An attempt to evaluate phosphorus fertilizer requirements of western Nigeria savannah soils

A greenhouse fertilizer trial was carried out on 60 surface soils of the western Nigeria savannah derived from basement complex rocks. Bray's P₁ available P in the soils varied between 1 and 112µg ml^–1. There was maize response to P addition and a critical P level of 12.7µg ml^–1 was calculated for the soils.For 22 of the soils, a laboratory incubation technique was used in evaluating changes in Bray's P₁ extractable P at various rates with time. The initial rapid decline in soil available P was completed between 28 and 84 days of incubation. A fertilizer factor, calculated from extracted P in treated and untreated soils varied between 1.5 and 16.7µg ml^–1 and was significantly correlated with soil pH and citrate-dithioniteextractable oxides of Fe and Al.Fertilizer rates based on critical soil P, available soil P and fertilizer factor, correlated significantly with greenhouse estimates for optimum yield obtained with the linear response plateau model (r = 0.91,p < 0.001). At ten field locations varying in available P content, response was only to P applications lower than 60 kg ha^–1 and the calculated P rates using a mean fertilizer factor of 3.0µg ml^–1 corresponded to P rates at which maximum yields were obtained in the sites.     

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.     

Evaluation of soil extractants for Mn availability and response of wheat to applied Mn in soils of semi-arid zone of Punjab, India

Seven chemical extractants were tested for their relative performance to predict the response of wheat to Mn application in coarse textured alkaline soils of semi-arid region. Five out of the seven extractants were found to be promising for the estimation of critical level of available Mn in these soils, as the amount of Mn extracted by these extractants was positively and significantly correlated with relative grain yield as well as Mn uptake. The critical deficiency level of soil available Mn with 0.005 M DTPA, 0.02% hydroquinone, 0.02 N sodium pyrophosphate, 0.1N H3PO4 and 0.05N HCl+0.025N H2SO4 was 3.1, 13.8, 23.5, 5.3 and 17.8 mg kg-1 soil, respectively. The 1N ammonium acetate and 0.01M CaCl2 were found to be unsuitable extractants for these soils. Further field trials at eight locations with varying levels of Mn deficiency showed successive increase in the grain yield of wheat with foliar Mn application, emphasizing the need for Mn fertilization when wheat is grown on Mn deficient soils.     

Adsorbed phosphorus partitioning in some benchmark soils from Northeast Brazil

The partitioning of adsorbed P between labile and non-labile pools by soils is fundamental to the residual effect of fertilizer-P added to soils. The main objective of the study was to determine the partitioning of adsorbed P between the labile and non-labile phases by some benchmark soils of northeast Brazil for which is little is known. Surface and subsurface samples of several soils: Non-Calcic Brown soil and Planosol (Haplustalfs), Cambisol (Ustropept), Lithosols (Orthents) and Alluvial soil (Tropaquept) were equilibrated with varying concentrations of KH₂PO₄. The readily exchangeable portion of the adsorbed P was determined by anion exchange resin (AER). Considerable hysteresis was observed between adsorbed P and AER-P. To quantify the extent of the hysteresis, a critical P concentration (P_crit), the amount of P adsorbed at zero desorption by AER, was defined. The P_crit of the soils averaged across the soil depths followed the order: Non-Calcic Brown soil > Planosol > Alluvial > Cambisol > Lithosol. The P_crit correlated with clay and oxalate Fe (Fe_o). The P affinity index (K) estimated by Langmuir adsorption model accounted for 66% of the variance in P_crit. A sequential extraction with 0.5M NaHCO₃, 0.1M NaOH and 11.5M HC1 to remove the labile, moderately labile and non-labile P forms respectively, indicated that between 63 and 99% of adsorbed P was in the labile pool (AER-P + HCO₃-P + OH-P), suggesting that the soils might have high potential for residual fertilizer-P responses.     

Critical levels for soil pH,available P,K, Zn and Mn and maize ear-leaf content of P,Cu and Mn in sedimentary soils of South-Western Nigeria

In the sedimentary soils of South-western Nigeria, actual and expected relative yields of maize were plotted against soil physical factors, soil avalilable nutrients and ear-leaf content of maize. These were used to set critical ranges of these factors for optimum production. Regression equations were obtained for each of the soil and plant factors for predicting yield, thereby making possible yield prediction with levels of each of these factors in these soils if all other factors are constant.The critical range concept combined with the soil physical and chemical properties and plant nutrient content could be a useful diagnostic tool for soil ammendment in crop production. Critical ranges were set as follows:pH, 6–6.5; available P (Bray's Pl), 10–16 mg Kg^–1; Exchangeable K, 0.6–0.8 me K100g^–1; available Zn, 5–10mg kg^–1; available Mn, about 25 mg Kg^–1; Ear-leaf P, 2.5–3.0%; Ear-leaf Cu, 10–20 mg Kg^–1; Earleaf Mn, about 50 mg Kg^–1.     

Chemical assessment of the zinc status of some soils of the semi-arid region of India

To find critical tissue levels of Zn for wheat, and to evaluate various chemical extractants, a screen-house experiment was conducted on 21 diverse soils representing semi-arid regions in Haryana State, India. The extractants differed in the amounts of Zn extracted and the order was: 0.1 N HC1 > EDTA-NH₄OAc > EDTA-(NH₄)₂CO₃ > DTPA + CaCl₂. The amounts (mg kg^–1) of extractable Zn associated with a yield reduction of 20% were: DTPA + CaCl₂, 0.60; EDTA — (NH₄)₂CO₃, 0.80; EDTA-NH₄OAc, 0.92 and 0.1 N HCl, 1.20. The corresponding critical Zn concentration in ten weeks old plants was found to be 17 mg gm^–1. The DTPA + CaCl₂ method gave the best correlation (r = 0.85) between extracted Zn and Bray's per cent yield. It is recommended for assessing Zn status of soils of semi-arid region.     

Soil test calibration for upland rice in south western Nigeria

The relationship between soil test values and yield response due to fertilization is usually described by response models and would indicate test value where response is expected. Upland rice grain yield was used to define relative yield in terms of yield from control (no fertilizer; or minus nutrient) treatment expressed as percent of complete treatment.Regression analyses show that soil properties were related to both relative yield definitions. Critical nutrient levels and calibration obtained with Cate and Nelson's technique were indicative for soil nutrients. Although relative yield based on fertilizer and control (no fertilizer) treatments has the advantage that many nutrients can be calibrated at the same time, lower R2 values observed in this study with upland rice mean that it cannot be recommended as replacement to conventional relative yield.     

A decade of fertilizer research on groundnuts (Arachis hypogaea L) in the savannah zone of Nigeria

Field experiments on fertilizer requirements of groundnuts carried out between 1970 and 1980 were reviewed and discussed. Results of the various studies seem to suggest that existing recommendations which are based on short term annual trials carried under non-intensive or semi-intensive traditional farming systems are becoming inadequate under a system of continuous intensive cultivation. Besides phosphorus and sulphur which have long been known to be deficient in the Nigerian savannah soils, the availability of the other nutrients in these soils can no longer be taken for granted. Potash and micronutrient deficiencies, notably those of B and Mo, hitherto rare, are now showing up with increasing frequency.As a P source, single superphosphate appears to be the most suitable fertilizer for groundnuts in the Nigerian savannah at present partly because it also contains S and partly because of its solubility. Nitrogen fertilizer application tends to depress pod and kernel yield although it enhances haulm production. Groundnut haulm constitutes an important livestock feed in the West African savannah and the role of fertilizer N may have to be viewed in this context. Calcium as a nutrient does not appear to cause any serious fertility problem. Where gypsum application has produced large increases in kernel and haulm yields the dominant effect has been attributed to the sulphate component. Magnesium response has so far not been reported on groundnuts in Nigeria.It is suggested that soil fertility research in the savannah zone of West Africa should move away from short-term annual trials to long-term studies designed to tackle problems that are likely to arise under continuous cultivation in these poorly buffered sandy soils.     

Residual phosphate fertilizer compounds in soils

A variety of P compounds can accumulate in soils as residues of fertilizer and may influence soil test versus plant yield relationships. This work evaluates specific chemical extractants for their capacity to identify such Al, Fe and Ca phosphates in soils as a basis for increasing the precision of yield prediction. Aluminium phosphate, iron phosphate, calcium phosphate (apatite) and P sorbed onto gibbsite, goethite and calcite were added to four Western Australian lateritic soils. These soils were then subjected to sequential selective extraction using a modified Chang and Jackson procedure in order to evaluate the selectivity of these extractants for the different forms of P with the sequence of extraction: 1 M NH₄Cl, 0.5 M NH₄F, 0.1 M NaOH + 1 M NaCl, citrate-dithionite-bicarbonate (CDB), 1 M NaOH and 1 M HCl. The results show that the procedure is not sufficiently specific and thus might be of little value for estimating the forms and amounts of residues of phosphate rock fertilizers in soils.     

The Olsen P method as an agronomic and environmental test for predicting phosphate release from acid soils

The role of soil phosphorus (P) in the eutrophication of fresh water systems is well established. It is crucial therefore to assess the potential loss of P from soil in the various scenarios where soil can come into contact with water. To date, such assessment has often been based on soil P tests that are used for agronomic purposes (e.g. fertilizer recommendations). The purpose of this work was to examine the usefulness of one such test (viz. the Olsen test, which is based on extraction with bicarbonate) for predicting not only the amount of soil P available to plants, but also that which can be desorbed to water in a group of 32 Portuguese soils, of which 29 were acid and 3 calcareous. To this end, we (i) assessed the total amount of phytoavailable P in soil by successively pot-cropping Chinese cabbage, buckwheat and rye; and (ii) measured the amount of phosphate-P desorbed to a dilute electrolyte mimicking fresh water over periods of up to 218 days at soil:solution ratios of 1:100, 1:1000 and 1:10000. Total phytoavailable P and Olsen P were found to bear a quadratic relationship, with Olsen’s extractant underestimating the content in phytoavailable P of soils with high Olsen P contents relatively to soils with low contents. The “change point” at which phytoavailable P began to increase rapidly per unit change in Olsen P was 53 mg Olsen P kg⁻¹ soil. For the acid soils, a significant quadratic relationship was found between the amount of P desorbed to water and Olsen P at the three soil:solution ratios studied. However, these relationships became less significant when only the soils with an Olsen P value of less than 50 mg kg⁻¹ were considered. For the acid soils, the change point at which P input to water began to increase rapidly per unit change in Olsen P was 20, 61 and 57 mg kg⁻¹ at the 1:100, 1:1000 and 1:10000 ratio, respectively. At comparable Olsen P values, the calcareous soils released more phosphate to water than the acid soils. On the basis of our results, we suggest the following environmental threshold values for Olsen P in acid soils: 20 mg kg⁻¹ for P desorption scenarios where the soil:solution ratio is high (e.g. drainage water) and 50 mg kg⁻¹ for desorption scenarios where the soil:solution ratio is low (e.g., runoff, water in reservoirs). Both values are higher than the agronomic threshold above which plants are well supplied with P.     

直接提取过磷酸钙有效磷方法的研究

研究采用乙二胺四乙酸二钠 (EDTA)溶液直接提取过磷酸钙中有效磷的条件 ,实验结果是 :用 10 0 mlEDTA溶液 (37.5 g/ L)提取 ,在 6 0℃± 1℃恒温下振荡 6 0 m in的测定值 ,与标准方法测定值的绝对差 <0 .15 %。该方法精度 RSD<0 .10 % ,具有快速 ,简便 ,实用的特点。     

Soil testing for available phosphorus in soils where phosphate rock-based fertilizers are used

Traditional soil tests for phosphorus (P) were developed to arrive at fertilizer recommendations when water-soluble P fertilizers have been used. When slowly water-soluble fertilizers such as phosphate rock (PR) have been used, soil tests using acidic extractants overestimate bioavailability of P, whereas alkaline extractants underestimate it. Therefore, separate calibration curves are needed for soluble and PR-based fertilizers. There are two soil tests that show promise as suitable tests in soils fertilized with soluble as well as PR-based fertilizers. These are the iron oxide impregnated paper (P _i ) test and the ion exchange resin paper test. In both cases, the strips act as a sink for P mobilized in a soil solution, and P measured depends only on the concentration of P mobilized in the solution and not on the source of P or properties of the soil. Both tests somewhat simulate the sorption of P by plant roots without disturbing the chemical equilibrium, unlike other tests that extract P by the destructive dissolution of specific soil P compounds. In both cases, P measured from soils fertilized with PR-based fertilizers has shown very good correlation with plant response. Field calibration with crops under different pedological and agroecological regimes is needed for using these soil tests in developing fertilizer recommendations.     

Does the Pi strip method allow assessment of the available soil P?: Comparison against the reference isotope method

Experiments have been carried out in acid soils developed under tropical climates with and without phosphate rock (PR) addition to assess the ability of the Pi strip method to extract available soil P compared with the Isotopic Exchange Kinetics method (IEK). In the Pi strip method strips of filter paper, previously impregnated with iron hydroxide acting as a sink for phosphate ions from soil components, are added to an aqueous soil suspension. The extracted phosphate ions are eluted in diluted sulfuric acid and quantified by a colorimetric method. Available soil P, defined as the amount of phosphate ions that can move from the soil particles to soil solution, is described by three factors: an intensity factor, a quantity factor and a capacity factor. These three factors were determined by the IEK-method. Following the addition of carrier-free ³²PO₄-ions to soil, the ability of the Pi strip to extract available soil P was assessed: (i) by comparing the quantity of instantaneously exchangeable P (E₁) to the quantity extracted with the Pi strip; (ii) by determining the fraction of ³²P extracted with the Pi strip, and (iii) by comparing the specific activity (SA) of P present as phosphate ions extracted by the Pi strip to the specific activity of P in the soil solution. It was observed that (i) E₁ and the amount extracted with the Pi strip are highly correlated, (ii) the recovery of ³²P extracted by the Pi strip varies between 17 and 66%, and (iii) the specific activity of P extracted by the Pi strip is of the same order of magnitude as that of P in the soil solution. In acid soils low in available P, part of the P in aqueous KCl-extracts is presumably not only present as free phosphate ions but also occluded in the form of a soluble complex, whose isotopic exchangeability is significantly lower than that of phosphate ions transferred to the Pi strip. It is concluded from the results that the Pi strip method can be recommended in routine analysis for the determination of the quantity factor. However, this method cannot provide intensity or capacity factors and therefore needs to be complemented by the IEK-method for full characterization of the available soil P status.     

Some factors associated with the use of the extractants 2% citric acid and 2% formic acid as estimators of available phosphorus in fertiliser products

The influences of calcium chloride, calcium sulphate, monocalcium phosphate, monosodium phosphate, calcium carbonate, sodium carbonate and iron and aluminium sulphates on the solubilities of two phosphate rocks (both ground and unground) in 2% citric and 2% formic acids have been examined. All of these potential impurity elements can affect the solubility of the phosphate rock through a variety of mechanisms, including common ion effects, complexation, acid-base reactions and reduction. 2% formic acid solubilities were slightly more influenced by these effects than 2% citric acid figures, militating against the use of this extractant for assessing mixed component products at present being manufactured in New Zealand.Solid to solvent and P to solvent ratios were also found to affect the solubilities of raw phosphate rocks and a correction for the differences in total P content of the phosphate rocks to produce a consistent P to solvent ratio in solubility tests is suggested. This study has indicated that alternative methods for assessing phosphate rocks in admixture with other components should be examined in future.     

Influence of soil organic carbon on the interpretation of soil test P for wheat grown on alkaline soils

Organic carbon is known to alter crop response to applied phosphorus (P) but that fact has not been incorporated in soil test interpretations. To achieve this objective, field experiments with wheat were conducted for four years on alkaline soils of Punjab, India. The experimental soils ranged from loamy sand to loam in texture, 7.4 to 9.6 in pH, 0.16 to 0.75% in organic carbon (OC) and 2 to 40 mg Olsen extractable P kg^–1 soil. Response of wheat to fertilizer phosphorus application was related to the combined effect of Olsen P and soil OC content. At a given Olsen P level, wheat yield was a function of soil OC content. Multiple regression analysis of the data showed that OC content <0.2% did not affect yield significantly. At values >0.6%, OC along with Olsen P accounted for 97% of the variation in yield and there was no response to applied fertilizer P. Yield isoquants for 4 and 5 tons grains ha^–1 showed that for a given Olsen P level, as OC content increased the amount of fertilizer P required to achieve a yield target decreased. It was shown that OC may be used to approximate the contribution of organic P mineralization to plant available soil P during a growing season. The reliability of fertilizer recommendations based on Olsen P may be improved on some alkaline soils by consideration of soil OC content.     

Phosphorus availability to soyabean from an indigenous phosphate rock sample in soils from southwest Nigeria

Laboratory and grecnhouse experiments were conducted on 12 soil samplesin Ogun State of Nigeria to evaluate the comparative transformation of Ogunphosphate rock (OPR), single Superphosphate (SSP) and OPR: SSP mixture withinsoils, and to determine the response of soyabean to the direct application ofthese sources. The OPR sample was found to contain 0.3% P soluble in water(2.7%of total P) and 3.4% P soluble in 2% formic acid (28.5% of total). Changes inthe values of the P fractions in soils were affected significantly by soiltype,P source and rate of application. The percent increase in Olsen P from OPRafter12 weeks of incubation averaged 44.5, 39.6 and 36.7% in Entisols, Alfisols andUltisols, respectively. All the soils responded to OPR, OPR: SSP and SSPapplication. Responses to OPR were found to be related inversely to soil pH (r=0.82). At the application rate of 60kg Pha^–1, the relative agronomic effectiveness of OPRand OPR: SSP varied from 11.6 to 92.3% (mean 61.3%) and 11.6 to 101.1% (mean51.4%), respectively. It was concluded that direct application of OPR as Psource for soyabean in soils from southwest Nigeria is feasible.     

采用微波辅助提取技术快速检测过磷酸钙中有效磷

用微波辅助EDTA提取过磷酸钙中的有效磷,并在显色过程中用微波辅助加热,用比色法分析过磷酸钙中有效磷。实验研究用EDTA微波提取4.5min,加4mL硝酸控制溶液pH0.38,将待测溶液加热80s,显色,冷却至20℃,比色,所测浓度与标准值相当,全程检测仅需25min左右。     

Dissolution of granulated CuZn-superphosphate in soils

Granules and pressed pellets of ordinary superphosphate and CuZn superphosphate were drilled and surface applied to two moist soils in the field. Residues were collected at intervals up to 1 year and chemically analyzed. The water-soluble P, Cu and Zn fractions dissolved within 7 days. Dissolution of S, Ca, remaining Cu and citratesoluble P was slow with approximately half the initial contents remaining in pellets and large granules after 1 year.All constituents dissolved more rapidly from small granules and in the heavier-textured soil. Crystalline dicalcium phosphate was not detected in residues despite its reported formation in analogous laboratory studies.     

Influence of rates of reactive phosphate rock and sulphur on potentially available phosphorous in organically managed soils in the south-eastern near-Mediterranean cropping region of Australia

Reactive phosphate rock (RPR) is the only phosphorous (P) fertiliser allowed for organically managed, broad-acre crop-pasture systems in southern Australia. However, soils are usually deficient in P, and the soils, climate, and plant species grown, do not promote extensive dissolution of RPR so the fertiliser is poorly effective for crop and pasture production. Biological oxidation of elemental sulphur (S) mixed and applied with RPR may sufficiently increase dissolution of P from RPR to improve its effectiveness as a P fertiliser. However, this needs to be confirmed in field studies in the region. Rates of RPR and S required to optimise dissolution of RPR are not known for the soils, environments, and agricultural systems used. Both pot and field studies showed that mixing RPR and S, and incorporating the mix into soil (top 10 cm for field studies), significantly increased Olsen P and soil solution P, even in strongly acidic soils (pH_Ca < 4.6). In general, Olsen P increased linearly with the applied rate of P up to 42–70 kg P ha⁻¹ and the rate of change in Olsen P per unit of applied P increased with the applied rate of S up to 400 kg S ha⁻¹. This interaction suggested that the effectiveness of RPR + S may be compromised by segregation of RPR and S. In addition, there was evidence that S application may not necessarily create a more acidic soil environment necessary for enhanced dissolution of RPR.     

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.