Impacts of fertilization practices on pH and the pH buffering capacity of calcareous soil

Modern intensive agricultural practices, particularly the use of nitrogen fertilizers, have accelerated soil acidification on a global scale. The soil pH buffering capacity (pHBC) is often used to quantify the soil acidification rate. Calcareous soils have relatively higher pH and pHBC, reflecting the presence of carbonate minerals; however, the impact of long-term fertilization treatment on pH and pHBC is poorly understood for calcareous soils. Here, calcareous soil samples (0–20 cm) were collected from fields receiving six different fertilization treatments for 22 years: control (CK, unfertilized but planted); nitrogen (N); nitrogen and phosphorus (NP); nitrogen, phosphorus and potassium (NPK); combined manure and NPK (NPKM); and combined corn-stover and NPK (NPKS). Both pH and pHBC significantly decreased for all treatments relative to CK. NPKS treatment had the lowest soil pH. Compared with CK, the soil pHBC decreased 5.7 to 17.3% under different treatments. The calcium carbonate (CaCO₃) content was significantly reduced by fertilization treatments, with a maximum decrease under the NPKS treatment. Structural equation model (SEM) analysis revealed that calcium carbonate and soil organic matter (SOM) made important contributions to effective cation exchangeable capacity (ECEC). Soil pHBC was directly controlled by ECEC, while CaCO₃ and SOM indirectly contributed to the pHBC through ECEC. These results indicated that NPKS treatment induces more severe soil acidification, reflecting the higher H⁺ input and lower pHBC under this treatment.     

Silicon availability of the nursery bed soils and effects of silicon fertilizer applied on the growth and silicon uptake of rice Oryza sativa L. seedlings

(pp. 41–46)

Silicon availability in 36 commercial nursery bed soils was evaluated by four methods the phosphate buffer (pH 6.2, 40 mmol L^?1), incubation, supernatant and acetate buffer (pH 4.0, 1 mol L^?1) Methods. The influence of silicon availability in the nursery bed soils on the silicon uptake of rice Oryza sativa L. cv. Hitomebore seedlings and the effect of silicon fertilizer application were examined in a glass house in 2002.

The results revealed that the best correlation between silicon content in rice seedlings and available silicon in soils was obtained with the phosphate buffer-solution method (r = 0.86). More precise evaluation of available silicon was achieved by grouping soils based on these phosphate absorption coefficients (PAC). The correlation coefficients between silicon content in rice seedlings and available silicon in soils were 0.92 and 0.72 for volcanic soils (PAC > 1500) and non volcanic soils (PAC < 1500), respectively.

We concluded that the phosphate buffer method is the most easily adjusted method for estimation of silicon availability in nursery bed soils, and silicon fertilizers should be applied when silicon availability in non-volcanic nursery bed soils goes below 200 mg kg^?1, whereas the level is less than 350 mg kg^?1 in volcanic soils.     

Soil phosphorous buffer coefficient as influenced by time and rate of P addition

Abstract

This study was conducted to investigate the effect of time and rate of phosphorus (P) addition on phosphorus availability and phosphorus buffer coefficient in some calcareous soils. Phosphorus was added to the samples at rates of 0, 50, 100, 200, 400, 600 and 800 mg P kg^?1 soil. The samples were incubated for 0.041, 1, 7, 14, 21, 30, 60 and 90 days at constant temperature and moisture. Extractable phosphorus was determined after the incubation. The results showed a sharp decrease in available P within 1 h after P addition. There was a linear relation between added P and extractable P in all soils. The buffer coefficients of soils were estimated by Olsen P for above incubation periods. Generally the buffer coefficient decreased with increasing time of incubation. The results indicated that inputs of between 23 – 59 mg kg^?1 are required to raise Olsen P by 10 mg kg^?1 in these calcareous soils, which assuming 2500 t soil ha^?1, gives a required input of 58 – 148 kg P ha^?1.     

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.     

Evaluation of sodium tetraphenylboron (NaBPh4) as a soil test of potassium availability

Non-exchangeable potassium (K_nex) contributes to soil K availability and several extractants are used to access its contribution. This study evaluated sodium tetraphenylboron (NaBPh₄) as a soil test of K availability in 20 soils from Northern Greece. Winter wheat (Triticum aestivum L. var. ‘Yecora’) was sown in a greenhouse pot experiment and five cropping cycles were carried out until K-depletion. Soils were analyzed with NH₄OAc and NaBPh₄ (1 and 5 min incubation periods). Critical levels of K ranged between 130–140 and 330–340 mg K kg^?1 of soil for NH₄OAc and NaBPh₄ (1 min incubation period), respectively, and between 32 and 35 g K kg^?1 of wheat dry matter. NaBPh₄-K (1 min) related better with K concentration and uptake compared to NH₄OAc for each cropping cycle (r² = 0.45–0.83 and 0.44–0.89) and for all soils (r² = 0.58 and 0.51). Similar results obtained in soils low in exchangeable K (r² = 0.41 and 0.39). Correlation between NH₄Oac- and NaBPh₄-extractable K was weaker among soils below the critical level (r = 0.70) compared to those above (r = 0.93). Inclusion of illitic K and cation exchange capacity in a multiple linear regression between NH₄OAc- and NaBPh₄-extractable K showed that they significantly contributed to NaBPh₄-extractable K.     

Sodium removal from a calcareous saline–sodic soil through leaching and plant uptake during phytoremediation

Saline–sodic and sodic soils are characterized by the occurrence of sodium (Na⁺) to levels that can adversely affect several soil properties and growth of most crops. As a potential substitute of cost‐intensive chemical amelioration, phytoremediation of such soils has emerged as an efficient and low‐cost strategy. This plant‐assisted amelioration involves cultivation of certain plant species that can withstand ambient soil salinity and sodicity levels. It relies on enhanced dissolution of native calcite within the root zone to provide adequate Ca²⁺ for the Na⁺ Ca²⁺ exchange at the cation exchange sites. There is a lack of information for the Na⁺ balance in terms of removal from saline–sodic soils through plant uptake and leaching during the phytoremediation process. We carried out a lysimeter experiment on a calcareous saline–sodic soil [pH of saturated soil paste (pH_s) = 7.2, electrical conductivity of the saturated paste extract (EC_e) = 4.9 dS m⁻¹, sodium adsorption ratio (SAR) = 15.9, CaCO₃ = 50 g kg⁻¹]. There were three treatments: (1) control (without application of a chemical amendment or crop cultivation), (2) soil application of gypsum according to the gypsum requirement of the soil and (3) planting of alfalfa (Medicago sativa L.) as a phytoremediation crop. The efficiency of treatments for soluble salt and Na⁺ removal from the soil was in the order: gypsum ≈ alfalfa > control. In the phytoremediation treatment, the amount of Na⁺ removed from the soil through leaching was found to be the principal cause of reduction in salinity and sodicity. Copyright © 2003 John Wiley & Sons, Ltd.     

Phosphorus-induced zinc deficiency in wheat pot-grown on noncalcareous and calcareous soils of different properties

High levels of phosphorus (P) often induce zinc (Zn) deficiency in plants grown on Zn-poor soils. We investigated P-induced Zn deficiency in durum wheat (Triticum durum L. ‘Carpio’) grown on 16 noncalcareous and 31 calcareous soils differing in levels of available (Olsen) P and available (diethylenetriaminepentaacetic acid (DTPA)-extractable) Zn using micropots. A completely randomized factorial design with two levels of P (0 and 40 mg P kg^?1 soil) and Zn (0 and 3 mg Zn kg^?1 soil), i.e. four treatments (‘control’, + P, + Zn, and + PZn), were used. Grain yield of control plants depended mainly on the Olsen P level. Phosphorus had a negative effect on yield in 6 soils with Olsen P/Zn_DTPA > 25, and Zn a positive one in 5 soils with Olsen P/Zn_DTPA > 50; and the + PZn treatment generally resulted in the highest yield. Grain Zn concentration of control plants was negatively correlated with growth and Olsen P. Calcareous soils were less sensitive to P-induced Zn deficiency than noncalcareous soils because phosphate is sorbed by calcite rather than being co-adsorbed with Zn on the Fe oxides. Co-application of P and Zn to soil at low and application of Zn at high Olsen P ensured both maximum yield and grain Zn bioavailability.     

Potassium fixation of some calcareous soils after short term extraction with different solutions

This investigation was done to determine the release of potassium (K) from five calcareous soils of southern Iran using 0.025 M CaCl₂, HCl and citric acid during six successive extractions and to study the K fixation capacity of the soils after K release experiment. Mineralogical study indicated that Vertisols and Mollisols were dominated with smectites; while other soils had illite, chlorite, palygorskite and smectite. Results indicated that citric acid extracted more K than CaCl₂ and HCl (137 vs. 111 and 113 mg kg^?1, respectively). The analysis of calcium (Ca), magnesium (Mg) and K concentrations in the solutions suggests that the exchange of K with soluble Ca and Mg (originated from dissolution of carbonates by acidic solutions) is the main mechanism of K release, but citrate is able to dissolve K-bearing minerals and release K in slightly calcareous soils. Soils with more illite released more K. Potassium fixation capacity of soils increased after extractions of soils with different extractants from 324 to 471 mg kg^?1, with no significant difference. It is suggested to apply more K fertilizers in K-depleted calcareous soils and use of different solutions for extracting K from soil minerals may be a temporary and short term solution.     

Effect of Oxalic and Citric Acids on Zinc Release Kinetic in Two Calcareous Soils

Rate of zinc (Zn) release from solid to solution phase by organic acids can influence Zn availability in calcareous soils. The objective of the present study was to investigate the effect of different concentrations (1.1, 2.2, and 3.3 mM) of oxalic acid and citric acid on the kinetic release of Zn from two calcareous soils from Eastern Iran. The two organic acids showed significant difference in Zn release from studied soils. Cumulative Zn release during 72 h ranged from 5.85 to 10.4 mg kg^?1 in soil 1 and ranged from 8.7 to 16.9 mg kg^?1 in soil 2 using different concentrations of oxalic acid. The amount of cumulative Zn release after 72 h in soil 1 ranged from 13.65 to 28.77 mg kg^?1 and from 17.63 to 23.13 mg kg^?1 when different concentrations of citric acid was used. In general, Citric acid released 38% more Zn from soils than oxalic acid. The release of Zn from soils increased with citric acid concentration but decreased with increasing of oxalic acid concentrations in the solution. The simplified Elovich equation best described Zn release as a function of time (r² = 0.93 and SE = 0.78). From the present study, Zn release from soils can be limited by the higher concentration of oxalic acid, while citric acid is suitable for enhancing soil lability of Zn.     

Zinc Deficiency in Rainfed Wheat in Pakistan: Magnitude,Spatial Variability,Management, and Plant Analysis Diagnostic Norms

Abstract

Zinc (Zn) deficiency is a widespread micronutrient disorder in crops grown in calcareous soils; therefore, we conducted a nutrient indexing of farmer‐grown rainfed wheat (Triticum aestivum, cv. Pak‐81) in 1.82 Mha Potohar plateau of Pakistan by sampling up to 30 cm tall whole shoots and associated soils. The crop was Zn deficient in more than 80% of the sampled fields, and a good agreement existed between plant Zn concentration and surface soil AB‐DTPA Zn content (r=0.52; p≤0.01). Contour maps of the sampled areas, prepared by geostatistical analysis techniques and computer graphics, delineated areas of Zn deficiency and, thus, would help focus future research and development. In two field experiments on rainfed wheat grown in alkaline Zn‐deficient Typic Haplustalfs (AB‐DTPA Zn, 0.49–0.52 mg kg^?1), soil‐applied Zn increased grain yield up to 12% over control. Fertilizer requirement for near‐maximum wheat grain yield was 2.0 kg Zn ha^?1, with a VCR of 4∶1. Zinc content in mature grain was a good indicator of soil Zn availability status, and plant tissue critical Zn concentration ranges appear to be 16–20 mg kg^?1 in young whole shoots, 12–16 mg kg^?1 in flag leaves, and 20–24 mg Zn kg^?1 in mature grains.     

Evaluation of Extractants and Quantity–Intensity Relationship for Estimation of Available Potassium in Some Calcareous Soils of Western Iran

Accurate estimation of the available potassium (K⁺) supplied by calcareous soils in arid and semi‐arid regions is becoming more important. Exchangeable K⁺, determined by ammonium acetate (NH₄OAc), might not be the best predictor of the soil K⁺ available to crops in soils containing micaceous minerals. The effectiveness of different extraction methods for the prediction of K‐supplying capacities and quantity–intensity relationships was studied in 10 calcareous soils in western Iran. Total K⁺ uptake by wheat grown in the greenhouse was used to measure plant‐available soil K⁺. The following methods extracted increasingly higher average amounts of soil K⁺: 0.025 M H₂SO₄ (45 mg K⁺ kg^?1), 1 M NaCl (92 mg K⁺ kg^?1), 0.01 M CaCl₂ (104 mg K⁺ kg^?1), 0.1 M BaCl₂ (126 mg K⁺ kg^?1), and 1 M NH₄OAc (312 mg K⁺ kg^?1). Potassium extracted by 0.01 M CaCl₂, 1 M NaCl, 0.1 M BaCl₂, and 0.025 M H₂SO₄ showed higher correlation with K⁺ uptake by the crop (P < 0.01) than did NH₄OAc (P < 0.05), which is used to extract K⁺ in the soils of the studied area. There were significant correlations among exchangeable K⁺ adsorbed on the planar surfaces of soils (labile K⁺) and K⁺ plant uptake and K⁺ extracted by all extractants. It would appear that both 0.01 M CaCl₂ and 1 M NaCl extractants and labile K⁺ may provide the most useful prediction of K⁺ uptake by plants in these calcareous soils containing micaceous minerals.     

Nitrification Inhibition in Iowa Soils Treated with Stay‐N 2000 as Affected by Temperature and Matric Potential

The effects of temperature and water potential on nitrification were investigated in two Iowa soils treated with Stay‐N 2000. The soils were incubated at 10, 20, and 30 °C after soil water potentials of ?1, ?10, or ?60 kPa were applied to each soil. A first‐order equation was used to calculate the maximum nitrification rate (K _max), duration of lag period (t′), period of maximum nitrification (Δt), and termination period of nitrification (t _s). The highest K _max were 18 and 24 mg kg^?1 d^?1 nitrate (NO₃ ^?)–nitrogen (N), respectively, at 30 °C and ?10 kPa in both the Nicollet (fine‐loamy, mixed, superactive, mesic Aquic Hapludoll) and Canisteo (fine‐loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll) soils and reduced to 4 and 16 mg kg^?1 d^?1 NO₃ ^?‐N when Stay‐N 2000 was added. The extension of t′ due to the addition of Stay‐N 2000 was as high as 7 d in the Nicollet soil at 10 °C and ?1 kPa and as little as 2 d in the Canisteo soil at 20 °C and ?10 kPa.     

Efficiency of gypsum application to acid Andosols estimated using aluminum release rates and plant root growth

Abstract

A great deal of information on the efficiency of gypsum or phosphogypsum to ameliorate acidity in highly weathered soils is available, but only limited information is available on the efficiency in acid Andosols, which possess large amounts of active aluminum (Al). We examined the effectiveness of gypsum application to non-allophanic Andosols (one humus-rich A horizon and two B horizons poor in humus) using extractable soil Al analyses (batch and continuous extraction methods) and a cultivation test using burdock (Arctium lappa). With gypsum amendment, pH(H₂O) values of the soil decreased from 4.5–4.7 to 4.2–4.4, whereas the treatment made almost no difference to the values of pH(KCl). Total active Al (acid oxalate-extractable Al) was hardly affected by gypsum for all samples. Potassium chloride-extractable Al definitely decreased with the addition of gypsum in all soils; however, the decrease was small (0.1–1.4 cmol_c kg^?1) and the values still exceeded “the threshold of 2 cmol_c kg^?1” for inducing Al toxicity in sensitive plants (4.4–8.6 cmol_c Al kg^?1). The change in Al solubility with gypsum application represented by Al release rates from soils using continuous extraction methods with a dilute acetate buffer solution (10^?3 mol L^?1, pH 3.5) differed greatly among the soil samples: The release rate of one of the B horizon samples decreased by 71%, certainly showing the insolubilization of Al compounds, whereas the release rates of the A horizon sample showed almost no change. These changes in Al solubility were well correlated with the plant root growth. Root growth was improved with gypsum in the B horizon sample, whereas improvement was not observed in the A horizon soil. The decrease in the rate of Al release of another B horizon soil with gypsum treatment was smaller (by 20–34%), possibly because of lower pH values after gypsum application (pH[H₂O] of 4.2–4.3). In the B horizon soil, root growth improved only slightly. Thus, the effectiveness of gypsum application to acid Andosols appeared to be largely influenced by soil humus contents and slight differences in soil pH values, and corresponded to a decrease in Al release rates using the continuous extraction method.     

Comparison of Prewashing Procedures for the Assessment of Phosphate Rock Dissolution in Soils

When evaluating phosphate rock (PR) dissolution, previous to the extraction with sodium hydroxide (NaOH), dry soil samples with PR were extracted with three solutions to remove exchangeable and solution calcium (Ca) [sodium chloride (NaCl) 1 M, buffered NaCl with ethylenediaminetetraacetic acid (EDTA) (NaCl–EDTA), and NaCl buffered at pH 7 with triethanolamine (TEA) (NaCl–TEA)] for comparison with the extraction of soil samples without any prewash. In acidic soils, up to 51% of applied P was recovered during the NaCl extraction because of the high exchangeable acidity released during the extraction. In soils with exchangeable Ca>2 cmol₍₊₎kg^?1, high EDTA quantities also promoted PR dissolution. The NaCl–TEA solution efficiently removed Ca, avoiding PR dissolution and P retention by calcium hydroxide [Ca(OH)₂] during the NaOH extraction. Thus, when evaluating PR dissolution we recommend the use of NaCl–TEA to remove Ca. We also recommend the same procedure when applying the Chang and Jackson fractionation to calcareous soils and soils submitted to PR application.     

Effect of Field Aging on Nickel Concentration in Soil Solutions

Abstract

The effect of field aging on nickel (Ni) concentration in soil solutions was studied on three soils, with pH 4.5, 6.1, and 7.6, that were spiked with Ni and aged outdoors for 5, 10, and 15 months. Field aging resulted in a minor decrease in total Ni content and a dramatic decrease in Ni concentration in soil solution. Nickel release isotherms in field‐aged soils differ from those in freshly spiked ones. The decrease in soluble Ni in noncalcareous soils at Ni loadings from 25 to 4800 mg kg^?1 followed a first‐order kinetic equation. In calcareous soil the observed relationships imply that soluble Ni may be controlled by dissolution of a surface precipitate either on the surface of soil carbonates [NiCO₃ or NiCO₃ · 2Ni(OH)₂] or on the surface of clay minerals (Ni‐aluminum double‐layer hydroxide).     

Factors affecting potassium fixation in calcareous soils of southern Iran

Potassium fixation capacity and mineralogical analysis of 24 representative soils, collected from southern Iran, were studied. Potassium fixation analysis was performed by adding six rates of K from 0 to 1000 mg kg^?1 soil in a plastic beaker and shaking for 24 h. Mineralogical analysis showed that the clay fractions were dominated by smectite, chlorite, mica, palygorskite, vermiculite and quartz. In general, the studied soils fixed 8.5–55% of the added K. The potassium fixation capacity of the studied soils was significantly correlated with smectite content (r ² = 0.87), clay content (r ² = 0.60), cation-exchange capacity (r ² = 0.79) and NH₄OAc-K. Wetting and drying treatment and incubation time had significant effects on K fixation. The average percentage increase in K fixation following the wetting and drying treatment was 24 and 30% for surface and subsurface soils, respectively. The average percentage increase in K fixation with increasing residence time was 79 and 56% for surface and subsurface soils, respectively. Because K fixation is a diffusion process, time and increased concentration of soluble K (because of soil drying) are factors affecting the rate of K diffusion from a soil solution to the interlayer positions of the expansible 2:1 clay minerals.     

Inorganic and organic ligands induced changes in Pb sorption of arid ‎clay and calcareous loamy sand soils

The objective of this study was to investigate sorption, desorption, and immobilization of Pb in the clay and calcareous loamy sand soils treated with inorganic ligands (NO₃^?, Cl^? and H₂PO₄^?). Pb sorption was also determined in the presence of oxalate and citrate. The maximum Pb sorption capacities (q) ranged from 42.2 to 47.1 mmol kg^?1 for the clay soil, and from 45.2 to 47.0 mmol kg^?1 for loamy sand soil. It was observed that the binding energy constant (k) for Pb sorbed onto loamy sand soil (528–1061) is higher than that for clay soil (24.38–55.29). The loamy sand soil-sorbed greater quantities of Pb compared to the clay soil when initial pH was ≥ 3. However, it had lower sorption capacity at the lowest initial pH of 2. Additionally, the greatest Pb sorption and immobilization occurred in the soil treated with H₂PO₄. In the clay soil, the sorption of Pb was depressed at 0.1 mol kg^?1 of Cl^?, as compared with other ligands. Concerning organic acids, citrate ligand showed the highest decrease in Pb sorption. It could be concluded that the nature of Pb sorption can depend on the type and quantity of ligands present, as well as the soil type.     

Distribution of iron oxides forms on a transect of calcareous soils,north-west of Iran

The iron oxides fractions of four major physiographic units obtained from a transect of calcareous materials were studied to assess the effects of key pedogenic processes and local hydrology conditions as well as physiographic units in controlling iron oxides forms in the north-west of Iran. Samples from different horizons belonging to six pedons were selected and analyzed for soil physicochemical properties, clay minerals, and Fe oxides forms (Fe_d, Fe_o, Fe_p). In general, the soils indicated some variation in the concentration of iron oxides that could be related to rate of weathering, pedogenic accumulations, geomorphologic conditions (as results of different in physiographic units), wet and dry cycle, and organic matter. A wide relative variation in mean values of Fe_d (6.4–9.9 g kg^?1), Fe_o (2.9–4 g kg^?1), and Fe_p (0.68–1.3 g kg^?1) was observed among physiographic units. On the plateau unit, the presence of the most stable geomorphologic conditions and high rate in situ weathering (reflected in clay content), coupled with minor deposition of sediment suggest that the soils have more dynamic conditions than other units, reflecting in the greatest amount Fe_d and the lowest Fe_o/Fe_d ratio. Fe_d content of the soils containing less clay content (15–25%) was significantly different from those with greater clay content (25–35%).     

Laboratory Assessment of Nostoc 9v (Cyanobacteria) Effects on N2 Fixation and Chemical Fertility of Degraded African Soils

The potential of Nostoc 9v for improving the nitrogen (N)₂–fixing capacity and nutrient status of semi‐arid soils from Tanzania, Zimbabwe, and South Africa was studied in a laboratory experiment. Nostoc 9v was inoculated on nonsterilized and sterilized soils. Inoculum rates were 2.5 mg dry biomass g^?1 soil and 5 mg dry biomass g^?1 soil. The soils were incubated for 3 months at 27 °C under 22 W m² illumination with a photoperiod of 16 h light and 8 h dark. The moisture was maintained at 60% of field capacity. In all soils, Nostoc 9v proliferated and colonized the soil surfaces very quickly and was tolerant to acidity and low nutrient availability. Cyanobacteria promoted soil N₂ fixation and had a pronounced effect on total soil organic carbon (SOC), which increased by 30–100%. Total N also increased, but the enrichment was, in most soils, comparatively lower than for carbon (C). Nitrate and ammonium concentrations, in contrast, decreased in all the soils studied. Increases in the concentration of available macronutrients were produced in most soils and treatments, ranging from 3 to 20 mg phosphorus (P) kg^?1 soil, from 5 to 58 mg potassium (K) kg^?1 soil, from 4 to 285 mg calcium (Ca) kg^?1, and from 12 to 90 mg magnesium (Mg) kg^?1 soil. Positive effects on the levels of available manganese (Mn) and zinc (Zn) were also observed.     

The Effect of Vermicompost on Transformation Rate of Available P Applied as Chemical Fertilizer in a Calcareous Clay Soil

The effects of vermicompost (VC) (0% and 1% w/w) on treated calcareous clay soil with 0 and 50 mg phosphorus (P) kg^?1 as calcium phosphate [Ca(H₂PO₄)₂.H₂O] was investigated. The soil samples were incubated for 7, 30, 60, 120, and 150 d at 25 ± 1°C and Olsen-P was measured after each incubation time. Results showed that Olsen-P increased 36% and 38% after VC addition in treated soil with 0 and 50 mg P kg^?1, respectively. Recovery of Olsen-P in treated soils with VC, combined fertilizer VC + P, and fertilizer P was 42%, 42%, and 17%, respectively. The rate coefficient in treated soils with fertilizer, VC, and combined fertilizer VC + P was 0.033, 0.026, and 0.023 mg kg^?1 d^?1/2, respectively. It seems that the process that leads to the decrease in available P in amended soils, is controlled by P diffusion into sorption sites in micropores of aggregates.