Effectiveness Of Reactive Phosphate Rocks Research Articles

Influence of rates of reactive phosphate rock and sulphur on potentially available phosphorous in organically managed soils in the south-eastern near-Mediterranean cropping region of Australia

Reactive phosphate rock (RPR) is the only phosphorous (P) fertiliser allowed for organically managed, broad-acre crop-pasture systems in southern Australia. However, soils are usually deficient in P, and the soils, climate, and plant species grown, do not promote extensive dissolution of RPR so the fertiliser is poorly effective for crop and pasture production. Biological oxidation of elemental sulphur (S) mixed and applied with RPR may sufficiently increase dissolution of P from RPR to improve its effectiveness as a P fertiliser. However, this needs to be confirmed in field studies in the region. Rates of RPR and S required to optimise dissolution of RPR are not known for the soils, environments, and agricultural systems used. Both pot and field studies showed that mixing RPR and S, and incorporating the mix into soil (top 10 cm for field studies), significantly increased Olsen P and soil solution P, even in strongly acidic soils (pHCa < 4.6). In general, Olsen P increased linearly with the applied rate of P up to 42–70 kg P ha−1 and the rate of change in Olsen P per unit of applied P increased with the applied rate of S up to 400 kg S ha−1. This interaction suggested that the effectiveness of RPR + S may be compromised by segregation of RPR and S. In addition, there was evidence that S application may not necessarily create a more acidic soil environment necessary for enhanced dissolution of RPR.

Developments in some aspects of reactive phosphate rock research and use in New Zealand

Summary. There has been over 50 years of use and research into the agronomic effectiveness of reactive phosphate rocks (RPR) directly applied to New Zealand pastures. In recent years RPR-carrying fertilisers made up about 16% of phosphatic fertiliser sales in the North Island of New Zealand. Most is applied, as maintenance fertiliser, to hill country sheep and beef farms. Use has been recommended on soils with pH &lt;6 and in annual rainfall regimes &gt;800 mm. This is based on the poor performance of Sechura phosphate rock in summer dry areas receiving &lt;750 mm of rainfall annually. Phosphate rocks that have more than 30% of their total phosphate soluble in 2% citric acid have been classed as ‘reactive’ and suitable for direct application. More recent research indicates that extraction with 2% formic acid, or a dissolution test performed in a simulated soil solution at a fixed pH, will provide improved measures of RPR quality. Field trials, undertaken by the New Zealand Ministry of Agriculture and Fisheries [MAF; now AgResearch Crown Research Institute (CRI)] and others, to evaluate the relative agronomic effectiveness of RPR versus soluble P fertilisers in adequate to marginally P-deficient soils have proven to be a painstaking task. Long periods (3–6 years) of fertiliser withdrawal were required for pasture growth on some soils to become significantly responsive to applied P. Only then did differences between P sources become significant. This problem has encouraged efforts to relate measurements of the extent of RPR dissolution in soils to their agronomic effectiveness. Three main modelling approaches have been used to achieve this objective: Kirk and Nye (1986a, 1986b, 1986c); Sinclair et al. (1993a); and Watkinson (1994b). These models are reviewed and their explanation of RPR dissolution in mowing trials tested. Components of each model have then been combined to produce models to predict the agronomic effectiveness of RPR. The development of P tests for soils receiving RPR-containing fertilisers is reviewed. Separate Olsen P test–yield response calibration curves are required for soils fertilised with soluble P fertilisers and soils fertilised with sparingly soluble P sources or soluble P in the presence of heavy lime applications. Whereas alkaline P tests such as Olsen or Colwell underestimate the amount of plant-available P in these soils, acid P tests such as Bray 1 are likely to overestimate the available P. Tests involving cation and anion exchange resin membranes appear to be more appropriate for soils with unknown histories of soluble P and RPR use and may permit the use of single calibration curves. Trends observed in Olsen P soil test values, from farms on the North Island of New Zealand that have a history (3–15 years) of RPR use are presented. A predictive dissolution model is used to explain these trends but it is evident that spatial and temporal variation in soil test results on farmers’ paddocks will be a major constraint to the precision to which this or similar models may be used. The model, however, may provide the basis for sound advice on the strategic use of RPR for direct application to New Zealand pasture soils. It may prove useful in explaining the variation in RPR effectiveness in a wider range of climates and soils.

The agronomic effectiveness of reactive phosphate rocks 3. A comparison of application strategies for soluble phosphorus and reactive phosphate rock fertilisers

Summary. An investigation into the effectiveness of large (up to 80 kg P/ha), single dressings (capital applications) of different phosphorus (P) fertilisers, compared with smaller annual applications, was undertaken in the National Reactive Phosphate Rock Project. Yield comparisons were made at 23 permanent pasture sites across Australia using triple superphosphate, the highly reactive North Carolina phosphate rock and a partially acidulated form of North Carolina rock. Over 4 years, 19 of 23 sites showed no significant reduction in mean annual pasture yield with a single, large dressing applied in year 1 only, compared with an equivalent amount of total P applied in 4 annual applications. At a site in North Queensland, where the initial soil P level was very low, the large year-1 application of each fertiliser resulted in increased pasture production over that obtained from smaller annual applications because of increased pasture production in the early years. This amounted to an annual increase of between 1500 and 3000 kg dry matter/ha for the capital application strategy. The effectiveness of capital applications depended on soil, pasture and climatic conditions, and on the type of P fertiliser. Soil and environmental factors which appear to be important in determining the effect of capital applications are whether soils allow leaching of P (a function of rainfall and texture), whether they are P-sorbing (a function of clay content and soil mineralogy), the soil P content and how quickly it is being utilised by the pasture. Capital applications can be considered where P is not leached from the soil profile or where P sorption is low, and are most effective where soil P is low and there is a responsive pasture species present. Capital applications of water-soluble P fertiliser should not be considered on free-draining, low P-absorbing soils. Average annual pasture dry matter losses of about 2000 kg/ha occurred with a capital application of triple superphosphate compared with annual applications, at one such site in Tasmania. North Carolina phosphate rock was found to be the most effective P fertiliser for large capital applications, especially on free-draining, low P-absorbing soils.

An appraisal of the National Reactive Phosphate Rock Project

Summary. An expert system has been developed, using the results from the National Reactive Phosphate Rock Project, to determine whether reactive phosphate rock is likely to be an effective substitute for water-soluble superphosphate fertiliser for a given pasture environment. The evaluation is made from site information [annual rainfall, pasture composition and the likelihood of phosphorus (P) leaching], and soil information (pH, Colwell P, soil colour and field texture). The expert system can determine the effectiveness of both highly reactive and moderately reactive phosphate rocks. Observed substitution values of triple superphosphate for the highly reactive North Carolina phosphate rock (ratio of the respective P levels required to produce 50% of the maximum observed yield response to triple superphosphate) were closely related to values predicted by the expert system (r = 0.92); the relationship between observed and predicted substitution values of single superphosphate for the moderately reactive Hemrawein phosphate rock was also close (r= 0.86). The expert system gives one of 4 different recommendations for reactive phosphate rock based on the magnitude of the predicted substitution values. These are ‘immediately effective’, ‘effective in the medium term’, ‘marginally effective’, and ‘not effective’. The system was validated using the results from independent field experiments that provided measures of the effectiveness of reactive phosphate rock at different pasture sites.

The agronomic effectiveness of reactive phosphate rocks 2. Effect of phosphate rock reactivity

Summary. The chemical reactivity of apatite phosphate rock (PR) from Sechura (Bayovar), North Carolina, Egypt (Hamrawein), Morocco (Khouribja) and Queensland (Duchess), was estimated using the following methods (indices): percentage of the phosphorus (P) present in the PR that is soluble in one extraction with 2% citric acid and 2% formic acid, and variation in crystal chemistry determined using X-ray diffraction to measure the length of the unit cell a dimension. The indices were related to the agronomic effectiveness of the PRs for pasture using percentage of the maximum (relative) yields for PR and superphosphate. At most sites there was a statistically significant trend for the agronomic effectiveness of the PRs to increase with increasing reactivity of the PRs as assessed by the 3 indices. However, this was not the case for sites where: (i) leaching of water-soluble P from freshly-applied superphosphate occurs in wet years; and (ii) the agronomic effectiveness using SV 50 values of all the PRs was uniformly low so that no differences between the PRs could be detected. The relationship between the agronomic effectiveness (SV50) of PR and chemical reactivity of the PR were generally different each year and in most cases this variability was related to the variation in total annual rainfall. From this study it is concluded that PRs in which greater than 65–70% of their total P content is soluble in 2% formic acid are likely to be highly reactive and therefore agronomically effective P fertilisers for direct application to appropriate soils in suitable environments.

Building an expert system to advise on the use of reactive phosphate rock on Australian pastures

Summary. An expert system has been developed, using the results from the National Reactive Phosphate Rock Project, to determine whether reactive phosphate rock is likely to be an effective substitute for water-soluble superphosphate fertiliser for a given pasture environment. The evaluation is made from site information [annual rainfall, pasture composition and the likelihood of phosphorus (P) leaching], and soil information (pH, Colwell P, soil colour and field texture). The expert system can determine the effectiveness of both highly reactive and moderately reactive phosphate rocks. Observed substitution values of triple superphosphate for the highly reactive North Carolina phosphate rock (ratio of the respective P levels required to produce 50% of the maximum observed yield response to triple superphosphate) were closely related to values predicted by the expert system (r = 0.92); the relationship between observed and predicted substitution values of single superphosphate for the moderately reactive Hemrawein phosphate rock was also close (r= 0.86). The expert system gives one of 4 different recommendations for reactive phosphate rock based on the magnitude of the predicted substitution values. These are ‘immediately effective’, ‘effective in the medium term’, ‘marginally effective’, and ‘not effective’. The system was validated using the results from independent field experiments that provided measures of the effectiveness of reactive phosphate rock at different pasture sites.

The agronomic effectiveness of reactive phosphate rocks 5. The effect of particle size of a moderately reactive phosphate rock

Summary. The effect of particle size on the agronomic effectiveness of a moderately reactive phosphate rock (from the Hamrawein deposit in Egypt) was investigated at 4 of the field sites in the National Reactive Phosphate Rock Project. The aim of these experiments was to determine whether the agronomic effectiveness of this fertiliser was increased by removing particles greater than 0.5 mm in diameter (these coarser particles constituted 28% of the fertiliser). The more reactive North Carolina phosphate rock was used as a reference fertiliser. Soils from 3 of the field sites were also used for glasshouse experiments. The forms of Hamrawein phosphate rock used in these experiments were the less than 0.50 mm particle size fraction (as was used in the field experiments), the 0.075–0.15 mm fraction, and the less than 0.50 mm particle size fraction finely ground. Subterranean clover was used as the test plant and North Carolina phosphate rock and monocalcium phosphate were used as reference fertilisers. A series of laboratory experiments were also undertaken to investigate the effect of particle size. The agronomic effectiveness of Hamrawein phosphate rock was not affected by the different particle size treatments used in the field and glasshouse experiments. The Hamrawein phosphate rock was about equal in effectiveness to North Carolina phosphate rock at the high rainfall leaching site that had acidic sandy soil, and was less effective than North Carolina phosphate rock at the other 2 field sites. The Hamrawein phosphate rock was similar in effectiveness to North Carolina phosphate rock and monocalcium phosphate in the glasshouse experiments with the 2 low P-sorbing soils and was much less effective than North Carolina phosphate rock and monocalcium phosphate in the high P-sorbing soil. Laboratory studies found that there were minimal differences in the amount and rate of dissolution of different particle size fractions of the Hamrawein phosphate rock in moist incubated soil. Free carbonate, most likely in the form of dolomite, was found in all particle size fractions of the Hamrawein phosphate rock and was particularly concentrated in the particle size fractions less than 0.15 mm. This fine dolomite fraction may have suppressed the extent that Hamrawein phosphate rock dissolves in soil, and may have negated any potential beneficial effect that reducing particle size by sieving or by grinding may have had on the agronomic effectiveness of Hamrawein phosphate rock in the field and glasshouse experiments.

The agronomic effectiveness of reactive phosphate rocks 1. Effect of the pasture environment

Summary. The agronomic effectiveness of directly applied North Carolina reactive phosphate rock was determined for 4 years from annual dry matter responses at 26 permanent pasture sites across Australia as part of the National Reactive Phosphate Rock Project. Fertiliser comparisons were based on the substitution value of North Carolina reactive phosphate rock for triple superphosphate (the SV50). The SV50 was calculated from fitted response curves for both fertilisers at the 50% of maximum yield response level of triple superphosphate. The reactive phosphate rock was judged to be as effective as triple superphosphate in the 1st year (and every year thereafter) at 4 sites (SV50 &gt;0.9), and was as effective by the 4th year at 5 sites. At another 9 sites the reactive phosphate rock was only moderately effective with SV50 values between 0.5 and 0.8 in the 4th year, and at the final 8 sites it performed poorly with the 4th year SV50 being less than 0.5. Pasture environments where the reactive phosphate rock was effective in the 1st year were: (i) those on sandy, humic or peaty podsols with an annual rainfall in excess of 850 mm; (ii) those on soils that experienced prolonged winter inundation and lateral surface flow; and (iii) tropical grass pastures in very high rainfall areas (&gt;2300 mm) on the wet tropical coast on North Queensland. The highly reactive North Carolina phosphate rock became effective by the 4th year at sites in southern Australia where annual rainfall exceeded 700 mm, and where the surface soil was acidic [pH (CaCl2) &lt;5.0] and not excessively sandy (sand fraction in the A1 horizon &lt;67%) but had some phosphorus (P) sorption capacity. Sites that were unsuitable for reactive phosphate rock use in the medium term (up to 4 years at least) were on very high P-sorbing krasnozem soils or high P-sorbing lateritic or red earth soils supporting subterranean-clover-dominant pasture, or on lower rainfall (&lt; 600 mm) pastures growing on soils with a sandy A1 horizon (sand component &gt;84%). No single environmental feature adequately predicted reactive phosphate rock performance although the surface pH of the soil was most closely correlated with the year-4 SV50 (r = 0.67). Multiple linear regression analysis found that available soil P (0–10 cm) and the P sorption class of the surface soil (0–2 cm), together with annual rainfall and a measure of the surface soil"s ability to retain moisture, could explain about two-thirds of the variance in the year-4 SV50 . The results from this Project indicate that there are a number of specific pasture environments in the higher rainfall regions of Australia where North Carolina reactive phosphate rock can be considered as an effective substitute P fertiliser for improved pasture.

The agronomic effectiveness of reactive phosphate rocks 4. Early season lag in herbage production when reactive phosphate rock is used as a pasture fertiliser

Summary. The agronomic effectiveness of highly reactive North Carolina phosphate rock (NCPR), relative to triple superphosphate (TSP), was determined for individual harvests at 26 permanent pasture sites in the National Reactive Phosphate Rock Project from 1992 to 1995. The aim was to determine whether the performance of NCPR relative to TSP was consistent over the growing season. Poor performance of NCPR early in the growing season (early seasonal lags), and its subsequent improvement later in the season, was observed at 15 of the 26 sites in this project and for 20 of the 103 possible site x year combinations. At 3 additional sites the apparent early season lag in NCPR performance was followed by harvests where there was no phosphorus (P) response. At one P-leaching site with a long growing season the relative yields from NCPR plots increased as the year progressed and exceeded those from TSP late in th e season. The duration of the early season lag phase was associated with the rate of build-up in plant-available P over the 4-year duration of the trials. The lag disappeared after 1 or 2 years at sites with low P-sorbing, sandy soils where there was a significant build-up in Colwell P, but persisted for 4 years on high P-sorbing soils where limited increases in Colwell P occurred. The average early season yield reduction associated with use of NCPR, compared with using TSP, was 28%. Colwell soil test values alone did not provide a good indication of the likelihood of a seasonal effect occurring. The occurrence of an early season lag phase needs to be taken into account by farmers considering the use of RPR as reductions in feed supply at this time of the year have the potential to reduce stocking rates and farm profitability. The use of partially acidulated NCPR, or large applications of NCPR applied in the first year, were management strategies that minimised the occurrence of these seasonal effects. The suggestion that the early season lag effect is due to the inability of NCPR to meet P demands from newly establishing pasture legumes, and increased P requirements by pasture plants in cold conditions is also discussed.

The long-term effectiveness of reactive phosphate rock as a phosphate fertiliser for New Zealand pastures

Sechura reactive phosphate rock (SPR) and triple superphosphate (TSP) have been compared as phosphate fertihsers for pastures in a series of 19 field trials dispersed throughout New Zealand, each continuing for 3-6 years. Both fertilisers were applied annuaIly at 4 rates. In the first 2 years SPR was much less effective at increasing dry matter yields than TSP, but its performance improved markedly with time. In year 6 SPR outyielded TSP at alI application rates for all sites combined. The relative performance of SPR and TSP differed markedly at different sites. Very high soil phosphate retention and a combination of low moisture and high pH appeared to be unfavourable for SPR performance. In most sites SPR was equal or superior to TSP by year 6. SPR substantially raised molybdenum concentration in clovers, and this may have contributed to its superior performance at some sites. As TSP and single superphosphate were equally effective, the good performauce of SPR and the relative cheapness of reactive phosphate rocks suggest an important role for the latter on New Zealand pastures. Keywords Sechura phosphate rock, reactive phosphate rock, RPR, superphosphates, phosphate fertilisers