EFFECTIVE NUTRIENT SOURCES FOR PLANT GROWTH ON BAUXITE RESIDUE: II. EVALUATING THE RESPONSE TO INORGANIC FERTILIZERS JUDY EASTHAM1,2 and TIM MORALD1 1

Alcoa World Alumina Australia, Perth, Western Australia. 2 Address of corresponding author: Private Bag 5, Wembley, WA 6913, Australia. e-mail: [email protected]; Tel: +68 8 9333 6674, Fax: +68 8 9387 8991

(Received 25 November 2005; accepted 7 December 2005)

Abstract. An integral part of managing dust emissions from bauxite residue storage areas in Western Australia is the establishment of native vegetation and dust control crops. Recent changes to local health department regulations preclude the routine use of poultry manure, the previous standard fertilizer for growing dust control crops on bauxite residue sand. This paper reports on a field evaluation of different forms of inorganic fertilizer, to assess their effectiveness as alternatives to poultry manure for supplying nutrients to dust control crops. We compared plant growth and nutrient uptake under different forms of nitrogen (N) and phosphorus (P) fertilizers with additional potassium (K) and trace elements. A diammonium phosphate (DAP) based fertilizer blend which supplied 270 kg N ha−1 of N and 307.5 kg P ha−1 was found to be more effective than a superphosphate based blend containing the same amounts of these nutrients. The DAP treatment did not respond to topdressing with different N fertilizers, but plant growth in the superphosphate treatment was responsive to topdressing with N. Of the three different nitrogenous fertilizers evaluated for topdressing the superphosphate treatment (ammonium sulphate, diammonium phosphate, and urea), the ammonium based fertilizers were most effective. The DAP blend was the most cost effective of all the fertilizers studied, costing only A$1070 ha−1 compared with A$2473 ha−1 for the superphosphate blend and A$1600 ha−1 for poultry manure. We concluded that the DAP fertilizer blend could be used as an effective replacement for poultry manure for growing dust control crops on bauxite residue sand. Keywords: Bauxite residue, dust control, fertilizers, plant nutrients, rehabilitation.

1. Introduction Establishment of native vegetation and dust control crops form an integral part of managing dust emissions and improving the appearance of bauxite residue storage areas in Western Australia. Bauxite residue, produced from the extraction of alumina from bauxite using the Bayer process, is highly alkaline, saline, sodic and low in organic matter and nutrients (Bell et al., 1997). Reduction in alkalinity, salinity and sodicity to levels suitable for plant growth can be achieved through application of gypsum and leaching of sodium by rainfall (Wong, 1990). However, problems of low nutrient status and availability need to be addressed, so that crops and native Water, Air, and Soil Pollution (2006) 171: 315–331 DOI: 10.1007/s11270-005-9055-8

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vegetation can be grown. In the past, poultry manure has routinely been used successfully to grow cover crops and native vegetation. However, its transport, storage and use as a soil amendment have recently been restricted by the Western Australian Health Department because of problems with stable fly. This has precluded its use at residue storage areas. A previous study to find alternatives to poultry manure for supplying nutrients to dust control crops found that organic fertilizers (compost and composted manure) were ineffective at economically viable application rates, whilst inorganic fertilizer gave adequate growth (Eastham et al., 2006). Furthermore, organic amendments gave no measurable improvement in residue properties compared with the inorganic fertilizer. The effectiveness of different inorganic fertilizers with similar elemental composition applied to alkaline bauxite residue may vary depending on the type of fertilizer applied. This is partly because the nature and size of nutrient losses from different forms of fertilizer may be expected to vary under the particular environmental conditions found on residue. For example, many researchers have concluded that volatilisation of ammonia (NH3 ) is greater from ammonium containing fertilizers and urea than other N compounds (Ali and Stroehlein, 1991), and studies have shown that significant NH3 losses from inorganic fertilizers can occur on alkaline soils (Devine and Holmes, 1964; Fenn et al., 1981; Ali and Stroehlein, 1991). Different fertilizers may have indirect effects on plant growth through changing the properties of the residue medium such as pH. For example, urea and ammonium salts cause soil acidification through oxidation of NH+ 4 to + form H+ and NO− . On alkaline soils, uptake of N in the form of NH may en3 4 hance proton extrusion (Jarvis and Robson, 1983) and amelioration of rhizosphere pH. Changing pH of the rhizosphere can affect the availability or solubility of nutrients such as P, Fe, Mn, Zn, Cu and Al (Hedley et al., 1982; Gahoonia, 1993). There may be preferential uptake by plants of cationic over anionic forms of nu− trients, and more rapid uptake of NH+ 4 over NO3 has been documented (Clarkson et al., 1986; Atwell, 1992). Plant uptake of P sources with an initially acidic pH can also be more rapid than alkaline sources on alkaline soils (Beaton and Read, 1963). This effect may be particularly important on bauxite residue, as phosphorus availability is often low in calcareous or alkaline soils (Rowell, 1988; Goos and Johnson, 2001). This may be due to availability of Ca or Mg for precipitation reactions, adsorption of P onto CaCO3 or clay surfaces, and the form of orthophosphate in the soil solution being predominantly HPO2− 4 , which is taken up more slowly than H2 PO− (Olsen and Flowerday, 1971; Barber, 1980; Goos and Johnson, 4 2001). The general aim of the experiment reported in this paper was to develop a cost effective management strategy for applying nutrients to grow cover crops for dust control on bauxite residue storage areas. Specific aims of the experiment were: (i) to compare crop responses to poultry manure, a diammonium phosphate based fertilizer blend and a superphosphate blend of similar elemental composition.

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(ii) to assess the effectiveness of topdressing with different nitrogen fertilizers. (iii) to evaluate the cost effectiveness of different fertilizer treatments. 2. Materials and Methods 2.1. EXPERIMENTAL

SITE AND TREATMENTS

The experiment was carried out in 2002 at Alcoa’s residue storage area at Wagerup, 122 km south of Perth in Western Australia (32◦ 55 S, 115◦ 55 W). The experiment was established on a sand embankment which had been hydraulically deposited as slurry in the previous year. To reduce the alkalinity, salinity and sodicity of the residue, gypsum was applied to the site at 50 t ha−1 , and the area deep ripped using a D9 dozer. Three main fertilizer treatments were arranged in a randomised block design in 40 × 15 m plots, with each treatment replicated 3 times (Figure 1). Main fertilizer treatments were poultry manure and 2 inorganic fertilizer blends based on DAP and superphosphate fertilizers. Four different topdressing treatments (no topdressing, diammonium phosphate, ammonium sulphate and urea) were later applied to subplots (5 × 5 m) of each of the DAP and superphosphate main treatment plots (Figure 1).

Figure 1. Experimental layout showing main plots with poultry manure, diammonium phosphate and superphosphate treatments and subplots of topdressing treatments with different nitrogenous fertilizers. The figure is not drawn to scale.

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TABLE I Rates of application of elements in the superphosphate, diammonium phosphate (DAP) and poultry manure treatments. One standard error of the application rate of elements in poultry manure is also shown (n = 3). Element Organic N Nitrogen (NO3 ) Nitrogen (NH4 ) Phosphorus Potassium Sulphur Calcium Sodium Iron Magnesium Boron Manganese Molybdenum Zinc Copper ∗

Superphosphate blend kg ha−1

DAP blend kg ha−1

∗

135a ∗ 135a 307.5a 300.0a 572.7a 675.8a

30.0a 1.5a 15.0a 0.25a 16.0a 10.0a

270b 307.5a 300.0a 211.1b

30.0a 1.5a 15.0a 0.25a 16.0a 10.0a

Poultry manure kg ha−1 748.2 ± 93.1 1.8b ± 0.2 57.9c ± 9.4 297.5a ± 25.1 256.8a ± 28.5 108.1c ± 18.3 483.0b ± 60.3 94.3 ± 19.8 10.9 ± 2.1 86.0b ± 14.4 7.7b ± 1.5 7.2b ± 2.0 16.4a ± 0.9 4.7b ± 0.5

Nitrogen applied separately as ammonium nitrate applied at 810 kg ha−1 . Mean values in rows not sharing the same letter are significantly different at p = 0.05 or greater significance level.

Both organic and inorganic fertilizers were applied to main treatment plots on 27 May, and incorporated with harrows. Poultry manure was applied at 50 m3 ha−1 , equivalent to an application rate of 24,033 kg ha−1 . The rates of application of the DAP and superphosphate blend were 2,571 and 4,450 kg ha−1 respectively, with the elemental application rates shown in Table I. Ammonium nitrate was also applied to the superphosphate treatment at 810 kg ha−1 , to give the same application rate of N as the DAP blend (270 kg N ha−1 ). Micronutrients were applied in similar amounts in both fertilizer blends (Table I). The amount of each of these macro and micronutrients applied was derived to address deficiencies measured in crops fertilized with inorganic fertilizer in a previous field experiment (Eastham et al., 2006). The topdressing fertilizer treatments were applied by hand to subplots on July 7 2002. The fertilizers included as topdressing treatments (40 kg N ha−1 ) were DAP (218 kg ha−1 ), ammonium sulphate (190 kg ha−1 ) and urea (87 kg ha−1 ). One subplot in each replicate plot without any topdressing treatment was also included in the experiment. All treatments were sown with a seed mix applied at 40 kg ha−1 comprising cereal rye (Secale cereale, 20 kg ha−1 ) and Wimmera rye (Lolium rigidum, 20 kg ha−1 ) on May 30.

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2.2. PLANT

319

BIOMASS YIELD AND LEAF NUTRIENT CONCENTRATIONS

Early season plant growth is an important indicator for effective dust control. Plant growth was determined by sampling the above ground biomass from 3 × 0.5 m lengths of row sampled from each main plot on July 7, and from each topdressing subplot on August 14. Samples were returned to the laboratory and oven dried at 70◦ C prior to weighing to determine their dry weights. A single sample of randomly selected green leaves of cereal rye was also collected from each sub plot for leaf nutrient analyses. Leaves were oven dried, ground, mixed and digested in nitric and perchloric acid in a closed pressurised vessel, heated in a microwave. The digested material was diluted and analysed using inductively coupled plasma atomic emission spectroscopy for trace element and macro element analysis (P, K, S, Mg, Ca, Na, Cu, Zn, Mn, Fe, B) (McQuaker et al., 1979). For total nitrogen, ground leaf material was combusted in oxygen analysed by a Leco analyser. Nitrate (NO3 ) and chlorides were analysed by mixing the ground plant material with deionised water and analysing the extract colorimetrically using a Lachat flow injection analyser. Leaf nutrient concentrations measured in all treatments were compared to those in Reuter and Robinson (1997) to determine whether they were at adequate or below adequate levels for plant growth. 2.3. ORGANIC

FERTILIZER COMPOSITION

Three samples were randomly taken from the poultry manure stockpile to determine their chemical composition. Total nitrogen was determined by a combustion method (Association of Official Analytical Chemists, 1990), using a LECO FP428 Nitrogen determinator. Phosphorus, potassium, calcium, magnesium, sulphur, boron, copper, iron, manganese and zinc were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) after digestion with a mixture of nitric and perchloric acids (McQuaker et al., 1979). Nitrate-nitrogen was determined using a modification of the autoanalyser procedure of Best (1976). Aqueous extracts were dialysed and nitrate was reduced to nitrite, followed by diazotisation and coupling before being determined colorimetrically. Measurements of pH (H2 O) and electrical conductivity at 25◦ C were made on a 1:5 extract of organic material and deionised water, using a pH and conductivity meter respectively. Water content was determined as the loss on drying at 105◦ C. Total organic carbon was measured combustimetrically using a LECO carbon analyser. Bulk density was determined by weighing the material which had been gently compacted into a known volume. Bulk density measurements were used to calculate application rates of various elements from their measured concentrations in poultry manure. 2.4. DATA

ANALYSIS

The statistical significance of responses to the different fertilizer treatments were investigated using analysis of variance. Responses in plant biomass, foliar nutrient

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concentrations and nutrient uptake to the main fertilizer treatments (poultry manure, superphosphate and DAP) prior to application of topdressing treatments (July 2002) were analysed using a single factor analysis of variance. Fishers F test statistic (Zar 1999) was used to determine the statistical significance of differences in responses to these main fertilizer treatments. Responses in plant biomass, foliar nutrient concentrations and nutrient uptake after application of topdressing treatments (August 2002) were analysed using a 2 way analysis of variance. Where there were significant interactions between main fertilizer and topdressing treatment, the Tukey multiple comparison test (Zar, 1999), was carried out to test for differences between individual treatments combinations. All differences reported in the text are significant at the 5% probability level, or greater.

3. Results 3.1. NUTRIENT

APPLICATION RATES IN THE DIFFERENT FERTILIZER TREATMENTS

Rates of application of all nutrients were similar for both inorganic fertilizer treatments, except for greater amounts of calcium and sulphur applied in the superphosphate blend (Table I). More nitrogen (N) was applied in poultry manure than in inorganic fertilizers, but N was predominantly in organic forms, and applications of inorganic N were less than for the other treatments. Similar amounts of phosphorus (P) and potassium (K) were applied in poultry manure and inorganic fertilizer treatments, but more magnesium (Mg) was applied in poultry manure. Of the trace elements, more boron (B) was applied in poultry manure, but less manganese (Mn) and copper (Cu) were applied compared with the inorganic treatments. 3.2. PLANT

GROWTH RESPONSES TO MAIN PHOSPHORUS FERTILIZER TREATMENT AND NITROGEN TOPDRESSING TREATMENTS

Early growth of crops fertilized with poultry manure and the diammonium phosphate (DAP) fertilizer blend was significantly greater than those receiving the superphosphate blend (Figure 2a). Growth was similar in the poultry manure and DAP treatments which produced 0.75 and 0.72 t ha−1 of dry matter, respectively, by July 7. In contrast, the superphosphate treatment produced only 0.36 t ha−1 of dry matter by July. Two way analysis of variance of plant biomass responses to the main phosphorus fertilizer treatments and nitrogen topdressing treatments in August indicated a significant interaction between phosphorus and nitrogen fertilizer treatments. Tukey multiple comparison tests of individual treatment means indicated that growth responses to main fertilizer treatments without topdressing were similar to those observed in July. Biomass yields were similar for the poultry manure (1.51 t ha−1 of dry

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Figure 2. Response in growth of dust control crops to main phosphorus fertilizer treatments (no topdressing) measured in July 2002 (a) and August 2002 (b). Growth responses to nitrogen topdressing treatments (no topdressing, ammonium sulphate, diammonium phosphate and urea) measured in August 2002 are also shown in Figure 2b. Vertical bars indicate one standard error of the mean.

matter) and DAP treatments (1.44 t ha−1 ), and significantly less for the superphosphate treatment (0.71 t ha−1 ) (Figure 2b). Topdressing the DAP fertilizer treatment with ammonium sulphate, diammonium phosphate or urea had no significant effect on biomass production, as yields were similar for all topdressing treatments. However, topdressing the superphosphate fertilizer treatment with ammonia-based fertilizers (ammonium sulphate and diammonium phosphate) significantly increased biomass to levels similar to those measured in the poultry manure and DAP fertilizer treatments without topdressing. Topdressing the superphosphate fertilizer treatment with urea had no significant effect on biomass production compared with the treatment which received no topdressing (Figure 2b).

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TABLE II Effect of different fertilizer treatments on foliar nutrient concentrations of the poultry manure, diammonium phosphate and superphosphate treatments measured in July 2002. One standard error of the mean concentration of each nutrient is also shown (n = 3).

Nitrogen (%) Phosphorus (%) Potassium (%) Sulphur (%) Sodium (%) Calcium (%) Magnesium (%) Chloride (%) Copper mg kg−1 Zinc mg kg−1 Manganese mg kg−1 Iron mg kg−1 Nitrate mg kg−1 Boron mg kg−1

Poultry manure

Diammonium phosphate

Superphosphate

6.74a ± 0.13 0.62a ± 0.04 3.69a ± 0.22 0.61a ± 0.02 0.42a ± 0.02 0.71a ± 0.02 0.24a ± 0.01 2.14a ± 0.04 12.39a ± 0.67 75.4a ± 12.6 18.8a ± 1.5 1779.7a ± 525.3 130.0a ± 26.0 5.79a ± 0.40

6.34ab ± 0.17 0.75a ± 0.07 3.80a ± 0.15 0.60a ± 0.01 0.43a ± 0.03 0.67a ± 0.02 0.20b ± 0.00 2.00b ± 0.01 10.60a ± 0.40 44.6a ± 4.2 33.4b ± 4.9 1627.7a ± 189.8 91.4a ± 2.2 7.55b ± 0.38

6.19b ± 0.06 0.70a ± 0.11 3.77a ± 0.18 0.60a ± 0.03 0.44a ± 0.05 0.71a ± 0.07 0.20ab ± 0.01 2.18b ± 0.16 10.59a ± 0.6 49.0a ± 7.1 31.2a,b ± 9.1 2527.3a ± 466 108.0a ± 16.2 8.12ab ± 1.60

Mean values in rows not sharing the same letter are significantly different at p = 0.05 or greater significance level. Foliar nutrient concentrations for particular nutrients shown in bold text are below levels considered adequate for plant growth (Reuter and Robinson, 1997).

3.3. LEAF

NUTRIENT CONCENTRATIONS IN JULY AND AUGUST

2002

Leaf concentrations of all nutrients were similar for all main P fertilizer treatments in July (Table II), with the following exceptions: Concentrations of Mg and Cl in leaves of the poultry manure treatment were greater than in the DAP treatment, and N was greater in the poultry manure than in the superphosphate treatment. Also, the foliar concentration of Mn in the poultry manure treatment was significantly less than for both inorganic fertilizers. Leaf concentrations of all measured nutrients were adequate in all treatments in July, except for Mn concentrations in leaves of the poultry manure treatment which were below adequate (Reuter and Robinson, 1997). Two way analysis of variance of responses in leaf N and P concentrations to the main P fertilizer treatments and N topdressing treatments in August indicated no significant effect of either the main P fertilizer treatment or N topdressing treatment on foliar N or P (Table III, IV and V). There was also no significant interaction between main P fertilizer and N topdressing treatments on foliar N or P. Leaf nutrient concentrations measured in August in main P fertilizer treatments (Table III); and in topdressing treatments on DAP (Table IV) and superphosphate

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TABLE III Effect of different fertilizer treatments on foliar nutrient concentrations of the poultry manure, diammonium phosphate and superphosphate treatments measured in August 2002. One standard error of the mean concentration of each nutrient is also shown (n = 3).

Nitrogen (%) Phosphorus (%) Potassium (%) Sulphur (%) Sodium (%) Calcium (%) Magnesium (%) Chloride (%) Copper mg kg−1 Zinc mg kg−1 Manganese mg kg−1 Iron mg kg−1 Nitrate mg kg−1 Boron mg kg−1

Poultry manure

Diammonium phosphate

Superphosphate

3.36 ± 0.51 0.53 ± 0.04 3.77 ± 0.22 0.39 ± 0.05 0.12 ± 0.04 0.40 ± 0.01 0.13 ± 0.00 0.84 ± 0.13 7.98 ± 0.59 41.9 ± 2.2 19.5 ± 2.1 126.2 ± 22.6 51.0 ± 4.1 2.90 ± 0.51

2.43 ± 0.10 0.38 ± 0.01 2.99 ± 0.12 0.22 ± 0.03 0.05 ± 0.01 0.41 ± 0.02 0.08 ± 0.01 0.63 ± 0.03 4.80 ± 0.11 17.8 ± 1.9 28.5 ± 7.2 187.2 ± 64.8 43.9 ± 1.8 3.34 ± 0.59

2.28 ± 0.23 0.38 ± 0.01 2.91 ± 0.09 0.19 ± 0.01 0.04 ± 0.00 0.39 ± 0.01 0.08 ± 0.01 0.66 ± 0.02 5.17 ± 0.28 21.0 ± 1.9 34.0 ± 18.0 207.2 ± 39.6 43.2 ± 0.7 3.52 ± 0.72

Foliar nutrient concentrations for particular nutrients shown in bold text are below levels considered adequate for plant growth (Reuter and Robinson, 1997). TABLE IV Response in foliar nutrient concentrations of the diammonium phosphate treatment to topdressing with ammonium sulphate, diammonium phosphate and urea, measured on 14 August. One standard error of the mean concentration of each nutrient is also shown (n = 3).

Nitrogen (%) Phosphorus (%) Potassium (%) Sulphur (%) Sodium (%) Calcium (%) Magnesium (%) Chloride (%) Copper mg kg−1 Zinc mg kg−1 Manganese mg kg−1 Iron mg kg−1 Nitrate mg kg−1 Boron mg kg−1

None

Ammonium sulphate

Diammonium phosphate

Urea

2.43 ± 0.10 0.38 ± 0.01 2.99 ± 0.13 0.22 ± 0.03 0.05 ± 0.01 0.41 ± 0.02 0.08 ± 0.01 0.63 ± 0.03 4.80 ± 0.11 17.8 ± 2.0 28.5 ± 7.6 187.2 ± 68.8 43.9 ± 1.9 3.34 ± 0.63

2.51 ± 0.19 0.39 ± 0.03 3.02 ± 0.12 0.23 ± 0.01 0.04 ± 0.01 0.41 ± 0.02 0.08 ± 0.01 0.66 ± 0.04 5.00 ± 0.18 19.0 ± 2.5 28.0 ± 11.3 180.6 ± 7.0 43.6 ± 2.3 3.62 ± 1.13

2.53 ± 0.24 0.43 ± 0.02 3.02 ± 0.12 0.23 ± 0.03 0.05 ± 0.01 0.40 ± 0.03 0.08 ± 0.01 0.67 ± 0.05 5.15 ± 0.29 21.9 ± 2.8 30.5 ± 3.8 220.8 ± 39.9 41.7 ± 0.9 3.23 ± 0.88

2.54 ± 0.12 0.42 ± 0.03 3.19 ± 0.18 0.24 ± 0.02 0.05 ± 0.02 0.43 ± 0.04 0.09 ± 0.01 0.73 ± 0.03 5.35 ± 0.17 21.9 ± 2.6 31.5 ± 3.8 296.2 ± 76.4 46.1 ± 1.1 4.24 ± 0.58

Foliar nutrient concentrations for particular nutrients shown in bold text are below levels considered adequate for plant growth (Reuter and Robinson, 1997).

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TABLE V Response in foliar nutrient concentrations of the superphosphate treatment to topdressing with ammonium sulphate, diammonium phosphate and urea, measured on 14 August. One standard error of the mean concentration of each nutrient is also shown (n = 3).

Nitrogen (%) Phosphorus (%) Potassium (%) Sulphur (%) Sodium (%) Calcium (%) Magnesium (%) Chloride (%) Copper mg kg−1 Zinc mg kg−1 Manganese mg kg−1 Iron mgkg−1 Nitrate mg kg−1 Boron mg kg−1

None

Ammonium sulphate

Diammonium phosphate

Urea

2.28 ± 0.23 0.38 ± 0.01 2.91 ± 0.10 0.19 ± 0.02 0.04 ± 0.00 0.39 ± 0.01 0.08 ± 0.01 0.66 ± 0.02 5.17 ± 0.29 21.0 ± 2.0 34.0 ± 19.0 207.2 ± 42.0 43.2 ± 0.8 3.52 ± 0.76

2.42 ± 0.25 0.41 ± 0.05 3.08 ± 0.20 0.23 ± 0.02 0.05 ± 0.01 0.41 ± 0.00 0.10 ± 0.00 0.67 ± 0.05 5.44 ± 0.39 21.7 ± 3.5 25.6 ± 11.7 196.1 ± 55.8 16.4 ± 0.0 3.42 ± 0.96

2.61 ± 0.24 0.41 ± 0.01 3.02 ± 0.09 0.25 ± 0.03 0.09 ± 0.03 0.45 ± 0.05 0.10 ± 0.01 0.71 ± 0.06 5.61 ± 0.61 21.7 ± 0.2 21.4 ± 4.2 204.5 ± 55.2 43.2 ± 2.3 3.54 ± 0.56

2.37 ± 0.34 0.37 ± 0.02 2.84 ± 0.16 0.20 ± 0.03 0.05 ± 0.01 0.40 ± 0.05 0.08 ± 0.01 0.68 ± 0.01 5.26 ± 0.33 20.0 ± 2.6 32.1 ± 10.1 268.2 ± 91.2 45.0 ± 2.5 3.35 ± 0.74

Foliar nutrient concentrations for particular nutrients shown in bold text are below levels considered adequate for plant growth (Reuter and Robinson, 1997).

(Table V) indicate that growth in all treatments may have been limited to some extent by nutrient deficiencies. Foliar concentrations of one or more nutrients were found to be below adequate by August (Tables III, IV and V). All treatments except the DAP treatment topdressed with urea had inadequate foliar concentrations of boron. Mn concentrations were below adequate in the poultry manure treatment, which received only 7.2 kg ha−1 Mn compared with 15 kg ha−1 in the inorganic fertilizer treatments (Table I). Foliar Mg was adequate in the poultry manure treatment (Table III) and below adequate in all inorganic fertilizer treatments (Tables IV and V). Cu levels were low in the DAP with no topdressing treatment (Table III), but were adequate in all other treatments (Tables III, IV and V). 3.4. NITROGEN

AND PHOSPHORUS UPTAKE IN JULY AND AUGUST

2002

There were differences between treatments in their uptake of N and P measured in July (Figures 3a and 3b). Uptake of P was significantly greater for plants growing on the DAP (4.8 kg ha−1 ) compared with the superphosphate treatment (2.4 kg ha−1 ). Plant uptake of N was significantly greater for both the poultry manure (50.9 kg ha−1 ) and DAP (45.6 kg ha−1 ) treatments than superphosphate treatment (21.6 kg ha−1 ) Two way analysis of variance of responses in plant N and P uptake to the main P fertilizer treatments and N topdressing treatments in August (Figures 4a and 4b)

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Figure 3. Plant uptake of nitrogen (a) and phosphorus (b) in above ground biomass of the poultry manure, diammonium phosphate and superphosphate fertilizer treatments measured in July 2002. Vertical bars indicate one standard error of the mean.

indicated a significant effect of P fertilizer treatment on N and P uptake. There were also significant interactions between main P fertilizer and N topdressing treatment on N and P uptake. Tukey multiple comparison tests of individual treatment means indicated uptake of both N and P to be similar for the poultry manure and DAP treatments with no topdressing, and greater than N and P uptake in the superphosphate treatment with no topdressing. Uptake of both N and P was similar for all DAP treatments with and without topdressing. Topdressing with ammonium sulphate and diammonium phosphate increased N and P uptake in the superphosphate treatment, but topdressing with urea had no significant effect on N and P uptake compared with the superphosphate treatment without topdressing. 3.5. A

COMPARISON OF COSTS OF DIFFERENT FERTILIZER TREATMENTS

Of the three main fertilizer treatments, the superphosphate treatment was the most expensive (A$2473/ha), and the DAP treatment the most economical (A$1070/ha)

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Figure 4. Plant uptake of nitrogen (a) and phosphorus (b) in all fertilizer treatments measured in August 2002. Vertical bars indicate one standard error of the mean.

when costs of both the materials and application in the field were considered (Figure 5). Differences in the concentration of P in the inorganic fertilizer blends were the key determinant of the differences in the treatment costs, with the DAP blend having a higher phosphorus concentration (12%) than the superphosphate blend (7%). The DAP blend with the greater P concentration was the cheaper option because of the lower application rate (2571 kg ha−1 ) required to give the same P application as the superphosphate blend (4450 kg ha−1 ). Extra costs were also incurred in the superphosphate blend treatment by the application of ammonium nitrate at 810 kg ha−1 . This was not required in the DAP treatment which also supplied nitrogen (10.5%). The cost of applying poultry manure was intermediate to cost of the inorganic fertilizer treatments at A$1600 ha−1 . Poultry manure used in rehabilitation operations and for this experiment costs $24 m−3 , and at an application rate of

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Figure 5. Costs of materials and application of the various fertilizer treatments.

50 m3 ha−1 the total cost of the organic material was $1200 ha−1 . This compared with a materials cost of $2064 ha−1 for the superphosphate blend and $870 ha−1 for the DAP blend. Spreading costs were least for the DAP blend ($200 ha−1 ), and similar for the poultry manure ($400 ha−1 ) and the superphosphate blend ($409 ha−1 ).

4. Discussion Application of a diammonium phosphate fertilizer blend to ryegrass grown for dust control gave comparable growth to ryegrass fertilized with poultry manure, the standard fertilizer used to grow ryegrass on residue embankments (Figure 2). Growth was less under a superphosphate treatment, which supplied the same amount of all nutrients as the DAP blend except for more Ca and S in the superphosphate blend (Table I). Better growth in the poultry manure and DAP main fertilizer treatments could be partly attributed to improved N and P uptake compared with the superphosphate treatment (Figures 3 and 4), though foliar N and P concentrations were adequate and similar for all treatments (Tables II, III and IV). Previous studies have indicated greater effectiveness of ammonium compared with calcium (superphosphate) forms of P fertilizer on alkaline soils (Mandal et al., 1982; Wild 1988b; Begum et al., 2004). This may result from nitrification of the ammonium ions, decreasing the pH of the rhizosphere and increasing phosphate solubility (Hedley et al., 1982; Gahoonia, 1993). Better growth of the DAP compared with the superphosphate treatment may also be partly attributed to different forms of fertilizer N applied at establishment. DAP − + supplied NH+ 4 , whilst equal amounts of NO3 and NH4 were supplied by ammonium nitrate applied in the superphosphate treatment. Differences in the effectiveness of + + NO− 3 and NH4 fertilizers at different soil pH have previously been reported. NH4

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fertilizers may be more effective at high pH, as cations are taken up faster than anions in alkaline solutions (Wild, 1988a). Also, experiments with culture solutions − have shown that more P is taken up when nitrogen is supplied as NH+ 4 than NO3 + (Wild 1988b). Improved growth on residue in response to NH4 fertilizers is also supported by differential responses in the superphosphate treatment to topdressing with different forms of nitrogen fertilizers. Growth was improved by topdressing with ammonium phosphate and ammonium sulphate, but there was no response to topdressing with urea. Growth of plants in one or more treatments may have been limited by inadequate levels of boron, manganese, magnesium or copper. Deficiencies in boron have previously been found to limit plant growth under alkaline conditions due to increasing absorption (Davies and Jones, 1988). Manganese deficiency has also been reported in plants grown on bauxite residue and attributed to high alkalinity (Fuller et al., 1982). Low foliar magnesium in all inorganic fertilizer treatments (Tables IV and V), may have resulted from high Na and Ca levels in sodic residue amended with gypsum. These ions may have adversely affected the availability of exchangeable and soluble forms of similar ionic species such as Mg through antagonism and competition (Chapman, 1966). In a previous experiment comparing responses of dust control crops to organic and inorganic fertilizers (Eastham et al., 2006) low foliar N, P, K, Mg, Zn and Cu were observed in most treatments, including a superphosphate based fertilizer blend. Low foliar N, P, K and Zn have been addressed in this experiment. Increasing N application from 80 to 270 kg ha−1 ; K from 50 to 300 kg ha−1 ; and Zn from 9 to 16 kg ha−1 has been effective in increasing foliar N, K and Zn. However, both inorganic fertilizer treatments still had low foliar Mg despite increasing Mg additions from 4.5 to 30 kg ha−1 . The DAP treatment still had below adequate foliar Cu, despite increasing Cu additions from 1.8 to 10 kg ha−1 . In addition, all treatments had below adequate levels of B, and the poultry manure treatment below adequate foliar Mn. Further experiments to investigate strategies to address low foliar Mg, Cu and B may be warranted to ensure a reliable growth response to inorganic fertilizers each year. The DAP based fertilizer blend developed for dust control crops may also be suitable for growing native vegetation, the final land use proposed for bauxite residue storage areas. However, field assessment will be required to investigate its capacity to supply the nutritional requirements of perennial vegetation throughout its growth and development into a sustainable ecosystem. The timing and amount of nutrients required to supply the short term rapid growth required for dust control crops may be quite different than for growth of perennial native species over a much longer timeframe. Some native species may be sensitive to high rates of applied P, whilst requirements of others may be satisfied at much smaller P applications, because of enhanced uptake through mycorrhizal associations. Different plant species + may also differ in their preference for NO− 3 or NH4 . For example, wheat plants − were found to grow better when supplied with both NH+ 4 than as NO3 (Cox and

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Reisenauer, 1973). Calcifuges are reported to prefer N as NH+ 4 and calcicoles to prefer NO− (Haynes and Goh 1978). 3 Thus a fertilizer for growing native plants on bauxite residue may require different forms of nutrients than one derived for growing dust control crops. In developing the composition of a fertilizer that will meet the needs of a sustainable vegetation cover requiring minimal management input, accumulation of a pool of soil organic matter and nutrient cycling processes will also need to be considered. 5. Conclusions It was concluded that a DAP based fertilizer blend could be used as an effective and economical replacement for poultry manure for growth of dust control crops on bauxite residue. Further research is required to correct low foliar Mg, Cu and B to ensure a reliable growth response each year. The fertilizer recommendation developed for growth of dust control crops will also require field assessment of its suitability for growth of perennial native vegetation, which may have quite different nutritional requirements to annual crops. Acknowledgements We would like to thank Steve Gibbs, residue supervisor at the Wagerup refinery for his support in setting up these field experiments. We would also like to thank Doug Styles and Brad Towns for their contributions in establishing and managing the crops. The staff at the CSBP laboratories are responsible for the plant analyses presented in this paper, and we are appreciative of their excellent service. We thank the Government Chemical Centre for their analyses of the organic fertilizers used in these experiments.

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