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
The 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,
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.