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Factors affecting the change in extractable phosphorus following the application of phosphatic fertiliser on pasture soils in southern Victoria.

Abstract

Nine pasture soils from high rainfall zones of southern Victoria were analysed for a range of chemical and physical properties before receiving a single application of P fertiliser in the form of triple superphosphate (TSP), single superphosphate (SSP), or TSP and lime (5 t/ha) at amounts ranging from 0 to 280 kg P/ha. Soils were analysed for bicarbonate-extractable P concentration, using both the Olsen P and Colwell P methods, 6 and 12 months after fertiliser application. A strong positive linear relationship existed at all sites between the amount of P applied and both the Olsen P and Colwell P concentrations. The slopes of these relationships measured the change in extractable P concentration ([Delta]EP) per unit of P applied, whilst the inverse of the [Delta]EP value indicated the amount of P fertiliser required above maintenance to increase the extractable P concentration by 1 mg/kg. These values ranged from 5 to 15 kg P/ha, depending on soil type. The [Delta]EP measured by the Olsen ([[Delta]EP.sub.Olsen]) method was closely related to selected soil properties and P sorption measures, whilst the [[Delta]EP.sub.Colwell] values were also closely related to selected soil properties and P sorption measures, but only when one particular site, an acidic sand, with a high organic carbon content was excluded from the analysis. In general, simple, direct measures of soil P sorption could allow the estimation of [Delta]EP values on different soil types.

The application of P in the form of SSP resulted in a trend for higher [Delta]EP values than occurred with TSP. This difference was significant on 3 sites (P [is less than] 0.05), but depended on the method of extraction and the

time after fertiliser application. The application of lime significantly (P [is less than] 0.001) increased soil pH ([H.sub.2]O and Ca[Cl.sub.2]) and decreased the concentration of exchangeable Al, 6 months after treatments were applied, but generally had little impact on [Delta]EP values.

Additional keywords: buffering, sorption, Olsen, Colwell, single superphosphate, triple superphosphate, lime.

Introduction

Australian soils are generally deficient in phosphorus (P) in their native state (Leeper and Uren 1993). They require the application of P fertiliser to increase the concentration of extractable soil P to satisfy the requirement of productive plant species for P. However, fertiliser P forms a range of reaction products in soil through sorption and precipitation reactions, decreasing the availability of fertiliser P for plant uptake (Moody and Bolland 1999). The extent of the reactions between a soil and P fertiliser depends on the physical and chemical properties of the soil (Leeper and Uren 1993) and will determine the amount of P fertiliser required to increase P availability for plant growth.

The amounts of P fertiliser required to increase extractable P concentrations in different soil types that are used for dairy pasture production in southern Victoria are poorly defined. This information is important as it enables farmers to apply the correct amount of P fertiliser to achieve the desired levels of P availability in their soils, as measured by extraction with bicarbonate using the Olsen (Olsen et al. 1954) or Colwell (1963) procedures. Improving P fertiliser recommendations may also prevent excessive P fertilisation, which may lead to the loss of P from pastures, and to eutrophication and algal blooms in waterways (Sharpley et al. 1987).

The slope of the relationship between the amount of P fertiliser applied and the increase in extractable P concentration in the soil represents the change in extractable P ([Delta]EP value) per unit of P applied. The inverse of the [Delta]EP value is therefore a measure of the amount of P fertiliser required above maintenance to increase the bicarbonate-extractable P test value by 1 mg/kg. Accurate determination of [Delta]EP values requires detailed field studies, which are expensive and time-consuming. Should close relationships between [Delta]EP and selected soil properties exist, it may be possible to use simpler laboratory measures to predict [Delta]EP and P requirements for soils outside this study.

The [Delta]EP value indicates the extent to which P fertiliser reacts with soil and is likely to be related to the P sorption capacity of soil (Moody and Bolland 1999). The P sorption capacity is commonly measured as P buffering capacity (PBC), which is the soil's capacity to moderate changes in P solution concentration when P is added, to or removed from, the soil (Ozanne 1980). The relationship between [Delta]EP in field studies and laboratory measures of PBC, however, has not been well defined. A glasshouse trial by Bolland et al. (1994) and a field study by Fleming et al. (1997) were unable to find relationships between [Delta]EP measured by the Colwell method, and PBC, or other soil properties. In contrast, Dear et al. (1992) measured [Delta]EP values of glasshouse soils and showed that [Delta]EP decreased with increasing PBC, indicating that relationships between [Delta]EP and PBC may be worth further investigation. Other soil properties such as oxalate-extractable concentrations of Fe and Al (Lewis et al. 1981; Toreu et al. 1988) and clay content (Toreu et al. 1988) have been shown to be good surrogate measures of PBC in laboratory studies, but their relationships with [Delta]EP have not been well defined.

The effect of P fertiliser form on [Delta]EP values for different soils is also not known. There is an increasing trend in Australia toward the use of high analysis P fertilisers such as triple superphosphate (TSP), which contains lower concentrations of sulfur and calcium compared with traditional single superphosphate (SSP). Lime is also commonly applied to dairy pasture soils in southern Victoria but its effect on [Delta]EP values for different soils is uncertain. The application of lime has been reported to increase, decrease, or have no effect on bicarbonate-extractable P concentrations in different soil types (Haynes 1984).

In the present study, we determined the [Delta]EP values for 9 different pasture soils, 6 and 12 months after P fertiliser application, using both the Olsen P and Colwell P methods, and assessed the relationships between [Delta]EP values and measures of P sorption capacity and other selected soil properties. In addition, we determined if the form of fertiliser such as SSP or TSP, or the application of lime influenced [Delta]EP values for these soils.

Materials and methods

Soil characterisation

Nine field sites were established on rain-fed commercial grazing properties across the major dairy areas of southern Victoria. Sites 1-6 were established in south and south-west Gippsland, whilst Sites 7-9 were located in south-western Victoria (Table 1). Soil from each site was characterised by taking 54 soil cores, 10 cm in depth, randomly across the site. This sampling procedure was repeated to give a replicate sample, and replicates were bulked, dried at 40 [degrees] C for 48 h, and passed through a 2-mm sieve before analysis. Samples were analysed for [PBC.sub.R&H] (Rayment and Higginson 1992), [PBC.sub.O&S] (Ozanne and Shaw 1968), a single point P sorption index ([PSI.sub.800]) adapted from Rayment and Higginson (1992), pH ([H.sub.2]O and Ca[Cl.sub.2]) and exchangeable Al (Rayment and Higginson 1992), organic carbon (C) (Walkley and Black 1934), effective cation exchange capacity (Gillman and Sumpter 1986), oxalate-extractable Fe and Al (Schwertmann 1964), and mechanical analysis (Mikhail and Briner 1978). The results are presented in Table 1.
Table 1. Chemical and physical properties of field study soils
prior to treatment

Characteristic Out- Ath- Yarra- Ellin- Strze-
 trim lone gon bank lecki

 1 2 3 4 5

Coarse sand (%) 17.8 6.8 4.3 3.1 2.5
Fine sand (%) 40.8 37.4 30.2 20.2 18.4
Silt (%) 18.3 31.3 37.8 27.0 32.5
Clay (%) 5.3 12.3 17.5 35.8 30.8
pH([H.sub.2]0) 4.95 5.31 5.92 5.50 5.31
pH(Ca[Cl.sub.2) 3.72 4.75 5.41 4.83 4.62
Olsen P (mg/kg) 8.1 15.5 12.5 7.7 10.0
Colwell P (mg/kg) 18.7 71.1 38.6 30.4 28.5
Organic C (%) 9.6 4.9 3.5 5.7 4.6
Exch. Al ([cmol.sub.c]/kg) 0.36 0.31 0.15 0.44 0.98
ECEC(A) ([cmol.sub.c]/kg) 41.2 25.2 22.2 37.9 33.9
Oxalate-extr. Fe (mg/kg) 549 2125 4381 8587 5058
Oxalate-extr. Al (mg/kg) 558 855 1253 3949 2286
[PBC.sub.R&H](B) (L/kg) 4.7 13.4 16.0 35.8 24.9
[PBC.sub.O&S](C) (L/kg) 0.6 10.1 10.5 44.7 19.8
[PSI.sub.800](D) 8.9 75.7 92.7 234.8 152.8

Characteristic Tynong Curdi- Glen- Terang
 evale ormiston

 6 7 8 9

Coarse sand (%) 12.8 22.0 10.3 3.7
Fine sand (%) 15.3 53.6 15.8 50.2
Silt (%) 17.0 11.3 22.8 20.8
Clay (%) 31.0 6.0 32.3 15.3
pH([H.sub.2]0) 5.24 6.18 6.04 5.60
pH(Ca[Cl.sub.2) 4.56 5.12 5.51 4.82
Olsen P (mg/kg) 11.9 8.3 24.4 19.7
Colwell P (mg/kg) 41.2 21.6 117.2 56.9
Organic C (%) 8.7 2.5 5.3 3.1
Exch. Al ([cmol.sub.c]/kg) 2.06 0.15 0.11 0.28
ECEC(A) ([cmol.sub.c]/kg) 43.9 15.0 49.5 18.7
Oxalate-extr. Fe (mg/kg) 8793 1943 8784 3561
Oxalate-extr. A1 (mg/kg) 6734 597 8000 1060
[PBC.sub.R&H](B) (L/kg) 54.2 9.2 38.6 16.9
[PBC.sub.O&S](C) (L/kg) 92.5 7.0 40.0 14.6
[PSI.sub.800](D) 277.9 68.1 210.4 111.1

(A) Effective cation exchange capacity.

(B) phosphorus buffering capacity (Rayment and Higginson 1992).

(C) Phosphorus buffering capacity (Ozanne and Shaw 1968).

(D) Phosphorus sorption index when 800 mg P/kg is added to soil
solution (adapted from Moody et al. 1988; Rayment and Higginson 1992).


The [PBC.sub.R&H] and [PBC.sub.O&S] P buffering measures were determined from a P sorption curve as described by Rayment and Higginson (1992). Briefly, soil was equilibrated with 0.01 M Ca[Cl.sub.2] with varying concentrations of P as [KH.sub.2][PO.sub.4] to give 9 different initial P concentrations ranging from 0 to 800 mg P/kg. Approximately 50 [micro]L of chloroform (0.25% v/v) was added to equilibrating solutions to reduce microbial activity. A 1:10 soil to solution ratio was used and the mixtures were shaken end-over-end (14 rpm) for 17 h at 25 [degrees] C. The mixtures were centrifuged at 3000 rpm and the concentration of P in the supernatants measured by the colorimetric method of Murphy and Riley (1962).

The amount of P sorbed by the soil (mg P/kg) was calculated as the difference between P added and P in the equilibrating solution, and these data were used to plot sorption curves. The [PBC.sub.O&S] was calculated by fitting the Freundlich equation to the curve and determining the slope of the curve between 0.25 and 0.35 (mg P/L) equilibrium P concentrations (Ozanne and Shaw 1968). The [PBC.sub.R&H] was measured from the slope of P sorbed (mg P/kg) against [log.sub.10] of the equilibrium concentration of P in the equilibrating solution (mg P/L). The [PSI.sub.800] was calculated by dividing the amount of P sorbed at a single P addition of 800 mg P/kg by the [log.sub.10] of the equilibrium P concentration ([micro]g P/L).

Treatments and site management

At each of the 9 sites, 18 treatments of P fertiliser and lime (Table 2) were applied in 3 replicates to plots 2 m by 10 m, in a randomised resolvable incomplete block design. The design contained 9 blocks with 6 treatments per block, with a total of 54 plots per site. Treatments 17 and 18 (TSP applied at 35 and 70 kg P/ha) were reapplied every 6 months, and therefore, results after the first 6 months only are included in this paper. Treatments 1-16 received a single application of TSP [containing 20.2% P, 16% calcium (Ca), and 1% sulfur (S)], SSP (containing 8.8% P, 22% Ca, and 12% S), and TSP with burnt lime (CaO) in April 1998. Burnt lime was applied in preference to common agricultural lime (Ca[CO.sub.3]), because it has a higher neutralising value.
Table 2. Amount of triple superphosphate (TSP), single superphosphate
(SSP), and lime treatments

Treatments 1-16 were applied as a single application in April 1998 and
treatments 17 and 18 were reapplied every 6 months

Treatment Treatment
 number

 1 TSP, 0 kg P/ha
 2 TSP, 17.5 kg P/ha
 3 TSP, 35 kg P/ha
 4 TSP, 52.5 kg P/ha
 5 TSP, 70 kg P/ha
 6 TSP, 105 kg P/ha
 7 TSP, 140 kg P/ha
 8 TSP, 210 kg P/ha
 9 TSP, 280 kg P/ha
 10 SSP, 35 kg P/ha
 11 SSP, 70 kg P/ha
 12 SSP, 140 kg P/ha
 13 TSP, 0 kg P/ha, with lime (5 t/ha)
 14 TSP, 35 kg P/ha, with lime (5 t/ha)
 15 TSP, 70 kg P/ha, with lime (5 t/ha)
 16 TSP, 140 kg P/ha, with lime (5 t/ha)
 17 TSP, 35 kg P/ha (reapplied every 6 months)
 18 TSP, 70 kg P/ha (reapplied every 6 months)


All sites had permanent pasture, comprising predominantly perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), with Sites 2, 3, 6, and 7 rotationally grazed by dairy cows and the remaining sites either set-stocked or rotationally grazed by beef cattle. Stocking rates varied between sites; however, P removal due to grazing and product exports were considered minimal compared with estimates of soil P sorption. All sites were situated in zones where the mean annual rainfall was [is greater than] 800 mm (40-year average).

Soil sampling and analyses

Initial variation in extractable soil P concentrations between each plot of the field experiments was measured immediately prior to the application of the treatments by taking 15 randomly spaced soil cores, 10 cm in depth, from each of the 54 plots. Soils from each plot were bulked, dried at 40 [degrees] C for 48 h, and passed through a 2-mm sieve before being analysed for Olsen P (Olsen et al. 1954) and Colwell P (Colwell 1963). Soils were similarly sampled and analysed for Olsen P and Colwell P a further 6 and 12 months after fertiliser treatments were applied. Treatment numbers 1, 5, 13, 14, 15, and 16 were also analysed for pH ([H.sub.2]O and Ca[Cl.sub.2]) and exchangeable Al (Rayment and Higginson 1992), to monitor changes with lime treatment 6 and 12 months after treatment application, using the same method of sampling as above.

Statistical analyses

Analyses were performed using CENSTAT 5 for Windows (GENSTAT 5 Committee 1993). Extractable P concentration data at 6 and 12 months were described using a generalised linear model (McCullagh and Nelder 1983) with gamma error distribution to accommodate increasing variance with the mean. An identity link was included in the model to retain the linear relationship between the amount of P applied and extractable P concentration. Terms included sequentially into the model were site, blocks over and within sites, and treatment terms. These treatment terms were defined to compare effects of the quantity and form of P applied, and the effect of lime. A nested treatment structure separating out amounts of 0, 35, 70, and 140 kg P/ha was defined to undertake an appropriate comparison between slopes under SSP, TSP, and TSP with lime (Table 2). Extractable P concentrations measured on the plots before the application of fertiliser were included as covariates in the analyses. Accumulated analysis of deviance tables were used to test terms for statistical significance.

Selected treatments which were analysed for pH and exchangeable Al concentrations were analysed, ignoring blocks within reps, as a randomised complete block within sites design, to test the effects of applying lime. The pH and exchangeable Al concentrations of soil before lime was applied were included as covariates in the analysis. Analysis of variance (ANOVA) was used to test terms for statistical significance. Excel (1997) was used to perform regressions between amount of P applied and extractable P concentration, and [Delta]EP and selected soil properties. Correlation coefficients between [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] were also determined using this program.

Results

Soil characterisation

The soils examined in this study represented the broad range of soil types used for dairy pasture production in southern Victoria, with soil textures including sandy loams, loams, clay loams, and clays. Sites were selected for their low to moderate extractable P concentrations (Olsen P concentration range 8-22 mg/kg, Colwell P concentration range 19-117 mg/kg). Phosphorus buffering capacity measures indicate that the soils covered a wide range of P sorption capacities (Table 1). Most of the soils were moderately acidic, but Site 1 was very acidic with a pH(Ca[Cl.sub.2]) of 3.72. Organic C percentages ranged from 2.5 to 9.6%.

Relationships between amount of P applied and Olsen P and Colwell P concentration

The mean Olsen P and Colwell P concentrations at both the 6- and 12-month samplings increased essentially linearly with increasing amounts of P fertiliser (mean of both TSP and SSP forms), with coefficients of determination for the linear regression ranging from 0.81 to 0.99. Graphs for the 6-month sampling are shown in Fig. 1. These plots show the variation in slopes of the linear relationships between sites and with the 2 extraction procedures. The [Delta]EP values for 6 and 12 months are presented in Table 3.

[GRAPH OMITTED]
Table 3. Change in extractable P ([Delta][EP.sub.olsen] and
[Delta][EP.sub.Colwell]) values six and 12 months after fertiliser
treatment

Change in extractable P is the slope of the linear relationship between
amount of P applied (kg P/ha) as TSP and SSP (mean for both P forms)
and the extractable P concentration measured by the Olsen P and Colwell
P methods (mg/kg). Standard errors based on linear regression of means
are recorded in parentheses.

Coefficient of determination values ([r.sup.2]) ranged from 0.81 to 0.99

Site [Delta][EP.sub.Olsen

 6 months 12 months Change

 1 0.15 ([+ or -] 0.005) 0.10 ([+ or -] 0.006) -0.05
 2 0.18 ([+ or -] 0.010) 0.16 ([+ or -] 0.010) -0.02
 3 0.13 ([+ or -] 0.009) 0.08 ([+ or -] 0.005) -0.05
 4 0.09 ([+ or -] 0.004) 0.08 ([+ or -] 0.006) -0.01
 5 0.11 ([+ or -] 0.005) 0.09 ([+ or -] 0.003) -0.02
 6 0.07 ([+ or -] 0.003) 0.06 ([+ or -] 0.003) -0.01
 7 0.14 ([+ or -] 0.007) 0.12 ([+ or -] 0.006) -0.02
 8 0.10 ([+ or -] 0.009) 0.08 ([+ or -] 0.010) -0.02
 9 0.15 ([+ or -] 0.007) 0.13 ([+ or -] 0.011) -0.02

Site [Delta][EP.sub.colwell]

 6 months 12 months Change

 1 0.26 ([+ or -] 0.014) 0.17 ([+ or -] 0.011) -0.09
 2 0.40 ([+ or -] 0.025) 0.39 ([+ or -] 0.033) -0.01
 3 0.46 ([+ or -] 0.033) 0.38 ([+ or -] 0.016) -0.08
 4 0.34 ([+ or -] 0.024) 0.30 ([+ or -] 0.024) -0.04
 5 0.38 ([+ or -] 0.020) 0.33 ([+ or -] 0.015) -0.05
 6 0.27 ([+ or -] 0.008) 0.26 ([+ or -] 0.010) -0.01
 7 0.41 ([+ or -] 0.025) 0.36 ([+ or -] 0.017) -0.05
 8 0.30 ([+ or -] 0.042) 0.28 ([+ or -] 0.038) -0.02
 9 0.45 ([+ or -] 0.026) 0.39 ([+ or -] 0.039) -0.06


Comparison of [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell]

The correlations between [Delta]EP values measured by the Olsen and Colwell extraction methods ([[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell]) were weak 6 and 12 months after fertiliser application (Fig. 2). Site 1 appeared to be responsible for the poor correlation. When the data from Site 1 were included, correlations between [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] values were not significant (P [is greater than] 0.05) after 6 (r = 0.46) and 12 months (r = 0.42). When the Site 1 data were excluded from the correlation, there was a significant (P [is less than] 0.05) correlation between [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] values at 6 (r = 0.76) and 12 months (r = 0.73) after fertiliser application (Fig. 2).

[GRAPH OMITTED]

There was a discrepancy between the ranking of [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] values after 6 and 12 months on some sites (Table 3). Site 1 ranked equal second highest in terms of its [[Delta]EP.sub.Olsen] value after 6 months, but fell in the mid range after 12 months. In contrast, Site 1 had the lowest [[Delta]EP.sub.Colwell] value of all 9 sites after 6 and 12 months. The [Delta]EP values for Site 3 were also inconsistent, as the [[Delta]EP.sub.Olsen] value ranked fourth and equal sixth after 6 and 12 months, whereas this site ranked in the higher range with [[Delta]EP.sub.Colwell] after 6 and 12 months.

In general, Sites 2 and 9 had the highest [Delta]EP values and therefore their extractable P concentrations increased readily with P fertiliser treatment, whilst extractable P concentrations at Sites 4, 6, and 8 increased at a lower rate. Sites 1 and 3 showed the largest decrease in [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] at 12 months after fertiliser application, whereas Sites 4, 6, and 8 showed a small decrease over the same time.

Relationships between selected soil properties and [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell]

The [[Delta]EP.sub.Olsen] values were more closely related to selected soil properties than the [[Delta]EP.sub.Colwell] values when Site 1 was included in the analyses, for the 9 sites investigated in this study (Table 4). All of the linear regressions between [[Delta]EP.sub.Olsen] and selected soil properties at 6 or 12 months after P fertiliser treatment were significant (P [is less than] 0.05). In contrast, there were no significant relationships between [[Delta]EP.sub.Colwell] and selected soil properties at 6 and 12 months after treatment. Relationships between both [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] values, and selected soil properties, decreased markedly when soils were analysed 12 months after fertiliser treatment, with coefficient of determination values ranging from 0.41 to 0.52 and 0.00 to 0.10, respectively. In comparison, the coefficient of determination values ranged from 0.62 to 0.78 and 0.08 to 0.34, respectively, 6 months after fertiliser was applied. The soil properties that were most closely related to [[Delta]EP.sub.Olsen] at 6 months after fertiliser treatment were [PBC.sub.R&H], oxalate-extractable Fe concentration, and [PSI.sub.800]. After 12 months, oxalate-extractable Fe, [PBC.sub.R&H], and clay content were closely related to [[Delta]EP.sub.Olsen]. Relationships between selected soil properties and [[Delta]EP.sub.Colwell] were weak ([r.sup.2] [is less than] 0.34) when Site 1 was included in the analyses.
Table 4. Coefficients of determination ([r.sup.2] value) for linear
regressions between [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.colwell],
and selected soil properties, 6 and 12 months after fertiliser
treatment

Coefficients of determination when Site 1 is excluded from the
analyses are reported in parentheses

Selected soil properties [[Delta]EP.sub.Olsen]

 6 months 12 months

[PBC.sub.R&H](A)(L/kg) 0.78(*) (0.76(*)) 0.47(*) (0.54(*))
[PBC.sub.O&S](B) (L/kg) 0.72(*) (0.70(*)) 0.41(*) (0.45(*))
[PSI.sub.800](C) 0.76(*) (0.79(*)) 0.43(*) (0.56(*))
Oxalate-extr. Fe(mg/kg) 0.77(*) (0.78(*)) 0.52(*) (0.66(*))
Oxalate-extr. Al (mg/kg) 0.62(*) (0.59(*)) 0.41(*) (0.43(*))
Clay content (%) 0.66(*) (0.64(*)) 0.46(*) (0.56(*))

Selected soil properties [[Delta]EP.sub.Colwell]

 6 months 12 months

[PBC.sub.R&H](A)(L/kg) 0.22 (0.82(*)) 0.03 (0.85(*))
[PBC.sub.O&S](B) (L/kg) 0.28 (0.76(*)) 0.06 (0.75(*))
[PSI.sub.800](C) 0.13 (0.78(*)) 0.01 (0.86(*))
Oxalate-extr. Fe(mg/kg) 0.12 (0.67(*)) 0.01 (0.81(*))
Oxalate-extr. A1 (mg/kg) 0.34 (0.81(*)) 0.10 (0.84(*))
Clay content (%) 0.08 (0.51(*)) 0.00 (0.64(*))

(*) P < 0.05.

(A) Phosphorus buffering capacity (Rayment and Higginson 1992).

(B) Phosphorus buffering capacity (Ozanne and Shaw 1968).

(C) Phosphorus sorption index when 800 mg P/kg is added to soil
solution (adapted from Moody et al. 1988; Rayment and Higginson 1992).


There was a marked improvement in the negative linear relationship between [[Delta]EP.sub.Colwell] and selected soil properties when Site 1 was excluded from the analyses in Table 4. All of the relationships between [[Delta]EP.sub.Colwell] values and selected soil properties were significant (P [is less than] 0.05) and, excluding Site 1, resulted in similar coefficient of determination values between [[Delta]EP.sub.Colwell] and selected soil properties after 6 months to those measured for [[Delta]EP.sub.Olsen]. Relationships between soil properties and [[Delta]EP.sub.Colwell] values remained consistent after 12 months, in contrast to relationships with [[Delta]EP.sub.Olsen], which decreased 12 months after fertiliser treatment (Table 4). However, Site 3 is the likely cause of this decline, as the [[Delta]EP.sub.Olsen] value at this site was lower than expected after 12 months (Table 3). In general, strong relationships were determined between oxalate-extractable Al concentration, [PSI.sub.800], and [PBC.sub.R&H], and [[Delta]EP.sub.Colwell] at 6 months after fertiliser treatment (Table 4). After 12 months, [PSI.sub.800], was most closely related to [[Delta]EP.sub.Colwell], followed by [PBC.sub.R&H] and oxalate-extractable Al concentration, when Site 1 was excluded (Table 4).

Effect of fertiliser form on [Delta]EP

The use of SSP resulted in a general trend for higher respective [Delta]EP values than TSP, using both bicarbonate-extractable P tests (Table 5). On some sites the increases in [Delta]EP values associated with SSP were significant (P [is less than] 0.05). These included the [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] at Site 7 after 6 months (Fig. 3), [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] at Site 8 after 12 months, and [[Delta]EP.sub.Colwell] at Site 5 at 6 months after fertiliser treatment (Table 5). When the effect of fertiliser form was compared across all sites, 6 months after fertiliser treatment, an average of 8.4 kg P/ha of SSP was required to increase the Olsen P value by 1 mg/kg, compared with a requirement of 10.1 kg P/ha of TSP. After 12 months, the difference between fertiliser forms decreased, with an average of 11.4 kg P/ha of SSP required whilst 12.2 kg P/ha of TSP was required to increase the Olsen P value by 1 mg/kg.

[GRAPH OMITTED]
Table 5. Difference between the [Delta]EP values, for P applied as
SSP and TSP

Site [[Delta]EP.sub.(SSP)] - [[Delta]EP.sub.(TSP)]

 Olsen Colwell

 6 months 12 months 6 months 12 months

 1 +0.035 +0.011 +0.005 -0.015
 2 -0.003 +0.005 +0.001 +0.012
 3 +0.022 -0.008 +0.070 -0.004
 4 +0.021 -0.001 +0.037 +0.007
 5 +0.024 +0.001 +0.119(*) -0.012
 6 +0.003 -0.013 -0.027 -0.017
 7 +0.042(*) +0.015 +0.147(*) +0.044
 8 +0.021 +0.049(*) +0.096 +0.228(*)
 9 -0.003 +0.041 -0.008 +0.097

(*) P < 0.05.


Effect of adding lime on pH, exchangeable Al and [Delta]EP

The application of lime (5 t/ha) did not have a major effect on the [Delta]EP values. Lime application resulted in a significant (P [is less than] 0.05) decrease in [[Delta]EP.sub.Olsen] at Site 2 when measured after 12 months (data not presented), but the decrease was small (mean decrease in Olsen P concentration of 6.1 mg/kg). The application of lime significantly increased (P [is less than] 0.05) [[Delta]EP.sub.Colwell] at Site 1 after 6 and 12 months. This increase was associated with a large increase in the Colwell P concentration (increase of 113 mg/kg after 6 months and 45 mg/kg after 12 months) when the highest amount of P fertiliser was applied (140 kg P/ha). For additions of P fertiliser of [is less than] 140 kg P/ha, lime had little or no effect on increasing the [[Delta]EP.sub.Colwell] value at Site 1.

The addition of 5 t/ha of lime, with and without TSP treatments, significantly (P [is less than] 0.001) increased the pH([H.sub.2]O) and pH(Ca[Cl.sub.2]) and decreased exchangeable Al concentrations across all sites when measured 6 and 12 months after treatment (data not presented). There was a significant (P [is less than] 0.001) site x lime interaction, with the greatest increases in pH(Ca[Cl.sub.2]) measured at sites 9 and 3, where the pH increased 1.89 and 1.72 units respectively after 6 months. The pH(Ca[Cl.sub.2]) increased least at Sites 1 and 8, with increases of 0.77 and 0.81, respectively. The application of lime resulted in a decrease in exchangeable Al concentration of 1.83 and 0.71 ([cmol.sub.c]/kg) at Sites 6 and 5. The smallest change was measured at Sites 8 and 3, which decreased by 0.01 and 0.03 units, respectively. The addition of TSP did not significantly influence the effect of lime (P [is greater than] 0.05) in changing the pH or the exchangeable Al concentration.

Discussion

The addition of similar amounts of P fertiliser resulted in different [Delta]EP values at each of the 9 sites. The [[Delta]EP.sub.Olsen] values, after 6 months (Table 3), indicated that the amount of P above maintenance to increase the Olsen P concentration by 1 mg/kg on a soil with a low P sorption capacity such as Site 2 (Table 2) was approximately 5 kg P/ha. A soil of moderate P sorption capacity such as Site 5 required 9 kg P/ha, whereas a soil with a high P sorption capacity such as Site 6 required 15 kg P/ha. In contrast, the [[Delta]EP.sub.Colwell] (Table 3) indicated that the amount of P fertiliser required above maintenance to raise the Colwell P concentration by 1 mg/kg ranged from approximately 2 kg P/ha on soils with a low P sorption capacity to 4 kg P/ha on soils with a high P sorption capacity. Similarly, the decrease in [Delta]EP values between 6 and 12 months (Table 3) was generally less on sites that had a high P sorption capacity (Table 2).

The poor correlation between the [Delta]EP values determined using the Olsen and Colwell extraction methods (Fig. 2, Table 3) was attributed to the Colwell P concentrations measured at Site 1. The Colwell P concentration indicated that Site 1 was the least responsive to P fertiliser addition of the 9 soils studied. Considering that Site 1 had the lowest [PBC.sub.R&H], [PBC.sub.O&S], and [PSI.sub.800], it was expected that this soil would have consistently been the most responsive to P fertiliser treatment. Site 1 was atypical in that it had the lowest pH in Ca[Cl.sub.2] (3.72), the lowest PBC, and the highest organic C content (9.6%) of the 9 soils studied. It is possible that the alkaline (pH 8.5) bicarbonate (0.5 M Na[HCO.sub.3]) extracting solution used in the Colwell P method produced a reaction between solubilised organic C and inorganic P in solution during the 16-h extraction period, making the inorganic P less detectable using the molybdate blue colour reaction. Attempts were made to remove solubilised organic C using carbon black, but despite the organic colloid being cleared, the addition of C did not affect the P concentration in the bicarbonate extract. Another explanation for the discrepancy in the [[Delta]EP.sub.Colwell] values at Site 1 is that some of the P applied as fertiliser was leached beyond the sampling depth of 10 cm, resulting in this P being undetected by the Colwell P method. This explanation however, is inconsistent with the Olsen P concentration measured for Site 1.

The above results for site 1 suggest that the longer extraction time of the Colwell test, in contrast to the Olsen test, is likely to be less effective in determining extractable P concentrations on acidic sands with a high organic C content. Such soils are quite widespread in high rainfall areas of Australia (Sale et al. 1997; Simpson et al. 1997), suggesting that the Colwell P test may not be an effective indicator of soil P fertility on these particular soils. Although this aberration has not been reported from field studies before, Barrow (1967) suggested that the Colwell P test may have under-estimated the extractable P concentration on low P sorbing soils and over-estimated it on high P sorbing soils in a greenhouse study involving 42 soils.

Selected soil properties and measures of P sorption were effective in predicting [[Delta]EP.sub.Olsen] values for the 9 sites (P [is less than] 0.05), and in predicting [[Delta]EP.sub.Colwell] when Site 1 was excluded from analyses (P [is less than] 0.05) (Table 4). Although [PBC.sub.R&H] and [PBC.sub.O&S] were highly related with [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] (when Site 1 was excluded), the PBC measures were derived from the slope of a multi-point P sorption curve. By contrast, the single-point P sorption index ([PSI.sub.800]) was closely related to both [[Delta]EP.sub.Olsen] and [[Delta]EP.sub.Colwell] values and offers potential in terms of a simple laboratory measure to predict the [Delta]EP value of soils. The oxalate-extractable Fe concentration was also closely related with [[Delta]EP.sub.Olsen], whilst the oxalate-extractable Al concentration was closely related with [[Delta]EP.sub.Colwell] at 6 and 12 months after fertiliser application (Table 4). In other studies, however, relationships between oxalate-extractable Al and Fe and PBC varied depending on the parent material of the soils being studied (Lewis et al. 1981; Toreu et al. 1988; Singh and Gilkes 1991). We also found that relationships between [Delta]EP and clay content were generally poorer than those observed with oxalate-extractable Al and Fe concentration when Site 1 was excluded from the analyses (Table 4). Thus, it would appear that a simple, direct measure of P sorption capacity such as [PSI.sub.800] may be more reliable across a broad range of soil types in predicting [Delta]EP than soil properties that indicate P sorption capacity.

In contrast to our findings, Fleming et al. (1997) was unable to identify relationships between selected soil properties and measures of P sorption and [Delta]EP for 27 soils collected from all States of Australia. Unlike our study, Fleming et al. (1997) compared PBC values measured from the 0-2 cm layer with soil texture measured from the Al horizon and [Delta]EP values measured from the 0-10 cm layer. The use of different samples from the upper layer of the soil could have confounded the comparison with our study. In addition, Fleming et al. (1997) included 3 acidic sandy soils which had organic C contents of [is greater than] 8.8%. In our study, the Colwell P method was unable to accurately describe the extractable P concentration on an acidic sand with a high organic C content. This would provide another explanation for the differences observed.

Coefficients of determination between selected soil properties and [[Delta]EP.sub.Olsen] decreased 12 months after fertiliser application (Table 4), possibly due to the low [[Delta]EP.sub.Olsen] value measured at Site 3 after 12 months (Table 3). This site is regularly subject to waterlogging and it is suggested that the fertiliser reaction products occurring at this site, 12 months after P fertiliser treatment, may have decreased the amount of P extracted by the Olsen method.

There was a general trend for SSP to be somewhat more effective than TSP in increasing the [Delta]EP value (Table 5). However, the differences between these fertiliser forms were only significant (P [is less than] 0.05) for some sites. The application of SSP was significantly more effective than TSP at Site 5 after 6 months using the Colwell test, at Site 7 for both the Olsen and Colwell tests, and at Site 8 for both tests after 12 months. It is important to note, however, that the high amounts of SSP, above 70 kg P/ha, were generally responsible for the differences between fertilisers measured (Site 7 illustrated in Fig. 3). It should also be noted that the application of 70-140 kg P/ha in the form of SSP would apply 93-187 kg S/ ha, which is in excess of the current recommendations (Roberts and Morton 1999).

One explanation for the greater effectiveness of SSP in increasing [Delta]EP values when more than 70 kg P/ha is applied could be the higher sulfate

concentrations in SSP (12%) than TSP (1%). The displacement of adsorbed phosphate by the higher concentration of sulfate from SSP may have increased the bicarbonate-extractable P concentration on some soil types (Table 5). Whilst phosphate and sulfate are similar in size and valence, and are likely to be adsorbed on to similar sites on the mineral surfaces of soils (Parfitt and Smart 1978), Barrow (1970) reported that sulfate adsorption in general was much weaker than that of phosphate. He also found that sulfate adsorption increases relative to phosphate, as soil pH decreases. Clearly, the effect of sulfate on extractable P concentrations using different P fertiliser products requires further investigation, particularly as the use of high analysis and blended fertilisers increase.

The application of lime (5 t/ha) with P fertiliser in this study generally had a minimal and inconsistent effect on [Delta]EP. This finding contrasts with the common perception that liming invariably decreases soil P sorption capacity and increases the concentration of extractable P for plant uptake (Sanchez and Uehara 1980), but is supported by the review findings of Haynes (1984).

Conclusion

The amount of P fertiliser required above maintenance to increase the Olsen P concentration by 1 mg/kg (the inverse of the [Delta]EP value) ranged from 5 to 15 kg P/ha across 9 different pasture soils in southern Victoria. These results demonstrate the importance of targeting P fertiliser recommendations to specific soil types, in order to improve the economic and environmental efficiency of P fertiliser use on pastures in this region.

In general, the relationships between [Delta][EP.sub.Olsen] and [Delta][EP.sub.Colwell] were well correlated, with the exception of Site 1. When Site 1 was excluded from the analyses there were strong relationships between [Delta][EP.sub.Colwell] and selected soil properties, indicating that the Colwell method was unable to accurately determine the [Delta]EP value of the acidic sand, with a high organic C content. We suggest that this resulted from an interference from the organic C during the 16-h Colwell P extraction period. This has important implications for Australian soil testing laboratories which commonly analyse soils with these properties.

Simple, direct measures of P sorption capacity, such as [PSI.sub.800], were closely related to [Delta]EP values and may be more reliable indicators of [Delta]EP compared with other soil properties. This is a key finding which could allow the estimation of [Delta]EP on different soil types, without the need for detailed field studies and time-consuming laboratory measurements.

The application of SSP in this study was found to be more effective for some soils in increasing the [Delta]EP in comparison with TSP, at very high P additions, due possibly to competition between the phosphate and sulfate anions. Lime, by comparison, generally had very little impact on [Delta]EP values.

Acknowledgments

The authors sincerely thank Bernie Watt, Beverley and Bill Lyons, Lynette and Chris Vaughan, Gwen and Barry Gilbert, Dorril and Kevin Beveridge, Brian Gleeson and family, Shirley and Colin Irvine, Josie and Neil Black, and Rosemary and Pat Roache for the generous use of their properties in this field study. We also acknowledge the assistance of Graeme Ward for locating suitable sites and Sue Laidlaw and the staff at the Agriculture Victoria Ellinbank Laboratory for their dedicated assistance in soil analyses. This project was a component of the Phosphorus for Dairy Farms project and was funded by Pivot Ltd, the Dairy Research and Development Corporation, the Department of Natural Resources and Environment, La Trobe University, and the University of Melbourne.

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Manuscript received 28 August 2000, accepted 3 January 2001

L. L. Burkitt(A)(B)(C), C. J. P. Gourley (A), P. W. G. Sale(B), N. C. Uren(B), and M. C. Hannah(A)

(A) Agriculture Victoria Ellinbank, RMB 2460, Hazeldean Rd, Ellinbank, Vic 3821, Australia.

(B) Department of Agricultural Sciences, La Trobe University, Bundoora, Vic 3083, Australia.

(C) Corresponding author; e-mail: Lucy.Burkitt@nre.vic.gov. au
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Author:Burkitt, L. L.; Gourley, C. J. P.; Sale, P. W. G.; Uren, N. C.; Hannah, M. C.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Geographic Code:8AUVI
Date:Jul 1, 2001
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