Acidulated Phosphate Rock Research Articles

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.

Solubility and agronomic effectiveness of partially acidulated phosphate rocks as influenced by their iron and aluminium oxide content

Partially acidulated phosphate rock (PAPR) has been shown to be an effective source of P for plants grown on acid soils. Less information in available, however, regarding the effect of the phosphate rock (PR) source on the solubility and agronomic effectiveness of PAPR. The effect of Fe2O3 + Al2O3 content in PR on the quality of PAPR produced was investigated in this study. Nine sources of PR from Africa, Latin America, and the United States, representing a range of Fe2O3 + Al2O3 from 0.7% to 12.4%, were used. In a single-step process, the finely ground PRs were partially acidulated with H2SO4 at the 30% or 50% acidulation level and granulated (−3.35 + 1.18 mm or −6 + 14 mesh). It was found that the water-soluble P content in PAPR decreased with increasing Fe2O3 + Al2O content in the PR used. Apparently, the presence of Fe2O3 + Al2O3 resulted in a reversion of some of the water-soluble P to citrate-soluble P and sometimes even to citrate-insoluble P. A short-term (6 weeks) greenhouse study was conducted to evaluate crop response to PAPRs and single superphosphate (SSP); maize, the test crop, was grown on an acid soil (pH 4.5)—Hartsells silt loam (Typic Hapludults). The agronomic effectiveness of PAPRs with respect to SSP (in terms of dry-matter yield of maize) decreased with increasing Fe2O3 + Al2O3 content in PRs. Phosphorus uptake by maize from PAPRs was found to correlate well with water solubility but not with citrate solubility. The results obtained in this study show that the detrimental effect of Fe2O3 + Al2O3 content on the solubility and P availability of PAPR should be considered when selecting a PR for PAPR production.

Comparison of Bray I and Pi tests for evaluating plant-available phosphorus from soils treated with different partially acidulated phosphate rocks

Phosphate rocks from seven locations in Africa and Latin America were partially acidulated with H2SO4 at 30% or 50% acidulation level and were applied to an acid silt loam. Dry-matter yield of and P uptake by maize grown on the soil were correlated with P measured by Bray I soil test and Pi soil test in which P is extracted by shaking a soil sample with 0.01M CaCl2 solution and a strip of iron hydroxide impregnated filter paper. There was a highly significant correlation between water-soluble P in the partially acidulated phosphate rocks (PAPR) and P measured by the Pi test (r=0.92**). The dry-matter yield correlated significantly with P measured by the Pi test (r=0.91**) as well as by the Bray test (r=0.73**). Phosphorus uptake correlated highly significantly with Pi−P (r=0.94**); correlation with Bray I−P was less significant (r=0.67*). Bray I solution extracted more P than Pi did, which resulted in an overestimation of plant-available P. Bray I therefore was less effective than the Pi soil test in evaluating P availability from different PAPRs applied to the soil.

Substitution rates as measures of the relative effectiveness of alternative phosphorus fertilizers

A procedure for representing the effectiveness of fertilizers relative to a standard fertilizer by the estimation of substitution rates is demonstrated with yield data from a fertilizer experiment with triple superphosphate and a partially acidulated rock phosphate. The substitution rate is estimated by an iterative regression procedure and the accuracy of the estimate indicated by a 5% confidence interval.

The nature of the residual apatites remaining after partial acidulation of phosphate rocks with phosphoric and sulphuric acids

The nature of the residual apatites remaining after the partial acidulation of North Carolina, Jordan and Nauru phosphate rocks have been studied using chemical and physical methods of analysis and the agronomic efficiency of residual North Carolina apatite compared to that of unacidulated North Carolina phosphate rock using a glasshouse pot trial technique.

WHITE CLOVER CULTIVAR RESPONSES TO PHOSPHATE FERTILISERS OF DIFFERING SOLUBILITY

A glasshouse study investigated the dry matter response of Grasslands Kopu and Grasslands Tahora white clover, cultivars of differing origins and morphologies, to three phosphate fertilizers, monocalcium phosphate (MCP), partially acidulated phosphate rock (PAPR) and a reactive phosphate rock, North Carolina phosphate rock (NCPR), of differing water solubility. Kopu responded more rapldly and reached Its maximum dry matter yield at a lower phosphorus (P) level than Tahora when MCP and NCPR were the P fertilizers. With PAPR as the P fertilizer, their response was similar. The shoot dry matter response by Kopu to all three P sources was reflected more in leaf weight and size than leaf number or stolon length. In contrast Tahora responded to added P by increasing leaf number and stolon length to a much greater extent than Kopu. Keywords: Trifolium repens, monocalcium phosphate, partially acidulated phosphate rock, reactive phosphate rock.

Partially acidulated phosphate rock as fertiliser and dissolution in soil of the residual rock phosphate

Abstract A field trial was conducted for two years to determine the effectiveness of partially acidulated phosphate rock (PAPR) — in this instance, a reactive North Carolina rock acidulated with 20, 30, and 50% of the phosphoric acid needed for complete acidulation. The soil was a yellow-brown loam (Typic Vitrandept), and triple superphosphate (TSP) was used as the standard fertiliser. The fertilisers were applied at four rates to a ryegrass-white clover permanent pasture. The soil was highly P retentive and the dominant clay was allophane. The trial design enabled measurement of the residual effectiveness of the fertilisers in the second year. The ability of soil tests (namely, Olsen, Bray-I 1 min, Bray-I 30 min, and anion exchange resin) to indicate soil P availability to plants was assessed. The rate of dissolution of unacidulated rock phosphate in PAPR was calculated from inorganic P fractionation in soil samples collected at intervals. Dry matter yields and P uptake indicated that, as a maintenance fertiliser, even PAPR 20% was as effective as TSP from the first pasture cut. The residual effectiveness of fertilisers in the second year ranged from 97 to 108% of the effectiveness of freshly added TSP. All soil tests significantly correlated with dry matter yield and P uptake. In the Bray-I test, extractable P decreased with increasing shaking time and the values stabilised after 15 min shaking. Bray-I 30 min was found to be desirable for convenience and accuracy. No measurable amount of unacidulated rock phosphate in PAPRs dissolved in the first 3 months after fertiliser addition. The rate of dissolution in the next 15 months was 60% per year of that initially added.

Long-term greenhouse evaluation of partially acidulated phosphate rock fertilizers

Twelve granular partially acidulated phosphate rock (PAPR) fertilizers were compared with unacidulated phosphate rocks (PR) and superphosphate at five rates of total P in the presence and absence of supplementary sulfate and plant residue recycling treatments in a long-term green-house experiment with lucerne (Medicago sativa L., cv. CUF101). The PAPRs were prepared from two PRs (Christmas Is. A grade and Duchess, Queensland) and acidulated at two rates (25% and 50% on an H2SO4 to single superphosphate basis) with either H2SO4 or H3PO4. Six harvests (each bulked from three cuttings) were collected over a 2-year period. It was generally found that lucerne response to PAPRs depended closely on their water-soluble plus citrate-soluble P contents which increased with increased degree of acidulation. The H3PO4 tended to yield more soluble P on acidulation of PR than H2SO4 and acidulation of Christmas Is. PR yielded more soluble P than did acidulation of Duchess PR. There was little evidence for enhanced availability of P due to action of the triple point solution in hydrolyzing granules on residual PR in those granules.

Effect of solubility of applied phosphate on the growth of narrow-leafed Lupin (Lupinus angustifolius L.)

The response of narrow-leafed lupins (Lupinus angustifolius L. cv. Illyarrie) to phosphorus (P) fertilizer applied as single superphosphate (super), as an elemental-sulphur-enriched, partially acidulated rock phosphate (esparp) and as biosuper, was investigated in a glasshouse pot trial. Measurements of plant yield and P uptake were recorded 41 and 76 days after sowing. Both dry matter production and P uptake from esparp and super were proportional to the amount of water soluble P added to the pots indicating that the non-water-soluble fractions from these fertilizers were not taken up by the lupin plants. A small amount of initially insoluble P from the higher rates of biosuper was taken up by day 76. Thus superphosphate was clearly the most effective P source for this lupin species followed by esparp while biosuper was ineffective. Similar responses were recorded for pod and nodule dry weights. It was concluded that less soluble P sources were not suitable as fertilizers in lupin production due to the poor response in P uptake, dry matter accumulation and nodulation.

Status of fertilizer products under development in India

This paper describes the status of various fertilizer materials in the process of development and testing. These include a range of products such as urea supergranules, urea coated with an array of coating agents, nitrogen solution, partially acidulated phosphate rocks, polyphosphates, compost-enriched or cattle-dung incubated rock phosphates, and single superphosphates fortified with boron or zinc. To the extent possible, economic aspects and future needs are also commented upon.

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.

The use of chemical solubility tests in comparing phosphate fertilisers

The ‘standard’ solubility tests as used in New Zealand for assessing single superphosphate have been applied to alternative high analysis fertilisers such as triple superphosphate and partially acidulated phosphate rocks using phosphoric acid. Under differing experimental conditions it has been established that the phosphate rock component has a variable solubility in 2% citric acid at all levels, and in neutral ammonium citrate at high levels of PR content, making comparisons between percentages of soluble P derived from different fertilisers inappropriate for chemical or agronomic assessment. Only water, which extracts all the monocalcium phosphate component but none of the phosphate rock, can be used to compare the efficiency of the acidulation process, and neutral ammonium citrate can be used to assess the amount of phosphate rock remaining when the acidulation level is in excess of 50%. 2% formic acid produced almost identical results to 2% citric acid for partially acidulated materials and therefore cannot be recommended as an alternative, improved extractant for fertilisers containing large proportions of residual phosphate rock.

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.

Reducing fertilizer requirements for hybrid squash (Cucurbita maxima L.) with localized applications of phosphorus and potassium and use of partially acidulated phosphate rock

Two experiments examined options for reducing the inputs of P and K fertilizers for hybrid squash (Cucurbita maxima L.) at Pukekohe, New Zealand. The first experiment examined the effects of elevating the NaHCO3-soluble P from 32 to 130 mg kg−1 and the exchangeable K from 140 to 350 mg kg−1 within strips from 0 to 0.75 m around rows of hybrid squash planted 1.5 m apart. From both P and K, crop yield increased as the width of the fertilized strip was increased up to 0.25 m, while wider fertilized strips had no further effect. These results followed similar effects on plant dry matter and tissue P or K concentration during early growth, and are explained in terms of the P and K accumulation by the crop, the decline during growth of the sensitivity of the crop to soil P and K fertility associated with declining rates of P and K uptake per unit length of root. Implications for fertilizer management for hybrid squash are also discussed. The second experiment compared the effects of partially acidulated phosphate rock and triple-superphosphate on soil P fertility, growth and yield of hybrid squash. Partially acidulated phosphate rock had smaller effects than those of triple-superphosphate on NaHCO3-soluble P levels in the soil, plant dry weight and tissue P concentration soon after emergence, and subsequently crop yield. On average, partially acidulated phosphate rock increased crop yield by about 70% of that following the application of the same quantity of P as triple-superphosphate. This lower effectiveness of partially acidulated phosphate rock for hybrid squash is explained in terms of its lower solubility and hence smaller effect on NaHCO3-soluble P in the soil during early growth, when the crop is most sensitive to soil P fertility.

Long-term greenhouse evaluation of partially acidulated phosphate rock fertilizers

Abst~ract. The effect of cogranulation with elemental sulfur (S) on the availability of phosphorus (P) from partially acidulated phosphate rock (PAPR) prepared from Christmas Island or Duchess phosphate rock (PR) was studied as part of a larger experiment examining factors affecting the agronomic effectiveness of PAPR sources. PAPRs cogranulated with (10% w/w) or without elemental S were compared at four rates of P (excluding controls) in the presence and absence of supplementary sulfate (SO4) from gypsum on a P-deficient pasture soil with no previous history of elemental S applications. Lucerne (Medicago sativa L., cv. CUF101) yields and P and S uptake were determined in six bulked harvests collected over a 2-year period. Large responses to P rates were recorded but only minor and inconsistent responses to SO4-S occurred during the experiment. In contrast, elemental S in PAPR granules resulted in greater lucerne yields and P and S uptake by lucerne tops; these effects, which did not appear until 6 months after application, were attributed to the acidulation of residual PR in PAPR granules by action of H 2 SO 4 produced by the oxidation of elemental S. These effects increased with increasing rates of PAPR applied and were greater for the Christmas Island PAPRs and at the 50% acidulation rate. This was attributed to the development of a more prolific root system at the higher P rates and the higher levels of soluble P provided by 50% PAPRs which enabled plants to more fully exploit P solubilized from S-granulated PAPR products.

Controlled and renewable release of phosphorous in soils from mixtures of phosphate rock and NH 4-exchanged clinoptilolite

A controlled and renewable release fertilization system is proposed that employs NH4 saturated clinoptilolite to aid in the dissolution of phosphate rock (apatite), and thereby to release soluble N, P, and exchangeable Ca for uptake by plants. The system is based on the principle that exchangers can sequester Ca ions released by the dissolution of apatite, thereby leading to further dissolution of the apatite. Experiments show that the quantity of P released by this method is an order of magnitude more than that released by apatite alone. This system offers an alternative technology to the acidulation of phosphate rock, and may circumvent problems associated with highly soluble fertilizers.

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.

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.

On the use of phosphorus solubility parameters for the evaluation of phosphate plant availability

Abstract Several phosphate solubility and availability parameters were obtained for calcareous soil samples fertilized with superphosphate, partially acidulated phosphate rock and a mixture of the two. Plant response to the fertilizer application was studied in a greenhouse experiment. The ionic product of calcium carbonate phosphate [Ca3(HCO3)3PO4] correlated best with the plant response parameters. The correlations of the plant response parameters with the other solubility parameters (CaPO4, H2PO4 or HPO4 potentials) were higher than that obtained with conventionally determined available soil phosphorus (P).

Effect of nutrient seed coating on the emergence and early growth of perennial ryegrass

Perennial ryegrass (Lolium perenne L.) seeds were coated with either lime, reverted superphosphate plus nitrogen as isobutylidene diurea (RSN), or partially acidulated rock phosphate plus sulfur (ESPARP). In one experiment emergence rates and total emergence were calculated by fitting Mitscherlich-type curves to the data and then conducting statistical tests on the three parameters found for each curve. There was a significant interaction between soil moisture and coating treatment for all three parameters. In a glasshouse experiment, the same coated and uncoated seed treatments were sown either in soil supplied with nitrogen, phosphorus and sulfur or in soil with nutrient deficiencies of either N, P, or S or NPS, and the early growth of the plants and their P and N uptakes were studied. The two nutrient-coated treatments (RSN and ESPARP) produced more tillers and dry matter yields of roots and shoots than did lime-coated or uncoated seed when grown in soils deficient in P, S or NPS. Nitrogen deficiency reduced yields of all plants, even those derived from RSN seeds. N uptake in the RSN treatment was not significantly different from the ESPARP treatment, indicating that the N in isobutylidene diurea was not available to the plant.