Phosphorus fractions and adsorption characteristics in grassland soils of varied soil phosphorus status

Characterization of phosphorus (P) in soils is important both agronomically and environmentally, although the outcome may depend on the technique applied. Consequently, we evaluated fractionation and adsorption, individually and jointly, and relevant ancillary soil attributes, to determine the dominant functional characteristics of soil P in 32 fertilized temperate grassland Inceptisols classified by eight soil series, and by two soil-P index and parent material groups. Residual P was low (30.7%) and organic P (Po) prominent, 42.0% vs. 17.5% for equivalent soils in unfertilized natural ecosystems. Labile fractions comprised 6.8% inorganic P (Pi) and 9.1% Po. The proportional increase in high vs. low index soils (Morgan P > 6.0 mg l⁻¹ vs. ≤ 6.0 mg l⁻¹) was higher for Pi, and highest for labile and moderately labile fractions. Only moderately labile Pi and Po differed significantly between soils of limestone and non-limestone origin. Oxalate extractable Fe (Fe_ox) and buffering (EBC) were higher in the latter. The equilibrium P concentration (EPC) was substantially higher in the high index group, and EBC and binding energy (k) substantially lower, with no significant difference in sorption maximum (Pmax). EBC equated with weak to strong buffering in different soil series, and conformed better than k to ancillary attributes. Pmax correlated in order Al_ox > clay > OC > Fe_ox, and more broadly reflected sorption attributes than oxalate-based sorption capacity (PSC). Principal component (PC) analysis showed consistent differentiation of P fractions, mostly labile and moderately labile, in PC 1 vs. adsorption and ancillary attributes in PC 2. However, scatterplots of PC scores showed that adsorption characteristics provided better functional differentiation than P fractions for distinguishing individual soil series, which may have implications in selection and interpretation of extractants not only for environmental but also for agronomic soil-tests.     

Phosphorus supplying capacity of heavily fertilized soils I. Phosphorus adsorption characteristics and phosphorus fractionation

Studies were conducted to investigate the P sorption characteristics and P fractions in eight intensively fertilized soils collected from southern and central Norway. Adsorption of P at the initial P concentrations in the soil solution was very high in the Særheim clay loam soil which contained high amounts of organic C and clay. Adsorption data were fitted well to the classical Langmuir equation. The P affinity constant (k), adsorption maximum (b) and maximum buffer capacity (mbc) calculated from this equation differed considerably among soils. The P affinity constant (r=0.96,p=0.01) and maximum buffer capacity (r=0.97,p=0.01) were highly and positively correlated to organic C. None of the soil parameters were related to adsorption maximum. Phosphorus desorption from the heavily fertilized soils varied widely and depended on the initial P status of the soil and soil texture. The ratio between desorbed P and total P was significantly correlated to sorption parameters. Multiple regression analysis showed that total P positively and organic C negatively affected P desorption in the soils. Iron-P was a major P sink in these soils and it was related to clay content (r=0.69,p=0.1) and organic P (r=0.76,p=0.0.5), but it did not relate to average P removed per harvest (RPH). Calcium-P and Al-P were not related to any of the soil parameters but these fractions were the major contributors to RPH as expressed by a multiple regression equation: RPH=0.397+0.0016 × Ca-P + 0.0012 × Al-P (r=0.84,p=0.05). High content of inorganic fractions shows that most of the residual P may be plant available, albeit at reduced rate with time, in these soils but the availability will depend on soil types.     

Effects of Soil Compaction and Moisture Regime on the Root and Shoot Growth and Phosphorus Uptake of Barley Plants Growing on Soils with Varying Phosphorus Status

The effects of soil compaction state and level of water supply on plant growth and phosphorus (P) uptake were studied in a pot experiment with spring barley (Hordeum vulgare L.). Three soils (loamy, clay loam and silt texture) were used, each with three levels of plant available P in soil (low, medium and high P-AL). The soils were compacted to two levels, 75 and 90% of the standard degree of compactness attained with free drainage under a load of 200 kPa, and they were maintained at two levels of water supply (20 and 30% mass wetness). Total root length (RL) was 59% lower at the higher of the two compactness levels, mainly due to fewer roots with diameter <0.5 mm. However, both shoot growth and plant P uptake were reduced a lesser degree then RL. This means that 69% more shoot dry-matter (DM) per unit RL was produced in the compacted soil, and that the P uptake per unit RL was 75% higher in the more compacted soil. Thus, the plants must have compensated for reduced RL by increasing their uptake of water and nutrients per unit RL, although this did not fully compensate for the reduced RL. The effects of moisture regime on shoot growth varied between the different soils. On the clay loam soil, there was a significant yield increase with high water supply at alt three soil P-AL levels. The same was true on the loam soil at medium and high soil P-AL levels. On the silt soil, the high level of water supply increased yield by 62%, but only at the highest soil P-AL level. The plants grown on the silt soil at low and medium levels of plant available P showed marked symptoms of P deficiency throughout the growth period.     

Phosphorus budget and phosphorus availability in soils under organic and conventional farming

The aim of this work was to assess to which extent organic farming practices would affect the accumulation of total and available phosphorus (P) in a cropped soil in comparison to conventional practices. In order to achieve this, soil samples were taken from a long-term field trial comparing a non-fertilised control (NON), two conventionally cultivated treatments (MIN, CON), and two organically cultivated treatments (ORG, DYN). Soil samples were taken from each treatment at two depths (0-20 and 30-50 cm) before starting the field trial (1977) and at the end of every three crop rotations (1984, 1991 and 1998). They were then analysed for total P (P_t), total inorganic P (P_i), total organic P (P_o) and isotopically exchangeable P_i. After 21 years, the average P input-output budget reached -20.9 kg P ha^–1 a^–1 for NON, -7.8 for DYN, -5.7 for ORG, -5.0 for MIN and +3.8 for CON. Total P, P_i as well as the amount of P_i isotopically exchangeable within 1 minute (E₁) were positively correlated to the P budget. Comparison between P budget and P_t in the top- and subsoils of the fertilised treatments suggested a net transfer of P from the 0–20 to the 30–50 cm layers between 13 and 26 kg P ha^–1 a^–1during the first rotation and between 3 and 12 kg P ha^–1 a^–1during the second rotation. During the third rotation a net upward movement of P from the subsurface to the topsoil ranging between 3.7 and 10.5 kg P ha^–1 a^–1was estimated. In the topsoil, E₁decreased from an initial value of 12 mg P kg^–1 to 11 in CON, 8 in MIN, 6 in ORG, 5 in DYN and 2 in NON after 21 years. In the subsoil, E₁ increased from an initial value of 2 mg P kg^–1 to 4 in MIN, ORG, DYN and NON and to 6 in CON. These results show that, with the exception of NON, all treatments had still an adequate level of available P after 21 years of trial and that, in this low to moderately P sorbing soil, an equilibrated input-output budget allows to maintain P availability at a constant level. In the organic systems, yields have so far partly been attained at the expense of soil reserves or residual P from earlier fertiliser applications.     

Phosphorus utilisation efficiency and depletion of phosphate fractions in the rhizosphere of three tea (Camellia sinensis L.) clones

Tea (Camellia sinensis L.) is mostly grown on highly weathered acidic Ultisols in the humid and sub-humid tropics. Phosphorus (P) availability in Ultisols is naturally low due to it's low diffusivity caused by high P-fixation of Fe and Al oxides. Tea is generally fertilised with low-cost reactive phosphate rocks (RPR) because of enhanced solubility of RPR under acidic conditions. In many countries, new tea clones have been developed to improve yield, drought tolerance and resistance to pest and diseases, but the effectiveness of these clones in utilising P from RPR and native soil P forms has not been studied. A study was conducted to investigate the effects of triple superphosphate (TSP) and a sparingly soluble phosphate rock (Eppawala phosphate rock, EPR) on plant P uptake and soil P fractions in the rhizosphere of three tea clones developed in Sri Lanka (S 106, TRI 2023 and TRI 2025). Phosphate uptake by TRI 2023 (3.3 ± 0.02 mg P plant-1) and TRI 2025 (2.6 ± 0.08 mg P plant-1) was significantly greater than S 106 (1.1 ± 0.01 mg P plant-1) for both P treatments. However the type of P fertiliser did not show any significant difference in P uptake by any of the clones. In all tea clones, the rhizosphere soil pH decreased significantly compared to that of the bulk soil in both P fertiliser treatments. The decrease of pH near the rhizoplane (0–0.5 mm) for TRI 2023, TRI 2025 and S 106 were 0.30 ± 0.02, 0.19 ± 0.03 and 0.17 ± 0.05 respectively. Dissolution of EPR in the rhizosphere of TRI 2023 and TRI 2025 was greater than S 106, which could be attributed to enhanced H+ efflux. TRI 2023 and TRI 2025 depleted more resin-P, NaOH–P _i and H2SO4–P _i in the rhizosphere compared to S 106, which is consistent with the P uptake results. NaOH–P _o accumulated in the rhizosphere of all clones suggesting that soluble inorganic P was transformed into organic P forms possibly as a result of increased microbial activity in the rhizosphere. TRI 2023 and TRI 2025 showed a higher external P efficiency (total P uptake) compared to S 106 due to higher root surface area and higher P uptake per unit root surface area. The higher P uptake per unit root surface area in TRI 2023 and TRI 2025 compared to S 106 may be due to higher rhizosphere acidification, root exudation and mycorrhizal associations. Between TRI 2023 and TRI 2025, the former had higher external P efficiency due to greater root surface area. S 106 had higher internal P efficiency (shoot dry matter production per unit plant P) compared to the other two clones. In future tea breeding programmes, attempts should be made to combine these two traits to maximise P utilisation efficiency from tea clones.     

Chemical Nature and Diversity of Phosphorus in New Zealand Pasture Soils Using <Superscript>31</Superscript>P Nuclear Magnetic Resonance Spectroscopy and Sequential Fractionation

Information on the phosphorus (P) forms of pasture soils is central to agricultural and environmental sustainability. We used a combination of ³¹P nuclear magnetic resonance (NMR) spectroscopy of NaOH–EDTA extracts and sequential fractionation to investigate P forms, with an emphasis on organic P in relation to environmental and biophysical variables, in 24 diverse pasture soils taken from around New Zealand (organic C 19–102 g kg⁻¹, total P 116–2746 mg kg⁻¹, pH 5.2–7.0). Soils were separated by cluster analysis of soil physicochemical properties and sequentially extracted P pools into those either derived from volcanic-ash materials or not. This separation was also evident for organic P species in NaOH–EDTA extracts, which removed about 75% of total soil organic P. The major organic P compound class was monoesters (24–60% of extracted P), made up of 14 to 91% myo-inositol hexakisphosphate. The next largest organic P class was diesters (0–4% of P extracted), which were enriched in volcanic-ash soils (monoester to diester ratio = 14) compared to non volcanic-ash soils (ratio = 30). Correlation analysis indicated that mean annual temperature had a significant negative and positive effect on monoester and diester concentrations, respectively. This was attributed to better physical protection of monoesters (especially inositol phosphates) from mineralization, and increased diesters from microbial activity and biological turnover. The anomalous behaviour of volcanic-ash soils was attributed to slow mineralization and the sampling of soils at different times of year without full knowledge of the large and highly dynamic microbial biomass P pool which is predominantly diesters.     

Correlations Between Cloud Point and Compositional Properties of Palm Oil and Liquid Fractions from Dry Fractionation

Dry fractionation of palm oil can be conducted as a multi-step process; this gives rise to new softer and harder fractions having multitudes of applications in fat foods. This work focuses on the liquid fractions obtained following a triple-step fractionation; the cloud point was measured on a set of 125 palm oil, olein, super olein and top olein samples, with the intention to correlate this value to the compositional properties. The Mettler cloud point, the diacylglycerol content, some selected single or groups of triacylglycerols and the iodine value (in total, 11 variables per sample) were measured and entered in a statistical model. A principal component analysis (PCA) was first carried out from which different sub-groups were highlighted. In each sub-group, various regressions (simple linear, simple non-linear and multiple linear) were applied and 28 significant equations were derived. Out of these, one multiple linear regression involving the iodine value, the UUU and PPP contents showed the best correlation with the Mettler cloud point. This equation was selected to predict the cloud point since it was further successfully validated by using a set of 25 other independent samples. A partial least square (PLS) regression was tested and also considered adequate to predict the cloud point.     

Relative agronomic merit of fused calcium phosphate

The dissolution of fused calcium phosphate (FP) and transformation of P were monitored in a silt loam, sandy loam, sandy clay and clay soil (pH in water = 5.3 – 5.8) incubated with 0, 108 or 324 mg P, as FP kg^–1 soil dry wt, for 10, 30 or 60 d. Soluble P was fractionated by sequential extraction using a chloride anion exchanger (P-resin) and solutions of NaHCO₃ (P_tCO₃), NaOH (P_tOH) and HCl (P-HCl). Inorganic P (P_iCO₃, P_iOH) and organic P (P_oCO₃, P_oOH) were also determined.The FP treatments significantly (P = 5%) increased both P-resin and P-HCl levels. Over time, the differences between P-resin or P-HCl levels and the controls declined, except in the silt loam soil in which P-resin levels nearly doubled. P_tCO₃ and P_tOH levels did not markedly respond to FP applications. As incubation proceeded P_tOH levels significantly (P = 5%) increased both as P_iOH and P_oOH. This P_tOH buildup was accompanied by small declines in P_tCO₃ levels, with the declines being greater in the P_oCO₃ than in P_iCO₃ fraction. P-resin, P_iOH and P_oOH levels were negatively correlated with those of p-HCl (P = 5%, n = 36). P-resin levels were also negatively correlated with those of P_oCO₃. Of the small and large FP doses, 12–45% and 51–56%, respectively, had yet to react by the end of the experiment.     

Phosphorus availability from partial acidulation of two phosphate rocks

Two phosphate rocks, one from Pesca (Colombia) and the other from Togo, were acidulated to various degrees with H₂SO₄ and H₃PO₄ for evaluation in varying granule size ranges. Products acidulated with H₂SO₄ were also prepared using different drying temperatures. Phosphorus availability was measured by dry-matter yield and P uptake in greenhouse experiments with maize.It was observed that partial acidulation with H₂SO₄ was effective in increasing the water-soluble P level of phosphate rock when the drying temperature of the product was not excessive. Crop response and P uptake were both highly correlated to the water solubility of the product. The relative agronomic effectiveness (RAE) of Togo rock increased from 3% when unacidulated to 33%, 47%, and 52% when 20%, 30%, and 40%, respectively, of the H₂SO₄ required to make SSP was added. Similar results were obtained with Pesca rock. No consistent effect due to granule size was observed.Twenty percent acidulation of Pesca rock with H₃PO₄ was 53–76% as effective as TSP with a single crop and 79–90% as effective over three cropping periods, showing a potential for high residual value.     

Phosphorus sorption in relation to soil properties in some cultivated Swedish soils

Phosphorus (P) sorption properties are poorly documented for Swedish soils. In this study, P sorption capacity and its relation to soil properties were determined and evaluated in 10 representative Swedish topsoils depleted in available P. P sorption indices were estimated from sorption isotherms using Langmuir and Freundlich equations (X_m and a_F, respectively) and P buffering capacity (PBC). X_m ranged from 6.0 to 12.2 mmol kg^–1. All indices obtained from sorption isotherms were significantly correlated with each other (r=0.96^*** to r=0.99^***). Two single-point sorption indices (PSI₁ and PSI₂) were also determined, with additions of 19.4 and 50 mmol P kg^–1 soil, respectively. Both PSI indices were well correlated with X_m (r0.98^***), with PSI₁ giving the highest correlation. As isotherms for determining P sorption capacities involve laborious analytical operations, PSI₁ would be preferable for routine analyses. X_m was significantly correlated with Fe extracted by sodium pyrophosphate and ammonium oxalate, to Al extracted by ammonium oxalate and dithionite-citrate-bicarbonate and to organic c. X_m was also significantly correlated with the sum of Fe and Al extracted by ammonium oxalate. The best prediction of X_m through multiple regression was obtained when Fe extracted in ammonium oxalate and Al extracted in dithionite-citrate-bicarbonate were used. Based on the results obtained, both PSI₁ and oxalate-extractable Fe plus Al can be used for predicting P sorption capacity in Swedish soils.     

Sorption of Pyrene to Two Estonian Soils

Two typical Estonian soils, Brown pseudopodzolic and Rendzina, were used for sorption studies with pyrene. The soil samples were subjected to a variety of chemical and physical tests. The specific surface area reached 3.05 m² g^?1 for Rendzina, and 0.87 m² g^?1 for Brown pseudopodzolic soil. The Rendzina type of soil was characterized also by a high organic matter content (22.9%) compared with the Brown pseudopodzolic soil (6.64%). A batch technique was used to perform all adsorption studies. K_oc values 2.7 × 10⁴ mg g^?1 for Rendzina and 4.5 × 10⁴ mg g^?1 for Brown pseudopodzolic were obtained. The results suggets that pyrene sorption behaviour could be in some instances mainly influenced by the composition of organic compounds in the soil, not so much by the overall organic matter content. Also the mineral part of the soil can have a impact to pyrene sorption through Ca-binded humic substances.     

Yield response of lentil to directly applied and residual phosphorus in a Mediterranean environment

The response of lentil grown under rainfed conditions to directly applied and residual phosphorus (P) was described by a modified Mitscherlich equation, accounting for the effects of rainfall on (1) potential yield, and (2) the availability of soil-P to the crop. The response of lentil yield to directly applied and residual P was studied in two-course cereal–lentil rotational trials under rainfed conditions in a Mediterranean-type environment. Cereal crops were grown at different P application rates during 4 growing seasons at 3 sites, representing different rainfall zones in northwest Syria. Lentil (Lens culinaris Med.) was grown during 4 seasons at the same sites, each lentil crop following a cereal crop. In 3 out of 4 lentil-growing seasons, additional P was applied to lentil in subplots to compare the residual and direct effects of P application. The initial contents of extractable soil-P (P-Olsen) were low at all sites, in the range of 2–5 ppm P. Under the conditions of the experiments, lentil appeared to benefit slightly more from P applied to the preceding wheat crop (residual P) than from directly applied P. It is shown that the modified Mitscherlich equation could be used as a basis for P fertilizer recommendations for rainfed farming. As for lentil, it was concluded that a single application of P to the wheat crop in a wheat/lentil rotation could reduce the cost of lentil production, without reducing lentil yield.     

Effect of phosphate rock and triple superphosphate on soil phosphorus fractions and their plant-availability and downward movement in two volcanic ash soils under <Emphasis Type="Italic">Pinus radiata</Emphasis> plantations in New Zealand

Changes in phosphorus (P) fractions and their plant-availability and downward movement in two strongly P fixing acidic Andosols (Allophanic and Pumice Soils) under Pinus radiata plantations in New Zealand were studied 2 years after triple superphosphate (TSP) and a phosphate rock (BGPR, origin Ben Guerir, Morocco) application, each at four rates, to determine the fate and plant availability of fertilizer-derived P in these soils. The rate of increase of the concentrations of the P fractions was highest for NaOH-P_i (inorganic P associated with Fe and Al oxides and allophane) when TSP was applied and highest for H₂SO₄-P_i (predominantly calcium phosphates or apatite-type P minerals) when BGPR was applied. The largest pool of soil P, the NaOH-P_o (labile organic P), was unaffected by the P fertilizer applications. The rate of NaOH-P_i concentration increase was higher in the higher P fixing Allophanic Soil than in the Pumice Soil. Both types of fertilizers increased resin-P_i (Inorganic P freely available to the plant) and Bray-2 P concentrations but only the TSP application increased Olsen P concentration. Phosphorus derived from TSP and BGPR applications moved down to 10–20 cm soil depth within 2 years of application in the Pumice Soil, but did not move below 10 cm depth in the higher P fixing and less porous Allophanic Soil. The fertilizers significantly increased needle P concentrations 2, 3 and 4 years after fertilizer application, but did not have any significant effect on tree growth.     

Phosphorus requirement of a maize-cowpea sequential cropping on a paleudult

A field experiment was conducted for 6 growing seasons on a Paleudult to assess the influence of P rate and frequency on crop yield and P fractions in a maize-cowpea sequential cropping system. Although cropping depleted soil organic P by up to 42.1% over the cropping seasons perhaps through mineralisation, the major portion of added P and mineralised P were not utilised by the crops. Transformations were probably more to the insoluble Fe-P than Al.P. Cumulative recovery of P in maize and cowpea was never above 75% at a single application of 30kg P ha^–1 over a period of 6 seasons while cumulative recovery of annually applied P ranged from 17.1 to 31.8% over the same period. Lower values were recorded for seasonally applied P plots. Annual application of P at 30kg ha^–1 kept the available P above the critical level and maintained maize and cowpea yields at near maximum over the cropping periods.     

Phosphorus supplying power of some thermally promoted reaction products of phosphate rocks

The P availability in soil and agronomic efficiency of the products of non-premium grade, unreactive Purulia phosphate rock (PPR) heated alone or with Na₂CO₃ or KCl at different temperatures were investigated in two P deficient soils. The heated products of PPR alone did not improve the P availability in soil or P utilisation by rice over the original PPR. The products of PPR-KCl mixtures heated at 300-900°C were not effective at all. Out of several products of PPR with Na₂CO₃, the product prepared from PPR and Na₂CO₃ mixture in the weight ratio 2:1 heated at 900°C was comparable to superphosphate (SP) with respect to P availability in soil, straw and grain yield and P uptake by rice. The effectiveness of the products of PPR-Na₂CO₃ mixtures heated at 700°C though inferior to SP were superior to that of the original PPR in the highly acidic P deficient soil from Choudwar. However, products of another phosphate rock (PR) from Jordan and NA₂CO₃ mixtures heated at 900°C were less effective in comparison to SP. The amount of inherent silica present in Jordan PR was inadequate to promote the apatite-NA₂CO₃-SiO₂ reaction towards completion thus leading to an inferior product. On the other hand, similar products of non-premium grade Kasipatnam and Mussoorie PRs which are not suitable for direct application were comparable to SP in their effectiveness when these PRs were fused with Na₂CO₃ in the weight ratio 2:1 at 900°C. X-ray diffraction studies indicated presence of water and citrate soluble phosphate phases viz., Na₃PO₄, NaCaPO₄ and possibly Ca₇ (PO₄)₂ (S10₄)₂ in these products of PR-Na₂CO₃ mixture heated at 900°C. The water and citrate soluble phases of these products could release adequate P for absorption by crop.     

Fate of phosphorus applied in slurry and mineral fertilizer: accumulation in soil and release into surface runoff water

Phosphorus (P) accumulation on the soil surface and its effect on the concentration of dissolved orthophosphate P (PO₄-P) in surface runoff water were studied after three years of surface application of slurry and mineral fertilizer to grass ley on a sandy soil, poor in P. The total amount of P applied was 107–143 kg ha^–1>, of which 72–119 kg ha^–1> was applied on the soil surface during two or three years without incorporation or mixing. The addition of slurry and mineral fertilizer resulted in an increase in inorganic P in the 0–5 cm but not the 5–25 cm soil layer, but organic P was not affected. The measured changes in inorganic P deviated only by 4–6 kg ha^–1> from the values derived from inputs and outputs of P (crop uptake + losses in surface runoff and drainage water). The increase in inorganic-P was accompanied by increases in the degree of P saturation (DPS) and in P extracted with acid am monium acetate (P_Ac ), sodium bicarbonate (P_Olsen) and anion-exchange resin (P_Resin). In surface runoff, 10–18 months after the last surface application of P, the mean flow-weighted concentration of PO₄-P was linearly increased with the values of DPS, P_Ac, P_Olson and P_Resin in the 0–5 cm soil layer. PO₄-P was lowest (0.033 mg l^–1> ) in the control plots and highest (0.62 mg l^–1>) in the plot where 143 kg ha^–1> P had been applied in slurry and fertilizer. On that plot, the corresponding values of DPS, P_Ac, P_Olson and P_Resin were 16%, 13 mg kg^–1>, 85 mg kg^–1> and 71 mg kg^–1 , even within a few years, and multiply the P loading to surface runoff from the site. A very shallow soil sampling (< 5 cm) is needed to assess P loading potential in a soil where P has been surface-applied.     

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.     

Association of soil-test phosphorus with phosphorus fractions and adsorption characteristics

There is incomplete understanding, based on a single comprehensive study, of the relationship between empirical extractants of soil-test P (STP) and fundamental measurements of soil-P status such as inorganic (Pi) and organic (Po) fractions, P adsorption and relevant ancillary soil attributes. Consequently, we assessed these relationships for the extractants Morgan, Mehlich-3, Olsen, Bray-1, lactate–acetate, CaCl₂ (1:2 and 1:10 soil:solution) and resin. Multiple regression analysis indicated that STP extracted with Morgan and CaCl₂ related dominantly with the most labile resin Pi fraction, Mehlich-3 and Olsen with labile NaHCO₃ Pi, Bray-1 with moderately labile NaOH Pi and lactate-acetate with relatively stable Ca-bound HCl Pi, for example. Especially for Morgan and CaCl₂ (1:2), and except for Bray-1, the best relationship of STP with adsorption characteristics was with equilibrium P concentration in solution (EPC). Buffering capacity (EBC) and binding energy (k) did not have significant effects, as indicated by regression, whereas the effects of clay and oxalate-extractable Fe (Fe_ox) were generally negative and Al_ox and organic carbon (OC) positive. Principal component analysis (PCA) highlighted many similarities in the extractants. However, regression of STP against soil-P pools, integrated as principal component scores, inadequately revealed relationships, which were better facilitated by PCA ordinations. For ordinations of STP with P fractions, eigenvalues for the first two axes explained 88.6% of the variance. Closest associations were residual Pi with HCl Pi, CaCl₂ (1:10) and to a lesser extent Morgan with OC and clay, and Mehlich-3 and Olsen with NaHCO₃ Pi and resin Pi. For ordinations of STP with P adsorption, eigenvalues for the first two axes explained 97.8% of the variance. The STP extractants grouped in specific, but distinctly different, combinations. For example, strongest inverse relationships were EBC to EPC and Morgan, and k to resin and Olsen, and to OC and clay, indicative of weak P bonding on these surfaces. These distinctions are consistent with, and provide a rationale for, the relevance of Morgan as an environmental P test vis-a-vis other extractants of STP.     

Availability and residual effects of phosphate rocks and inorganic P fractionation in a red soil of Central China

The agronomic effectiveness, phosphorus utilization efficiency and inorganic fractions of phosphorus, applied once to an Ultisol in Xianning, Hubei province, and then cropped 9 seasons, were measured. The results showed: (1)forall crops, the relative agronomic effectiveness (RAE) of three phosphate rocks (PRs), compared with triple superphosphate (TSP), reached 103.1–154.9% at three rates of P applied; (2) phosphorus utilization ratios during 5 years were 33.7–59.3% with the average of 35.0–52.7% for three PRs, greater than TSP ; (3)the total amountof inorganic phosphorus in soils which received phosphate fertilizers were 395.5–492.8 mg/kg, however, it was 387.4 mg/kg in soil given no phosphorus fertilizer. The most remarkable increases were occluded P(O-P) and calcium P(Ca-P) fractions at P 35 level (i.e. soil extractable P reached 35 mg/kg). (4) after 9 crops were harvested, the balance of phosphorus in the soils of control (CK) and all P 15 level treatments was negative, however, for soils treated with P 25 level and P 35 level, the balance was positive. This revised version was published online in June 2006 with corrections to the Cover Date.     

The relationship between Bray's P1, modified NaHCO3, New Mehlich and NH4F. HF extractants for P in savannah soils of western Nigeria

Sixty surface (0–15 cm) soil samples and 20 field sites with different cropping histories were selected from the major soil types of the western Nigeria basement complex savannah. Available P was evaluated with six different extractants, namely Bray's P₁, 0.1N AHDF (pH 4.1–4.3), 0.1N AHDF (pH 5.0), 0.05N AHDF (pH 7.0), New Mehlich and modified Olsen's 0.5M NaHCO₃ (pH 8.5).The highest amount of P was extracted by 0.1N AHDF (pH 4.1–4.3), while 0.05N AHDF (pH 7.0) extracted the lowest. In the greenhouse, the New Mehlich extractant had the best correlation with P uptake (r = 0.95,p < 0.001), which was not different from Bray's P₁, 0.1N (pH 4.1–4.3), 0.1N AHDF (pH 5.0) and modified 0.5M NaHCO₃ extractants. Field experiments in the various locations showed a significant correlation between relative maize yield and soil tests. The 0.5M NaHCO₃ had the highest correlation (r = 0.70,p < 0.01) while 0.1N AHDF (pH 4.1–4.3) had the lowest (r = 0.53,p < 0.05).All extractants seem to extract the same forms of P. Although Fe-P contributes the highest amount of active P extracted with the Chang and Jackson procedure, its utilization by the crop seems to be relatively lower than the other P forms. The 0.1N AHDF (pH 4.1–4.3) extracted 38% of this active P form while Bray's P₁, New Mehlich, 0.1N AHDF (pH 5.0), modified 0.5M NaHCO₃ and 0.05N AHDF (pH 7.0) extracted 29, 27, 22, 21 and 7% respectively. It is therefore concluded that any of the following four extractants, namely New Mehlich, 0.5M NaHCO₃, 0.1N AHDF (pH 5.0) and Bray's P₁, could be adopted for routine soil testing for P in the savannah zone of western Nigeria.