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.     

The initial and residual agronomic effectiveness of some Indian,USA and Australian rock phosphates

The initial and residual agronomic effectiveness of six apatite rock phosphates from India, one from the USA (North Carolina) and one from Australia (Queensland) were evaluated in a pot trial with wheat on a lateritic soil. All of the Indian rock phosphates were very poor sources of phosphorus. Values of initial effectiveness relative to monocalcium phosphate ranged from < 0.0001 to 0.02 and from < 0.0001 to 0.008 for measurements based on yield and phosphorus uptake respectively. The residual effectiveness relative to freshly applied monocalcium phosphate was determined by growing a second crop on the fertilized soils. The effectiveness of the Indian rock phosphates remained very low ranging from < 0.0001 to 0.002 and from < 0.0001 to 0.0004 for yield and phosphorus uptake respectively. Queensland and North Carolina rock phosphates were much superior to the Indian sources with initial effectiveness values in terms of yield of 0.08 and 0.37 and residual effectiveness values of 0.02 and 0.15 respectively. For each crop there was a single relationship between yield and phosphorus uptake (i.e. internal efficiency) for all phosphorus sources showing that variations in yield response were due solely to differences in phosphorus availability. Sodium bicarbonate extractable phosphorus values for fertilized soils sampled shortly after fertilization were not predictive of yield unless different calibration curves were used for the different phosphorus fertilizers.     

Comparative study of reactivity and agronomic effectiveness of Indian,US, African and Middle East rock phosphates for growing rice

Comparative reactivity and efficiency of eight Indian, six US, two African and one Middle East sources of rock phosphates for growing rice on laterite, red and alluvial soils under flooded conditions were evaluated in pot and laboratory experiments. When applied to moist aerobic soils, 15 days prior to flooding and transplanting rice, all the Indian sources were as poor as theno phosphate control in the three soil types, in respect of P availability in soil, grain yield response and P uptake by rice. North Carolina rock phosphate used in this study was as good as superphosphate in the laterite and red soils, but was also as poor as control in the black soil.NH₄ -citrate was found to interfere in the colorimetric determination of citrate soluble P by the vanado-molybdate colour method. A modified sulpho-molybdate-Sn Cl₂ blue colour method could successfully be used to determine 2–8µg P in the presence of 0.02 to 0.2 meq NH₄ -citrate, especially in rocks containing small amounts of citrate soluble P. All the Indian, as well as Idaho, Missouri and Tennessee rock phosphates were found to be less reactive as they contained much lower amounts of citrate extractable P in eight successive extracts as compared to North Carolina rock phosphate.The cumulative citrate soluble P of 10 rock phosphates determined experimentally in eight successive extracts was significantly correlated with their reported a₀ (length of a axis of unit apatite crystal), mole ratio of CO₃/PO₄ or weight ratio of F/P₂O₅ of rocks. In the absence of X-ray and computer facilities, these regression equations were used to calculate the a₀, CO₃/PO₄, F/P₂O₅, ACS, empirical formula and the apatite content of the unknown Indian rock samples. The Indian rock phosphates had a lower degree of CO₃ and F substitution for PO₄ in the apatite crystal, giving low ACS values and hence were less reactive. This might explain their lower efficiency for direct application for growing rice, obtained in the present experiment. These Indian rock phosphates had also lower apatite content. The use of the statistical method was limited to francolites alone. Scope for the use of this method for other unknown francolite rock phosphate samples has been discussed.     

Field evaluation of commercial triple superphosphate fertilizers

Field studies were conducted for three years (1987–1989) at two locations to evaluate 4 commercial triple superphosphate (TSP) fertilizers containing various levels of water-soluble P. The fertilizers had been produced from phosphate rock deposits located in Florida, North Carolina and Morocco. AOAC available P was 81 to 94% water-soluble. Water-soluble P was inversely related to the level of Fe and Al in the fertilizers. Phosphorus from each source was applied to a Malbis soil (Plinthic Paleudults) and a Hartsells soil (Typic Hapludults) at rates of 0, 25, 49 and 99 kg ha^–1. Potato (Solanum tuberosum L.) yields were increased by the application of P, except for the Malbis soil in 1988. Yields were not affected by the source of added P on either soil during the three years of the study. Fertilizer performance was not affected by the level of water-soluble P or the content of Fe and Al when band applied to potatoes under field conditions in the Southeastern United States.     

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.     

Partial acidulation of an ‘unground’ phosphate rock: II. Plant availability of phosphate

Four greenhouse experiments were conducted using three soils to determine the availability to plants of P from unground North Carolina phosphate rock (PR) treated with 20% to 50% of the H₃PO₄ required for complete acidulation. The influences of soil P retention, P status, the method of preparation of partially acidulated phosphate rocks (PAPRs) and the granule size of the products were investigated. Perennial ryegrass was grown as the test plant for up to 8 months. Triple superphosphate (TSP) was used as the standard fertilizer and unground North Carolina rock was included for comparison.The dry matter yield and P uptake response curves showed that in all experiments PAPRs were markedly superior to the PR. P status of soils appeared to influence the effectiveness of PAPRs to a greater extent than P retention. In soils of low P status the degree of acidulation required for PAPR to be nearly or as effective as TSP was 50% whereas in a soil of high P status even 30% PAPR applied as a maintenance fertilizer was effective. There was a significant positive correlation between water soluble P of fertilizers and P uptake by ryegrass. However, in general PAPRs were more effective per unit of water soluble P than TSP. Granule size (< 1 mm and 1–2 mm) and method of preparation of PAPRs did not alter the effectiveness of PAPRs.     

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.     

Long-term residual value of North Carolina and Queensland rock phosphates compared with triple superphosphate

The effectiveness of large single applications of North Carolina reactive rock phosphate, Queensland non-reactive rock phosphate, and Calciphos, were compared to the effectiveness of superphosphate in field experiments in south-western Australia for up to 11 years after application. As measured using plant yield, superphosphate was the most effective fertilizer in the year of application, and relative to freshly-applied superphosphate, the effectiveness of the superphosphate residues declined to be about 15 to 65% as effective in the year after application, and 5 to 20% as effective 9 to 10 years after application. Relative to freshly-applied superphosphate, all the rock phosphates were 10 to 30% as effective in the year of application, and the residues remained 2 to 20% as effective in the 10 years after application. The bicarbonate soil test reagent predicted a more gradual decrease in effectiveness of superphosphate of up to 70% 10 years after application. For rock phosphate, the reagent predicted effectiveness to be always lower than for superphosphate, being initially 2 to 11% as effective in the year after application, and from 10% to equally as effective 10 years later. Therefore rock phosphates are unlikely to be economic alternatives to superphosphate in the short or long term on most lateritic soils in south-western Australia.     

Partially acidulated phosphate rocks: Controlled release phosphorus fertilizers for more sustainable agriculture

Phosphate rocks partially acidulated either with H₃PO₄ or H₂SO₄ were compared against SSP or TSP as phosphate fertilizers for permanent pasture. Eleven field trials were conducted over periods of up to 6 yrs. Fertilizers were surface applied annually. Initial soil pHw values ranged from 5.5–6.3 and Soil P retention from 25% to 97%. The PRs used for partial acidulation were unground or ground North Carolina PR, ground Khouribga PR, and a blend of ground PRs of North Carolina, Arad and Khouribga PRs. From the DM yields, fertilizer substitution values were calculated: fertilizer substitution value was the ratio of total P applied as superphosphate to total P as PAPR required to produce the same DM yield.Rates of dissolution of the PR component of PAPRs were also determined in soils collected from two trials.Agronomic results demonstrated that 30% acidulated phosphoric PAPRs (about 50% of total P as water-soluble P) were as effective as TSP, when the PR acidulated was from unground North Carolina PR. Results from one field trial indicated that when PAPR was from ground North Carolina PR, 20% acidulated product (water-soluble P 30–40% of total P) was equally effective as TSP. Replacement of ground North Carolina PR by a less reactive Khouribga PR did not appear to decrease the yield. Results indicated that per unit P released into soil solution, PAPRs were more efficient fertilizers than TSP. With annual applications, fertilizer substitution value of PAPR 30% tended to increase with time.Sulphuric PAPRs prepared from North Carolina PR were generally inferior to phosphoric PAPRs containing similar amounts of water-soluble P. This was attributed to the presence of CaSO₄ coatings.Abbreviations DM Dry matter - PAPR Partially acidulated phosphate rock - PR Phosphate rock - SSP Single superphosphate - TSP Triple superphosphate     

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.     

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.     

Comparison of efficiency of Mussoorie partially acidulated phosphate rock and single superphosphate in a shallow Alfisol of the Indian semiarid tropics

The agronomic effectiveness of a partially acidulated phosphate rock (PAPR) was measured in a field experiment with sorghum (Sorghum bicolor cv. CSH-6) in a shallow Alfisol at the ICRISAT farm, Patancheru (Hyderabad), India. The experiment compared PAPR with single superphosphate. The PAPR was made by acidulating an indigenous Indian phosphate rock (Mussoorie) with H₂SO₄ at 50% acidulation level. P response was evaluated at a single relatively high N rate (120 kg ha^–1) with five rates of P (0, 2.2, 4.4, 8.8, and 17.6 kg P ha^–1). A significant response to P was obtained at rates up to 17.6kg P ha^–1.There was no significant difference due to source of P in terms of sorghum grain yield or total P uptake. Both Olsen and Bray 1 soil tests underestimated P availability from PAPR with respect to that from SSP.A rapid rate of P uptake was observed during grain filling to maturity (81–102 days), when 40% of the total P was taken by the plant. The internal efficiency of both P sources was the same.     

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.     

Partial acidulation of an ‘unground’ phosphate rock: I. Preparation and characteristics

Partially acidulated phosphate rocks were prepared from unground North Carolina phsophate rock and H₃PO₄ by (i) mixing phosphate rock with the requisite amount of H₃PO₄, (ii) mixing with a portion of the acid followed by adding the remaining acid during granulation and (iii) single-step acidulation and granulation. The degrees of acidulation were 20, 30, 40 and 50%. Only 20% and 30% acidulations were done by method (iii). The phosphate rock granulated readily on addition of H₃PO₄ either as in method (ii) or (iii) and the products did not need external drying before storage. The citric and water soluble P showed that from the viewpoint of acid-phosphate rock interaction all three methods of preparation were satisfactory. The granules were equally strong as or stronger than commercially available single or triple superphosphate samples tested. The percent degradation on abrasion was less than 4% compared to about 8% for superphosphate and 0.4% for triple superphosphate. A sand incubation study suggested an interaction in the partially acidulated phosphate rock between the monocalcium phosphate component and unreacted phosphate rock which initially increased the solubility of P.     

Single superphosphate-reactive phosphate rock mixtures. 1. Factors affecting chemical composition

The effect of additon of reactive phosphate rock (RPR — North Carolina) on the degree of acidulation of unreactive phosphate rocks (PRs — Nauru and Christmas Island A) during the manufacture of single superphosphate (SSP) was examined using³²P in isotopic dilution studies. Acidulation of unreactive PR during SSP manufacture continued through denning, granulation and drying. Even after 3 hours drying, between 20 and 30% of the total P remained as free phosphoric acid in the reaction mixture. The addition of North Carolina phosphate rock (NCPR) to ex-den SSP reaction mixture (3:7 NCPR:SSP reaction mixture) preferentially consumed the free phosphoric acid remaining in the reaction mixture. This resulted in reduced acidulation of the unreactive PR in the reaction mixture and partial acidulation (10–23%) of the RPR. Hence the SSP-RPR mixture contains more residual, unreactive PR than is present in SSP.The extent of partial acidulation of the RPR when mixed with SSP was determined by the nature of free acid remaining in the SSP reaction mixture, which in turn is affected by the type of unreactive PR used for SSP manufacture. The free acid in the Christmas Island A reaction mixture contained approximately 8 and 12 times as much Fe and Al respectively as that in the Nauru reaction mixture, and was only half as effective at converting the P in RPR to soluble P. Unless made with extended denning times and carefully chosen PR, SSP-RPR mixtures can contain (a) undesirable amounts of unreactive PR residues, and (b) low quality partially acidulated RPR, both of which have low agronomic value.     

Effects of different phosphate fertilizers on yield of barley and rape seed on reddish brown soils of the Ethiopian highlands

The effectiveness of six phosphorus sources at 4 rates were tested for two seasons on reddish brown soil at Holetta, Ethiopia, using barley (Hordeum vulgare L.) and rape (Brassica napus L.) as test crops. The fertilizer sources include: basic slag (BS), bone meal (BM), Ethiopian rock phosphate (ERP), Gafsa rock phosphate (GRP), triple superphosphate (TSP) and mixture of TSP and GRP in the ratio 1:4 (MIX). Yield, P uptake by both crops as well as available soil P showed a marked response to the application of the various P sources. On continuously cropped field, grain yield increase over the unfertilized plot ranged from 2.5 to 16.4 dt ha^–1 for barley and rape respectively. On newly cleared field no significant effects of the different P sources on barley were observed. On the other hand for rape, a grain yield increase over the unfertilized plot ranging from 10.6 to 17.8 dt ha^–1 was recorded. The highest agronomic effectiveness relative to TSP (RAE) for both crops was obtained with BS. Rape was found to utilize P not only from the reactive rock phosphate (RP) but also from the unreactive one, which had a total P content of only 3% and 0.4% ammonium citrate soluble P. Barley, on the contrary, could not utilize P from this magmatic rock phosphate and failed to grow.     

Evaluation of the fertilizer value of some indigenous rock phosphates and basic slag of India

Summary Field and pot culture experiments were conducted in terai acid soil (Haplaquoll) to evaluate the fertilizer value of one basic slag and two rock phosphates, such as Purulia rock phosphate (Igneous) and Mussoorie rock phosphate (sedimentary). In the field experiments two crop sequences were followed: (i) Rice — wheat — greengram (ii) Greengram — rice — wheat. In terms of crop yield and P uptake Purulia rock phosphate did not show any significant effect, except in case of greengram grown as the third crop after its application. Mussoorie rock phosphate increased the yield and P uptake through its direct and residual effect in all the crops, except in rice. Irrespective of crop species and crop sequences basic slag showed considerable direct and residual effect in increasing the crop yield and P uptake. Its effect was at par with that of superphosphate. By total yield increase of three consecutive crops due to added P the efficiencies of the fertilizers were graded as basic slag > superphosphate = Mussoorie rock phosphate > Purulia rock phosphate for rice — wheat — greengram rotation and superphosphate > basic slag > Mussoorie rock phosphate > Purulia rock phosphate for greengram — rice —wheat rotation. Composting improved the efficiency of all the insoluble phosphatic fertilizers.     

Development and evaluation of an improved soil test for phosphorus: 1. The influence of phosphorus fertilizer solubility and soil properties on the extractability of soil P

The objective of this work was to develop and evaluate a soil test suitable for estimating the phosphorus status of soils whether they were fertilized with soluble or sparingly soluble P fertilizers or both. Four New Zealand soils of contrasting P sorption capacity and exchangeable Ca content were incubated alone or with monocalcium phosphate (MCP), reactive North Carolina (NC) phosphate rock or unreactive Florida (FRD) rock, at 240 mg P kg^–1 soil, to allow the P sources of different solubilities to react with each soil and provide soil samples containing different amounts of extractable P, Ca and residual phosphate rock. The phosphorus in the incubated soils was fractionated into alkali soluble and acid soluble P fractions using a sequential extraction procedure to assess the extent of phosphate rock dissolution. Eight soil P tests [three moderately alkaline — Olsen (0.5M NaHCO₃) modified Olsen (pretreatment with 1M NaCl) and Colwell; three acid tests — Bray 1, modified Bray 1 and Truog; and two resin tests — bicarbonate anion exchange resin (AER) and combined AER plus sodium cation exchange resin (CER)] were assessed in their ability to extract P from the incubated soils.The 0.5M NaHCO₃ based alkaline tests could not differentiate between the Control and FRD treatments in any soil nor between the Control, NC and FRD treatments in the high P sorption soils. The acid extractants appeared to be affected by the P sorption capacity of the soil probably because of reabsorption of dissolved P in the acid medium. The AER test gave results similar to Olsen. Only the combined AER + CER test extracted P in amounts related to the solubility of the P sources incubated with each soil. Furthermore, when soil samples were spiked with FRD and NC and extracted immediately, the P extracted by the AER + CER test, over and above the control soils, increased with the amount and chemical reactivity of the rocks. There was no extraction of rock P by any of the alkaline extractions.Increases in the amounts of P extracted (P) by each soil test from the fertilized soils, over and above the control soils were compared with the amounts ofP dissolved from the fertilizers during incubation (measured by P fractionation). Soil P sorption capacity had least influence on the amounts of P extracted by the AER + CER and Colwell tests. However, the Colwell test was unable to differentiate between all P sources in all four soils and suffered from the disadvantage of producing coloured extracts. The AER + CER test appeared to have the potential to assess the available P status of soils better than the other tests used because of its ability to extract a representative portion of residual PR (in accordance with the amount and reactivity) and dissolved P, and thus to differentiate between fertilizer treatments in all four soils.     

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.     

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.