S. H. Chien

Factors affecting the agronomic effectiveness of phosphate rock for direct application

Phosphorus (P) is critically needed to improve soil fertility for sustainable crop production in large areas of developing countries. In recent years, phosphate rock (PR) for direct application has been tested in tropical acid soils as a potential alternative to conventional water-soluble P fertilizers like single superphosphate (SSP) and triple superphosphate (TSP). Some developing countries have PR deposits which, if used to supplement other imported P fertilizers, would allow a saving of much needed foreign exchange. Solubility of P fertilizers is not the only criterion in selection of the most suitable P fertilizer. This paper discusses the results of experiments to compare the relative agronomic effectiveness (RAE) of various PR sources with respect to SSP or TSP as influenced by four important factors: PR sources, soil properties, management practices, and crop species. Under certain conditions, PRs can be agronomically effective.

Agronomic and environmental aspects of phosphate fertilizers varying in source and solubility: an update review

This review discusses and summarizes the latest reports regarding the agronomic utilization and potential environmental effects of different types of phosphate (P) fertilizers that vary in solubility. The agronomic effectiveness of P fertilizer can be influenced by the following factors: (1) water and citrate solubility; (2) chemical composition of solid water-soluble P (WSP) fertilizers; (3) fluid and solid forms of WSP fertilizers; and (4) chemical reactions of P fertilizers in soils. Non-conventional P fertilizers are compared with WSP fertilizers in terms of P use efficiency in crop production. Non-conventional P fertilizers include directly applied phosphate rock (PR), partially acidulated PR (PAPR), and compacted mixtures of PR and WSP. The potential impacts of the use of P fertilizers from both conventional (fully acidulated) and non-conventional sources are discussed in terms of (1) contamination of soils and plants with toxic heavy metals, such as cadmium (Cd), and (2) the contribution of P runoff to eutrophication. Best practices of integrated nutrient management should be implemented when applying P fertilizers to different cropping systems. The ideal management system will use appropriate sources, application rates, timing, and placement in consideration of soil properties. The goal of P fertilizer use should be to optimize crop production without causing environmental problems.

Iron oxide impregnated filter paper (Pi test): a review of its development and methodological research

Iron oxide impregnated filter paper (FeO paper) has been used to study the availability of phosphorus (P) to plants and algae, P desorption kinetics and P dynamics in the field. Since its initial development a number of differences in the method of preparation of the paper and its application have been implemented. The purpose of this article is to (i) critically review studies on the various aspects of both preparation and use of the FeO paper and (ii) to suggest a standardized procedure.Type of filter paper, FeO impregnation of the filter paper, neutralization of FeCl3, washing and drying of the impregnated paper, all have a significant effect on the amount of P sorbed. It is suggested that the method for neutralization of FeCl3 is checked on the release of acid when FeO paper is used, because the acid can release P compounds that would otherwise not desorb. Washing after neutralization is essential to remove loose FeO particles, which otherwise will cause incomplete P recovery. Modifications of the desorption procedure itself also affect the amount of P sorbed onto the paper. The ionic strength of the shaking solution, surface area of the FeO paper and the number and position of the paper in the shaking solution, and shaking strength and time all may introduce variations in the amount of P on the FeO paper. Differences in the dissolution of the Fe compounds on the filter paper and the subsequent determination of P in the solute may contribute to variations in P recovery. It is illustrated that the existing variation in the preparation and use of the FeO paper will cause differences in the amount of P extracted from soils, impeding the comparison of different studies.Finally, we recommend a standardized procedure for the preparation and use of the FeO paper. Such a procedure will allow comparison between studies and may reduce or circumvent possible artefacts.

Development and evaluation of the Pi soil test for plant‐available phosphorus

Abstract The Pi test is a new soil test for phosphorus (P), which uses a strip of iron oxide‐impregnated paper as a sink to adsorb P mobilized in a soil‐solution suspension. The paper strips are prepared by immersing hardened filter paper circles in a 10% solution of ferric chloride; then they are dried and exposed to ammonia vapor. The paper is then cut into strips 10 cm long and 2 cm wide. To extract P, 1 g of soil is shaken for 16 h with 40 mL 0.01 M CaCl2 and a Pi strip, which is enclosed in a nylon mesh bag. Phosphorus retained on the strip is dissolved in 0.1 M H2SO4 and measured. The amount of P extracted by the paper strips has been found to correlate well with yield and P uptake by maize (Zea Mays L., Var. Funk 5757) grown on acidic, alkaline, and calcareous soils and in soils fertilized with soluble fertilizers and phosphate rock. To evaluate the suitability of the method to measure plant‐available P contained in a large number of soils, greenhouse experiments were carried out using 18 soils wit...

A discussion of the methods for comparing the relative effectiveness of phosphate fertilizers varying in solubility

Because various phosphate (P) fertilizers differ widely in their solubility, it is commonly observed that crop response to P fertilizers varies under the same soil and crop conditions. Furthermore, a major problem encountered in the methods for determining the relative effectiveness (RE) of water-insoluble P fertilizer (e.g., phosphate rock) with respect to water-soluble P fertilizers, e.g., single superphosphate (SSP) and triple superphosphate (TSP), is that their growth response curves are usually nonlinear and often do not share a common maximum yield. In this paper, we review and discuss the advantages and disadvantages of the three most commonly used methods for calculating the RE of phosphate rock with respect to TSP (or SSP). The three methods are vertical comparison, horizontal (substitution rate) comparison, and linear-response comparison.

Comparison of Bray I and Pi tests for evaluating plant-available phosphorus from soils treated with different partially acidulated phosphate rocks

Phosphate rocks from seven locations in Africa and Latin America were partially acidulated with H2SO4 at 30% or 50% acidulation level and were applied to an acid silt loam. Dry-matter yield of and P uptake by maize grown on the soil were correlated with P measured by Bray I soil test and Pi soil test in which P is extracted by shaking a soil sample with 0.01M CaCl2 solution and a strip of iron hydroxide impregnated filter paper. There was a highly significant correlation between water-soluble P in the partially acidulated phosphate rocks (PAPR) and P measured by the Pi test (r=0.92**). The dry-matter yield correlated significantly with P measured by the Pi test (r=0.91**) as well as by the Bray test (r=0.73**). Phosphorus uptake correlated highly significantly with Pi−P (r=0.94**); correlation with Bray I−P was less significant (r=0.67*). Bray I solution extracted more P than Pi did, which resulted in an overestimation of plant-available P. Bray I therefore was less effective than the Pi soil test in evaluating P availability from different PAPRs applied to the soil.

Iron oxide-impregnated filter paper (Pi test): II. A review of its application

The iron oxide impregnated filter paper test (Pi test) is a recently developed soil test for phosphorus (P) in which the FeO paper acts as an infinite sink for P mobilized in a soil solution. Several papers have been published evaluating the effectiveness of the test for predicting plant availability of P under different soil conditions. The use of FeO paper to predict algal availability of P in water bodies and runoffs has also been studied.The purpose of this paper is to review studies on the use of the Pi test to evaluate plant availability of P in soils, and predict availability of P to algae in an aquatic environment. Phosphorus extracted by the FeO paper is primarily physically bound extractable (resin P) and correlates significantly with Bray I and Mehlich P in acid soils and Olsen P in calcareous soils. Dry-matter yield and P uptake by maize (Zea mays L), kidney beans (Phaseolus vulgaris L), and upland rice (Oryza sativa L) grown in acidic soils correlated well with Pi-P. Likewise, in calcareous soils, Pi-P was as good as Olsen-P in predicting crop response. Field trials have shown that the Pi test is a good predictor of plant yield in soils with wide ranging properties. Compared to the standard method to measure bioavailable P to algae in waters and agricultural runoffs involving lengthy algal essays culturing selenastrum capricornutum with sediment samples, the Pi method is a faster and easier method to estimate P that may be potentially available for uptake by algae.

Sorption of phosphorus by the iron oxide-impregnated filter paper (Pi soil test) embedded in soils

Incubation experiments were carried out to evaluate the feasibility of extracting phosphorus from soil by embedding iron oxide-impregnanted filter paper strips (Pi strips) in soils having a wide range in pH, texture, and extractable-P contents. Under flooded conditions, the amount of P extracted by the Pi strips increased with the period of submergence and embedding time of the Pi strips. Under unsaturated conditions, the Pi strips were found to extract P from soils over a wide range in moisture conditions; however, keeping the soil at moisture level between saturation and field capacity was found to result in maximal sorption of P by the strips. An embedding time of 16 h was found to be adequate.Phosphorus extracted by embedding Pi strips in soil columns for 16 h at field capacity moisture level correlated significantly with P extracted by shaking the soil with 0.01 M CaCl2 solution and a Pi strip for 16 h in the laboratory (r=0.94**). The P extracted by embedding Pi strips correlated best with Bray 1 P in acid soils (r=0.97**) and with Olsen P in alkaline and calcareous soils (r=0.96**). The results of the studies demonstrate the feasibility of developing a nondestructive method of monitoring changes in plant-available P in situ under field conditions.

Solubility assessment for fertilizer containing phosphate rock

Various methods have been used worldwide for measuring the solubility of phosphate rock (PR) and fertilizers containing PR. In this review, chemical, mineralogical, and physical factors affecting various measurements of PR solubility are discussed. The most important factors are (1) PR solubility expression, (2) mineralogical composition of PR, (3) free carbonate (calcite dolomite) effect, (4) effect of apatite crystallinity and cementing of apatite with silica, and (5) effect of monocalcium phosphate and gypsum in the mixture of PR and TSP or SSP.

Phosphorus availability from partial acidulation of two phosphate rocks.

Two phosphate rocks, one from Pesca (Colombia) and the other from Togo, were acidulated to various degrees with H2SO4 and H3PO4 for evaluation in varying granule size ranges. Products acidulated with H2SO4 were also prepared using different drying temperatures. Phosphorus availability was measured by dry-matter yield and P uptake in greenhouse experiments with maize.It was observed that partial acidulation with H2SO4 was effective in increasing the water-soluble P level of phosphate rock when the drying temperature of the product was not excessive. Crop response and P uptake were both highly correlated to the water solubility of the product. The relative agronomic effectiveness (RAE) of Togo rock increased from 3% when unacidulated to 33%, 47%, and 52% when 20%, 30%, and 40%, respectively, of the H2SO4 required to make SSP was added. Similar results were obtained with Pesca rock. No consistent effect due to granule size was observed.Twenty percent acidulation of Pesca rock with H3PO4 was 53–76% as effective as TSP with a single crop and 79–90% as effective over three cropping periods, showing a potential for high residual value.