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.     

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.     

Agronomic effectiveness of partially acidulated rock phosphate and fused calcium-magnesium phosphate compared with superphosphate

The agronomic effectiveness of two partially acidulated rock phosphate (PARP) fertilizers, made from either North Carolina or Moroccan apatite rock phosphate, and a fused calcium-magnesium phosphate (thermal phosphate or TP), was compared with the effectiveness of superphosphate in two glasshouse experiments. A different lateritic soil from Western Australia was used for each experiment. Oats (Avena sativa) were grown in one experiment and triticale (×Triticosecale) in the other. Fertilizer effectiveness was measured using (i) yield of dried tops, (ii) P content (P concentration in tissue multiplied by yield) of dried tops, and (iii) bicarbonate-extractable soil P (soil test value).The following relationships differed for the different fertilizers: (i) yield of dried tops and P content in the dried tops; (ii) yield and soil test values. Consequently the fertilizer effectiveness values calculated using yield data differed from those calculated using P content or soil test data. Freshly-applied superphosphate was always the most effective fertilizer regardless of the method used to calculate fertilizer effectiveness values. For one of the soils, as calculated using yield data, relative to freshly-applied superphosphate, the PARP and TP fertilizers were 15 to 30% as effective for the first crop, and 20 to 50% as effective for the second crop. The second soil was more acidic, and for the first crop the PARP and TP fertilizers were 80 to 90% as effective as freshly-applied superphosphate, but all fertilizers were only 5 to 15% as effective for the second crop. For each soil, the two PARP fertilizers had similar fertilizer effectiveness values. Generally the TP fertilizer was more effective than the PARP fertilizers.     

Assessment of methods for studying the dissolution of phosphate fertilizers of differing solubility in soil

Relationships between plant response and rates of dissolution of ground (< 150µm) North Carolina phosphate rock (NCPR), NCPR 30% acidulated with phosphoric acid (NCPAPR) and monocalcium phosphate (MCP) were assessed in pot experiments. The three fertilizers were incubated for 1, 50 and 111 days, at the rates of 75, 150 and 750µg P g^–1 soil, using two soils with different P-retention capacity. After each period of incubation, four pots were set up from each treatment, and perennial ryegrass (Lolium perenne) was grown in a growth chamber for about six weeks to assess the agronomic effectiveness of the fertilizers. Results in dry matter yield and P uptake showed that immediately following application (1 day incubation), the MCP (solution) was supplying more P to plants than either the NCPR or the NCPAPR applied at the same rate. After 50 and 111 days of incubation, the NCPR and NCPAPR were just as effective in the lower P-retention Tekapo soil. The relative agronomic effectiveness (RAE) of the NCPR and NCPAPR compared with MCP was generally poorer in the higher P-retention Craigieburn soil than in the Tekapo soil shortly after application, but improved with time of incubation. Ryegrass responses to the application of the three fertilizers corresponded to the changing trends of exchangeable P in the soils, measured by the isotopic method.Regressions were made between plant P uptake and indices describing the intensity factor (water extractable P), quantity factor (Bray I P, Olsen P, 0.5M NaOH extractable P and isotopic exchangeable P) and the kinetic factor (F_in) of soil P supply to plants in the Tekapo soil. The percentage of variation in plant P uptake explained by individual indices was generally less than 80%, no matter which of the three single variable models, the Mitscherlich, the quadratic or the power function was fitted. However, more than 96% of the variation in plant P uptake in the Tekapo soil could be explained by the power function models involving two variables. The rate of P dissolution (F_in) determined by the isotopic dilution method was included in all the two variable models. The results suggest that assessment of soil P supply to plants should consider the kinetic factor in addition to the intensity and quantity factors, particularly where P fertilizers with differing solubility are applied.     

Effectiveness of different phosphatic fertilizers measured using labelled superphosphate and phosphorus taken up by plants

The agronomic effectiveness of three P fertilizers (diamonium phosphate, rock phosphate and compost) was studied in a greenhouse experiment using wheat. A radioisotopic method, using triple superphosphate labelled with³²P, was used to evaluate the P in dried tops that was derived from i) the soil, ii) labelled superphosphate and iii) the fertilizer being studied.The ratio between P uptake from each fertilizer and P uptake from the soil was used to compare the effectiveness of the different fertilizers. P derived from diammonium phosphate was greater than P derived from the soil, except in one soil. P derived from rock phosphate was always lower than P derived from the soil. The effectiveness of compost depended on soil type. Compost can produce two kind of effects: i) a direct P contribution and ii) an indirect effect improving P uptake from the soil. The radioisotopic method can be used to study the effectiveness of fertilizers even when there are no differences in yield.     

Effectiveness of Ecophos compared with single and coastal superphosphates

Ecophos is a possible alternative phosphorus (P) fertilizer to single and coastal superphosphate for clover pasture (Trifolium subterraneum) on P leaching, sandy, humic podzols in the > 800 mm annual average rainfall areas of south-western Australia. Ecophos and coastal superphosphate are partially acidulated rock phosphates (PARP) fertilizers. Ecophos is made from calcium iron aluminium (crandallite millisite) rock phosphate. Coastal superphosphate is made from apatite. The sandy humic podzols are known to promote extensive dissolution of rock phosphates, including the untreated rock phosphate present in PARP fertilizers. In this field study (early April 1992 to end of October 1994), the effectiveness of the PARP fertilizers was calculated relative to the effectiveness of single superphosphate (relative effectiveness or RE), using yield and P content of dry clover herbage. The RE of the PARP fertilizers varied markedly between assessments, both within and between years, from being much less effective than single superphosphate, to equally or much more efective. This great diversity in RE is attributed to the different extents P can be leached in the soil, depending on seasonal conditions. It is concluded that Ecophos is a suitable alternative P fertilizer for the soil and environment studied.     

The agronomic effectiveness of reactive rock phosphate,partially acidulated rock phosphate and monocalcium phosphate in soils of different pH

The initial and residual fertilizer effectiveness of North Carolina RP (rock phosphate), monocalcium phosphate and partially acidulated RP (made from North Carolina RP at 30% acidulation), both granulated and non-granulated, were measured in a glasshouse experiment. Triticale (xTriticosecale) was grown for 30 days on a soil that had been adjusted to three pH values (4.2, 5.2 and 6.2). Two crops were grown with a six month interval between crops. The effectiveness of the different fertilizers was compared using relationships between (1) yield of dried tops and the amount of P applied and (2) P content (P concentration in tissue multiplied by yield) and the amount of P applied. For the first crop, relative effectiveness (RE) of the fertilizers was calculated relative to granulated monocalcium phosphate, the most effective fertilizer. Monocalcium phosphate was not applied to the second crop, so relative residual effectiveness (RRE) was estimated for each fertilizer relative to the residual effectiveness of granulated monocalcium phosphate.The relative effectiveness of granulated monocalcium phosphate (band application) was greater (RE = 1.00) than of North Carolina RP (0.01–0.02) and partially acidulated RP (0.45–0.76) for all three soil pH values for the first crop. Granulation and band application increased the effectiveness of monocalcium phosphate and partially acidulated RP, but reduced the effectiveness of North Carolina RP. Both non-granulated monocalcium phosphate and partially acidulated RP were less effective than granulated partially acidulated RP for both crops. For the second crop granulated monocalcium phosphate was most effective and the RRE of non-granulated partially acidulated RP (0.16–0.32) and North Carolina RP (0.19–0.28) was greater than for non-granulated monocalcium phosphate (0.12). For the more acidic soil the RE of non-granulated North Carolina RP was four times higher than for the high pH soil for the first crop and 60% higher for the second crop, but it was still poorly effective relative to granulated monocalcium phosphate. Granulated North Carolina RP was least effective among all the fertilizers for all soil pH values and for both crops.     

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.     

Identification of residual P compounds in fertilized soils using density fractionation,X-ray diffraction,scanning and transmission electron microscopy

Residual fertilizer grains were recovered using density fractionation from three Western Australian soils fertilized five years previously with North Carolina apatitic rock phosphate (RP), Queensland apatitic RP and calcined calcium-iron-aluminium RP (Calciphos). Electron micrographs, energy dispersive X-ray analyses (EDS) and unit cell parameters derived from X-ray diffraction patterns (XRD) of recovered apatitic grains do not differ from those for the original fertilizers. Dissolution of these apatitic fertilizers in soils proceeds by surface etching of apatite crystals and is congruent with no formation of intermediate P compounds. Calciphos grains also did not differ in mineralogy and composition but they had developed a porous fabric due to dissolution along subgrain boundaries. Poorly soluble superphosphate residues were mainly composed of dicalcium phosphate dihydrate, FeAl phosphate and apatite together with quartz and other soil minerals. The P concentrations in various particle size fractions of recovered fertilizer materials were related to the particle size distribution of fertilizers originally added to the soils. These results support the agronomic results that the low agronomic effectiveness of RP fertilizers in these soils is primarily due to the small extent of dissolution of RP.     

Effect of soil pH on the requirement for water-soluble phosphorus in triple superphosphate fertilizers

A greenhouse study was conducted to determine if soil pH affects the requirement for water-soluble P and the tolerance of water-insoluble impurities in TSP fertilizers. Two commercial TSP fertilizers were selected to represent a range in phosphate rock sources and impurities. Phosphate fertilizer impurities were isolated as the water-washed fraction by washing whole fertilizers with deionized water. TSP fertilizers with various quantities of water-soluble P (1.2 to 99% water-soluble P) were simulated by mixing the water-washed fertilizer fractions or dicalcium phosphate (DCP) with reagent-grade monocalcium phosphate (MCP). The fertilizers were applied to supply 40 mg AOAC available P kg^–1 to a Mountview silt loam (fine-silty, siliceous, thermic Typic Paleudults). Wheat (Triticum aestivum (L.)) was harvested at 49 and 84 days after planting. Soil pH values at the final forage harvest were 5.4±0.16 and 6.4±0.15. At a soil pH of 5.4, the TSP fertilizers required only 37% water-soluble P to reach maximum yields while at pH 6.4 the fertilizers required 63% water-soluble P. Results of this study show that higher levels of water -insoluble P can be tolerated in TSP fertilizers when applied to acid soils. Phosphorus uptake was not affected by soil pH, but for the mixtures containing the fertilizer residues the source having the lowest level of Fe and Al had a higher relative agronomic effectiveness.     

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.     

Differential availability of partially sulphuric and phosphoric acidulated phosphate rocks I. Plant response

A glasshouse study was conducted to determine the influence of soil pH on the agronomic effectiveness of partially phosphoric (Phos-PAPR) and partially sulphuric (SA-PAPR) acidulated phosphate rocks (PR). For Phos-PAPR ground North Carolina PR (NCPR) was acidulated with 10, 30 and 50% of acid needed for complete acidulation. For SA-PAPR a blend of NCPR, Arad and Khouribga PRs were acidulated with 60% of the acid needed. The relative agronomic effectiveness of these PAPRs were compared with superphosphate (SSP) and ground NCPR. A highly phosphate (P) retentive and P deficient pasture soil was used. Prior to addition of fertilizers to soil, the pH of soil was adjusted to 5.1 (initial soil pH) 5.4, 5.7 and 6.1 by applying varying amounts of Ca(OH)₂. Ryegrass (Lolium perenne) was grown as the test plant over a period of eight months. Fertilizers were applied at three rates plus control. Soil pH was monitored and continuously adjusted to the desired levels throughout the experimental period.The dry matter yields and P uptake in SSP treated pots were not influenced by soil pH. With increasing soil pH, agronomic performance of Phos-PAPRs and NCPR significantly (P<0.01) decreased but that of SA-PAPR was not affected. On the basis of per unit water-soluble P applied, uptake of P by plants was greater from PAPRs than SSP. Using the P uptake values of SSP and NCPR (which was used to prepare the PAPRs), the dissolution of P from the residual PR component of the PAPRs were calculated. The residual PR component of the Phos-PAPRs apparently dissolved in greater quantities than unacidulated NCPR. Dissolution of the residual PR was enhanced with increasing degree of acidulation. However, in the case of SA-PAPR, the agronomic performance of the PAPR was mostly dependent on the water-soluble P component of the PAPR. The uptake of P from the residual PR component of the SA-PAPR was insignificant.     

The effects of nitrogen fertilizer form on the plant availability of phosphate from soil,phosphate rock and mono-calcium phosphate

A glasshouse trial using lettuce as the test crop, and laboratory incubations were used to evaluate the influence of various nitrogen fertilizers on the availability of phosphate from an unfertilized loamy sand soil and from the same soil fertilized with Sechura phosphate rock or monocalcium phosphate. The order in which nitrogen fertilizer form increased plant yield and P uptake from soil alone and from soil fertilized with the rock was ammonium sulphate > sulphurised urea > ammonium nitrate > urea > potassium nitrate. For each rock application (both 30 and 60 mg/pot) and for soil alone, increased P uptake by the plant correlated well with decreased soil pH. In soil fertilized with the soluble P form, monocalcium phosphate, the form of the nitrogen fertilizer had little effect on plant P uptake. Subsequent laboratory incubation studies showed that increased dissolution of soil-P or Sechura phosphate rock did not occur until acidity, generated by nitrification or sulphur oxidation of the fertilizer materials, had lowered soil pH to below 5.5. A sequential phosphate fractionation procedure was used to show that in soils treated with the acidifying nitrogen fertilizers, ammonium sulphate and urea, there was considerable release of Sechura phosphate rock P to the soil, amounting to 42% and 27% of the original rock P added, respectively.     

Isotopic assessment of soil phosphorus fertility and evaluation of rock phosphates as phosphorus sources for plants in subtropical China

Soil phosphorus (P) deficiency is a major factor limiting crop productivity in many tropical and subtropical soils. Due to the acidic nature of these soils, rock phosphate (RP)-based P fertilizers that are cheaper than manufactured water-soluble P fertilizers can be an attractive alternative under certain conditions. Assessment of the efficacy of these alternative P fertilizers and a rational management of local P resources for sustainable agricultural production require an understanding of the dynamics of P in the soil–plant system and the interactions of various P sources in soils and monitoring of soil available P levels. The present work was conducted to test the applicability of the ³²P isotopic kinetic method to assess the soil P fertility status and evaluate the agronomic effectiveness of local rock phosphates in subtropical China. A series of experiments was carried out in the laboratory, greenhouse and field conditions with the following specific objectives: (a) to evaluate the suitability of this isotopic kinetic method in evaluating soil P fertility in 32 soil samples collected across southern China, (b) to test and further develop chemical extraction methods for routine soil P testing, (c) to monitor the dissolution kinetics of local low to medium grade rock phosphate sources and their effect on soil properties and (d) to evaluate their agronomic effectiveness in greenhouse and field experiments. Since most of the studied soils had very low concentrations of soluble P and high P-fixing capacities, the isotopic kinetic method was found unsuitable for evaluating soil P fertility and to predict plant P uptake. In contrast, the proposed chemical extraction method (NaHCO₃-NH₄F) predicted very well plant P uptake, suggesting that this extraction method can be routinely used to evaluate soil bioavailable P in similar soils in subtropical China. From the incubation study, it was found that although the local low to medium grade RPs were inferior to the reactive NCPR in increasing soil available P levels, they have the potential to improve soil chemical properties. Field experiments indeed demonstrated that the medium grade Jinxiang RP significantly increased crop yield, suggesting that local low to medium grade RPs could be used as P sources to provide P to plants and also to improve soil chemical properties. Overall, these results provide important information for a rational management of P resources for sustainable agriculture in subtropical China.     

Cultivation reduces fertilizer residual effectiveness and affects soil testing for available phosphorus

The agronomic effectiveness of superphosphate and two rock phosphates that had been applied once only to the soil surface 8 to 12 years previously was measured in a field experiment with oats on a lateritic soil in south-western Australia. The soil was either undisturbed or cultivated with a rotary hoe before sowing. The rock phosphates were Christmas Island C-grade ore (C-ore, a calcium ironaluminium rock phosphate), and C-ore calcined (heated) at about 500°C (Calciphos).Cultivation reduced the effectiveness for all three fertilizers by 20 to 50%. The effectiveness of phosphorus (P) applied as superphosphate decreased with increasing period from time of application whereas the effectiveness of the rock phosphates increased but they were always much less effective than superphosphate.The relationship between grain yield and P concentration of plant tissue (i.e. the internal efficiency of P use curve) was similar regardless of fertilizer type, year of application of fertilizer, and whether or not the soil was cultivated. Thus differences in fertilizer residual effectiveness were solely due to the amount of P taken up by the plants.Values of bicarbonate-soluble P (i.e. soil test for P values) for superphosphate treated soil were reduced by about 20 to 25% when the fertilizer was incorporated into the soil whereas for the rock phosphate treated soils the values were little affected by cultivation. The relationship between yield and soil test for P values varied depending on cultivation treatment and fertilizer.We conclude that cultivation decreases the effectiveness of residual fertilizer P and that cultivation and fertilizer type influence the accuracy of yield prediction from soil test values.     

32P isotopic techniques for evaluating the agronomic effectiveness of rock phosphate materials

Tropical soils are often low in available P and therefore require inputs of P fertilizer for optimum plant growth and production of food and fiber. The cost of applying imported or locally produced, water-soluble, P fertilizers is often greater than utilizing indigenous phosphate rock. Therefore quantifying the P availability of soils amended with phosphate rock-based products in a variety of crop management and environmental conditions in developing countries is desirable for making recommendations on best type and rate of fertilizers to use to obtain maximum agronomic and economic benefits. One adequate approach for evaluating the agronomic effectiveness of rock phosphate materials is through the use of³²P/³³P isotopic tracers. The present paper describes the principles and assumptions of the³²P isotopic techniques commonly used in the field and greenhouse for the agronomic evaluation of rock phosphate materials. An overview of the applications of these techniques is also given.     

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

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

The agronomic value of co-granulated Christmas Island Grade C phosphate rock and elemental sulphur

Certain low grade phosphate rocks have low agronomic value as direct applied fertilizers and make poor quality superphosphates. With some rocks there is potential to increase their solubility in soils and hence the fertilizer value by mixing with finely divided elemental sulphur (S°). The agronomic value of a prototype low cost, granular P and S fertilizer, Christmas Island Grade C phosphate rock/elemental sulphur (Xmas C/S°), was compared against single superphosphate (SSP) and reactive phosphate rock (RPR)/S° dry blends, North Carolina phosphate rock/S° (NCPR/S°) and NCPR/50% S-super (granular SSP + 44% S°). An eight- month glasshouse trial using ryegrass (Lolium perenne L. cv. Nui), grown in a central yellow-grey earth (Aeric Fragiaqualf), indicated that fertilization with Xmas C/S° produced dry matter yields, between 54 to 73% and 10 to 40% lower than SSP and NCPR/S° blends respectively, and lower plant P and S uptake. Fertilization with Xmas C/S° however produced significantly higher yield and P and S uptake than unfertilized pots and pots receiving Xmas C PR and S° alone. The two RPR/S° dry blends, namely NCPR/S° and NCPR/50% S-super, produced significantly lower yield and P and S uptake than the same two fertilizers granulated with water. The difference in yields and P and S uptake between blends and granulated forms increased with time. The recovery of fertilizer P and S by plants ranged from 3 to 35% and 2 to 45% respectively with the lowest recoveries for Xmas C PR alone and S° alone and the highest for SSP and SSP + S°. During the period of plant growth the percentage of S° oxidised from the S°, Xmas C/S° co-granule, NCPR/S° physical blend and NCPR/S° granular treatments were 37, 32, 32 and 45% respectively. Field evaluations, or use, of the co-granule should consider it's slow P and S release rates.     

Assessment of the relative agronomic effectiveness of phosphate rocks under glasshouse conditions

Six phosphate rocks (PRs) of varying reactivities were compared with monocalcium phosphate (MCP) in a glasshouse experiment growing perennial ryegrass (Lolium perenne cv. Nui) as the test plant on four soils of contrasting P sorption capacity and exchangeable Ca. The cumulative dry matter yield over 10 harvests showed a significant response to P application in all soils. Based on relative yield and P uptake, MCP was the most effective P fertilizer followed by the reactive phosphate rocks, which were superior to the unreactive rocks in all soils. The relative agronomic effectiveness (RAE) and substitution ratio (SR) of individual PR fertilizers, calculated with respect to MCP using the methods of vertical and horizontal comparison, respectively, were similar over a range of fertilizer rate. There was a decline or slight increase in the performance of PRs with time in the low P sorption soils but a consistent increase in the high P sorption soils. Some initial influence of exchangeable Ca content of the soils on the relative performance of PRs was also observed. Generally the PRs performed better in high P sorption soils than low P sorption soils and in low exchangeable Ca soils than high exchangeable Ca soils.     

Alleviating soil fertility constraints to increased crop production in West Africa: The experience in the Sahel

Lack of moisture limits crop production in semi-arid west Africa but poor soil fertility is a more serious constraint in the long run. Work done by IFDC and ICRISAT showed that phosphorus is the most limiting nutrient although response by millet to nitrogen when moisture and P are non-limiting can be substantial. A summary of the results of field trials in Niger showed that the sufficiency level for P estimated by regression analysis on the basis of Bray 1 extractable P is 7.9µg P/g of soil for 90% of maximum yield of millet. Application of 15–20 kg P/ha was usually adequate for optimum yields. Matam phosphate rock (PR) from Senegal, Tilemsi PR from Mali and Tahoua PR from Niger which are medium reactive were found to be suitable for direct application while partial acidulation (50% with sulfuric acid) of the less reactive phosphate rocks resulted in products with similar agronomic effectiveness as commercial superphosphates. Tests conducted by farmers showed that millet yields can be increased by more than 250% by the use of fertilizers. The need for constant monitoring of the crop needs for sulfur and potassium under continuous cultivation was highlighted.