Impact of long-term inorganic phosphorus fertilization on accumulation, sorption and release of phosphorus in five Swedish soil profiles

The impact of long-term fertilization with inorganic P was studied in soil profiles (0–100 cm) from five sites in Sweden. Accumulation of P was studied by comparing P extracted with ammonium lactate/acetic acid (P-AL) and NaHCO₃ (Olsen-P) in non-fertilized and fertilized soil profiles. The fertilized soils had received 42–49 kg P ha^–1y^–1 for more than 30 years. P-AL and Olsen-P were significantly higher in the fertilized than in the non-fertilized profiles down to 40 cm depth. The P sorption index (PSI₂) based on a single-point P addition of 50 mmol P kg^–1 soil was used to estimate P sorption capacity in the soils. The variation in PSI₂ with depth was not consistent between the five soil profiles. PSI₂ did not vary with depth in one soil, while it decreased in one and increased in the other three, and it was weakly but significantly correlated with the sum of Fe and Al extracted with ammonium oxalate (Fe_ox +Al_ox) (r = 0.65^**) and with clay content (r = 0.69^***). To estimate P release in the soils, P was extracted with CaCl₂ (CaCl₂-P) and water (P_w). CaCl₂-P and P_w were significantly higher in the fertilized treatment than in the non-fertilized treatment in the top 20 cm. Below 30 cm depth, CaCl₂-P was very low in all soils, while P_w was relatively high in two soils and low in the other three soils. To estimate the degree of P saturation, the ratio of P-AL/PSI₂ and Olsen-P/PSI₂ was calculated. P-AL/PSI₂ was significantly higher in the fertilized treatment in the 0–20 cm layer, while Olsen-P/PSI₂ was significantly higher in the fertilized treatment in the 0–40 cm layer. P-AL/PSI₂ was correlated with CaCl₂-P and P_w when all soils and horizons were included (r0.78^***), but the correlation increased markedly when only 0–40 cm was included (r0.94^***). Olsen-P/PSI₂ was well correlated with CaCl₂-P and P_w (r0.94^***) for all soils and depths. Thus the two indices, P-AL/PSI₂ and Olsen-P/PSI₂, were comparable in their ability to predict P release in the top 40 cm, whereas Olsen-P/PSI₂ was better when all depths were included. The overall conclusion was that P fertilization had an impact on P properties down to 40 cm depth, while the effects were small below this depth.     

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

The P_i 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).     

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

The iron oxide impregnated filter paper test (P_i 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 P_i 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 P_i-P. Likewise, in calcareous soils, P_i-P was as good as Olsen-P in predicting crop response. Field trials have shown that the P_i 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 P_i method is a faster and easier method to estimate P that may be potentially available for uptake by algae.     

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.Dr. A.G. Sinclair died on 3 December 1996 whilst this paper was in preparation.     

Phosphorus sorption in relation to soil properties in some cultivated Swedish soils

Phosphorus (P) sorption properties are poorly documented for Swedish soils. In this study, P sorption capacity and its relation to soil properties were determined and evaluated in 10 representative Swedish topsoils depleted in available P. P sorption indices were estimated from sorption isotherms using Langmuir and Freundlich equations (X_m and a_F, respectively) and P buffering capacity (PBC). X_m ranged from 6.0 to 12.2 mmol kg^–1. All indices obtained from sorption isotherms were significantly correlated with each other (r=0.96^*** to r=0.99^***). Two single-point sorption indices (PSI₁ and PSI₂) were also determined, with additions of 19.4 and 50 mmol P kg^–1 soil, respectively. Both PSI indices were well correlated with X_m (r0.98^***), with PSI₁ giving the highest correlation. As isotherms for determining P sorption capacities involve laborious analytical operations, PSI₁ would be preferable for routine analyses. X_m was significantly correlated with Fe extracted by sodium pyrophosphate and ammonium oxalate, to Al extracted by ammonium oxalate and dithionite-citrate-bicarbonate and to organic c. X_m was also significantly correlated with the sum of Fe and Al extracted by ammonium oxalate. The best prediction of X_m through multiple regression was obtained when Fe extracted in ammonium oxalate and Al extracted in dithionite-citrate-bicarbonate were used. Based on the results obtained, both PSI₁ and oxalate-extractable Fe plus Al can be used for predicting P sorption capacity in Swedish soils.     

Agronomic evaluation of fertilizer products derived from Sukulu Hills phosphate rock

Partial acidulation of phosphate rock (PR) or compaction of PR with soluble P fertilizers can improve the usefulness of unreactive PR for use as P fertilizer. A greenhouse study was conducted to evaluate nonconventional phosphate fertilizers derived from a low reactive Sukulu Hills PR from Uganda. Raw PR (which contained 341.0 g kg^–1 Fe₂O₃), beneficiated or concentrate PR, partially acidulated PR (PAPR) and PR compacted with triple superphosphate (TSP) were evaluated. Compacted materials had a P ratio of PR:TSP = 50:50. PAPR materials were made by 50% acidulation with H₂SO₄. TSP was used as a reference fertilizer. Fertilizers were applied to an acidic (pH = 5.4) Hiwassee loam (clayey, kaolinitic, thermic Rhodic Kanhapludults) at rates of 0, 50, 100, 200, 300 and 400 mg P kg^–1 soil. Two successive corn (Zea mays L.) crops were grown for 6 weeks. Compacted concentrate PR + TSP and raw PR + TSP were 94.4 and 89.7% as effective as TSP, respectively, in increasing dry-matter yields for the first corn crop. PAPR from the concentrate was 54.8% as effective as TSP. Raw PR, concentrate PR and the PAPR from the raw PR were ineffective in increasing dry-matter yields. The same trends were obtained when P uptake was used to compare effectiveness. Ineffectiveness of the raw PR and its corresponding PAPR was attributed to a high Fe₂O₃ content in the raw PR. Bray I and Pi paper were found to be nearly equally suitable at estimating available P in the soils treated with responsive fertilizer materials. Mehlich 1 overestimated available P in soil treated with raw PR, concentrate PR or the PAPR from the raw PR.     

Comparison of efficiency of Mussoorie partially acidulated phosphate rock and single superphosphate in a shallow Alfisol of the Indian semiarid tropics

The agronomic effectiveness of a partially acidulated phosphate rock (PAPR) was measured in a field experiment with sorghum (Sorghum bicolor cv. CSH-6) in a shallow Alfisol at the ICRISAT farm, Patancheru (Hyderabad), India. The experiment compared PAPR with single superphosphate. The PAPR was made by acidulating an indigenous Indian phosphate rock (Mussoorie) with H₂SO₄ at 50% acidulation level. P response was evaluated at a single relatively high N rate (120 kg ha^–1) with five rates of P (0, 2.2, 4.4, 8.8, and 17.6 kg P ha^–1). A significant response to P was obtained at rates up to 17.6kg P ha^–1.There was no significant difference due to source of P in terms of sorghum grain yield or total P uptake. Both Olsen and Bray 1 soil tests underestimated P availability from PAPR with respect to that from SSP.A rapid rate of P uptake was observed during grain filling to maturity (81–102 days), when 40% of the total P was taken by the plant. The internal efficiency of both P sources was the same.     

Phosphate availability in calcareous Vertisols and Inceptisols in relation to fertilizer type and soil properties

The availability to plants of fertilizer phosphorus (P) applied to soil, as measured by chemical extraction, is used to estimate P fertilizer needs. We studied the availability of P, applied as monocalcium phosphate (MCP) powder, ordinary superphosphate (OSP) granules and diammonium phosphate (DAP) granules in 24 calcareous Vertisols and Inceptisols of Andalusia, Spain, by using laboratory incubation techniques. The soils differed widely in their P adsorption- and Ca-phosphate precipitation-related properties. For MCP, availability (defined as the proportion of added P that is recovered by extraction with NaHCO₃ or is isotopically exchangeable) decreased markedly with incubation time and increasing addition rate. The mean recoveries after 180 d of incubation at field capacity at a rate of 246 mg P kg^–1 soil were 17% for Olsen P, 38% for Colwell P, and 16% for isotopically exchangeable P (IEP). Increasing the application rate to 2460 mg kg^–1 resulted in recoveries of 6% for Olsen P, 25% for Colwell P, and 4% for IEP. While IEP-based recovery was not significantly correlated to any soil property, that based on Olsen P (and, to a lesser extent, Colwell P) decreased sharply with increase in the ratio of clay (or Fe oxides) to total (or active) calcium carbonate equivalent. Accordingly, Olsen P might overestimate P availability in those soils relatively rich in carbonate and poor in clay and Fe oxides. On the other hand, recovery of applied P from soils containing more clay and Fe oxides, by a sequential extraction (with H₂O, two 0.5M NaHCO₃ treatments, 0.5M HCl), was lower than 100%, thereby suggesting phosphate occlusion by Fe oxides or clay.Availability of the fertilizers tested 90 d after application was found to decrease in the following order: MCP powder (rate, 246 mg kg^–1) > DAP granules (rate, 547 mg kg^–1) > MCP powder (rate, 738 mg kg^–1) > OSP granules (rate, 308 mg kg^–1). Differences between fertilizers tended to increase with increasing carbonate content in the soil. This may have been due to precipitation of Ca phosphates caused by the presence of Ca in the fertilizer and the high Ca- supplying capacity of the more calcareous soils.     

Influence of soil organic carbon on the interpretation of soil test P for wheat grown on alkaline soils

Organic carbon is known to alter crop response to applied phosphorus (P) but that fact has not been incorporated in soil test interpretations. To achieve this objective, field experiments with wheat were conducted for four years on alkaline soils of Punjab, India. The experimental soils ranged from loamy sand to loam in texture, 7.4 to 9.6 in pH, 0.16 to 0.75% in organic carbon (OC) and 2 to 40 mg Olsen extractable P kg^–1 soil. Response of wheat to fertilizer phosphorus application was related to the combined effect of Olsen P and soil OC content. At a given Olsen P level, wheat yield was a function of soil OC content. Multiple regression analysis of the data showed that OC content <0.2% did not affect yield significantly. At values >0.6%, OC along with Olsen P accounted for 97% of the variation in yield and there was no response to applied fertilizer P. Yield isoquants for 4 and 5 tons grains ha^–1 showed that for a given Olsen P level, as OC content increased the amount of fertilizer P required to achieve a yield target decreased. It was shown that OC may be used to approximate the contribution of organic P mineralization to plant available soil P during a growing season. The reliability of fertilizer recommendations based on Olsen P may be improved on some alkaline soils by consideration of soil OC content.     

Comparison of five soil testing methods to establish phosphorus sufficiency levels in soil fertilized with water-soluble and sparingly soluble-P sources

Field experiments were conducted in Niger with pearl millet (Pennisetum glaucum [L] R. Br.) in which the crop was fertilized with phosphate rock (PR) from two deposits from Niger (Tahoua and Parc W). The PR was applied either as ground rock or as partially acidulated phosphate rock (PAPR) and was compared to water soluble sources (TSP and SSP) in terms of millet yield response. The ability of five soil testing procedures (Bray P₁, Bray P₂, Mehlich 1, Olsen, and water extraction) to establish P sufficiency levels for millet was tested. The results of all soil testing methods were highly correlated amongst each other for the treatments receiving water-soluble fertilizers or PAPRs. None of the soil testing procedures which were evaluated was able to accurately measure available P when PRs were applied. Sufficiency levels were calculated for the PAPR and water-soluble fertilizers using nonlinear regression analysis and a graphic procedure for each of the P soil testing methods. The Bray P₁ method appeared to be the most reliable procedure and was used to study the effect of accumulated total or total water + citrate-soluble P rates on final P availability. A single quadratic function was able to describe this effect when the P rates were expressed as water + citrate-soluble P for both PAPRs and water-soluble fertilizers independently of the P fertilizer source.     

Evaluation of residual and cumulative phosphorus effects in contrasted Moroccan calcareous soils

Consideration of factors affecting the availability of applied P in soils could improve P fertilization recommendations. Little information is available on the effects of continuous P fertilizer applications under cropping systems in Morocco. A greenhouse study was conducted to evaluate the residual and cumulative P effects on three succeeding crops, wheat ( Triticum aestivum L.)-corn (Zea maize)-wheat, in contrasting calcareous soils from the arid and semiarid zones of Morocco. The treatments were the amount (0, 3.4, 6.7, or 13.4 mg P kg^-1 soil) and time of application of P. The residual P effects on grain yield, dry-matter production, and total P uptake were significant. In this study, the increase in yields as a function of applied P was explained by the model: Y = a + b*(P_ad)^0.5. The increase rate (constant b in the model) of dry-matter production of corn ranged from 0.56 (soil 10) to 2.89 (soil 11). At the same P fertilizer rate, single applications yielded less grain production than repeated applications. These results showed that if we want to take residual P into consideration in P fertilizer recommendations, the critical soil test P level should be lower than the one normally determined by soil test calibration method. Also, soils with low initial NaHCO₃-P levels had the lowest residual value, inferring that a large portion of added P is fixed in these soils. This study showed that a significant response of corn to residual P would occur in soils with initial NaHCO₃-P test levels less than 6 mg P kg^-1. The response would be inconsistent between 9 and 14 mg P kg^-1, and no response is expected above a soil test P level of 14 mg P kg^-1. In general, soils with more than 14 mg kg^-1 NaHCO₃-P could provide adequate P for maximum yield for three succeeding crops under greenhouse conditions.     

The Olsen P method as an agronomic and environmental test for predicting phosphate release from acid soils

The role of soil phosphorus (P) in the eutrophication of fresh water systems is well established. It is crucial therefore to assess the potential loss of P from soil in the various scenarios where soil can come into contact with water. To date, such assessment has often been based on soil P tests that are used for agronomic purposes (e.g. fertilizer recommendations). The purpose of this work was to examine the usefulness of one such test (viz. the Olsen test, which is based on extraction with bicarbonate) for predicting not only the amount of soil P available to plants, but also that which can be desorbed to water in a group of 32 Portuguese soils, of which 29 were acid and 3 calcareous. To this end, we (i) assessed the total amount of phytoavailable P in soil by successively pot-cropping Chinese cabbage, buckwheat and rye; and (ii) measured the amount of phosphate-P desorbed to a dilute electrolyte mimicking fresh water over periods of up to 218 days at soil:solution ratios of 1:100, 1:1000 and 1:10000. Total phytoavailable P and Olsen P were found to bear a quadratic relationship, with Olsen’s extractant underestimating the content in phytoavailable P of soils with high Olsen P contents relatively to soils with low contents. The “change point” at which phytoavailable P began to increase rapidly per unit change in Olsen P was 53 mg Olsen P kg⁻¹ soil. For the acid soils, a significant quadratic relationship was found between the amount of P desorbed to water and Olsen P at the three soil:solution ratios studied. However, these relationships became less significant when only the soils with an Olsen P value of less than 50 mg kg⁻¹ were considered. For the acid soils, the change point at which P input to water began to increase rapidly per unit change in Olsen P was 20, 61 and 57 mg kg⁻¹ at the 1:100, 1:1000 and 1:10000 ratio, respectively. At comparable Olsen P values, the calcareous soils released more phosphate to water than the acid soils. On the basis of our results, we suggest the following environmental threshold values for Olsen P in acid soils: 20 mg kg⁻¹ for P desorption scenarios where the soil:solution ratio is high (e.g. drainage water) and 50 mg kg⁻¹ for desorption scenarios where the soil:solution ratio is low (e.g., runoff, water in reservoirs). Both values are higher than the agronomic threshold above which plants are well supplied with P.     

Solubility and agronomic effectiveness of partially acidulated phosphate rocks as influenced by their iron and aluminium oxide content

Partially acidulated phosphate rock (PAPR) has been shown to be an effective source of P for plants grown on acid soils. Less information in available, however, regarding the effect of the phosphate rock (PR) source on the solubility and agronomic effectiveness of PAPR.The effect of Fe₂O₃ + Al₂O₃ content in PR on the quality of PAPR produced was investigated in this study. Nine sources of PR from Africa, Latin America, and the United States, representing a range of Fe₂O₃ + Al₂O₃ from 0.7% to 12.4%, were used. In a single-step process, the finely ground PRs were partially acidulated with H₂SO₄ at the 30% or 50% acidulation level and granulated (–3.35 + 1.18 mm or –6 + 14 mesh). It was found that the water-soluble P content in PAPR decreased with increasing Fe₂O₃ + Al₂O content in the PR used. Apparently, the presence of Fe₂O₃ + Al₂O₃ resulted in a reversion of some of the water-soluble P to citrate-soluble P and sometimes even to citrate-insoluble P.A short-term (6 weeks) greenhouse study was conducted to evaluate crop response to PAPRs and single superphosphate (SSP); maize, the test crop, was grown on an acid soil (pH 4.5)—Hartsells silt loam (Typic Hapludults). The agronomic effectiveness of PAPRs with respect to SSP (in terms of dry-matter yield of maize) decreased with increasing Fe₂O₃ + Al₂O₃ content in PRs. Phosphorus uptake by maize from PAPRs was found to correlate well with water solubility but not with citrate solubility. The results obtained in this study show that the detrimental effect of Fe₂O₃ + Al₂O₃ content on the solubility and P availability of PAPR should be considered when selecting a PR for PAPR production.     

Comparison of P- availability from monocalcium and diammonium phosphates using a mechanistic nutrient uptake model

Calcium and ammonium phosphates are the most commonly used phosphate fertilizers. Since they differ in some chemical aspects it is important to compare their ability for supplying P to plant roots in different soils. The objective of this research was to compare the predicted effectiveness of monocalcium phosphate (MCP) and diammonium phosphate (DAP) for supply of P to maize in 13 soils. Phosphorus was applied at rates varying with soil from 50 to 400 mg kg^–1. Thirty days later P, Ca, pH, and Al were measured in the soil solution and in the solid phase. We calculated buffer power (b) and effective diffusion coefficient (D_e) for P, and used them, together with solution P (C_li), in the Barber-Cushman mechanistic nutrient model to predict P uptake. Monocalcium phosphate and DAP were similarly effective in supplying P to plant roots. Predicted P uptake differed between fertilizers in only three soils, and maximum differences between fertilizers in C_li or resin-exchangeable P (C_si) in any one soil were always less than 30%. The determinations most highly correlated with predicted P uptake were D_e (r = 0.93^**) and C_li (r = 0.60^*). Resin-exchangeable P was not significantly correlated with C_li, D_e, b or P uptake. Calcium, Al, and pH varied with source of P and soil: soils treated with DAP had lower extractable Al, lower Al in solution, and higher soil pH than soils where MCP was applied. Monocalcium phosphate increased extractable Ca whereas DAP did not affect it.Contribution of Purdue Univ. Agric. Exp. Stn. Purdue Journal Paper No. 12094. Received 0000.     

Development and evaluation of an improved soil test for phosphorus. 2. Comparison of the Olsen and mixed cation-anion exchange resin tests for predicting the yield of ryegrass grown in pots

A glasshouse experiment was conducted on four soils contrasting in P sorption capacity and exchangeable Ca content with perennial ryegrass using six phosphate rock (PR) sources and a soluble P source applied at four rates (including a control). After three harvests (11 weeks) replicate pots of each treatment were destructively sampled and Olsen P and mixed cation-anion exchange resin (Resin P) extractions carried out. The remaining replicated treatments were harvested another seven times (during 41 weeks). Yields (for the last seven harvests) were expressed as percentages of the maximum yield attainable with MCP.In general, the Resin P test extracted more than twice as much P as the Olsen test. There was a significant increase in Resin P with an increase in the amount of each P source in all four soils, but Olsen P values were not significantly different for soils treated with different rates of each phosphate rock. The abilities of the Olsen and mixed resin soil P tests to predict the cumulative dry matter yield from 7 harvests and the relative yield of ryegrass were compared. Correlations between measured yield (for the last 7 harvests) and soil test for each soil, and relative yield and soil test for all four soils were assessed by regression analysis using Mitscherlich-type models.When dry matter yields were regressed separately against soil test values for each soil, the Resin P consistently accounted for 18–28% more of the variation in yield than did Olsen P. For Resin P a single function was not significantly different from the separate functions fitted to MCP and PR treatments. However, for Olsen P the separate functions for the MCP and PR treatments varied significantly from the single fitted function. The Resin P test (R² = 0.84) was a better predictor of relative yields over this range of soils than the Olsen test (R² = 0.75). Two regression models based on the regression of relative yield for MCP treatments against either Olsen or Resin were developed. These models were then fitted to the relative yield data on soils fertilized with PRs only. The Olsen P model was found to be a poorer predictor (R² = 0.41) than the Resin P model (R² = 0.73) because it underestimated the observed yield of the PR treatments.     

Greenhouse evaluation of agronomic potential of different sources of phosphate fertilizer in a typical concretionary soil of Northern Ghana

The concretionary soils of Northern Ghana, which are near neutral with respect to pH and which comprise mostly lateritic ferruginous nodules are known to sorb significant amounts of phosphate. Instead of imported superphosphate, the use of less expensive indigenous Togo rock phosphate (PR) or partially acidulated (50%) Togo rock phosphate (PAPR-50), are possible alternative phosphate fertilizer options for these soils. The objective of this research was to evaluate the effectiveness of freshly-applied SSP, PR and PAPR-50, and the effectiveness of the residues of these fertilizers in a glasshouse pot study. Laboratory studies were also undertaken to study the transformation of these fertilizers after their application to the concretionary ferruginous soils. In the greenhouse study, yield of dried tops and the P uptake by the tops of maize var. Dobidi (Zea mays) was used to measure fertilizer effectiveness. One level of P was applied for each fertilizer (26.4 kg P ha^–1). Plants were grown for 28 days. After harvesting the first crop, subsequent cropping was carried out to evaluate the effects of the residual P in the pots. The results showed that increases in dry matter yield of shoot and total P uptake followed the trend SSP > PAPR-50 > PR > control. The relative agronomic effciency (RAE) of PAPR-50 was 58% that of commercial SSP in increasing growth of the crop, while that of PR was only 23%. The residual effect of either PAPR-50 or PR on dry matter yield and total P uptake was found to be negligible compared with SSP, suggesting that apatitic P was poorly effective relative to SSP in the used soils. The P fractionation results confirmed that PR and PAPR-50 did not significantly increase any of the P fractions in either the soil fines or nodules after the first crop. By contrast, application of SSP increased all extractable Pi fractions, most of the P added being recovered from the nodules in forms associated with Fe (hydroxide and residual Pi).It is concluded that, relative to SSP, the P from residues of PAPR-50 and PR are poorly effective in the soils studied for sustainable plant production.     

Assessment of plant-available phosphate in limed,acid soils using several soil-testing procedures

A range of soil-testing procedures was used in a factorial glasshouse study to assess the plant-available phosphate (P) status of soils which had been treated with lime and added P. A close 1:1 relationship (r = 0.90^***) was obtained between plant P uptake and resin-extractable soil P. In contrast, Olsen-, Colwell-, Bray (I) and (II)-, and Mehlich-extractable P were only weakly correlated with P uptake. Inclusion of 4 different indices of P-buffer capacity did not improve the relationship between plant P uptake, and extractable P. The difficulty in relating plant P uptake data to extractable-soil P levels is attributed to the problems associated with extracting P from limed soils. There was no useful relationship between plant P uptake and isotopically-exchangeable P in the soils.     

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.     

An evaluation of water extraction as a soil-testing procedure for phosphorus I. Glasshouse assessment of plant-available phosphorus

A water extraction procedure was evaluated as a soil-testing procedure for phosphorus (P). In a glasshouse experiment using perennial ryegrass, the water extraction procedure was used to predict plant-available P in 20 New Zealand soils varying widely in P status and P retention capacity. Water-extractable P in the 20 soils was highly correlated with plant uptake of P (r = 0.90^**). Although plant uptake of P and Olsen-extractable P were equally well correlated (r = 0.90^**), relationships between plant uptake of P and Bray₁ — and Truog-extractable P, and isotopically exchangeable P were less close. The prediction of plant-available P using water extraction was not improved by inclusion of an estimate of P-buffering capacity (obtained from P retention capacity or the slope of the P desorption isotherm), in contrast to the finding for Olsen-extractable P. Because the interpretation of the results obtained appears to be independent of P-buffering capacity and soil type, the water extraction procedure may have advantages over the other soil-testing procedures for P for soils containing reasonable amounts of water-extractable P.     

Phosphorus effectiveness in fertilized soils evaluated by chemical solutions and residual value for wheat growth

Nineteen soils from the south east of the Province of Buenos Aires (Argentina) that had been fertilized with moderate amounts of P (10–40 kgP/ha) during the last 10 years were used to investigate the effect of time on the decline of P availability as measured by three soil tests (Bray 1, Bray 2, Olsen) and the null-point method. Differences in rates of P decline among soils and chemical methods were characterized by an exponential coefficient for time (b ₂) in equations which describe the changes of the added P retained by the soil (Pr =ac ^b1 t ^b2). The rate of decline of P for the nineteen soils calculated for the soil test methods was ordered decreasingly as: null-point > Olsen > Bray 1 > Bray 2. The ability of the chemical methods for assessing the residual value of P for wheat growth (RV) was tested in a pot experiment on seven of the soils that differed in their individual rates of reaction with P. Differences between soils in the rate of reaction with P as measured in the laboratory by the null-point method and by the Olsen test were reflected in different residual values for P fertilizer for wheat plants. Thus the value ofb ₂ for these methods was well correlated with the observed residual values. The soil properties commonly associated with the retention of P were not related to the value ofb ₂ suggesting that more than one soil property may be involved in the measure ofb ₂. The exponent for timeb ₂ may be used as an index of the ability of the soil test to reflect the decline of P availability with time.