Modelling the effects of pH on phosphate sorption by soils

Samples of six soils were incubated at 60°C for 24 h with several levels of either calcium carbonate or hydrochloric acid. Phosphate sorption was then measured on sub-samples of the treated soils over 24 h at 25°C. In one set of measurements on all soils, 0.01 M calcium chloride was used as the background electrolyte. In another set, on two soils, 0.01 M sodium chloride was used. An interpolation method was used to give points on the three-dimensional surface relating the final pH of the suspensions to sorption of phosphate at specified solution concentrations of phosphate. The effects of pH on phosphate sorption differed between soils. For unfertilized soils, increases in pH up to about pH 5.5 decreased sorption. Further increases in pH decreased sorption further in one soil and increased it in three others. For fertilized soils, measured sorption increased with pH. When sodium chloride was used instead of calcium chloride, there was a more marked trend for sorption to decrease as pH increased. Differences between the soils were ascribed to differences in two soil properties. One was the rate at which the electrostatic potential in the plane of adsorption decreased as pH increased. Only small differences in the rate of change of potential were needed to reproduce the observed differences between soils. The electrostatic potential would decrease more quickly in solutions of a sodium salt than in solutions of a calcium salt and this explains the observed differences between these media. The other soil property that affected observed sorption was the release of phosphate from the soil. The amount released was largest at low pH. Consequently, for fertilized soils, measured sorption increased with pH.     

Effect of zeolite application and soil pH on cadmium sorption in soils

Abstract

In this study the influence of zeolite application and soil liming on cadmium (Cd) sorption by soils in Greece was investigated. The zeolite was natural and consisted mainly of clinoptilolite. The soil samples were strongly acid surface horizons of an Alfisol limed from a pH of 4.0 to 8.5, and a neutral Bt horizon. The result showed that liming and zeolite application substantially increased sorption of Cd in the soils. Cadmium sorption was described adequately by the Freundlich equation whereas the Langmuir model failed to describe Cd sorption in the soils. The Freundlich constant K increased in value by zeolite application as well as by soil liming. A strong relationship was observed between this parameter and soil pH. A high percentage of cadmium sorbed was released in the desorption procedure. The amount of Cd released was reduced by zeolite application as well as by soil liming. It is concluded that zeolite application as well as soil liming increased Cd sorption by the soils.     

Dicalcium phosphate reaction with soils during 2N ammonium sulfate extraction

The consistent method for fractionation of soil phosphorus has already been recommended by M.L. Jackson et al. (1,2,3) based on the solubility product of different soil phosphates, and being widely applied for soil research and practice. Lately, some criticism of the method arose as to the probability of dissolution in 0.5 M ammonium fluoride solution of freshly formed dicalcium phosphate and other reaction products of fertilizer recently added to soil. Since dicalcium phosphate is of nature soluble in ammonium fluoride, as well as aluminium phosphate, if dicalcium phosphate was contained in appreciable amount in soil, so that the value of aluminum phosphate to be dissolved into ammonium fluoride must become higher beyond the real value. It is, therefore, significant to know the fate of applied phosphate fertilizer and the status of native labile phosphate.     

Effect of pH on enzyme stability in soils

The pH stability of urease, acid phosphatase, alkaline phosphatase and phosphodiesterase in soils was investigated by first incubating a soil sample at the indicated pH (1–13) for 24 h and then measuring the activity at its optimal pH under standardized conditions. Generally, the decline in enzyme activity in a pH-profile near the optimum pH range was due to a reversible reaction that involved ionization or deionization of acidic or basic groups in the active centre of the enzyme-protein. Irreversible inactivation of the enzyme was particularly evident at the lower and higher ranges of acidic and alkaline conditions. Results showed that the pH stability of soil enzymes was highly dependent on the soils being assayed. The variation among soils may be attributed to the diversity of vegetation, micro-organisms and soil fauna as sources contributing to the enzyme activity and to the protective sites which allowed entrapment of the enzyme within colloidal humus and organo-mineral complexes. Adherence of the enzyme-protein to the humic-clay fractions would allow some resistance to pH denaturation.     

Effect of liming and phosphate application on sudangrass growth and phosphorus availability in two temperate acid soils

Abstract

A pot experiment was carried out in the greenhouse with two loamy sand Dystric Cambisols derived from schist to investigate the effect of liming and phosphorus (P) application on plant growth and P availability and its assessment by four soil test methods: 0.01M calcium chloride (CaCl₂), cation anion exchange membrane (CAEM), Egnér‐Riehm, and Olsen procedures. Soils were first incubated for two weeks with lime at four levels, depending on their content of exchangeable aluminum (Al). Phosphorus was added at two rates (75 and 150 mg P kg^‐1) and the incubation proceeded for an additional two‐week period. Sudangrass (Sorghum sudanenses cv. Tama) was then planted and harvested four weeks later. During incubation and plant growth, soils were maintained at 70% of field moisture capacity. Although pH value and soil extractable P in original soils were similar, the results showed a significant difference on the effect of liming and P application. Acidity was the major limitation for DM yield in the soil with the highest amount of exchangeable Al, while P availability was the main constraint in the other soil. Liming above pH (0.01M CaCl₂) 5.3–5.5 did not increase DM yield in either soil and showed a negative effect on one soil (9.7 to 6.9 and 10.2 to 7.8 g pot^‐1). Phosphorus content and uptake by sudangrass increased with liming, revealing a positive effect of lime on the availability of P to plants. Added P showed a lower efficiency in the soil with highest amounts of Al compounds. Soil tests performed after the execution of the pot experiment showed variable tendencies to predict P availability, according to the nature of the procedures and soils. Soluble‐P in 0.01M CaCl₂ increased with the rise of soil pH. Extractable CAEM‐P and Egnér‐Riehm‐P also increased with liming, but reflected the soil depletion caused by plant uptake. Extractable Olsen‐P presented the most inconclusive results, suggesting the limitation of this method for acid soils which have been limed.     

Effect of pH,organic matter content and nitrate on the evolution of ethylene from soils

The evolution of C₂H₄ from soils was stimulated by air-drying, and still more by oven-drying at 105°C. The quantities evolved were closely correlated with organic matter content, with no significant difference in this relationship between grassland and arable soils, or between topsoil and subsoil. In arable soils only, the quantities of C₂H₄ also increased significantly with decreasing pH. No significant relationship could be found between NO₃^? concentrations in fresh soils and the quantities of C₂H₄ evolved, but for air-dried soils (arable only) there was a significant decrease with increasing NO₃^?. Artificial addition of NO₃^? only partially inhibited the evolution of C₂H₄, even at concentrations an order of magnitude higher than those found in the field. At normal soil concentrations the only effect of NO₃^? seems likely to be a short delay in the achievement of the maximum C₂H₄ concentrations.     

Effect of liming on phosphate sorption by acid soils

The sorption of phosphate (P) by four strongly acid Fijian soils from 0.01 M CaCl₂ decreased with increasing pH up to pH 5.5–6.0 and then increased again. The initial decrease in P sorption with increasing pH appears to result from an interaction between added P, negative charge, and the electrostatic potential in the plane of sorption. The results of a sorption study, involving KCl or CaCl₂ of varying concentrations as the background electrolyte and using Nadroloulou soil incubated with KOH or Ca(OH)₂, suggested that the increase in P sorption at pH values > 6.0 was caused by the formation of insoluble Ca-P compounds. For some soils this is consistent with the results of an isotopic-exchange study in which incubation with lime caused marked reductions in the amounts of exchangeable P at high pH.     

宁夏主要土壤的磷酸吸收系数及需磷量

用pH7.0的2%磷酸铵溶液测定了宁夏7种主要生产性土壤的磷酸吸收系数,结果表明,宁夏7种主要生产性土壤的磷酸吸收系数(PAC)介于8712～12998mg/kg之间,PAC值的大小与土壤物理性粘粒有极显著相关关系(r=0.8297)。主要生产性土壤黄绵土(黑垆土)、灌淤土、灰钙土依PAC的0.2%计算的小麦建议施磷量分别为53、54、59和43kg/hm2,与肥效函数法所确定的用量相近。     

Effect of pH on nitrogen mineralization in crop-residue-treated soils

Summary This study compares N mineralization in soils treated with crop residues [corn (Zea mays L.), soybean (Glycine max (L.) Merr.), sorghum (Sorghum vulgare Pers.)] or alfalfa (Medicago sativa L.) at three adjusted soil pH values (4, 6, and 8); pH was adjusted with dilute H₂SO₄ or KOH. A sample of soil (20 g) was treated with 0.448 g plant material (equivalent to 50t ha^–1), mixed with 20 g silica sand adjusted to the pH of the soil, and packed in a leaching tube. The soil-sand mixture was leached with 100 ml 5 mM CaCl₂ adjusted to the same pH as that of the treated soil to remove the initial mineral N, and incubated at 30°C. The leaching procedure was repeated every 2 weeks for 20 weeks. Results from three soils showed that N mineralization increased as the soil pH increased. In one soil (Lester soil), significant amounts of NH ₄ ⁺ -N accumulated at pH 4 during the first 12 weeks. Treatment with corn and soybean residues resulted in a marked reduction in N mineralization, especially at pH 4. The percentage of organic N mineralized from sorghum residue and alfalfa added to soils increased as the soil pH increased; the values ranged from 7.7% to 37.0% for sorghum and from 17.2% to 30.1% for alfalfa.     

Effect of phosphate status and pH on sulphate sorption and desorption

For soils from tea estates in northern India, sulphate sorption was of a similar magnitude to, and sometimes exceeded, phosphate sorption. Only a small part of this relatively large sulphate sorption was caused by the low pH of these soils. Most was caused by increased negative charge as a result of prior reaction over many decades with phosphate fertilizers. This decreased sorption of both phosphate and sulphate, but the effect on phosphate was larger. This is compatible with a model in which the mean location of the charge on the adsorbed phosphate ions is closer to the surface than for sulphate. On soils of low phosphate status, sulphate desorption curves showed hysteresis; on soils of high phosphate status, they did not. Further, on soils of high phosphate status, displacement of sulphate by phosphate solutions was faster. We interpret these observations as showing that, for low phosphate status soils, sulphate ions penetrated the surface, but for high phosphate status soils it did not because the pathways by which sulphate diffuses into the adsorbing material were blocked. We also show that, with increasing soil phosphate status, phosphate solutions were less effective in displacing sorbed sulphate. We think this also occurred because reaction with phosphate had decreased the affinity for phosphate more than it decreased the affinity for sulphate.     

Effect of wetting and drying of soils on phosphate adsorption and resin extraction of soil phosphate

Sixteen topsoils from Denmark and the UK were subjected to two wetting and drying treatments: (i) moist incubation (wet), (ii) eleven wetting and drying cycles (W/D). The W/D treatment resulted in larger P adsorption and resin extraction of soil P than the wet treatment. The differences in P adsorption at the final concentration of 800 μM P were mainly above 20 per cent, whereas the differences in amount of resin-extracted P were mainly less than 20 per cent. The effects were positively correlated with the cation exchange capacity of inorganic components. Furthermore, the increase in rapidly released P was positively correlated with pH. It is suggested that wetting and drying effects on P adsorption and desorption are associated with changes in soil structure caused by rewetting of dry samples.     

Effect of liming and phosphate additions on sulphate leaching in soils

The effect of lime (CaCO₃) and phosphate additions on surface charge characteristics and their effect on the leaching of sulphate were examined for two soils (Patua loam and Tokomaru silt loam) which differed in their adsorption capacities for sulphate.
Incubation of soils with either CaCO₃ (0–600 mmol kg⁻¹) or phosphate (0-208 mmol kg⁻¹) resulted in a two- to five-fold increase in the net negative charge and a similar decrease in the adsorption of sulphate. The effect of either lime or phosphate addition on both the surface charge and sulphate adsorption was more pronounced for the allophanic Patua soil than for the Tokomaru soil containing mainly vermiculite.
In a column experiment, liming induced the leaching of sulphur either by the desorp-tion of adsorbed sulphate or by the mineralization of organic sulphur. During a miscible displacement study, addition of either CaCO₃ or phosphate resulted in an early breakthrough of sulphate in the leachate. In a pulse experiment, in which soils were incubated with sulphate (3.12 mmol kg⁻¹) for 1 week and subsequently leached with water, more added sulphate was lost in the leachate of the soils previously incubated with either CaCO₃ or phosphate.     

不同形态磷酸盐及施用方式对石灰性土壤磷移动性和有效性的影响

  【目的】  磷的固定是石灰性土壤中磷肥效益低的重要原因,研究两种施肥方式下不同形态磷源在石灰性土壤中的迁移以及有效性,为实现磷肥减施增效提供理论基础。  【方法】  采用土柱模拟试验方法进行研究,供试土壤为粘质和壤质石灰性土壤。供试磷酸盐为磷酸脲、焦磷酸和聚磷酸,壤土施磷量为0.0581 g/柱,粘土为0.0594 g/柱。施用方式包括一次施用和分4次滴施,同时以不施用磷酸盐土柱为对照。于地下室内 (27 ± 1.0)℃培养28天后将土柱在?80℃条件下快速冷冻固形,从土表向下0—100 mm内每隔5 mm作为一个切割单元,100—300 mm间每隔20 mm作为一个切割单元,测定每层土壤的水溶性磷和Olsen-P含量。  【结果】  培养28天后,一次施用条件下,磷在壤土中的移动距离表现为聚磷酸 (90 mm) > 焦磷酸 (60 mm) > 磷酸脲 (50 mm),粘土中表现为聚磷酸 (80 mm) > 焦磷酸 (70 mm) > 磷酸脲 (60 mm)。分次滴施条件下,聚磷酸 (95 mm) 在壤土中的移动距离比磷酸脲 (65 mm) 和焦磷酸 (70 mm) 分别增加46.2%和35.7%,在粘土中聚磷酸 (90 mm) 的移动距离较磷酸脲 (70 mm) 和焦磷酸 (75 mm) 分别增加28.6%和20.0%。磷浓度下降到一半时所达到土柱深度 (半运移深度) 的结果表明,在壤土一次施用条件下,半运移深度表现为聚磷酸 (15.1 mm) > 焦磷酸 (11.4 mm) > 磷酸脲 (10.5 mm),分次滴施条件下半运移深度为聚磷酸 (20.0 mm) > 焦磷酸 (14.4 mm) > 磷酸脲 (14.3 mm)。在粘土一次施用条件下,半运移深度为聚磷酸 (17.7 mm) > 焦磷酸 (15.8 mm) > 磷酸脲 (14.8 mm),分次滴施条件下,聚磷酸、焦磷酸和磷酸脲的半运移深度依次为51.3、27.1和41.4 mm。相关性分析结果表明,不论一次施用还是分次滴施,聚磷酸和焦磷酸处理均随着水溶性磷含量的增加,有效磷含量在粘土上的增加量大于在壤土上的,分次滴施聚磷酸较一次施用在同样水溶性磷含量下,有效磷的含量在粘土和壤土中的差距减小,焦磷酸处理中水溶性磷与有效磷在两种土壤上较为接近。磷酸脲一次施用后,有效磷在粘土中随水溶性磷的变化量大于在壤土中,分次滴施结果则相反。  【结论】  在质地为壤土和粘土的石灰性土壤中,不论是一次性施用还是分次滴施,磷的移动性均表现为聚磷酸 > 焦磷酸 > 磷酸脲,且分次滴施3种磷源时磷的移动性和有效性均显著高于一次施用。同样水溶性磷含量条件下,粘土中磷的有效性高于壤土,分次滴施提高土壤磷素有效性的效果表现为粘土优于壤土。     

生物质炭对不同pH值土壤矿质氮含量的影响

为了揭示生物质炭作为土壤调理剂添加后对土壤矿质氮形态、含量等土壤性质的影响,该研究利用芒草分别在350和700℃裂解制得生物质炭,发现2个温度尤其是700℃制得的生物质炭,对NH4+有很强的吸附能力,但对NO3-的吸附能力很弱。将生物质炭分别加入到酸性(pH值为3.8)和碱性(pH值为7.6)土壤中,25℃下室内培养180d。结果表明,生物质炭提高了土壤全氮含量,酸性和碱性土壤分别平均提高了22%和17%;但使土壤铵态氮含量大幅降低至接近仪器检测限水平;生物质炭对土壤硝态氮含量的影响因生物质炭和土壤类型而异。生物质炭对土壤矿质氮形态和含量的影响,显然与生物质炭对铵的吸附作用、提高土壤pH值、增强氨挥发损失,以及形成微生物量氮等密切相关。该研究可为开展生物质炭基氮素新型肥料及制剂等方面的科学研究提供参考。     

Fixation and release of phosphate by and from soils

The availability of phosphatic fertilizers applied to soil may be governed by the fixation of phosphate to soil and the release of phosphate fixed be soil. The fixation of phosphate by soil has been studied by many researchers. On the other hand, the studies on the release of phosphate from soil are very few.     

Effect of liming and added phosphate on charge characteristics of acid soils

The effects of liming-induced pH changes on surface charge were studied with four acid Fijian soils. All the soils possessed both positive and negative charge. Much of this charge was pH-dependent, but some negative charge was present at pH values as low as 4.0 and small quantities of positive charge were detected at pH values as high as 8.2. Adsorption of phosphate (P) reduced positive charge at low pH and increased negative charge at high pH. The net amount of surface charge per unit of P adsorbed varied with pH and appeared to depend on the mineralogy of the soil.     

Effect of liming and air-drying on the adsorption of phosphate by some acid soils

The effect of liming the A and B horizons of a number of acid soils on the subsequent adsorption of phosphate by soils retained moist or allowed to dry was investigated. Air-drying increased the phosphate adsorption capacity but the reason was not clear. When A horizons were maintained moist, incubation with lime for six weeks increased phosphate adsorption by four samples and had no effect on another. When A horizons were air-dried, the effect was considerably reduced or reversed. For B horizons, which had considerably greater phosphate adsorption capacities than A horizons, liming decreased phosphate adsorption irrespective of whether the soils remained moist or were dried. The relative decrease in adsorption was, however, greater when the soils were dried. In a more detailed study using one acid soil it was shown that incubation of the soil with lime for six weeks had no effect on phosphate adsorption by moist A and B horizons but after 36 weeks incubation liming decreased adsorption by the moist samples. If soils were dried then liming decreased phosphate adsorption after six or 36 weeks incubation. Such relative effects of liming on phosphate adsorption were confounded by the fact that air-drying greatly increased the phosphate adsorption capacity of the unlimed soil. The drying effect was at least partially reversible following rewetting of the soil. Results were interpreted in terms of the lime-induced increase in soil pH causing (i) the surface charge conferred on soil oxide surfaces to become more negative (thus decreasing phosphate adsorption) and (ii) the precipitation of exchangeable Al as hydroxy-Al polymers resulting in the formation of new, highly active, adsorbing surfaces (thus increasing phosphate adsorption). Phosphate adsorption by moist limed soils can, therefore, be increased, decreased or unaffected depending on the relative magnitudes of these two processes. It is suggested that after liming, and/or air-drying, crystallization of amorphous materials progressively decreases their surface area and adsorbing capacity. Thus, liming tends to decrease phosphate adsorption when the soils are dried.     

Effect of iron oxide on phosphate sorption by calcite and calcareous soils

Pure calcite (AR grade CaCO₃) was treated with ferrous perchlorate solution to give a surface coating of iron (Fe) oxide. Maximum sorption (x_m) of phosphate (P) by the calcite increased from 18.2 to 160 mg P kg^?1 as the amount of coating increased from 0.00 to 16.0 g Fe₂O₃, kg^?1 CaCO₃. Evidence for Fe oxide coatings on carbonate minerals in two Sudanese soils was obtained by optical microscopy and electron-probe microanalysis. The relative contributions of carbonate and Fe oxide minerals, and Fe oxide coatings to P sorption in these soils were calculated, based on an assumed model of oxide distribution. Separate-phase Fe oxide was the major contributor (30–40%) to P sorption in the soils; the Fe oxide coatings on carbonate minerals were only minor contributors (< 6%), and the contribution of uncoated carbonate minerals was found to be negligible (<1 %). These results suggest a very minor role for carbonate minerals, even when coated with Fe oxide, in the sorption of P by these calcareous, Sudanese soils.