Yield and P uptake by wheat as affected by residual and applied P

The effect of residual (built up in continued fertilizer experiments) and applied P in Fatehpur loamy sand and Jodhan sandy loam soils of two long term experiments on its availability and uptake by wheat (Triticum aestivum L.) was studied using³²P in greenhouse experiment. Dry matter yield increased significantly with carry over P. Response in the absence of residual P was observed up to a direct application of 134 kg P ha^?1, whereas, in the presence of carry over P, the response was confined upto 67 kg P ha^?1 at site A. The response was however, observed up to 67 kg P ha^?1 irrespective of carry over P at site B. Total P uptake by wheat was significantly increased at all levels of residual P. However, it did not differ signficantly among levels of residual P when these were high. The uptake of soil P (residual) increased signficantly with increase in the dose of directly applied P, indicating that addition of inorganic P fertilizer tends to mobilize the native soil P. Per cent utilization of fertilizer P was enhanced in soils having residual P from continuous application of both inorganic and organic P sources.     

Transfers of phosphorus within three dairy farming systems receiving varying inputs in feeds and fertilizers

Inputs of phosphorus (P) above requirements for production on dairy farms lead to surplus P with increased risk of P transfer in land run-off to surface waters causing eutrophication. The impact of reducing surplus P inputs in purchased feeds and fertilizers on milk and forage production was investigated in a comparison of three dairy farm systems on chalkland soils in southern England over a 3-year period. In accordance with current commercial practice, no attempt was made to regulate P inputs in system 1, which accumulated an average annual surplus of 23 kg P ha^-1. Progressive reductions in purchased feed and/or fertilizer inputs into systems 2 and 3 decreased surplus P to 17 and 3 kg ha^-1, respectively, without apparently limiting either milk or herbage dry matter production. The estimated reduction in faecal P output from system 3 cows fed a low P diet compared to system 1 cows fed a high P diet was 26%. Milk P concentrations significantly (P <0.001) increased in systems 2 and 3 which included maize in the diet. Output of P in milk and meat products, as a proportion of the total dietary P inputs, increased from 28% in system 1 to 36% in system 3. Surplus P was greatest in continuous maize fields receiving both dairy manure and starter P fertilizer. Withholding P fertilizer in system 3 did not reduce P offtake in cut herbage on soils of moderate P fertility. Total annual losses of P in storm run-off and leaching were no greater than annual inputs of P from the atmosphere (0.5 kg ha^-1). The results indicate there is scope to reduce surplus P on commercial dairy farms without sacrificing production targets at least in the short term. Purchased feeds are the largest of the P inputs on intensive dairy farms, yet these are rarely quantified on commercial holdings.     

Processes controlling soil phosphorus release to runoff and implications for agricultural management

Phosphorus (P) loss from agricultural land to surface waters is well known as an environmental issue because of the role of P in freshwater eutrophication. Much research has been conducted on the erosion and loss of P in sediments and surface runoff. Recently, P loss in sub-surface runoff via agricultural drainage has been identified as environmentally significant. High soil P levels are considered as a potential source of P loss. However, without favourable hydrological conditions P will not move. In this paper, we review the basis of soil P release into solution and transport in surface and sub-surface runoff. Our objectives are to outline the role of soil P and hydrology in P movement and management practices that can minimize P loss to surface waters. Remedial strategies to reduce the risk of P loss in the short-term are discussed, although it is acknowledged that long-term solutions must focus on achieving a balance between P inputs in fertilizers and feed and P outputs in production systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

An evaluation of water extraction as a soil-testing procedure for phosphorus II. Factors affecting the amounts of water-extractable phosphorus in field soils

The effects of seasonal variation, sampling depth, and fertilizer P addition on water-extractable P values were investigated in two field experiments, involving soils of contrasting P retention capacity (Ramiha and Tokomaru) under permanent pasture over 12 months. The effects of the same parameters on Olsen-extractable P were also evaluated. The amounts of water-extractable P in soil were always lower than those of Olsen-extractable P. Over the 12-month period, the average value of water-extractable P in the unfertilized Ramiha soil (0–7.5 cm depth) was 1.8µg g^–1 soil compared to an Olsen-extractable P value of 12.6µg g^–1. The variability associated with water-extractable P at each sampling time was comparable with that for Olsen-extractable P. However, the relative seasonal variation over 12 months was larger for water-extractable P than for Olsen-extractable P. The results obtained with both extractants showed a seasonal fluctuation which was closely related to the pattern of pasture P uptake. The amounts of water- and Olsen-extractable P were higher in samples taken from the 0–4.0 cm than the 0–7.5 cm sampling depth. Fertilizer P addition resulted in larger increases in water-extractable P in the 0–4.0 cm sampling depth than in the 0–7.5 cm depth. The relative increase in water-extractable P following fertilizer P addition was larger than that of Olsen-extractable P. Seasonal changes in the soil microbial biomass P were not related to changes in either water-extractable P or plant uptake of P. Microbial biomass P may be a less sensitive index of soil P availability than is commonly thought.     

Assessment of plant-available phosphate in limed,acid soils using several soil-testing procedures

A range of soil-testing procedures was used in a factorial glasshouse study to assess the plant-available phosphate (P) status of soils which had been treated with lime and added P. A close 1:1 relationship (r = 0.90^***) was obtained between plant P uptake and resin-extractable soil P. In contrast, Olsen-, Colwell-, Bray (I) and (II)-, and Mehlich-extractable P were only weakly correlated with P uptake. Inclusion of 4 different indices of P-buffer capacity did not improve the relationship between plant P uptake, and extractable P. The difficulty in relating plant P uptake data to extractable-soil P levels is attributed to the problems associated with extracting P from limed soils. There was no useful relationship between plant P uptake and isotopically-exchangeable P in the soils.     

Organic and inorganic phosphorus forms in soil as affected by long-term application of organic amendments

Organic amendments contribute significantly to the phosphorus (P) supply in agroecosystems. However, their long-term effects on specific P forms in soils are not completely understood. The objective of this study was to investigate the concentration of organic P forms and inorganic P pools in soil and the activity of enzymes involved in the P turnover in a long-term field experiment running since 1998 in Northern Germany as affected by P amendments. The following treatments with different P supplies were sampled in 2012, 14 years after the establishment of the experiment: control (no P), cattle manure (manure), biowaste compost (compost), and biowaste compost in combination with triple-superphosphate (compost + TSP). The classification of organic P forms by using enzyme additions to NaOH–EDTA soil extracts showed non-hydrolyzable organic P as the dominant form in soil followed by inositol hexakisphosphate (Ins6P)-like P. Non-hydrolyzable and total organic P concentrations in soil were highest in the combined compost + TSP treatment, which received the highest amount of inorganic P. The values of the bioavailable P pools (water-extractable P and double lactate-extractable P) were in accordance with the P balance (P addition with the amendments minus P removal with harvested crops) independently of the type of amendment. The results of this research suggest that the distribution of soil P forms is more reliant on the turnover processes in the soil than on the forms of P added.     

Influence of soil phosphorus and manure on phosphorus leaching in Swedish topsoils

In Sweden, subsurface transport of phosphorus (P) from agricultural soils represents the primary pathway of concern for surface water quality. However, there are mixed findings linking P in leachate with soil P and limited understanding of the interactive effects of applied P sources and soil test P on P leaching potential. Identifying soils that are susceptible to P leaching when manure is applied is critical to management strategies that reduce P loadings to water bodies. Intact soil columns (20 cm deep) from five long-term fertilization trials across Sweden were used in leaching experiments with simulated rainfall to explore the interactive effects of dairy cow (Bos taurus L.) manure application, soil test P and cropping system. Strong relationships were observed between ammonium lactate-extractable P in soil and dissolved reactive P (DRP) concentrations in leachate, although regression slopes varied across soils. For three soils, application of manure (equal to 21–30 kg P ha^?1) to the soil columns significantly increased DRP leaching losses. The increase in DRP concentration was correlated to soil test P, but with wide variations between the three soils. For two soils leachate P concentrations after manure addition were independent of soil P status. Despite variable trends in P leaching across the different soils, P concentrations in leachate were always moderate from soils at fertilization rates equivalent to P removal with harvest. Results clearly stress the importance of long-term P balance to limit P leaching losses from Swedish agricultural soils.     

An evaluation of water extraction as a soil-testing procedure for phosphorus I. Glasshouse assessment of plant-available phosphorus

A water extraction procedure was evaluated as a soil-testing procedure for phosphorus (P). In a glasshouse experiment using perennial ryegrass, the water extraction procedure was used to predict plant-available P in 20 New Zealand soils varying widely in P status and P retention capacity. Water-extractable P in the 20 soils was highly correlated with plant uptake of P (r = 0.90^**). Although plant uptake of P and Olsen-extractable P were equally well correlated (r = 0.90^**), relationships between plant uptake of P and Bray₁ — and Truog-extractable P, and isotopically exchangeable P were less close. The prediction of plant-available P using water extraction was not improved by inclusion of an estimate of P-buffering capacity (obtained from P retention capacity or the slope of the P desorption isotherm), in contrast to the finding for Olsen-extractable P. Because the interpretation of the results obtained appears to be independent of P-buffering capacity and soil type, the water extraction procedure may have advantages over the other soil-testing procedures for P for soils containing reasonable amounts of water-extractable P.     

The phosphorus requirement of different crop species compared with wheat on lateritic soils

The phosphorus (P) requirement for grain production of different crop species (oats (Avena sativa), barley (Hordeum vulgare), triticale (xTriticosecale), narrow-leafed lupins (Lupinus angustifolius), and sandplain lupins (L. cosentinii) was compared with wheat (Triticum aestivum) in five field experiments on different lateritic soils in south-western Australia. Seven or eight levels of superphosphate were applied at the start of each experiment. The amount of P required to produce 70% (four experiments) or 90% (one experiment) of the maximum yield was used to compare P requirements. Large differences in the P requirements of the species were obtained.On P deficient soil in 3 experiments, oats required from 50 to 70% less P than wheat, but required 40% more P on a soil with a long history of superphosphate applications.Compared with wheat, in the year of P application, barley required 50% less P in one experiment, had similar P requirements in two experiments, and required 80% more P in another experiment. In the years after P application, barley required 20% less P in two experiments.On an acidic soil triticale required from 50% to 70% less P than wheat, but on less acidic soil it required 100% more P.In the year of P application, narrow-leafed lupins required 800% more P than wheat in one experiment, and 30% more P in the other experiment.In the year of P application, sandplain lupins required 70% less P than wheat in one experiment.     

Phosphorus status and cycling in native savanna and improved pastures on an acid low-P Colombian Oxisol

On acid low-phosphorus (P) Colombian Oxisols, improved pastures with acid-soil-tolerant grass and legume varieties have increased beef production by a factor of 10 to 15 with only modest P fertilizer inputs. This indicates that the efficiency of P fertilization could be greater than is commonly expected on such strongly P-sorbing soils. To understand the effect of improved pastures on P cycling and availability, we estimated P budgets, and characterized soil P by sequential fractionation, isotopic exchange and biological activity measurements on soil samples from unfertilized native savanna, and fertilized improved grass-only (Brachiaria decumbens cv. Basilisk) and grass-legume (B. decumbens + Pueraria phaseoloides, Kudzu) pastures established in 1978 on a medium-textured isohyperthermic, tropeptic haplustox. Comparison of calculated P budgets, based on inputs and exports, with total soil P contents showed that fertilization, as part of the improved pasture management, had resulted in a measurable increase of total P in the surface 0–20 cm soil layer of nearly 30 mg kg^-1 or about 20% over the savanna level. Sequential soil P fractionation of different seasonal samplings indicated that grass-legume maintained higher organic and available inorganic P levels with less temporal variation than the two other types. The linkage of organic P and available P was also reflected in soil biological activity. Estimates of P in microbial biomass and phosphatase activity were significantly higher in grass-legume than grass-only and savanna. The improvement in soil P availability, as measured by solution P concentration, P sorption and exchangeable P, was much greater in grass-legume than in grass-only. With comparable fertilizer inputs and greater product exports, improved P availability in grass-legume cannot be due to differences in budgets but can be attributed to changes in the overall biological activity in the soil-plant system caused by the presence of legumes in the vegetation cover. Total C, organic P content and macrofaunal activity were all significantly higher in grass-legume soils. Greater turnover of organic litter in grass-legume may provide for steadier organic P inputs and, therefore, higher P cycling and availability.     

Maintenance of Phosphorus Fertilizer and Residual Phosphorus Effect on Corn Production

Phosphorus (P) deficiency is invariably a common crop growth and yield-limiting factor in unfertilized soils, especially soils high in calcium carbonate, which reduces P solubility. Even when such soils are fertilized, adsorption and desorption lead towards a reversion to stable and less soluble P forms, thus reducing fertilizer use efficiency. Field trials that examine the implications of such P reactions and residual fertilizer P responses in the field are relatively rare in Mediterranean environments. A 5-year field experiment in southern Turkey examined the residual effects of repeated P fertilizer applications for corn production in a calcareous soil Typic Xerofluvent. Following the initial year's fertilization (0, 33, 66 and 99 kg P ha⁻¹) to establish a range of soil P levels in subsequent years, the main plots received 0, 9, 18, 27 and 36 kg P ha⁻¹ annually. Grain P uptake was calculated for each year and used in the prediction of P recovery. All plots were sampled and analyzed for available P prior to planting with a local corn hybrid. Soil P values increased with the initial P levels (8–24 mg kg⁻¹) but declined after 3 years (6–10 mg kg⁻¹). Only the lowest annual P application rate (9 kg P ha⁻¹) produced an available P level that was not in the sufficiency range. Grain yields across the main and subplots and years ranged from 6.6 to 13.2 t ha⁻¹. Overall corn yield averaged over the years increased by 8–33% compared to the control as the rates of applied P increased. However, P application had no effect in a year when below-average rainfall restricted crop growth. A residual P effect on grain yield occurred with higher P application levels in the last year. Leaf and grain P concentrations were in the sufficiency range in general. Grain P uptake was calculated for each year and used in the prediction of P recovery. Actual recovery was higher with low P application rates and ranged between 10.8 and 46.4%. The study indicated that under irrigated conditions, corn is likely to respond to P fertilization, but that buildup of available P can occur within a few years and adequate plant available P levels can be maintained by modest P fertilizer application rates.     

Assessment of fertilizer P residues in a calcareous vertisol

The amounts of available P in a Typic Pellustert containing different levels of residual fertilizer P were assessed in a field experiment and chemically with cation-anion-exchange resin, using: (i) P desorbed after 1 extraction, (ii) cumulative P from 8 extractions [Resin8-P] and (iii) the asymptote of the extraction curve. P exchangeable to³²P, Olsen-P and P extracted by dilute salt solution (0.03M KCl) were also measured. P available to sorghum in 1989 was strongly influenced by fertilizer P applied in 1988, but not by P applied in 1987. Olsen-P and Resin8-P both reflected well the effects of residual P on yield and P uptake and could therefore be used to predict available residual P. The amounts of Resin8-P were much larger than Olsen-P, so it was easier to observe the differences between soil treatments, but Olsen-P gave the better correlations with grain yield and P uptake.Multiple regressions of crop yield or P uptake with amounts of P applied in 1987 and 1988 gave the best correlations, without the need for practical work. Thus, the development of an appropriate model based on records of P fertilization is seen as the most effective way to predict availability of residual P. Assessments related to the P intensity and desorption rate using resin were not able to improve predictions of available P, indicating that within one soil the quantity of residual P is most important, but for comparisons between soils, account will need to be taken of differences in soil characteristics.Approved for publication as Journal Article No. 1309 of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)     

Decreases in Colwell bicarbonate soil test P in the years after addition of superphosphate, and the residual value of superphosphate measured using plant yield and soil test P

Decreases in Colwell bicarbonate soil test P in the years after applying single (ordinary) superphosphate, and the residual value of superphosphate, was measured in a long-term field experiment on a duplex (texture contrast) soil (sand over lateritic ironstone gravel clay sand at 10–15 cm), at Wongan Hills, Western Australia, typical of many soils used to grow crops in Western Australia. Ten levels of P (0–91 kg P ha-1) were applied once only in late May to different plots in different years from 1988 to 1993. Wheat (Triticum aestivum), or lupin ( Lupinus angustifolius)) were sown in late May of each year, when the P treatments applied that year were banded (drilled) with the seed. Soil samples were collected each June to measure soil test P. Seed (grain) yields of the crops were measured each December. The residual value (RV) of P applied in previous years was calculated relative to P applied in the current year, using grain yields (RV_yield) and soil test P (RV_soil). Soil test P measured on soil samples collected in June was related to yields measured in December that year to provide soil P test calibrations. Relative to P applied in the current year, soil test P decreased by between 15 to 30% for P applied one year previously, by 25 to 30% for P applied three years previously, and by 60 to 70% for P applied six years previously. Soil test P was affected by spatial variation, and it also varied in the different years, for P applied in the current year, one year previously, two years previously, etc. Compared with P applied in the current year, mean RV_yield determined in the different years decreased by about 40% one year after P application, followed by a further 20% decrease for P applied two years previously, followed by a further 20% decrease for P applied three to five years previously. Relative to current P, RV_soil decreased by about 25% one year after P application, followed by a further 20% for P applied two years previously, followed by a further 10% for P applied three years ago, and followed by a further 6% for P applied four and five years ago. As measured in the different years, the soil P test calibration varied between years for P applied one, two etc. years previously. This was so even when the same cultivar of wheat was grown at the same site in different years.     

Influence of Long-term Use of Fertilizers and Farmyard Manure on the Adsorption–Desorption Behaviour and Bioavailability of Phosphorus in Soils

An understanding of the adsorption–desorption behaviour of phosphate (P) in soils after three decades of long-term fertilization would be an invaluable supplement to our knowledge of the chemistry of P in soils and would assist in developing P application strategies for successive crops. With this objective and within the framework of a long-term experiment, we collected surface soil samples (0–0.15 m) from agricultural crop land on which a rotation of maize-wheat-cowpea (fodder) crops had been grown for 32 years. The soil samples were investigated for adsorption and desorption behaviour of P, and the buffering capacities of soil samples were computed from the adsorption data. Soils differed widely in their P supply characteristics. Phosphate adsorption increased while percentage P adsorbed decreased gradually with increasing levels of added P. Adsorption maxima and the extent of P adsorption was at its maximum in unfertilized soil, and P adsorption decreased with increased P applications. The incorporation of farmyard manure with optimal P levels decreased P adsorption even more than a super-optimal application of P fertilizers. Bonding energy and Freundlich constant ȁ8nȁ9 also decreased with increased P application and with the incorporation of farmyard manure. The adsorption and desorption of applied P were inversely related, and the soils that adsorbed P the most readily released it the least into the soil solution, and vice-versa. Computation of maximum buffering capacities (MBC) indicated that increased levels of P application decreased MBC and increased P supply in soil. The beneficial effects of an annual incorporation of farmyard manure with optimal P levels were clearly pronounced, and P availability in these soils was found to be even higher than the soil receiving the super-optimal application of P fertilizers. Various adsorption–desorption parameters were significantly related with the P uptake of different summer (rice, maize, soybean) and winter (wheat, berseem and raya) crops. Adsorption maxima, desorption maxima and maximum buffering capacity are the major parameters governing P availability in soils.     

Structural equation modeling for the estimation of interconnections between the P cycle and soil properties

The aim of this study was used a basic hypothetical structural model with latent variables to analyze the interconnections between the pools of stable P (inorganic P (Pi) and organic P (Po)), labile P (Pi and Po) and available P (Mehlich-1 P) and the pools of organic matter (OM) content and physicochemical properties in tropical soils of differing pedogenesis. We used structural equation modeling for designing models for two groups of soil: (1) mineral soils with low to medium organic matter content and (2) mineral soils with high organic matter content and organic soils. The proposed structural models were consistent with the hypothesis of dependence between the pools of P and organic matter as well as physicochemical properties in tropical soils. In general, stable and labile P pools acted as P sources for the available P pool; furthermore, the strength of these structural relationships was strongly associated with soil organic matter content. Yet the pool of physicochemical properties behaved as a sink of P for the labile P pool, however with a beneficial effect in maintaining the stable P pool. The pools of P and OM are strongly bonded in tropical soils under different pedogenesis. All structural models evidenced that various forms of P in different levels of lability could contribute in keeping the supply of bioavailable P, yet its magnitude would be regulated by P buffer capacity of each soil.     

Dissection of P2X4 and P2X7 Receptor Current Components in BV-2 Microglia

Microglia cells represent the immune system of the central nervous system. They become activated by ATP released from damaged and inflamed tissue via purinergic receptors. Ionotropic purinergic P2X4 and P2X7 receptors have been shown to be involved in neurological inflammation and pain sensation. Whether the two receptors assemble exclusively as homotrimers or also as heterotrimers is still a matter of debate. We investigated the expression of P2X receptors in BV-2 microglia cells applying the whole-cell voltage-clamp technique. We dissected P2X4 and P2X7 receptor-mediated current components by using specific P2X4 and P2X7 receptor blockers and by their characteristic current kinetics. We found that P2X4 and P2X7 receptors are activated independently from each other, indicating that P2X4/P2X7 heteromers are not of functional significance in these cells. The pro-inflammatory mediators lipopolysaccharide and interferon γ, if applied in combination, upregulated P2X4, but not P2X7 receptor-dependent current components also arguing against phenotypically relevant heteromerization of P2X4 and P2X7 receptor subunits.     

Meat bone meal and fox manure as P sources for ryegrass (Lolium multiflorum) grown on a limed soil

Phosphorus (P)-rich by-products, such as meat and bone meal (MBM) and fur animal manures, are potential P sources in plant production systems. However, the solubility of P and its availability to plants in these forms has not been evaluated. We characterized P solubility in MBM, fox manures (FoxM) and dairy manure (DairyM) by Hedley fractionation and assessed P availability for ryegrass in a pot experiment. Up to 81% of P was water-soluble in DairyM, but only about 3 and 5–28% was soluble in MBM and FoxM products, respectively. Of the P in MBM and FoxM, 90 and 65–89%, respectively, was soluble only in 1 M HCl. Most of the P was inorganic; DairyM contained the highest share (14%) of organic P. Based on ryegrass yields and P uptake in a 3-year pot experiment with three P levels (25, 50 and 100 mg kg⁻¹), P availability was equal in the DairyM and superphosphate (SP) treatments. Compared with the availability of P in DairyM and SP, 19 and 35–54% of the P in MBM and FoxM, respectively, was immediately available to the plant; for the 3-year period with ten ryegrass cuts, the respective P availabilities increased to 63 and 69–87%. Additions of the sparingly soluble P sources MBM and FoxM increased the acid-soluble P concentrations in the experimental soil, with MBM having the strongest effect. However, the acid-soluble P fraction decreased with time. Although the immediate bioavailability of P in sparingly soluble P sources was lower than that in DairyM and SP, our results suggest that their use as a long-term P supply for perennial plants in non-calcareous soils should be encouraged.     

Spectrum-stable hyperbranched polyfluorene with photocrosslinkable group

Two kinds of blue light-emitting hyperbranched crosslinkable polyfluorene P1 and P2 with different contents of branch unit were synthesized via the palladium-catalyzed Suzuki coupling reaction, and the linear crosslinkable polyfluorene P0 was also prepared for comparison. The molecular weight and thermal behavior of the polymers were measured by GPC, DSC and TGA. Compared with uncrosslinked P0, P1 and P2, the photoluminescence spectra of crosslinked P0, P1 and P2 were little influenced. After annealing for 3 h at 200 °C in air, the photoluminescence spectrum of P2 with higher contents of branch unit showed more excellent stability compared with that of P1 and P0. The spectrum-stable hyperbranched polyfluorene can be used in multilayer and multicolor PLED.     

Effect of incorporation of crop residues and organic manures on adsorption/desorption and bio-availability of phosphate

Transformations of applied phosphorus (P) to unavailable residual soil P is the major cause of limited P supply in most of the P-deficient soils. The effect of the incorporation of crop residues (rice straw [RS] and wheat straw [WS]) and organic manures (farmyard manure [FYM] and green manure [GM]) on P release in soil and its bio-availability to various summer and winter crops was investigated in laboratory and screen house experiments. Surface (0–0.15 m) soil samples collected after 32 years of differential fertilization to maize–wheat–cowpea fodder crop rotation, were examined for adsorption/desorption behavior of P, after incubating with organics of varying C:P ratios. Incorporation of crop residues increased P adsorption maxima as well as resistance to P release in soils. Increased buffering capacities in crop residue-incorporated treatments decreased P desorption in soil, whereas the incorporation of organic manures decreased P sorption, maximum buffering capacity (MBC), bonding energy, and increased P concentration in soil solution. Although the incorporation of crop residues decreased P release in soil its bio-availability in the soil–plant system was crop-specific and varied with the time of incorporation of organics. Raya showed increased P uptake with incorporation of both RS and WS, whereas in the case of berseem increased P uptake occurred only with wheat straw. Phosphorus uptake in rice, maize, and soybean decreased with the incorporation of both RS and WS. Incorporation of crop residues 1 day before the sowing of summer crops decreased P uptake, whereas incorporation 3 weeks prior to the sowing of winter crops improved P bio-availability. Incorporation of organic manures with a narrow C:P ratio, however, improved P uptake in all the crops under investigation, in both the seasons. The results thus emphasized that adsorption parameters calculated from the examination of soil samples should not be used independently for making fertilizer P recommendations. Crop effects (root exudates) and their interaction with P reaction products in soil and synchronization in P release from organics and crop uptake need to be considered to understand the virtual behavior of P bio-availability in the soil–plant system.     

Phosphorus availability from phosphate rock and sewage sludge as influenced by the addition of water soluble phosphate fertilizer

Phosphorus (P) inputs are required for sustainable agricultural production in most acid soils of the tropics and subtropics. Phosphate rocks (PR) and organic materials have been suggested as alternative P sources in these soils. Quantitative information on the P availability from sewage sludge (SL) is scanty. Methods to improve the effectiveness of PR such as partial acidulation and compaction with water-soluble P sources have been recommended. The objective of this greenhouse study was to evaluate the relative agronomic effectiveness (RAE) of Florida PR and sewage sludges (irradiated and non-irradiated) applied alone and in mixture with a water-soluble source (triple superphosphate, TSP) at two rates (50 and 150 mg P kg^–1 soil). The ³²P isotope dilution technique was utilised to determine the proportion of P in the plant taken up from the P fertilizer treatments. Wheat was grown on an acid loamy sand Dystric Eutrocrepts and harvested 6 weeks after planting. Results on total P uptake and the RAE of the P fertilizer sources tested indicated that the addition of 50 mg P kg^–1 soil as TSP was adequate in supplying P to the 6-week-old wheat plants as compared to PR and sewage sludge. Intermediate values were obtained for the mixtures. Similar responses were observed for the high P rate. For a given P rate, phosphorus uptake from PR and SL in presence of TSP was higher than P uptake from these sources alone, indicating an enhancement effect of TSP on the effectiveness of these non-readily available sources. With respect to P uptake from PR applied alone, the relative increases in P uptake from PR due to TSP influence were 52 and 67% for the low and high P rates, respectively. The relative increases in P uptake from SL due to TSP when compared to P uptake from SL alone were 102 and 59% for the low and high P rates of application. Application of a water-soluble P fertilizer together with a non-readily available P source shows an enhancement on the P uptake from the non-readily available P source by the wheat plants. In this experiment the estimated enhancement effects are very likely underestimated.