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.     

Agronomic effectiveness of partially acidulated rock phosphate and fused calcium-magnesium phosphate compared with superphosphate

The agronomic effectiveness of two partially acidulated rock phosphate (PARP) fertilizers, made from either North Carolina or Moroccan apatite rock phosphate, and a fused calcium-magnesium phosphate (thermal phosphate or TP), was compared with the effectiveness of superphosphate in two glasshouse experiments. A different lateritic soil from Western Australia was used for each experiment. Oats (Avena sativa) were grown in one experiment and triticale (×Triticosecale) in the other. Fertilizer effectiveness was measured using (i) yield of dried tops, (ii) P content (P concentration in tissue multiplied by yield) of dried tops, and (iii) bicarbonate-extractable soil P (soil test value).The following relationships differed for the different fertilizers: (i) yield of dried tops and P content in the dried tops; (ii) yield and soil test values. Consequently the fertilizer effectiveness values calculated using yield data differed from those calculated using P content or soil test data. Freshly-applied superphosphate was always the most effective fertilizer regardless of the method used to calculate fertilizer effectiveness values. For one of the soils, as calculated using yield data, relative to freshly-applied superphosphate, the PARP and TP fertilizers were 15 to 30% as effective for the first crop, and 20 to 50% as effective for the second crop. The second soil was more acidic, and for the first crop the PARP and TP fertilizers were 80 to 90% as effective as freshly-applied superphosphate, but all fertilizers were only 5 to 15% as effective for the second crop. For each soil, the two PARP fertilizers had similar fertilizer effectiveness values. Generally the TP fertilizer was more effective than the PARP fertilizers.     

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.     

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.     

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.     

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.     

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.     

Effect of fertilizer type,sampling depth,and years on Colwell soil test phosphorus for phosphorus leaching soils

The relationships between (i) soil test phosphorus (P) (Colwell sodium bicarbonate procedure) and the level of P applied (from 0 to 1000 kg total P ha^–1) (relationship 1), and (ii) yield and soil-test P (relationship 2, the soil P test calibration), were measured in two field experiments on very sandy, P-leaching soils in the high rainfall (> 800 mm annual average) areas of south-western Australia. The soils were humic sandy podzols, or haplohumods, comprising 97% sand (20 to 2000 m). The experiments started in April 1984 and were terminated at the end of 1990. Soil-test P, measured on soil samples collected to 5, 10 and 25 cm depth each January in the years after P application, was related to yields of dried clover (Trifolium subterraneum) herbage measured later in each year. The four P fertilizers studied were single superphosphate, coastal superphosphate (made by adding, just before granulation, extra rock phosphate together with elemental sulphur while manufacturing single superphosphate), apatite rock phosphate, and Calciphos.Relationship (1) was adequately described by a linear equation (R² > 0.80, most being > 0.90). The slope coefficient estimates the extractability of P from the soil by the Colwell procedure, and is called extractability. Relationship (2) was adequately described by the Mitscherlich equation (R² > 0.75, most being > 0.90). For relationship (2), use of percentage of the maximum (relative) yield eliminated differences due to different maximum yields and yield responses (maximum yield minus the yield for the nil-P treatment). Soil test P ranged from about 4 to 150 g Pg^–1 soil. Soil test P and extractability were generally higher for samples of the top 5 cm of the soil than the top 25 cm, and were largest for single superphosphate and lowest for apatite rock phosphate. Both extractability (relationship (1)) and the curvature coefficient of the Mitscherlich equation (relationship (2)), differed for different P fertilizers and different soil sample depths. The curvature coefficient also differed for different yield assessments (harvests) in the same or different years. Different soil P test calibrations were required for different P fertilizers, soil sample depths and harvest in the same or different years. It is concluded that soil P testing provides a crude estimate of the current P status of P-leaching soils in Western Australia.     

The effects of nitrogen fertilizer form on the plant availability of phosphate from soil,phosphate rock and mono-calcium phosphate

A glasshouse trial using lettuce as the test crop, and laboratory incubations were used to evaluate the influence of various nitrogen fertilizers on the availability of phosphate from an unfertilized loamy sand soil and from the same soil fertilized with Sechura phosphate rock or monocalcium phosphate. The order in which nitrogen fertilizer form increased plant yield and P uptake from soil alone and from soil fertilized with the rock was ammonium sulphate > sulphurised urea > ammonium nitrate > urea > potassium nitrate. For each rock application (both 30 and 60 mg/pot) and for soil alone, increased P uptake by the plant correlated well with decreased soil pH. In soil fertilized with the soluble P form, monocalcium phosphate, the form of the nitrogen fertilizer had little effect on plant P uptake. Subsequent laboratory incubation studies showed that increased dissolution of soil-P or Sechura phosphate rock did not occur until acidity, generated by nitrification or sulphur oxidation of the fertilizer materials, had lowered soil pH to below 5.5. A sequential phosphate fractionation procedure was used to show that in soils treated with the acidifying nitrogen fertilizers, ammonium sulphate and urea, there was considerable release of Sechura phosphate rock P to the soil, amounting to 42% and 27% of the original rock P added, respectively.     

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.     

The agronomic effectiveness of reactive rock phosphate,partially acidulated rock phosphate and monocalcium phosphate in soils of different pH

The initial and residual fertilizer effectiveness of North Carolina RP (rock phosphate), monocalcium phosphate and partially acidulated RP (made from North Carolina RP at 30% acidulation), both granulated and non-granulated, were measured in a glasshouse experiment. Triticale (xTriticosecale) was grown for 30 days on a soil that had been adjusted to three pH values (4.2, 5.2 and 6.2). Two crops were grown with a six month interval between crops. The effectiveness of the different fertilizers was compared using relationships between (1) yield of dried tops and the amount of P applied and (2) P content (P concentration in tissue multiplied by yield) and the amount of P applied. For the first crop, relative effectiveness (RE) of the fertilizers was calculated relative to granulated monocalcium phosphate, the most effective fertilizer. Monocalcium phosphate was not applied to the second crop, so relative residual effectiveness (RRE) was estimated for each fertilizer relative to the residual effectiveness of granulated monocalcium phosphate.The relative effectiveness of granulated monocalcium phosphate (band application) was greater (RE = 1.00) than of North Carolina RP (0.01–0.02) and partially acidulated RP (0.45–0.76) for all three soil pH values for the first crop. Granulation and band application increased the effectiveness of monocalcium phosphate and partially acidulated RP, but reduced the effectiveness of North Carolina RP. Both non-granulated monocalcium phosphate and partially acidulated RP were less effective than granulated partially acidulated RP for both crops. For the second crop granulated monocalcium phosphate was most effective and the RRE of non-granulated partially acidulated RP (0.16–0.32) and North Carolina RP (0.19–0.28) was greater than for non-granulated monocalcium phosphate (0.12). For the more acidic soil the RE of non-granulated North Carolina RP was four times higher than for the high pH soil for the first crop and 60% higher for the second crop, but it was still poorly effective relative to granulated monocalcium phosphate. Granulated North Carolina RP was least effective among all the fertilizers for all soil pH values and for both crops.     

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.     

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.     

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.     

Development and evaluation of an improved soil test for phosphorus. 2. Comparison of the Olsen and mixed cation-anion exchange resin tests for predicting the yield of ryegrass grown in pots

A glasshouse experiment was conducted on four soils contrasting in P sorption capacity and exchangeable Ca content with perennial ryegrass using six phosphate rock (PR) sources and a soluble P source applied at four rates (including a control). After three harvests (11 weeks) replicate pots of each treatment were destructively sampled and Olsen P and mixed cation-anion exchange resin (Resin P) extractions carried out. The remaining replicated treatments were harvested another seven times (during 41 weeks). Yields (for the last seven harvests) were expressed as percentages of the maximum yield attainable with MCP.In general, the Resin P test extracted more than twice as much P as the Olsen test. There was a significant increase in Resin P with an increase in the amount of each P source in all four soils, but Olsen P values were not significantly different for soils treated with different rates of each phosphate rock. The abilities of the Olsen and mixed resin soil P tests to predict the cumulative dry matter yield from 7 harvests and the relative yield of ryegrass were compared. Correlations between measured yield (for the last 7 harvests) and soil test for each soil, and relative yield and soil test for all four soils were assessed by regression analysis using Mitscherlich-type models.When dry matter yields were regressed separately against soil test values for each soil, the Resin P consistently accounted for 18–28% more of the variation in yield than did Olsen P. For Resin P a single function was not significantly different from the separate functions fitted to MCP and PR treatments. However, for Olsen P the separate functions for the MCP and PR treatments varied significantly from the single fitted function. The Resin P test (R² = 0.84) was a better predictor of relative yields over this range of soils than the Olsen test (R² = 0.75). Two regression models based on the regression of relative yield for MCP treatments against either Olsen or Resin were developed. These models were then fitted to the relative yield data on soils fertilized with PRs only. The Olsen P model was found to be a poorer predictor (R² = 0.41) than the Resin P model (R² = 0.73) because it underestimated the observed yield of the PR treatments.     

Phosphate rock and phosphate rock/sulphur granules as phosphate fertilizers and their dissolution in soil

A field trial was conducted for 3 years to evaluate phosphate rocks and phosphate rock/sulphur granules as fertilizers for permanent pastures. Two reactive phosphate rocks, North Carolina (USA) and Chatham Rise (New Zealand), and an unreactive Florida (USA) were used. The materials were applied to a highly phosphate retentive allophanic soil of medium to high phosphorus status. Single superphosphate was employed as the standard fertilizer. The fertilizers were applied at four rates including a control in the first year and again in the third year. The field design enabled measurement of residual effects as well. All the plots received blanket applications of sulphate. The rate of dissolution of phosphate rock was measured by determining soil inorganic phosphate fractions at the highest rate of fertilizer application.The reactive phosphate rocks applied with or without sulphur were as effective as superphosphate in the first and third year of the trial respectively under low and medium responsive conditions. The Florida rock was at the best only 55% as effective as superphosphate. When applied after granulating with sulphur the value increased to 72%. In the second year there was no greater residual effect from the phosphate rocks compared with superphosphate. However, in the third year reactive phosphate rocks gave a slightly greater residual effect; averaged over rates of application the yield increase was 23% over control compared with 18% for superphosphate.The reactive phosphate rocks, applied with or without sulphur, dissolved at the rate of 44% of that added in the first year and 62% of that remaining in the second year. The corresponding values for Florida rock were 27% and 30%, and for Florida with elemental sulphur 35% and 33%. Over 3 years about 96% of the reactive rocks dissolved compared with 56% and 78% in the case of Florida and Florida with sulphur respectively.     

The current and residual value of superphosphate for lupins grown in rotation with oats and wheat on a deep sandy soil

In a field experiment on a deep pale-yellow sand in a 600 mm per annum rainfall Mediterranean environment of south-western Australia, six levels of phosphorus (P) as superphosphate (O up to 546 kg P ha^–1) were applied once only, to the soil surface, before sowing lupins (Lupinus angustifolius). The lupins were grown in a continuous arable cropping rotation with, in successive years, oats (Avena sativa), wheat (Triticum aestivum), lupins. Five such rotations were started in the experiment from 1985 to 1989. The experiment continued until the end of 1990.The relationship between lupin seed (grain) yields and the level of P applied was measured in the year of P application for five successive years (1985 to 1989). The relationship had the same general form but it varied between years, largely due to different maximum yields (yield plateaux) in each year.The residual value of superphosphate applied three years previously was measured for lupins on two occasions (1988 and 1989) relative to superphosphate applied in the current year. The residual values was different in the two years. The superphosphate applied three years previously was about 30% as effective as freshly applied superphosphate in 1988, and 12% as effective in 1989.At each harvest, the relationship between grain yield and the P concentration in the grain differed for different species. However, for each species at each harvest, the relationship was similar regardless of when the P was applied in the previous years. Thus each species had the same internal efficiency of P use curve, and yields varied only with P concentration in tissue.Bicarbonate-extractable soil P was determined on soil samples taken in mid-July of 1989 and 1990. These soil test values were related to grain yields at harvest. The relationship between yield and soil test values had the same general form but varied for different species within years and for each species between years. It also varied for each species within years depending on the year the P was applied.     

The long-term residual value of rock phosphate and superphosphate fertilizers for various plant species under field conditions

In three, long-term field experiments on different lateritic soils in south-western Australia, the effectiveness of superphosphate and rock phosphate fertilizers applied 10 years (one experiment) or 4 years previously was measured relative to the effectiveness of freshly-applied superphosphate (relative effectiveness or RE) using several different plant species. For the species comparisons, RE values were estimated using the initial slope of the relationship between yield and the level of P applied. In addition, RE values were also determined for different levels of application to test whether RE values for previously-applied fertilizer changed with increasing level of application. Soil samples were collected 3–5 months before sowing for a soil test for phosphate (P) and the soil test values were related to plant yields measured later that year. At each site, the RE value of previously-appliedrock phosphate was calculated using initial slopes and was mostly consistently low and was similar (0.04–0.18) for all plant species. The exceptions were that the RE value about doubled for barley in one experiment and for another experiment the effectiveness of calcined (heated) C-grade ore (Calciphos) was about 2–3 times that of the untreated (i.e. unheated) fertilizer. In most cases, the RE value of previously appliedsuperphosphate at each site was similar (0.23–0.34) regardless of plant species. The exceptions were that the RE value was about double for barley in one experiment and about half for triticale in another. Rock phosphates applied 4 or 10 years previously were between about one twentieth to one quarter as effective as freshly applied superphosphate. Superphosphate applied 4 or 10 years previously was between about a quarter to one third as effective as freshly-applied superphosphate. At each site, the yield of each species was closely related to the P content of plant tissue and the relationship was independent of the fertilizer type or when the fertilizer was applied. At each site and for each plant species, the RE value of the previously-applied rock phosphate was estimated for different levels of application and generally decreased with increasing level of application, whereas the RE value for previously-applied superphosphate mostly remained approximately constant. At each site, the relationship between yield and soil test values (i.e. soil test for P calibrations) differed depending on the fertilizer type and the plant species.     

Assessment of methods for studying the dissolution of phosphate fertilizers of differing solubility in soil

Relationships between plant response and rates of dissolution of ground (< 150µm) North Carolina phosphate rock (NCPR), NCPR 30% acidulated with phosphoric acid (NCPAPR) and monocalcium phosphate (MCP) were assessed in pot experiments. The three fertilizers were incubated for 1, 50 and 111 days, at the rates of 75, 150 and 750µg P g^–1 soil, using two soils with different P-retention capacity. After each period of incubation, four pots were set up from each treatment, and perennial ryegrass (Lolium perenne) was grown in a growth chamber for about six weeks to assess the agronomic effectiveness of the fertilizers. Results in dry matter yield and P uptake showed that immediately following application (1 day incubation), the MCP (solution) was supplying more P to plants than either the NCPR or the NCPAPR applied at the same rate. After 50 and 111 days of incubation, the NCPR and NCPAPR were just as effective in the lower P-retention Tekapo soil. The relative agronomic effectiveness (RAE) of the NCPR and NCPAPR compared with MCP was generally poorer in the higher P-retention Craigieburn soil than in the Tekapo soil shortly after application, but improved with time of incubation. Ryegrass responses to the application of the three fertilizers corresponded to the changing trends of exchangeable P in the soils, measured by the isotopic method.Regressions were made between plant P uptake and indices describing the intensity factor (water extractable P), quantity factor (Bray I P, Olsen P, 0.5M NaOH extractable P and isotopic exchangeable P) and the kinetic factor (F_in) of soil P supply to plants in the Tekapo soil. The percentage of variation in plant P uptake explained by individual indices was generally less than 80%, no matter which of the three single variable models, the Mitscherlich, the quadratic or the power function was fitted. However, more than 96% of the variation in plant P uptake in the Tekapo soil could be explained by the power function models involving two variables. The rate of P dissolution (F_in) determined by the isotopic dilution method was included in all the two variable models. The results suggest that assessment of soil P supply to plants should consider the kinetic factor in addition to the intensity and quantity factors, particularly where P fertilizers with differing solubility are applied.     

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.