Investigations on interactions of phosphorus compounds in partially acidulated phosphate rock and fertilizer effectiveness

Khouribga phosphate rock was partially acidulated with 50 and 70% of the required H₂SO₄ for complete acidulation. The unreacted rock residue was isolated by subsequent extractions with water and alkaline ammonium citrate solution. P solubility in 2% formic acid of this residues was reduced as compared to the original Khouribga phosphate rock. This loss in reactivity consistently increased with the degree of acidulation. Plant response to fertilizer application emphasized the negative effect of partial acidulation in an acid soil. Mixtures of superphosphate and phosphate rock were more effective than partially acidulated phosphate rock.Applications of apatitic P did not affect P efficiency on a neutral soil. Differences between mixed and partially acidulated phosphate rock could therefore not be observed. The effectiveness of the products was due to their content of acidulated P.Hydrolysis of monocalciumphosphate caused a further acidulation of the residual apatite in moist incubated granules. The extent of these reactions, however, was too low to improve P efficiency significantly.     

Influence of manufacturing variables on characteristics and the agronomic value of partially acidulated phosphate fertilizers

Partially acidulated phosphate fertilizers are manufactured either by direct partial acidulation of phosphate rocks (PRs) with sulphuric and/or phosphoric acid (directly acidulated PAPR) or indirectly by mixing reactive phosphate rocks (RPRs) with single superphosphate (SSP-RPR mixture). This form of low cost fertilizer manufacture is suitable for improving the agronomic value of unreactive PRs or production of high analysis fertilizers that can have agronomic values similar to fully acidulated phosphate fertilizers.The solubility characteristics of the directly acidulated PAPRs are affected by the type, composition and concentration of the acid used for acidulation, degree of acidulation, nature and fineness of PR and the method of manufacture. In general, partial acidulation with phosphoric acids which contain minimum amounts of metallic impurities acidulates more PR and results in more soluble P in the product. In the case of SSP-RPR mixtures made by adding RPR to immature SSP, the nature of PR used for SSP manufacture and the time of addition of RPR to ex-den SSP mixture affects the quality of the product. In order to minimize the selective reaction of the RPR with residual acid present in the ex-den SSP reaction mixture, RPR should not be added until PR acidulation (used for SSP) is essentially complete.The agronomic value of partially acidulated phosphate fertilizers is affected by the amount of water soluble P and the solubility of residual PR. None of the single extraction tests such as 2% citric acid, 2% formic acid and neutral ammonium citrate appear to be appropriate as indicators of plant available P in these fertilizers. Double extraction procedures which remove both the soluble P and the residual P have been investigated, but need to be correlated with agronomic data before they can be adopted as quality tests.     

Evaluation of double extraction techniques as solubility tests for fertiliser products containing phosphate rock components

Alternative extraction techniques for assessing the available P content in multi-component fertilisers such as partially acidulated phosphate rocks have been examined. Two types of double extraction method have been investigated, one involving sequential extraction procedures, and a second examining both the initial fertiliser and its residue independently, using either water or cold neutral ammonium citrate as the first extractant, and either 2% citric or 2% formic acid as the second solvent. The latter method, which used cold neutral ammonium citrate as the initial extractant, produced the most consistent results and provided a reproducible assessment of the solubility of the phosphate rock residue; it was not obvious which second solvent was preferable. Comparisons between the original phosphate rocks and their extracted residues indicated that there were many subtle influences which affected the solubility of the phosphate rock components, but a general deactivation of phosphate rock residues on acidulation as previously suggested was not apparent. Before any particular method can be recommended as an alternative to existing one step extraction techniques it is necessary to confirm its validity through agronomic trials.     

Single superphosphate-reactive phosphate rock mixtures. 2. The effect of phosphate rock type and denning time on the amounts of acidulated and extractable phosphate

Ground samples of Nauru (N), Christmas Island A (X), Jordan (J) or North Carolina (NC) phosphate rocks (PRs) were acidulated with³²P spiked sulphuric acid to produce single superphosphate (SSP) reaction mixtures. Subsequently, single superphosphatereactive phosphate rock (SSP-RPR) mixtures were manufactured by adding reactive phosphate rock (RPR) as either ground or unground NCPR or ground JPR to SSP reaction mixtures that had been denned for either 22 or 47 minutes after acid addition. The solubility of P in the final SSP-RPR products was assessed either by extraction with water, 2% citric acid, 2% formic acid or 1M neutral ammonium citrate (NAC), or by calculation of the exchangeable P content of the fertilizer by isotopic dilution techniques. The measurement of exchangeable P allowed calculation of the amounts of acidulated P in the ex-den SSP and the amount of RPR P acidulated on addition to ex-den SSP containing free phosphoric acid.Among the PRs used for SSP manufacture, the highest degree of acidulation at the ex-den stage was obtained for NCPR (92%) and the lowest was obtained for XPR (75%). As a consequence, the presence of XPR in the SSP reaction mixture decreased the amount of exchangeable P in the SSP-RPR mixtures. Whereas initially the conversion of PR P increased with time of acidulation at 22 minutes and 47 minutes (i.e. the time of addition of RPR) the differences in the degree of acidulation of PR in the ex-den SSP were not large and hence had no significant effect on the extractability of P in the SSP-RPR mixtures.The nature of the RPR added to the ex-den SSP reaction mixture had a significant effect on the solubility of P in the SSP-RPR mixtures. SSP-RPR mixtures with added unground NCPR or ground JPR had lower P solubility than when ground NCPR was added. RPR P constituted between 38 and 46% of the total P in the SSP-RPR mixtures and at acid/PR (A/R) ratios of 0.60 to 0.70, between 28 to 49% of the RPR P was acidulated by the free acid in the SSP reaction mixture during manufacture.The results also indicate that RPR mixtures made using ex-den SSP made from unreactive PRs will always contain more unreactive PR residue than those made with mature SSP. However, given the practical difficulties of producing the SSP-RPR mixtures with mature SSP, denning times should be extended for as long as practicably possible.     

An investigation of extraction procedures for assessing the phosphate rock residue in multicomponent fertilisers

Five chemical extractants-water, neutral ammonium acetate, neutral ammonium citrate (hot and cold) and alkaline ammonium citrate-have been assessed on their ability to efficiently isolate the residual phosphate rock present in multicomponent fertilisers without affecting the phosphate rock. Phosphoric acid-based partially acidulated phosphate rocks (PAPRs) were manufactured along with several LONGLIFE (LL, single superphosphate to which reactive phosphate rock is added ex-den) materials for the assessment of these extractants.Each extractant was assessed by analysing the resultant residual phosphate rock isolated from each multicomponent fertiliser and comparing its chemical properties with the original phosphate rock used to manufacture the products. An analysis of extractable phosphorus was also undertaken. X-ray diffraction analysis was performed on selected phosphate rock residues as an independent method of examining the extraction techniques.Water and neutral ammonium acetate were found to be unsatisfactory for use as extractants; neither removed calcium sulphate when present, while the latter also did not appear to extract all the monocalcium phosphate component. The remaining three extractants all removed soluble phosphorus components and impurity species from the fertiliser products, leaving the residual phosphate rock relatively unaffected. Residue analyses were only slightly modified to those of the original rocks, although there were some inconsistencies in results obtained using hot neutral ammonium citrate. X-ray diffraction analysis showed no significant differences between residues produced from neutral ammonium citrate or alkaline ammonium citrate extractions, but water and neutral ammonium acetate residues confirmed the presence of calcium sulphate in LL and the presence of monocalcium phosphate in a neutral ammonium acetate extracted PAPR.From this study, cold neutral ammonium citrate and alkaline ammonium citrate appear to be equally suitable for isolating the phosphate rock from a multicomponent fertiliser, leaving the residual rock substantially unmodified from the original phosphate rock. However, there was a slight preference for cold neutral ammonium citrate due to its ease of preparation and use.     

Factors affecting the solubility of phosphate rock residues in 2% citric acid and 2% formic acid

The 2% citric and formic acid solubilities of phosphate rock residues extracted from partially acidulated materials (20 and 35% acidulation) manufactured from two phosphate rocks (both ground and unground) in the presence of monocalcium phosphate, monosodium phosphate, calcium chloride, calcium sulphate, calcium carbonate, sodium carbonate and aluminium and iron sulphates have been examined. Such figures have been compared with previous results obtained for the equivalent unprocessed phosphate rocks. In general the effects of additives on the solubility of the phosphate rock residues were similar to those found for unprocessed phosphate rocks. However, the solubility figures were considerably lower for the 20% acidulated residues, while 2% formic acid solubilities for the 35% acidulated residues were also reduced. This would indicate that some deactivation of the phosphate rock had taken place during acidulation, the degree of which was dependent on the acidulation level and could be more readily observed in changes in 2% formic acid solubility than in 2% citric acid figures.     

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.     

Chemical effects in commercial and laboratory mixtures of ‘reactive’ phosphate rock and acidulated fertilisers

The chemical analyses of different size fractions of a variety of commerical and laboratory prepared samples of partially acidulated phophate rocks and mixtures of reactive phosphate rock and single superphosphate (called LONGLIFE in New Zealand) have been studied. Whereas only minor chemical segregation effects have been observed for partially acidulated products quite a large bias has been established for LONGLIFE materials, and more especially commercial samples, where larger proportions of phosphate rock were found in the lower size fractions. This inhomogeneity was considered to arise from poor mixing of components and subsequent inconsistent granulation; more stringent rejection criteria for undersize material would greatly assist in improving the product quality. Chemical deactivation of the phosphate rock residue in LONGLIFE materials was also observed; this can be partially explained by a selective reaction of the reactive phosphate rock component with acid still present at the time of mixing with the single superphosphate component.     

Properties of unreacted rock residues in partially acidulated phosphate rocks affecting their reactivity

The nonacidulated P fraction of partially acidulated phosphate rocks (PAPR) was obtained by extraction with alkaline ammonium citrate solution. Investigations on this unacidulated rock residues using standard analytical techniques and electronbeam microanalysis showed a surface coating with highly increased fluorine content surrounding the unreacted phosphate rock particles. The coating may be responsible for low reactivity of the residues and their inferior agronomic effectiveness as compared to the original mother rock phosphate. Furthermore, the existence of dicalciumphosphate and Fe—Al—P compounds in PAPR products make solubility tests based on alkaline ammonium citrate appropriate to gauge the acidulated and easily plant available P fraction. Only after the removal of these compounds with citrate solution is an assessment of the potential agronomic value of the nonacidulated rock residues in 2% formic acid possible.     

Long-term greenhouse evaluation of partially acidulated phosphate rock fertilizers

The agronomic effectiveness (yield and P uptake) of twelve granular, partially acidulated phosphate rock fertilizers (PAPR) and two finely ground, unacidulated phosphate rocks (PR) were compared to that of a single superphosphate in a long-term greenhouse experiment with lucerne (Medicago sativa L., cv. CUF101), grown in a low P sorbing, moderately acid, sandy loam soil of moderate P status (Paleustaf). The PAPRs were prepared from two unreactive PRs (Christmas Is. A grade and Duchess rock from Queensland) and acidulated at two rates (25% and 50% on a H₂SO₄ to single superphosphate basis) with either H₂SO₄ or H₃PO₄. Additional products included H₂SO₄ PAPRs cogranulated with elemental S (10% w/w).Superphosphate was consistently superior to all PRs and PAPRs in agronomic effectiveness throughout this two-year study. The most effective of the PAPRs were those that were 50% acidulated with H₂SO₄ and cogranulated with elemental S; this type of fertilizer from both rocks was approximately 2/3 as effective as superphosphate when relative agronomic effectiveness indices (RAE) were calculated from cumulative yields. The increase in agronomic effectiveness relative to superphosphate (RAE value) by the partial acidulation of the PR could be attributed to its effect of increasing the P solubility in the PAPR. A curvilinear relationship existed between the RAE values of PRs and PAPRs, measured from cumulative yield or P uptake data, and the percentage of the total P in each fertilizer that was in a soluble (water + citrate soluble) form. Cogranulation with elemental S (10% w/w) significantly displaced this relationship upwards by increasing the RAE of H₂SO₄ PAPRs by more than 50%. The maximum cumulative recovery of applied P by lucerne tops after five bulked harvests (fifteen consecutive harvests) was 61.5%, which occurred at the low application rate of superphosphate. The decline in the substitution value of PRs for superphosphate, that occurred with increasing P rates tended to be offset both by increasing the level of acidulation and by cogranulating the PAPR with elemental S.     

Effects of partial acidulation on chemical and mineralogical characteristics of residual phosphate rocks

Original phosphate rocks (PR) and water insoluble residues (WIR) from mixtures of reactive PRs and single superphosphate, known commercially as longlife single superphosphate (LLSSP), and from partially acidulated PRs (PAPR), were compared in terms of their elemental content, chemical reactivity as indicated by the apatite unit cell a dimension and solubility. Phosphate rock reactivity is known to be inversely related to the a dimension. Partial acidulation (20%) with commercial grade phosphoric acid resulted in an increase in aluminium (Al), iron (Fe) and fluoride (F) concentrations in the WIRs. The apatite a dimensions of WIRs from LLSSPs were greater than those of the respective original North Carolina (NC), Khouribga (KR), Jordan (JR), Sechura (SE) and Arad (AR) PRs added to single superphosphate (SSP), made from Nauru PR (NR)) to produce the LLSSPs. This was attributed to the presence of the less reactive NR in the WIRs left-over from the SSP. Partial acidulation with phosphoric acid increased the apatite a dimensions of NC and ElHassa (EH) PRs. The increase in apatite a dimension of NC and EH was probably due to selective dissolution of a more reactive fraction of the PRs during partial acidulation. Changes in the apatite a dimension following partial acidulation with phosphoric acid were not significant for the other PRs studied, e.g. Gafsa (GF), KR and AR, although differential X-ray diffractograms (DXRD) indicated that the material dissolved during partial acidulation was more reactive than the WIRs and the original PRs. The apatite a dimension of NC PR was not affected by pretreatment with 2% or 4% citric acid (CTA). The contrasting response in a of NC PR to acidulation with phosphoric and citric acids may be related to differences in the strength of these acids, and/or to the differing environments under which the reactions took place.The 2% CTA and formic acid (FMA) solubilities of the WIRs from LLSSPs and PAPRs were markedly lower than those of the original PRs. This reduction in solubility of PRs following partial acidulation was probably related to changes in mineralogical and chemical composition of the WIRs as indicated by the increases in apatite a dimension of some residual PRs and shifts in peak positions in DXRD, to increases in the concentrations of Fe, Al and F compounds, and to coating effects of PR particles by Fe, Al and F compounds. This, in turn, may reduce the agronomic value of the residual PR component of PAPR and LLSSP fertilizers, particularly over the short-term.The solubility of residual PRs following pretreatment with 2% or 4% CTA was slightly lower than that of the original PRs. The pretreatment caused no significant change in the apatite a dimension of NC PR. The complexing effects of CTA and its lack of Fe and Al impurities may have prevented the formation of Fe, Al and F compounds. The effect of citric acid on PR reactivity is thus quite different from that of the mineral acids used to prepare LLSSPs and PAPRs.     

Partially acidulated phosphate rocks made from phosphoric acid using direct acidulation-granulation techniques

Partially acidulated phosphate rocks were produced by spraying phosphoric acid onto North Carolina phosphate rock of three finenesses (unground, medium, or finely ground) in a pan or drum granulator. This direct acidulation-granulation procedure resulted in free-flowing granular products using laboratory acid up to stoichiometric levels of 30% (unground), 45% (medium) and in excess of 50% (fine). However, when less pure works grade acids were used the maximum levels of acidulation were reduced to 15%, 30% and 45% respectively with a corresponding greater difficulty in producing good granules.Mature product analyses indicated an analytical bias with granule size, the larger granules containing greater quantities of soluble phosphate in most instances. Physical tests on mature products indicated that all granules produced were inferior to those of the traditional New Zealand fertiliser single superphosphate.     

Processing aspects of production of partially acidulated phosphate rock fertilisers using phosphoric acid

Several factors which could affect the rate of reaction between phosphate rock and phosphoric acid in the production of partially acidulated phosphate rock fertilisers (20 to 50% of stoichiometric) were examined. Fineness of the phosphate rock had predictably the largest effect, with rates increasing markedly with increasing fineness. Quality of the phosphoric acid also had a noticeable effect, especially in the acidulation of unground phosphate rock, where use of poorer quality acids caused reductions in reaction rate and consequent detrimental effects on the physical condition of products. Acid concentration was an important factor in determining the physical condition of the product but had only a small influence on reaction rate.None of the above factors had a major influence on the total or percentages of soluble phosphorus in matured products. Slightly higher phosphorus percentages at the same level of acidulation were obtained with the finer phosphate rock and better quality phosphoric acids, but overall any variations in the phosphorus analyses attained at any specific acidulation level were considered of minor importance in comparison to processing and agronomic limitations of partially acidulated phosphate rock fertilisers in the New Zealand agricultural situation.     

Evaluating the efficacy of various phosphate fertiliser sources for oil palm seedlings

Isotope dilution techniques were used in a glasshouse experiment to compare seven P sources for oil palm seedlings grown on Rengam series soil (Typic Paleudult). The P sources were triple superphosphate (TSP) and six phosphate rocks from North Carolina, USA (NCPR), Tunisia (Gafsa PR), Jordan (JPR), Morocco (MPR), Christmas Island (CIPR) and China (CPR). The percent P derived from fertilisers (%PdfF) in the 3, 6, 9 and 12 months of growth ranged from 81% to 99%, indicating the poor P supplying power of the soil used. TSP was far superior than PR in supplying the required P at all times of measurement. Total amount of P taken up during the 12 months growing period was equivalent to 15.0% of the added P as TSP, it was 5.2% from NCPR, 4.2% from JPR, 4.1% from MPR, 3.2% from GPR, 4% from CIPR and 2.2% from CPR. The PR effectiveness based on the amounts of fertilizer P taken up by the oil palm seedlings at 12 months of growth was in the sequence of triple superphosphate > North Carolina PR > Gafsa PR Jordan PR Morocco PR Christmas Island PR > China PR. This was due to the reactivity of these P sources when applied into the soil, triple superphosphate being water soluble is immediately available. PR sources reacted with the soil solution with time, making P slowly available. PR solubilised by neutral ammonium citrate (NAC) expressed as percentage of rock was shown to correlate better than 2% citric acid and 2% formic acid with plant P uptake. Thus this method of extracting P from PR can be used as a basis for comparing PR effectiveness to oil palm seedlings.     

Chemical effects in the assessment of phosphate rock residues extracted from multicomponent fertilisers

Methods of obtaining reactive phosphate rock residues from multicomponent phosphate fertilisers which contain soluble phosphorus components have been examined. Extraction using cold neutral ammonium citrate has been found to be more suitable than water, as chemical reactions appear to occur in water extraction solutions to modify the residue, and more impurity species such as calcium sulphate and iron and aluminium phosphates remain in the residues to complicate the process of obtaining reliable solubility figures.Deactivation of the phosphate rock residues (as measured by decreased 2% formic acid solubilities) was found to be due to reactions occurring during the maturing process, and was dependent on the phosphate rock used, with North Carolina phosphate rock-based residues significantly affected, but Arad-based residues almost unaffected. However, for neutral ammonium citrate extractions, only North Carolina-based residues produced solubility figures lower than the original phosphate rock. Some selectivity of reaction in single superphosphate-reactive phosphate rock mixtures was indicated which further complicated assessment of the deactivation effect; however such an effect appeared to be much less significant than previously suggested.     

The effects of different manufacturing techniques on the availability of sulfate to pasture from mixtures of elemental sulfur with either triple superphosphate or partially acidulated reactive phosphate rock

Six different preparations of elemental sulfur incorporated into triple superphosphate and a single mixture of elemental sulfur with partially acidulated reactive phosphate rock were evaluated for the rate of release of sulfatesulfur to pasture at two sites.The method of preparing the mixtures affected the size of sulfur particles released on dispersion of fertilizer granules. The rates of release of sulfate from oxidation of the elemental sulfur was controlled by the fineness of the particles of elemental sulfur. For rapid release of sulfate by oxidation a mixture prepared by sintering elemental sulfur with mature triple superphosphate was the most effective. It contained sulfur particles of diameter less than 75 m. The mixtures which comprised synthetic blends of elemental sulfur of particle size less than 150 m or 150–250 m, each with mature triple superphosphate, released sulfate at a progressively slower rate than that with particles less than 75 m. However, both proved effective sulfur fertilizers releasing sulfate over a 1–2 year period. In contrast, mixtures prepared by adding molten elemental sulfur during the process of manufacturing triple superphosphate resulted in a coarser distribution of sulfur particles and a slow release of sulfate to plants.The incorporation of sulfur into a mixture with partially acidulated reactive phosphate rock was made via the dispersion of molten sulfur into phosphoric acid which was then used to partially acidulate phosphate rock. The resultant sulfur particles ranged from finely divided (e.g. 38% < 150 m) to moderately fine (81% < 500 m), and the release of sulfate was predictable based on the particle size distribution.Differences between the two experimental sites in uptake of plant sulphate reflected the different pastures grown. At the cool temperate coastal site a ryegrass/clover pasture rapidly took up sulfate as it was released into the topsoil. Inland, in the summer-dry environment, the uptake of released sulfate by the deep-rooted lucerne pasture was delayed for 12–18 months, indicating that the sulfate uptake increased after the nutrient had leached beyond the surface soil layer.     

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.     

An assessment of the agronomic efficiency of partially acidulated phosphate rock fertilisers

The literature comparing the efficiency of partially acidulated phosphate rock fertilisers with that of a single or triple superphosphate is briefly summarised and found to be conflicting. Various theories purported to explain why partially acidulated phosphate rock products are as efficient are examined. An alternative hypothesis, which appears to reconcile the conflicting evidence, is presented. This depends in part on the dissolution of the phosphate rock component of a partially acidulated product in the soil; the factors influencing the dissolution of phosphate rock in the soil are reviewed in relation to the conflicting statements about the efficiency of partially acidulated phosphate rock products.     

Evaluation of partially acidulated phosphate fertilisers and reactive phosphate rock for hill pastures

The initial and residual effectiveness of two partially acidulated fertilisers, a single superphosphate (SSP): reactive phosphate rock (RPR) physical mix (SSP:RPR) and a partially acidulated phosphate rock (PAPR), and a RPR, North Carolina, were compared with SSP at two phosphate (P)-responsive sites in hill country. One site had received small annual inputs of SSP (125 kg ha^–1 y^–1) fertiliser for 10 years (LF) and the other site no SSP in the past 5 years (NF). The SSP, PAPR and RPR were applied at 3 rates (20, 40 and 60 kg P ha^–1) and SSP:RPR at one rate (40 kg P ha^–1) once only in the first year. Fertiliser treatments were applied with or without Grasslands Huia white clover (Trifolium repens L.) seed. Initial and residual pasture and legume responses were measured over two years.In the first year, pasture and legume response to applied P was much greater at the LF than NF site. A deterioration in legume content and vigour brought about by withholding fertiliser, rather than a difference in soil-fertiliser reactions, appears to be the main reason for the different response at the two sites in the first year. At the LF site the fully (SSP) and partially (SSP:RPR and PAPR) acidulated fertilisers were far more effective in stimulating legume growth than the RPR, while at the NF site no differences in pasture or legume production were found between fertilisers in the first year. Where fertiliser has been withheld for a number of years the use of SSP appears to be a wasteful and inefficient use of a processed fertiliser.Residual effects of RPR were greater than those of SSP, as shown by the greater yield of legume at both sites in the second year. The residual effectiveness of both the partially acidulated materials was much less than that of the RPR. Mixing and sowing white clover with the fertilisers had some beneficial effects on legume content at the NF site in both years and improved legume production at this site in the second year.     

Effectiveness of partially acidulated phosphate rock as a source to plants in calcareous soils

In a series of greenhouse experiments granulated phosphate fertilizers prepared by mixing triple superphosphate with phosphate rock and partially acidulated phosphate rock, ranging in their content of water souble P from 95 to 17 per cent of total P were applied to neutral and slightly alkaline (pH 6.9–7.8), sandy loam to clay soils ranging in calcium carbonate content from 2 to 35 percent. Dry matter yield of clover, alfalfa, millet or maize were obtained, P uptake determined and sodium bicarbonate extractable P in soil measured. In one field experiment triple superphosphate was compared to mixture of triple superphosphate and phosphate rock on maize. X ray difraction on one triple superphosphate — phosphate rock mixture and on one partially acidulated phosphate rock showed that both fertilizers contain mainly monocalcium phosphate and fluorapatite. After incubation in soil the dicalcium phosphate content rose and the monocalcium phosphate disappeared.Parameters received in greenhouse experiments and in the field indicate that phosphate fertilizers composed of superphosphate and up to 50 percent phosphate rock are as efficient source of P to plants on calcareous and slightly alkaline soils as superphosphate. If this indication would be proven in extensive field experimentation it would lead to savings in acid consumption and in fertilizer manufacturing plant capacity for calcareous soils.