The effect of phosphate withdrawal by plant and by an anion-exchange resin on the phosphate potential of a soil

Summary Varying degrees of phosphate depletion by plants and AER were brought about in two acid soils. After depletion, the monocalcium phosphate potential was determined in soil suspended in dilute CaCl₂ solution.The results showed that phosphate depletion either by plants or AER did not change the phosphate potential of a soil. It seemed that some of the labile phosphate removed by plants was replaced by a mobilization of non-labile soil phosphate fraction in the soil.The constancy of the phosphate potential indicated that the concentration of phosphate in the soil solution was controlled by the solubility of sparingly soluble soil phosphates. The data implied that it was the solubility of hydroxyapatite which determined the phosphate potential of both soils.     

Effects of phosphate sorptivity on long-term plant recovery and effectiveness of fertilizer phosphate in soils

Summary A greenhouse experiment was carried out on 30 soils of eastern Australia in the first year and 15 soils in the following years to determine the effects of soil phosphate sorptivity and phosphate removal in harvested material on effectiveness and recovery of fertilizer phosphate by white clover during a four year period.Phosphate recovery by the clover and fertilizer effectiveness were primarily negative functions of phosphate sorptivity. After the first year there was a progressive decrease in the sorptivity effect such that the subsequent decreases in phosphate recovery and effectiveness were largest on weakly sorptive soils and smallest on strongly sorptive soils. In the long term the cumulative effects of the apparently slow immobilizing reactions on the weakly sorptive soils tended to equalize the effects of faster reactions on strongly sorptive soils so that the actual recovery and effectiveness in the final year were similar on all soils regardless of their sorptivities. The cumulative recovery and cumulative effectiveness in the long term, however, remained strongly negative functions of sorptivity. Phosphate removal in harvested material caused large decreases in phosphate effectiveness on all soils and particularly on weakly sorptive soils.     

The residual value of fertilizer phosphate applied in two field experiments

Summary Of the phosphate applied to two soils in the field, 42 and 100 per cent respectively remained in the top six inches three years later. The loss from the organic soil is believed to have been due to leaching. In the soils from both sites, half the residual phosphate (measured by isotopic dilution) was still labile, and within each soil the uptake of phosphate by ryegrass was highly correlated with the L-value. However, the phosphate in the labile pools of the two soils differed: in the soil that had lost phosphate, a greater fraction of the pool was in the soil solution and a greater proportion was taken up by ryegrass grown in pots. It is suggested that such differences in the behaviour of the phosphate within the labile pool may yield information on the mechanism of phosphate retention.     

Immobilization and phytoavailability of cadmium in variable charge soils. I. Effect of phosphate addition

The effect of phosphate on the surface charge and cadmium (Cd) adsorption was examined in seven soils that varied in their variable-charge components. The effect of phosphate on immobilization and phytoavailability of Cd from one of the soils, treated with various levels of Cd (0–10 mg Cd kg^–1 soil), was further evaluated using mustard (Brassica juncea L.) plants. Cadmium immobilization in soil was evaluated by a chemical fractionation scheme. Addition of phosphate, as KH₂PO₄, increased the pH, negative charge and Cd adsorption by the soils. Of the seven soils examined, the three allophanic soils (i.e., Egmont, Patua and Ramiha) exhibited greater increases in phosphate-induced pH, negative charge and Cd²⁺ adsorption over the other four non-allophanic soils (i.e., Ballantrae, Foxton, Manawatu ad Tokomaru). Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Addition of phosphate effectively reduced the phytotoxicity of Cd. There was a significant inverse relationship between dry matter yield and Cd concentration in soil solution. Addition of phosphate decreased the concentration of the soluble + exchangeable Cd fraction but increased the concentration of inorganic-bound Cd fraction in soil. The phosphate-induced alleviation of Cd phytotoxicity can be attributed primarily to Cd immobilization due to increases in pH and surface charge.     

Influence of poultry manure on phosphorus availability and the standard phosphate requirement of crop estimated from quantity-intensity relationships in different soils

Twelve samples comprising of non-calcareous, calcareous and acidic soils were used to study P availability parameters and standard phosphate requirement (SPR) in the presence and absence of poultry manure (PM). Adsorption of P and phosphate potential decreased while Olsen extractable P increased by PM addition, the extent varying with the type of soil. The calculated SPR to attain soil solution concentration of 0.2 microg P ml(-1) was invariably higher in fine textured soils. Acid and calcareous soils registered higher SPR compared to non-calcareous soils. PM addition lowered the SPR in all the soils. The observed P supplementation through PM ranged from 11.6% to 100% signifying no fertilizer P need in the latter. Calibration curves were drawn for calculating the SPR for a given soil solution P concentration without and with the addition of PM.     

Leaf litter inputs decrease phosphate sorption in a strongly weathered tropical soil over two time scales

In strongly weathered soils, leaf litter not only returns phosphorus (P) to the soil environment, it may also modify soil properties and soil solution chemistry, with the potential to decrease phosphate sorption and increase plant available P. Using a radioactive phosphate tracer (³²P) and 1 h laboratory incubations we investigated the effect of litter inputs on phosphate sorption over two time scales: (1) long-term field litter manipulations (litter addition, control and litter removal) and (2) pulses of litter leachate (i.e. water extracts of leaf litter) from five species. Leachate pulse effects were compared to a simulated throughfall, which served as a control solution. Soil receiving long-term doubling of leaf litter maintained five-fold more phosphate in solution than the litter removal soil. In addition to the quantity of phosphate sorbed, the field litter addition treatment decreased the strength of phosphate sorption, as evaluated through extraction of sorbed ³²P using a weakly acidic ammonium fluoride solution (Bray 1). In litter removal soil, leachate pulses significantly reduced phosphate sorption in comparison to the throughfall control for all five species evaluated. However, the ability of leachate pulses to reduce phosphate sorption decreased when soil had received field litter inputs. Across soils the effect of leachate pulses on phosphate sorption increased with net sorption of dissolved organic C, with the exception of leachate from one species that had a higher index of aromatic C concentration. These results demonstrate that litter inputs, as both long-term inputs and short-term leachate pulses, can decrease the quantity and strength of phosphate sorption, which may increase the biological availability of this key nutrient.     

Different performances of soil phosphate tests for reflecting the effects of buffering capacity on uptake of native phosphate with time

The ability of two sodium bicarbonate (Colwell and Olsen) and two ammonium fluoride (Bray I and Bray II) soil tests to reflect the effect of phosphate buffering capacity of the soil on plant growth through time was studied on ten Argentine soils. The soils were divided into three groups (low, medium and high buffering capacity) according to a buffering index calculated from the slope of the Freundlich equation. The relation between phosphate extracted by soil tests and both relative yield and phosphate uptake of rye grass plants was affected by the phosphate buffering capacity of the soil. The effect of buffering on that relation was more marked for the sodium bicarbonate tests (specially Colwell) than for the Bray tests. This effect was consistent with time. Hence, adjustment for buffering would be more important for the sodium bicarbonate tests than for the Bray tests. Soils with high buffering capacity were able to sustain a greater rate of phosphate uptake. The effect of buffering on the relation between soil tests and both relative yield and phosphate uptake was greatest when the plants were young and decreased with time. This effect would therefore be very important for the early nutrition of annual pasture or crop species.     

Effects of phosphate buffer capacity of soil on the phosphate requirements of plants

Summary Plant-uptake and yield data for ryegrass in a greenhouse experiment are used to estimate the theoretical fertilizer phosphate requirement (Pf) of 24 Sherborne soils. Pf is shown to be a function of three parameters: (i) quantity of P required by the plant (Pr) for optimum yield; (ii) quantity of soil P (Qr) required to maintain a non-limiting soil solution concentration (Ir); (iii) quantity of labile soil P (Q). Because of its large effect on Qr and Ir, the phosphate buffer capacity has an important effect on Pf. However Pf cannot be directly related to phosphate buffer capacity if Q is ignored. On soils of similar Q, increasing buffer capacity will always have a positive effect on Pf, but on soils of the same I, it may have a positive or negative effect on Pf. Consequently, Pf can only be simply, but inversely, related to Q or I on a group of soils of similar phosphate buffer capacity. re]19750513     

Use of partially acidulated rock phosphate as a possible means of minimising phosphate fixation in acid soils

Summary To examine the possibility of minimising phosphate fixation the lateritic soil at various levels of liming was incubated with phosphate rock from U.A.R. acidulated to different degree viz. 0, 10, 20, 50, and 100 per cent both with phosphoric and nitric acid. The soil was incubated for 90 days on addition of different phosphate carriers at the rate of 100 ppm total P₂O₅ containing different proportion of water-soluble, citrate-soluble and insoluble phosphorus. Samples were drawn at an interval of 30 days. Bray's p₁ and pH of the soil samples were measured. The dry-matter yield and uptake of phosphorus by two successive crops of maize grown in pots, the treatments being same as in incubation study, were well correlated with the Bray's p₁. Ground rock phosphate and 10 per cent acidulated material were effective in minimising the fixation in soil of pH 4.0 whereas 50 per cent acidulation was suitable for soils of higher pHi.e. 5.6 and 6.5. H₃PO₄ acidulated material was proved superior to HNO₃ acidulated product. The use of partially acidulated rock phosphate for acid soils may be recommended to receive economic return. Associate Professor Senior Research Assistant.     

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.     

Inability to solubilize phosphate in limestone soils—key factor controlling calcifuge habit of plants

Germination, seedling establishment and growth of calcifuge plants in Swedish limestone soils of Archean and Ordovician age were studied. As previously demonstrated for Viscaria vulgaris, establishment of Rumex acetosella and Silene rupestris did not succeed unless CaHPO₄ (at the rate of 10 mmol dm^-3 of soil) was supplied. Growth of Deschampsia flexuosa was enhanced by phosphate addition, whereas establishment success of Jasione montana was poor, regardless of phosphate treatment. Establishment and growth in an acidic gneiss soil, used as a reference for the species studied, was good. Total, total inorganic, exchangeable, and soil solution P were considered in all soils and treatments. It is proposed that the calcifuge behaviour of plants is quite often caused by inability to solubilize the native phosphate of limestone soils.     

The dependence of isotopically exchangeable phosphate (L-value) on phosphate uptake

Summary L-values were determined using clover and ryegrass, grown either separately or together, in soil or soil/sand media. Where the two species were grown in the same pot, there was no interspecific effect on L-value. However, as the experiment progressed L-values increased and clover and ryegrass grown separately gave different values. The increases can be explained by P uptake, single regressions of L-value on phosphate uptake accounting for 86 and 88 per cent of the variations in L-value on the two soils used, with little improvement for treating the two crops separately. The most likely explanation for the observation is that the phosphate stress placed on the soil system caused mobilization of previously non-labile P by the dissolution of sparingly soluble phosphate compounds.     

Effect of flooding and cropping on the changes in the inorganic phosphate fractions in some rice soils

Summary Greenhouse experiments were conducted to evaluate the significance to flooded rice of the various forms of inorganic soil phosphate on Aborlan, Luisiana and Pili soils with and without phosphate. Changes in the various inorganic soil phosphate fractions with time under cropped and uncropped conditions were measured. Results showed that the iron phosphate fraction was the most important source of phosphate to the rice plant. It was also evident that phosphate added to the soils was slowly converted into the iron phosphate form. Transformation of soil phosphate seemed to be greatly affected by the presence of plant roots. Changes in the phosphate fractions were marked in the cropped soils.Data presented in this paper form part of Ph.D. dissertation submitted by the first author to the University of the Philippines, College of Agriculture, Los Banos, The Philippines.Professor of Agr. Chemistry, Dept. of Agr. Chemistry, Univ, of the Philippines, College of Agriculture.Rice Agronomist, Univ. of Agr. Sciences, Regional Research Station, Mandaya, Mysore, India.     

磷酸盐修复重金属污染土壤的研究进展

从研究方法、反应机理以及风险评价等方面综述了磷酸盐修复重金属污染土壤的研究进展,分析和讨论了其中存在的问题和不足,提出了今后加强研究的重点。目前磷酸盐修复重金属污染土壤时,使用的主要研究方法有化学形态提取法、化学平衡形态模型法和光谱及显微镜技术,各个方法都有其优缺点,应该结合使用并探索新方法。磷酸盐稳定重金属的作用机理主要有3个:磷酸盐诱导重金属吸附、磷酸盐和重金属生成沉淀或矿物和磷酸盐表面吸附重金属,但磷酸盐与重金属反应的机理十分复杂,人们尚不完全清楚,因此难以有效区分和评价诱导吸附机理和沉淀机理或其它固定机理,相应地对磷酸盐修复重金属的长期稳定性难以预测。磷酸盐修复重金属污染土壤时由于其较高的施用量可能会造成磷的积聚从而引发一些环境风险,如磷淋失造成水体富营养化,营养失衡造成作物必需的中量和微量元素缺乏以及土壤酸化等。所以应该谨慎选择磷肥种类和用量,最好是水溶性磷肥和难溶性磷肥配合、磷肥与石灰物质等配合施用。今后应着重研究磷酸盐与重金属相互作用的机理区分和评价;关注磷酸盐修复重金属污染土壤时存在的潜在风险,特别是加强植物长期不断吸收磷或其它环境条件变化致使土壤磷素持续减少过程中稳定的重金属溶解性和移动性的研究,磷酸盐修复重金属污染土壤的长期田间实践等。     

Phosphate adsorption and availability plant of phosphate

Summary The effects of phosphate buffer capacity on the plant-availability of labile soil phosphate, when measured as intensity (I) or quantity (Q), are described and tested using results from a greenhouse experiment on 24 Sherborne soils. In multiple regression studies, phosphate buffer capacity with I or Q measurements as independent variables accounted for up to 94% of the variance in P uptake by ryegrass, the maximum buffer capacity being generally more useful than the equilibrium buffer capacity.When the quantity of soil P is measured (Q), its availability (i.e. ease of desorption) to plant roots is inversely related to the Langmuir bonding energy parameter and the buffer capacity. When the intensity of soil P is measured (I), its availability (i.e. resistance to change) is directly related to the adsorption and buffer capacities. The levels of Q or I, therefore, which are optimal for plant uptake vary with the buffer capacity of the soil. There is little or no correlation between the adsorption capacity and the bonding energy in many soils and consequently phosphate buffer capacity is only poorly correlated with the total adsorption capacity.     

Studies on the phosphate potentials of soils

Summary When a sample of Upper Greensand soil was shaken in dilute solutions of calcium chloride containing orthophosphate, microbial activity was sufficiently stimulated to alter the measured value of Schofield's phosphate potential. The effect was more rapid and more marked for air-dried than for field-moist soils. However, the effect on the potential^SP was not significant during the first 2 to 4 hours of shaking a moist sample, and probably not significant during the first 1/2 to 1 hour of shaking an air-dried sample.It was also shown that the changes in the phosphate potential^SP during the first few hours of shaking an untreated field sample are not due to microbial interference. These changes appear to result from the co-existence, in field soils, of micro-volumes of soil of potential^SP higher and lower than the mean for the soil (Part I, and Part III, in preparation).     

Ecological restoration on former agricultural soils: Feasibility of in situ phosphate fixation as an alternative to top soil removal

In Europe, high phosphorus (P) concentrations form the most important constraint on the ecological restoration of biodiverse vegetation on former agricultural soils, because they lead to dominance of highly competitive species like Juncus effusus or to algal blooms in flooded situations. Top soil removal is often not sufficient or not possible, so alternative methods have to be found. We therefore investigated whether modified bentonite clay to which 5% lanthanum had been added (LMC) and lime could effectively decrease bioavailable P and phosphate mobilization to the water layer in different soil types.A container experiment was performed using peaty and sandy soils with different Olsen-P concentrations, mixed with different doses of LMC and lime. The soils were exposed to two different common water regimes (moist and flooded). J. effusus seedlings were used as phytometers.Addition of LMC and lime lowered extractable P concentrations in some of the P-rich sandy soils. Only the highest LMC dose was able to decrease phosphate mobilization to the water layer in the sandy soils. However, neither LMC nor lime was sufficiently effective in reducing Olsen-P concentrations and J. effusus growth. Lime addition eventually even led to additional nutrient mobilization by alkalinization and increased mineralization of the soil.Our experiments therefore show that LMC and lime are not feasible alternatives to top soil removal, because they are inefficient in preventing dominance of highly competitive species under moist or shallowly flooded conditions. LMC may only be used to prevent phosphate mobilization to the water layer in deeply flooded situations, which may allow for a more biodiverse vegetation development.     

Effects of vesicular-arbuscular mycorrhiza on the size of the labile pool of soil phosphate

Summary Inoculation of lettuce, onion and clover with VA mycorrhizal fungus (Glomus mosseae) increased plant yields and phosphate uptake in three soils that had been depleted in phosphate. From two soils in which the labile pool of phosphate had been labelled with³²P, the specific activity of plant phosphate was the same whether the plants were mycorrhizal or non-mycorrhizal. In a third soil (Sonning) the specific activity was lower in lettuce and clover when the plants were mycorrhizal. When the experiment was repeated with the same soil under conditions that gave lower growth rates, the specific activity was the same in mycorrhizal and non-mycorrhizal plants. The lower specific activity in lettuce and clover in the first experiment is atributed to greater release of slowly exchanging phosphate (which is not in equilibrium with the added³²P), caused by the high uptake of phosphate by the mycorrhizal plants. When they occur, lower specific activities in mycorrhizal plants may therefore not necessarily indicate a solubilizing effect of the mycorrhiza on soil phosphate.     

Phosphate potential and phosphate capacity of soils

Summary The relationship between the phosphate potential (I) and the amount of phosphate (Q), added to the soil has been examined by equilibrating soil samples with 0.001M or 0.01M CaCl₂ solutions containing various amounts of phosphate. For one neutral and two alkaline soils the Q/I relationship depends on the CaCl₂ concentration and the pH in such a way that the apparent values of I decrease when the CaCl₂ concentration increases from 0.001 M to 0.01M. The difference between the two values increases when the pH increases. When correction is made for the formation of the soluble calcium phosphate complex, CaHPO₄, the Q/I relationship becomes independent of the CaCl₂ concentration. The initial phosphate potential (I₀) determined by interpolation, is also found to be independent of the CaCl₂ concentration. The necessary amount of phosphate to be added or removed per gram of soil in order to obtain a certain alteration of the phosphate potential is designated the differential phosphate potential buffering capacity, DPBC. For ten soils DPBC-values are determined on the basis of the Q/I relationships, (ΔQ/ΔI)_Io, and found to be independent of the CaCl₂ concentration. The content of colloids and of inorganic phosphate accounts for a significant part of the variation in the DPBC for different soils. The importance of the DPBC for characterization of the phosphate status of soils in respect to phosphate supply to plants is briefly discussed. The author is indebted to professor, Dr. H. C. Aslyng, head of the department for his suggestions and helpful criticism during the progress of this work.     

Measurement of available phosphate content of some Spanish soils

Summary Eight Spanish and seven English soils were used to examine methods of determining available soil phosphate. The reference value for the amount of phosphate available was that taken up by six successive cuts of ryegrass grown in a pot experiment.The L-value had the best overall correlation (r=0.94) but is a lengthy determination. Its laboratory analogue, the E-value, was more rapid and equivalent, with the exception of high-clay soils where it gave anomalous values. An anion-exchange resin technique was most suitable for routine measurement and predicted available phosphate (r=0.88) satisfactorily. re]19730122