Efficiency of HCl- and H2SO4- acidulated rock phosphates for rice (Oryza Sativa L.) on acid soils

Summary Rock phosphates from Udaipur (India), North Carolina, Florida, Tennessee and Missouri (USA) were acidulated with HCl or H₂SO₄, to the extent of 25, 50, 75 or 100 per cent of the requirement for complete conversion into single superphosphate. Partial acidulation resulted in the formation of a mixture of water- and citrate-soluble and -insoluble phosphates, the proportion of the first two increasing with increasing degree of acidulation, at the expense of the insoluble phosphate. For a given degree of acidulation, the P compounds formed with HCl or H₂SO₄ were of comparable composition, except that the HCl-acidulated products were hygroscopic. Pot experiments with acid P-deficient soils showed that the behaviour of the HCl- or H₂SO₄- acidulated products in respect of P availability in soil, grain yield response and P uptake by rice was more or less similar. Partially acidulated rock phosphate to the extent of 50 per cent with either of the acids was found to be suitable for growing rice under flooded soil conditions. There is thus scope for the use of HCl- acidulated rock phosphate as a substitute for the H₂SO₄- acidulated product, for growing rice in acid soils. The scope for use of lower degree HCl-acidulated product in an upland crop — rice rotation in acid soils is also discussed.     

Effectiveness of some iron phosphates as sources of phosphorus for plants

Summary Two well-characterized crystalline ferric phosphates, two colloidal ferric phosphates, and fluorapatite were tested under greenhouse conditions as sources of phosphorus for corn over a 3-cropping period. The selected compounds are representative of those expected to form in soils as reaction products from more soluble phosphate fertilizers.Strengite, FePO₄·2H₂O, was completely unavailable in acid soils and gave only a slight phosphorus response on soils limed to pH 7.6. Uptake of P from hydrogen ammonium ferric phosphate, H₈NH₄Fe₃(PO₄)₈·6H₂O, was approximately 70 per cent that from MCP, and increased with cropping.The colloidal ferric phosphates were approximately 78 per cent as available as MCP and became more available with liming and cropping. In the soil limed to pH 6.5, their effectiveness increased from 47 per cent that of MCP in the first crop to 100 per cent as effective by the third crop.Fluorapatite, included as an insoluble calcium phosphate source, was completely unavailable.     

Agronomic effectiveness of partially acidulated phosphate rock as influenced by soil phosphorus-fixing capacity

A greenhouse study compared the effect of soil P-fixing capacity on the relative argonomic effectiveness (RAE) of partially acidulated phosphate rock (PAPR) and water-soluble P. Such information is lacking in the literature. Six soils varying widely in P-fixing capacity (5.6%–56.1%) were used. A phosphate rock (Huila PR) from Colombia was acidulated with H₂SO₄ at 50% of the level necessary to achieve full conversion to single superphosphate (SSP). Rates of P applied from PAPR or SSP were 0,05, 100, and 300 mg P kg⁻¹. The P fertilizers were mixed with the soils, and maize was grown for 6 weeks before harvest. The results show that the effectiveness of PAPR in increasing dry-matter yield and P uptake over yield and uptake obtained with SSP linearly increased as the soil P-fixing capacity increased. PAPR and SSP were equally effective in increasing dry-matter yield or P uptake at P-fixing capacities of 28% or 36%, respectively. PAPR was found to be more effective than SSP in soils (treated with Fe-gel) with P-fixing capacity higher than these values. The internal efficiency, which is defined as the ratio between dry-matter yield and P uptake, was the same for both PAPR and SSP in all the soils.     

Effect of varying static and changing moisture levels during incubation on extractability of soil phosphate

Summary A sandy loam (pH 6.5) was incubated at 28°C at static moisture levels, ranging from 10 per cent saturation to 133 per cent saturation (waterlogging), for 6 and 12 weeks; other samples covering the same moisture range were first incubated for 6 weeks, and after changing all moisture levels to 50 per cent saturation were incubated for a further 6 weeks.With increasing static soil moisture level during incubation there was a slight reduction in Morgan-extractable phosphate up to 70 per cent saturation, but thereafter, due to anaerobic effects, there were considerable increases in extractable phosphate with increasing moisture level.With changing moisture level during incubation the effects of anaerobiosis became apparent where original moisture level was greater than 50 per cent saturation; extractable phosphate was reduced to levels lower than those occurring where the soil was maintained continuously at 50 per cent saturation. The extent of reduction in extractable phosphate increased with original soil saturation level.     

Chemical evaluation of partially acidulated phosphate rocks and their impact on dry matter yield and phosphorus uptake of maize

Previous studies investigated the direct application of phosphate rock and its partially acidulated to enhance its solubility compared to soluble fertilizers. However, the interaction between the effect of particles diameter and partial acidulation of phosphate rock on phosphorus (P) availability and its effect on dry matter yield and P uptake is still elusive. This study was conducted to assess the effect of partially acidulated Egyptian phosphate rocks with different particle size diameters on P availability and its effect on dry matter yield and P uptake of maize (Zea mays L.). A pot experiment was conducted on maize plants grown on light clay soil for 42 days. Acidulation was done by mixing phosphate rock with single superphosphate or triple superphosphate at a total rate of 200 mg P kg^?1 with five acidulation mix ratios (100:0, 75:25, 50:50, 25:75, and 0:100). Different particle size diameters of phosphate rocks (500, 212, 75, and <45 µm included nano-particles ranged from 69.3 to 25.7 nm) were used. We found that dry matter yield and P uptake increased significantly due to the use of partially acidulated phosphate rocks especially when triple superphosphate was used for acidulation and the mixing ratio of 50:50 was the best. We also found that maize yield and P uptake increased significantly with decreasing particle size. It is recommended to use finely grounded partially acidulated phosphate rocks with particles diameter less than 45 µm at acidulation ratio 50% and no need to increase acidulation ratio above that as a slow-release phosphate fertilizer.     

The influence of radioactive phosphate level on the absorption of phosphate by plants and on the determination of labile soil phosphate

Summary Previous work has suggested that the presence of P³² in fertilizers inhibits the uptake of the applied phosphate from the soil by plants, and also that if the applied phosphate is not incorporated uniformly in the soil there will be preferential uptake from regions of low specific activity. This made it desirable to determine the effect of P³²-level on phosphate uptake and the determination ofL-values in pot experiments in which the labelled phosphate source is added as discrete particles of the phosphate form of an anion-exchange resin.Increasing the level of P³² from 0.05 to 1.25 mo per gram of phosphorus in the added phosphate did not have a significant effect on the fresh weight, dry weight or total phosphorus uptake of the ryegrass crop. The measuredL-value showed a significant increase, about 15 per cent for a five-fold increase in P³² level, on each of the four soil types used, as would be expected if P³² depressed the uptake of labelled fertilizer phosphate.Although a significant effect of P³² was observed this does not invalidate a comparison of soils with respect toL-value.     

Determination of threshold value of iron in soils and plants for the response of rice and lentil to iron application in calcareous soil

Summary In a pot experiment with 26 calcareous soils, the critical limit of Fe in soils and plants was evaluated. DTPA-extractable Fe was found significanty correlated with Bray's per cent yield in rice. The Fe²⁺ (iron) in rice and lentil was also found significantly correlated with DTPA-extractable Fe as well as Bray's per cent yield showing thereby the superiority of Fe²⁺ (iron) in leaves over DTPA-extractable soil Fe to differentiate Fe responsive soils from non-responsive ones. The total Fe content in plant tissues does not seem correlated with the occurrence of Fe deficiency. The threshold values of DTPA-extractable soil Fe and Fe²⁺ (iron) in rice and lentil leaves were 6.95, 44 and 74.5 ppm, respectively below which appreciable responses to Fe application were observed. The optimum Fe level for these soils was found to be 10 ppm in which the dry matter yield response in all the 19 rice soils and 16 lentil soils ranged from 14.28 to 56.16 (Av. 25.75%) and 13.31 to 53.97 (Av. 22.47%), respectively.     

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.     

Phosphorus availability to maize from partially acidulated phosphate rocks and phosphate rocks compacted with triple superphosphate

Two phosphate rocks (PR), moderately reactive Hila PR from Colombia and slightly reactive Capinota PR from Bolivia, were compacted with triple superphosphate (TSP) such that 50% of total P in the mixture was in water-soluble form. The effectiveness of these materials as phosphorus sources was compared with that of partially acidulated phosphate rocks (PAPR) at 50% acidulation with sulfuric acid and TSP in Hartsells silt loam (pH 4.5) with maize as the test crop. Huila PAPR and Huila PR compacted with TSP were as effective as TSP as phosphorus sources. Huila PR was only half as effective. The slightly reactive Capinota PR with 8.8% Al₂O₃ + Fe₂O₃ content was not suited for direct application, and Capinota PAPR was only half as effective as TSP. Capinota PR compacted with TSP, however, was as effective as TSP. PR compacted with TSP, urea, and KCl was no more effective as phosphorus source than PR compacted with TSP alone.     

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.     

Studies on the nutrition of forage legumes

Summary Surface samples of six Ontario soils were selected to provide a range in pH, texture and carbonate content. Phosphorus was added to the soil samples at the rate of 200, and 2000 pounds. P₂O₅ per acre and the inorganic phosphates associated with aluminum (Al-P), iron (Fe-P) and calcium (Ca-P) were analysed 15, 30, and 335 days after treatment. Samples of the check soils were leached with the equivalent of 7 litres of soil-percolated water and then analysed for phosphate fractions. In 5 soils with added phosphorus the Al-P fraction increased at both rates of added phosphorus, Fe-P increased only at the higher rate of added phosphorus and Ca-P did not increase at either rate. In the sixth soil (69.9% CaCO₃ equivalent) Ca-P increased at both rates of added phosphorus, but proportionately less than the water-soluble phosphorus. These changes in phosphorus values existed relatively unchanged for 335 days after the addition of phosphorus. Leaching generally removed phosphorus from all phosphorus fraction. At or below pH 7.1 (soil paste) the Al-P decreased proportionately more than the Fe-P and Ca-P. Above pH 7.1 the Ca-P decreased proportionately more than the other two fractions upon leaching.Contribution from Dept. of Soil Science, O.A.C., Guelph, Canada. Part of thesis submitted by junior author to the Graduate School, University of Toronto, in partial fulfilment of the requirements for the M.S.A. degree.     

Response of cowpea (Vigna unguiculata) to inoculation with VA-mycorrhizal fungi and to rock phosphate fertilization in some unsterilized Nigerian soils

Summary The effects on cowpea of inoculation with vesicular-arbuscular (VA) mycorrhizal fungi and rock phosphate (RP) fertilization were studied in pots using Alagba and Araromi series soils and in the field on Alagba, Apomu and Egbeda series soils. Inoculation of the plants with VA-mycorrhizal fungi caused very rapid infection of the roots. A higher per cent mycorrhizal infection was maintained during subsequent plant growth in the field. RP application reduced the degree of infection without affecting plant growth in the field and in pot experiments. Nodulation, nitrogen fixation and utilization of RP were increased by inoculation with mycorrhizal fungi in the pot experiments but not in the field experiments. In the pot experiments, inoculated plants supplied with RP flowered earlier, and took up more phosphorus than either inoculated plants without RP or uninoculated plants. The largest response to inoculation in terms of shoot dry matter, nodule yield and nitrogen content of shoots was obtained in Alagba soil under both pot and field conditions.IITA Journal Series Paper No. 136.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

Solubility of phosphorus added to four Manitoba soils with different calcium and magnesium contents

Summary The solubility of phosphorus was found to approximate that of dicalcium phosphate dihydrate and/or dimagnesium phosphate trihydrate when KH₂-PO₄, H₃PO₄ and K₂HPO₄ were added to four Manitoba soils. Eighty to one hundred, seventy to ninety and sixty to eighty per cent of the phosphorus added remained in solution when H₃PO₄, KH₂PO₄ and K₂HPO₄ were added, respectively. The solubility of the added phosphorus was high in all samples and relatively soluble compounds, dicalcium phosphate dihydrate and dimagnesium phosphate trihydrate, were most likely formed in the samples indicating that phosphorus added to these soils would be readily available to plants. Associate Professor and Professor respectively.     

Influence of phosphate rock sources and rates on rice and common bean production in an Oxisol

A field experiment was conducted for five consecutive years to determine upland rice (Oryza sativa L.) and common bean (Phaseolus vulgaris L.) response to eight P sources at three P rates in an Oxisol of Central Brazil. The P sources tested were triple superphosphate (TSP), Arafertil phosphate partially acidulated (APPA), phosphate of Patos partially acidulated (PPPA), phosphate of Araxa concentrated (PAC), phosphate of Catalao (PC), phosphate of Jacupiranga (PJ), phosphate of Patos de Minas (PPM), and phosphate of Abaete (PA). All phosphate rock sources were of Brazilian origin. The P rates used were 87, 174 and 262 kg P ha^-1. Yield response to P sources and rates varied from crop to crop. Rice and bean yields were significantly correlated with Bray 1 P, but not Mehlich 1 P. In the first year, TSP and the two partially acidulated phosphate rocks (APPA, PPPA) produced higher grain yields. In the second year and all remaining years of the experiment, the efficiency of phosphate rock sources as measured by grain yield was equivalent to TSP or partially acidulated P sources. The results suggest that these phosphate rock sources could be used in rice-bean rotations on Brazilian Oxisols. Yield losses in the first year could be partially offset by the addition of a small amount of soluble P.     

Influence of alternate submergence and drying of rice soils prior to resubmergence on available phosphorus

Summary Four acid rice-soils (0–20 cm, air-dry, and <2 mm) were subjected to 3 cycles, each consisting of 30-day submergence followed by air-drying at room temperature or oven-drying at 105°C for 3 days. The available phosphorus was determined colorimetrically by Bray's P-2 extractant (0.03N NH₄F+0.1N HCl) using 1:20 soil extractant ratio. The beneficial effect of 30-day submergence followed by drying of soil prior to resubmergence on native phosphorus was apparent in all soils studied except Shirgaon clay soil, which was probably due to presence of relatively more active Fe in that soil.     

Interactions of an acid tolerant strain of phosphate solubilizing bacteria with a few acid tolerant crops

Phosphate solubilizing bacteria (PSB) were isolated from sixty soil samples of various soil classes and cropping histories in Himalayan regions of Uttar Pradesh, India by enrichment culture techniques. Phosphate solubilization and acid tolerance of each strain was estimated. A strain (PAS-2) isolated froma pasture and waste land of pH 4.8, organic matter 2.6% available N 265kg ha^-1, available P₂O₅(Bray's II) 2.3kg ha^-1 and available K₂O 353 kg ha^-1 had the highest P-solubilization (45 µg P per mL per day) and also highest acid tolerance rating 42. The strain was identified as Bacillus sp. Seed inoculation of this bacterial strain resulted in significant increases in grain and vegetative yield of fingermillet (Elosine coracana), maize (Zea mays), amaranth (Amaranthus hypochondriacus), buckwheat (Fagopyrium esculentum), frenchbean (Phaseolus vulgaris) with or without added P sources. The significant grain yield (quintol ha^-1) with phosphate and seed inoculation ranged from 33.85 in maize, 26.33 in frenchbean, 22.41 in buckwheat, 20.71 in amaranth and 19.19 in fingermillet as compared to controls. The highest response was observed with frenchbean followed by fingermillet, buckwheat, amaranth and maize. Phosphate use efficiency was highest in frenchbean followed by maize and lowest and almost at par in buckwheat, amaranth and fingermillet. Available phosphate was also highest in frenchbean cultivated plot followed by amaranth, fingermillet, buckwheat and maize. The MPN count of phosphate solubilizing bacteria were also influenced by seed inoculation of strain PAS-2. Frenchbean exerted greaterrhizosphere effect followed by pseudocereals and cereals. Likewise, phosphate nutrition of crops were also improved through seed inoculation irrespective of added P sources. The study thus demonstrated that selection of efficient strain of PSB from acid soil and its seed inoculation in selected crop genotype is beneficial in boosting up crop yield in low productive hill soil. Seed inoculation also created greater rhizosphere effect over uninoculation which improved P-nutrition of crops and also available soil P.     

Effect of rock phosphate on soil properties and apparent phosphorus recovery in acid soil of Sumatra

The effects of rock phosphate (RP) on soil properties and apparent P recovery of corn in acid soil of Sumatra were investigated. The soil was dominated by kaolinitic minerals, very acid and low in extractable phosphorus. The experiment was conducted in two steps: incubation and greenhouse. The treatments were rock phosphate and triple superphosphate (TSP) at 0, 200, 400 and 600 µg P g^-1 for the incubation experiment, and at 0, 50, 100, 150 and 200 µg P g^-1 for greenhouse experiment In the incubation experiment, rock phosphate reduced exchangeable Al and increased pH_H2O better than TSP, and supplied Bray-1 extractable P as much as did TSP. In the greenhouse experiment, apparent P recovery values of rock phosphate were very close to those of TSP and the values tended to decrease at higher rates of P. Although corn yields of the rock phosphate treatment were lower than TSP, its relative agronomic effectiveness was quite high.     

Organic acid production and plant growth promotion as a function of phosphate solubilization by Acinetobacter rhizosphaerae strain BIHB 723 isolated from the cold deserts of the trans-Himalayas

An efficient phosphate-solubilizing plant growth–promoting Acinetobacter rhizosphaerae strain BIHB 723 exhibited significantly higher solubilization of tricalcium phosphate (TCP) than Udaipur rock phosphate (URP), Mussoorie rock phosphate (MRP) and North Carolina rock phosphate (NCRP). Qualitative and quantitative differences were discerned in the gluconic, oxalic, 2-keto gluconic, lactic, malic and formic acids during the solubilization of various inorganic phosphates by the strain. Gluconic acid was the main organic acid produced during phosphate solubilization. Formic acid production was restricted to TCP solubilization and oxalic acid production to the solubilization of MRP, URP and NCRP. A significant increase in plant height, shoot fresh weight, shoot dry weight, root length, root dry weight, and root, shoot and soil phosphorus (P) contents was recorded with the inoculated treatments over the uninoculated NP₀K or NP_TCPK treatments. Plant growth promotion as a function of phosphate solubilization suggested that the use of bacterial strain would be a beneficial addition to the agriculture practices in TCP-rich soils in reducing the application of phosphatic fertilizers.     

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.