Reactive Phosphate Rock Research Articles

Soil fauna in sheep-grazed hill pastures under organic and conventional livestock management and in an adjacent ungrazed pasture

Abstract Organic pasture management includes a focus on mixed livestock grazing, restrictions on nutrient inputs and livestock pest control. These are all factors which influence the environment of soil invertebrates. In this study, soil macrofauna, mesofauna and microfauna were collected from duplicate 11 and 20 year old organic and conventional legume-based sheep-grazed pasture systems. Pastures in both systems had received the same annual input of reactive phosphate rock and elemental sulphur and were stocked with the same numbers of sheep. The major difference between the two systems was absence of the chemical control of livestock pests on the organic system, resulting in lower sheep liveweights. Two ungrazed pastures which had not received any nutrient inputs for 20+ years were also sampled as part of the study to provide an additional contrast. No significant differences in either the diversity or abundance of the soil invertebrate community were found between the organic and conventional systems, despite the use of chemicals in the conventional system. The lack of difference suggests that many of the observed and reported responses to organic management reflect altered nutrient inputs and grazing management (which were the same in the current study), rather than the cessation of chemical controls in the organic system. In contrast, the invertebrate community in the unfertilised, ungrazed pasture was distinctly different from both the grazed systems. The ungrazed pasture had a higher diversity with more New Zealand endemic species than the grazed pasture. This included twice as many large Oribatida as soil pore size increased. The lower litter quality from the lack of nutrient inputs in the ungrazed pasture was reflected in both lower earthworm abundance and the Nematode Channel Ratio (indicative of a higher proportion of fungal- than bacterial-feeding nematodes), than the grazed pasture. We conclude that organic management when limited to a comparison of livestock pest control is not beneficial to soil invertebrates. Management practices such as stocking rates and fertiliser regime, by altering the soil physical environment and food resources, are more important in influencing invertebrate populations in this soil. Some would argue these altered management practices are an integral part of an organic production system.

Phosphorus fertilizer form affects phosphorus loss to waterways: a paired catchment study

AbstractTwo catchments were used to test the hypothesis that using reactive phosphate rock (RPR), a low water soluble fertilizer, instead of superphosphate, could decrease phosphorus (P) export in streams. The sheep‐grazed catchments, each ca. 12 ha in size, received 20–25 kg P/ha either as RPR or superphosphate for 3 yr. Filterable (<0.45 μm) reactive P (FRP) and total P (TP) in stream samples were then compared against two subsequent years when both catchments received 20 kg P/ha as superphosphate. On average, during the 5‐yr study, loads of FRP and TP in stream flow were low (110 g FRP/ha and 358 g TP/ha), typical of dry sheep‐grazed hill country. However, FRP (58%) and TP (38%) in stream flow were significantly less when RPR was applied instead of superphosphate. Where appropriate climatic and soil conditions exist to maintain agronomic targets, RPR could be used to decrease P export in streams.

Production, nutritional status and chemical properties of soils with addition of cattle manure, reactive natural phosphate and biotite schist in Massai cultivar

The objective of this work was to evaluate the effects of cattle manure, reactive natural phosphate and biotite schist on the soil fertility, yield and nutritional status of Megathyrsus spp. cv. Massai. The experiment was conducted under field conditions, in a dystrophic Red Yellow Latosol (Oxisol). It was used a randomized block experimental design with the following treatments: three natural reactive rock phosphate from Algeria (Djebel-Onk) doses (0, 100 and 200 kg ha-1 of P2O5), three biotite schist doses (0, 150 and 300 kg ha-1 of K2O) and three cattle manure doses (0, 20 and 40 t ha-1). The application of natural phosphate increased dry matter yield, however, application of cattle manure and biotite schist did not influence this variable. Foliar levels of nitrogen, potassium and magnesium (cattle manure), phosphorous and boron (natural rock phosphate) and potassium (biotite schist) were influenced by the applied fertilizer doses. Only the levels of phosphorous in the soil and in the plant and levels of magnesium and boron in the plant show interaction with dry matter yield of Massai cultivar.

Availability of Cadmium and Zinc as Affected by the Use of Reactive Phosphate Rock, Lime, and Chicken Manure on an Indonesian Acidic Upland Soil under Field Conditions

We assessed cadmium (Cd) and zinc (Zn) availability when applying reactive phosphate rock (RPR) in combination with lime and chicken manure on Indonesian acidic upland soils. Maize plants were grown on unamended soil and soils treated with several combinations of 2 tons dolomite ha–1, 2 tons of chicken manure ha–1, 1 ton ha–1 of RPRL (reactive phosphate rock containing 4 mg Cd kg–1 and 224 mg Zn kg–1), and 1 ton ha–1 of RPRH (RPR containing 69 mg Cd kg–1 and 745 mg Zn kg–1). In addition to its positive effect on plant yield, application of RPR in combination with chicken manure did not result in toxic Cd concentrations. Although liming is effective to reduce plant Cd concentrations, it results in more soil Cd accumulation and more plant Zn deficiency. Cadmium and Zn concentrations in shoots and grains can be predicted well from amounts extracted from the soil by 0.5 M ammonium (NH4) acetate + 0.02 M ethylenediaminetetraacetic acid (EDTA) at pH 4.65.

Evaluation of two management options to improve the water quality of Lake Brunner, New Zealand

Lake Brunner, located on the west coast of the South Island of New Zealand, has been identified as threatened by phosphorus (P) inputs from neighbouring dairy farms. A two-year study was undertaken to determine if the application of reactive phosphate rock (RPR) with or without the addition of aluminium (Al) sulphate (alum) at 20 kg Al ha−1 could decrease P losses in surface runoff from dairy pastures compared to the same rate of P applied as superphosphate (30 kg P ha−1). The study was conducted on nine humped and hollowed plots (1100–3500 m2). Frequent and reliable rainfall year round (totalling 4000–5000 mm yr−1) translated into more than 50 surface runoff events per year and large losses of P from superphosphate-treated plots (flow-weighted mean filterable reactive P (FRP) = 0.072 mg l−1; mean load 8.2 kg P ha−1 yr−1). In contrast, the flow-weighted mean FRP concentration in the RPR-treated plots was significantly less (0.034 mg l−1; mean load 4.0 kg P ha−1 yr−1). No effect was found on other P forms. Alum did not decrease P losses, most likely due to Al being washed off in surface runoff before it could bind to the soil. Enhanced P losses from superphosphate-treated plots were attributed to the greater availability of P within 21 days of application (water solubility 80–95%) compared with RPR (≈1% water soluble), and the occurrence of surface runoff during this period (especially in year 1). The use of RPR is recommended as one management practice to decrease FRP inputs to Lake Brunner and may be useful in other regions where rainfall and soil pH allow.

Crescimento e produção de grãos da soja sob diferentes doses e fontes de fósforo em solos distintos

A fertilidade dos solos das regiões tropicais é limitada pela baixa disponibilidade de fósforo e pela indisponibilização de boa parte do mesmo fornecido na adubação. Ainda existem muitas divergências sobre a melhor forma de utilização das diversas fontes fosfatadas disponíveis no Brasil. Objetivou-se, neste experimento, avaliar a produção de soja, sob a aplicação de diferentes doses e adubos fosfatados em dois tipos de solo. O trabalho foi realizado em casa de vegetação do Departamento de Ciência do Solo da Universidade de Lavras, em vasos de 3 dm³. Utilizaram-se dois solos sob cerrado: Neossolo Quartzarênico e Latossolo Vermelho Distroférrico típico. Foram testadas oito fontes (superfosfato triplo-ST, termofosfato magnesiano-TF, quatro fosfatos reativos importados: FR1, FR2, FR3 e FR4 e dois fosfatos naturais nacionais- FN1 e FN2) e quatro doses (0, 100, 300 e 600 mg dm-3) de fósforo, utilizando-se a soja como planta-teste. As fontes de solubilidade mais elevada ocasionaram as maiores produções e índices de eficiência em relação aos fosfatos naturais. Os fosfatos naturais importados e o FN2 apresentaram-se mais promissores em fornecer P para a cultura da soja, em relação ao superfosfato triplo, mesmo no primeiro cultivo, principalmente em solo com maior poder tampão de fósforo. A capacidade tampão de fósforo dos solos influenciou a eficiência das fontes de fósforo na cultura da soja.

Eficiência de fosfato natural reativo aplicado em misturas com superfosfato triplo em milho e soja

O objetivo deste trabalho foi avaliar o efeito do modo de aplicação do superfosfato triplo (SFT), do fosfato natural reativo de Gafsa e da mistura entre essas fontes no aproveitamento do fósforo (P) por plantas de milho e de soja. Os experimentos foram realizados em casa de vegetação, e as plantas foram cultivadas em vasos com Latossolo Vermelho-Amarelo distrófico. As fontes fosfatadas foram aplicadas isoladamente ou em misturas (proporção 1:1), à dose de 90 mg kg-1 de P, de modo não localizado ou localizado no solo. Para a quantificação do P absorvido pelas plantas das diferentes fontes, empregaram-se tratamentos com o solo e com o SFT marcados com 32P. O aproveitamento do fósforo do superfosfato triplo aplicado de forma localizada (9,8%) foi superior ao da não localizada (7%). A aplicação não localizada do fosfato natural reativo proporcionou maior aproveitamento do fósforo (3,8%), em comparação à localizada (0,5%). Em comparação à aplicação isolada do fosfato de Gafsa, o aproveitamento do fósforo pelas plantas de milho e soja aumentou quando essa fonte foi aplicada como mistura compactada; em presença do superfosfato triplo, a aplicação do fosfato natural reativo de Gafsa de modo não localizado proporcionou maior aproveitamento do P pelas plantas de soja. O aproveitamento do fósforo do fosfato natural de Gafsa aumenta quando aplicado ao solo com o superfosfato triplo, e esse efeito depende do método de aplicação das fontes fosfatadas.

New fertiliser options for managing phosphorus for organic and low-input farming systems

Plant-available phosphorus (P) has been found to be limiting crop and pasture production in Australian dryland, broadacre, organic farming systems. The present review examines the mechanisms that act to provide organic sources of P to soil or mobilise P stored within the soil. A range of products is available to exploit one or more of these mechanisms to achieve a claimed improvement in P fertility. These products are described, and where possible, scientific research of their effectiveness is reviewed. The use of microbial inoculants, although successful in laboratory and glasshouse experiments, has returned varied results in field trials. The addition of organic fertilisers, such as composted or elemental sulfur (S) enriched reactive phosphate rock (RPR), tended to produce more reliable results. The variable nature of the composting process creates complexity in the production of composted RPR. The increased dissolution of RPR by the oxidation of added S has been successful in increasing available P content above that of RPR alone. This is especially significant to low-rainfall areas where RPR tend to be ineffective. This paper highlights the need for development and optimisation of the many organic fertilisers and additives available to organic producers. In all cases, products still require rigorous field and economic evaluation so that organic producers can be confident in making decisions that are informed, correct, and profitable with regard to P fertility. The alleviation of P deficiency is vital to the increased adoption and sustainability of boardacre organic farming in Australia.

Phosphorus management on extensive organic and low-input farms

A synthesis of the Australian literature reporting soil and plant phosphorus (P) status under organic methods of broadacre farming provides clear evidence that available soil P is lower in organic systems, although there have been no reports of farm P balances that might help to explain the lower P concentrations. There is also evidence, which is largely circumstantial, to suggest that P deficiency significantly reduces productivity of broadacre organic farms, but few experiments prove this conclusively because of other confounding factors. An overview of international literature suggests similar findings for mixed farms. Nine case studies further examined the constraints imposed by P on broadacre organic and low-input farms in Australia. Two farms on fertile soils had negative P balances but maintained productivity without fertilisers by ‘mining available’ P reserves. Five extensive organic farms on inherently less fertile soils had positive P balances because P fertiliser was used. Four of these farmers reported low productivity, which was supported by comparisons of wheat yields with estimated water-limited potential yields. Low productivity appeared to be related to P deficiency despite the use of allowable mineral fertilisers, mostly reactive phosphate rock (RPR), on these farms. The apparent ineffectiveness of RPR is most likely due to the modest rainfall at these farms (380–580 mm/year). The highest research priority is to develop effective, allowable fertilisers. Until this has been achieved, or ways of using less labile P have been developed, there is a case for derogation in the Certification Standards to allow the use of soluble forms of P fertiliser on soils with low soil solution P and high soil P-sorption. Two low-input farms practicing pasture-cropping had approximately balanced P budgets and from this perspective were the most sustainable of the farms studied.

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.

Characterization and Greenhouse Evaluation of Brazilian Calcined Nonapatite Phosphate Rocks for Rice

Little information is available on the agronomic effectiveness of calcined nonapatite phosphate rock (PR) sources containing crandallite minerals in the form of Ca–Fe–Al–P for flooded and upland rice (Oryza sativa L.). We conducted laboratory and greenhouse studies to (i) characterize the mineralogical composition, (ii) investigate the solubility and dissolution behavior, and (iii) evaluate the agronomic effectiveness of two nonapatite PR sources (Juquia and Sapucaia) from Brazil and compared them with (i) a highly reactive Gafsa PR (Tunisia) containing apatite in the form of Ca–P and (ii) a reference water‐soluble triple superphosphate (TSP) for flooded and upland rice. After calcination at 500°C for 4 h, the solubility of Juquia PR and Sapucaia PR in neutral ammonium citrate (NAC) significantly increased from almost nil to a maximum of 39.3 and 114 g P kg−1, respectively. X‐ray diffraction showed that crystalline crandallite mineral was transformed to an amophorus form after calcination. The solubility behavior of the two calcined PR sources followed the same trend as Gafsa PR, that is, P release decreased with increasing equilibrium pH in the 0.01 M KCl solution (pH 3.0–8.0). At pH 3, the solubility followed: Gafsa PR &gt; calcined Sapucaia PR &gt; calcined Juquia PR. No P release was detected from any of the PR sources at pH ≥ 5.0 in the solution, indicating the Ca–P characteristic of the Ca–Fe–Al–P mineral controlled P dissolution of the calcined PR. Without calcination, both Juquia PR and Sapucaia PR were totally ineffective for upland rice grown on a Hiwassee clay loam (fine, kaolinitic, thermic Rhodic Kanhapludult) with pH 5.4 whereas a significant P response was observed with the calcined PR samples. For flooded rice grown on Hiwassee soil, the calcined Juquia PR and Sapucaia PR were 66 and 72%, respectively, as effective as TSP in increasing rice grain yield whereas Gafsa PR was ineffective. For upland rice grown on the unlimed soil, Gafsa PR was as effective as TSP in increasing rice grain yield whereas calcined Juquia PR and Sapucaia PR were 89 and 83% of TSP. The effectiveness of Gafsa PR was reduced to 0% after the soil was limed to pH 7.0 whereas the two calcined PR sources were reduced to 49% of TSP. Soil available P extracted by iron oxide impregnated filter paper (Pi test) or anion‐exchange resin after rice harvest correlated well with P uptake by rice grain for flooded and upland rice.

Frações de fósforo em solo adubado com fosfatos em diferentes modos de aplicação e cultivado com milho

O histórico de uso e o manejo da adubação influenciam a capacidade de suprimento de P pelo solo. Neste trabalho, foram avaliadas as frações de P num Argissolo Vermelho da região do Cerrado, adubado com fosfatos de reatividade distinta, em diferentes modos de aplicação e cultivado com milho durante três safras. A área, que já havia sido adubada em épocas passadas, estava coberta por braquiária. O delineamento experimental foi o de blocos casualizados com quatro repetições, num esquema fatorial 4 x 3 + 1, envolvendo quatro fontes de P, na quantidade de 180 kg ha-1 de P2O5 (superfosfato triplo, termofosfato magnesiano, fosfato natural reativo de Arad e fosfato natural de Araxá), três formas de aplicação (a lanço em área total no primeiro ano, localizada no sulco de plantio no primeiro ano e parcelada anualmente no sulco) e uma testemunha (sem P) como tratamento adicional. O P foi fornecido considerando-se os teores totais do nutriente nas fontes. Após três cultivos de milho, foram retiradas amostras de solo na profundidade de 0-20 cm para o fracionamento do P. A interação entre fontes e modos de aplicação de P interfere nas frações do nutriente no solo no final do cultivo de milho. De modo geral, o parcelamento da adubação condiciona maior teor do nutriente nas frações inorgânica (todas as fontes) e orgânica (fontes mais reativas). O fato de a área ter sido anteriormente adubada e ocupada por braquiária influenciou os efeitos dos tratamentos, pois a produção de milho foi relativamente elevada onde não houve aplicação de P. Houve evidências da participação de frações consideradas pouco lábeis no suprimento de P para o milho, em função da diminuição dessas frações no solo.

Phosphorus sources and fractions in an oxisol under no-tilled soybean

Phosphorus dynamics in soil can be modified by the use of no-till systems. Brazilian farmers have applied phosphorus fertilizers onto the soil surface to optimize machine field operations despite the lack of research supporting this practice. An experiment was conducted to study the effects of the application of two P sources onto soil surface and soybean seed furrows. The treatments consisted either of the application or not of 80 kg ha-1 of total P2O5 as natural reactive rock phosphate and superphosphate spread on the soil surface over the standing plant residues. At soybean planting, additional treatments (80 kg ha-1 of Natural Phosphate or Superphosphate) were applied at seed furrows. Soil was sampled down to 40 cm deep before soybean planting and after harvest. A control sample was taken from an adjacent non-cropped area. Phosphorus contents increased down to 40 cm after the soybean crop, and the increase was observed mainly in Ca-bound P and organic phosphorus. However, there was a decrease in Fe-bound P, showing that P availability to soybeans was likely related also to this fraction. Phosphorus fertilization with both phosphates decreased occluded P contents on the soil surface layer when compared with the non-cropped area.

Enhancing the agronomic effectiveness of natural phosphate rock with poultry manure: a way forward to sustainable crop production

Phosphorus inputs are required in highly weathered tropical soils for sustainable crop production. However, high cost and limited access to mineral P fertilizers limit their use by resource-poor farmers in West Africa. Direct application of finely ground phosphate rock (PR) is a promising alternative but low solubility of PR hampers its use. Co-application of PR with manure could be a low cost means of improving the solubility of natural PR and improve their agronomic effectiveness. Our objective was to quantitatively estimate the enhancement effect of poultry manure on P availability from low reactive PR (Togo phosphate rock) applied to highly weathered soils. We utilized two highly weathered soils from Ghana and Brazil for this greenhouse study. Using 32P isotopic tracers, the agronomic effectiveness of poultry-manure-amended Togo rock phosphate (TPR) was compared with partially acidulated Togo rock phosphate (PAPR) and triple superphosphate (TSP). Four rates of poultry manure: 0, low (30 mg P kg−1 soil), high (60 mg P kg−1 soil) and very high (120 mg P kg−1 soil) were, respectively, added to a constant amendment (60 mg P kg−1 soil) of the P sources and applied to each pot of 4 kg soil. A Randomized Complete Block Design was used for the greenhouse experiment and Maize (Zea mays L.) was used as a test crop. The plants were grown for 42 days after which the above ground biomass was harvested for analysis. Without poultry manure addition, the agronomic effectiveness, represented by the relative agronomic effectiveness (RAE) and proportion of P derived from fertilizer (% Pdff) was in the order TSP > PAPR > TPR = control (P0). In the presence of low rate poultry manure addition, the agronomic effectiveness followed the order TSP > PAPR = PR > P0. However, at the high and very high rates of poultry manure addition, no significant differences in agronomic effectiveness were observed among the P sources, suggesting that at this rate of poultry manure addition, PR was equally as effective as TSP. The study showed that direct application of PR co-applied with poultry manure at a 1:1 P ratio will be a viable option for P replenishment. Thus a combination of PR and poultry manure could be a cost-effective means of ensuring sustainable agricultural production in P-deficient, highly weathered tropical soils.

Agronomic application of olive mill wastewaters with phosphate rock in a semi-arid Mediterranean soil modifies the soil properties and decreases the extractable soil phosphorus

Olive mill wastewater (OMW), a by-product of the olive-mill industry, is produced in large amounts in Mediterranean countries. The presence of indigenous phosphate deposits in some countries like Tunisia 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 present in OMW represents a new perspective in PR research and a possible solution for the recycling of the OMW. The present work was aimed at evaluating, under natural situations (field of olive trees), the effects of agronomic application of OMW (amounts applied: 30, 60 m 3 ha −1) with PR (amounts applied: 150 kg ha −1) on olive trees soil properties. We measured organic C, nitrogen (N), extractable phosphorus (P), exchangeable calcium (Ca), and exchangeable potassium (K), as well as other properties (pH and electrical conductivity). Our data provide evidence that agronomic application of OMW with PR has important effects on soil properties. Increases in organic C, total N, extractable P and exchangeable potassium (K) were found after the first agronomic application of OMW and PR. These increases were only temporary, following the second agronomic application of OMW and PR, significant reductions were detected in the extractable soil P (19.67 mg kg −1 in the control soil vs. 8.99 mg kg −1 in the amended soil). Changes in the extractable soil P could alter plant productivity and plant community structure because shifts in nutriment availability can affect the balance between limiting and non-limiting nutrients.

Phosphorus Uptake from Green Manures and Phosphate Fertilizers Applied in an Acid Tropical Soil

Quantifying the relative contribution of different phosphorus (P) sources to P uptake can lead to greater understanding of the mechanisms that increase available P in integrated P management systems. The 32P–33P double isotope labeling technique was used to determine the relative contribution of green manures (GMs) and P fertilizers to P uptake by Setaria grass (Setaria sphacelata) grown in an amended tropical acid soil (Bungor series) in a glasshouse study. The amendments were factorial combinations of GMs (Calopogonium caeruleum, Gliricidia sepium and Imperata cylindrica) and P fertilizers [phosphate rocks (PRs) from North Carolina (NCPR), China (CPR) and Algeria (APR), and triple superphosphate (TSP)]. Dry matter yield, P uptake, and P utilization from the amendments were monitored at 4, 8, and 15 weeks after establishment (WAE). The GMs alone or in combination with P fertilizers contributed less than 5% to total P uptake in this soil, but total P uptake into Setaria plants in the GM treatments was three to four times that of the P fertilizers because the GMs mobilized more soil P. Also, the GMs markedly increased fertilizer P utilization in the combined treatments, from 3% to 39% with CPR, from 6–9% to 19–48% with reactive PRs, and from 6% to 37% with TSP in this soil. Both PGM and the other decomposition products were probably involved in reducing soil P‐retention capacity. Mobilization of soil P was most likely the result of the action of the other decomposition products. These results demonstrate the high potential of integrating GMs and PRs for managing P in tropical soils and the importance of the soil P mobilization capacity of the organic components. Even the low‐quality Imperata GM enhanced the effectiveness of the reactive APR more than fourfold.

Influence of Tithonia diversifolia and triple superphosphate on dissolution and effectiveness of phosphate rock in acidic soil

AbstractAn incubation and a pot experiment were conducted to evaluate the dissolution and agronomic effectiveness of a less reactive phosphate rock, Busumbu soft ore (BPR), in an Oxisol in Kenya. Resin (anion and anion + cation)‐extractable P and sequentially extracted P with 0.5 M NaHCO3, 0.1 M NaOH, and 1 M HCl were analyzed. Dissolution was determined from the increase in anion resin (AER)–, NaHCO3‐, and NaOH‐extractable P in soil amended with PR compared with the control soil. Where P was applied, resin P significantly increased above the no‐P treatment. Busumbu‐PR solubility was low and did not increase significantly in 16 weeks. Anion + cation (ACER)‐extractable P was generally greater than AER‐P. The difference was greater for PR than for triple superphosphate (TSP). The ACER extraction may be a better estimate of plant P availability, particularly when poorly soluble P sources are used. Addition of P fertilizers alone or in combination with Tithonia diversifolia (TSP, BPR, TSP + Tithonia, and BPR + Tithonia) increased the concentration of labile inorganic P pools (NaHCO3‐ and NaOH‐Pi). Cumulative evolved CO2 was significantly correlated with cumulative N mineralized from Tithonia (r, 0.51, p &lt; 0.05). Decrease in pH caused NH$ _4^+ $‐N accumulation while NO$ _3^- $‐N remained low where Tithonia was incorporated at all sampling times. However, when pH was increased, NH$ _4^+ $‐N declined with a corresponding rise in NO$ _3^- $‐N. Tithonia significantly depressed soil exchangeable acidity relative to control with time. A significant increase (p &lt; 0.05) was observed for P uptake but not dry‐mass production in maize where BPR was applied. The variations in yield and P uptake due to source and rates of application were statistically significant. At any given P rate, highest yields were obtained with Tithonia alone. Combination of Busumbu PR with TSP or Tithonia did not enhance the effectiveness of the PR. The poor dissolution and plant P uptake of BPR may be related to the high Fe content in the PR material.

Application of reactive phosphate rock and sulphur fertilisers to enhance the availability of soil phosphate in organic farming

Plant available soil phosphate is frequently deficient for crop and pasture production on organic farms in southern Australia. Improved P management, including developing a fertiliser product conforming to organic farming regulations, is required to sustain and increase production on these farms. Reactive phosphate rock (RPR) and elemental sulphur (S) are natural products. Field and pot experiments were established to measure the impact of ground RPR, and co-treatment of RPR with finely ground S, on available soil phosphate (Olsen P), plant dry matter, and the P concentration (%) and content (kg P ha−1) of the dry matter. Under dry-land field conditions characteristic of cropping regions in southern Australia ( 1,000 mm) and soil organic carbon concentration (OC 11%). It was deduced that the S is probably required to overcome the constraint on dissolution of RPR resulting from frequent periods of low soil moisture. It was concluded that for the south-eastern Australian cropping zone, co-treatment of ground reactive phosphate rock with finely ground elemental S, at ratios (RPR:S) of at least 2:1, depending on soil pH, is required for effective use␣of RPR, even in strongly acidic soil (pHCa < 4.5). It was recommended that ‘organic’ farmers may recover soil P fertility by applying RPR + S fertiliser to the most acidic fields, postponing soil liming, and managing the fields to conserve soil moisture.

Factors Influencing the Dissolution of Phosphate Rock in a Range of High P‐Fixing Soils from Cameroon

A laboratory incubation experiment was conducted to evaluate the soil factors that influence the dissolution of two phosphate rocks (PRs) of different reactivity (Gafsa, GPR, reactive PR; and Togo‐Hahotoe, HPR, low reactivity PR) in seven agricultural soils from Cameroon having variable phosphorus (P)‐sorption capacities, organic carbon (C) contents, and exchangeable acidities. Ground PR was mixed with the soils at a rate of 500 mg P kg−1 soil and incubated at 30°C for 85 days. Dissolution of the PRs was determined at various intervals using the ΔNaOH‐P method (the difference of the amount of P extracted by 0.5 M NaOH between the PR‐treated soils and the control). Between 4 and 27% of HPR and 33 and 50% of GPR were dissolved in the soils. Calcium (Ca) saturation of cation exchange sites and proton supply strongly affected PR dissolution in these soils. Acid soils with pH‐(H2O)<5 (NKL, ODJ, NSM, MTF) dissolved more phosphate rock than those with pH‐(H2O)>5 (DSC, FGT, BAF). However, the lack of a sufficient Ca sink in the former constrained the dissolution of both PRs. The dissolution of GPR in the slightly acidic soils was limited by increase in Ca saturation and that of HPR was constrained by limited supply in protons. Generally, the dissolution of GPR was higher than that of HPR for each soil. The kinetics of dissolution of PR in the soils was best described by the power function equation P=AtB. More efficient use of PR in these soils can be achieved by raising the soil cation exchange capacity, thereby increasing the Ca sink size. This could be done by amending such soils with organic materials.

Phosphorus leaching from pastures can be an environmental risk and even a significant fertiliser expense

While it is true that leaching is usually not a strong pathway for phosphorus (P) loss under many systems, is it true for all? Two studies reported in this paper sought to establish if significant phosphorus leaching can occur under normal pastoral production systems. Undisturbed-core lysimeters collected from a Wharekohe silt loam from Northland were treated with fertiliser P (reactive phosphate rock and superphosphate) then leached from August to November, 2005. In a second study, soil profiles under pasture for sheep/beef and dairy production in the catchments of the Rotorua lakes were sampled to depths of 1.5 m (28 sites), and soil Olsen P and P retention capacity index were determined down these profiles. Phosphorus losses from the Wharekohe soil to 25 cm depth were up to 33% of the P applied (superphosphate applications of 50 and 100 kg P/ha). Some Rotorua soils displayed enriched P concentrations at depth (to 1.5 m), often coupled with moderate or low P sorption capacities. If connectivity exists between leaching pathways and surface water bodies these observations indicate that alternative management strategies need to be developed and adopted for soils that leach significant quantities of P. The Wharekohe silt loam is one such soil. Keywords: phosphorus, leaching