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.     

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.     

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.     

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.     

Evaluation of the agronomic effectiveness of natural and partially acidulated phosphate rocks in several soils using32P isotopic dilution techniques

The agronomic effectiveness of two natural phosphate rocks (PRs) from North Carolina (USA) and Togo and their 50% partially acidulated products (PAPRs) was evaluated in two greenhouse experiments using³²P isotopic dilution techniques, namely L and A_L values.In the first experiment rye grass was grown in a soil from Ghana. While the proportion of P in the plant derived from the P fertilizer (Pdff) ranged on. the average from about 10% for the PRs up to 80% for the PAPRs, the P fertilizer recovery was less than 1% for a 60-day growth period. In the second experiment, average values of P in the maize plants derived from the PAPRs ranged from 35% to 75% in 3 different soils. Both PRs were ineffective with the exception of North Carolina PR in the Seibersdorf soil. The P fertilizer recovery was 0.25% for the North Carolina PR in this soil whereas the recovery values ranged from 1.2% to 1.6% for the PAPRs.Mean values of the relative fertilizer efficiency estimated from the L values of each soil were less than 1% for the PRs whereas the values for the PAPRs which were dependent on soil type ranged from 20% up to 45%. The coefficient of relative effect of partial acidulation, that was calculated from the ratio of A_L values for PR and PAPR in each soil indicated that partial acidulation increased the effectiveness of the natural PRs in all soils under study.This study showed that the use of³²P isotope dilution techniques allows an accurate measurement of the P availability from natural and modified PR products to crops. Another advantage is that quantitative comparison of the P sources under study, PRs and PAPRs in this case, can be made even in soils where there is no response to the applied P sources.     

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.     

Induced phosphorus fixation and the effectiveness of phosphate fertilizers derived from Sukulu Hills phosphate rock

A greenhouse study was conducted with two surface, acidic soils (a Hiwassee loam and a Marvyn loamy sand) to measure the effect of increasing P-fixation capacity, on the relative agronomic effectiveness (RAE) of phosphate fertilizers derived from Sukulu Hills phosphate rock (PR) from Uganda. Prior to fertilizer application, Fe-gel was added to increase P-fixation capacity from 4.4 to 14.3% for the Marvyn soil and from 37.0 to 61.5% for the Hiwassee soil. Phosphate materials included compacted Sukulu Hills concentrate PR + Triple superphosphate (CTSP) at a total P ratio of PR:TSP = 50:50; 50% partially acidulated PR (CPAPR) from Sukulu Hills concentrate PR made with H₂SO₄; and Sukulu Hills concentrate PR (PRC) made by magnetically removing iron oxide from raw PR ore. Triple superphosphate (TSP) was used as a reference fertilizer. After adjusting soil pH to approximately 6, P sources were applied at rates of 0, 50, 150, and 300 mg total P kg^–1 soil. Two successive crops of 5 week old corn seedlings (Zea mays L.) were grown. The results show that the RAE of the phosphate materials measured using dry-matter yield or P uptake generally decreased as P-fixation capacity was increased for both soils. CTSP was more effective in increasing dry-matter yield and P uptake than CPAPR. PRC alone was an ineffective P source. Soil chemical analysis showed that Bray 1 and Mehlich 1 extractants were ineffective on the high P-fixation capacity Fe-gel amended Hiwassee soil. Mehlich 1 was unsuitable for soils treated with PRC since it apparently solubilizes unreactive PR. When all of the soils and P sources were considered together, Pi paper was the most reliable test for estimating plant available P.     

Rape genotypic differences in P uptake and utilization from phosphate rocks in an Andisol of Chile

A main constraint to agricultural productivity in the southern regionsof Chile is the low available soil P exacerbated by the high P sorptioncapacityof the predominant Andisols. Therefore, substantial amounts of P fertilizersmust be applied to obtain optimum growth and crop yields. One cost-effectivestrategy followed to supply P to crops grown in these soils is the directapplication of the local Bahia Inglesa PR source. However, a more sustainablestrategy would be to combine the use of the local PR with the crop species andcultivars that are able to grow in these acid soils and can utilize efficientlyPR. Rape is reported to be very efficient in utilising P from PR sources due toits capacity to exude organic acids to the rhizosphere. Therefore, the presentstudy was conducted to evaluate the ability of five rape cultivars grown in anAndisol of southern Chile in utilising P from two PR sources (Bahia Inglesa andBayovar) and triple superphosphate, a water-soluble P fertilizer. It was foundthat rape was able to absorb significant amounts of P from the PR sources andmuch less from the TSP and soil P. Both Bahia Inglesa and Bayovar PRs werefoundto be as effective as TSP for the rape genotypes in the Andisol Pemehue. Theuseof the ³²P isotope technique enabled to assess the ability of thegenotypes tested to utilize P from the different P fertilizers applied. Thegenotypes G2 and G3 showed increased P acquisition from the PR than thegenotypeG5. Combined utilization of P efficient genotypes and direct application of theBahia Inglesa PR seems to be a promising technology for attaining sustainableagricultural productivity in the Andisols of Chile. Further field trials forvalidating these findings at the level of cropping systems are needed. Thisagronomic testing should be accompanied by in-depth studies to assess therelative importance of the morphological and physiological traits determining ahigher P efficiency.     

Phosphorus fertility recapitalization of nutrient-depleted tropical acid soils with reactive phosphate rock: An assessment using the isotopic exchange technique

A soil P fertility recapitalization initiative utilizinglarge rates of phosphate rocks (PRs) was proposed to improve the soil P statusand increase the sustainable food production in acid and P-deficient tropicalsoils. Two series of experiments were carried out using five tropical acidsoilstreated with heavy applications of Gafsa phosphate rock (GPR). In the firstseries, the soils were mixed with GPR at the following application rates: 0,500, 1000 and 2000 mg P·kg^–1, andincubatedfor one month in moist conditions. In another series, 1000 mg Pkg^–1 applied as GPR was added to three soils andincubated for 1.5 month; thereafter 50 mg P kg^–1as triple superphosphate (TSP) were added. The ³²P isotopic exchangemethod was utilized to assess the contribution of GPR to the available soil P.Changes in amounts, E, of P transferred with time as phosphate ions from thesoil particles to the soil solution as well as changes in pH, calcium andphosphate concentrations in soil suspensions were determined. It was foundthat:(i) the contribution of P from GPR to recapitalization of soil P fertility wasmainly assessed by E pool size, pH, calcium and phosphate concentrations; othervariables were not significant at the 0.1 level; (ii) heavy applications of GPRdid not saturate all the P sorption sites, P freshly applied as water-soluble Pwas still sorbed; (iii) recapitalization of soil P fertility using GPR waspartly obtained in some acid tropical soils; (iv) Upon dissolution, GPRprovidedcalcium ions to crops and to soils, thus reducing Al toxicity, but its limingeffect was limited. To explain these effects with heavy application rates ofGPR, it was postulated that a coating of Al and Fe compounds is formed aroundPRparticles with time, thus reducing further dissolution.     

Development and evaluation of an improved soil test for phosphorus, 3: field comparison of Olsen, Colwell and Resin soil P tests for New Zealand pasture soils

Soil tests suitable for estimating the phosphorus (P) status of soils fertilised with soluble or sparingly soluble P fertilisers (reactive phosphate rock) were evaluated using the New Zealand Ministry of Agriculture Technology (NZMAFTech) National Series forms of phosphate trials on permanent pastures located throughout NZ. This included a common core of treatments comparing Sechura phosphate rock (SPR) with triple superphosphate (TSP). At each site, a re-application of twice maintenance TSP was superimposed on one-half plots that previously had received six annual applications of increasing amounts of P (0, 0.5, 0.75, 1.0 and 2.0 times the maintenance rate) in the form of TSP or SPR. Before the re-application of TSP, soil samples (0–30 and 0–75 mm depths) were collected from each plot. All the trials were run for 1 year during which seven to ten harvests were taken. Pasture response was expressed as percent increase in yield obtained with re-application over the previous treatment.The 0.5 NaHCO₃ based (Olsen P) extractant with different combinations i.e. soil volume (Olsen (v)), soil weight (Olsen (w)), shaking time variations (Olsen (16 h)) and soil:solution ratio (Colwell), and Resin P soil tests were conducted on soils taken from the plots prior to re-application of TSP. The Olsen (v), Olsen (16 h) and Colwell P values increased with increasing rates of P applied in all soils with values for sparingly soluble P materials being less than where soluble P fertiliser had been previously applied. The Resin P values showed similar increases with P applied regardless of the solubility of previously applied fertiliser. When the yield increases caused by TSP application to all treatments (irrespective of fertiliser source) were regressed against soil test values, Resin P explained 76% of the variation in yield response, compared to 50% by Olsen (v), 42% by Olsen (w), 39% by Olsen (16 h) and 40% by Colwell P. Partitioning the data according to fertiliser source slightly improved the coefficient of determination for Resin P for both the soluble (R²=0.81) and sparingly soluble (R²= 0.80) P fertilisers. With 0.5 M NaHCO₃ (Olsen) extractants, R² values consistently indicated a poorer prediction for the SPR treatments. A Resin P model was able to account for more variance in yield response to re-applied TSP, than an Olsen P model because the Olsen model underestimated the yield response to re-applied TSP on the PR treatments. The Resin test is more suitable than the current Olsen test for assessing the plant available P status of soils previously fertilised with fertilisers of varying solubility.Dr. A.G. Sinclair died on 3 December 1996 whilst this paper was in preparation.     

Enhancing the agronomic effectiveness of natural phosphate rock with poultry manure: a way forward to sustainable crop production

Phosphorus inputs are required in highly weathered tropical soils for sustainable crop production. However, high cost and limited access to mineral P fertilizers limit their use by resource-poor farmers in West Africa. Direct application of finely ground phosphate rock (PR) is a promising alternative but low solubility of PR hampers its use. Co-application of PR with manure could be a low cost means of improving the solubility of natural PR and improve their agronomic effectiveness. Our objective was to quantitatively estimate the enhancement effect of poultry manure on P availability from low reactive PR (Togo phosphate rock) applied to highly weathered soils. We utilized two highly weathered soils from Ghana and Brazil for this greenhouse study. Using ³²P isotopic tracers, the agronomic effectiveness of poultry-manure-amended Togo rock phosphate (TPR) was compared with partially acidulated Togo rock phosphate (PAPR) and triple superphosphate (TSP). Four rates of poultry manure: 0, low (30 mg P kg⁻¹ soil), high (60 mg P kg⁻¹ soil) and very high (120 mg P kg⁻¹ soil) were, respectively, added to a constant amendment (60 mg P kg⁻¹ soil) of the P sources and applied to each pot of 4 kg soil. A Randomized Complete Block Design was used for the greenhouse experiment and Maize (Zea mays L.) was used as a test crop. The plants were grown for 42 days after which the above ground biomass was harvested for analysis. Without poultry manure addition, the agronomic effectiveness, represented by the relative agronomic effectiveness (RAE) and proportion of P derived from fertilizer (% Pdff) was in the order TSP > PAPR > TPR = control (P₀). In the presence of low rate poultry manure addition, the agronomic effectiveness followed the order TSP > PAPR = PR > P₀. However, at the high and very high rates of poultry manure addition, no significant differences in agronomic effectiveness were observed among the P sources, suggesting that at this rate of poultry manure addition, PR was equally as effective as TSP. The study showed that direct application of PR co-applied with poultry manure at a 1:1 P ratio will be a viable option for P replenishment. Thus a combination of PR and poultry manure could be a cost-effective means of ensuring sustainable agricultural production in P-deficient, highly weathered tropical soils.     

Ability of a dynamic model of phosphorus to account for observed patterns of response in a series of phosphate fertilizer trials on pasture

A national series of rates × forms of phosphate fertilizer trials has provided a large amount of data on patterns of response of pasture production and soil P tests to forms, rates and frequencies of P fertilizer application over time. A simple mechanistic model of P in pastoral systems has been developed in an attempt to account for the observed patterns. The model considers active P in the soil to be in one of two compartments: undissolved fertilizer P (P_F) and a pool of labile, plant-available P (P_A). P dissolving from P_F enters P_A, and P is lost from P_A by immobilisation in the soil and by net removal above ground through non-recycled uptake by plants. Both these transfers are taken to follow first order kinetics, with rate constants designated K₁ and K₂ respectively. Pasture dry matter (DM) production in any year is related to the mid-year value of P_A by the diminishing-returns Mitscherlich equation, with nil pasture yield being associated with nil P_A.Data from different sites with the same design were combined to give generalised response patterns with which to assess the model. Only the control treatment and treatments receiving triple superphosphate (TSP) and Sechura phosphate rock (SPR) were considered. With a value for K₁ corresponding to almost immediate dissolution, the model accounted well for the Olsen soil P test patterns from TSP when K₂ corresponded to an annual loss of 16.3% of P in P_A and a factor of 10 was used to convert Olsen P to kg P/ha in P_A. Pasture DM yield response curves to rates of TSP in individual years were well accounted for when the mid-year value of P_A required for 90% relative yield was taken as 130 kg P/ha. The model also accounted well for the difference between response patterns to annual and triennial TSP applications for both soil P tests and DM production. The declines in soil P tests and DM production which occurred in nil P controls were less than model predictions: this could be due to unfertilised pasture making more use of P from greater soil depth or less available forms.Olsen P tests with the higher rates of SPR were always lower than model predictions using a value for K₁ derived from chemical measurement of SPR dissolution rates in the trials. However, DM yields from SPR were generally close to model predictions. These divergent results support independent data suggesting that available soil P from reactive phosphate rocks (RPRs) is underestimated by the Olsen P test.The relative success of this simple compartmental model in accounting for generalised response patterns in such a large body of data suggests that it could be a useful starting point for a dynamic P fertilizer recommendation model.     

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.     

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

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

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 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.     

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.     

Phosphorus bioavailability of fertilizers: A predictive laboratory method for its evaluation

A laboratory method and a laboratory index is proposed to estimate the phosphorus taken up by plants that is derived from fertilizers (Pdff). Pdff values were measured using greenhouse experiments and³²P labelling technics. The laboratory index estimates the proportion of PO₄-ions derived from the fertilizer in the soil solution and is measured by means of an isotopic exchange of³²PO₄-ion procedure. This indicator was named JCF. Two typical soil-fertilizer conditions were studied. One concerned measurement of Pdff and JCF values for freshly-applied phosphorus as diammonium phosphate (DAP) at levels of 15, 30, 45, 60 and 90 mg P kg^–1 soil. The other concerned measures of Pdff and JCF values for two types of P residues previously applied in soils as concentrated superphosphate (CSP) or Gafsa rock phosphate (GRP) applied at 0 and 43.7 kg.ha^–1 each year over a 15 yr period.For freshly-applied DAP a linear relationship between Pdff and JCF values was obtained over the range of 0 to 90 mg P (kg soil)^–1 levels of application: JCF = 1.16 Pdff + 1.78, (r ² = 0.98). For the P residues, JCF and Pdff values were not significantly different for a given residual treatment. However JCF and Pdff pair data for CSP treatments (56.0, 65.9) were about tenfold superior to those for GRP treatments (5.3, 4.6)). Consequently the nearly 1:1 ratio between JCF and Pdff values that was obtained for the two different soil-fertilizer conditions suggests that the proposed laboratory method can be used to predict availability of P fertilizers to plants. Thus it deserves to be considered in helping to estimate P fertilizer applications.     

Dissolution of phosphate rocks in soils. 2. Effect of pH on the dissolution and plant availability of phosphate rock in soil with pH dependent charge

The effect of soil pH on the dissolution of phosphate rocks (PRs) and the subsequent availability of the dissolved inorganic phosphorus (Pi) to plants was examined in a volcanic soil adjusted to different pH values. Potassium dihydrogen orthophosphate (KH₂PO₄) and three PRs, Nauru (NPR), Jordan (JPR) and North Carolina (NCPR) were incubated with the pH-amended soils at a rate of 800µg P g^–1 soil for 84 days. The extent of PR dissolution was determined by measuring the increases in the amount of 0.5 M NaOH extractable Pi (NaOH-P) in the PR treated soil over the control soil. The amount of plant available P was measured either by extracting with 0.5 M NaHCO₃ or by growing ryegrass in soil samples incubated with the phosphate sources.At each pH the order of the extent of PR dissolution followed NCPR > JPR > NPR, which was consistent with the decreasing order of their chemical reactivities. As the pH decreased from 6.5 to 3.9 the dissolution of PRs increased from 29.3% to 83.5%, from 18.2% to 78.9%, and from 12.5% to 60.3% for NCPR, JPR and NPR, respectively. In contrast, as the soil pH decreased from 6.5 to 3.9, the proportion of the dissolved P extracted by 0.5 M NAHCO₃ decreased from 38% to 5% and the proportion taken up by ryegrass plants decreased from 46% to 7%. This decrease in plant available P corresponded to an increase in the adsorption of inorganic P with a decrease in pH. However, the uptake of P from PR relative to that from KH₂PO₄ was higher at low pH than at high pH. Further, the amount of P taken up by plants was more closely related to the amount of NaHCO₃ extractable P than to the amount of dissolved P present in the soil.     

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.