The role of isotopic techniques on the evaluation of the agronomic effectiveness of P fertilizers

Many isotopic techniques can be applied to determine the relative immediate and residual effectiveness of P fertilizers. Using isotopes as tracers, the percentage of utilization by plants of the P derived from a fertilizer can be determined. However this is only possible during the three or four months after the application. Therefore, the P fertilizers may be classified only according to their relative immediate effectiveness. To also evaluate residual effect, which can be observed when more P is applied than is removed with harvest, isotopes of phosphorus can be used. This residual effect is determined by comparing pool sizes of bioavailable soil P in soils with and without P fertilizer aged in soil. The bioavailable soil P pool may be analyzed according to three isotopic experimental procedures which give access to either the A value, or the E value or the L value. The aims, the similarities and the differences between these three procedures, are examined. Some of the theoretical and practical constraints of each method are described in this paper; they must be followed in order to obtain reliable information for agronomic purposes. A method involves measuring the rate of isotopic exchange of phosphate ions in soil-solution systems maintained in steady-state. It is now possible to predict the effectiveness of P fertilizers, whatever their chemical form when this method is applied on soil samples where P fertilizers were applied.This paper was originally submitted as part of the special issue on Evaluation of the Agronomic Effectiveness of Phosphate Fertilizers through the use of Nuclear Related Techniques edited by F. Zapata     

Phosphorus fertility recapitalization of nutrient-depleted tropical acid soils with reactive phosphate rock: An assessment using the isotopic exchange technique

A soil P fertility recapitalization initiative utilizinglarge rates of phosphate rocks (PRs) was proposed to improve the soil P statusand increase the sustainable food production in acid and P-deficient tropicalsoils. Two series of experiments were carried out using five tropical acidsoilstreated with heavy applications of Gafsa phosphate rock (GPR). In the firstseries, the soils were mixed with GPR at the following application rates: 0,500, 1000 and 2000 mg P·kg^–1, andincubatedfor one month in moist conditions. In another series, 1000 mg Pkg^–1 applied as GPR was added to three soils andincubated for 1.5 month; thereafter 50 mg P kg^–1as triple superphosphate (TSP) were added. The ³²P isotopic exchangemethod was utilized to assess the contribution of GPR to the available soil P.Changes in amounts, E, of P transferred with time as phosphate ions from thesoil particles to the soil solution as well as changes in pH, calcium andphosphate concentrations in soil suspensions were determined. It was foundthat:(i) the contribution of P from GPR to recapitalization of soil P fertility wasmainly assessed by E pool size, pH, calcium and phosphate concentrations; othervariables were not significant at the 0.1 level; (ii) heavy applications of GPRdid not saturate all the P sorption sites, P freshly applied as water-soluble Pwas still sorbed; (iii) recapitalization of soil P fertility using GPR waspartly obtained in some acid tropical soils; (iv) Upon dissolution, GPRprovidedcalcium ions to crops and to soils, thus reducing Al toxicity, but its limingeffect was limited. To explain these effects with heavy application rates ofGPR, it was postulated that a coating of Al and Fe compounds is formed aroundPRparticles with time, thus reducing further dissolution.     

IspG-catalyzed positional isotopic exchange in methylerythritol cyclodiphosphate of the deoxyxylulose phosphate pathway: mechanistic implications

H(2)(18)O under the bridge: Recently, the deoxyxylulose phosphate (DXP) pathway was discovered to be a second pathway supplying isoprenoid biosynthetic precursors. One of steps is an IspG-catalyzed reductive deoxygenation of methylerythritol cyclodiphosphate (MEcPP) to 4-hydroxyl-3-methyl-2-(E)-1-diphosphate (HMBPP). Using [2-(13) C,(18) O]-MEcPP, we detected the positional isotopic exchange for the bridging oxygen in MEcPP.     

Phosphate fixing capacity of soils: A survey,using the isotopic exchange technique,or soils from north-eastern France

A large variety of soil samples was collected from farms located in two north-eastern regions of France. Their phosphate fertility was assessed by the isotopic exchange kinetics method which allows for the measurement of a r₁/R ratio, where r₁ is the quantity of radioactivity still present in solution one minute after the injection of a quantity of radioactivity R into a soil-solution system at steady-state. The r₁/R parameter gives an estimate of the soil fixing capacity for phosphate ions (PFC). Results of the analysis of 233 soil samples revealed that the great majority of soils from these areas exhibit a high PFC. Neither the soil type nor mode of utilization appeared sufficient for prediction of the PFC. Attempts to express the PFC as a function of other soil parameters (pH, clay, organic matter, calcium carbonate and exchangeable cations), resulted in a significant regression equation involving soil clay content and soluble phosphate. Due to high PFC of soils in the Lorraine area, phosphate fertilizer could be applied at a time as close as possible to the plant requirements.     

Evaluation of the agronomic effectiveness of natural and partially acidulated phosphate rocks in several soils using32P isotopic dilution techniques

The agronomic effectiveness of two natural phosphate rocks (PRs) from North Carolina (USA) and Togo and their 50% partially acidulated products (PAPRs) was evaluated in two greenhouse experiments using³²P isotopic dilution techniques, namely L and A_L values.In the first experiment rye grass was grown in a soil from Ghana. While the proportion of P in the plant derived from the P fertilizer (Pdff) ranged on. the average from about 10% for the PRs up to 80% for the PAPRs, the P fertilizer recovery was less than 1% for a 60-day growth period. In the second experiment, average values of P in the maize plants derived from the PAPRs ranged from 35% to 75% in 3 different soils. Both PRs were ineffective with the exception of North Carolina PR in the Seibersdorf soil. The P fertilizer recovery was 0.25% for the North Carolina PR in this soil whereas the recovery values ranged from 1.2% to 1.6% for the PAPRs.Mean values of the relative fertilizer efficiency estimated from the L values of each soil were less than 1% for the PRs whereas the values for the PAPRs which were dependent on soil type ranged from 20% up to 45%. The coefficient of relative effect of partial acidulation, that was calculated from the ratio of A_L values for PR and PAPR in each soil indicated that partial acidulation increased the effectiveness of the natural PRs in all soils under study.This study showed that the use of³²P isotope dilution techniques allows an accurate measurement of the P availability from natural and modified PR products to crops. Another advantage is that quantitative comparison of the P sources under study, PRs and PAPRs in this case, can be made even in soils where there is no response to the applied P sources.     

Dissolution of phosphate rocks in soils. 1. Evaluation of extraction methods for the measurement of phosphate rock dissolution

The dissolution of phosphate rock (PR) in soil was determined by measuring the amount of either the dissolved inorganic phosphate (Pi) and Ca or the residual P and Ca remaining as undissolved PR in the soil. The dissolved Pi was extracted by either 0.5M NaOH or 0.5M NaHCO₃ and the dissolved Ca was extracted using either 0.5M BaCl₂/triethanolamine (TEA) or 1M NH₄OAc. Undissolved PR was extracted by 1M HCl following the extraction with either 0.5M NaOH or 0.5M BaCl₂/TEA.From the mixture of PR with dry soil, the 0.5M NaOH extracted between <0.1 and 4.7% of the undissolved Pi and the 0.5M BaCl₂/TEA extracted between 2.8 to 7.8% of the undissolved Ca. From PR incubated with moist soil, these reagents extracted most of the dissolved Pi or Ca (95 to 99%). The 0.5M NaHCO₃ extracted between < 0.1 to 4.8% of the undissolved Pi and the 1M NH₄OAc extracted between 5.2 to 10.5% of the undissolved Ca from the PR/dry soil mixture. However the extraction of dissolved Pi and Ca from PR incubated with moist soil by these reagents was incomplete (30 to 80%). The 0.5M NaHCO₃ and 1M NH₄OAc extractants tended to underestimate the extent of PR dissolution when the levels of dissolved Pi and Ca were high. The difference between the amounts of 0.5M NaOH extractable Pi (P) and 0.5M BaCl₂/TEA extractable Ca (Ca) in the PR-treated soil and in the soil alone gave a more accurate estimate of the extent of PR dissolution in laboratory incubated soils. However, under field conditions where the Pi and Ca released during the dissolution of PR can be removed by plant uptake and leaching, these two methods, which measure the amount of the dissolution products, may underestimate the extent of dissolution. Under these conditions, the amount of residual P or Ca extracted by 1M HCl is recommended as the best estimate of the amount of undissolved PR remaining in soil.     

An attempt to evaluate phosphorus fertilizer requirements of western Nigeria savannah soils

A greenhouse fertilizer trial was carried out on 60 surface soils of the western Nigeria savannah derived from basement complex rocks. Bray's P₁ available P in the soils varied between 1 and 112µg ml^–1. There was maize response to P addition and a critical P level of 12.7µg ml^–1 was calculated for the soils.For 22 of the soils, a laboratory incubation technique was used in evaluating changes in Bray's P₁ extractable P at various rates with time. The initial rapid decline in soil available P was completed between 28 and 84 days of incubation. A fertilizer factor, calculated from extracted P in treated and untreated soils varied between 1.5 and 16.7µg ml^–1 and was significantly correlated with soil pH and citrate-dithioniteextractable oxides of Fe and Al.Fertilizer rates based on critical soil P, available soil P and fertilizer factor, correlated significantly with greenhouse estimates for optimum yield obtained with the linear response plateau model (r = 0.91,p < 0.001). At ten field locations varying in available P content, response was only to P applications lower than 60 kg ha^–1 and the calculated P rates using a mean fertilizer factor of 3.0µg ml^–1 corresponded to P rates at which maximum yields were obtained in the sites.     

The agronomic effectiveness of reactive rock phosphate,partially acidulated rock phosphate and monocalcium phosphate in soils of different pH

The initial and residual fertilizer effectiveness of North Carolina RP (rock phosphate), monocalcium phosphate and partially acidulated RP (made from North Carolina RP at 30% acidulation), both granulated and non-granulated, were measured in a glasshouse experiment. Triticale (xTriticosecale) was grown for 30 days on a soil that had been adjusted to three pH values (4.2, 5.2 and 6.2). Two crops were grown with a six month interval between crops. The effectiveness of the different fertilizers was compared using relationships between (1) yield of dried tops and the amount of P applied and (2) P content (P concentration in tissue multiplied by yield) and the amount of P applied. For the first crop, relative effectiveness (RE) of the fertilizers was calculated relative to granulated monocalcium phosphate, the most effective fertilizer. Monocalcium phosphate was not applied to the second crop, so relative residual effectiveness (RRE) was estimated for each fertilizer relative to the residual effectiveness of granulated monocalcium phosphate.The relative effectiveness of granulated monocalcium phosphate (band application) was greater (RE = 1.00) than of North Carolina RP (0.01–0.02) and partially acidulated RP (0.45–0.76) for all three soil pH values for the first crop. Granulation and band application increased the effectiveness of monocalcium phosphate and partially acidulated RP, but reduced the effectiveness of North Carolina RP. Both non-granulated monocalcium phosphate and partially acidulated RP were less effective than granulated partially acidulated RP for both crops. For the second crop granulated monocalcium phosphate was most effective and the RRE of non-granulated partially acidulated RP (0.16–0.32) and North Carolina RP (0.19–0.28) was greater than for non-granulated monocalcium phosphate (0.12). For the more acidic soil the RE of non-granulated North Carolina RP was four times higher than for the high pH soil for the first crop and 60% higher for the second crop, but it was still poorly effective relative to granulated monocalcium phosphate. Granulated North Carolina RP was least effective among all the fertilizers for all soil pH values and for both crops.     

Effects of organic manure on solubility and mobility of different phosphate fertilizers in two paddy soils

The solubility and mobility of three phosphate fertilizers [superphosphate (SP), Ca-Mg phosphate (Ca-Mg-P) and diammonium phosphate (DAP)] with or without combination of farmyard manure (FYM) and/or farmyard manure juice (FYMJ) in two paddy soils (Ultisol and Entisol) from the Zhejiang Province of China were determined. Incubation experiments were conducted under controlled greenhouse conditions in the Institute of Agricultural Chemistry, Bonn University, Germany. The results showed that water soluble P and Olsen P in both the application band and the adjacent zones (10 and 20 mm from the application band) was highest with DAP, and lowest with Ca-Mg-P. Both FYM and FYMJ increased water soluble P (100–300%) and Olsen P (80–300%) for the application band in both soils. Moreover, the combination of FYM significantly increased water soluble P and Olsen P for the adjacent zones (20 mm for the Ultisol and 10 mm for the Entisol), but such effects were not significant in combination with FYMJ. Consequently, the main effect of FYM is due to organic substances from decomposition rather than organic compounds in the FYMJ. Also, the combination of FYMJ with SP and DAP significantly increased the pH value in the application band. The pH in the adjacent zone (10 mm) was slightly increased by the combination of FYM with each of three P fertilizers. The solubility and mobility of phosphate in soils differed greatly among phosphate fertilizers. The combinations of organic manure with mineral phosphate fertilizers increased solubility and mobility of phosphate in both paddy soils, especially in the Ultisol.This paper is dedicated to Professor Xi Sun.     

Soil testing for available phosphorus in soils where phosphate rock-based fertilizers are used

Traditional soil tests for phosphorus (P) were developed to arrive at fertilizer recommendations when water-soluble P fertilizers have been used. When slowly water-soluble fertilizers such as phosphate rock (PR) have been used, soil tests using acidic extractants overestimate bioavailability of P, whereas alkaline extractants underestimate it. Therefore, separate calibration curves are needed for soluble and PR-based fertilizers. There are two soil tests that show promise as suitable tests in soils fertilized with soluble as well as PR-based fertilizers. These are the iron oxide impregnated paper (P _i ) test and the ion exchange resin paper test. In both cases, the strips act as a sink for P mobilized in a soil solution, and P measured depends only on the concentration of P mobilized in the solution and not on the source of P or properties of the soil. Both tests somewhat simulate the sorption of P by plant roots without disturbing the chemical equilibrium, unlike other tests that extract P by the destructive dissolution of specific soil P compounds. In both cases, P measured from soils fertilized with PR-based fertilizers has shown very good correlation with plant response. Field calibration with crops under different pedological and agroecological regimes is needed for using these soil tests in developing fertilizer recommendations.     

Comparison of greenhouse and 32P isotopic laboratory methods for evaluating the agronomic effectiveness of natural and modified rock phosphates in some acid soils of Ghana

Phosphorus deficiency is one of the major constraints for normal plant growth and crop yields in the acid soils of Ghana and therefore addition of P inputs is required for sustainable crop production. This is often difficult, if not impossible for small-scale farmers due to the high cost of mineral P fertilizers and limited access to fertilizer supplies. Direct application of finely ground phosphate rocks (PRs) and their modified forms have been recommended as alternatives for P fertilization. The direct application of the natural and modified PRs to these acid soils implies the need to predict their agronomic effectiveness of the PRs in the simplest and most cost-effective manner. In this study the classical greenhouse pot experiment was compared to the ³²P isotopic kinetics laboratory method for evaluating the agronomic effectiveness of natural and modified Togo PR in six highly weathered Oxisols from southwest Ghana. In the ³²P isotopic kinetics laboratory experiment the six soil samples were each fertilised at the rate of 50 mg P kg^–1 soil in the form of triple superphosphate (TSP), Togo PAPR-50%, and Togo PR, respectively. Controls without P amendment were also included. Isotopic exchange kinetics experiments were carried out on two sets of samples, immediately after P fertilizer additions (without incubation) and after 6 weeks of incubation under wet conditions and at a room temperature of 25 °C. In the greenhouse pot experiment, P fertilizers in the form of Togo PR, Togo PAPR, Mali PR and TSP were each applied to the six soils at rates equivalent to 0, 30, 60, and 120 kg P ha^–1, respectively. The P fertilizers were mixed with the soils and maize (Zea mays L.) variety Obatanpa was grown for 42 days before harvest. The isotopic kinetics data of the control samples indicated that 5 of the studied soils had very low P fertility status as reflected by their low P concentrations in solution (C_P<0.02 mg P l^–1) and low exchangeable P (E₁min < 5 mg P kg^–1). The capacity factor and the fixation index of the soils were variable. Application of water-soluble P as TSP increased both the C_P and E₁ values of all the soils above the critical levels. Togo PR was least effective among the fertilizers tested for all soil soils, except in Boi soil. Acidulation of Togo PR (Togo PAPR-50%) was an effective means to increase its agronomic effectiveness. Direct application of natural Togo PR would be only feasible in the Boi soil series as reflected by its high Pdff% value in soil solution. Incubation with the P fertilizers caused an increase in the soil pH and a decline in the effectiveness of the applied P fertilizers, irrespective of the soil and the fertilizer utilized. Based upon the results of the greenhouse pot experiment, the relative crop response index (RCRI) in terms of increasing dry matter yield and P uptake followed the order of TSP > PAPR = Mali PR >Togo PR = Control. Both the laboratory index, Pdff% in soil solution derived from the isotopic method and the RCRI values obtained from the pot experiment produced similar results in ranking the P fertilizers tested according to their agronomic effectiveness. The isotopic kinetic method may be considered as an alternative to both greenhouse and field methods in the evaluation of agronomic effectiveness of P fertilizers in tropical acid soils when it offers comparative advantages in assessing the soil P status and its changes. But trained staff and adequate laboratory facilities are needed to perform this technique. Also the method can be used as a reference for comparison purposes as in this case. Further research is needed to assess the overall agronomic effectiveness (immediate and residual effects) of PR sources in predominant cropping systems of this region of Ghana.     

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

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

Availability and residual effects of phosphate rocks and inorganic P fractionation in a red soil of Central China

The agronomic effectiveness, phosphorus utilization efficiency and inorganic fractions of phosphorus, applied once to an Ultisol in Xianning, Hubei province, and then cropped 9 seasons, were measured. The results showed: (1)forall crops, the relative agronomic effectiveness (RAE) of three phosphate rocks (PRs), compared with triple superphosphate (TSP), reached 103.1–154.9% at three rates of P applied; (2) phosphorus utilization ratios during 5 years were 33.7–59.3% with the average of 35.0–52.7% for three PRs, greater than TSP ; (3)the total amountof inorganic phosphorus in soils which received phosphate fertilizers were 395.5–492.8 mg/kg, however, it was 387.4 mg/kg in soil given no phosphorus fertilizer. The most remarkable increases were occluded P(O-P) and calcium P(Ca-P) fractions at P 35 level (i.e. soil extractable P reached 35 mg/kg). (4) after 9 crops were harvested, the balance of phosphorus in the soils of control (CK) and all P 15 level treatments was negative, however, for soils treated with P 25 level and P 35 level, the balance was positive. This revised version was published online in June 2006 with corrections to the Cover Date.     

Forms and exchangeability of inorganic phosphate in composted solid organic wastes

Switzerland yearly produces more than 260,000 Mg of compost, two thirds of which is recycled in agriculture and horticulture. This research was undertaken to examine the forms and availability of inorganic P (Pi) in Swiss composts made from solid kitchen and garden wastes using the isotopic exchange kinetic technique, a sequential Pi extraction and magic angle spinning (MAS) solid-state ³¹P nuclear magnetic resonance (NMR) spectroscopy. The different approaches described in this paper demonstrate the presence of a complex mixture of Pi species in the studied composts. Isotopic exchange experiments and sequential extraction showed that these composts contained relatively large concentrations of rapidly available Pi. Significant correlations were observed between the concentration of water-soluble Pi (Cp), and the total N, C and P content of composts suggesting that organic substances partly controlled the amount of rapidly available Pi. Significant correlations were observed in alkaline composts between the amount of Pi which can not be exchanged within 3 months and the total P and Ca content. In alkaline composts solid-state MAS ³¹P NMR results suggested the presence of a range of slightly soluble and poorly crystallized Ca-P compounds such as apatites or octacalcium phosphates and of organic P compounds. The slowly or non-exchangeable Pi present in these composts could therefore be bound to Ca in the form of apatites or octacalcium phosphates.     

Residual effect of fertilizer phosphorus in lowland rice

To evaluate residual recoveries of fertilizer phosphorus (P) two field experiments were conducted in Quezon Province, Philippines during 1991 dry season, under irrigated condition. Average grain yield increase was 0.50–0.9 t/ha due to residual effects of inorganic fertilizer P, regardless of source. Residual effects of fertilizer P with Sesbania rostrata or alone increased grain yield by 0.3–1.0 t/ha over control. Significantly higher yields were obtained with S. rostrata fertilized with Morocco phospate Rock (MPR) and S. rostrata + MPR applied on rice than the control. Results revealed that there is a promising effect of residual P from the applied P sources in increasing rice grain yield. Total P uptake increased due to residual P from fertilizer P applied. Results suggest that increased P uptake increased dry matter and grain yield.     

Effect of phosphate on the sorption,desorption and plant-availability of selenium in soil

Phosphate, applied at 5µg P cm^–3, decreased selenite sorption by from 30–70% in three soils studied. Both maximum sorption (X_m) and the binding-energy of sorption as indicated by the binding-energy related constant (k) or the molar free energy (G) of the sorption reaction derived from the Langmuir equation were considerably decreased. On the other hand, phosphate sorption was decreased by increasing concentration of selenite from 0.2µg Se cm^–3 to 1.0µg Se cm^–3 in the initial solution. The competitive sorption of phosphate with selenite was likely the main mechanism involved in the P-Se interactions. The competitively sorbed selenite exhibited much larger desorption in 0.01M CaCl₂ solution, more readily extractable to 0.5M NaHCO₃ and significantly higher isotopic exchangeability compared to that sorbed without the competing anion. Results from pot trial using ryegrass indicated that phosphate application increased more efficiently the plant-availability of applied fertilizer Se than that of indegeneous Se in soil.     

Seasonal patterns of phosphate uptake in relation to comparisons between fertilizers

Pot experiments, and field trials with temperate pastures, were used to compare pelleted Citraphos (ground calcined Christmas Island C rock phosphate) with soluble phosphates. The relative effectiveness of Citraphos increased markedly with time during a growing season. This increase occurred with residual Citraphos, topdressed twelve months previously and more, as well as with freshly applied Citraphos. It was not observed when Citraphos was compared with residual superphosphate.The apparent improvement of Citraphos thus derived very largely from the seasonal pattern of phosphorus uptake associated with soluble phosphate sources. As the availability of the latter decreased progressively after the early part of the season, the effectiveness of the more constant Citraphos improved by comparison. There is considerable arbitrariness in values of relative effectiveness assigned to slow acting phosphate fertilizers on the basis of single measurements. Direct assessments of the residual values of phosphatic fertilizers, by comparison with freshly applied soluble phosphates, appear to suffer similar limitations.The sharply decreasing availability of soluble phosphates has implications for the production of improved, long lasting phosphatic fertilizers.     

Response of maize (Zea mays) to phosphorus application on basaltic soils in Northwestern Cameroon

A field trial was conducted on two P sorptive, basaltic soils commonly used for maize production in Northwestern Cameroon. The objective was to determine the maintenance P rates required for adequate P supply in the soils for maize after initial capital dressing applications of P (0, 22, 44, 88 and 132 kg ha^-1) in 1991. These were followed by three supplementary P rates – 0, 44 and 88 kg ha^-1 in 1992. Three crops of maize (cv COCA) were grown to monitor the availability of the residual P. Experimental design was randomized complete block with four replications in 1991 and a split-plot in 1992 and 1993. The soils at the experimental sites were rich in organic P which formed 67% and 57% of total P at Mfonta and Babungo respectively. Laboratory P sorption studies indicated high P requirements by the basaltic soils used in the study. The amounts of P sorbed to attain 0.2 ug g^-1 in soil solution were 1200 ug g^-1 at Mfonta and 600 ug g^-1 at Babungo. In spite of these high P sorption capacities, significant responses to small rates of P application were observed. It was concluded that a sizable proportion of P released from organic P mineralization was used to satisfy P sorption capacity of the soils, resulting in maize response to small rates of fertilizer P application. Residual P effects on maize yield were related to applied P. Bray 1 extractable soil P was weakly related to grain yields (r = 0.136 at Mfonta and r = 0.186 at Babungo). A critical value of 5.5 mg kg^-1 Bray 2 extractable P in the soil was established for maize at Mfonta site. About 44 kg P ha^-1 was recommended for maize at this site when Bray 2 soil P test was below this critical value.     

磷在土壤中的固定机制和磷肥的高效利用

磷肥当季利用率只有10%～25%,通过系统分析文献研究进展表明,磷肥低效利用的主要原因是土壤对磷的固定,固定方式包括吸附固定和化学反应固定。土壤对磷的固定取决于土壤中碳酸钙、铁铝氧化物、土壤粘粒的含量以及土壤中磷的初始浓度,在施肥初期发生大量的吸附固定,持续数小时至数十小时,后期逐渐转化为化学反应固定,使有效态磷转化为无效态磷,持续数月乃至数年。磷肥在土壤中的固定动力学可用Elovich方程描述。要减弱土壤对磷的固定,需要降低土壤中水溶性磷的浓度。本文结合文献中报道的嵊县红壤对磷的固定规律和油菜对磷的吸收特性,定量分析了磷在土壤中的释放、固定、吸收过程,通过建立模型计算得出,与单施普通磷肥相比,混施普通磷肥和包膜磷肥可大幅度降低施肥量和提高磷的当季利用率。     

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.