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Item The development and assessment of alternative techniques to improve the agronomic value of Dorowa phosphate rock (Zimbabwe) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, School of Agriculture and Environment, College of Sciences, Massey University Palmerston North, New Zealand(Massey University, 2021) Tumbure, AkinsonAgronomically effective P fertilisers are unavailable to most smallholder farmers in Zimbabwe due to the high costs of manufacture and transportation. While phosphorus (P) deficiency remains widespread in these smallholder farming areas, farmers have limited options to ameliorate their soils leading to recurring food insecurity problems. The aim of this thesis was to develop alternative P sources with good agronomic value using locally available materials and alternative techniques such as thermal alteration, co-pyrolysis, and acid leaching. In-order to design alternative techniques, chemical and physical characterisation of the local Dorowa phosphate rock (DPR) was conducted. The DPR contained 89% and 3.5% apatite (hydroxy-fluorapatite) and calcite (CaCO₃) respectively and had a total P (TP) content of 16.5%. With less than 13.5% of TP soluble in 2% citric acid, DPR has limited agronomic value as a direct application phosphate rock. The cadmium (Cd) and fluoride (F) content in DPR was low at 0.16 mg kg⁻¹ and 2% respectively, indicating reduced F and Cd soil contamination issues in final fertiliser products. The DPR generally contained about half the amount of Fe, Al, Mg and K that is reported in literature and this was because the current sample contained less gangue materials. When compared to previous reports on DPR, the observed differences in the current sample were likely as a result of improvements in the mining and beneficiation process, or the current grade of the ore had a higher apatite content. To improve the agronomic value of DPR, the effect of thermal alteration of DPR in the presence of silicate materials (dunite, serpentine and recycled glass) was investigated. Sintering (heating at sub fusion temperatures) was chosen as a less energy intensive process compared to fusion. The sintered DPR mixtures (50% initial DPR content) had an increase of citric soluble P of up to 45, 53, and 73% when mixed with dunite, serpentine and recycled glass, respectively, compared to the unamended DPR. Increases in citric soluble P suggested isomorphous substitution of PO₄³⁻ in fluoro-hydroxyapatite by SiO₄⁴⁻ and or Mg²⁺/Na⁺ for Ca²⁺ and Fe²⁺. The sintered products that had high citric soluble P indicated that they might have improved agronomic value and were recommended for further testing in a glasshouse. Another technique where the DPR was added to maize stover residues (stems + leaves) and pyrolysed at 450 oC was developed and assessed for potential to improve the agronomic value of DPR. A suite of biochar-based fertilisers (BBFs) were obtained from pyrolysis of DPR + maize residues mixed at w/w ratios of 1:2, 1:4, 1:6, and 1:8 (DPR/ maize residues). Except for the 1:2 mixture, co-pyrolysis DPR with maize stover resulted in increases in biochar yield, carbon retention and nitrogen retention of at least 26, 43, and 26% respectively, compared to the pyrolysis of maize stover alone. The 1:6 and 1:8 mixtures produced biochar with more than a 30% increase in citric soluble P compared to the unamended DPR. The results showed that there was potential for on-farm co-pyrolysis of crop wastes with DPR to produce a P source with greater agronomic value. From these results, the 1:6 mixture that had 5.6% total P and 33.6% of the total P citric soluble, was recommended for testing in the glasshouse. The potential of using pyrolysis condensate as a cheaper acid source to recover P from DPR using sequential extractions was also evaluated. Before pyrolysis condensate could be used there was need to ascertain how much P could be recovered from DPR by the common organic acids; citric, acetic, and oxalic acids at various pH values, and then sequentially leached. Results showed that a suspension pH of 3 was necessary for maximum P recovery with citric and oxalic acids solubilising about 21.9 and 46.3% of the total P in DPR respectively, after 3 extractions. The greater P recovery under oxalic acid was attributed to the acid’s ability to remove Ca from solution as evidenced by the Ca:P molar ratio in oxalic acid leachates, which was at least 3 times less than that of other acids tested. Given this potential, a mixture of organic acids in pyrolysis condensate produced from maize stover were evaluated for their P recovery ability. Despite the high acidity and a pH of 3 maintained in leachates, sequential leaching extractions with the aqueous phase pyrolysis liquid over 26 hours was relatively ineffective, solubilising less than 14% of the total P in DPR. Four of the alternative P sources that were developed exhibited high potential agronomic value and were further evaluated for agronomic effectiveness in the glasshouse using broccoli and ryegrass as test crops. After 6 harvests, ryegrass that had been fertilised with DPR: biochar (1:6) or sintered DPR + recycled glass (50%), had similar P uptake and produced at least 95% of the biomass produced when monocalcium phosphate (MCP) was applied at the same citric soluble P rate. The same alternative P sources produced broccoli biomass yields and P uptake that was either comparable to or higher than when MCP was applied. The DPR co-pyrolysis biochar and recycled glass (50%) sintered P sources would provide a good option for smallholder farmers around the Dorowa area in Zimbabwe where PR is mined. However, larger scale field studies are recommended.Item An investigation of the agronomic value of fine grinding and granulating reactive phosphate rocks : a thesis presented in partial fulfilment of the requirements for the degree of Master of Horticultural Science in soil science at Massey University, New Zealand(Massey University, 1990) Officer, Sally JaneThe future trends in the use of reactive phosphate rocks in New Zealand may be dependent on improving the handling characteristics of these fine sand- and powder-like materials. Granulation of these materials has been suggested as one option. The effect of fine grinding and granulating reactive phosphate rocks on their agronomic performance was evaluated using a range of phosphate rocks, in laboratory studies and in field and glasshouse trials. North Carolina, Arad, Sechura and White Youssafia phosphate rocks, in forms normally imported into New Zealand (sand sized material, majority <2mm particle size), were characterised in terms of origin, composition, particle size, and solubility in 2% formic acid. In a 30 minute formic acid extraction of the imported material, White Youssafia phosphate rock at 44% solubility was found to be less reactive than the other phosphate rocks, which ranged from 47% to 55% in formic solubility In preliminary field trials a very finely ground North Carolina phosphate rock (100% <42µm particle size) was granulated with K2S04. The ungranulated phosphate rock, and granules of 0.5-1mm, 1-2mm and 2-4mm diameter, were evaluated on permanent pasture on the Tokomaru silt loam, using an inverse isotopic dilution technique in which the field soil, at the 1.5-6cm depth, was uniformly labelled with by a novel injection method. No plant yield response to fertiliser was observed but significant differences in herbage phosphate content and specific activity indicated a phosphate uptake response to fertiliser. Despite careful selection of areas of sward which had a similar plant content and vigour, the large variability in data from replicate treatments limited the amount of information which could be drawn from the results but the data indicated that the agronomic performance of the finely ground North Carolina phosphate rock was not limited by granulating to 0.5-1mm (mini-granules). A further range of granulation agents, including neutral salts, iii organic and mineral acids, their salts, and tallow, were tested for their ability to form strong mini-granules from unground North Carolina phosphate rock. The best granulation agent was a 1 :0.6 mixture of citric acid and magnesium sulphate, producing 0.5-1mm mini-granules which had an arbitrary crushing strength of 0.Skg/granule. The production of mini-granules involved pre-drying a phosphate rock/granulation agent slurry until it was just unsaturated, followed by cutting the wet mix through a 0.710mm seive, granulation at high speed for 30 seconds, and drying of the granules at 80°C for approximately 2 hours. This granulation process was then used to manufacture granules from unground Sechura and Arad phosphate rocks, as well as ground North Carolina and Arad phosphate rocks. Ground North Carolina phosphate rock was also granulated using tallow, by melting the fat and mixing in the phosphate rock, followed by setting the mix in a mould. Granulated materials, including a commercially prepared product ("Hyphos"), and ungranulated phosphate rocks (including White Youssafia), were evaluated in a glasshouse pot trial. The fertiliser was applied to the surface of pots of established "Nui" perennial ryegrass, with 7 harvests over three and half months. In general, at the common application rate of 60kgP/ha, the phosphate rock materials were never more than 70% as effective as mono calcium phosphate. The yeilds derived from unground, ungranlated Sechura, North Carolina, Arad and White Youssafia phosphate rocks were similar, the only significant difference being that the yield derived from Sechura phosphate rock·was greater than the yield derived from North Carolina phosphate rock. The effect of mini-granulation on agronomic performance varied with with the type and particle size of the phosphate rock used to make the granules. For example, mini-granulation of "as received" North Carolina and Sechura phosphate rocks caused no reduction in phosphate availability from these materials, however, mini-granulated "as received" and works ground Arad phosphate rock caused a significant reduction in phosphate availability. The agronomic performance of North Carolina phosphate rock was improved by grinding to less than 250µm in particle size but no further improvement occurred if the phosphate rock was more finely ground (<42µm particle size). The agronomic performance of Arad phosphate rock was not improved by grinding. The sequential fractionation of soil phosphate (1MNaOH followed by 1MHC1) indicated that only approximately 8% of the works ground North Carolina phosphate rock fertiliser had dissolved in the soil at the 5th harvest (10 weeks). A comparison of yields derived from pots fertilised with different rates of K2HPO4 sprayed onto chromite (whch had a similar particle size distribution to the unground phosphate rocks) indicated that the dissolved phosphate in the soil from the phosphate rock had a similar agronomic value to the K2 P04 . The low amount of phosphate rock dissolution and the absence of increased of yield response when works ground North Carolina phosphate rock was applied to soil at rates greater than 40 kgP/ha indicated that soil factors were limiting the dissolution of phosphate rock in this experiment. The extent of the limitation varied depending on the phosphate rock type and also the type of pot used (the black polythene bag used for the majority of treatments was enclosed in a galvanised steel cylindar for an inverse isotopic dilution experiment). The variable effects of grinding and granulation were attributed to the limitation of the phosphate rock dissolution. The type of granulation agent (including partial acidulation) had no significant effect on the agronomic performance of the granulated materials, except when tallow was used as a granulation agent and reduced the availability of works ground North Carolina phosphate rock. Unground White Youssafia phosphate rock requires further testing under more rigorous conditions before conclusions can be made about its agronomic availability. Two isotopic techniques were utilised in the glasshouse experiment in an attempt to quantify the extent of phosphate rock dissolution in the soil. The surfaces of some phosphate rock treatments were sprayed with a carrier free solution of P3 2 , and the inverse isotopic dilution technique used in the field was used again on some treatments. The use of labelled K2 H P 3 2 0 4 as a control for the surface labelled experiment provided sufficient information to allow differentiation of phosphate in the plant which was derived from soil and the fertiliser but the model developed could not be directly applied to results from the phosphate rock treatments. The dissolution of different forms of phosphate rock could not be compared using this labelling technique. The inverse isotopic dilution technique was re-evaluated in the glasshouse trial, by uniformly injecting the pots of ryegrass with a carrier free P 2 solution. The fertiliser treatments unpredictably stimulated uptake of labelled soil phosphate, so that the changes in herbage specific activity provided little meaningful information. These two unsuccessful attempts to derive quantitative information from the introduction of the P3 2 isotope into the phosphate rock soil-plant system demonstrated the difficulties involved in using isotopic dilution techniques to examine phosphate rock dissolution in field soils.Item An evaluation of Chatham Rise phosphorite as a direct-application phosphatic fertilizer : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University(Massey University, 1982) Mackay, Alec DonaldChatham Rise phosphorite (CRP) occurs as nodules on the sea floor some 800 km to the east of the South Island of New Zealand. The phosphate component is a carbonate fluorapatite and the material contains approximately 9% phosphorus (P) and 25% CaCO3. Several lines of evidence suggest that CRP has potential as a direct-application phosphatic fertilizer for pasture. In an initial evaluation in the glasshouse, CRP was found to be an effective source of P for ryegrass when compared to superphosphate over six harvests with four soils. The form (powdered or pelletised) and method (surface applied or incorporated) of application of CRP were found to have a marked effect on the agronomic effectiveness of this P source in the glasshouse. The effectiveness of CRP, when compared at 90% of the yield maxima obtained with superphosphate, which was assigned a value of 100, decreased in the order of powdered and incorporated (100 to 106) > powdered and surface applied (96 to 100) > pelletised and surface applied (85 to 104) > pelletised and incorporated (83 to 90). Results from a comprehensive, long-term field evaluation of CRP at four contrasting sites under permanent pasture over 3 years confirmed and extended the findings of the preliminary glasshouse study with CRP. Apart from some initial differences, pelletised CRP was as effective as superphosphate at all four sites and at two of the hill-country sites (Ballantrae and Wanganui) it showed a marked residual effect in the third year. This was particularly pronounced in the clover component of the sward at these two sites. In fact at these two sites a single, initial application of 70 kgP ha-1 as CRP was agronomically as effective in the third year as three annual applications of 35 kgP ha-1 as superphosphate. This finding has implications to the strategy of fertilizer use. The origin of the marked residual effect shown by CRP at Ballantrae and Wanganui in the third year appears to result from the effect of CaCO3 on the rate of release of P from CRP. The findings that pelletised CRP was almost always as effective as both powdered CRP and superphosphate in the field contrasts with the results of the preliminary glasshouse study with four soils. This discrepancy probably results from the fact that in glasshouse studies a number of factors which can operate in the field and which may contribute to an increased effectiveness of a surface-applied, pelletised phosphate rock (PR) material are excluded (e.g. earthworms). In a glasshouse study, earthworms increased the effectiveness of CRP as a source of P to ryegrass by 15 to 30% over seven hervests. Subsequent studies showed that both the burrowing and casting activity of earthworms indirectly increased the availability to ryegrass of P in the PR by improving the physical distribution and degree of contact of the PR particles with the soil. Interestingly, good agreement was found between the agronomic effectiveness of pelletised CRP in the field and in the glasshouse when earthworms were included as a treatment in the glasshouse. Consequently, care must be taken in extrapolating to the field situation, the results obtained with pelletised PR materials in the glasshouse in the absence of biological mixing. In a comparison in the glasshouse, using six soils and both ryegrass and white clover as indicator species, CRP was as effective as North Carolina phosphate rock (NCPR) and Sechura phosphate rock (SPR), both of which are reactive PR materials. The agronomic data from this glasshouse study were used to evaluate a number of conventional, single chemical-extraction procedures used for assessing the likely agronomic effectiveness of PR materials. Of these, 2% formic acid appears to offer the most promise. However, sequential extraction appears to be necessary with PR materials which contain appreciable amounts of CaCO3. A procedure involving a single extraction with 0.5M NaOH was developed for measuring the extent of dissolution of a PR in soil. Because apatite minerals are largely insoluble in dilute NaOH and because this reagent extracts sorbed inorganic P, increases in 0.5M NaOH-extractable P in a soil to which a PR is added, provide a good estimate of the amount of P dissolved and retained on sorption sites. The extent of dissolution of SPR, measured by NaOH extraction, was found to vary from 22% of added P on the low P-sorbing Tokomaru soil to 48% on the high P-sorbing Egmont soil during incubation at 15°C for 90 days. A high correlation (r = 0.935**) was obtained for the relationship between the dissolution of SPR, measured by NaOH extraction, and the P-sorption capacity of the six soils used. Whereas increasing the P status of the Wainui soil, by the addition of KH2PO4, had no measurable effect on the extent of dissolution of SPR, increasing addition of Ca(OH)2 markedly decreased the dissolution of SPR in this soil. Of the decrease measured in the dissolution of SPR on liming the Wainui soil from pH 5.2 to 6.9, 75-79% of the decrease could be accounted for by the effect of Ca, which also increases on liming. Recults with the Egmont soil indicate that a PR can dissolve at pH 6.5. This suggests that the effect of a higher pH on dissolution is decreased in a soil of high P-sorption capacity. Although the extent of dissolution or SPR increased as the P-sorption capacity of the soils increased, the amounts of water-, Bray-, and bicarbonate-extractable P in the same soils decreased. Of these three estimates of plant-available P, both the Bray and bicarbonate procedures were found to be useful indicators of short-term, plant-available P when SPR and CRP were added to three contrasting soils. Of the two procedures, the Bray procedure accounted for more of the variability, possibly reflecting the difference in the mechanisms by which these two extractants remove P from soil. In contrast, a single water-extraction procedure grossly underestimated the amount of short-term, plant-available P in the soil to which a PR was added. A simple model, based on a modified Mitscherlich equation, was developed to describe and predict the dissolution of SPR in soil. The model, which was developed and evaluated using contrasting soils, appears to have good practical application and should prove useful in future studies of the reactions of PR materials in soils. Although not yet commercially available, CRP appears to have very good potential as a direct-application P fertilizer for pasture and, of particular relevance to hill country farming, it shows a good residual effect. A possible disadvantage is the relatively low P content.