Reactive Phosphate Rock Research Articles

Effect of serpentine rock and its acidulated products as magnesium fertilisers for pasture, compared with magnesium oxide and Epsom salts, on a Pumice Soil. 1. Dry matter yield and magnesium uptake

Low pasture magnesium (Mg) intakes by grazing dairy and beef cows is a main cause of hypomagnesaemia in these animals soon after calving. Magnesium fertiliser application to pasture can increase pasture mixed‐herbage (hereafter “herbage") Mg concentration and reduce the risk of this disorder. However, the effectiveness of Mg fertilisers is variable depending on fertiliser type and soil properties. A 32‐month Mg fertiliser field trial was conducted on dairy pasture on an Immature Orthic Pumice Soil (near Taupo, New Zealand; exchangeable Mg 0.4 cmolc kg–1, pasture Mg 0.13%) to compare the effectiveness of a range of Mg fertilisers in elevating herbage Mg concentration. The fertilisers used were ground serpentine rock, three acidulated serpentine rock products, Epsom salts, and E‐mag (magnesium oxide). All fertilisers were applied at 100 kg Mg ha–1 at the start of the trial. The trial also had two treatments where Epsom salts was applied at 25 and 50 kg Mg ha–1 and a treatment where serpentine rock was reapplied during the second and third year at 100 kg Mg ha–1. Magnesium fertiliser treatments did not affect pasture dry matter yield significantly at any harvest, however, the effect of treatments on herbage Mg concentration was highly significant (P < 0.001) at all harvests. The single application of serpentine significantly increased herbage Mg concentration at 10 harvests, with five of these harvests having Mg concentrations ≥ 0.20%, which is generally considered to be the minimum “threshold level” for stock health. In comparison, the Mg concentration in pasture not receiving Mg fertiliser remained below the minimum threshold level throughout the trial. Application of serpentine rock annually further increased herbage Mg concentration and accumulation. The herbage Mg concentrations for this treatment were similar to those of the Epsom salts (50 kg Mg ha–1) treatment at the majority of the harvests. These results indicate that the annual application of serpentine rock may have potential as an alternative strategy to the application of more soluble Mg sources for raising herbage Mg concentration to levels considered adequate to meet stock health requirements. The more soluble Mg fertilisers—Epsom salts, the two serpentine‐superphosphate products, and an acidulated serpentine rock plus reactive phosphate rock product—had the greatest effect on increasing herbage Mg concentration and accumulation. These treatments increased herbage Mg concentration to ≥0.20% within 3–6 months after application and for the majority of harvests over the rest of the trial period. The effect of E‐mag on herbage Mg concentration was similar to that of serpentine rock during the first year, but the subsequent performance of E‐mag was similar to the more soluble Mg fertilisers.

Alternative fertilisers and management to decrease incidental phosphorus loss

Incidental phosphorus loss is a concern for surface water quality. Here we showed that the risk of incidental P loss can be minimised, even from highly soluble superphosphate fertiliser, by timing application when overland flow is unlikely. Moreover, we demonstrated that the risk of incidental P loss can be estimated from water solubility, decreasing the need for expensive field trials. As such, we suggest that slowly available fertilisers such as reactive phosphate rock or serpentine super could be used in situations where incidental losses need to be decreased and conditions are suitable e.g., soil pH less than 6 and rainfall greater than 800 mm for reactive phosphate rock.

Phosphate sources for pasture production on summer‐dry soils in eastern New Zealand

In the period 1988–96, staff of Ravensdown Fertiliser Co‐operative Ltd undertook a series of pastoral trials with the primary purpose of examining the performance of commercially and locally available fertiliser products containing various forms of phosphorus (P) and sulphur (S). Treatments included Superphosphate (SSP), Longlife Super, two partially acidulated phosphate rocks (PAPRs), and North Carolina reactive phosphate rock (NCPR), all containing where required additional sulphur (as elemental S, So). These were applied annually for 6 years to supply the current requirements for P and S at each site. The sites were predominantly in eastern New Zealand. This paper covers 12 pastoral sites representing the summer‐dry Pallic soils (yellow‐grey earths and their intergrades to yellow‐brown earths). In mowing trials, 10 sites responded significantly in dry matter production to the commercial fertilisers; the remaining two sites had high initial Olsen P values or did not last 6 years. Superphosphate, Longlife Super, and the PAPR products Hyphos Supreme and Phospar/So produced significantly more dry matter than the control in all years, as did NCPR/So from year 2. Superphosphate gave the highest mean dry matter in all years, producing significantly more dry matter than NCPR/So in all years, Longlife Super in year 4, and the PAPR treatments in years 2–4. By year 5, the Longlife Super and PAPR treatments had reached 80–90% and the NCPR/So 60–70% of the relative effectiveness of SSP. There was some variation in the relative performance of individual fertiliser treatments with each site, however the causes could not be clearly isolated due to the interaction of climate on P and/or S plant availability. Results suggest NCPR/ So treatments were not suitable for these soils, due to their inability to release P quickly, and the high requirement for S on these soils. Longlife Super and PAPR treatments performed well but only on some sites. Currently they are not cost effective compared with SSP. Rainfall, in particular summer rainfall, was considered the most important driver of the treatments in releasing P (and S) on these soils.

Grain mineral concentrations and yield of wheat grown under organic and conventional management

AbstractOn the low‐P soils in southeastern Australia, organic crops differ from conventional ones primarily in the use of relatively insoluble, as opposed to soluble, P fertilisers and in the non‐use of herbicides. As organic management, particularly elimination of soluble fertilisers, is often claimed to enhance grain mineral concentrations, we examined grain from wheat on paired organic and conventional farms in two sets of experiments: (1) four pairs of commercial crops (1991–1993); and (2) fertiliser experiments on one farm pair where nil fertiliser was compared with 40 kg ha−1 of P as either relatively insoluble reactive phosphate rock or more soluble superphosphate (1991 and 1992). All wheat was grown following a 2–6 year legume‐based pasture phase. Both conventional management and the superphosphate treatment greatly increased yields but reduced colonisation by mycorrhizal fungi. While only minor variations occurred in grain N, K, Mg, Ca, S and Fe concentrations, conventional grain had lower Zn and Cu but higher Mn and P than organic grain. These differences were ascribed to: soluble P fertilisers increasing P uptake but reducing mycorrhizal colonisation and thereby reducing Zn uptake and enhancing Mn uptake; dilution of Cu in heavier crops; and past lime applications on the organic farm decreasing Mn availability. These variations in grain minerals had nutritional implications primarily favouring the organic grain; however, organic management and, specifically, elimination of soluble fertilisers did not induce dramatic increases in grain mineral concentrations. In addition, organic management was coupled with yield reductions of 17–84 per cent due to P limitation and weeds. The impact of large regional variations in the characteristics of organic and conventional systems on the general applicability of the results from this study and other similar studies is discussed. Copyright © 2004 Society of Chemical Industry

Potential phosphorus losses in overland flow from pastoral soils receiving long‐term applications of either superphosphate or reactive phosphate rock

The forms and concentrations of P in overland flow were measured from intact pastoral soils obtained from the Winchmore long‐term P fertiliser trial. Treatments under evaluation were soils that received either 0, 188, 250 or 376 kg superphosphate ha−1 yr−1, or 175 kg reactive phosphate rock (RPR) ha−1 yr−1. The objective was to determine the magnitude of potential P transfers from soil to water following P fertilisation, and to determine if losses were different following RPR fertilisation compared with superphosphate. Overland flow was induced by the application of artificial rainfall at 15 mm h−1, maintained for 1 h after flow commenced. Concentrations of dissolved reactive P (DRP) and total P (TP) mirrored the long‐term application rates, although prior to a fresh application of P, soils with P applied in RPR form lost more P during an event than soils with the same rate of P applied as superphosphate. After a fresh application of RPR and superphosphate treatments, up to 5.4 mg TP litre−1 was lost in flow from the 376 kg superphosphate ha−1 yr−1 treatment, while P in flow from soils fertilised with RPR were commonly c. 0.11 mg litre−1, but still greater than from the unfertilised control soils (0.02 mg litre−1). Regression analysis indicated that DRP concentrations in flow from the fertilised soils were elevated above that lost before fertiliser application for a period of approximately 60 days. These results support earlier studies that demonstrate the greater risks of incidental P losses from soluble P fertilisers such as superphosphate (up to 60 days), and conversely the potential environmental benefits from RPR fertilisation of soils “at‐risk” of P loss (e.g., where much overland flow occurs such as in very wet soils and near stream channels). However, if good management practice is followed then the difference in P loss between superphosphate and RPR treated soils should be minimal over a period of a year.

Evaluation of Rape Response to Different Sources of Phosphate Rock in an Alkaline Soil

A greenhouse experiment was conducted to evaluate the agronomic effectiveness of one unground, as received from Tunisia, and eight finely ground phosphate rock (PR) sources from Syria, Kazakhstan, Jordan, Russia, Mali, Burkina Faso, Tanzania, and China as compared with TSP for rape grown on an alkaline soil (pH 7.7). The P rates applied were 0, 5, 10, 25, and 50 mg P kg−1. The rape was grown to maturity. The yield of rapeseed obtained with each P source was fitted to a linear function with P rate. The relative agronomic effectiveness (RAE) of each PR with respect to TSP was calculated as the ratio of the slopes of the response function. The results show that RAE ranged from 0 to 88%, depending on the reactivity of PR. RAE correlated highly with the solubility of all PR sources as measured by 2% citric acid (CA) (R2=0.947) in a semi-log function. When the unground, highly reactive PR was excluded in the correlation, R2 was increased to 0.957 with 2% CA. The correlation was also very high (R2=0.963) when the RAE of the unground PR was plotted against its solubility measured in the finely ground form along with other finely ground PR sources. Low-reactivity PR with 2% CA-soluble P as low as 2.5% P can be 50% as effective as TSP in increasing yield of rapeseed even in alkaline soils. Use of indigenous PR sources, especially high reactive, for rapeseed production is feasible in those countries where agricultural soils are alkaline.

Effect of low-molecular weight organic acids on phosphorus release and phytoavailabilty of phosphorus in phosphate rocks added to soils

Phosphorus (P) is critically needed to improve soil fertility in many parts of the world. The use of water-soluble P fertilizers, e.g., single super-phosphate and triple super-phosphate in developing countries to improve crop production has been limited primarily by their high cost. The presence of indigenous phosphate deposits in some countries provides an incentive for direct application or local chemical treatment at low cost to improve the solubility of low reactive phosphate rocks (PRs). The use of naturally occurring low-molecular weight organic acids (LMWOAs) that are produced in soil as microbial metabolites or plant exudates from dead or living cells represents a new perspective in PR research. The LMWOAs contain various functional groups that may play a significant role in PR dissolution. Little information is available, however, about the potential of LMWOAs in releasing P from PRs. This study reports P release from 12 PRs, four each of low, medium, and high reactivity, obtained from various deposits (Kodjari, Tahoua, North Carolina, Gafsa, Khouribga, Tilemsi Valley, Central Florida, Sechura, Minjingu, North Florida, Hahotoe, and Parc W) using nine LMWOAs containing mono-, di-, and tri-carboxylic groups and a mineral acid (H 2SO 4). Laboratory studies showed that the organic acids are effective in releasing P from low and medium reactive PRs, but very ineffective in releasing P from high reactive PRs. The average amounts of P released by all the organic acids from the three types of PRs were 65.5 mmol kg −1 PR from the low reactive PRs, 55.1 mmol kg −1 from the medium, and 11.1 mmol kg −1 from the high; those released from across all the PRs were 21.9 by the mono-carboxylic acid group, 54.2 by the di-carboxylic acid group, and 57.0 mmol P kg −1 by the tri-carboxylic acid group. The P released was negatively correlated with the equilibrium pH, but positively correlated with Ca released from the PRs. Laboratory incubation studies on the release of P from PRs added to soils with or without organic acids and incubated at 25 °C for 15, 30, and 45 days showed that the percentage of plant-available P released varied considerably from 0.95 in the Kodjari PR to 40.1% in the North Carolina PR and was related to PR reactivity. A greenhouse study with corn ( Zea mays L.) grown for 60 days on soils treated with PRs or with PRs mixed with organic acids showed that corn response to addition of oxalic or citric acid varied with P rates and PR sources, suggesting that organic acids have potential as amendments for increasing plant-available P in PR-treated soils.

Herbage Productivity and Botanical Composition of Hill Pasture as a Function of Clipping and Site Features

Complex topography and varied soil of hill‐land pastures create microsite conditions that support an array of floristic associations and herbage production patterns. This complicates management for forage and livestock production because the seasonal distribution and quantity of forage vary. Our objective was to determine herbage production and floristic composition of a hill pasture as a function of site characteristics and canopy management. An existing 3‐ha hill pasture watershed was oversown with white clover (Trifolium repens L.) and orchardgrass (Dactylis glomerata L.) and fertilized with reactive phosphate (PO4) rock (PR). Replicated plots on each of four sites were clipped once (stockpiled), twice (hay harvest), or three times (long rotation) annually. Site had a significant impact on cumulative herbage production, whereas the influence of clipping was mixed. The least (1.9 Mg ha−1) amount of herbage production in a given season occurred on a northeast (NE)‐facing site and the greatest (4.6 Mg ha−1) in a natural drainage area (ND) traversing the pasture. Herbage production increased by about 80% with overseeding and PR, but the relative ranking of production among sites stayed the same. Botanical composition was also strongly influenced by site, with velvetgrass (Holcus lanatus L.) predominating in ND and red fescue (Festuca rubra L.) occurring primarily on the NE site. The stockpiled treatments became dominated by grasses and weeds 4 yr after treatments were imposed, regardless of site, and were similar to the least productive site (NE‐facing) in the pasture. Our results suggest that application of amendments to the more productive portions of a site are likely to have greater return.

Zeolite/rock phosphate—a novel slow release phosphorus fertiliser for potted plant production

A glasshouse study was undertaken to determine if the zeolite mineral clinoptilolite from an Australian deposit in combination with rock phosphate (RP) could significantly enhance the uptake of P by sunflowers. The zeolite/RP combination was intended to act as an exchange-fertiliser, with Ca 2+ exchanging onto the zeolite in response to plant uptake of nutrient cations (NH 4 + or K +) enhancing the dissolution of the RP. A reactive RP (Sechura) and a relatively non-reactive RP (Duchess) were examined. Zeolite was used in Ca 2+-, K +- and NH 4 +-saturated forms at ratios of 3.5:1 and 7:1 with RP; Ca 2+-zeolite was considered the control, with exchange-induced dissolution possible from K +- and NH 4 +-zeolite. The zeolite/RP mixture was applied as a vertical band adjacent to the sunflower seedling. In addition, N was supplied as urea in an effort to determine if RP dissolution resulted from H + release by nitrification. Phosphorus supply from the zeolite/RP system was compared with an available P source (KH 2PO 4). The experiment clearly demonstrated greatly enhanced plant uptake of P from RP when applied in combination with NH 4-zeolite, though the P uptake was lower than that from the soluble P source. The zeolite/RP interaction was much more effective with the reactive RP than the non-reactive material. Within the NH 4 +-zeolite/RP band, root proliferation was greatly increased, as would be expected in an exchange-fertiliser system. The K +-zeolite system did not produce a significantly greater yield than the Ca 2+-zeolite control, probably because adequate K + supply from the basal application reduced uptake within the zeolite/RP band, thus reducing the extent of exchange-induced dissolution. Nevertheless, increased root proliferation within the band was observed, implying that exchange-induced dissolution may also be possible from this system. The zeolite/RP system offers the considerable advantage of P release in response to plant demand and is unique in this regard.

Long-term field evaluation of phosphate rock and superphosphate use strategies in acid soils of Hungary: Two comparative field trials

The effect of two P-forms and the P fertilization system were studied in field trials set up on two moderately acidic Hungarian soils. Reactive Algerian rock phosphate and Kola superphosphate doses were based on the phosphorus equivalence. The experimental design makes it possible to compare the effect of annual 35 kg/ha P doses with initial one-time application of the 175 kg/ha P level in a five-year interval. Ammonium-lactate (AL)-, NaHCO3 (Olsen)- and DW-P contents as well as Lakanen- Ervio (LE)- soluble Cd, Cr and Sr contents were also determined. The results of the first five-year period are reported in the paper. Responses to P fertilization were related to the original P supply of the soils. There was no significant difference between the two P forms and between the P fertilization systems on both grain yield and P-uptake. While AL- method overestimated, and Olsen-method – on the other hand – underestimated the P supply of reactive Algerian rock phosphate, distilled water (DW)-soluble P contents indicated the soil P status more accurately. Phosphorus balances were positive after the fifth year of the trials in the P treated plots. The soluble Cd and Cr contents did not increase in the Algerian rock phosphate treated plots. On the other hand, Kola superphosphate application at 175 kg/ha P level resulted in higher LE-Sr contents in soils. The Algerian rock phosphate is an economic alternative P source on the moderately or strongly acidic Hungarian soils.

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.

Herbage Productivity and Botanical Composition of Hill Pasture as a Function of Clipping and Site Features

Complex topography and varied soil of hill-land pastures create microsite conditions that support an array of floristic associations and herbage production patterns. This complicates management for forage and livestock production because the seasonal distribution and quantity of forage vary. Our objective was to determine herbage production and floristic composition of a hill pasture as a function of site characteristics and canopy management. An existing 3-ha hill pasture watershed was oversown with white clover (Trifolium repens L.) and orchardgrass (Dactylis glomerata L.) and fertilized with reactive phosphate (PO4) rock (PR). Replicated plots on each of four sites were clipped once (stockpiled), twice (hay harvest), or three times (long rotation) annually. Site had a significant impact on cumulative herbage production, whereas the influence of clipping was mixed. The least (1.9 Mg ha−1) amount of herbage production in a given season occurred on a northeast (NE)-facing site and the greatest (4.6 Mg ha−1) in a natural drainage area (ND) traversing the pasture. Herbage production increased by about 80% with overseeding and PR, but the relative ranking of production among sites stayed the same. Botanical composition was also strongly influenced by site, with velvetgrass (Holcus lanatus L.) predominating in ND and red fescue (Festuca rubra L.) occurring primarily on the NE site. The stockpiled treatments became dominated by grasses and weeds 4 yr after treatments were imposed, regardless of site, and were similar to the least productive site (NE-facing) in the pasture. Our results suggest that application of amendments to the more productive portions of a site are likely to have greater return.

The influence of soil properties on the effectiveness of phosphate rock fertilisers

Eighteen soils from south-western Australia were used to measure the effectiveness of 2 reactive phosphate rocks (RPR) [North Carolina and Sechura (Bayovar) RPRs] relative to superphosphate (relative effectiveness, or RE) using (i) yield of dried shoots of 30-day-old wheat plants (REyield ), (ii) P content of the dried shoots (RE Pcontent ), and (iii) bicarbonate soil test P (RE bicP ) as measures of effectiveness. The RE values were positively related to PR dissolution in soil, titratable acidity, and to P and Ca retention by soil, and were negatively related to soil pH. No single soil property adequately predicted REyield , RE Pcontent, or RE bicP . Stepwise multiple regression indicated that: (1) PR dissolution, soil pH, and organic carbon together accounted for about half the variation in RE yield ; (2) organic C, soil pH, and titratable acidity together accounted for about 67% of the variation in RE Pcontent ; (3) PR dissolution, P retention capacity, and titratable acidity together accounted for about 71% of the variation in RE bicP . We conclude that the agronomic effectiveness of phosphate rock fertilisers is a consequence of complex interactions of phosphate rock with soil that cannot be adequately predicted by measurements of a single soil property.

Comparison of behavior of natural and synthetic phosphate fertilizers in moderately water‐repellent sandy loam soil under winter rainfall

This study compares the mobility of phosphate‐phosphorus (PO4‐P) and total P derived from two commercial phosphate blends, namely a naturally reactive rock phosphate containing 12.5% P, and an synthetic phosphate containing 9% P, in small, undisturbed soil cores extracted from a paddock in a dairy farm approximately 35 km southeast of Warmambool in the Western Region of Victoria, Australia. Since both fertilizers contained major ions, the transport of sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg) were also studied. The farm's soil, a water repellent, acid, dark brownish grey, sand loam is typical of many found in this rich beef and dairy region, and is used for pasture. Our results suggest that there was little difference in chemical dynamics, and by inference chemical bioavailability and ecological safety, between the two commercial phosphate materials.

Determination of residual reactive phosphate in soil

Soil P tests are not reliable for the estimation of soil phosphorus status where reactive phosphate rock (RPR) has been used as a direct application fertiliser. This is because the soil tests used either do not include the residual RPR in the soil, or do not take into account the influence of soil and environmental factors on the rate of RPR dissolution in the soil. An alternative approach is use of a RPR dissolution model that incorporates effects of the fertiliser, soil and environmental characteristics. The model requires an estimate of residual RPR in soil accumulated from previous fertiliser additions. This paper proposes an analytical method for this that does not have the complications of the P fractionation procedures normally used. Extraction of soil with a pH 4 buffer enables differentiation between soil fluorapatite‐P and RPR‐P. In the proposed method this extraction is followed by addition of NaOH to make the suspension alkaline. A parallel sample of the same soil is treated similarly, but without the pH 4 buffer extraction to provide a correction for non‐apatite P extracted. Dissolved RPR‐Ca and exchangeable Ca are complexed with citrate to prevent precipitation of P during the alkaline extraction. Using this method small background levels (0–30 μg P g‐1 soil), due to slight dissolution of soil fluorapatite, were found for most soils. 100% of the most reactive RPRs were recovered from soil/RPR mixtures.

Agronomic effectiveness of partially acidulated phosphate rock fertilizers in an allophanic soil at near‐neutral pH

The agronomic effectiveness of five partially acidulated phosphate rocks (PAPRs) and an unground phosphate rock (PR) were compared against single superphosphate (SSP) in a glasshouse experiment using a high phosphorus (P) retention soil at a near‐neutral pH (pH 6.5), and corn (Zea mays L.) as the test crop. The PAPRs were prepared by acidulating unground North Carolina PR with either phosphoric or sulphuric acid (expressed as Phos‐PAPR and SA‐PAPR, respectively) and at three levels of acidulation (20, 33, and 50%). The relative agronomic effectiveness (RAE) and substitution value (SV) of the test fertilizers, calculated with respect to SSP using the standard “vertical”; and “horizontal”; comparisons, showed that 50% phosphoric acidulated PAPR performed as effectively as SSP whereas the other fertilizers were less effective. The PR treatment showed a small yield response. The dry matter yield and P uptake were linearly related to water‐soluble P of the fertilizers up to 66% of total P and there was no advantage in acidulating fertilizers above this level of water‐soluble P using reactive PR. Whereas very little of the directly‐applied PR dissolved (3.4% of PR applied), PR applied as a component of PAPRs dissolved up to 22%. The dissolved proportion of added PR component increased with increasing water‐soluble P content of the fertilizer. The results suggest a greater efficiency of PAPR than SSP as a P supplier to plants.

Comparison of phosphate rock and triple superphosphate on a phosphorus‐deficient Kenyan soil

Soil phosphorus (P) deficiency is a constraint to crop production in many regions of sub‐Saharan Africa, which could be overcome through use of either soluble P fertilizer or sufficiently reactive phosphate rock (PR). A field study was conducted with corn (Zea mays L.) for three growing seasons (18 months) on a P‐deficient, acid soil in Kenya to compare a soluble P source (triple superphosphate, TSP) and relatively reactive Minjingu PR from Tanzania. In the 18 months following application of 250 kg P ha‐1, bicarbonate extractable inorganic soil P (Pi) was higher for application of TSP than PR, but Pi extracted with a mixed anion‐cation resin was comparable for TSP and PR. Inorganic P extracted by 0.1M NaOH, without prior extraction of resin and bicarbonate Pi, decreased during the 18 months following TSP application, but increased following PR application. After 18 months, about 7% of the added PR‐P remained as Ca‐bound P that was extracted with 1M HCl. The 1M HCl extractable P., however, underestimated residual PR‐P that gradually dissolved and supplied plant‐available P, as indicated by recovery of <40% of PR‐P added to soil in laboratory incubations even though PR solubility in HCl was >90%. Minjingu PR was an effective source of P for corn. Corn yields were comparable for TSP and PR, and the relative agronomic effectiveness of PR averaged 107% in Season 1 and 79% in Season 3. Anion resin and mixed anion‐cation resin appeared to be superior to bicarbonate and NaOH as a soil P test for use with both TSP‐ and PR‐treated soils.

Phosphorus utilisation efficiency and depletion of phosphate fractions in the rhizosphere of three tea (Camellia sinensis L.) clones

Tea (Camellia sinensis L.) is mostly grown on highly weathered acidic Ultisols in the humid and sub-humid tropics. Phosphorus (P) availability in Ultisols is naturally low due to it's low diffusivity caused by high P-fixation of Fe and Al oxides. Tea is generally fertilised with low-cost reactive phosphate rocks (RPR) because of enhanced solubility of RPR under acidic conditions. In many countries, new tea clones have been developed to improve yield, drought tolerance and resistance to pest and diseases, but the effectiveness of these clones in utilising P from RPR and native soil P forms has not been studied. A study was conducted to investigate the effects of triple superphosphate (TSP) and a sparingly soluble phosphate rock (Eppawala phosphate rock, EPR) on plant P uptake and soil P fractions in the rhizosphere of three tea clones developed in Sri Lanka (S 106, TRI 2023 and TRI 2025). Phosphate uptake by TRI 2023 (3.3 ± 0.02 mg P plant-1) and TRI 2025 (2.6 ± 0.08 mg P plant-1) was significantly greater than S 106 (1.1 ± 0.01 mg P plant-1) for both P treatments. However the type of P fertiliser did not show any significant difference in P uptake by any of the clones. In all tea clones, the rhizosphere soil pH decreased significantly compared to that of the bulk soil in both P fertiliser treatments. The decrease of pH near the rhizoplane (0–0.5 mm) for TRI 2023, TRI 2025 and S 106 were 0.30 ± 0.02, 0.19 ± 0.03 and 0.17 ± 0.05 respectively. Dissolution of EPR in the rhizosphere of TRI 2023 and TRI 2025 was greater than S 106, which could be attributed to enhanced H+ efflux. TRI 2023 and TRI 2025 depleted more resin-P, NaOH–Piand H2SO4–Piin the rhizosphere compared to S 106, which is consistent with the P uptake results. NaOH–Po accumulated in the rhizosphere of all clones suggesting that soluble inorganic P was transformed into organic P forms possibly as a result of increased microbial activity in the rhizosphere. TRI 2023 and TRI 2025 showed a higher external P efficiency (total P uptake) compared to S 106 due to higher root surface area and higher P uptake per unit root surface area. The higher P uptake per unit root surface area in TRI 2023 and TRI 2025 compared to S 106 may be due to higher rhizosphere acidification, root exudation and mycorrhizal associations. Between TRI 2023 and TRI 2025, the former had higher external P efficiency due to greater root surface area. S 106 had higher internal P efficiency (shoot dry matter production per unit plant P) compared to the other two clones. In future tea breeding programmes, attempts should be made to combine these two traits to maximise P utilisation efficiency from tea clones.

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.

The relative effectiveness of superphosphate and rock phosphate for soils where vertical and lateral leaching of phosphate occurs

Laboratory studies have shown that up to 70% reactive rock phosphate dissolves in three soil types found in the high rainfall (> 800 mm annual average) area of south-western Australia. Three field experiments were undertaken on these soils to compare reactive apatite rock phosphate from North Carolina (NCRP) with single superphosphate (SSP) as fertilizers for subterranean clover (Trifolium subterraneum) pasture. Vertical leaching of phosphorus (P) occurs in one soil, a deep, very sandy, acid peaty sand. Lateral leaching of P occurs in the second soil, a shallow (3 cm) sand over a slowly permeable sandy clay loam. No leaching of P occurs in the third soil, a uniform, permeable red sandy loam with a moderate capacity to sorb P. All the soils remained moist to very wet for the 6 to 8 month growing season. Fertilizers were applied once only to different plots over a four-year period (1992 to 1995). Each year fertilizer effectiveness was determined relative to the effectiveness of freshly-applied (current) SSP using yield and P content of dried clover herbage and bicarbonate-soluble P extracted from the soil (soil test P) as indices of effectiveness.