Agronomic evaluation of fertilizer products derived from Sukulu Hills phosphate rock

Partial acidulation of phosphate rock (PR) or compaction of PR with soluble P fertilizers can improve the usefulness of unreactive PR for use as P fertilizer. A greenhouse study was conducted to evaluate nonconventional phosphate fertilizers derived from a low reactive Sukulu Hills PR from Uganda. Raw PR (which contained 341.0 g kg^–1 Fe₂O₃), beneficiated or concentrate PR, partially acidulated PR (PAPR) and PR compacted with triple superphosphate (TSP) were evaluated. Compacted materials had a P ratio of PR:TSP = 50:50. PAPR materials were made by 50% acidulation with H₂SO₄. TSP was used as a reference fertilizer. Fertilizers were applied to an acidic (pH = 5.4) Hiwassee loam (clayey, kaolinitic, thermic Rhodic Kanhapludults) at rates of 0, 50, 100, 200, 300 and 400 mg P kg^–1 soil. Two successive corn (Zea mays L.) crops were grown for 6 weeks. Compacted concentrate PR + TSP and raw PR + TSP were 94.4 and 89.7% as effective as TSP, respectively, in increasing dry-matter yields for the first corn crop. PAPR from the concentrate was 54.8% as effective as TSP. Raw PR, concentrate PR and the PAPR from the raw PR were ineffective in increasing dry-matter yields. The same trends were obtained when P uptake was used to compare effectiveness. Ineffectiveness of the raw PR and its corresponding PAPR was attributed to a high Fe₂O₃ content in the raw PR. Bray I and Pi paper were found to be nearly equally suitable at estimating available P in the soils treated with responsive fertilizer materials. Mehlich 1 overestimated available P in soil treated with raw PR, concentrate PR or the PAPR from the raw PR.     

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.     

Factors affecting the agronomic effectiveness of phosphate rock for direct application

Phosphorus (P) is critically needed to improve soil fertility for sustainable crop production in large areas of developing countries. In recent years, phosphate rock (PR) for direct application has been tested in tropical acid soils as a potential alternative to conventional water-soluble P fertilizers like single superphosphate (SSP) and triple superphosphate (TSP). Some developing countries have PR deposits which, if used to supplement other imported P fertilizers, would allow a saving of much needed foreign exchange. Solubility of P fertilizers is not the only criterion in selection of the most suitable P fertilizer. This paper discusses the results of experiments to compare the relative agronomic effectiveness (RAE) of various PR sources with respect to SSP or TSP as influenced by four important factors: PR sources, soil properties, management practices, and crop species. Under certain conditions, PRs can be agronomically effective.     

Agronomic evaluation of modified phosphate rock products

Phosphorus (P) is critically needed to improve the soil fertility for crop production in large areas of developing countries. The high cost of conventional, water-soluble P fertilizers constrains their use by resource-poor farmers. Finely ground phosphate rock (PR) has been tested and used as a direct application fertilizer on tropical acid soils as a low-cost alternative where indigenous deposits of PR are located. However, direct application of PR with low reactivity or with inappropriate soil/crop combinations does not always give satisfactory results. Partial acidulation of PR (PAPR) or compaction with triple superphosphate (PR + TSP) or single superphosphate (PR + SSP) represent technologies that can be used to produce highly effective P fertilizers from those indigenous deposits. Numerous field trials conducted by IFDC in Asia, sub-Saharan Africa, and Latin America have demonstrated that PAPR at 40-50% acidulation with H₂SO₄ or at 20% with H₃PO₄ approaches the effectiveness of SSP or TSP in certain tropical soils and crops. This paper discusses how the agronomic effectiveness of PAPR is affected by mineralogical composition and reactivity of PR used and by soil properties and soil reactions. The paper also indicates that if a PR has high Fe₂O₃ + Al₂O₃ content, it may not be suitable for PAPR processing because of the reversion of water-soluble P to water-insoluble P during the PAPR manufacturing process. Under these conditions, compaction of PR with water-soluble P fertilizers (e.g. SSP, TSP) at P ratio of approximately 50:50 can be agronomically and economically attractive for utilizing the indigenous PRs in developing countries.     

Phosphorus availability from phosphate rock and sewage sludge as influenced by the addition of water soluble phosphate fertilizer

Phosphorus (P) inputs are required for sustainable agricultural production in most acid soils of the tropics and subtropics. Phosphate rocks (PR) and organic materials have been suggested as alternative P sources in these soils. Quantitative information on the P availability from sewage sludge (SL) is scanty. Methods to improve the effectiveness of PR such as partial acidulation and compaction with water-soluble P sources have been recommended. The objective of this greenhouse study was to evaluate the relative agronomic effectiveness (RAE) of Florida PR and sewage sludges (irradiated and non-irradiated) applied alone and in mixture with a water-soluble source (triple superphosphate, TSP) at two rates (50 and 150 mg P kg^–1 soil). The ³²P isotope dilution technique was utilised to determine the proportion of P in the plant taken up from the P fertilizer treatments. Wheat was grown on an acid loamy sand Dystric Eutrocrepts and harvested 6 weeks after planting. Results on total P uptake and the RAE of the P fertilizer sources tested indicated that the addition of 50 mg P kg^–1 soil as TSP was adequate in supplying P to the 6-week-old wheat plants as compared to PR and sewage sludge. Intermediate values were obtained for the mixtures. Similar responses were observed for the high P rate. For a given P rate, phosphorus uptake from PR and SL in presence of TSP was higher than P uptake from these sources alone, indicating an enhancement effect of TSP on the effectiveness of these non-readily available sources. With respect to P uptake from PR applied alone, the relative increases in P uptake from PR due to TSP influence were 52 and 67% for the low and high P rates, respectively. The relative increases in P uptake from SL due to TSP when compared to P uptake from SL alone were 102 and 59% for the low and high P rates of application. Application of a water-soluble P fertilizer together with a non-readily available P source shows an enhancement on the P uptake from the non-readily available P source by the wheat plants. In this experiment the estimated enhancement effects are very likely underestimated.     

Field evaluation of partially acidulated phosphate rocks in a Ferralsol from Cuba

Phosphorus (P) is needed in large areas of developing countries toimprove soil fertility for crop production. The use of phosphate rock (PR) isan alternative to costly soluble P fertilizers, but it is ineffective usuallyin non-acid soils unless it is modified i.e. partially acidulated (PAPR). Alaboratory incubation study using the isotopic exchange kinetic method of³²P and field experiments were undertaken on a neutral Ferralsol ofCuba to evaluate the effectiveness of PAPRs as fertilizers for common bean(Phaseolus vulgaris, L.). Sulfuric-acid based PAPR using40%, 50% and 60% of the acid required to produce singlesuperphosphate were studied. In the laboratory experiment Trinidad de GuedesPAPR was effective in providing P to the soil, through increases inisotopicallyexchangeable P and the percentage of P derived from fertilizer (%Pdff). In the three field experiments carried out to compare the P sources,yields of common bean were increased by PAPR, though the response was less thanwith triple superphosphate (TSP). The relative agronomic effectiveness (RAE) ofPAPR was greater than that of unacidulated PR. Taking into account the RAEvalues and the current cost of the P sources, the choice of Trinidad de GuedesPAPR instead of TSP could be economic, although the RAE value for PAPR waslowerthan that of TSP. This result indicates that PAPR could be used in thesoil understudy to obtain the best economic return. DM yield, P uptake and grain yield ofcommon bean were significantly increased by applying P as 50% PAPR. Lowcost improvement of the agronomic value of PR can be achieved by partialacidulation, so this modification of the phosphate rock show promise forutilization of PR reserves indigenous to developing countries.     

Effect of phosphate rock sources on biological nitrogen fixation by soybean

Very little information is available concerning the effect of phosphate rock (PR) sources on biological nitrogen fixation (BNF) in legume crops. In a greenhouse study, the¹⁵N isotopic dilution technique was used to compare the effectiveness of three sources of PR (Hahotoe rock, Togo; Tilemsi rock, Mali; and Sechura rock, Peru) with that of triple superphosphate (TSP) in increasing soybean seed yield and the amounts of N fixed by the soybean crop. The acid Hartsells slit loam was limed to pH 5.2 and incubated with 8.5 mg N kg^–1 as K¹⁵NO₃ and sucrose for 2 months prior to planting. Then fertilizer P was incorporated into the soil at 12.5, 25, 50, and 100 mg P kg^–1 rates.The relative agronomic effectiveness (RAE) of the three PRs with respect to TSP (RAE = 100%) in terms of increasing seed yield was Hahotoe rock = 6.0%, Tilemsi rock = 45.9%, and Sechura rock = 75.2%; this trend followed the same trend as PR reactivity, i.e., Sechura rock > Tilemsi rock > Hahotoe rock. BNF was affected significantly by all the P treatments. Of the total N derived from the three N sources (atmosphere, N_dfa; fertilizer K¹⁵NO₃, N_dff; and soil, N_dfs), N_dfa was highest with TSP and lowest with Hahotoe rock, whereas the reverse was found with N_dfs. Among various plant parts, more N_dfa was translocated and stored in seeds than in stems + leaves and roots. The RAE values of the three PRs with respect to TSP (RAE = 100%) in terms of influencing the amount of BNF were Hahotoe rock = 3.0%, Tilemsi rock = 43.4%, and Sechura rock = 71.2%. A linear relationship was found between the amount of BNF by the whole soybean plant and the soybean seed yield.     

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.     

Comparison of the agronomic effectiveness of a phosphate rock and triple superphosphate as phosphate fertilisers for tea (Camellia sinensis L.) on a strongly acidic Ultisol

Phosphorus deficiency is a major problem affecting tea production in the highly weathered acid soils of humid and sub-humid tropics which are known to have high P fixing capacities. As many of these soils are strongly acidic and receive high rainfall, low-cost phosphate rock (PR) may effectively supply the plant P needs and limited preliminary experiments suggest this is so. A long-term glasshouse trial was conducted on 8-month old tea seedlings to compare the agronomic effectiveness of a locally available PR (Eppawala phosphate rock, EPR) with triple superphosphate (TSP) applied to a strongly acidic (pH water 4.55) marginally P deficient Rhodustult from Sri Lanka at six rates ranging from 10 to 60 kg P ha⁻¹. The results showed that TSP or EPR fertiliser at a rate as low as 10 or 20 kg P ha⁻¹ was sufficient to obtain maximun tea yield. The agronomic effectiveness of EPR was equal to that of TSP at the 5- and 10-month samplings. The concentration of soil P extracted by a cation–anion exchange resin membrane (resin-P) was higher in the TSP treated soil at 5 months due to its greater solubility but at 10 months, the EPR produced higher resin-P due to its increased dissolution over time. In the presence of tea plants, 52% of P from the EPR applied at the rate of 10 kg ha⁻¹, was dissolved at 5 months compared to 75% of dissolution at the 10-month sampling. In the absence of plants, the corresponding dissolution figures were 40% at 5 months and 55% at 10 months. The concentration of inorganic P extracted by 0.1 M NaOH (NaOH-P_i) (loosely characterising Fe + Al bound P) was significantly higher in the TSP treated soil and concentration of P extracted by 0.5 M H₂SO₄ (Ca bound P) was higher in the EPR treated soil. The results suggest that the low-cost, locally available EPR may be used profitably as a maintenace P fertiliser for tea plantations in moderately P deficient soils, which need to be confirmed by field studies. This revised version was published online in June 2006 with corrections to the Cover Date.     

Unacidulated and Partially acidulated phosphate rock: Agronomic effectiveness and the rates of dissolution of phosphate rock

The agronomic effectiveness of unground North Carolina phosphate rock (PR) and partially acidulated phosphate rocks (PAPR) prepared by acidulation of the PR with 30%, 40% and 50% of the phosphoric acid needed for complete acidulation, was determined in a 4 year field experiment on permanent pastures. The soil developed from volcanic ash, and was highly P retentive. The rate of dissolution in soil of the PR component in PAPR and of PR applied directly was measured, together with bicarbonate extractable P. The priming effect of the monocalcium phosphate (MCP) component of PAPR on root growth was also investigated.Pasture yields showed that even the 30% acidulated PAPR was as effective as fully acidulated triple superphosphate (TSP), mainly due to the high reactivity of the PR used. The 50% acidulated PAPR tended to be superior to TSP. Soluble P in PAPR caused a marked increase in root proliferation, and dry matter yields were greater than predicted from the amounts of MCP and PR in PAPR. Directly applied PR was inferior to TSP in years 1 and 2 but was equal in year 4. (There was no pasture response to application of P fertilizers in year 3.)Dissolution rates of the PRs were determined applying a cubic model to PR dissolution data. The rate of dissolution increased with increasing acidulation and this is tentatively ascribed to increased root proliferation around PAPR granules and acidification of the clover rhizosphere during nitrogen fixation.     

Extractable phosphorus to predict agronomic effectiveness of ground and unground phosphate rocks

The reactivity of 11 phosphate rocks (PRs), both in their finely ground and unground forms, and containing up to 12% of CaCO₃ were estimated from the P extracted with 2% citric acid, 2% formic acid and neutral ammonium citrate. With the neutral ammonium citrate, PRs were also extracted sequentially. To determine the relative agronomic effectiveness (RAE) of PRs a greenhouse experiment was conducted using three soils of different P retention, and growing ryegrass as the test plant. PRs were applied at three rates plus control. Ground Sechura PR was employed as the standard PR and single superphosphate was included to determine maximum dry matter production. Three cuts of ryegrass were taken over 4 months in 2 soils, and 8 cuts over 10 months in one soil. RAE of the PRs was calculated from dry matter yields obtained at a fertiliser P rate where superphosphate gave 90% of the maximum attainable yield, and also from the yield and P uptake values averaged over rates of application.Amount of P extracted by the different reagents correlated closely with each other (R² 0.82–0.99). Chemical extractable P integrated the effect of particle size as well as the inherent chemical reactivity of PRs. Formic P expanded the scale and thus was the most sensitive indicator to rank PRs. The ranking of PRs were similar between soils. RAEs between soils correlated significantly (R² = 0.88–0.91). For predicting the agronomic potential of PRs, RAEs calculated from 4 cuts of ryegrass over 4 months appeared to be as good as those calculated from 8 cuts, over 10 months.The total P of PRs was smaller in the large and very small size fractions than in the intermediate ones. With unground rocks the extractable P decreased with increasing particle size mainly due to their decreasing specific surface area. Practical implications of these results are discussed. Extractable P did not decline with increasing total P in the different size fractions of PRs. End-over-end shakers, unlike wrist-action shakers, gave consistent extractable P values.Formic-P was the best predictor of the agronomic effectiveness of PRs even when the data from ground and unground PRs, and from 3 soils of differing P retention were treated as one population (R² = 0.87–0.92).     

Greenhouse evaluation of phosphorus availability from compacted phosphate rocks with urea or with urea and triple superphosphate

The possible effect of urea hydrolysis on the availability of phosphorus (P) from phosphate rock (PR) was evaluated in two greenhouse experiments with maize, using two sources of PR — Pesca (Colombia) and Bayovar (Peru) — representing low and high chemical reactivity, respectively.In Experiment I, on a neutral Josephine silty clay loam (pH 6.2) (Typic Haplozerult), Bayovar PR compacted with urea (Bayovar PR + urea) performed better than Bayovar PR compacted with NH₄Cl (Bayovar PR + NH₄Cl) in increasing dry-matter yield at a rate of 100 mg P kg^–1 but not at rates of 50 and 200 mg P kg^–1. It was also found that the dry-matter yield obtained with compacted Bayovar PR products was significantly higher when the N ratios of urea: NH₄Cl were 1:1 or higher than when the ratios were below 1:1. Although Bray I–P can overestimate available P from PR with respect to that from TSP, a good relationship was observed between Bray I–P and dry-matter yield from various compacted Bayover PR products with a small particle size (–0.43 + 0.15 mm).In Experiment II an acid Bladen sandy loam (pH 4.5) (Typic Albaquult) was used. Finely ground Bayovar PR (– 0.07 mm) was about 66% as effective as TSP in increasing dry-matter yield, whereas Pesca PR was ineffective. When Pesca PR was partially acidulated with H₃PO₄ at 20% level (PAPR), it became 70% as effective as TSP. Granulated PAPR and Pesca PR compacted with TSP (Pesca PR + TSP) were found to be equally effective in increasing dry-matter yield when both products had the same particle size and the same water-soluble and citrate-soluble P as percent of total P, and when prilled urea was used as the N source. However, when urea was compacted with Pesca PR and TSP, the product's effectiveness was further increased by 30% and to the same level as TSP.In summary, the results tend to support the suggestion that urea hydrolysis can be beneficial in increasing the availability of P from PR to plants in soils having medium to high organic matter contents.     

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     

Greenhouse evaluation of agronomic potential of different sources of phosphate fertilizer in a typical concretionary soil of Northern Ghana

The concretionary soils of Northern Ghana, which are near neutral with respect to pH and which comprise mostly lateritic ferruginous nodules are known to sorb significant amounts of phosphate. Instead of imported superphosphate, the use of less expensive indigenous Togo rock phosphate (PR) or partially acidulated (50%) Togo rock phosphate (PAPR-50), are possible alternative phosphate fertilizer options for these soils. The objective of this research was to evaluate the effectiveness of freshly-applied SSP, PR and PAPR-50, and the effectiveness of the residues of these fertilizers in a glasshouse pot study. Laboratory studies were also undertaken to study the transformation of these fertilizers after their application to the concretionary ferruginous soils. In the greenhouse study, yield of dried tops and the P uptake by the tops of maize var. Dobidi (Zea mays) was used to measure fertilizer effectiveness. One level of P was applied for each fertilizer (26.4 kg P ha^–1). Plants were grown for 28 days. After harvesting the first crop, subsequent cropping was carried out to evaluate the effects of the residual P in the pots. The results showed that increases in dry matter yield of shoot and total P uptake followed the trend SSP > PAPR-50 > PR > control. The relative agronomic effciency (RAE) of PAPR-50 was 58% that of commercial SSP in increasing growth of the crop, while that of PR was only 23%. The residual effect of either PAPR-50 or PR on dry matter yield and total P uptake was found to be negligible compared with SSP, suggesting that apatitic P was poorly effective relative to SSP in the used soils. The P fractionation results confirmed that PR and PAPR-50 did not significantly increase any of the P fractions in either the soil fines or nodules after the first crop. By contrast, application of SSP increased all extractable Pi fractions, most of the P added being recovered from the nodules in forms associated with Fe (hydroxide and residual Pi).It is concluded that, relative to SSP, the P from residues of PAPR-50 and PR are poorly effective in the soils studied for sustainable plant production.     

Cowpea varietal differences in uptake of phosphorus from Gafsa phosphate rock in a low-P Ultisol

A greenhouse experiment was conducted to evaluate varietal differences in the uptake and availability of P from Gafsa phosphate rock (PR) to five cowpea cultivars grown in a low-P Paleustult soil from Ghana, using the A value technique. The³²P radioisotope used as a tracer was³²P-labelled triple superphosphate (TSP). Each cultivar received sole or a combined application of the two fertilizers (TSP, PR). From these treatments it was possible to estimate for each cultivar, A_R + A_S, A_S and by difference A_R (A_R and A_S stand for A values for phosphate rock and soil, respectively). Using this approach we measured significant genotypic differences in P uptake from PR and A_R values. The ranking of the cultivars in P uptake from PR was the same as for A_R, i.e. Asontem > Vallenga > Soronko > IT81D-1137 > Amantin. Similarly, ranking for uptake from soil P was the same as A_S, i.e. Vallenga, Soronko, IT81D-1137, Asontem and Amantin. Thus although PR could not be labelled directly, using the A value approach it was possible to distinguish between P availability from PR and soil to the plant. The recoveries of applied TSP ranged from 8.0% to 9.4% and those of PR from 2.3% to 3%.The other advantage of the A value method is that it made it possible for the different genotypes tested to be compared directly in units of a standard fertilizer, TSP in this case. Thus for Vallenga in this soil 3.01 kg of P in Gafsa PR was capable of supplying the same amount of P that could be supplied by 1 kg P of TSP; whereas for Amantin a higher amount i.e. 3.5 kg P as Gafsa PR was needed. This information is useful for adjusting application rates to be recommended for different P fertilizer sources in field trials so as to achieve similar effects.     

Combining Tithonia diversifolia and minjingu phosphate rock for improvement of P availability and maize grain yields on a chromic acrisol in Morogoro,Tanzania

A 2-year field experiment was conducted to evaluate the effects of Tithonia diversifolia green manure combined with either Minjingu phosphate rock (MPR) or triple super phosphate (TSP) on soil chemical properties that influence P availability, P pools and maize grain yields, on a Chromic Acrisol in Morogoro, Tanzania. Leafy biomass of tithonia was applied before maize planting for two consecutive growing seasons. Treatments compared were the control, MPR and TSP each at 80 kg P ha⁻¹; tithonia alone at 2.5, 5.0, and 7.5 Mg ha⁻¹ dry matter and tithonia combined with MPR or TSP at 40 kg P ha⁻¹. Tithonia led to significant increases in soil pH, exchangeable Ca, labile (resin and NaHCO₃-Pi), and moderately labile inorganic P (NaOH-Pi). It reduced exchangeable Al and P sorption. Application of MPR alone had liming effects and resulted in increase in labile P. Combining tithonia with MPR had similar but more intense effects. Triple superphosphate alone led to acidification and this was reversed when TSP was co-applied with tithonia. Increasing the application rates of tithonia either alone or in combination with TSP or MPR led to more pronounced liming effects but the differences between 2.5 and 5.0 Mg tithonia ha⁻¹ were not significant due to moisture stress that was experienced during the season. The P and Ca concentrations of the maize plants at tasselling increased with the application of tithonia alone or combined with MPR or TSP, and were significantly correlated with maize grain yields (r = 0.75 and 0.64 for MPR and TSP, respectively). Tithonia added consecutively for 2 years increased total maize grain yields by 70% compared to that in the control. The relative agronomic effectiveness (RAE) of MPR increased from 46% in the first year of application to > 142% in the second year, indicating that the initially slow dissolution of MPR improved by combined application of tithonia and MPR, attributed to reduction of P sorption. It is concluded that tithonia can enhance P availability from the Chromic Acrisol through modification of soil properties associated with P transformation and availability. In cases where tithonia is found within the farmers’ fields its combined application with MPR can increase maize yields at a much-reduced cost associated with tithonia procurement.     

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.     

Comparison of greenhouse and 32P isotopic laboratory methods for evaluating the agronomic effectiveness of natural and modified rock phosphates in some acid soils of Ghana

Phosphorus deficiency is one of the major constraints for normal plant growth and crop yields in the acid soils of Ghana and therefore addition of P inputs is required for sustainable crop production. This is often difficult, if not impossible for small-scale farmers due to the high cost of mineral P fertilizers and limited access to fertilizer supplies. Direct application of finely ground phosphate rocks (PRs) and their modified forms have been recommended as alternatives for P fertilization. The direct application of the natural and modified PRs to these acid soils implies the need to predict their agronomic effectiveness of the PRs in the simplest and most cost-effective manner. In this study the classical greenhouse pot experiment was compared to the ³²P isotopic kinetics laboratory method for evaluating the agronomic effectiveness of natural and modified Togo PR in six highly weathered Oxisols from southwest Ghana. In the ³²P isotopic kinetics laboratory experiment the six soil samples were each fertilised at the rate of 50 mg P kg^–1 soil in the form of triple superphosphate (TSP), Togo PAPR-50%, and Togo PR, respectively. Controls without P amendment were also included. Isotopic exchange kinetics experiments were carried out on two sets of samples, immediately after P fertilizer additions (without incubation) and after 6 weeks of incubation under wet conditions and at a room temperature of 25 °C. In the greenhouse pot experiment, P fertilizers in the form of Togo PR, Togo PAPR, Mali PR and TSP were each applied to the six soils at rates equivalent to 0, 30, 60, and 120 kg P ha^–1, respectively. The P fertilizers were mixed with the soils and maize (Zea mays L.) variety Obatanpa was grown for 42 days before harvest. The isotopic kinetics data of the control samples indicated that 5 of the studied soils had very low P fertility status as reflected by their low P concentrations in solution (C_P<0.02 mg P l^–1) and low exchangeable P (E₁min < 5 mg P kg^–1). The capacity factor and the fixation index of the soils were variable. Application of water-soluble P as TSP increased both the C_P and E₁ values of all the soils above the critical levels. Togo PR was least effective among the fertilizers tested for all soil soils, except in Boi soil. Acidulation of Togo PR (Togo PAPR-50%) was an effective means to increase its agronomic effectiveness. Direct application of natural Togo PR would be only feasible in the Boi soil series as reflected by its high Pdff% value in soil solution. Incubation with the P fertilizers caused an increase in the soil pH and a decline in the effectiveness of the applied P fertilizers, irrespective of the soil and the fertilizer utilized. Based upon the results of the greenhouse pot experiment, the relative crop response index (RCRI) in terms of increasing dry matter yield and P uptake followed the order of TSP > PAPR = Mali PR >Togo PR = Control. Both the laboratory index, Pdff% in soil solution derived from the isotopic method and the RCRI values obtained from the pot experiment produced similar results in ranking the P fertilizers tested according to their agronomic effectiveness. The isotopic kinetic method may be considered as an alternative to both greenhouse and field methods in the evaluation of agronomic effectiveness of P fertilizers in tropical acid soils when it offers comparative advantages in assessing the soil P status and its changes. But trained staff and adequate laboratory facilities are needed to perform this technique. Also the method can be used as a reference for comparison purposes as in this case. Further research is needed to assess the overall agronomic effectiveness (immediate and residual effects) of PR sources in predominant cropping systems of this region of Ghana.     

Use of partially acidulated phosphate rocks as phosphate fertilizers

Partially acidulated phosphate rocks (PAPRs) are manufactured by acidulation of PRs with less than the stoichiometric amounts of, usually, phosphoric or sulphuric acids. Products of similar composition to PAPRs are also prepared by cogranulating superphosphate with PRs. For most crops the agronomic value of PAPRs is determined by the availability to plants of their water-soluble P as well as their PR P component. The acid unreacted PR present in the directly acidulated PAPR, is considered to be less reactive than the original PR. This is probably the result of surface coatings of chemical compounds formed during acidulation. Under some soil conditions, in the presence of plants, the PR component probably dissolves faster than the original PR. For seasonal crops, except for fast growing ones such as squash (Cucurbita maxima), reactive PRs partially acidulated so that the final products contain about 50% of its total P in water-soluble form, are generally as effective as fully acidulated superphosphate. For permanent pastures the water P content may be reduced to about 40% of total P without reducing their agronomic effectiveness of the product. In medium P retentive soils pH seems to have little or no influence on the agronomic effectiveness of PAPRs. In highly P retentive soils increasing soil pH reduces the agronomic effectiveness of phosphoric PAPRs apparently by reducing the solubility of the PR component of PAPRs. Even at low pH the dissolution of unreacted PR in sulphuric PAPRs is less than that in phosphoric PAPRs, probably due to the possible coating of calcium sulphate on the residual PR in sulphuric PAPRs. Results on the agronomic effectiveness of PAPRs prepared from unreactive rocks were highly variable and no generalisation could be made regarding the degree of acidulation needed for the products to be consistently effective. Single superphosphate (SSP) cogranulated with reactive rocks (SSP/PR) was agronomically less effective than SSP, and also than phosphoric PAPRs of similar water-soluble P.     

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.