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Changes in soil pH from a long-term experiment at Hamilton, Vic., associated with time, fertiliser application, and stocking rate were assessed. The pH was measured in a 1 :5 suspension of soil in water (pHW) from 1980 onwards and in a 1:5 suspension of soil in 0.01 M CaCl2 (pHCa) from 1984 onwards. Topsoils (0–10 cm) were sampled regularly from 1980 to 1999, and the soil profile to a depth of 80 cm in 1994. The site was sown to perennial ryegrass, phalaris, and subterranean clover in 1977. Treatments fertilised with different amounts of superphosphate were grazed by sheep at stocking rates of 7–19 dry sheep equivalents (DSE)/ha. Average applications of phosphorus (P) ranged from 0.5 to 38.7 kg P/ha.year. The pHCa of the topsoil in 1984 was 4.9. It decreased at an average rate of 0.005 pHCa or 0.008 pHW units/year, with little variation due to fertiliser or stocking rate. Measurements in 1994 revealed subtle but statistically significant (P < 0.01) trends in soil pHCa that were associated with grazing pressure, inputs of fertiliser, and whether or not areas sampled were used by the sheep for camping. The top 0–5 cm of soil was slightly less acidic (+0.07 pHCa units) in the camp areas compared with non-camp areas. Below 5 cm to a depth of 80 cm, camp areas were more acidic (–0.19 pHCa units). At the highest stocking rates, heavier applications of superphosphate were associated with greater subsoil acidity: –0.06 pHCa units per 100 kg phosphorus (P) applied. There was no relationship at medium stocking rates. At low stocking rates, higher P applications were associated with more alkaline subsoils. Net removal of product from 1979 to 1994 (wool and meat removed from plots and excreta transferred to camps) was estimated to be equivalent to 140–380 kg/ha of lime over this 15-year period. The organic carbon (OC) content of the topsoil did not change over 20 years of records from 1979, and was unaffected by inputs of P. In 1994, the OC content of the 0–5 cm layer of topsoil was greater than the 5–10 cm layer (mean values 5.5% and 3.8% respectively; P < 0.001). The OC content of camp areas was higher than that of non-camp areas, this difference being more pronounced in the 0–5 cm layer (P < 0.01). It was concluded that the rate of change of pH was slow because of the high pH buffering capacity of the soil, the small amount of alkalinity removed in product, and the generally high perennial grass content of the pastures. Soils shown to be at greatest risk of acidification from this study were those under camp areas, and where high fertiliser rates were applied to pastures with a low perennial grass content. Rotational grazing should diminish these problems by reducing the concentration of excreta in camp areas, and favouring perennials over annuals in both camp and non-camp areas. Inputs of lime may eventually be needed to compensate for the acidifying effect of product removal. perennial grass, pasture, stocking rate, superphosphate, acidification.

Fluoride (F) is an impurity in phosphatic fertilisers and significant amounts of F will be added to soils with long-term application of phosphatic fertiliser to soil. There is a risk that F concentrations in soil may increase to levels toxic to plants or grazing animals. If F added to soil remains in the upper soil profile, due to high reactivity with soil, grazing animals could increase their intake of F through inadvertent ingestion of F-enriched soil. This study determined the distribution of F in the soil profile of medium (>8 years) and long-term permanent pastures (>50 years) where F had been applied as an impurity in phosphatic fertiliser, and assessed the potential bioavailability of fertiliser F in soil. Long-term application of superphosphate was linked to increased concentrations of the more soluble fraction of F in soil. The more soluble F fraction in the soil surface led to increased F concentrations in herbage (from 11 to 22 mg F/kg). Fluoride concentrations in herbage were not high enough to be toxic to plants or grazing animals. Fluoride applied as an impurity in phosphatic fertiliser was generally found to accumulate in the upper soil profile (0–6 cm depth). Such increases could also increase F ingestion by ruminants through incidental soil ingestion while grazing. The risk of fluorosis to grazing animals is low at present, but with continued application of fertiliser to soil containing high concentrations of F, and depending on the bioavailability of fertiliser-derived F in soil, we estimate that fluorosis may become an issue in the next 25–50 years in highly fertilised pasture soils. superphosphate, aluminium, plant, ingestion.

A nutrient audit was conducted on a long-term grazed fertiliser experiment at Hamilton in south-western Victoria to determine the fate of applied phosphorus (P) and sulfur (S). Single superphosphate had been applied at rates averaging between 1 and 33 kg P/ha.year since the start of the experiment in 1977. Soil samples were taken in 1994 by coring to a depth of 80 cm, and analysed for total soil nutrient concentration. Most (80%) applied P was in the top 43 cm of the soil profile. A further 6.5% had been transferred to sheep camp areas and 6.5% had been exported as product. It was estimated that <0.4% of applied P left the site in surface water movement. Unaccounted P (6.6%) was probably in the soil, but could not be detected because of the relatively wide confidence margin for total soil P. Only 31% of applied S was detected in the top 43 cm, 3.6% had been transferred to sheep camps, and 4.9% exported in product. Unaccounted S (60%) had probably moved deeper into the soil where it could not be detected from background levels of total soil S. Bulk density in the 0–5-cm layer increased by 1% for each additional ewe per ha, but decreased by up to 0.4% for each kg/ha.year of P fertiliser. Soil nitrogen (N) accumulated at 46 kg N/ha.year at the highest P application rate.Differences in total potassium (K) between low and high fertility treatments indicated that 20 kg K/ha.year had moved out of the 5–19-cm soil layer of the high fertility treatment. This was attributed to competition for exchange sites from calcium (Ca) in the superphosphate. It was concluded that fertilisers with a higher P : S ratio and a lower Ca content than superphosphate are more appropriate for the basalt-derived duplex soils because they would reduce problems associated with displacement of K in the soil profile. phosphorus movement, cation movement, bulk density, nitrogen fixation.

Summary. The agronomic effectiveness of directly applied North Carolina reactive phosphate rock was determined for 4 years from annual dry matter responses at 26 permanent pasture sites across Australia as part of the National Reactive Phosphate Rock Project. Fertiliser comparisons were based on the substitution value of North Carolina reactive phosphate rock for triple superphosphate (the SV50). The SV50 was calculated from fitted response curves for both fertilisers at the 50% of maximum yield response level of triple superphosphate. The reactive phosphate rock was judged to be as effective as triple superphosphate in the 1st year (and every year thereafter) at 4 sites (SV50 >0.9), and was as effective by the 4th year at 5 sites. At another 9 sites the reactive phosphate rock was only moderately effective with SV50 values between 0.5 and 0.8 in the 4th year, and at the final 8 sites it performed poorly with the 4th year SV50 being less than 0.5. Pasture environments where the reactive phosphate rock was effective in the 1st year were: (i) those on sandy, humic or peaty podsols with an annual rainfall in excess of 850 mm; (ii) those on soils that experienced prolonged winter inundation and lateral surface flow; and (iii) tropical grass pastures in very high rainfall areas (>2300 mm) on the wet tropical coast on North Queensland. The highly reactive North Carolina phosphate rock became effective by the 4th year at sites in southern Australia where annual rainfall exceeded 700 mm, and where the surface soil was acidic [pH (CaCl2) <5.0] and not excessively sandy (sand fraction in the A1 horizon <67%) but had some phosphorus (P) sorption capacity. Sites that were unsuitable for reactive phosphate rock use in the medium term (up to 4 years at least) were on very high P-sorbing krasnozem soils or high P-sorbing lateritic or red earth soils supporting subterranean-clover-dominant pasture, or on lower rainfall (< 600 mm) pastures growing on soils with a sandy A1 horizon (sand component >84%). No single environmental feature adequately predicted reactive phosphate rock performance although the surface pH of the soil was most closely correlated with the year-4 SV50 (r = 0.67). Multiple linear regression analysis found that available soil P (0–10 cm) and the P sorption class of the surface soil (0–2 cm), together with annual rainfall and a measure of the surface soil"s ability to retain moisture, could explain about two-thirds of the variance in the year-4 SV50 . The results from this Project indicate that there are a number of specific pasture environments in the higher rainfall regions of Australia where North Carolina reactive phosphate rock can be considered as an effective substitute P fertiliser for improved pasture.

Summary. The agronomic effectiveness of highly reactive North Carolina phosphate rock (NCPR), relative to triple superphosphate (TSP), was determined for individual harvests at 26 permanent pasture sites in the National Reactive Phosphate Rock Project from 1992 to 1995. The aim was to determine whether the performance of NCPR relative to TSP was consistent over the growing season. Poor performance of NCPR early in the growing season (early seasonal lags), and its subsequent improvement later in the season, was observed at 15 of the 26 sites in this project and for 20 of the 103 possible site x year combinations. At 3 additional sites the apparent early season lag in NCPR performance was followed by harvests where there was no phosphorus (P) response. At one P-leaching site with a long growing season the relative yields from NCPR plots increased as the year progressed and exceeded those from TSP late in th e season. The duration of the early season lag phase was associated with the rate of build-up in plant-available P over the 4-year duration of the trials. The lag disappeared after 1 or 2 years at sites with low P-sorbing, sandy soils where there was a significant build-up in Colwell P, but persisted for 4 years on high P-sorbing soils where limited increases in Colwell P occurred. The average early season yield reduction associated with use of NCPR, compared with using TSP, was 28%. Colwell soil test values alone did not provide a good indication of the likelihood of a seasonal effect occurring. The occurrence of an early season lag phase needs to be taken into account by farmers considering the use of RPR as reductions in feed supply at this time of the year have the potential to reduce stocking rates and farm profitability. The use of partially acidulated NCPR, or large applications of NCPR applied in the first year, were management strategies that minimised the occurrence of these seasonal effects. The suggestion that the early season lag effect is due to the inability of NCPR to meet P demands from newly establishing pasture legumes, and increased P requirements by pasture plants in cold conditions is also discussed.

Summary. Field-based cutting trials, which formed part of the National Reactive Phosphate Rock Project, were established across Australia in a range of environments to evaluate the agronomic effectiveness of 5 phosphate rocks, and 1 partially acidulated phosphate rock, relative to either single superphosphate or triple superphosphate. The phosphate rocks differed in reactivity. Sechura (Bayovar) and North Carolina phosphate rocks were highly reactive (>70% solubility in 2% formic acid), whilst Khouribja (Moroccan) and Hamrawein (Egypt) phosphate rock were moderately reactive. Duchess phosphate rock from Queensland was relatively unreactive (<45% solubility in 2% formic acid). Phosphate rock effectiveness was assessed by measuring pasture production over a range of phosphorus levels, and by monitoring bicarbonate-soluble phosphorus extracted from soil samples collected before the start of each growing season. Other treatments included single large applications of triple superphosphate, partially acidulated phosphate rock and North Carolina phosphate rock applied at 2 rates, and the application of monocalcium phosphate and North Carolina phosphate rock sources without sulfur to evaluate the importance of sulfur in the potential use of phosphate rock fertilisers at each site. A broad range of environments were represented over the 30 sites which were based on pastures using annual and/or perennial legumes and perennial grasses. Rainfall across the network of sites ranged from 560 to 4320 mm, soil pH (CaCl2) from 4.0 to 5.1, and Colwell-extractable phosphorus ranged from 3 to 47 µg/g before fertiliser application. Two core experiments were established at each site. The first measured the effects of phosphate rock reactivity on agronomic effectiveness, while the second measured the effects of the degree of water solubility of the phosphorus source on agronomic effectiveness. The National Reactive Phosphate Rock Project trials gave the opportunity to confirm the suitability of accepted procedures to model fertiliser response and to develop new approaches for comparing different fertiliser responses. The Project also provided the framework for subsidiary studies such as the effect of fertiliser source on soil phosphorus extractability, cadmium and fluorine concentrations in herbage, evaluation of soil phosphorus tests, and the influence of particle size on phosphate rock effectiveness. The National Reactive Phosphate Rock Project presents a valuable model for a large, Australia-wide, collaborative team approach to an important agricultural issue. The use of standard and consistent experimental methodologies at every site ensured that maximum benefit was obtained from data generated. The aims, rationale and methods used for the experiments across the network are presented and discussed.

The response of pastures based on Lolium perenne L. and Trifolium subterraneumL. to single superphosphate was assessed at Hamilton, Victoria, by measuring the growth of pastures during winter, spring, and summer over 7 years from 1979 to 1987. The seasons were defined by the pattern of pasture production, rather than by calendar months. Winter was the period of constant growth rate following the autumn rain; spring was the period of accelerating growth rate until growth rate changed abruptly following the onset of dry summer weather. Pastures were grazed with sheep at a low, medium, or high grazing pressure, corresponding generally to stocking rates of 10, 14, or 18 sheep/ha. At each level of grazing pressure, single superphosphate was applied at 5 rates from 1979 to 1982; the highest rate, expressed as elemental phosphorus (P), was reduced from 100 to 40 kg/ha during this time. In addition there was an unfertilised treatment. In 1984, fertiliser was applied at 6 rates from 4 to 40 kg P/ha. No fertiliser was applied in the remaining years, including 1983. Pasture production was measured from 1979 to 1982 and from 1985 to 1987. Total pasture dry matter (DM) accumulation per year at the highest stocking rate was less than the other treatments in 4 of the years. Averaged over all years and fertiliser treatments, the annual net production was 10·1, 10·1, and 9·0 t DM/ha (P < 0·05) for plots grazed at low, medium, and high stocking rates, respectively. The amount of fertiliser required to reach a given proportion of maximum yield response did not vary between winter and spring in any year, but the greater potential yield in spring (P < 0 ·001) meant that as more fertiliser was applied, the disparity between pasture grown in winter and pasture grown in spring increased. Differences in this disparity between extreme levels of P ranged from 1·4 t DM/ha in a drought to about 7 t DM/ha in a good season. The implications for managing farms when pastures are fertilised at higher rates than currently practised by district farmers are that systems of animal production with a requirement for plentiful good quality pasture in spring, such as ewes lambing in spring, should be used. The benefit of spring lambing over autumn lambing was supported when the 2 systems were compared over 26 years using the GrassGro decision support system. Well fertilised pasture systems will also allow more scope for conserving pasture as hay or silage, and increase opportunities for diversification in the farming enterprise, such as spring-growing crops. phosphate, sheep, simulation, GrassGro.

The effect of 3 successive yearly applications of single superphosphate (SSP) to pastures on bicarbonate- extractable soil phosphorus (Olsen P) was measured. The soil was a duplex derived from basalt and the pastures, based on perennial ryegrass and subterranean clover, were continuously stocked with sheep. Six levels of SSP were compared at 3 stocking rates. The amount of P applied annually varied from 0 to 100 kg/ha. These data were used to create an empirical model which used the current value for Olsen P (Olsen Pn ), the amount of P applied as fertiliser that year (fert Pn ), and a lower limit for Olsen P for basalt-derived duplex soils (Olsen Plow ) to predict the Olsen P for the following year (Olsen Pn+1 ). The model had the form: Olsen Pn+1 = Olsen P low + afert Pn + b(Olsen Pn – Olsen Plow). Olsen P low was fixed at 3 mg P/kg soil, and the coefficients a and b were 0.0995 and 0.8020. The model accounted for 96.6% of the variance in Olsen Pn+1 . This model was tested at the same site at 2 other periods: when fertiliser was withheld for 3 years and again after applications of SSP were resumed. The model was also tested against data from another experiment conducted on a similar soil. The model can estimate the amount of fertiliser required to maintain the P status of the soil and predicts that to increase Olsen P by 1 unit in the following year it is necessary to apply 10 kg P/ha in excess of soil maintenance requirement.

The productivity of spring-lambing fine wool Merino sheep grazing pastures sown in 1977 to perennial ryegrass and subterranean clover was assessed from 1989 to 1998. The pastures were fertilised each autumn with single superphosphate at 6 levels, and were stocked at a low, medium, or high stocking rate (SR) at each level of fertiliser. The average phosphorus (P) applied annually since sowing (P ) ranged from 1.6 to 32.9 kg/ha. The SRs used varied with fertiliser level in that they were higher where more fertiliser had been applied, so that the highest SR at each level of fertiliser ensured that the pastures were well utilised. Each ewe raised 1 lamb, which was removed at weaning. The influence of fertiliser on the productivity of the sheep at 4 classes (1–4) of SR (mean SR = 7.1, 10.1, 12.6, and 18.2 ewes/ha for classes 1–4, respectively) was described by: y = A−BCP, where y represents production per sheep (kg), and A, B, and C are constants. For greasy fleece weight, estimates of B and C were 1.59 and 0.84; and for SR classes 1–4, the estimates of A were 5.06, 4.89, 4.78, and 4.46, respectively. For weaning weight of lambs, estimates of B and C were 8.4 and 0.82, and estimates of A were 23.5, 22.7, 21.5, and 20.9 for SR classes 1–4. The mean fibre diameter (µm) of the wool was described by: D = 14.18+1.48 GW, where GW is the mean greasy wool produced annually per sheep (kg) averaged over all sheep and years for each of the 18 treatments. The price (cents/kg) of wool with a fibre diameter D (P D) was given by: P D = 12197+4.94P2 + 688D−0.1945P20D − 5810√D, where 20 µm wool is P20 cents/kg. Supplements were fed if the body condition of ewes fell to a predetermined level. The supplement fed per ewe each year (S), expressed as metabolisable energy (in MJ) was described by: S = −602 − 44.1S R + 178.5P + 8.71S R P +539 √SR− 338.5√P−70.8P√SR, where S R and P represent the mean stocking rate (ewes/ha) and mean P applied annually. When a current set of costs and prices was applied to these equations, the maximum gross margin for a SR of 7.1 ewes/ha was $AU119/ha with 8.6 kg P/ha applied annually, and $AU262/ha for SR of 18.2 ewes/ha with 17.6 kg P/ha applied annually. If income derived from sheep is maintained constant, intensifying the sheep enterprise from the low to the high SR system would involve increasing sheep numbers by about 17%, but would release about 55% of the farm’s area for another purpose. phosphate, economics, wool quality, pasture, farming systems.