Adsorption and desorption of phosphate in some semi-arid tropical Indian Vertisols

Adsorption and desorption of phosphorus in soils are among the key processes governing its availability to crops. There have been very few studies on the phosphorus adsorption and desorption characteristics of Vertisols. The P adsorption and desorption characteristics of four Vertisols belonging to three agriculturally important soil series were studied. The amounts of P adsorbed by the soils at 0.2µg ml^–1 equilibrium solution P concentration was low and ranged from 34.3 to 79.5µg g^–1 soil. The phosphate adsorption was very well described by Langmuir and Freundlich isotherms. The P adsorbed by a Vertisol (BR-1) fertilized with different rates of P in the previous season (0, 10, 20 and 40 kg P ha^–1) was similar (34.3–41.3µg g^–1 soil) indicating little effect of fertilization on P adsorption. The correlation studies indicated that the DTPA-extractable Fe was the most important factor accounting for P adsorption in these soils. Clay and CaCO₃ content were found to be relatively less important factors affecting P adsorption in the soils studied.The capacity of the two extractants and EUF (electro-ultrafiltration) to desorb the adsorbed P followed the order: EUF (400V, 80°C)>sodium bicarbonate>EUF (200V, 20°C)>calcium chloride. The average amounts of P desorbed from the four Vertisols using these methods were 74, 63, 50, and 3% respectively of the adsorbed P. In the Begamganj soil, the amount of P desorbed by EUF (400V, 80°C) exceeded 100%, indicating that all of the adsorbed P was desorbable including some native P.In conclusion the results of our study show that the Vertisols studied have low phosphate adsorption capacity and that the P they adsorbed is easily desorbable.Approved for publication as Journal Article No. 983 by International Crops Research Institute for the Semi-Arid Tropics (ICRISAT).     

Long-term changes in phosphorus fractions in growers’ paddocks in the northern Victorian grain belt

Increases in P fertiliser use in Australian grain production systems over the past decades have not necessarily coincided with improved nutrient-use efficiency by crops because only a small proportion of applied P has been directly used by crops, leaving large amounts of the P in soil. Information on the transformation of applied P and the residual effectiveness of P fertiliser in growers’ paddocks under their crop rotations in a wide range of soil types may help to develop improved management practises for P fertilisers. The present study examined the size and changes of P pools in soils in the major regions used for grain production across northern Victoria. Soil samples were collected from 43 sites representing all the major soil types. In addition, samples were collected from adjacent ‘reference’ areas across fence-lines where remnant native vegetation was present to examine long-term changes in soil fertility as a result of cropping. Highly positive P balance in 27 growers’ paddocks indicates potentially high rates of P fertilization in excess of that exported/lost from these paddocks. The accumulated P was transformed into both labile and non-labile pools depending on the general chemical–physical properties of a particular soil. In particular, in soils with a sandy texture and low oxalate-extractable Al and Fe such as the sandy Calcarosols, the residual P fertilizer was preferentially transformed into labile and moderately labile pools. These P fractions can be available for subsequent crops. In contrast, a large proportion of P fertilizer applied to other soil types has been transformed into the non-labile P pools where crops have difficulty in accessing. It is suggested that P application rates should be reduced to avoid the loss of P through sorption/precipitation, leaching or run-off while still meet crop demand.     

Fertilizer P transformations and P availability in hillslope soils of northern Ghana

Alfisols of the Savannas in northern Ghana have high base saturation and moderate P sorption capacities. Lateritic nodules are common, occurring in highly variable quantities at different landscape positions. Such nodules can have high P sorption capacities, and therefore effectiveness of P fertilisation may depend on landscape position. The objective of this study was to investigate the effectiveness of Togo rock phosphate (TRP), 50% acidulated TRP (PAPR) and single super phosphate (SSP) in providing and maintaining available P.Phosphorus supplying ability of soils from upper and lower slopes of three locations in northern Ghana was studied with and without fertilizer addition by repeated desorption with anion exchange resin membrane (AEM) burial in the laboratory and by measuring dry matter yield (DMY) and P uptake of sorghum for six successive greenhouse croppings. Transformations of the applied fertilizers were studied by sequential extraction.Phosphate desorbed with AEM after 38 days, the DMY and the P uptake of sorghum all followed the order SSP > PAPR > TRP = control. The relative agronomic effectiveness of the PAPR was 63% of SSP. Although half the applied TRP was transformed to other forms than acid extractable apatite, this did not liberate P to the AEM. Less than 10% of the SSP was absorbed by the AEM.In the nodule-rich upper slope soils, initial availability of added P was higher, but decreased more rapidly than in lower slope soils. We attributed this to an initial concentration of fertilizer in the smaller volume of soil fines followed by slow sorption into the nodules. At the upper slope, much of the TRP and PAPR were transformed to less available forms, while at the lower slope more TRP remained as untransformed apatite.     

Pathways of native and fertilizer phosphorus in Argentine soils

Management of soil phosphate fertility in sustainable agriculture depends on transformations of applied fertilizers as an input to correct soil defficiencies. This research investigated the changes of P pools of different extractability with fertilization in an incubation experiment. Sequential fractionation was used in 5 native argentine soils: Entisol, Andisol, Vertisol, Mollisol and Ultisol, with 0 and 45 kg P ha^-1 added as KH₂PO₄ and incubated for 90 days. In our experimental conditions, labile inorganic P (LIP) content of control soils increased for Entisol and Mollisol (75% and 35% respectively), while moderately resistant inorganic fractions (MRIP) were greater in Andisol (95%) and Ultisol (39%) following incubation. This increase was related to a decrease in labile organic fractions (LOP) in Andisol (-73%), Mollisol (-36%) and Ultisol (-36%). Moderately resistant organic forms (MROP) were significatively lower for all soils except Mollisol.As a consequence of P fertilization, LIP increased for Entisol (146%), Vertisol (23%) and Mollisol (39%), and MRIP showed the same tendence in Ultisol (57%) and Andisol (97%). LOP was signifcatively lower for all soils, except Andisol. MROP decreased in all soils except Mollisol, with the greatest variation in Andisol (-56%). In this experiment, labile P, the agronomically important pool, showed a similar pathway for native and fertilizer P for each taxonomic Order, with a significative increase in Mollisol and Entisol. P for each taxonomic Order, with a significative increase in Mollisol and Entisol.The main reservoir for fertilizer P was IP, mainly LIP in Mollisol and Entisol, and MRIP in Vertisol, Andisol and Ultisol.Organic P tended to decrease with incubation, and the highest values of organic fractions were found in younger soils (Entisol and Andisol), followed by Ultisol. Residual effect of fertilizer could be higher in Andisol and Ultisol due to transformation into non-labile forms.     

Effect of rate and source of N fertilizer on yield,quality and N recovery of bromegrass grown for hay

Isotopic exchange of phosphate was investigated in soils which had received 0–176 kg P ha^–1 as triple superphosphate in the field. The soils were kaolinitic oxisols from near Manaus, Brazil, and Pescador, Colombia, and an allophanic andept with much organic matter from Popayán, Colombia. When fresh phosphate was added to these soils in the laboratory the residues of triple superphosphate decreased the intensity of phosphate in the liquid phase that was associated with a specific quantity of exchangeable phosphate. However, residues of triple superphosphate increased phosphate taken up by maize at Manaus. Residues did not cause this effect in soils from Colombia in which the major minerals were halloysite and imogolite.The cause of the anomalous results in kaolinitic soils may be that in the field, during long contact between soil and phosphate, the latter penetrated into the mineral crystal so that, although it maintained adequate concentration for crops, exchange of the residues was slower in the laboratory than exchange of the fresh phosphate that was added for the measurements. In the andept the effect may have been caused by strong sorption of fertilizer phosphate onto hydrous oxides of aluminium released from organic matter by lime.The results indicated that although short term exchange data may be useful to differentiate phosphate sorption characteristics in untreated soils they may not, at least in some soils, represent long term behaviour of phosphate residues in relation to phosphate taken up by crops.     

Response of sorghum to fertilizer phosphorus and its residual value in a Vertisol

The response of crops to added P in Vertisols is generally less predictable than in other soil types under similar agroclimatic conditions. Very few studies have considered the residual effects of P while studying responses to fresh P applications. Field experiments were conducted for three years to study the response of sorghum to fertilizer P applied at 0, 10, 20 and 40 kg P ha^–1, and its residual value in a Vertisol, very low in extractable P (0.4 mg P kg^–1 soil), at the ICRISAT Center, Patancheru (near Hyderbad), India. In order to compare the response to fresh and residual P directly in each season, a split-plot design was adopted. One crop of sorghum (cv CSH6) was grown each year during the rainy season (June-September).The phenology of the sorghum crop and its harvest index were greatly affected by P application. The days to 50% flowering and physiological maturity were significantly reduced by P application as well as by the residues of fertilizer P applied in the previous season. In the first year of the experiment, sorghum grain yield increased from 0.14 t (no P added) to 3.48 t ha^–1 with P added at the rate of 40 kg P ha^–1. Phosphorus applied in the previous year was 58% as effective as fresh P but P applied two years earlier was only 18% as effective as fresh P.     

Transformations of residual fertilizer P in a semi-arid tropical soil under eight-year peanut-wheat rotation

Long-term transformations of residual phosphorus (P) governs the availability of phosphorus to crops. Very limited information is available on the transformations of residual fertilizer P in semi-arid tropical soils under long-term crop rotations. Therefore, using sequential phosphorus fractionation procedure, we studied changes in labile and stable forms of inorganic and organic P in a semi-arid alluvial soil (Typic Ustisamments) after eight years of annual fertilizer P application either to one crop (alternate) or to both crops (cumulative) in a peanut (Arachis hypogaea) — wheat (Triticum aestivum) rotation.Total residual fertilizer P in soil (P recovered from P-fertilized minus control plots) ranged from 62 to 176 mg P kg^–1. In the alternate P treatments (P applied to peanut or wheat only), on an average of 3 rates of applied P (13, 26 and 39 kg P ha^–1), in surface (0–15 cm) and subsurface (15 to 30 cm) soil, respectively, residual fertilizer P consisted of 14.8 and 2.2% resin-P, 8.6 and 2.8% NaHCO₃-P, 6.3 and 0% microbial-P, 31.4 and 4.2% NaOH-P, 7.8 and 3.0% aggregate protected-P, 12.5 and 3.0% HCl-P, 3.4 and 0% H₂SO₄-P. The corresponding values for surface and subsurface soils of cumulative P treatments (P applied to both peanut and wheat) were: 12.8 and 1.6% resin-P, 6.9 and 2.3% NaHCO₃-P, 4.7 and 0% microbial-P, 32.5 and 4.2% NaOH-P, 5.6 and 2.0% aggregate protected-P, 14.8 and 3.8% HCl-P, 6.7 and 2.1% H₂SO₄-P. Considerable lower values for the 15–30 cm depth indicate only a very small movement of residual P to the subsoil.Significantly lower amount of fertilizer P (28% and 44%) found in labile (resin, NaHCO₃ and microbial P) and semi-labile (NaOH and sonicated NaOH P) fractions for the cumulative P treatment than alternate P treatment (35 and 46%, respectively) suggests that increased rates and frequency of applied P tend to enhance the conversion of residual P to stable forms which are less available to plants. About 12 to 19% of residual fertilizer P found as organic P in labile and semi-labile forms confirmed that organic P increased with long-term fertilizer management. In conclusion, the results of our study suggest that the alternate application of fertilizer P to a crop, as is shown for wheat, helps reduce the transformations of residual P to stable P forms.     

Phosphate enrichment in the sandy loam soils of West-Flanders, Belgium

The last three decades, pig breeding has evolved towards a specialised, large scaled, land independent bio-industry in the province of West-Flanders. Subsequently, in certain regions, very high amounts of liquid pig manure are produced each year. This pig slurry is used as a fertilizer at a rate which very often exceeds normal agricultural practices. Because of the nonequilibrium between the phosphorus crop requirements and the P-inputs, phosphates accumulate in the soil. However, the phosphate sorption capacity of a soil is limited. Once the sorption capacity is exceeded, phosphates will start leaching through the soil profile. Since, during winter, in these areas, the groundwater table is situated at a depth of less than 1.0 m, phosphate breakthrough might take place. In the sandy loam soil region (± 1000 km²) of the province, an inventory of the P status of the soil was made. The region was sampled according to a regular grid with 2 km intervals. At random, some sample points were only 500 m apart. This resulted in a total of 296 samplings. In view of fertilizer recommendations, lactate extractable P of the plough layer (0-30 cm) was determined. A maximum value of 101 mg P 100 g^–1 of air dry soil, a minimum value of 6 mg P 100 g^–1 and a median value of 31 mg P 100 g^–1 were found, indicating that for half of the spots monitored, the P status of the soil is high to very high. An oxalate extraction was done to investigate the phosphate saturation of the soil profile (0-90 cm). Based on a critical phosphate saturation degree of 30%, more than half of the soil profiles are phosphate saturated. Phosphate leaching at a rate higher than 0.1 mg ortho-P 1^–1 at a depth of 90 cm can be expected. Therefore, a restriction of the P fertilization should be highly recommended. The geostatistical processing of the data using block kriging resulted in a spatial continuous estimate of the phosphate saturation degree. A good agreement was found between the pig density and the phosphate saturation degree of the soil profile.     

Soil order and management practices control soil phosphorus fractions in managed wetland ecosystems

Phosphorus, an essential element for plant and animal growth, can also impair water quality. Understanding management effects on P dynamics can aid in the management of these systems to reduce nonpoint source pollution and improve fertilizer use efficiency. A sequential P fractionation procedure was used to evaluate labile to recalcitrant inorganic (Pi) and organic (Po) fractions in wetland rice soils. In this study we evaluated 71 wetland soils in the Sacramento Valley, California, consisting of different soil orders (Alfisols, Entisols, Mollisols and Vertisols) and different management systems (conventional rice, organic rice and natural wetlands). Total soil P ranged from 165 to 784 μg g⁻¹ and averaged 415 μg g⁻¹. Mollisols contained significantly more Pi and Po than all other soils; especially more HCl-Pi and Po. Although most studies ignore Po in the HCl fraction, 9% of total P was recovered in this fraction, suggesting that this fraction should not be ignored in studies aimed at quantifying and understanding organic P. The HCl-Po fraction was closely correlated with HCl-Pi, suggesting that it may be Ca bound P. Soils managed under organic rice production had higher NaHCO₃-Pi and NaOH-Pi levels than conventional rice systems; while the natural wetland systems showed intermediate amounts. Organic or synthetic P fertilizers applied in excess of the amount of P that is removed during harvest was recovered as NaHCO₃-Pi and NaOH-Pi.     

Assessment of the relative agronomic effectiveness of phosphate rocks under glasshouse conditions

Six phosphate rocks (PRs) of varying reactivities were compared with monocalcium phosphate (MCP) in a glasshouse experiment growing perennial ryegrass (Lolium perenne cv. Nui) as the test plant on four soils of contrasting P sorption capacity and exchangeable Ca. The cumulative dry matter yield over 10 harvests showed a significant response to P application in all soils. Based on relative yield and P uptake, MCP was the most effective P fertilizer followed by the reactive phosphate rocks, which were superior to the unreactive rocks in all soils. The relative agronomic effectiveness (RAE) and substitution ratio (SR) of individual PR fertilizers, calculated with respect to MCP using the methods of vertical and horizontal comparison, respectively, were similar over a range of fertilizer rate. There was a decline or slight increase in the performance of PRs with time in the low P sorption soils but a consistent increase in the high P sorption soils. Some initial influence of exchangeable Ca content of the soils on the relative performance of PRs was also observed. Generally the PRs performed better in high P sorption soils than low P sorption soils and in low exchangeable Ca soils than high exchangeable Ca soils.     

Exchangeable and non-exchangeable phosphate sorption in Portuguese soils

Total amounts of phosphate (P) sorbed were measured for 6 Portuguese soils of widely varying properties.³²P was used to assess the isotopically exchangeable and non-exchangeable sorbed P. Total sorbed and exchangeable P were described by modified Freundlich equations and non-exchangeable P by a Temkin equation. The Langmuir equation also proved to fit the data for non-exchangeable P well. The amount of total sorbed P required to attain 0.2 mg P 1^–1 in solution ranged from 5.3 to 819 mg P kg^–1. At this concentration exchangeable and non-exchangeable P values varied from 62.4 to 536.6 and from 0.4 to 322.1 mg P kg^–1 respectively.There were highly significant (p < 0.01) correlations between soil organic matter and all forms of sorbed P (total sorbed, exchangeable and non-exchangeable). The P sorption parameters with correlation coefficients greater than 0.967 were parametersa andb of the modified Freundlich equation b_n of Temkin and parametersa of the Langmuir equation. Aluminium extracted by acid oxalate (Al_ox) and dithionite (Al_di) showed highly significant correlation coefficients (r = 0.972) with the same sorption parameters. But P sorption was not closely related to the clay content Fe_ox and Fe_di. It was concluded that extractable aluminium (Al_ox and Al_di) had the most important effects on P sorption in these soils.     

The movement into soil of P from superphosphate grains and its availability to plants

The movement of P applied as grains of triple superphosphate into two soils (laterite and podzol) of differing P sorption capacities was studied in a laboratory experiment. The availability of this P for plant growth was evaluated by measuring the P desorption characteristics of the fertilized soil and also through a plant growth experiment. Four weeks after fertilizer application to the soil 45% and 72% of the fertilizer P had dissolved for the laterite and podzol, respectively. For both soils all the added P was retained within 80 mm of the fertilizer grain and was considered to occur in the soil in three discrete zones. These zones consist of: (1) the residual grain and a small adjacent zone of soil where most P occurs as insoluble fertilizer compounds and possibly as compounds precipitated from fertilizer solution (2) an inner region where both precipitates and P adsorbed on to the soil at about the maximum adsorption value are present and (3) an outer region where all the added P is adsorbed on to the soil at levels less than the maximum adsorption value.The desorption of fertilizer P from soil in 0.01M CaCl₂ solution at different solution:soil ratios as a function of total soil P followed a relationship of the type Y = aX^b where Y is desorbed P and X is adsorbed P. For both soils the values of exponent (b) decreased and tended to unity as the solution:soil ratio increased. A much higher proportion of total P (1.5–3 fold) was desorbed from the podzol as compared to the laterite.The results of the greenhouse trial showed that P from soil reacted at three P concentrations corresponding to the three discrete zones surrounding fertilizer grains was equally available. This result was obtained for two successive wheat crops for both the soils. When the P fertilized soil was banded it was much more effective (about 3 to 5 times for the laterite and 2 to 3 times for the podzol) than when mixed through the soil.     

Relationship between extracted phosphorus and sorghum yield in a Vertisol and an Alfisol under rainfed cropping

Little attention has been devoted to calibrating the soil tests for P in the field for crops grown under rainfed conditions in different soil types. Field experiments were conducted during the 1990 rainy season (June-September) at the ICRISAT Center, Patancheru (near Hyderabad), India on nearby Vertisol and Alfisol sites having a range in extractable P, for establishing relationships between extractable P and sorghum yield.In the Vertisol, 90% relative grain yield of sorghum was obtained at 2.8 mg kg^–1 Olsen extractable P while in the Alfisol, 90% relative grain yield was achieved at 5.0 mg P kg^–1 soil. These results suggest that a single critical limit of available P does not hold true for grain sorghum in the two soil types under similar agroclimatic conditions and that the critical limit is lower for the clayey Vertisol than the sandy Alfisol.     

Dissolution of phosphorus from animal bone char in 12 soils

Heat-treated animal bone char (ABC) has not previously been evaluated for its potential as a phosphorus (P) fertilizer. ABC, Gafsa phosphate rock (GPR) and triple superphosphate fertilizer (TSP) were incubated in 12 soils. Dissolved-P was assessed by extraction with NaOH and bioavailability with the Olsen extractant. The rate of P dissolution from ABC was described almost equally well by the Elovich and Power equations. After 145 days, the fraction of P dissolved ranged from 0 to 73% and to 56% for ABC and GPR, respectively. The most important soil properties determining P dissolution from ABC were pH and P sorption. P dissolution was not significant at soil pH >6.1 (ABC) and >5 (GPR) and the lower the pH, the greater the Dissolved-P. Dissolved-P also correlated positively and significantly with inorganic P sorption, measured by the Freundlich isotherm and the P sorption index of Bache and Williams (1971). Soil pH and P sorption index could be combined in multiple regression equations that use readily measured soil properties to predict the potential for ABC dissolution in a soil. Dissolution of P from GPR correlated with soil pH and exchangeable acidity. In comparison with GPR, ABC was a better source of available P, assessed by Olsen-P. In most soils, ABC increased Olsen-P immediately after application, including soils of relatively high pH in which GPR was ineffective. ABC is a P fertilizer of solubility intermediate between GPR and TSP.     

Phosphorus sorption by three cultivated savanna alfisols as influenced by pH

An earlier study of phosphate sorption by some savanna soils from Nigeria suggested that increased P sorption when pH was raised might be due to precipitation of exchangeable Al as amorphous polymeric Al species with increased sorption sites. But these savanna soils have Ca as the dominant cation in their exchange sites, and low exchangeable Al. The objective of this study was to determine the role played by Ca in pH-induced P sorption of three savanna soils under continuous cultivation. Phosphorus sorption increased when pH was raised from 4.5 to 7.0. Similarly, Ca retention increased with increasing pH. Regression of P sorption on Ca retention indicated a significant linear relationship in the three soils. Three possible mechanisms were proposed to explain the increasing P sorption with increasing pH: precipitation of Ca-phosphates, Ca-induced P sorption or co-adsorption of Ca and H₂PO ₄ ^– or HPO ₄ ^2– as ion pairs or complexes. Available evidence suggests that all three mechanisms can operate together to enhance P retention as pH increases. The paper proposes that increased P sorption by savanna soils when pH is raised is likely to be related to the chemistry and retention of Ca rather than to hydrolytic reactions of Al.     

Combining Tithonia diversifolia and minjingu phosphate rock for improvement of P availability and maize grain yields on a chromic acrisol in Morogoro,Tanzania

A 2-year field experiment was conducted to evaluate the effects of Tithonia diversifolia green manure combined with either Minjingu phosphate rock (MPR) or triple super phosphate (TSP) on soil chemical properties that influence P availability, P pools and maize grain yields, on a Chromic Acrisol in Morogoro, Tanzania. Leafy biomass of tithonia was applied before maize planting for two consecutive growing seasons. Treatments compared were the control, MPR and TSP each at 80 kg P ha⁻¹; tithonia alone at 2.5, 5.0, and 7.5 Mg ha⁻¹ dry matter and tithonia combined with MPR or TSP at 40 kg P ha⁻¹. Tithonia led to significant increases in soil pH, exchangeable Ca, labile (resin and NaHCO₃-Pi), and moderately labile inorganic P (NaOH-Pi). It reduced exchangeable Al and P sorption. Application of MPR alone had liming effects and resulted in increase in labile P. Combining tithonia with MPR had similar but more intense effects. Triple superphosphate alone led to acidification and this was reversed when TSP was co-applied with tithonia. Increasing the application rates of tithonia either alone or in combination with TSP or MPR led to more pronounced liming effects but the differences between 2.5 and 5.0 Mg tithonia ha⁻¹ were not significant due to moisture stress that was experienced during the season. The P and Ca concentrations of the maize plants at tasselling increased with the application of tithonia alone or combined with MPR or TSP, and were significantly correlated with maize grain yields (r = 0.75 and 0.64 for MPR and TSP, respectively). Tithonia added consecutively for 2 years increased total maize grain yields by 70% compared to that in the control. The relative agronomic effectiveness (RAE) of MPR increased from 46% in the first year of application to > 142% in the second year, indicating that the initially slow dissolution of MPR improved by combined application of tithonia and MPR, attributed to reduction of P sorption. It is concluded that tithonia can enhance P availability from the Chromic Acrisol through modification of soil properties associated with P transformation and availability. In cases where tithonia is found within the farmers’ fields its combined application with MPR can increase maize yields at a much-reduced cost associated with tithonia procurement.     

The effect of various phosphorus fertilizers on yields,phosphorus uptake by plants and conversion of phosphorus fertilizers in long-term experiments on different soils

Long-term experiments have been undertaken to investigate the effects of various types of phosphorus fertilizers on yields, phosphorus uptake and fertilizer conversion in the soil. Optimum effects were obtained from water-soluble phosphate, whereas finely ground soft rock phosphate had little effect even with large amounts of phosphorus fertilizers and with a pH value of the soil of around 5.4. Partially decomposed and sintered phosphates performed well, as did NPK fertilizers (100% water soluble) and Thomas phosphate. The addition of large amounts of phosphorus fertilizers (to cereals, 26.4 kg/ha, to other crops 44 kg/ha) led to yields being increased by 7% and phosphorus extraction being increased by 11%. Smaller amounts of phosphorus fertilizers (13.2 and 17.6 kg/ha) did not have this effect. Increases in the amount of double-lactate-soluble phosphorus in the soil depended on the level of fertilization and did not differ significantly according to the type of phosphorus fertilizer used.     

Extractable iron in two soils of contrasting pH fertilized with ferrous sulfate,FeEDTA and FeEDDHA

The behaviour of FeSO₄, FeEDTA and FeEDDHA added to a Vertisol (pH 8.3) and an Alfisol (pH 5.8) was studied by periodically monitoring DTPA extractable Fe in soil samples incubated at — 33 kPa soil moisture at 30°C for 8 weeks. It was found that FeEDDHA was most effective in both Alfisol and Vertisol in maintaining high amounts of extractable Fe during 8 weeks. Both FeSO₄ and FeEDTA were completely ineffective in the Vertisol though they were moderately effective in the Alfisol. These results suggest that FeEDDHA is the most effective source of iron for soil application in the high pH Vertisols.     

Phosphate availability in calcareous Vertisols and Inceptisols in relation to fertilizer type and soil properties

The availability to plants of fertilizer phosphorus (P) applied to soil, as measured by chemical extraction, is used to estimate P fertilizer needs. We studied the availability of P, applied as monocalcium phosphate (MCP) powder, ordinary superphosphate (OSP) granules and diammonium phosphate (DAP) granules in 24 calcareous Vertisols and Inceptisols of Andalusia, Spain, by using laboratory incubation techniques. The soils differed widely in their P adsorption- and Ca-phosphate precipitation-related properties. For MCP, availability (defined as the proportion of added P that is recovered by extraction with NaHCO₃ or is isotopically exchangeable) decreased markedly with incubation time and increasing addition rate. The mean recoveries after 180 d of incubation at field capacity at a rate of 246 mg P kg^–1 soil were 17% for Olsen P, 38% for Colwell P, and 16% for isotopically exchangeable P (IEP). Increasing the application rate to 2460 mg kg^–1 resulted in recoveries of 6% for Olsen P, 25% for Colwell P, and 4% for IEP. While IEP-based recovery was not significantly correlated to any soil property, that based on Olsen P (and, to a lesser extent, Colwell P) decreased sharply with increase in the ratio of clay (or Fe oxides) to total (or active) calcium carbonate equivalent. Accordingly, Olsen P might overestimate P availability in those soils relatively rich in carbonate and poor in clay and Fe oxides. On the other hand, recovery of applied P from soils containing more clay and Fe oxides, by a sequential extraction (with H₂O, two 0.5M NaHCO₃ treatments, 0.5M HCl), was lower than 100%, thereby suggesting phosphate occlusion by Fe oxides or clay.Availability of the fertilizers tested 90 d after application was found to decrease in the following order: MCP powder (rate, 246 mg kg^–1) > DAP granules (rate, 547 mg kg^–1) > MCP powder (rate, 738 mg kg^–1) > OSP granules (rate, 308 mg kg^–1). Differences between fertilizers tended to increase with increasing carbonate content in the soil. This may have been due to precipitation of Ca phosphates caused by the presence of Ca in the fertilizer and the high Ca- supplying capacity of the more calcareous soils.     

Development and evaluation of an improved soil test for phosphorus: 1. The influence of phosphorus fertilizer solubility and soil properties on the extractability of soil P

The objective of this work was to develop and evaluate a soil test suitable for estimating the phosphorus status of soils whether they were fertilized with soluble or sparingly soluble P fertilizers or both. Four New Zealand soils of contrasting P sorption capacity and exchangeable Ca content were incubated alone or with monocalcium phosphate (MCP), reactive North Carolina (NC) phosphate rock or unreactive Florida (FRD) rock, at 240 mg P kg^–1 soil, to allow the P sources of different solubilities to react with each soil and provide soil samples containing different amounts of extractable P, Ca and residual phosphate rock. The phosphorus in the incubated soils was fractionated into alkali soluble and acid soluble P fractions using a sequential extraction procedure to assess the extent of phosphate rock dissolution. Eight soil P tests [three moderately alkaline — Olsen (0.5M NaHCO₃) modified Olsen (pretreatment with 1M NaCl) and Colwell; three acid tests — Bray 1, modified Bray 1 and Truog; and two resin tests — bicarbonate anion exchange resin (AER) and combined AER plus sodium cation exchange resin (CER)] were assessed in their ability to extract P from the incubated soils.The 0.5M NaHCO₃ based alkaline tests could not differentiate between the Control and FRD treatments in any soil nor between the Control, NC and FRD treatments in the high P sorption soils. The acid extractants appeared to be affected by the P sorption capacity of the soil probably because of reabsorption of dissolved P in the acid medium. The AER test gave results similar to Olsen. Only the combined AER + CER test extracted P in amounts related to the solubility of the P sources incubated with each soil. Furthermore, when soil samples were spiked with FRD and NC and extracted immediately, the P extracted by the AER + CER test, over and above the control soils, increased with the amount and chemical reactivity of the rocks. There was no extraction of rock P by any of the alkaline extractions.Increases in the amounts of P extracted (P) by each soil test from the fertilized soils, over and above the control soils were compared with the amounts ofP dissolved from the fertilizers during incubation (measured by P fractionation). Soil P sorption capacity had least influence on the amounts of P extracted by the AER + CER and Colwell tests. However, the Colwell test was unable to differentiate between all P sources in all four soils and suffered from the disadvantage of producing coloured extracts. The AER + CER test appeared to have the potential to assess the available P status of soils better than the other tests used because of its ability to extract a representative portion of residual PR (in accordance with the amount and reactivity) and dissolved P, and thus to differentiate between fertilizer treatments in all four soils.