Comparison of single and coastal superphosphate for subterranean clover on phosphorus leaching soils

Coastal superphosphate, a partially acidulated rock phosphate (PARP), is being considered as an alternative fertilizer to single superphosphate for pastures in high rainfall (> 800 mm annual average) areas of south-western Australia. The effectiveness of single and coastal superphosphate, as P fertilizers, was measured in two field experiments using dry herbage yield of subterranean clover (Trifolium subterraneum). The experiments were started in April 1990 and were terminated at the end of 1993. In the years after P applications, soil samples were collected each January to measure Colwell soil-test P, which was related to plant yields measured later on that year, to provide soil P test calibrations.Relative to freshly-applied single superphosphate, the effectiveness of freshly-applied coastal superphosphate and the residues of previously-applied single and coastal superphosphate were less effective in some years (from 3% as effective to equally effective), and up to 100% more effective in other years. This large range in effectiveness values in different years is attributed to different climatic conditions. Soil P test calibrations were different for soils treated with single or coastal superphosphate. The calibrations were also different for different yield assessments (harvests) in the same year, and in different years. Consequently soil P testing can only provide a very crude estimate of the current P status of the soils.     

Summary of research on soil testing for rock phosphate fertilizers in Western Australia

The relationship between plant yield and values of soils tests for phosphorus (P) was studied in long-term field experiments in south-western Australia for soil previously fertilized with rock phosphate and superphosphate. The rock phosphates studied were: Queensland (Duchess) apatite rock phosphate; reactive apatite rock phosphate from North Carolina; and rock phosphate from Christmas Island (as either C-grade ore or Calciphos). The P fertilizers were applied once only at the start of each experiment, and in subsequent years, soil samples were collected in January-March to measure soil test values. These were compared with plant yields measured later on in that year. The Colwell alkaline bicarbonate soil test was used in all years in all experiments. Olsen, Bray, lactate and Troug tests were used in some years in some experiments. For all soil tests the relationships between yield and soil test values was generally different for rock phosphate and superphosphate. For a given source of P, none of the different soil test reagents was significantly superior for predicting plant yields. The relationship between yield and soil test value was also generally different for different plant species. At one site cultivation was included as a treatment and the relationship varied depending on the cultivation treatment of the topsoil before sowing oats (Avena sativa). The relationship between yield and soil test also differed between years.     

Development and evaluation of an improved soil test for phosphorus, 3: field comparison of Olsen, Colwell and Resin soil P tests for New Zealand pasture soils

Soil tests suitable for estimating the phosphorus (P) status of soils fertilised with soluble or sparingly soluble P fertilisers (reactive phosphate rock) were evaluated using the New Zealand Ministry of Agriculture Technology (NZMAFTech) National Series forms of phosphate trials on permanent pastures located throughout NZ. This included a common core of treatments comparing Sechura phosphate rock (SPR) with triple superphosphate (TSP). At each site, a re-application of twice maintenance TSP was superimposed on one-half plots that previously had received six annual applications of increasing amounts of P (0, 0.5, 0.75, 1.0 and 2.0 times the maintenance rate) in the form of TSP or SPR. Before the re-application of TSP, soil samples (0–30 and 0–75 mm depths) were collected from each plot. All the trials were run for 1 year during which seven to ten harvests were taken. Pasture response was expressed as percent increase in yield obtained with re-application over the previous treatment.The 0.5 NaHCO₃ based (Olsen P) extractant with different combinations i.e. soil volume (Olsen (v)), soil weight (Olsen (w)), shaking time variations (Olsen (16 h)) and soil:solution ratio (Colwell), and Resin P soil tests were conducted on soils taken from the plots prior to re-application of TSP. The Olsen (v), Olsen (16 h) and Colwell P values increased with increasing rates of P applied in all soils with values for sparingly soluble P materials being less than where soluble P fertiliser had been previously applied. The Resin P values showed similar increases with P applied regardless of the solubility of previously applied fertiliser. When the yield increases caused by TSP application to all treatments (irrespective of fertiliser source) were regressed against soil test values, Resin P explained 76% of the variation in yield response, compared to 50% by Olsen (v), 42% by Olsen (w), 39% by Olsen (16 h) and 40% by Colwell P. Partitioning the data according to fertiliser source slightly improved the coefficient of determination for Resin P for both the soluble (R²=0.81) and sparingly soluble (R²= 0.80) P fertilisers. With 0.5 M NaHCO₃ (Olsen) extractants, R² values consistently indicated a poorer prediction for the SPR treatments. A Resin P model was able to account for more variance in yield response to re-applied TSP, than an Olsen P model because the Olsen model underestimated the yield response to re-applied TSP on the PR treatments. The Resin test is more suitable than the current Olsen test for assessing the plant available P status of soils previously fertilised with fertilisers of varying solubility.Dr. A.G. Sinclair died on 3 December 1996 whilst this paper was in preparation.     

A comparison of seven soil P tests for plant species with different external P requirements grown on soils containing rock phosphate and superphosphate residues

Seven soil tests for phosphate (P) (Bray 1, Bray 2, Truog, ammonium oxalate, Colwell, iron oxide-strip (Pi) and resin-strip soil tests) were evaluated for predicting the yield of plant species which have very different external P requirements. Two acid, sandy soils that had been fertilized six years previously with superphosphate and three rock phosphates were used. A glasshouse pot experiment with lettuce, wheat and maize was used to calibrate the soil tests.For some soil P tests, different calibrations relating yield to soil P test values were required for each plant species, P fertilizer and soil combination. The Bray 2 and Truog soil P tests were the worst predictors of yield for both soils and all plant species. The Pi and ammonium oxalate tests were the most predictive tests for one soil when data for all fertilizers were considered. The Bray 1 and Colwell soil P tests were the most predictive for the other soil. The resin-strip P test was poorly predictive of yield of lettuce and wheat for both the soils. The accuracy in prediction of yield on the basis of P test value decreased in the sequence maize > wheat > lettuce. This rank is opposite to the increasing external P requirements of these species.     

Evaluation of the Bray 1, calcium acetate lactate (CAL), Truog and Colwell soil tests as predictors of triticale grain production on soil fertilized with superphosphate and rock phosphate

In a field experiment in Western Australia, six different levels of three different phosphorus (P) fertilizers (triple superphosphate, TSP; Queensland (Duchess) rock phosphate, QRP; North Carolina rock phosphate, NCRP) were applied at the start of the experiment in 1984. Grain yield of triticale (×Triticosecale) was measured from 1984 to 1988. In February-March of each year from 1985 to 1988, soil samples were collected to measure soil extractable P (soil test values) using four reagents (Bray 1, calcium acetate lactate (CAL), Truog and Colwell). Soil test values were related to triticale grain yields, determined either as absolute yield or percentage of the maximum yield, produced later on in each year. The relationship differed with fertilizer type, reagent and year. All four soil test reagents were equally predictive of yield. It is concluded that these soil P tests provide crude predictions of plant yield regardless of the reagent used.     

The residual value of superphosphate and rock phosphates for lateritic soils and its evaluation using three soil phosphate tests

The residual value of superphosphate and several rock phosphates was measured in three field experiments in Western Australia. The rock phosphates were Christmas Island C-grade ore, calcined C-grade ore (Calciphos) and apatite rock phosphates. The predictive capacity of the Colwell, Olsen and Bray 1 soil tests for phosphate were also evaluated.As measured by yields of variously wheat, oats, barley or clover, the effectiveness of an initial application of superphosphate decreased to about 50% of that of newly applied superphosphate between years 1 and 2, and further decreased to about 20% over subsequent years. At low levels of application, all the rock phosphates were between 10–20% as effective as superphosphate in the year of application for all experiments. Relative to newly applied superphosphate their effectiveness remained approximately constant in subsequent years for two experiments and doubled for the other experiment.The Colwell soil test predicted that the effectiveness of superphosphate decreased to about 45% between years 2 and 3, followed by a more gradual decrease to approximately 15%. At low levels of application, the effectiveness of the rock phosphates as predicted by the Colwell soil test values was initially very low relative to superphosphate (2–30%), and remained low in subsequent years (2–20%). For superphosphate treated soil, the proportion of the added phosphorus extracted generally increased as the level of application increased. By contrast, for rock phosphate treated soil, the proportion of added phosphorus extracted decreased as the level of application increased.For all three experiments there were highly significant positive correlations between amounts of P extracted by the three soil tests. Consequently all soil tests were equally predictive of yield but usually for each soil test separate calibrations between yield and soil test values were required for the different fertilizers and for each combination of fertilizer and plant species and for each year.     

Effect of water supply on the response of wheat and triticale to applications of rock phosphate and superphosphate

The effect of water supply on the response of wheat (Triticum aestivum) and triticale (×Triticosecale) to levels of freshly-applied rock phosphate and superphosphate, and the residues of these fertilizers applied 9 years previously in the field, was studied in three glasshouse experiments. The < 2 mm fraction of the top 10 cm of soil was used (1.8 kg soil per pot), and in one experiment, freshly-applied fertilizer was also added to the more acidic subsoil (10 to 20 cm). There were two water treatments: the soil was returned to field capacity by watering to weight, either daily (W1, adequate water) or weekly (W2, water stress). Yield of dried tops was used to calculate fertilizer effectiveness. The phosphorus (P) concentration in dried tops was used to determine critical P, which is the P concentration related to 90% of the maximum yield. Just before sowing, soil samples were collected to measure bicarbonate-extractable (soil test) P which was related to plant yield.Water stress reduced yields and maximum yield plateaus by 20 to 40%. Water stress reduced the effectiveness of all P fertilizers by between 20 to 60%, largely because of a reduction in the maximum yield potentials. In the field, water supply is seasonally dependent and it can affect the yield response of plants to freshly-applied rock phosphate and superphosphate and the residues of these fertilizers applied to the field in previous years. Relative to placing fertilizer in the topsoil, placing fertilizer in the subsoil improved effectiveness by about 26% for rock phosphate and 12% for superphosphate.The relationship between yield and P concentration in dried tops, and critical P, differed for W1 and W2. The soil test P calibration, which relates yield to soil test P, and the soil test P required to produce the same yield also differed for W1 and W2. Consequently critical P and soil test P calibrations depend on water supply, which in the field varies within and between growing seasons. This is so for freshly- and previously-applied rock phosphate and superphosphate.     

A comparison of five soil phosphorus tests for five crop species for soil previously fertilized with superphosphate and rock phosphate

The P_i, Colwell, Bray 1, calcium acetate lactate (CAL) and Truog phosphorus (P) soil test reagents were assessed in two field experiments on lateritic soils in Western Australia that had been fertilized four years previously (1984) with triple superphosphate, North Carolina rock phosphate, Queensland rock phosphate, and in one experiment, Calciphos. Soil samples to measure soil P test were collected February 1987. Soil P test was related to seed (grain) yields measured later in 1987. Different crop species were grown on different sections of the same plot at each site. The species were lupins (Lupinus angustifolius), barley (Hordeum vulgare) and oats (Avena sativa) at one site, and lupins, oats, triticale (×Triticosecale) and rapeseed (Brassica napus) at the other site. For each reagent, the soil P test calibration, which is the relationship between yield, expressed as a percentage of the maximum yield, and soil P test, generally differed for different plant species and for different fertilizer types. Variations in soil P test required to produce half the maximum yield of each species at each site was least for the CAL reagent followed by the Colwell reagent.     

Reactive rock phosphate fertilizers and soil testing for phosphorus: The effect of particle size of the rock phosphate

North Carolina rock phosphate (NCRP) (highly carbonate—substituted apatite) was ground to produce three samples with different particle size distributions. The effectiveness of these fertilizers was compared with the effectiveness of superphosphate in a field experiment and three glasshouse experiments using lateritic soils from south-western Australia. Non-reactive Queensland rock phosphate (low carbonate-substituted apatite from the Duchess deposit) was also used in the pot experiments. Bicarbonate-soluble phosphorus extracted from the soil is widely used in Western Australia to predict plant yields from previously-applied fertilizer dressings. For both field and pot experiments bicarbonate-extractable phosphorus (soil test value) was measured and related to subsequent plant yields.As calculated from the initial slope of the relationship between yield and the level of P applied, finely powdered NCRP was about 5–32% as effective as freshly-applied superphosphate in the year of application and also for two years after application in the field experiment, and for two successive crops in the three pot experiments. For both field and pot experiments, finely powdered NCRP, was at best, 1.5–2.0 times as effective as granular NCRP. Relative to freshly-applied superphosphate, the effectiveness of rock phosphates usually decreased with increasing level of application.For each of the crops in the field experiment, the relationships between yield and phosphorus content of plants (i.e. internal efficiency curves) were similar for all fertilizers. Thus the low effectiveness of the rock phosphates relative to superphosphate was solely due to much less phosphorus being taken up by plants. By contrast, in the pot experiments internal efficiency curves differed for different fertilizers. This is attributed to differences in the rate of phosphorus uptake by plant roots during the early stages of plant growth.For both field and pot experiments, soil test calibrations (the relationship between yield and soil test value) differed for rock phosphates and superphosphate. For superphosphate, soil test calibrations also differed for the three different years after the initial application of this fertilizer in the field experiment. For the second crop in the pot experiment, soil test calibrations differed for superphosphate applied at different times (before the first and the second crop). These results point out the difficulty of applying soil testing procedures to soils that have experienced different histories of fertilizer application.     

Cultivation reduces fertilizer residual effectiveness and affects soil testing for available phosphorus

The agronomic effectiveness of superphosphate and two rock phosphates that had been applied once only to the soil surface 8 to 12 years previously was measured in a field experiment with oats on a lateritic soil in south-western Australia. The soil was either undisturbed or cultivated with a rotary hoe before sowing. The rock phosphates were Christmas Island C-grade ore (C-ore, a calcium ironaluminium rock phosphate), and C-ore calcined (heated) at about 500°C (Calciphos).Cultivation reduced the effectiveness for all three fertilizers by 20 to 50%. The effectiveness of phosphorus (P) applied as superphosphate decreased with increasing period from time of application whereas the effectiveness of the rock phosphates increased but they were always much less effective than superphosphate.The relationship between grain yield and P concentration of plant tissue (i.e. the internal efficiency of P use curve) was similar regardless of fertilizer type, year of application of fertilizer, and whether or not the soil was cultivated. Thus differences in fertilizer residual effectiveness were solely due to the amount of P taken up by the plants.Values of bicarbonate-soluble P (i.e. soil test for P values) for superphosphate treated soil were reduced by about 20 to 25% when the fertilizer was incorporated into the soil whereas for the rock phosphate treated soils the values were little affected by cultivation. The relationship between yield and soil test for P values varied depending on cultivation treatment and fertilizer.We conclude that cultivation decreases the effectiveness of residual fertilizer P and that cultivation and fertilizer type influence the accuracy of yield prediction from soil test values.     

Effectiveness of different phosphatic fertilizers measured using labelled superphosphate and phosphorus taken up by plants

The agronomic effectiveness of three P fertilizers (diamonium phosphate, rock phosphate and compost) was studied in a greenhouse experiment using wheat. A radioisotopic method, using triple superphosphate labelled with³²P, was used to evaluate the P in dried tops that was derived from i) the soil, ii) labelled superphosphate and iii) the fertilizer being studied.The ratio between P uptake from each fertilizer and P uptake from the soil was used to compare the effectiveness of the different fertilizers. P derived from diammonium phosphate was greater than P derived from the soil, except in one soil. P derived from rock phosphate was always lower than P derived from the soil. The effectiveness of compost depended on soil type. Compost can produce two kind of effects: i) a direct P contribution and ii) an indirect effect improving P uptake from the soil. The radioisotopic method can be used to study the effectiveness of fertilizers even when there are no differences in yield.     

Residual value of superphosphate for oat and barley grown on a very sandy,phosphorus leaching soil

In a field experiment in a Mediterranean climate (474 mm annual rainfall, 325 mm (69%) falling in the May to October growing season) on a deep sandy soil near Kojaneerup, south-western Australia, the residual value of superphosphate was measured relative to freshly-applied superphosphate. The grain yield of five successive crops (1988–1992) was used to measure the residual value: barley (Hordeum vulgare), barley, oat (Avena sativa), lupin (Lupinus angustifolius), and barley. There was no significant yield response to superphosphate applied to the first crop (barley, cv. Moondyne). There were no results for the second crop (barley) due to weeds or the fourth crop (lupin) due to severe wind erosion which damaged the crop. The residual value of superphosphate was measured using grain yields of the third crop (oat, cv. Mortlock) for superphosphate applied one and two years previously, and the fifth crop (barley, cv. Onslow) for superphosphate applied one, two, three and four years previously. In February 1992, before sowing the fifth crop, soil samples were collected to measure bicarbonate-extractable phosphorus (P) (soil test P) which was related to the subsequent grain yields of that crop. This relationship is the soil test P calibration used to estimate the current P status of soils when providing P fertilizer recommendations.The residual value of superphosphate declined markedly. For the third crop (oat), it was 6% as effective as freshly-applied superphosphate one year after application, and 2% as effective two years after application. For the fifth crop (barley), relative to freshly-applied superphosphate, the residual value of superphosphate in successive years after application was 46%, 6%, 3% and 2% as effective. The soil has a very low capacity to sorb P, and P was found to leach down the soil profile. The largest yield for P applied one and two years previously in 1990, and two, three and four years previously in 1992, was 35 to 50% lower than the maximum yield for freshly-applied P.Soil test P was very variable (coefficient of variation was 32%) and mostly less than 8µg P/g soil. The calibration relating yield (y axis) to soil test P (x axis) differed for soil treated with superphosphate one year previously compared with soil treated two, three and four years previously. The top 10 cm of soil was used for soil P testing, the standard depth. P was leached below this depth but some of the P leached below 10 cm may still have been taken up by plant roots. Consequently soil test P underestimated the P available to plants in the soil profile. The soil test P calibration therefore provided a very crude estimate of the current P status of the soil.     

The influence of seasonal conditions,plant species and fertilizer type on the prediction of plant yield using the Colwell bicarbonate soil test for phosphate

Testing for soil phosphate (P) using the Colwell procedure is widely used in south-western Australia to estimate fertilizer applications required for crops and pastures. The relationship between plant yield, expressed as a percentage of the maximum yield, and soil test values is assumed to be constant in different years for the same soil type and plant species. Data from 11 long-term field experiments in south-western Australia show that regardless of whether percentage of maximum or absolute yield is used, the relationship between yield and soil test values is different (1)in different years, for the same site and where the same P fertilizer type has been used. This occurred irrespective of whether the same or different plant species were grown in different years; (2)where different types of P fertilizer had been used, for the same site, same year and same plant species; (3)for different plant species, for the same site, same year, and same type of P fertilizer. We conclude that considerable errors in the recommendation of fertilizer rates may result from the assumption that there is a constant relationship between soil test and yield.     

Comparison of the Pi,Colwell, Bray 1, calcium acetate lactate (CAL) and Truog soil phosphorus test for predicting growth of oats,barley, triticale and clover in the field in lateritic soils fertilised with superphosphate and rock phosphate

The P_i soil test for phosphorus (P), which uses an iron oxide impregnated paper, was evaluated in three field experiment on lateritic soils in south-western Australia fertilised with triple superphosphate, North Carolina rock phosphate, Queensland rock phosphate, and in one experiment, Calciphos. Soil samples were collected February to March from 1985 to 1988. The P_i, Colwell, Bray 1, calcium acetate lactate (CAL) and Truog soil P tests were used. Soil test P values were related to yields of triticale (×Triticosecale) or oats (Avena sativa), barley (Hordeum vulgare) or dry herbage yields of subterranean clover (Trifolium subterraneum). The Colwell soil test, which is commonly used in Australia, and the Pi soil test were almost equally predictive, but showed considerable error in prediction of yield. For each soil test and plant species the relationship between yield and soil test P differed with fertilizer type and year. For combined data for all sites, fertilizers and years, the CAL soil test was the most predictive and the Truog soil test was least predictive of plant yield.     

Residual value of superphosphate measured using yields of different pasture legume species and bicarbonate — extractable soil phosphorus

The residual value of superphosphate was measured in three glasshouse pot experiments using three different lateritic soils (pH CaCl₂: 4.8–5.3) from south-western Australia. The residual value was estimated relative to levels of freshly-applied superphosphate using yield of dried tops and bicarbonatesoluble P extracted from the soil (soil test values). Up to five successive crops were grown. In each experiment, four different pasture legume species fertilized with mineral nitrogen were grown in rotation with a cereal species. The legume species includedMedicago polymorpha, M. murex, Trifolium subterraneum, Ornithopus compressus, O. perpusillus andO. pinnatus. The cereal species includedTriticum aestivum, ×Triticosecale, andHordeum vulgare. The comparative phosphorus (P) requirement of the different pasture legumes was estimated from the amount of P required to produce 50 or 90% of the maximum yield measured for each species at each harvest. Soil samples for the soil test were collected just before sowing each crop, and were related to the plant yields of that crop.Relative to freshly-applied superphosphate, the residual value of superphosphate measured using plant yield was similar for all pasture legume species, and decreased markedly, by about 50 to 80% between the first and second crop, and by a further 5 to 30% for subsequent crops. The decrease in residual value estimated using soil test values was less marked. For freshly-applied superphosphate, and for the same plant species, the relationship between yield and the level of P applied differed for different crops.There was no consistent, systematic trend for the comparative P requirement of the different legume species within and between crops of the three experiments and soils.For all crops, the relationship between yield of dried tops and P concentration in dried tissue generally differed for the different legume species, indicating the different species usually have different internal efficiency of P use curves. However, for each experiment, when the same cereal species was grown in all the pots, the relationship between yield and P concentration in tissue was similar for previously- and freshly-applied superphosphate, regardless of the pasture legume species grown in previous crops.The relationship between yield and soil test values usually differed, within each crop, for different plant species and for previously- and freshly-applied superphosphate. For the same plant species, the relationship also differed between different crops.     

An assessment of soil tests for phosphate for the prediction of cereal yields on a sandy soil in Western Australia

Nine soil tests for phosphate were evaluated for predicting the yield and P content of wheat, barley and oats grown on a sandy soil in Western Australia: Olsen, modified Olsen 1 (soil:solution ratio 1:5), modified Olsen 2 (soil:solution ratio 1:50), Colwell, Bray 1, Bray 2, modified Bray 2T (shaking time 10 minutes), modified Bray 2C (pH 3.7) and lactate. The soil had been fertilized 5 years previously with 20 levels each of superphosphate (OSP, range 0 to 400 kg P ha^–1) and Queensland rock phosphate (QRP, range 0 to 20 000 kg P ha^–1). For each species and fertilizer taken separately, all the tests, except for lactate, gave a good prediction of yield. When data for OSP and QRP were pooled, Bray 2 and modified Bray 2T tests were unsatisfactory predictors of both yield and P content.A linear relationship (P < 0.05) between mean soil tests value () and the standard deviation ( ) of the test value was observed for each soil test. For QRP, the results for lactate were the most variable (i.e./ was greatest) followed by modified Olsen 2 > Bray 1 > Bray 2 > Olsen > modified Bray 2C > modified Olsen 1 > modified Bray 2T > Colwell. The order for OSP fertilized soil was Bray 1 > modified Bray 2T > Bray 2 > Olsen > Colwell > modified Bray 2C > modified Olsen 1 > lactate > modified Olsen 2. For combined OSP and QRP data, the results of the Olsen 1 and Colwell extractions were the least variable.Errors in the prediction of yield ( _Y ) for all crops resulting from an error in soil test values () were calculated. For OSP-fertilized soil variability in values for the Bray-1 test provided the highest error (about 16%) in the prediction of the yield, followed by Bray 2 (12%) > Bray 2T (10%) > Olsen (8%) > Colwell (7%) > modified Bray 2C (6%) > lactate (4%). Maximum error was at yields of about 65% of maximum yield. For soil fertilized with QRP, lactate provided the highest error (about 10%) in the prediction of yield, followed by the other tests (< 6%). Maximum error was at yields of about 35% of maximum yield.The Colwell soil test gave the most accurate overall prediction of yield for both fertilizers.     

The effect of water supply on the response of subterranean clover,annual medic and wheat to superphosphate applications

The effect of water supply on the response of subterranean clover (Trifolium subterraneum), annual medic (Medicago polymorpha) and wheat (Triticum aestivum) to levels of phosphorus (P) applied to the soil (soil P) was studied in four glasshouse experiments. P was applied as powdered superphosphate. In one experiment, the effect on plant yield of P concentration in the sown seed (seed P) was also studied. There were two water treatments: the soil was returned to field capacity, by watering to weight, either daily (adequate water, W1) or weekly (water stress, W2). In three experiments: (i) P concentration or content (P concentration × yield) in plant tissue was related to plant yield, and (ii) soil samples were collected before sowing to measure bicarbonate-extractable P (soil test P) which was related to subsequent plant yields.Compared with W1, water stress consistently reduced yields of dried tops and the maximum yield plateau for the relationship between yield and the level of P applied, by up to 25 to 60% in both cases. Compared with W1, the effectiveness of superphosphate for producing dried tops decreased for W2 by 11 to 45%, for both freshly-applied and incubated superphosphate. Consequently in the field, water supply, which varies with seasonal conditions, may effect plant yield responses to freshly — and previously — applied P fertilizer.Seed P increased yields, for W1, by 40% for low soil P and 20% for high soil P; corresponding values for W2 were 20 and 12%. Consequently proportional increases due to seed P were smaller for the water-stressed treatment.The relationship between yield and P concentration or content (internal efficiency of P use) differed for W1 and W2, so that the same P concentration or content in tissue was related to different yields. Estimating the P status of plants from tissue P values evidently depends on water supply, which in the field, differs in different years depending on seasonal conditions.The relationship between yield and soil test P differed for W1 and W2. Predicting yields from soil test P can only provide a guide, because plant yields depend on both P and water supply, which in the field may vary depending on seasonal conditions.     

The initial and residual value for subterranean clover of phosphorus from crandallite rock phosphates,apatite rock phosphates and superphosphate

The residual value of phosphorus from superphosphate, crandallite rock phosphate (Christmas Island C-grade ore), 500°C calcined crandallite rock phosphate (Calciphos) and apatite rock phosphate from Queensland, Australia, was measured in a 6 year field experiment sited on lateritic soil in south-western Australia. Different amounts of each fertilizer were applied at the commencement of the experiment, and either left on the soil surface or mixed through the soil by cultivating to a depth of about 10 cm. Dry matter production of subterranean clover measured in spring (August) and bicarbonate-extractable phosphorus determined from soil samples collected in summer (January–February) were used as indicators of fertilizer effectiveness.The effectiveness values calculated for each fertilizer each year were similar for the treatments that were left on the soil surface and those which were mixed through the soil. The effectiveness of both ordinary and triple superphosphate were similar each year. They were the most effective fertilizers for the duration of the experiment. Using pasture yield as an indicator, the effectiveness of the superphosphates decreased by about 50% from year 1 to year 2, and by a further 10% over the remaining 4 years. Using bicarbonate-extracted soil phosphorus the effectiveness of both superphosphates decreased in a more uniform fashion by about 60% from year 2 to year 6. The effectiveness of all the rock phosphate fertilizers was approximately constant through time. As calculated from yield and bicarbonate-soluble phosphorus values, C-grade ore, Calciphos and the Queensland apatite were respectively 5%, 20% and 7% as effective as freshly applied superphosphate.The proportion of the total phosphorus content present in the rock phosphates which was initially soluble in neutral ammonium citrate was a poor predictor of the effectiveness of the phosphorus from these fertilizers determined using herbage yield or the amount of bicarbonate — soluble phosphorus extracted from the soil.The bicarbonate soil test did not predict the same future production for superphosphate and some of the rock phosphates in years 2 and 3 of the experiment, indicating that different soil test calibration curves are needed for the different fertilizers.     

Effectiveness of Ecophos compared with single and coastal superphosphates

Ecophos is a possible alternative phosphorus (P) fertilizer to single and coastal superphosphate for clover pasture (Trifolium subterraneum) on P leaching, sandy, humic podzols in the > 800 mm annual average rainfall areas of south-western Australia. Ecophos and coastal superphosphate are partially acidulated rock phosphates (PARP) fertilizers. Ecophos is made from calcium iron aluminium (crandallite millisite) rock phosphate. Coastal superphosphate is made from apatite. The sandy humic podzols are known to promote extensive dissolution of rock phosphates, including the untreated rock phosphate present in PARP fertilizers. In this field study (early April 1992 to end of October 1994), the effectiveness of the PARP fertilizers was calculated relative to the effectiveness of single superphosphate (relative effectiveness or RE), using yield and P content of dry clover herbage. The RE of the PARP fertilizers varied markedly between assessments, both within and between years, from being much less effective than single superphosphate, to equally or much more efective. This great diversity in RE is attributed to the different extents P can be leached in the soil, depending on seasonal conditions. It is concluded that Ecophos is a suitable alternative P fertilizer for the soil and environment studied.     

Evaluation of two rock phosphates and a calcined rock phosphate as maintenance fertilizers for crop — pasture rotations in Western Australia

Two long-term (11 and 12 y) field experiments in south-western Australia are described that measured the relative effectiveness of three rock phosphate fertilizers (C-grade ore, Calciphos and Queensland (Duchess) rock phosphate), single, double and triple superphosphate. The experiments were on established subterranean clover (Trifolium subterraneum) — based pasture that had received large, yearly, applications of single superphosphate for many years before the experiments began so that in the first year the nil phosphorus (P) treatment produced 80 to 90% of the maximum yield. The experiments were conducted using a rotation of one year cereal crop (oats,Avena sativa at one site, and barley,Hordeum vulgare, at the other): 2 y pasture, a typical rotation on farms in the region. Five levels of each P fertilizer were applied every third year with the crop. Grain yield of cereals, P content of grain, pasture yield, and bicarbonate-soluble P extracted from the soil (available P) were used to estimate fertilizer effectiveness values.The three superphosphate fertilizers had identical values of fertilizer effectiveness. Superphosphate was always the most effective fertilizer for producing grain. The rock phosphate fertilizers were one-seventh to one-half as effective per kg P as superphosphate when assessed on the yield or P content (P concentration × yield) of grain within each cropping year. Bicarbonate-extractable soil P values demonstrated that superphosphate was two to fifteen times as effective as the rock phosphate fertilizers. The relationship between grain yield and P content in grain (i.e. the internal efficiency of P use curve) was similar for the different P fertilizers. Thus for all P fertilizers yield was not limited by other factors as it varied solely in response to the P content, which in turn presumably depended on the P supply from the fertilizers.The relative agronomic effectiveness of rock phosphates is greater for marginally P deficient soils than for highly P deficient soils but rock phosphate remains less effective than superphosphate. We conclude that the rock phosphates studied should not be substituted for superphosphate as maintenance fertilizers for soils in Western Australia that are marginally deficient in P. This result is consistent with the results of many field experiments on highly P deficient soils in south-western Australia. These have shown that a wide variety of rock phosphate fertilizers are much less effective than superphosphate in both the short and long term.