Phosphate sorption isotherms in relation to P nutrition of wheat (Triticum aestivum)

The phosphate sorption isotherms are needed to explain differential plant responses to P fertilization in soils. Laboratory and greenhouse experiments investigated the use of phosphorus sorption isotherms in relation to P fertilizer requirement of wheat in ten benchmark soils of Punjab, India. The modified Mitscherlich Equation (3) was used to describe plant response observed in different soils. Maximum obtainable yield (MOY) ranged from 11.6 g pot^–1 in Gurdaspur (I) sandy clay loam to 7.0 g pot^–1 in Nabha sandy clay loam. Response to P applied @ 25 mg P kg^–1 soil was maximum (77%) in Bathinda sand and minimum in Chuharpur clay loam (33%). The response curvature varied from 3.74 × 10^–2 in Nabha sandy clay loam to 4.43 × 10^–2 in Kanjli sandy loam. The soil solution P required to produce optimum yield (90% MOY) varied from 1.61 µg ml^–1 in Bathinda sand to 0.10 µg ml^–1 in Sadhugarh clay. Dry matter yield obtained at 0.2 µg ml^–1 solution P concentration ranged from 55% in Bathinda sand to 85% of MOY in Gurdaspur (II) clay loam. At the same solution P concentration (0.1 µg P ml^–1), dry matter yield was 91% in Sadhugarh clay, 80% in Gurdaspur (II) clay loam and, 43% of MOY in Bathinda sand and eventually coincided with the decreasing maximum buffer capacity (MBC) in these soils. At the same level of sorbed P (100 mg P kg^–1 soil) the yield was observed to be inversely proportional to MBC. The study, therefore, concludes that, soils should be grouped according to their P sorption characteristics and MBC before using critical soil solution P as a criterion for obtaining optimum yields.     

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).     

Critical manganese deficiency level for soybean grown in Ustochrepts

A greenhouse study with 15 soils, having a range in DTPA extractable Mn, was conducted to determine the critical deficiency level of Mn in Ustochrepts for predicting response of soybean to Mn application. Soil application of 10 mg Mn kg^–1 soil significantly increased the dry matter yield in deficient soils. Soil Mn was significantly related with Bray's per cent yield (r = 0.72^**) and Mn uptake (r = 0.75^**). Both graphical and statistical models of Cate and Nelson indicated the critical level to be 3.3 mg kg^–1 soil of DTPA extractable Mn. Critical Mn deficiency level in recently matured terminal leaflet blade at V6 growth stage in soybean plant was 22.0µg g^–1 dry matter. The predictability of soil and plant critical Mn level was 87 per cent.     

The interpretation of Olsen extractable P values in relation to recommendations for the fertilizer requirements of crops

A greenhouse experiment, with Okra (Abelmoschus esculentus L.) as the test crop, was conducted on twenty-one soils ranging in Olsen's extractable phosphorus from 1.8 to 15.5µg Pg^–1 soil. The experiment was conducted at Punjab Agricultural University, Ludhiana, India. The soils were nonsaline with pH ranging from 7.7 to 8.6. A critical level of 2.55µg Pg^–1 soil was predicted by Cate and Nelson's (1971) statistical procedure. Because of a wide range in relative yields, this value did not accurately predict response to applied P. An approach to compute minimum response to applied fertilizer, which is likely to be obtained at a particular Olsen P level, has been presented. It involves calculation of lower 60 percent confidence limits for relative yield and fitting log_e-linear regression to the transformed data. The regression was tested on a published data set and was found to hold well.     

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

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

Polyphosphates as sources of phosphorus for plants

Polyphosphates vary in their rates of hydrolysis and degree of sorption by soil constituents, which could affect the efficiency of P recovery from these compounds by plants. In this study, four linear oligophosphates (P₂, P₃, P₁₅, and P₄₅) and one cyclic polyphosphate (trimetaphosphate) were tested in greenhouse experiments for their ability to supply P to plants, compared with that of the conventional P source, orthophosphate (P₁). Annual ryegrass (Lolium multiflorum Lam.) and corn (Zea mays) were used as the indicator plants. Ryegrass was grown on four Iowa soils (Clarion, Webster, Primghar, and Ida), treated at the rates of 0, 20, 40, and 80 mg P kg^–1 soil (1.8 kg of soil pot^–1) and harvested four times at 30-day intervals. The soils of the control pots were recovered, repotted (150 g pot^–1), and seeded with corn after treatment with polyphosphates at the same rates as given. The corn plants were harvested after 35 d. With all P sources, dry matter yield and P yield increased with increasing rates of P application for both ryegrass and corn. Statistical analysis (Duncan's Multiple Range Test) of P yields of ryegrass and corn grown in soils amended with the various P sources indicated that there were no P compounds that consistently showed significantly greater amounts of P recovery in plant tissue. Considering all the P sources and rates of P application, dry matter yield was significantly correlated (exponentially) with P uptake by ryegrass from all four soils. The dry matter yield of corn was significantly correlated (linearly) with rates of P application with all P sources, indicating that the effectiveness of a unit of P taken up by plants in increasing the dry matter yield was similar among the polyphosphates and the conventional P fertilizer, orthophosphate. Measurements of extractable P (Bray-Kurtz I on the acid Clarion, Webster, and Primghar soils and Olsen's test on the alkaline Ida soil) upon completion of each experiment indicated that the residual P effects were similar among the P compounds after the short cropping period with corn (35 days) and the long cropping period with ryegrass (120 days).     

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.     

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.     

Effect of poultry litter and composts on soil nitrogen and phosphorus availability and corn production

Environmental problems associated with raw manure application might bemitigated by chemically or biologically immobilizing and stabilizing solublephosphorus (P) forms. Composting poultry litter has been suggested as a means tostabilize soluble P biologically. The objectives of this study were to assessthe nutrient (N, P) value of different-age poultry litter (PL) compostsrelativeto raw poultry litter and commercial fertilizer and determine effects ofpoultrylitter and composts on corn (Zea mays) grain yield andnutrient uptake. The research was conducted for two years on Maryland'sEastern Shore. Six soil fertility treatments were applied annually to aMatapeake silt loam soil (Typic Hapludult): (1) a check without fertilizer, (2)NH₄NO₃ fertilizer control (168 kg Nha^–1), (3) raw poultry litter (8.9 Mgha^–1), (4) 15-month old poultry litter compost (68.7Mg ha^–1), (5) 4-month old poultry litter compost(59 Mg ha^–1) and (6) 1-month old poultry littercompost (64 Mg ha^–1). We monitored changes inavailable soil NO₃-N and P over the growing season and post harvest.We measured total aboveground biomass at tasseling and harvest and corn yield.We determined corn N and P uptake at tasseling.Patterns of available soil NO₃-N were similar between raw PL-and NH₄NO₃ fertilizer-amended soils. LittleNO₃-N was released from any of the PL composts in the first year ofstudy. The mature 15-month old compost mineralized significant NO₃-Nonly after the second year of application. In contrast, available soil P washighest in plots amended with 15-month old compost, followed by raw PL-amendedplots. Immature composts immobilized soil P in the first year of study. Cornbiomass and yields were 30% higher in fertilizer and raw PL amendedplotscompared to yields in compost-amended treatments. Yields in compost-amendedplots were greater than those in the no-amendment control plots. Corn N and Puptake mirrored patterns of available soil NO₃-N and P. Corn Puptakewas highest in plots amended with 15-month old compost and raw PL, even thoughother composts contained 1.5–2 times more total P than raw PL. There wasalinear relationship between amount of P added and available soil P, regardlessof source. The similar P availabilities from either raw or composted PL,coupledwith limited crop P uptake at high soil P concentrations, suggest that raw andcomposted PL should be applied to soils based on crop P requirements to avoidbuild-up of available soil P.     

Predicting external sulphur requirement of alfalfa from sulphate sorption isotherm of Tanzanian soils

For Tanzanian soils dominant in hydrous oxides of iron and amorphous ferri-alumino silicate, a 48-hour (hr) mixing period with the sulphate (SO₄) solution was adequate for a near-equilibrium condition. Although differing in their SO₄ sorption capacity, all the soils sorbed SO₄ at or beyond 1µg ml^–1 sulphur (S) concentration in the supernatant. Hydroxyl (OH) ions were displaced during SO₄ sorption as indicated by a significant positive correlation between the amount of sorbed SO₄ and the difference in pH values determined in 0.1N K₂ SO₄ and 0.1N KCl, i.e. the dpH values.In a greenhouse experiment, alfalfa was grown on eight soils at six adjusted S concentrations. Sulphur deficiency symptoms appeared in the control pots of those soils which were low in native sorbed SO₄, SO₄ sorption capacity and initial soil solution S concentration. Sulphur fertilization increased dry matter (DM) yield as well as response to applied S. The external S concentration, i.e. adjusted S concentration required for 95% of the maximum DM yield, ranged from 0.8 to 8.2µg S ml^–1 with values less than 2.0 on most of the soils. The external S concentration decreased hyperbolically as the SO₄ sorption capacity of the soils increased. The total amount of fertilizer S required to obtain the external S concentration in solution, and at the same time satisfy the SO₄ sorption capacity of the soil at the external S concentration (determined from the sorption isotherm) was defined as the external S requirement for the specified yield level of alfalfa. The external S requirement for 95% of the maximum yield of alfalfa varied from soil to soil due to differences in their capacity and intensity for S nutrition.Part of a thesis by the senior author for the MSc (Agric) degree of the University of Dar es Salaam     

Calibration of available P in soils from derived savannah zone of western Nigeria

Extractants for determining soil available P are useful in fertilizer recommendations when calibrated with actual yield responses. The relationships between relative (or percent) yield and soil P test levels can be described using various mathematical models, and the relevant critical P levels estimated.Critical P values for 60 cultivated savannah surface soils estimated in a greenhouse study and for 20 soils in 20 field observations varied with extractants. Most of the soils studied were deficient in P with 4–10 mg kg^–1 available P range. Calibration of P in soil using the Cate and Nelson's analysis of variance method, Mitscherlich and quadratic equations compared favourably. The advantages of the Cate and Nelson's procedure were discussed.     

Comparison of greenhouse and 32P isotopic laboratory methods for evaluating the agronomic effectiveness of natural and modified rock phosphates in some acid soils of Ghana

Phosphorus deficiency is one of the major constraints for normal plant growth and crop yields in the acid soils of Ghana and therefore addition of P inputs is required for sustainable crop production. This is often difficult, if not impossible for small-scale farmers due to the high cost of mineral P fertilizers and limited access to fertilizer supplies. Direct application of finely ground phosphate rocks (PRs) and their modified forms have been recommended as alternatives for P fertilization. The direct application of the natural and modified PRs to these acid soils implies the need to predict their agronomic effectiveness of the PRs in the simplest and most cost-effective manner. In this study the classical greenhouse pot experiment was compared to the ³²P isotopic kinetics laboratory method for evaluating the agronomic effectiveness of natural and modified Togo PR in six highly weathered Oxisols from southwest Ghana. In the ³²P isotopic kinetics laboratory experiment the six soil samples were each fertilised at the rate of 50 mg P kg^–1 soil in the form of triple superphosphate (TSP), Togo PAPR-50%, and Togo PR, respectively. Controls without P amendment were also included. Isotopic exchange kinetics experiments were carried out on two sets of samples, immediately after P fertilizer additions (without incubation) and after 6 weeks of incubation under wet conditions and at a room temperature of 25 °C. In the greenhouse pot experiment, P fertilizers in the form of Togo PR, Togo PAPR, Mali PR and TSP were each applied to the six soils at rates equivalent to 0, 30, 60, and 120 kg P ha^–1, respectively. The P fertilizers were mixed with the soils and maize (Zea mays L.) variety Obatanpa was grown for 42 days before harvest. The isotopic kinetics data of the control samples indicated that 5 of the studied soils had very low P fertility status as reflected by their low P concentrations in solution (C_P<0.02 mg P l^–1) and low exchangeable P (E₁min < 5 mg P kg^–1). The capacity factor and the fixation index of the soils were variable. Application of water-soluble P as TSP increased both the C_P and E₁ values of all the soils above the critical levels. Togo PR was least effective among the fertilizers tested for all soil soils, except in Boi soil. Acidulation of Togo PR (Togo PAPR-50%) was an effective means to increase its agronomic effectiveness. Direct application of natural Togo PR would be only feasible in the Boi soil series as reflected by its high Pdff% value in soil solution. Incubation with the P fertilizers caused an increase in the soil pH and a decline in the effectiveness of the applied P fertilizers, irrespective of the soil and the fertilizer utilized. Based upon the results of the greenhouse pot experiment, the relative crop response index (RCRI) in terms of increasing dry matter yield and P uptake followed the order of TSP > PAPR = Mali PR >Togo PR = Control. Both the laboratory index, Pdff% in soil solution derived from the isotopic method and the RCRI values obtained from the pot experiment produced similar results in ranking the P fertilizers tested according to their agronomic effectiveness. The isotopic kinetic method may be considered as an alternative to both greenhouse and field methods in the evaluation of agronomic effectiveness of P fertilizers in tropical acid soils when it offers comparative advantages in assessing the soil P status and its changes. But trained staff and adequate laboratory facilities are needed to perform this technique. Also the method can be used as a reference for comparison purposes as in this case. Further research is needed to assess the overall agronomic effectiveness (immediate and residual effects) of PR sources in predominant cropping systems of this region of Ghana.     

Evaluation of ammonium thiosulfate as a soil urease inhibitor

Interest in use of ammonium thiosulfate (ATS) in conjunction with urea as a fertilizer has been stimulated by recent reports that this compound retards hydrolysis of urea by soil urease and thereby reduces volatilization of urea N as ammonia from soils fertilized with urea. We evaluated ATS as a soil urease inhibitor by studying its effects on urea hydrolysis, seed germination, and early seedling growth in soil. We found that ATS significantly retarded urea hydrolysis only when applied at rates as high as 2,500 or 5,000µg g^–1 soil, whereasN-(n-butyl) thiophosphoric triamide (NBPT) (a patented inhibitor of urea hydrolysis in soil) caused substantial retardation of urea hydrolysis when applied at rates as low as 1µg g^–1 soil. We also found that ATS had an adverse effect on germination of corn or wheat seeds in soil when applied at the rate of 2,500 or 5,000µg g^–1 soil and caused a dramatic reduction of early seedling growth of corn or wheat when applied at the rate of 1,000, 2,500, or 5,000µg g^–1 soil. These findings indicate that ATS has little, if any, potential value for retarding hydrolysis of urea fertilizer in soil.     

Agronomical evaluation of phosphate fertilizer as a nutrient source of phosphorus for crops: Isotopic procedure

The agronomic effectiveness of P fertilizers, as sources of phosphorus for crops, was evaluated using the quantities, P_f, of phosphorus taken up byLolium perenne grown on 14 soils during greenhouse experiments in pot cultures. The P_f quantities were determined using³²P-labelled fertilizers. Data were analysed using a new concept: the Isotopic Relative Agronomic Effectiveness (IRAE). The IRAE value was defined as the ratio of the P_f quantity, taken up by a crop, of a tested fertilizer over the P_f quantity, taken up by a crop, of a fertilizer used as standard. In our experiments diammonium phosphate (DAP) was used as standard P fertilizer and two rock phosphates, the North Carolina rock phosphate (NCPR) and a calcium-iron-aluminium phosphate (Phospal), were tested. As a linear relationship between P_f(NCPR) quantities and P_f(DAP) quantities was obtained, with r² = 0.95, when the application rates increased from 15 mgP (kg soil)^–1 to 200 mgP (kg soil)^–1, it is conciuded that IRAE values for a given fertilizer, other than the standard fertilizer, could be determined with a single rate of application. As regards soil pH in the range 4.7 to 8.2 the IRAE_NCPR is related to soil pH by a curvilinear relationship: log IRAE_NCPR = –(0.44) pH + 4.05 with r² = 0.89. The average of IRAE_phospal values was 0.15 with a standard error = 7% irrespective of soil pH. Then a logarithmic relationship was obtained between IRAE values of the two tested fertilizers and their water P-solubility determined at the soil pH where they were applied.     

Phosphorus sorption capacity of low activity clay soils of South Western Nigeria and its usefulness in evaluating P requirement of rice

Field and laboratory experiments were conducted on 15 low activity clay soils in Ogun State of Nigeria to evaluate the relationships between P sorption capacity and some soil properties and the use of sorption indices in evaluating the P requirement of rice. Langmuir adsorption capacity (b) varied from 30.9 to 414.3 µg g^–1. Although adsorption capacity was related significantly to a number of soil properties, citrate dithinonite bicarbonate (CDB) extractable Fe was the most important variable accounting for 99% of the variation in adsorption capacity. The solution P concentration (SPC) required to achieve 95% maximum grain yield of rice varied from 0.03 in a sandy clay soil to 0.19 µg ml^–1 in a sandy soil, while the quantity of fertilizer P required to attain the solution P concentration (Standard Phosphate requirement, SPR) varied from 14.1 to 88.7 kg ha^–1. Highly significant power function relationships were obtained between SPC and b (r=0.93) and between SPR and b (r=0.93). The P buffering capacity (PBC) of the soils indicated that the soils are moderately buffered. However, SPR accounted best for the variation in grain yield of rice on the field (R²=0.90). The use of P sorption indices in estimating P needs of rice appears superior to the use of chemical extractants.     

Evaluation of chromium release during the decomposition of leather meal fertilizers applied to the soil

Greenhouse studies of 14 soils, having a range in DTPA extractable Mn, were made to determine the critical deficiency level of Mn in ustochrepts for predicting response of green gram to Mn application. Soil Mn was significantly related with Bray's per cent dry matter yield (r = 0.68^**). Soil application of 20 mg Mn kg^–1 soil significantly increased the yield. Both graphical and statistical models of Cate and Nelson indicated the critical level to be 2.9 mg kg^–1 soil of DTPA extractable Mn. The critical deficiency level in youngest matured terminal leaf (YML) of 40 day green gram plants was 19.0µg g^–1. The predictability of soil and plant critical Mn level was 93 per cent.     

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.     

Induced phosphorus fixation and the effectiveness of phosphate fertilizers derived from Sukulu Hills phosphate rock

A greenhouse study was conducted with two surface, acidic soils (a Hiwassee loam and a Marvyn loamy sand) to measure the effect of increasing P-fixation capacity, on the relative agronomic effectiveness (RAE) of phosphate fertilizers derived from Sukulu Hills phosphate rock (PR) from Uganda. Prior to fertilizer application, Fe-gel was added to increase P-fixation capacity from 4.4 to 14.3% for the Marvyn soil and from 37.0 to 61.5% for the Hiwassee soil. Phosphate materials included compacted Sukulu Hills concentrate PR + Triple superphosphate (CTSP) at a total P ratio of PR:TSP = 50:50; 50% partially acidulated PR (CPAPR) from Sukulu Hills concentrate PR made with H₂SO₄; and Sukulu Hills concentrate PR (PRC) made by magnetically removing iron oxide from raw PR ore. Triple superphosphate (TSP) was used as a reference fertilizer. After adjusting soil pH to approximately 6, P sources were applied at rates of 0, 50, 150, and 300 mg total P kg^–1 soil. Two successive crops of 5 week old corn seedlings (Zea mays L.) were grown. The results show that the RAE of the phosphate materials measured using dry-matter yield or P uptake generally decreased as P-fixation capacity was increased for both soils. CTSP was more effective in increasing dry-matter yield and P uptake than CPAPR. PRC alone was an ineffective P source. Soil chemical analysis showed that Bray 1 and Mehlich 1 extractants were ineffective on the high P-fixation capacity Fe-gel amended Hiwassee soil. Mehlich 1 was unsuitable for soils treated with PRC since it apparently solubilizes unreactive PR. When all of the soils and P sources were considered together, Pi paper was the most reliable test for estimating plant available P.     

Phosphorus effectiveness in fertilized soils evaluated by chemical solutions and residual value for wheat growth

Nineteen soils from the south east of the Province of Buenos Aires (Argentina) that had been fertilized with moderate amounts of P (10–40 kgP/ha) during the last 10 years were used to investigate the effect of time on the decline of P availability as measured by three soil tests (Bray 1, Bray 2, Olsen) and the null-point method. Differences in rates of P decline among soils and chemical methods were characterized by an exponential coefficient for time (b ₂) in equations which describe the changes of the added P retained by the soil (Pr =ac ^b1 t ^b2). The rate of decline of P for the nineteen soils calculated for the soil test methods was ordered decreasingly as: null-point > Olsen > Bray 1 > Bray 2. The ability of the chemical methods for assessing the residual value of P for wheat growth (RV) was tested in a pot experiment on seven of the soils that differed in their individual rates of reaction with P. Differences between soils in the rate of reaction with P as measured in the laboratory by the null-point method and by the Olsen test were reflected in different residual values for P fertilizer for wheat plants. Thus the value ofb ₂ for these methods was well correlated with the observed residual values. The soil properties commonly associated with the retention of P were not related to the value ofb ₂ suggesting that more than one soil property may be involved in the measure ofb ₂. The exponent for timeb ₂ may be used as an index of the ability of the soil test to reflect the decline of P availability with time.     

Phosphorus supplying capacity of heavily fertilized soils II. Dry matter yield of successive crops and phosphorus uptake at different temperatures

Nine heavily fertilized soils were collected from southern and central Norway. A greenhouse experiment in the phytotron was conducted to evaluate the P supplying capacities of these soils at different temperatures (9, 12 and 18 °C). The crops were grown in succession and the sequence was oat, rye grass (cut twice), oat, rape and oat. Effect of temperature on dry matter (DM) yield and P uptake was more marked up to the fourth crop but the effect varied among crops. The DM yields of oat and rape increased with increasing temperature but the opposite was the case with rye grass. The yield differences among soils at 12 °C were highly significant (p < 0.01) in contrast to 9 and 18 °C. The amount of P taken up by plants in these soils was highest at 18. °C. The P supplying capacity was highest in the soils with higher content of organic P. Generally, the soils of very fine and coarse texture classes failed to supply enough P to crops to avoid P deficiency in the successive crops. Soil P test (P-NH₄-lactate) values in most of the soils increased with increasing temperatures. The highest temperature effect was seen in the Særheim sand soil. Soil P test extractants P-AL, Bray-1 and Colwell-P were used to determine P in the soil after each harvest and the soil P test values were compared with P uptake by crops. Only the P-AL extractant was significantly correlated to cumulative P removal (CPR) by plants in most of the soils. Regression equation was calculated for each soil. The value of removed P per harvest (RPH) varied from 10.33 to 20.87 mg P kg^–1 soil. Phosphorus drawdown slope was determined for each soil and the number of consecutive harvests necessary to reduce the P-AL value to a normal level (110 mg P kg^–1 soil) was calculated. The drawdown slope varied widely (1.257–2.801) and this reflected the P buffer capacity and the number of crops required to lower the soil test P value to a normal level. The highest drawdown slope was found in the soils with higher P supplying capacities. The Bray-1 extractant was significantly correlated in the soils with higher buffer capacity but the Colwell-P method did not show significant correlation in any of the soils.