Phosphorus fertilizer recovery from calcareous soils amended with humic and fulvic acids

Precipitation of Ca phosphates negatively affects recovery by plants of P fertilizer applied to calcareous soils, but organic matter slows the precipitation of poorly soluble Ca phosphates. To study the effect of high molecular weight organic compounds on the recovery of applied P, a mixture of humic and fulvic acids was applied to calcareous soils with different levels of salinity and Na saturation which were fertilized with 200 and 2000 mg P kg^–1 as NH₄H₂PO₄. Recovery was measured as the ratio of increment in Olsen P-to-applied P after 30, 60 and 150 days, and associated P forms were studied using sequential chemical fractionation and ³¹P NMR spectroscopy. Application of the humic-fulvic acid mixture (HFA) increased the amount of applied P recovered as Olsen P in all the soils except in one soil with the highest Na saturation. In soils with high Ca saturation and high Olsen P, recovery increased from < 15% in the absence of amendment to > 40% at a 5 g HFA kg^–1 amendment rate (30 days incubation and 200 mg P kg^–1 fertilizer rate). This is ascribed to inhibition of the precipitation of poorly soluble Ca phosphates, consistent with the sequential chemical extraction (reduction of the HCl extractable P) and P concentration in 0.01 M CaCl₂ (1:10 soil:solution ratio) extracts. ³¹P NMR spectra revealed that in non-amended samples, most spectral shifts were due to poorly soluble P compounds (carbonate apatite); on the other hand, at the 5 g HFA kg^–1 rate, significant amounts of amorphous Ca phosphate and dicalcium phosphate dihydrate (DCDP) were identified. The increase in the recovery of applied P due to HFA reveals a positive effect of the application of organic matter as soil amendments on the efficiency of P fertilizers and also explains that manures and other organic sources of P were more efficient increasing available P than inorganic P fertilizers in calcareous soils.     

Effects of liming on phosphate availability in acid soils

Summary The critical factors involved in the plant-soil-phosphorus-lime interaction are outlined and discussed. Conflicting reports suggest that the prior liming of highly weathered acid soils can result in an increase, a decrease, or no change in the availability of applied phosphate. Adsorption of phosphate by amphoteric soil surfaces generally decreases slowly as the pH is raised from 4.0 to 7.0. However, in soils initially high in exchangeable Al³⁺, liming results in the formation of new, highly active, phosphate adsorbing surfaces as the Al³⁺ ions precipitate as insoluble polymeric hydroxy-Al cation species. Thus, if an acid soil is reacted with lime and then phosphate, without intervening air drying, liming can increase phosphate adsorption. If the same limed soil is air dried before reaction with phosphate (e.g. adsorption isotherm studies), liming decreases phosphate adsorption. Apparently, air drying alters the surface characteristics of recently limed soils, probably by promoting the crystallization of the hydroxy-Al cation polymers as gibbsite.An important phenomenon, which is often overlooked, is that liming can increase phosphate availability by stimulating mineralization of soil organic phosphorus. However, at high soil pH values, the precipitation of insoluble calcium phosphates can decrease phosphate availability. Since Al toxicity is characterised by the inhibition of the uptake, translocation and utilization of phosphate by plants, liming often increases the utilization of soil phosphate by plants through amelioration of Al toxicity.When making lime recommendations or interpreting the data collected from lime-phosphate experiments, it is important to consider all the complex interacting soil and plant factors involved.     

Improving phosphorus availability in two upland soils of Vietnam using Tithonia diversifolia H.

Phosphorus was added to two acidic upland soils (a Cambisol and a Ferralsol) at two rates (9 mg P kg⁻¹ and 145 mg P kg⁻¹) either in an inorganic P form (KH₂PO₄) or as a green manure (Tithonia diversifolia H. at 2.5 g kg⁻¹ and 40 g kg⁻¹). The effect of P source on the chemical availability of P was assessed in an incubation experiment by measuring resin extractable P, soluble molybdate reactive (DMR-P) and unreactive P (DMU-P). Soil pH and extractable Al were monitored during the incubation period of 49 days. Green manure addition caused an immediate and sustained increase in soil pH and an immediate and sustained decrease in extractable Al. Labile P (resin P + DMR-P + DMU-P) was increased more by P added as a green manure than when added in inorganic form in one soil (Ferralsol), while it decreased or did not differ in the other one (Cambisol). In both soils, the concentrations of soluble DMU-P were frequently higher where Tithonia had been added. The effects of green manure amendment on physical factors governing the phosphorus supply through diffusive transport were also investigated. Aggregate size distribution was substantially changed by green manure amendment due to a shift in the percentage of microaggregates (<250 μm in diameter) to larger sizes. Changes in soil aggregation as a consequence of green manure amendment led to a reduction in specific surface area (SSA) of the whole soil. Coupled with the large increase in effective cation exchange capacity caused by green manure amendment in both soils, and the decrease in SSA, there was an increase in the net negative surface charge density in both soils. In summary, at a large addition rate – and in addition to the well-known effect derived from the extra supply in P, green manure amendment may improve the chemical availability and diffusive supply of P through the following mechanisms: (i) an increase in soil pH increasing the solubility of phosphate sources; (ii) a decrease in extractable Al reducing the fixation of added P; (iii) increased macro-aggregation and reduced specific surface area and porosity leading to fewer sorption sites for P and hence enhanced diffusion rates; and (iv) increased negative charges and reduced positive charges at the soil surface resulting in a net increase in repulsive force for P. The induced changes in most measured soil properties were smaller in the Ferralsol than in the Cambisol. This revised version was published online in June 2005 with a corrected article title.     

Effect of Sodium and Magnesium on Kinetics of Phosphorus Release in Some Calcareous Soils of Western Iran

Knowledge of the rate of release of phosphorus (P) from soils resulting from poor water quality application is essential for long-term planning of crop production while minimizing the impact on groundwater quality. In this study, we examined the effect of sodium adsorption ratio (SAR) and Ca:Mg ratio of water on P release of some calcareous soils from western Iran. Nine different solutions at a total electrolyte concentration of 100 mmol_c l^?1 and three levels of SAR (5, 15, 45) each with Ca:Mg ratios of 1:3, 1:1, or 3:1, prepared using solutions of NaCl, CaCl₂, and MgCl₂, were used to extract P from the soils. The geochemical Visual MINTEQ was used to calculate saturation indices and P species at the initial and end of P release. Significantly different quantities of P were extracted by the solutions. The maximum (average of five soils) (233.6 mg kg^?1) and the minimum (162.9 mg kg^?1) P were extracted by an SAR 45 solution with a Ca:Mg ratio of 1:3 and SAR 15 solution with Ca:Mg ratio of 3:1, respectively. Elovich model adequately described P release. The release rate for SAR 15 with Ca:Mg of 3:1 and SAR 45 with Ca:Mg 1:3 ranges from 16.3 to 31.3 mg kg^?1 h^?1 and from 20.0 to 32.8 mg kg^?1 h^?1, respectively. In the initial stage of P release the solution samples in most soils were saturated with respect to hydroxyapatite, octacalcium phosphate, ß-tricalcium phosphate, and undersaturated with respect to dicalcium phosphate dihydrate, dicalcium phosphate, and mangnesium phosphates. At the end of P release, all solutions were saturated with respect to hydroxyapatite and under saturated with respect to other phosphate minerals. The results imply that P release from soils could be increased during use of saline and sodic irrigation water containing high Mg concentration and that P fertilization management may need modification.     

The effect of phosphate rock dissolution on soil chemical properties and wheat seedling root elongation

Soils of the Appalachian region of the United States are acidic and deficient in P. North Carolina phosphate rock (PR), a highly substituted fluoroapatite, should be quite reactive in these soils, allowing it to serve both as a source of P and a potential ameliorant of soil acidity. An experiment was conducted to evaluate the influence of PR dissolution on soil chemical properties and wheat (Triticum aestivum cv. Hart) seedling root elongation. Ten treatments including nine rates of PR (0, 12.5, 25, 50, 100, 200, 400, 800, and 1600 mg P kg^-1) and a CaCO₃ (1000 mg kg^-1) control were mixed with two acidic soils, moistened to a level corresponding to 33 kPa moisture tension and incubated for 30 days. Pregerminated wheat seedlings were grown for three days in the PR treated soils and the CaCO₃ control. Root length was significantly (P<0.05) increased both by PR treatments and CaCO₃, indicating that PR dissolution was ameliorating soil acidity. The PR treatments increased soil pH, exchangeable Ca, and soil solution Ca while lowering exchangeable Al and 0.01 M CaCl₂ extractable soil Al. Root growth in PR treatments was best described by an exponential equation (P<0.01) containing 0.01 M CaCl₂ extractable Al. The PR dissolution did not reduce total soil solution Al, but did release Al complexing anions into soil solution, which along with increased pH, shifted Al speciation from toxic to nontoxic forms. These results suggest that North Carolina PR should contribute to amelioration of soil acidity in acidic, low CEC soils of the Appalachian region.     

Effect of phosphate on aluminium-inhibited growth and signal transduction pathways in Coffea arabica suspension cells

In acid soils, aluminium (Al) toxicity and phosphate (Pi) deficiency are the most significant constraints on plant growth. Al inhibits cell growth and disrupts signal transduction processes, thus interfering with metabolism of phospholipase C (PLC), an enzyme involved in second messenger production in the cell. Using a Coffea arabica suspension cell model, we demonstrate that cell growth inhibition by Al toxicity is mitigated at a high Pi concentration. Aluminium-induced cell growth inhibition may be due to culture medium Pi deficiency, since Pi forms complexes with Al, reducing Pi availability to cells. Phosphate does not mitigate inhibition of PLC activity by Al toxicity. Other enzymes of the phosphoinositide signal transduction pathway were also evaluated. Aluminium disrupts production of second messengers such as inositol 1,4,5-trisphosphate (IP₃) and phosphatidic acid (PA) by blocking PLC activity; however, phospholipase D (PLD) and diacylglycerol kinase (DGK) activities are stimulated by Al, a response probably aimed at counteracting Al effects on PA formation. Phosphate deprivation also induces PLC and DGK activity. These results suggest that Al-induced cell growth inhibition is not linked to PLC activity inhibition.     

Mineral nutrition and growth of tropical maize as affected by soil acidity

Soil constraints linked to low pH reduce grain yield in about 10% of the maize growing area in tropical developing countries. The aim of this research was to elucidate the reasons for this maize yield reduction on an oxisol of Guadeloupe. The field experiment had two treatments: the native non-limed soil (NLI, pH 4.5, 2.1 cmol Al kg^–1, corresponding to 20% Al saturation), and the same soil limed 6 years prior to the experiment (LI, pH 5.3, 0 cmol Al kg^–1). The soils were fertilized with P and N. The above-ground biomass, root biomass at flowering, grain yield and yield components, leaf area index (LAI), light interception, radiation-use-efficiency (RUE), P and N uptake, soil water storage, and soil mineral N were measured during the maize cycle. The allometric relationships between shoot N concentration, LAI and above-ground biomass in LI were similar to those reported for maize cropped in temperate regions, indicating that these relationships are also useful to describe maize growth on tropical soils without Al toxicity. In NLI, soil acidity severely affected leaf appearance, leaf size and consequently the LAI, which was reduced by 60% at flowering, although the RUE was not affected. Therefore, the reduction in the above-ground biomass (30% at flowering) and grain yield (47%) were due to the lower LAI and light interception. At flowering, the root/shoot ratio was 0.25 in NLI and 0.17 in LI, and the root biomass in NLI was reduced by 64% compared to LI. Nitrogen uptake was also reduced in NLI in spite of high soil N availability. Nevertheless, shoot N concentration vs aboveground biomass showed a typical decline in both treatments. In NLI, the shoot P concentration vs above-ground biomass relationship showed an increase in the early stages, indicating that P uptake and root-shoot competition for the absorbed P in the early plant stages controlled the establishment and the development of the leaf area.     

Changes in organic phosphorus composition in boreal forest humus soils: the role of iron and aluminium

Organic phosphorus (P) is an important component of boreal forest humus soils, and its concentration has been found to be closely related to the concentration of iron (Fe) and aluminium (Al). We used solution and solid state ³¹P NMR spectroscopy on humus soils to characterize organic P along two groundwater recharge and discharge gradients in Fennoscandian boreal forest, which are also P sorption gradients due to differences in aluminium (Al) and iron (Fe) concentration in the humus. The composition of organic P changed sharply along the gradients. Phosphate diesters and their degradation products, as well as polyphosphates, were proportionally more abundant in low Al and Fe sites, whereas phosphate monoesters such as myo-, scyllo- and unknown inositol phosphates dominated in high Al and Fe soils. The concentration of inositol phosphates, but not that of diesters, was positively related to Al and Fe concentration in the humus soil. Overall, in high Al and Fe sites the composition of organic P seemed to be closely associated with stabilization processes, whereas in low Al and Fe sites it more closely reflected inputs of organic P, given the dominance of diesters which are generally assumed to constitute the bulk of organic P inputs to the soil. These gradients encompass the broad variation in soil properties detected in the wider Fennoscandian boreal forest landscape, as such our findings provide insight into the factors controlling P biogeochemistry in the region but should be of relevance to boreal forests elsewhere.     

Studies on mineral ion absorption by plants

Summary The effect of Al and P on the growth of lucerne (Medicago sativa) was studied in nutrient solutions in which aluminium phosphate did not precipitate. Al and P retained in the free space of the roots was washed out with 0.1N HCl/O₄ at 5°C. The inhibitory effect of Al on growth was much less at pH 5 than at pH 4.5, although 3 to 4 times as much Al was found in the roots and shoots of the pH 5 plants.It is suggested that the low toxicity of high contents of Al was due to a portion of the uptake at pH 5 being in the form of polymeric aluminophosphate complexes of low net charge density. The optimum pH for the formation and polymerization of such complexes is around 5, and their composition depends on the P/Al mole ratio of the initial solutions. Washing³²P-labelled roots in unlabelled P solutions containing Ca showed that 12–43 per cent more of the total label diffused out of the Al-treated roots at pH 5 than from control roots. This was consistent with estimates by solution analysis of 16–36 per cent (depending on the P/Al mole ratio) of the P present in the original uptake solutions being complexed with Al.     

Cadmium and zinc bioaccumulation by Phytolacca americana from hydroponic media and contaminated soils

Abstract

Hydroponic, greenhouse and field experiments were conducted to explore the potential of pokeweed (Phytolacca americana L.) to accumulate Zn and Cd from nutrient solutions and contaminated soils. The hydroponic results confirmed that this native species is a strong Zn and Cd bioaccumulator that does not experience severe phytotoxicity until quite high root and shoot concentrations, approaching 4000 and 1600?mg?kg^?1 of Zn, and 1500 and 500?mg?kg^?1 of Cd, respectively. These high Zn and Cd concentrations were accompanied by increased sulfur and lower manganese in both shoots and roots. However, in field and greenhouse trials with soils historically contaminated by a number of heavy metals including Zn and Cd, concentrations of Zn and Cd in shoots of P. americana reached concentrations less than 30% and 10%, respectively, of those achieved with hydroponics. The main constraint to phytoremediation of soils by P. americana was the low concentrations of Zn and Cd in soil solution. Pretreatment of the metal-contaminated soil by oxalic acid increased soluble Cd and Zn but failed to increase plant uptake of either metal, a possible result of higher solubility of competing metal ions (Cu, Mn) or low bioavailability of Cd and Zn-oxalate complexes.     

INFLUENCE OF SOIL PROPERTIES AND PHOSPHATE ADDITION ON ARSENIC UPTAKE FROM POLLUTED SOILS BY VELVETGRASS (HOLCUS LANATUS)

Four kinds of soil material were used in a pot experiment with velvetgrass (Holcus lanatus). Two unpolluted soils: sand (S) and loam (L) were spiked with sodium arsenite (As III) and arsenate (As V), to obtain total arsenic (As) concentrations of 500 mg As kg^?1. Two other soils (ZS I, ZS III), containing 3320 and 5350 mg As kg^?1, were collected from Zloty Stok where gold and arsenic ores were mined and processed for several centuries. The effects of phosphate addition on plants growth and As uptake were investigated. Phosphate was applied to soils in the form of NH₄H₂PO₄ at the rate 0.2 g P/kg. Average concentrations of arsenic in the shoots of velvetgrass grown in spiked soils S and L without P amendment were in the range 18–210 mg As kg^?1 d.wt., whereas those in plants grown on ZS I and ZS II soils were considerably lower, and varied in the range 11–52 mg As kg^?1 d.wt. The addition of phosphate caused a significant increase in plant biomass and therefore the total amounts of As taken up by plants, however, the differences in As concentrations in the shoots of velvetgrass amended and non-amended with phosphate were not statistically significant.     

LEAD TOLERANCE AND ACCUMULATION IN PTERIS VITTATA AND PITYROGRAMMA CALOMELANOS,AND THEIR POTENTIAL FOR PHYTOREMEDIATION OF LEAD-CONTAMINATED SOIL

A field survey was conducted to search for Pb accumulation in fern species at Bo Ngam Pb mine, Thailand. Eleven fern species including Pteris vittata accumulated Pb in the range of 23.3–295.6 mg kg^?1 in the aboveground parts. Hydroponic, pot, and field trial experiments were carried out to investigate Pb-accumulation ability in ferns; including P. vittata and the ornamental species, Pityrogramma calomelanos, Nephrolepis exaltata cv. Gracillimum, and N. exaltata cv. Smirha. In hydroponic experiment, Pi. calomelanos accumulated the highest concentration of Pb (root 14161.1 mg kg^?1, frond 402.7 mg kg^?1). The pot study showed that P. vittata, Pi. calomelanos, and N. exaltata cv. Gracillimum grew well when grown in soil Pb at 92900 mg kg^?1. N. exaltata cv. Gracillimum accumulated the highest Pb concentration in the frond (5074 mg kg^?1) and P. vittata accumulated the highest Pb concentration in the root (16257.5 mg kg^?1). All fern species exhibited TF values less than 1 in both hydroponic and pot experiments. When P. vittata and Pi. calomelanos were grown at mine soils for 6 months, P. vittata tolerated higher soil Pb (94584–101405 mg kg^?1) and accumulated more Pb in frond (4829.6 mg kg^?1) and showed TF > 1 after 2 months of growth. These results indicated that P. vittata can be potentially useful for phytoremediation of Pb-contaminated soil.     

Interactions between aluminium,phosphorus and pH in the response of barley to soil acidity

Summary Two acid soils showing different Al solubility as a function of pH were limed to a range of pH values (in 10^–2 M CaCl₂) between 4.1 and 5.6. The apparent critical pH for the growth of barley in pots was 0.25 lower in the soil showing lower Al solubility. The addition of phosphate reduced exchangeable and soluble Al in the soils, and lowered the apparent critical pH by 0.35 while maintaining the difference between the soils. The Al concentration at the critical pH, measured after cropping to take account of the treatment effects on soil Al, also varied with soil and with phosphate addition. These apparent critical values of both pH and soluble Al varied linearly with available phosphate, over the range 18 to 73 mg P/kg soil, as follows: pH from 4.9 to 4.3; soluble Al, from 0.010 mM to 0.056 mM; and the soluble Ca/Al mole ratio, from 1270 to 214.     

Effects of biochar from slow pyrolysis of papermill waste on agronomic performance and soil fertility

The amendment of two agricultural soils with two biochars derived from the slow pyrolysis of papermill waste was assessed in a glasshouse study. Characterisation of both biochars revealed high surface area (115 m² g^?1) and zones of calcium mineral agglomeration. The biochars differed slightly in their liming values (33% and 29%), and carbon content (50% and 52%). Molar H/C ratios of 0.3 in the biochars suggested aromatic stability. At application rates of 10 t ha^?1 in a ferrosol both biochars significantly increased pH, CEC, exchangeable Ca and total C, while in a calcarosol both biochars increased C while biochar 2 also increased exchangeable K. Biochars reduced Al availability (ca. 2 cmol (+) kg^?1 to <0.1 cmol (+) kg^?1) in the ferrosol. The analysis of biomass production revealed a range of responses, due to both biochar characteristics and soil type. Both biochars significantly increased N uptake in wheat grown in fertiliser amended ferrosol. Concomitant increase in biomass production (250% times that of control) therefore suggested improved fertiliser use efficiency. Likewise, biochar amendment significantly increased biomass in soybean and radish in the ferrosol with fertiliser. The calcarosol amended with fertiliser and biochar however gave varied crop responses: Increased soybean biomass, but reduced wheat and radish biomass. No significant effects of biochar were shown in the absence of fertiliser for wheat and soybean, while radish biomass increased significantly. Earthworms showed preference for biochar-amended ferrosol over control soils with no significant difference recorded for the calcarosol. The results from this work demonstrate that the agronomic benefits of papermill biochars have to be verified for different soil types and crops.     

Microbially Available Phosphorus in Boreal Forests: Effects of Aluminum and Iron Accumulation in the Humus Layer

Soil microorganisms play an important role in the mobilization of phosphorus (P), and these activities may be beneficial for plant P utilization. We investigated the effects on microbial P availability of different combinations of aluminum and iron (Al + Fe) concentrations and different P pools in humus soils from boreal forest ecosystems. We measured respiration rates in laboratory incubations before and after additions of glucose plus (NH₄)₂SO₄ (Glu+N), with or without a small dose of KH₂PO₄. Glu+N was added in excess so that the availability of the inherent soil P would be growth-limiting for the microorganisms. The exponential increases observed in microbial growth after substrate additions (Glu+N) was slower for humus soils with high Al+Fe concentrations than for humus soils with low Al+Fe concentrations. Adding a small dose of KH₂PO₄ to humus soils with high Al+Fe concentrations did, however, increase the exponential growth, measured as the slope of the log-transformed respiration rates, by more than 200%. By contrast, the average increase in exponential growth was only 6% in humus soils with low Al+Fe concentrations. Almost eight times more carbon dioxide (CO₂) was evolved between the substrate additions and the point at which the respiration rate reached 1 mg CO₂ h^–1 for soils with high Al+Fe concentrations compared to humus soils with low Al+Fe concentrations. The amount of CO₂ evolved was positively related to the Al+Fe concentration of the humus soils (r ² = 0.86, P < 0.001), whereas the slope was negatively related to Al+Fe concentration (r ² = 0.70, P < 0.001). Easily available P forms were negatively related to the Al+Fe concentration, whereas organic P showed a strong positive relationship to Al+Fe (r ² = 0.85, P < 0.001), suggesting that other forms of P, as well as inorganic P, are affected by the increased sorption capacity. The results indicate that P mobilization by microorganisms is affected by the presence of sorption sites in the humus layer, and that this capacity for sorption may relate not only to phosphate but also to organic P compounds.     

P uptake characteristics and root morphological responses in the mining ecotype of Polygonum hydropiper under high organic P media

Understanding plant phosphorus (P) assimilation and its root morphological responses is important to acquire an ideal material for remediation of P-enriched environments. Pot experiments were conducted to explore P accumulation and root morphological traits in a mining ecotype (ME) and non-mining ecotype (NME) of Polygonum hydropiper under different organic P (Po) sources (G1P, AMP, ATP, IHP) and inorganic P (Pi) source (KH₂PO₄), and also their responses to a high level of IHP for different growth periods. Both ecotypes showed higher biomass in Pi and IHP treatments than other Po sources. P accumulation in seedlings were in the order of Pi > IHP > other Po media. Extending the growth period increased biomass and P accumulation in both ecotypes. The ME demonstrated 1.11–1.46 times higher P accumulation than the NME. Seedlings fed with IHP demonstrated significantly greater morphological parameters of fine, medium, and thick roots compared to other Po sources. Total root length, surface area, and volume of both ecotypes significantly increased with the prolonged growth period. The ME has a higher ability to develop root system and exhibits better distribution of fine roots to enhance P accumulation from high P media, and thus it is a worthy material for P-phytoextraction.     

Root phosphate exudation and pH shift in the rhizosphere are not responsible for aluminum resistance in rice

A series of hydroponic experiments and an agar culture experiment were carried out to investigate aluminum (Al) accumulation and translocation in two rice (Oryza sativa L.) cultivars (Kasalath and Koshihikari) that differ in Al resistance. Al-resistance mechanisms, including Pi exudation under Al stress and pH shifts in the rhizosphere, were also studied. Al content in rice shoots was 41 mg kg⁻¹ on average and did not differ between the two cultivars, which demonstrated that the rice cultivars were not Al accumulators. The majority of Al (95–97%) accumulated in roots. Al content in roots in the resistant cultivar (Koshihikari) was lower than that in the sensitive cultivar (Kasalath), which indicated that Al-exclusion mechanisms were mainly acting in rice. However, the rate of Pi exudation from the whole root or root tips was very low in both cultivars and was not significantly influenced by Al exposure, and thus seemed not to be the main Al-resistance mechanism. On the other hand, experiments with pH-buffered solution and color changes following culture in agar medium containing bromocresol purple revealed that the Al-induced pH increase could not explain the high Al resistance of rice. In addition, the Al content in shoots of Koshihikari was lower after the formation of iron plaque on the root surface, whereas that of Kasalath was not lower. These results suggested that rice roots cell wall components or root surfaces such as iron plaque, rather than pH changes and/or root exudates including organic acids and phosphate, play important roles in Al resistance in rice.     

Phosphate fertilization reduces zinc adsorption by calcareous soils

Summary Zinc adsorption isotherms were constructed for three calcereous soils which varied in carbonate contents, texture, and past history of phosphate fertilization. The equilibrium conditions were 25°C, 0.01 M CaCl₂ and 6 days.Higher phosphate fertilization of the soils reduced Zn adsorption. The effect of P was more in the soil with lower carbonate content which suggested that soil carbonates played a dominant role in the Zn adsorption characteristics of the soils.The adsorption data conformed to the Langmuir equation. Constants (k and b) calculated from the Langmuir isotherm showed that bonding energies (k) were inversely related to extractable P; i.e. higher Zn adsorption was associated with lower bonding energy. The Zn adsorption maxima (b) were higher for the soils with higher calcium carbonate equivalent.Adsorbed Zn was extracted with a single extraction of 0.005 M DTPA. The recovery was 91 percent for the Tandojam soil, 82 percent for the Tarnab soil, and 63 percent for the Kala shah Kaku soil, indicating that most of the adsorbed Zn is not irreversibly fixed by the soils and can be utilized by plant during growth.The results suggest that P-induced Zn deficiency could not be ascribed to precipitation of Zn as insoluble Zn-P compounds in soils. The increased Zn solubility with P fertilization is the evidence that P-Zn interaction does not reside in the growing medium external to plant.The work is part of Ph.D. thesis submitted to the University of Hawaii, Honolulu, U.S.A.     

Fate of urea-N in floodwater

One day after application, urea-N remaining in the floodwater and determined as water-soluble N (urea-N + NH₄ ⁺-N) was used to calculate the potential N loss from lowland rice soils. Actual N loss calculated from ¹⁵N balance measurements using forced air exchange (airflow rate: 20 L min^-1) in greenhouse pots. Conditions for variable potential N loss were created by manipulating the method of urea application and duration of presubmergence or by selecting soils with diverse cation exchange capacities (CEC). Potential N loss tended to be lower than actual N loss; the differences were, however, nonsignificant. The method of urea application that led to the lowest potential N loss from a Guthrie silty clay loam (Typic Fragiaquult) also led to the least ¹⁵N loss and vice-versa (r=0.99^**). Duration of presubmergence did not alter the relationship between potential and actual N loss although it influenced the rate of urea hydrolysis in floodwater. The primary depencence of actual N loss on water-soluble N was maintained in soils differing in CEC (r=0.83^**). The association between potential and actual N loss was closer for high-CEC soils ( 20 cmol [+] kg^-1 soil, r=0.91^**) than for low-CEC soils (<20 cmol [+] kg^-1 soil, r=0.85^**). Ammonia volatilization could be more closely predicted by potential N loss than could apparent denitrification.The results of this study suggest that potential N loss calculated from one-time determination of water-soluble N in floodwater can be a good index of actual N loss from flooded, puddled rice soils. Notable exceptions are to be expected for soils in which water-soluble N gets lost from floodwater either before (soils with fast urea hydrolysis in floodwater) or after (soils with steady leaching) determination of potential N loss.