R. G. Menon

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.

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...

Interlaboratory comparison of soil phosphorus extracted by various soil test methods

The widespread use of soil phosphorus (P) data, either in the context of agronomic or environmental management, requires an explicit understanding of potential errors related to soil P testing. This study compares a variety of soil P extraction methods, each performed by 9 separate laboratories on 24 soils from across the United States. Soil clay content ranged from 0 to 47%, pH from 4.2 to 8.6, and Mehlich-3 P concentration from 2 to 205 mg kg−1. Average interlaboratory coefficients of variation (CVs) ranged from 0.11 to 0.22 for solution extracts (Bray-1 P, Fe-strip P, Mehlich-3 P, and Olsen P) and from 0.11 to 0.70 for saturated paste extracts (resin capsules and resin membranes, incubated for 2, 4, and 7 days). For soil tests based upon solution extracts, Olsen P exhibited the greatest variability among laboratories (CV = 0.22), despite its reputed suitability for a wider range of soils than Bray-1 and Mehlich-3. Soil test data were highly correlated, with the lowest correlations occurring between Olsen and Bray-1 P or Olsen and Mehlich-3 P (r = 0.77 and 0.84, respectively) and the highest correlations occurring between Olsen P and Fe-strip P or Mehlich-3 and Bray-1 P (r = 0.94 for both correlations). Results indicate that some common soil test P protocols, when carefully conducted, yield data that may be reliably compared, such as in the compilation of regional and national soil databases.

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.

Effect of acidulation of high cadmium containing phosphate rocks on cadmium uptake by upland rice

Little information is available in literature on Cd uptake by crops from either phosphate rock (PR) or partially acidulated PR (PAPR). The purpose of this greenhouse experiment was to study the effect of acidulation of two PRs having high Cd content (highly reactive North Carolina PR and low-reactive Togo PR) on Cd uptake by upland rice. The degrees of acidulation with H2SO4 were 100% for North Carolina PR (NC-single superphosphate [SSP]) and 50% or 100% for Togo-PR (i.e., Togo PAPR or Togo-SSP). Separation of the confounding effect between P uptake and Cd uptake from various P sources was made by adding 200 mg P/kg as KH2PO4 to all the treatments. Rates of Cd added from various P sources were 50–400 µg Cd/kg. Upland rice (Oryza sativa L.) was grown on two acid soils (Hartsells, pH 5.0 and Waverly, pH 5.6) to maturity.The results show that Cd uptake by rice grains followed the order of NC-SSP> NC-PR and Togo SSP> Togo PAPR> Togo PR. The results also showed that most of the Cd uptake was retained in rice roots and straw. Total uptake of Cd, Ca, and P by rice plant (root, straw, and grain) was higher from NC-PR than from Togo-PR. Cd concentration in rice grains showed no significant difference between NC-PR and Togo-PR, whereas Cd concentrations in root and straw were higher with NC-PR than that with Togo-PR. There was a significant relationship between total Cd uptake by rice plant and Cd extracted by DTPA from soils treated with various P sources at 400 µg Cd/kg.

Agronomic evaluation of modified phosphate rock products

Phosphorus (P) is critically needed to improve the soil fertility for crop production in large areas of developing countries. The high cost of conventional, water-soluble P fertilizers constrains their use by resource-poor farmers. Finely ground phosphate rock (PR) has been tested and used as a direct application fertilizer on tropical acid soils as a low-cost alternative where indigenous deposits of PR are located. However, direct application of PR with low reactivity or with inappropriate soil/crop combinations does not always give satisfactory results. Partial acidulation of PR (PAPR) or compaction with triple superphosphate (PR + TSP) or single superphosphate (PR + SSP) represent technologies that can be used to produce highly effective P fertilizers from those indigenous deposits. Numerous field trials conducted by IFDC in Asia, sub-Saharan Africa, and Latin America have demonstrated that PAPR at 40-50% acidulation with H2SO4 or at 20% with H3PO4 approaches the effectiveness of SSP or TSP in certain tropical soils and crops. This paper discusses how the agronomic effectiveness of PAPR is affected by mineralogical composition and reactivity of PR used and by soil properties and soil reactions. The paper also indicates that if a PR has high Fe2O3 + Al2O3 content, it may not be suitable for PAPR processing because of the reversion of water-soluble P to water-insoluble P during the PAPR manufacturing process. Under these conditions, compaction of PR with water-soluble P fertilizers (e.g. SSP, TSP) at P ratio of approximately 50:50 can be agronomically and economically attractive for utilizing the indigenous PRs in developing countries.

Modified techniques for preparing paper strips for the new Pi soil test for phosphorus

The Pi soil test, a new approach to measure plant-available P, uses strips of filter paper impregnated with iron oxide as a collector for P in the soil suspension. A modified method for impregnating the paper strips with iron oxide was developed by exposing the FeCl3-treated paper to ammonia vapor instead of immersing it in NH4OH solution. The paper strips prepared by the vapor technique had a more uniform deposit of iron oxide, and the P extracted from four soils ranging in pH from 4.5 to 8.2 and fertilized with two P sources and three rates correlated significantly with dry-matter yield (r = 0.751) of maize.The vapor technique was effective in preparing strips impregnated with both Fe and Al oxides. The ratio of Fe and Al on the strips was about the same as that in the solutions in which the papers were immersed. Strips containing iron oxide alone extracted more P than did strips containing both Fe and Al oxides. The dry-matter yield and P uptake of maize correlated significantly with P extracted by the strips impregnated with Fe, Al, or (Fe + Al) oxides. Phosphorus extracted by the paper impregnated with Fe oxide, and a mixture of Fe and Al oxide in the ratio of 2:1 correlated best with dry-matter yield (r = 0.751 and r = 0.736) and P uptake (r = 0.776 and r = 0.777).

Comparison of Olsen and Pi soil tests for evaluating phosphorus bioavailability in a calcareous soil treated with single superphosphate and partially acidulated phosphate rock

The Pi test for phosphorus (P) is a new method in which strips of iron oxide impregnated filter paper are used as a sink to sorb and extract P from a soil solution. In a greenhouse experiment, the Olsen and Pi tests were compared for their effectiveness in evaluating P availability to maize on calcareous soils. Phosphate rock from Togo, partially acidulated with H2SO4 at 50% acidulation level (PAPR 50% H2SO4) and single superphosphate (SSP) were applied at different rates to a calcareous soil (Vernon Clay, pH 8.2, CaCO3 18.9%) which was preincubated with KH2PO4 to raise plant-available P to different levels. In soils treated with SSP, dry-matter yield of maize correlated equally well with Pi-P and with Olsen-P (r = 0.96***). P uptake correlated significantly with Pi-P (r = 0.94***) as well as Olsen-P (r = 0.97***). Likewise, in soils fertilized with PAPR, significant correlations were found between dry-matter yield and Pi-P (r = 0.97***) and between dry-matter yield and Olsen-P (r = 0.94***). When all the data were pooled, Pi-P and Olsen-P correlated equally well with both dry-matter weight (r = 0.97***) and P uptake (r = 0.94***). Phosphorus extracted by the Pi test correlated significantly with P extracted by the Olsen test (r = 0.99***).