasture was measured using a rising plate meter (Earle and McGowan 1979) on 22 Sept. 1992, 19 Oct. 1992, 14 Oct. 1993, 18 Sept. 1995, and 10 Oct The meter readings were calibrated with dry matter pasture yield cut from within quadrats. The pasture for the calibration was cut to ground level and dried at 70 C in a forced-draught oven before weighing. After each measurement the pasture was cut with a mower set at 3 cm above the ground to simulate grazing. The cut herbage was removed from the plots. The pasture was cut in this manner in winter to control weeds. Before measuring yield, samples of pasture tops were collected from random locations within each subplot. The samples from each subplot were bulked, dried, and used to determine the concentration of P in the plant tissue. Samples of the ground, dried plants were digested in sulfuric acid and hydrogen peroxide (Yuen and Pollard 1954). The P concentration in the digest was measured by the molybdovanadium phosphate method (AOAC 1975). Soil samples were collected each January February using a metal tube 2 cm in diameter and 10 cm deep. Ten core samples were collected from random locations within each of the subplots that were treated with superphosphate in previous years. The samples from each subplot were combined. The soil was analysed for bicarbonate-extractable P and potassium using an automated version of the Colwell (1965) procedure. In addition, the amount of P extracted by sulfuric acid [hereafter called P(H 2 SO 4 )] was also determined colorimetrically (Murphy and Riley 1962). The soil ph was determined in 1 : 5 soil : 0.01 M CaCl 2.

5 982 R. N. Summers et al. Analysis of data The relationship between yield and the amount of P applied was described by the Mitscherlich equation: y = a b exp( cx) where y is the yield of dried pasture (kg/ha), x the amount of P applied (kg P/ha), and a, b, and c are coefficients. Coefficient a provides an estimate of the asymptote or maximum yield plateau (kg/ha). Coefficient b estimates the difference between the asymptote and the intercept on the yield (y) axis at x = 0 and so estimates the maximum yield response to the added P (kg/ha). Coefficient c describes the curvature or shape of the response curve (ha/kg P) and governs the rate at which y approaches the maximum yield plateau as the value of x increases (Ratkowsky 1990), such that as the value of c increases, the response curve moves to the left and less P is required to produce the same yield. Mean data were fitted to the equation by non-linear regression using a computer program written in compiler BASIC (Barrow and Mendoza 1990). The Simplex method (Nelder and Mead 1965) was used to locate the least square estimate of the non-linear coefficients. Different maximum yield plateaus were mostly produced for the various red mud treatments and lime. Under the circumstances it is not valid to use the c coefficient of the Mitscherlich equation to estimate the effectiveness of the superphosphate treatments (Barrow and Campbell 1972). Instead, the initial slope of the relationship between yield and amount of P applied was used. As x tends towards zero, dy/dx tends to bc, so bc was used as an estimate of the initial slope (Barrow and Campbell 1972; Barrow 1975). The effectiveness of the P treatments was calculated relative to the effectiveness of freshly applied superphosphate on soil not treated with red mud or lime. This was done by using the initial slope (bc) of the fitted Mitscherlich s equation. The bc of each fertiliser treatment was divided by the bc for freshly applied superphosphate for the nil red mud and nil lime treatment to provide relative effectiveness (RE) values. Therefore, by definition, the RE of freshly applied P on the nil red mud and nil lime treatment is So if the RE for P on a red mud treatment is >1.00, then less P on that red mud treatment is required to produce the same herbage yield than P applied to the nil red mud and nil lime treatment and vice versa. For yields measured on 22 August 1992, applications of 80 and 160 kg P/ha reduced yield on the 0 t/ha of red mud treatment, and similarly for applications of 160 kg P/ha on the lime and 2 t/ha of red mud treatments, so the data were not fitted to the Mitscherlich equation and, hence, were not used to calculate RE. Differences between treatments were determined using ANOVA and Fisher s protected least significant differences (P < 0.05). The 95% confidence limits as shown in figures were derived from analyses of variance using the Fisher s protected least significant differences. Results Analysis of variance indicated highly statistically significant effects on dry matter yields due to amounts of red mud, lime, and P applied in the year of, and in the years after, application (P < 0.001); and the red mud P applied interactions were also significant (P < 0.05). Error bars were derived from non-linear regression and describe the deviation of the data from the model fit. Yield Yield increased with increasing application of red mud up to a maximum of 20 t red mud/ha. Red mud increased dry matter yield in the first herbage assessment (22 Sept. 92) by 48% from a maximum yield of 2.7 t/ha without red mud to 4 t/ha when 20 t/ha of red mud was applied (Fig. 1). When 40 t/ha of red mud was applied the maximum yield was not as great (3.2 t/ha or only 19% increase). Lime also improved the maximum yield by 15% (3.1 t/ha) but not as much as any of the red mud treatments. When no superphosphate was applied, red mud significantly (P < 0.05) increased yields from 1.9 t/ha without red mud to 2.3 t/ha when 20 t/ha of red mud was applied. Red mud applied at 40 t/ha reduced yields slightly, but not significantly, relative to nil red mud. Lime significantly (P < 0.05) increased production to 2.3 t/ha when no P was applied. Reductions in yield at the higher amounts of P application were evident at 0, 2,

6 Bauxite residue and yield of subterranean clover 983 Fig. 1. Dry matter yield response to freshly applied P fertiliser and freshly applied red mud 28.viii.92. (a), 0 t/ha of red mud, R 2 = 0.97, a (maximum) = 3020, b (max min) = 1090, c (slope) = 0.10;, 2 t/ha of lime (dashed line), R 2 = 0.80, a = 3609, b = 1246, c = 0.024; (b) 10 t/ha of red mud, R 2 = 0.98, a = 3755, b = 1597, c = 0.049; (c) 20 t/ha of red mud, R 2 = 0.91, a = 4016, b = 1716, c = 0.049; (d) 40 t/ha of red mud, R 2 = 0.95, a = 3307, b = 1426, c = and 5 t/ha of red mud and 2 t/ha of lime (beyond maxima at kg P/ha). This apparent phosphorus toxicity was only evident in the first assessment of the first year. The increase in dry matter yield was less at the second assessment (19 Oct. 92) when the maximum yield of the nil red mud was 2.4 t/ha, which significantly (P < 0.05) increased to 2.7 t/ha for the 20 t/ha red mud. When superphosphate was freshly applied after the red mud was 12 months old, the maximum yield significantly (P < 0.05) increased from 3.1 t/ha without red mud to 4.5 t/ha with 40 t/ha of red mud. For the lime treatment the increase in maximum yield to 3.1 t/ha was not as much as any of the red mud treatments. The red mud plots with P applied the year before showed a similar trend with a significant (P < 0.05) increase in maximum yield from 2.4 t/ha without red mud to 3.8 t/ha when 40 t/ha of red mud was applied. Lime significantly (P < 0.05) increased the maximum yield to 3.3 t/ha. No measures of yield were possible in Year 3 because of poor seasonal conditions. However, in Year 4 the red mud produced similar increases in yield with freshly applied superphosphate to those in the previous years. Residues of phosphorus applied in previous years also produced increase in yield with increasing red mud applications but yield responses to P applied in previous years were not as steep as for freshly applied P.

7 984 R. N. Summers et al. Fig. 2. Mitscherlich curves fitted to the yield of years , assessed 10.x.95, with effectiveness of applied phosphorus relative to freshly applied superphosphate. Phosphorus applications greater than 1 year old were scaled by multiplying by the relative effectiveness. (a), 0 t/ha of red mud, continuous line R 2 = 0.93, a = 1899, b = 454 c = 0.052; +, 2 t/ha of lime, dashed line ( only), R 2 = 0.93, a = 2103, b = 1598, c = 0.018; (b) 10 t/ha of red mud, R 2 = 0.92, a = 1891, b = 404, c = 0.15; (c) 20 t/ha of red mud, R 2 = 0.81, a = 1987, b =490, c = 0.085; (d) 40 t/ha of red mud, R 2 = 0.86, a = 2375, b = 674, c = Effective P, year 4 Effective P for freshly applied P is the amount of P applied. For P applied in previous years, effective P was calculated by multiplying the amount of P applied by the RE value. When yield (y axis) was plotted against effective P, then all of the data for P applied in previous years and current years were on the same curve (Fig. 2). The red mud increased the effectiveness of the P for all amounts of red mud. That is, less P was required to increase the yield. Yield maxima increased for increasing applications of red mud. Lime reduced the effectiveness of P although it increased the maximum yield. Yield potential The yield maxima, derived from the Mitscherlich s equation, were compared with the amount of red mud applied to show the maximum yield potential. Application of red mud increased the yield potential for both freshly applied P and P applied in previous years. Yield potential reached a plateau at t/ha of red mud. Where P was applied to freshly applied red mud (22 Aug P applied 1992, 18 Sept P applied 1992) there was a peak rather than a plateau, with the 40 t/ha of red mud having lower yields than the 20 t/ha.

8 Bauxite residue and yield of subterranean clover 985 Fig. 3. Yield response to the application of red mud and P fertiliser, totalled for all of the years., 0 kg P/ha;, 5 kg P/ha;, 10 kg P/ha;, 20 kg P/ha;, 40 kg P/ha;, 80 kg P/ha;, 160 kg P/ha. Note: only a subset of some of the fertiliser rates could be adequately fitted to the Mitscherlich curve. Error bar shows significant (P < 0.05) differences between means (symbols). Total yield, all years combined The greatest increase in total yield occurred below 5 t red mud/ha for all of the levels of P applied (Fig. 3). The highest levels of P ( kg/ha) reached a plateau nearer to 10 t/ha of red mud. Relative effectiveness Initially the RE of superphosphate decreased with increasing applications of red mud (RE decreased from 1 to 0.5 as red mud increased from 0 to 40 t/ha, on 22 Sept. 1992). Note that this was only the case for freshly applied red mud and freshly applied superphosphate. By the second harvest (19 Oct. 1992) the decrease in the RE was less marked and some increases were apparent (RE 1.7 at 10 t/ha of red mud). In 1993, for P applied 1 year previously, the RE of superphosphate increased from 0.4 to 0.9 as the amount of red mud applied in 1992 increased from 0 to 20 t/ha. This was also so for P freshly applied in 1993 (P applied in the current year, 1993), but the increase in RE (up to 2.4 at 40 t/ha of red mud) was greater for the freshly applied P. The RE of P applied for 1995 was up to 5 times greater than the RE of P applied in previous years. The RE for P applied before 1995 was up to 4 times greater when red mud was applied, reaching a plateau at 5 t/ha of red mud (18 Sept. 1995). P concentration in plant tissue Additions of fertiliser P in all years increased the concentration of P measured in dried pasture herbage. This was so for all amounts of red mud or lime applied (Fig. 4). In the year when red mud was first applied (1992), concentrations of P in dried pasture herbage generally decreased as the amount of red mud applied increased. In all subsequent years after red mud application, the concentration of P measured in dried herbage tended to increase with increasing applications of red mud. Soil P Bicarbonate-extractable and sulfuric acid extractable soil test P values tended to increase as more P was applied. This was so for P applied in the current or previous years and for soil treated with red mud or lime. The bicarbonate-extractable P, sulfuric acid-extractable P, and ph increased significantly (P < 0.05) with the application of red mud (Fig. 5). The bicarbonate-extractable P, sulfuric

9 986 R. N. Summers et al. Fig. 4. Phosphorus concentration in plant tissue up to 4 years since P was applied., 0 t/ha of red mud (fine broken line);, 2 t/ha of lime (coarse broken line);, 10 t/ha of red mud;, 20 t/ha of red mud;, 40 t/ha of red mud. Error bars show significant (P < 0.05) differences between means (symbols). acid-extractable P, and ph all decreased with time from application of red mud. Application of 10 t/ha of red mud initially increased the bicarbonate-extractable P by 5 µg/g (62% greater than no red mud) and by the fourth year the increase had fallen to 3 µg/g (66% greater than no red mud). Application of 10 t/ha of red mud initially increased the sulfuric acid extractable P by 22 µg/g (41% greater than no red mud) and by the fourth year the increase had diminished to 17 µg/g (45% greater than no red mud). Application of 10 t/ha of red mud initially increased the soil ph by 0.7 (17% greater than no red mud) and by the fourth year the increase had fallen to 0.6 (15% greater than no red mud). The relationship between the bicarbonate- and sulfuric acid-extractable soil test P and the amount of P applied was adequately (r 2 > 0.9) described by a linear equation. This was so for all red mud and the lime treatments. The slope of the line estimates the rate of change in soil test P per unit of P applied and so is called extractability. Extractability of P tended to improve as more red mud was applied (Fig. 6). Additions of lime also increased extractability. The extractability of P applied 1 year previously (P applied in May 1 year ago, soil samples to measure soil test P were collected each January February) tended to decrease between the first and the second year of application but was greatly increased for

10 Bauxite residue and yield of subterranean clover 987 Fig. 5. (a) Soil ph, (b) bicarbonateextractable soil P, and (c) sulfuric acidextractable soil P, as affected by red mud, lime, and the time since application of red mud. Soil P corresponds to the 80 kg P/ha treatment. P applied 2 years previously (in the same year the red mud was freshly applied, not shown here). As the amount of bicarbonate-extractable soil test P increased so did yield, which reached a plateau at 15 mg/kg P. For all of the rates of red mud, all of the points that related yield and soil test P were coincident on the same response curve. Discussion The application of red mud increased the residual value of fertiliser P as seen by the continued increase in P concentration of the pasture, the maintenance of improved growth up to 4 years after application of P, the soil test P, and extractability of soil test P with increasing amounts of red mud applied to the soil. Most of the increase in plant growth occurred where 2 5 t/ha of red mud was applied. This was for all levels of P applied, although there was a trend for large applications of P ( kg/ha) to reach maximum

11 988 R. N. Summers et al. Fig. 6. The bicarbonate-extractable soil P as affected by red mud and applied P (P applied 1992 and assessed Jan. 1993)., 0 t/ha of red mud (fine broken line);, lime 2 t/ha (coarse broken line),, 2 t/ha of red mud;, 5 t/ha of red mud;, 10 t/ha of red mud;, 20 t/ha of red mud;, 40 t/ha of red mud. yield plateau at 10 t/ha. Although the red mud increased the amount of P retained in the soil, the relationship between plant yield and bicarbonate-extractable soil test P was not altered by the addition of red mud. This shows that the prediction of yield from the soil test P (calibration) is not affected by the addition of red mud. However, the amount of P applied to reach the same level of P in the soil will be less as more red mud is applied. We attribute the initial increase in yield to the rapid increase in the ph of the soil caused by the alkalinity of red mud. The increase in ph directly affects plant growth, and it also indirectly affects growth through the increase in mineralisation and availability of nutrients from organic matter (Barrow 1964) We attribute the decrease in the initial RE of superphosphate to the greater retention of P by the freshly applied red mud treatments. The red mud was spread over the soil surface first in 1992 and 10 days later the superphosphates were spread on top of the red mud. It is a common farmer practice in the region where our experiment was done to spread both red mud and fertiliser to the soil surface and pasture. Therefore, the superphosphate was in close contact with bauxite residue that was recently applied to the surface of the soil. The alkali in the fresh red mud (ph > 9) had little opportunity to react with the acidic soil and formed a thin layer between the fertiliser and the soil. This initial increase in P retention was probably due to both the high initial ph in close proximity to the superphosphate precipitating P and a high number of strongly reactive sorption sites of fresh red mud. After the red mud was applied for more than a year, the P concentration of the pasture increased with increased applications of red mud. After the first year, the superphosphate was applied to red mud that had reacted with the soil through the action of rain, allowing the alkali to react with the soil and also incorporate the red mud into the surface, so reducing precipitation and retention of the P by the red mud. Consequently, the longer the red mud had been applied, the more reduced precipitation of the P by the red mud was observed. This was so for the current and previous years. Therefore, as the time since application of red mud increased, so the P was more effective as measured by yield, P concentration in tissue, and soil test P. The increase in yield on the plots treated with superphosphate the year before could be attributed primarily to the ph and an increase in retention of P. Once the initial impact of the alkaline material was reduced, red mud increased both maximum yield plateaus and yield response to fertiliser P (larger value of the c coefficient of the Mitscherlich equation, so less applied P was needed to produce the same yield). Although the response to P was

12 Bauxite residue and yield of subterranean clover 989 greatest below 10 t/ha of red mud, the 20 and 40 t/ha applications still created a greater response to P than the nil or the lime treatments. The lime increased the maximum yield plateau to P but actually decreased the yield response (the value of the c parameter increased, so more fertiliser P was needed) and had little effect on the P concentration of the pasture. When fresh superphosphate was applied to the red mud that had been in situ for 12 months there were improvements in yield, relative effectiveness, and relative yield. While the increase in soil ph was improving yield, the improvement of RE and yield potential with time we attributed to: redissolution of P that was precipitated by the high initial alkalinity; less precipitation of freshly applied P, because the red mud was less alkaline after reaction with the soil; mineralisation of P from organic sources due to the liming effect of the red mud (Barrow 1964, 1982). Some of this mineralised P was probably resorbed during the 12 months incubation with red mud, reducing the available sorption sites on the red mud, consequently reducing the adsorption of freshly applied superphosphate. The absolute retention of P fell over the 4 years of the trial. However, the proportional increase in retention caused by the red mud remained the same. The effect of increasing P levels in the soil is long lasting. The increased ph from the addition of red mud decreased over the 4 years of the experiment and it is likely that an application of 10 t/ha of red mud would not need to be re-applied for at least 8 years depending upon the initial ph of the soil (based on the measured rate of decline meeting the initial ph). References Allen DG, Jeffery RC (1990) Methods of analysis of P in Western Australian Soils. Report of Investigation No. 37, Chemistry Centre of Western Australia, East Perth. AOAC (1975) Official methods of analysis. 12th edn (Association of Official Agricultural Chemists: Washington, DC) Barrow NJ (1964) Some responses to lime on established pastures. Australian Journal of Agriculture and Animal Husbandry 4, Barrow NJ (1975) The response to phosphate of two annual pasture species. 1. Effect of the soils ability to adsorb phosphate on comparative phosphate requirement. Australian Journal of Agricultural Research 26, Barrow NJ (1982) Possibility of using caustic residue from bauxite for improving the chemical and physical properties of sandy soils. Australian Journal of Agricultural Research 33, Barrow NJ, Campbell NA (1972) Methods of measuring residual values of fertilizers. Australian Journal of Experimental Agriculture and Animal Husbandry 12, Barrow NJ, Mendoza RE (1990) Equations for describing sigmoid yield responses and their application to some phosphate responses by lupins and subterranean clover. Fertilizer Research 22, Colwell JD (1963) The estimation of phosphorus fertilizer requirements of wheat in southern New South Wales by soil analysis. Australian Journal of Experimental Agriculture and Animal Husbandry 3, Colwell JD (1965) An automatic procedure for the determination of phosphorus in sodium hydrogen carbonate extract of soil. Chemistry and Industry Earle DF, McGowan AA (1979) Evaluation and calibration of an automatic rising plate meter for estimating dry matter yield of pasture. Australian Journal of Experimental Agriculture and Animal Husbandry 19, Fixen PE, Grove JH (1991) Testing for P. In Soil testing and plant analysis. (Ed. RL Westerman) pp (Soil Science Society of America: Madison, WI) McArthur WM, Bettenay E (1974) The development and distribution of the soils of the Swan Coastal Plain, Western Australia. CSIRO, Melbourne, Australia, Publication No. 16.

13 990 R. N. Summers et al. Murphy J, Riley JP (1962) A modified single solution method for determining phosphate in natural waters. Analitica Chimica Acta 27, Nelder JA, Mead R (1965) A simples method for function minimisation. Computer Journal 7, Northcote K (1979) A factual key for the recognition of Australian soils. 4th edn (Rellim Technical Publications: Glenside, S. Aust.) Ozanne PG, Shaw TC (1967) Phosphate sorption by soils as a measure of the phosphate requirement for pasture growth. Australian Journal of Agricultural Research 18, Ratkowsky DA (1990) Handbook of linear regression models. (Marcel Dekker: New York) Schwertmann U (1964) Differenzierung der Eisenoxide des Bodens durch photochemishe Extraktion mit sauer Ammoniumoxalat-Losung. Zeitschrift fur Pflanzenernahr. und Bodenkunde 105, Soil Survey Staff (1975) Soil taxonomy a basic system of soil classification for making and interpreting soil surveys. (United States Department of Agriculture: Washington, DC) Summers RN, Guise NR, Smirk DD (1994) Bauxite residue (red mud) increases phosphorus retention in sandy soil catchments in Western Australia. Fertilizer Research 34, Summers RN, Guise NR, Smirk DD, Summers KJ (1996) Bauxite residue (red mud) improves pasture growth on sandy soils. Australian Journal of Soil Research 34, Vlahos S, Summers KJ, Bell DT, Gilkes RJ (1989) Reducing phosphorus leaching from sandy soils with red mud bauxite processing residues. Australian Journal of Soil Research 27, Ward SC (1983) Growth and fertiliser requirements of annual legumes on a sandy soil amended with fine residue from bauxite refining. Reclamation and Revegetation Research 2, Yuen SH, Pollard AG (1954) Determination of nitrogen in agricultural materials by the Nessler reagent. II. Micro determinations in plant tissue and soil extracts. Journal of the Science and Food and Agriculture 5, Manuscript received 24 September 1997, accepted 15 February