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TECHNICAL REPORT
Landscape and Watershed Processes

Timing of Phosphorus Fertilizer Application within an Irrigation Cycle for Perennial Pasture

^a Dep. of Natural Resources and Environment, Institute of Sustainable Irrigated Agriculture, Ferguson Rd., Tatura, Victoria, 3616, Australia
^b Water-Use Efficiency Unit, NSW Agriculture, P.O. Box 865, Dubbo, NSW, 2830, Australia

Corresponding author (garnc{at}bigpond.com)

Received for publication January 5, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION GENERAL DISCUSSION CONCLUSIONS REFERENCES

 
Irrigated pastures are significant contributors of phosphorus (P) to inland watercourses, with much of the P coming from applied fertilizer. It was hypothesized that the timing of P fertilizer application relative to irrigation regulates P concentrations in runoff and infiltrating water. To test this hypothesis, a two-by-two factorial experiment was conducted on twelve 8- x 30-m border-irrigated bays growing perennial pasture. Phosphorus fertilizer in the form of single superphosphate (44 kg P ha^-1) was surface-broadcast onto the bays when the nominal change in soil water deficit reached 0 or 50 mm (U.S. Class A pan evaporation minus rainfall). Following fertilizer application, the bays were again irrigated when the nominal soil water deficit between fertilizing and the subsequent irrigation reached either 0 or 50 mm. The volume of water applied, runoff volume, and changes in soil water content were recorded for the three irrigations following fertilizer application. Total phosphorus (TP) and filtrable reactive phosphorus (FRP, <0.45 µm) concentrations in runoff and at depths of 0.1, 0.3, and 0.6 m in the soil were also measured. Soil water content at fertilizer application had less effect on P concentrations in runoff and soil water than the additional time between fertilizing and irrigating. By allowing a deficit of 50 mm between fertilizer application and irrigation, the average concentration of P in runoff and moving below a soil depth of 0.1 m was approximately halved. To maximize fertilizer use efficiency and minimize environmental effects, a delay should occur between applying P fertilizer and irrigating perennial pasture.

Abbreviations: E_p, U.S. Class A pan evaporation • F, fertilizer application • _nF, where _n represents the nominal soil water deficit at fertilizer application • F_m, where _m represents the nominal change in soil water content between fertilizer application and the subsequent irrigation • FRP, filtrable reactive phosphorus (<0.45 µm) • I₁, first irrigation following fertilizer application • I₂, second irrigation following fertilizer application • I₃, third irrigation following fertilizer application • I_p, irrigation prior to experimental treatments being imposed (prior to fertilizer application) • R, rainfall • SE, standard error • TP, total phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION GENERAL DISCUSSION CONCLUSIONS REFERENCES

 
THE increased phosphorus (P) loads in Australia's inland watercourses have contributed to the increased frequency and intensity of algal blooms (Murray–Darling Basin Ministerial Council, 1994). Drainage from irrigated dairy pastures contributes considerable P loads to these inland watercourses (Gutteridge, Haskins and Davey, 1992; Murray–Darling Basin Ministerial Council, 1994). The load of P from irrigated pastures is the product of the volume of runoff water and the concentration of P in the water.

In the southern Murray–Darling Basin, perennial pasture is generally border- (often referred to as flood or surface) irrigated. Irrigation is applied every 7 to 10 d from August to May, resulting in approximately 20 irrigations during a season. On average, 20% of the applied water runs off the end of the irrigation bay (Austin, 1998; Nexhip and Austin, 1998). Runoff from irrigated pasture usually contains much higher P concentrations than natural waters (Murray and Philcox, 1995; Harrison, 1994). Some 90% of P in this runoff is nominally soluble (<0.45 µm) (Austin, 1998), referred to as filtrable reactive phosphorus (FRP) (Harrison and Lloyd, 1993). In this form, P is immediately available for uptake by algae (Sharpley, 1993).

Phosphorus is generally surface-broadcast onto border-irrigated pastures as superphosphate (Pulsford, 1989), either as a single or a split application in February or March and/or November to December (Cairns et al., 1999). The recommended P fertilizer application rate in northern Victoria is 50 kg P ha^-1 (Smith, 1993). Following superphosphate application, P concentrations in runoff can be two orders of magnitude greater than immediately prior to fertilizer application (Austin et al., 1996). Movement of P to depth has been less well documented than P movement in surface runoff. This may be a result of both difficulty of measurement and the general perception that P movement through the soil profile is minimal due to the high retention capacity of most soils (Heckrath et al., 1997).

Studies on the reaction between soil and P fertilizer have shown that superphosphate requires minimal moisture to dissolve P from the granule (Lawton and Vomocil, 1954). The studies have also shown that the majority of P dissolution occurs within a few days of fertilizer application (Lawton and Vomocil, 1954; Kolaian and Ohlrogge, 1959; Williams, 1971; Lauer, 1988). Once P has dissolved from the granule, it diffuses into the soil and reacts with soil minerals or cations (Morgan, 1997).

On commercial farms, the application of fertilizer in relation to the time of irrigation varies widely. Cairns et al. (1999) conducted a survey of 268 dairy farms in northern Victoria and reported that 45% of P fertilizer was applied immediately before irrigation, 21% directly after irrigation, and 34% at some other stage within the irrigation cycle. It has been shown that the interval between runoff and the application of swine and poultry manure to pasture influenced the concentration of total phosphorus (TP) in runoff (Westerman and Overcash, 1980). Field data from Nexhip and Austin (1998) showed that there was an exponential decay in P concentrations in runoff with days since fertilizer application. The field data indicated that leaving 3 d between fertilizer application and irrigating would approximately halve the concentration of P in runoff.

For this paper it was hypothesized that varying the timing of fertilizer applications with respect to irrigation would regulate the depth and distribution of fertilizer penetration into the soil, thus affecting P losses in runoff and deep percolation. The objective of this work was to measure the interactions between irrigation and the timing of P fertilizer application on P concentrations in surface runoff and infiltrating water.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION GENERAL DISCUSSION CONCLUSIONS REFERENCES

 
This work was conducted at the Institute of Sustainable Irrigated Agriculture, Tatura (36°26' S, 145°16' E, altitude 113 m), Victoria, Australia. The average annual rainfall (R) for the area is 491 mm and average annual U.S. Class A pan evaporation (E_p) is 1365 mm. The soil at the experimental site is a red-brown earth classified as Lemnos loam (Skene and Poutsma, 1962) or Calcic, Subnatric, Red Sodosol (Isbell, 1996). The soil comprises a 0.15-m-deep, dark-brown, heavy clay-loam A horizon, overlying a fine sandy clay from 0.15 to 0.20 m. A clear change to medium clay distinguishes the B21 horizon, from 0.2 to 0.7 m (Imhof et al., 1997). Organic carbon is typically less than 2% in the A horizon (Adem and Tisdall, 1984), and the clay factions are 30% at 0.1 m, 50% at 0.2 m, and 40% at 0.6 m deep (Imhof et al., 1997). The soil exhibits shrinking upon drying, resulting in the formation of cracks. Greenwood (1997) describes the soil properties in detail.

In November 1993, the site was deep-ripped to 0.3 m, then laser-graded to a slope of 13% (1-in-750). Twelve 8- by 30-m irrigation bays were established. In March 1994, the twelve bays were sown with a white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) pasture mix. Since establishment the pasture had been regularly mown and the clippings collected. The bays were mown 3 d prior to the experiment, resulting in standing biomass of 1500 kg dry matter ha^-1. The bays were border-irrigated whenever accumulated E_p minus rainfall (R) reached 50 mm.

The site had not been grazed since establishment, but prior to this experiment, had P fertilizer added in 1979, 1980, 1982, 1994, and 1995 and nitrogen in 1996 at application rates consistent with farm practices in northern Victoria (Cairns et al., 1999).

Experiment
In February 1997, a two-by-two factorial experiment was conducted to determine if the timing of P fertilizer application in relation to irrigation affected the concentrations of P in runoff and soil water. The 12 bays were allocated to four treatments in a randomized block design, replicated three times. Single superphosphate (Pivot Ltd., Geelong, VIC, Australia) [8.8% P (8.0% water-soluble P, 0.6% citrate-soluble P, 0.2% citrate-insoluble P); 55% calcium sulfate (gypsum: 19% Ca; 11% S); and 7% free moisture] was surface-broadcast over the 12 bays at 44 kg P ha^-1. The treatments were: ₀F₀, ₀F₅₀, ₅₀F₀, ₅₀F₅₀, where the first subscript (_nF) represents the nominal evaporative deficit, as described by E_p - R, at fertilizer application (F). Evaporative deficit is used to infer soil water content. The second subscript (F_m) represents the nominal change in evaporative deficit between fertilizer application and the subsequent irrigation, measured by the additional accumulated E_p - R (Fig. 1), and again, used as a surrogate for change in soil water content. These treatments represent the full range of irrigation–fertilizer scenarios likely to be encountered in practice.

View larger version (10K): [in this window] [in a new window]   Fig. 1. Irrigation schedule for each treatment. The dashed line represents the application of 44 kg P ha-1 as single superphosphate. Ep - R = accumulated U.S. Class A pan evaporation minus rainfall. For treatments, nFm is the fertilizer application, where n is the nominal soil water deficit as described by Ep - R at fertilizer application, and m is the nominal change in soil water content between fertilizer application and the subsequent irrigation and is described by the additional accumulated Ep - R. Ip is the irrigation that established Ep - R at fertilizer application. I1 is the first irrigation following fertilizer application. I2 and I3 are the second and third irrigations following fertilizer application. represents 11.4 mm of rainfall that fell 4 d after fertilizer application.

The depth of water that infiltrated into the soil (infiltrated depth) was estimated using two methods. First, infiltrated depth was calculated by volume balance (Eq. [1]). Second, soil water profiles were measured using gravimetric analysis (0 to 0.1 m) and a neutron probe (0.1 to 1.2 m), taken 1 h before irrigation and again after surface runoff had finished. Infiltrated depth was determined by the change in soil water content.

[1]

Sample Collection
Soil water and runoff samples were collected from the irrigation prior to fertilizer application (I_p) and from the three irrigations following fertilizer application (I₁, I₂, I₃) (Fig. 1). Runoff samples were collected manually from the weirs at 1, 30, and 60 min after runoff commenced. Soil water samples were collected using "poly-pipe" suction samplers when the surface water arrived at the samplers and again 30 min after the initial sample collection. The samplers consisted of polyethylene pipe (13 mm i.d. x 40 mm long) covered at both ends with fine Marix mesh (Therofilm, Melbourne, VIC, Australia). A length of polyethylene microtube attached to a 50-mL syringe was inserted into the pipe. The syringe provided suction to draw up to 50 mL of water out of the soil pores. The samplers were installed in the center of each irrigation bay at 0.1-, 0.3-, and 0.6-m depths in the soil profile. The sampler tubing was installed at a 60° angle with a plug of bentonite approximately 30 mm below the soil surface to minimize the potential for preferential flow around the tubes.

Phosphorus Analyses
A proportion of each sample was immediately filtered through a 0.45-µm filter. All samples were then frozen until analysis. The filtrate was analyzed for FRP colormetrically (Rayment and Higginson, 1992). The minimum detection limit was 0.01 mg P L^-1. The unfiltered samples were analyzed for TP using plasma emission spectroscopy in an inductively coupled plasma (ICP) system. The minimum detection limit was 0.1 mg P L^-1. For statistical analyses, values lower than the detection limits were assumed to be half of the detection limit.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION GENERAL DISCUSSION CONCLUSIONS REFERENCES

 
The irrigation supply water typically contained less than 0.1 mg TP and FRP L^-1 for all irrigations.

The water budget for the first irrigation following fertilizer application is shown in Table 1. The depth infiltrated in ₀F₀ was greater than calculated evapotranspiration (ET), probably as a result of surface storage and evaporation that occurred during the trial. The depth infiltrated was similar to calculated ET for treatments ₀F₅₀ and ₅₀F₀. With ₅₀F₅₀, considerably less water infiltrated than the calculated ET. Two probable causes were that the intake opportunity time was insufficient for infiltration to replenish the soil water deficit (Austin and Prendergast, 1997), and that the ET rate slowed as the soil dried (there was visible pasture stress), so that actual ET was lower than the calculated ET.

Pre-Fertilizer Concentrations in Runoff and Soil Water Solution
The average TP concentrations in runoff collected in the irrigation prior to fertilizer application (I_p) averaged 0.35 mg P L^-1 (standard error [SE] ±0.05). Filtrable reactive P concentrations in runoff averaged 0.48 mg P L^-1 (SE ±0.04). These values are lower than runoff P concentrations from highly fertile dairy pastures, which typically average 2.5 mg TP L^-1 (Nexhip and Austin, 1998). The lower values may be due to the absence of manure P, since the bays used in the experiment had never been grazed. Average TP and FRP concentrations in soil water at 0.1 m depth were 0.23 mg TP L^-1 (SE ±0.08) and 0.35 mg FRP L^-1 (SE ±0.06). At 0.3 m these declined to 0.13 mg TP L^-1 (SE ±0.04) and 0.25 mg FRP L^-1 (SE ±0.04), and at 0.6 m declined further to 0.09 mg TP L^-1 (SE ±0.03) and 0.11 mg FRP L^-1 (SE ±0.03). Filtrable reactive P concentrations sometimes exceeded TP concentrations, as the TP detection limit of 0.1 mg L^-1 was approached.

Irrigation One
Results
Total P and FRP concentrations in runoff and in the soil water solution during I₁ were greater (P < 0.05) than in I_p for all treatments. During I₁, P concentrations at all runoff collection times were affected (P < 0.002) by the delay between fertilizer application and the subsequent irrigation (F_m), with both F₀ treatments having significantly greater P runoff concentrations than the F₅₀ treatments (Fig. 2).

View larger version (34K): [in this window] [in a new window]   Fig. 2. Average total phosphorus (TP) and filtrable reactive phosphorus (FRP) concentrations sampled in runoff and in the soil during the first irrigation following fertilizer application. Different subscripts (a, b, c) above the bars indicate significant differences (P < 0.05) between treatments at each depth or time, calculated using Fisher's unrestricted least significant difference (P < 0.05).

Both antecedent (_nF) and subsequent (F_m) deficits had an effect (P < 0.03) on TP and FRP concentrations at 0.1 m depth. At 0.3 m depth only F_m had an effect (P = 0.001) on P concentrations and at 0.6 m depth there were no significant effects. Figure 2 shows that at 0.1 and 0.3 m depth, ₀F₀ had the highest (P < 0.05) concentrations of TP and FRP. Treatment ₅₀F₀ had the next highest concentrations and F₅₀ treatments were not significantly different from one another.

Discussion
The period between fertilizer application and subsequent irrigation (F_m) had a great effect on P concentrations in runoff and soil water solution at 0.1- and 0.3-m depths. Phosphorus concentrations at 0.1- and 0.3-m depths and in runoff were far greater (P < 0.003) for F₀ than F₅₀. Delaying irrigation (F₅₀) allowed the fertilizer to dissolve from the granule into the adjacent soil (Lauer, 1988). Therefore, there was less P available to dissolve in the irrigation water, and be carried off the border in surface runoff and to depth in the soil profile. Consequently, it is likely that a greater percentage of the P remained on the border within the top 0.1 m of the profile after I₁. As the majority of pasture roots are concentrated above 0.1 m (Kelly, 1985; Maher et al., 1995), it is intuitive that leaving a delay between fertilizer application and irrigation would benefit pasture production, as more of the P remains in the root zone. These results complement those of Westerman and Overcash (1980), who found a similar response in runoff TP concentrations by delaying irrigation for a few days following manure application.

Soil water content at fertilizer application (_nF) had an effect on runoff and soil water concentrations when irrigation immediately followed fertilizer application. The wetter the soil when fertilizer was applied the higher the P concentrations in runoff and at 0.1- and 0.3-m depths. The higher concentrations from ₀F₀ are likely to be a result of the shorter opportunity time for fertilizer adsorption (Fig. 1). Soil water content at fertilizer application had little effect on runoff and soil water concentrations when there was a 50-mm delay between fertilizer application and irrigation. This is possibly because similar amounts of P fertilizer from both F₅₀ treatments were adsorbed to the soil particles, either by moisture in the soil or atmosphere. Superphosphate can draw moisture from a relatively dry soil (>1520 kPa) toward the granule, forming a moist soil shell around the granule (Lawton and Vomocil, 1954). This may explain why no differences were observed.

The effect of leaving a delay between fertilizer application and irrigation (F₅₀) may have been accentuated by 11.4 mm of rainfall that fell on the experimental bays. The rain fell 4 d after fertilizer application, with no runoff resulting. It fell on treatments ₀F₅₀ and ₅₀F₅₀ before I₁, and coincided with the period between I₁ and I₂ for treatments ₀F₀ and ₅₀F₀ (Fig. 1). Therefore, the effects on treatments were not equal. It is unlikely that the rainfall had a large effect on the dissolution of P from the fertilizer granule, because the majority of P dissolution occurs within a few days of fertilizer application (Lawton and Vomocil, 1954). However, the rainfall on treatments ₀F₅₀ and ₅₀F₅₀ may have leached P that had dissolved from the granule further into the soil, making it less prone to runoff and further movement to depth.

Irrigation Two
Phosphorus concentrations in runoff and soil water from I₂ (Fig. 3) were considerably lower (P < 0.05) than those in I₁, but were higher than I_p concentrations. The lower P concentrations in runoff during I₂ were due to less P being available, since P was previously removed in runoff and infiltrating water during I₁, and the fact that P had more time to adsorb to soil particles and/or be used by the pasture.

View larger version (56K): [in this window] [in a new window]   Fig. 3. Average total phosphorus (TP) and filtrable reactive phosphorus (FRP) concentrations sampled in runoff and in the soil during the second irrigation following fertilizer application. Different subscripts (a, b, c) above the bars indicate significant differences (P < 0.05) between treatments at each depth or time, calculated using Fisher's unrestricted least significant difference (P < 0.05).

Irrigation Three
Results
Phosphorus concentrations in runoff and soil water from I₃ (Fig. 4) were lower than those in I₂ and were half, or less than half, of the concentration measured during I₁. Total P runoff concentrations, at all collection times, were affected (P = 0.042) by the length of the delay between fertilizer application and irrigation (F_m), with F₅₀ treatments having higher concentrations in runoff than F₀ (Fig. 4). This is the reverse of I₁, although concentrations were much lower. Filtrable reactive P concentrations were significantly influenced by F_m at the 30 and 60 min sampling with F₅₀ having greater (P = 0.04) FRP concentrations than F₀ (Fig. 4).

View larger version (65K): [in this window] [in a new window]   Fig. 4. Average total phosphours (TP) and filtrable reactive phosphorus (FRP) concentrations sampled in runoff and in the soil during the third irrigation following fertilizer application. Different subscripts (a, b, c) above the bars indicate significant differences (P < 0.05) between treatments at each depth or time, calculated using Fisher's unrestricted least significant difference (P < 0.05).

Discussion
By I₃, P runoff concentrations were higher from treatments that initially had a delay between fertilizer application and irrigating (F₅₀) than treatments irrigated immediately after fertilizer application (F₀). While this is the reverse of I₁, P concentrations measured during I₃ were significantly lower than during I₁. Phosphorus concentrations are higher from F₅₀ than F₀, possibly because less P was removed in runoff and to depth during I₁ from F₅₀ (Fig. 2), therefore it is likely that more P was retained on these treatments following I₁. In addition, the P was probably adsorbed nearer to the soil surface, possibly making it easier to desorb during subsequent irrigations. These results indicate that the timing of the first irrigation following fertilizer application affects the concentration of P in surface runoff two irrigations later. However, neither _nF nor F_m affected P concentrations at depth during I₃.

Phosphorus Loads
The runoff volumes from I₁ were reasonably similar from each treatment (Table 1), therefore the loads were largely proportional to the concentrations. The greatest (P < 0.05) P loads in I₁ were from ₀F₀, the next highest from ₅₀F₀, and then the two F₅₀ treatments, which were not significantly different from one another. During I₂ and I₃ there was no significant difference between treatments and P runoff loads. The total runoff P load for the three irrigations as a percentage of the P fertilizer applied was 35% for ₀F₀, 9% for ₀F₅₀, and 6% for the F₅₀ treatments. All treatments lost the majority of their P load during I₁ (94, 51, 84, and 61% for ₀F₀, ₀F₅₀, ₅₀F₀ and ₅₀F₅₀, respectively).

As the volume of runoff is directly proportional to the load of P, the authors caution readers against drawing conclusions about the P runoff loads, as the loads are an artifact of the management of the experiment. The P loads moving into the soil could not be estimated from the measurements taken in this experiment, because only water contents, not tensions and hence fluxes, were determined.

	GENERAL DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION GENERAL DISCUSSION CONCLUSIONS REFERENCES

 
The increased P concentrations in runoff following P fertilizer application support the work of Austin et al. (1996) and Nexhip and Austin (1998). However, increased P concentrations at depth following P fertilizer application differ from the findings of Sharpley and Menzel (1987). Sharpley and Menzel (1987) did not consider P losses from subsurface flow as significant, due to the high retention capacity of most soil. This finding was based on water and dissolved P moving slowly through the soil matrix, coming in close contact with the soil. However, the increased P concentrations measured at depth during this experiment were probably a result of preferential flow. Preferential flow represents a significant proportion of water movement through Lemnos loam soils (Prendergast, 1995). The water and dissolved P moved quickly through the soils to depth via macropores and/or cracks, allowing little time for the dissolved P and soil to interact. Gotoh and Patrick (1974) and Sims et al. (1998) also suggested that P might move to depth through cracks in the soil profile. The high P concentrations (up to 101 mg P L^-1) moving to 0.1 and 0.3 m in ₀F₀ probably occurred through saturated flow via cracks that were observed to remain in the soil profile after wetting. There is a possibility that the soil water P concentrations at depth may have been overestimated resulting from the extraction of water for the sample. This extraction may have increased the downward hydraulic gradient, potentially increasing P movement to depth.

The majority (>72%) of P in runoff or moving to depth for the three irrigations was as soluble FRP rather than particulate P. This was partly because the sediment loss from perennial pastures with gentle slopes is minimal (Sharpley and Menzel, 1987). The percentage of TP as FRP was greater from the F₀ treatments than the F₅₀ treatments. Delaying irrigation following fertilizer application (F₅₀) allowed the fertilizer time to dissolve from the granule and bind with the soil. This meant there was less soluble P available to dissolve in the irrigation water, therefore reducing the proportion of soluble FRP in TP. Leaving a delay between fertilizer application and irrigation reduced the quantity as well as the percentage of soluble P that could potentially be available for algal growth.

The findings from this replicated work, in association with the field data from Nexhip and Austin (1998), suggest that when applying P fertilizer to red-brown earths, a delay of at least 3 d should be left between applying fertilizer and irrigating to allow the nutrients to adsorb to the soil particles. A delay of 3 d or greater between applying fertilizer and irrigating will approximately halve the concentration of P in runoff and moving below 0.1 m in the soil for the three irrigations following fertilizer application.

The Hydrologic Research Group (1998) stated that 50 to 75% of P removed annually from a grazed irrigation bay is removed in the first three irrigations following fertilizer application. Therefore, by applying fertilizer 3 d prior to irrigation, there is a potential 25 to 37% reduction in P lost from the irrigation bay. The adoption of this management practice will substantially reduce the concentration and therefore the load of P from irrigated pastures entering irrigation drains and subsequently rivers, potentially reducing the frequency and intensity of algal blooms.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION GENERAL DISCUSSION CONCLUSIONS REFERENCES

 
The period between fertilizer application and irrigation (F_m) had a great effect on P concentrations in runoff and infiltrating water. Soil water content at fertilizer application (_nF) only affected the concentrations of P in runoff and moving to depth when irrigation immediately followed fertilizer application.

In the first irrigation after fertilizer application, runoff P concentrations and P moving below 0.1 m in the soil were greatest (P < 0.001) from treatments that were irrigated immediately after fertilizer application (F₀). Conversely, by the third irrigation, treatments that had a delay of 50 mm between fertilizer application and irrigation (F₅₀) had the greatest (P < 0.05) P concentrations in runoff. However, P concentrations measured during the third irrigation were, at most, half those in first irrigation following fertilizer application. Therefore, over the three irrigations, leaving a delay between applying single superphosphate fertilizer and irrigating significantly reduced P concentrations in runoff and in the soil water at 0.1 and 0.3 m depth.

To minimize P losses in runoff and moving below the majority of a pasture's roots, and therefore to maximize fertilizer use efficiency–pasture production and minimize detrimental environmental effects, it is recommended that there be a delay between applying single superphosphate and the subsequent irrigation.

	ACKNOWLEDGMENTS

 
We wish to thank Pivot Agriculture and the Murray–Darling Basin Commission through the Natural Resources Management Strategy for financial support. We thank Dr. Leigh Callinan for providing biometrical support and Pauline MacDonald and Brian O'Meara for technical assistance.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION GENERAL DISCUSSION CONCLUSIONS REFERENCES

 

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