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.     

Phosphate fixing capacity of soils: A survey,using the isotopic exchange technique,or soils from north-eastern France

A large variety of soil samples was collected from farms located in two north-eastern regions of France. Their phosphate fertility was assessed by the isotopic exchange kinetics method which allows for the measurement of a r₁/R ratio, where r₁ is the quantity of radioactivity still present in solution one minute after the injection of a quantity of radioactivity R into a soil-solution system at steady-state. The r₁/R parameter gives an estimate of the soil fixing capacity for phosphate ions (PFC). Results of the analysis of 233 soil samples revealed that the great majority of soils from these areas exhibit a high PFC. Neither the soil type nor mode of utilization appeared sufficient for prediction of the PFC. Attempts to express the PFC as a function of other soil parameters (pH, clay, organic matter, calcium carbonate and exchangeable cations), resulted in a significant regression equation involving soil clay content and soluble phosphate. Due to high PFC of soils in the Lorraine area, phosphate fertilizer could be applied at a time as close as possible to the plant requirements.     

The role of isotopic techniques on the evaluation of the agronomic effectiveness of P fertilizers

Many isotopic techniques can be applied to determine the relative immediate and residual effectiveness of P fertilizers. Using isotopes as tracers, the percentage of utilization by plants of the P derived from a fertilizer can be determined. However this is only possible during the three or four months after the application. Therefore, the P fertilizers may be classified only according to their relative immediate effectiveness. To also evaluate residual effect, which can be observed when more P is applied than is removed with harvest, isotopes of phosphorus can be used. This residual effect is determined by comparing pool sizes of bioavailable soil P in soils with and without P fertilizer aged in soil. The bioavailable soil P pool may be analyzed according to three isotopic experimental procedures which give access to either the A value, or the E value or the L value. The aims, the similarities and the differences between these three procedures, are examined. Some of the theoretical and practical constraints of each method are described in this paper; they must be followed in order to obtain reliable information for agronomic purposes. A method involves measuring the rate of isotopic exchange of phosphate ions in soil-solution systems maintained in steady-state. It is now possible to predict the effectiveness of P fertilizers, whatever their chemical form when this method is applied on soil samples where P fertilizers were applied.This paper was originally submitted as part of the special issue on Evaluation of the Agronomic Effectiveness of Phosphate Fertilizers through the use of Nuclear Related Techniques edited by F. Zapata     

Phosphorus fertility recapitalization of nutrient-depleted tropical acid soils with reactive phosphate rock: An assessment using the isotopic exchange technique

A soil P fertility recapitalization initiative utilizinglarge rates of phosphate rocks (PRs) was proposed to improve the soil P statusand increase the sustainable food production in acid and P-deficient tropicalsoils. Two series of experiments were carried out using five tropical acidsoilstreated with heavy applications of Gafsa phosphate rock (GPR). In the firstseries, the soils were mixed with GPR at the following application rates: 0,500, 1000 and 2000 mg P·kg^–1, andincubatedfor one month in moist conditions. In another series, 1000 mg Pkg^–1 applied as GPR was added to three soils andincubated for 1.5 month; thereafter 50 mg P kg^–1as triple superphosphate (TSP) were added. The ³²P isotopic exchangemethod was utilized to assess the contribution of GPR to the available soil P.Changes in amounts, E, of P transferred with time as phosphate ions from thesoil particles to the soil solution as well as changes in pH, calcium andphosphate concentrations in soil suspensions were determined. It was foundthat:(i) the contribution of P from GPR to recapitalization of soil P fertility wasmainly assessed by E pool size, pH, calcium and phosphate concentrations; othervariables were not significant at the 0.1 level; (ii) heavy applications of GPRdid not saturate all the P sorption sites, P freshly applied as water-soluble Pwas still sorbed; (iii) recapitalization of soil P fertility using GPR waspartly obtained in some acid tropical soils; (iv) Upon dissolution, GPRprovidedcalcium ions to crops and to soils, thus reducing Al toxicity, but its limingeffect was limited. To explain these effects with heavy application rates ofGPR, it was postulated that a coating of Al and Fe compounds is formed aroundPRparticles with time, thus reducing further dissolution.     

Evaluating the Effectiveness of Phosphate Fertilizers in some Venezuelan soils

In Venezuela, 70% of the soils are acid with low natural fertility where phosphorus is the most limiting element together with nitrogen and potassium for plant growth. The efficiency of phosphate fertilization is low. Greenhouse and field experiments were conducted to evaluate the efficiency of natural and modified rock phosphate using conventional and isotopic techniques. An incubation experiment was done to measure changes in available P on application of different phosphate fertilizers at a constant rate of 100 mg P/kg in ten acid soils of agricultural importance in Venezuela. In the greenhouse, two experiments were conducted to relate P fixation to soil P availability and the response of an index plant (Agrostis sp.). A high variability in P fixing capacity of the soils (r1/Ro = 0.02–0.76) was observed with the same level of available P. This fixation index is defined as the proportion of the added radioactivity (³²P) remaining in the soil solution after 1 min of exchange and a low fixing capacity is indicated by the values close to 1. The proportion of the total soil P that can possibly enter the soil solution and therefore is potentially available for plant uptake was measured using the traditional method (Bray I) and the isotopic method (E value). The high variability was also apparent in available P extracted by Bray I showing a range of 10 to 88% of the total P removed by the extracting solution. The incubation studies showed that the effectiveness of the P source for available P in the soil solution was related to their reactivity and the soil P fixing properties. The increase in the fixing capacity of the soils used caused a significant reduction in the E value, independent of the source of P used. A high positive and significant correlation between Bray I extracted P and the E value (r = 0.95) obtained from the different treatments, showed the relationship of the extractant for some forms of available P in soils where rock phosphate was applied. In the greenhouse experiment, the crop response was related to the P fixing properties of the soil, the initial availability and the solubility of the P source used. The P in plant derived from the fertilizer and the Utilization Coefficient decreased significantly as the P fixing capacity of the soils increases indicating a lower availability of P for the the index plant (Agrostis sp.). The P in plant derived from the P fertilizers calculated by using the specific activity of each treatment and the one of the check plot showed that triple superphosphate had the highest values with acidulated Riecito rock phosphate (40%) having intermediate values, and Riecito rock phosphate having the lowest value. The use of ³²P techniques as a powerful method to study soil P dynamics and P uptake from different P sources and the effectiveness of phosphate rocks (natural and modified) produced in Venezuela with respect to the water-soluble P source (imported), are some of the practical implications of this study.     

Isotopic assessment of soil phosphorus fertility and evaluation of rock phosphates as phosphorus sources for plants in subtropical China

Soil phosphorus (P) deficiency is a major factor limiting crop productivity in many tropical and subtropical soils. Due to the acidic nature of these soils, rock phosphate (RP)-based P fertilizers that are cheaper than manufactured water-soluble P fertilizers can be an attractive alternative under certain conditions. Assessment of the efficacy of these alternative P fertilizers and a rational management of local P resources for sustainable agricultural production require an understanding of the dynamics of P in the soil–plant system and the interactions of various P sources in soils and monitoring of soil available P levels. The present work was conducted to test the applicability of the ³²P isotopic kinetic method to assess the soil P fertility status and evaluate the agronomic effectiveness of local rock phosphates in subtropical China. A series of experiments was carried out in the laboratory, greenhouse and field conditions with the following specific objectives: (a) to evaluate the suitability of this isotopic kinetic method in evaluating soil P fertility in 32 soil samples collected across southern China, (b) to test and further develop chemical extraction methods for routine soil P testing, (c) to monitor the dissolution kinetics of local low to medium grade rock phosphate sources and their effect on soil properties and (d) to evaluate their agronomic effectiveness in greenhouse and field experiments. Since most of the studied soils had very low concentrations of soluble P and high P-fixing capacities, the isotopic kinetic method was found unsuitable for evaluating soil P fertility and to predict plant P uptake. In contrast, the proposed chemical extraction method (NaHCO₃-NH₄F) predicted very well plant P uptake, suggesting that this extraction method can be routinely used to evaluate soil bioavailable P in similar soils in subtropical China. From the incubation study, it was found that although the local low to medium grade RPs were inferior to the reactive NCPR in increasing soil available P levels, they have the potential to improve soil chemical properties. Field experiments indeed demonstrated that the medium grade Jinxiang RP significantly increased crop yield, suggesting that local low to medium grade RPs could be used as P sources to provide P to plants and also to improve soil chemical properties. Overall, these results provide important information for a rational management of P resources for sustainable agriculture in subtropical China.     

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

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

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.     

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.     

Application of reactive phosphate rock and sulphur fertilisers to enhance the availability of soil phosphate in organic farming

Plant available soil phosphate is frequently deficient for crop and pasture production on organic farms in southern Australia. Improved P management, including developing a fertiliser product conforming to organic farming regulations, is required to sustain and increase production on these farms. Reactive phosphate rock (RPR) and elemental sulphur (S) are natural products. Field and pot experiments were established to measure the impact of ground RPR, and co-treatment of RPR with finely ground S, on available soil phosphate (Olsen P), plant dry matter, and the P concentration (%) and content (kg P ha⁻¹) of the dry matter. Under dry-land field conditions characteristic of cropping regions in southern Australia ( < 600 mm rainfall, organic carbon < 3%), co-treatment of RPR with S was necessary to increase Olsen P, and higher values of Olsen P were generally associated with increased plant dry matter, together with P concentration or P content of the dry matter. The required amount of S was less the more acidic the soil, but greater than reported as being effective in situations of higher rainfall ( > 1,000 mm) and soil organic carbon concentration (OC 11%). It was deduced that the S is probably required to overcome the constraint on dissolution of RPR resulting from frequent periods of low soil moisture. It was concluded that for the south-eastern Australian cropping zone, co-treatment of ground reactive phosphate rock with finely ground elemental S, at ratios (RPR:S) of at least 2:1, depending on soil pH, is required for effective use␣of RPR, even in strongly acidic soil (pH_Ca < 4.5). It was recommended that ‘organic’ farmers may recover soil P fertility by applying RPR + S fertiliser to the most acidic fields, postponing soil liming, and managing the fields to conserve soil moisture.     

Effect of soil pH on the requirement for water-soluble phosphorus in triple superphosphate fertilizers

A greenhouse study was conducted to determine if soil pH affects the requirement for water-soluble P and the tolerance of water-insoluble impurities in TSP fertilizers. Two commercial TSP fertilizers were selected to represent a range in phosphate rock sources and impurities. Phosphate fertilizer impurities were isolated as the water-washed fraction by washing whole fertilizers with deionized water. TSP fertilizers with various quantities of water-soluble P (1.2 to 99% water-soluble P) were simulated by mixing the water-washed fertilizer fractions or dicalcium phosphate (DCP) with reagent-grade monocalcium phosphate (MCP). The fertilizers were applied to supply 40 mg AOAC available P kg^–1 to a Mountview silt loam (fine-silty, siliceous, thermic Typic Paleudults). Wheat (Triticum aestivum (L.)) was harvested at 49 and 84 days after planting. Soil pH values at the final forage harvest were 5.4±0.16 and 6.4±0.15. At a soil pH of 5.4, the TSP fertilizers required only 37% water-soluble P to reach maximum yields while at pH 6.4 the fertilizers required 63% water-soluble P. Results of this study show that higher levels of water -insoluble P can be tolerated in TSP fertilizers when applied to acid soils. Phosphorus uptake was not affected by soil pH, but for the mixtures containing the fertilizer residues the source having the lowest level of Fe and Al had a higher relative agronomic effectiveness.     

Reducing fertilizer requirements for hybrid squash (Cucurbita maxima L.) with localized applications of phosphorus and potassium and use of partially acidulated phosphate rock

Two experiments examined options for reducing the inputs of P and K fertilizers for hybrid squash (Cucurbita maxima L.) at Pukekohe, New Zealand. The first experiment examined the effects of elevating the NaHCO₃-soluble P from 32 to 130 mg kg^–1 and the exchangeable K from 140 to 350 mg kg^–1 within strips from 0 to 0.75 m around rows of hybrid squash planted 1.5 m apart. From both P and K, crop yield increased as the width of the fertilized strip was increased up to 0.25 m, while wider fertilized strips had no further effect. These results followed similar effects on plant dry matter and tissue P or K concentration during early growth, and are explained in terms of the P and K accumulation by the crop, the decline during growth of the sensitivity of the crop to soil P and K fertility associated with declining rates of P and K uptake per unit length of root. Implications for fertilizer management for hybrid squash are also discussed.The second experiment compared the effects of partially acidulated phosphate rock and triple-superphosphate on soil P fertility, growth and yield of hybrid squash. Partially acidulated phosphate rock had smaller effects than those of triple-superphosphate on NaHCO₃-soluble P levels in the soil, plant dry weight and tissue P concentration soon after emergence, and subsequently crop yield. On average, partially acidulated phosphate rock increased crop yield by about 70% of that following the application of the same quantity of P as triple-superphosphate. This lower effectiveness of partially acidulated phosphate rock for hybrid squash is explained in terms of its lower solubility and hence smaller effect on NaHCO₃-soluble P in the soil during early growth, when the crop is most sensitive to soil P fertility.     

Agronomy,modelling and economics of reactive phosphate rocks as slow-release phosphate fertilizers for grasslands

Reactive phosphate rocks (RPRs) from Sechura, Peru (SPR) and North Carolina, USA (NCPR) were compared with triple superphosphate (TSP) as phosphate (P) fertilizers for permanent grass/clover pastures in four field trials in New Zealand. Trial sites ranged in initial pH (in water) from 5.7 to 6.3 and in rainfall from 712 to 1338 mm yr^–1. SPR and NCPR were used in the unground as-received state. Fertilizers were applied annually for six years. Pasture was harvested by frequent mowing, and herbage dry matter (DM) yields were measured at each cut. Herbage P concentrations were measured at each cut in two trials and on most cuts in the other two.For all sites combined, DM production from RPRs was initially significantly less than from TSP but it improved relative to TSP with time. Substitution values of RPR relative to TSP, denoted by S.V. (TSP/RPR) and defined as the ratio of P in TSP to P in RPR required to produce the same plant response during a specified period of time, were estimated by relating yields from RPR treatments to the yield response curve for different application rates of TSP. For the four trials combined, S.V. (TSP/SPR) increased from 0.32 in year 2 to 0.85 in year 6. S.V. values for NCPR were similar. The site which had the lowest S.V. values (average 0.20) for total production over six years was the site with highest pH (6.3) and lowest rainfall (712 mm). Corresponding S.V. values for the other sites were 0.50 to 0.78.Herbage P concentrations showed a similar pattern of RPR performance relative to TSP to that shown by DM production except at the highest application rate where TSP always supported much higher herbage P concentrations than RPR.The pattern of DM production from RPR relative to TSP was explained on the basis of a model involving soil P pools of undissolved fertilizer P and plant-available P respectively, with the hypothesis that P dissolved from RPR entered the plant-available P pool and was used with the same efficiency as P entering by dissolution of TSP. Model predictions of substitution values using directly measured RPR dissolution rates agreed well with observed substitution values.The advantage of RPRs in comparison to soluble P fertilizers for permanent pastures was considered to lie in their lower price and not in greater nutrient efficiency. Economic advantage was calculated in terms of the return on investment from establishing and maintaining a pool of RPR in the soil large enough to release the required annual amount of plant-available P compared with the cost of annual applications of soluble P fertilizer.     

Phosphate adsorption and precipitation in calcareous soils: the role of calcium ions in solution and carbonate minerals

P-removal from soil solution is mainly due to adsorption and precipitation reactions. For calcareous soils two pathways have been proposed as being relevant: partitioning on soil surfaces and precipitation induced by Ca2+ ions in solution. To define P-speciation in soil and reduce P-immobilisation following fertilisation, the relative importance of these two reactions needs to be quantitatively established. This investigation, conducted on two calcareous soils, suggests that Ca-ion activity in the liquid phase is mainly responsible for the formation of insoluble Ca-P phases. Our study was carried out by determining: a) batch sorption isotherms at different slurry concentrations, times of contact, pH and indifferent electrolyte concentrations; b) supernatant isotherms on soil suspensions; c) insolubilisation kinetics of P added to soil columns. The shape of the sorption isotherms indicated that adsorption predominated at low concentrations (below approximately 0.5 mM); above this level precipitation became predominant. Precipitation from solution was demonstrated by adding increasing amounts of phosphate to soil suspension supernatants and precipitation levels comparable to those observed in sorption isotherms were obtained. Thus, carbonate mineral surfaces were not necessary for the induction of P precipitation. The formation of Ca-P mineral phases was increased with reaction time and was governed by the concentration of Ca-ions, pH and indifferent electrolyte concentration. P added at the top of soil columns was rapidly insolubilised: after 5 weeks the P-Olsen value was reduced to about 60% and P was not transported to the deepest layers but remained in the surface ones. These results suggest that, for soils with a high reservoir of exchangeable cations able to form insoluble P phases, precipitation is the predominant mechanism which reduces P availability for plants.     

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

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

Nutrient budgeting for phosphorus and potassium in a long-term fertilizer trial

Influence of N, P and K application through inorganic and organic fertilizers on P and K removal in crop plants, changes in soil fertility status and their balance in the soil-plant (maize-wheat-cowpea fodder) studied for the first 22 years of a long-term experiment at Punjab Agricultural University farm, Ludhiana, India. The results showed that P uptake by wheat was about 1.5 times that of maize, whereas K uptake by wheat was only 1.1 times that of maize. The apparent P recovery by both maize and wheat depended on the rates of N, P and K application. Fertilizer rates greater than the recommended (150% NPK) resulted in lower P recoveries. At optimum level of N, P and K application (100% NPK) the mean P recovery (for 22 years) was 30.3% (±5.47) in wheat as compared to 20% (±11.35) in maize. The apparent P recovery in maize declined as the number of cropping years progressed. In 100% NPK plots, it declined from 45.6% in 1973 to 12.5% in 1992. The decline in P recovery was due to the accumulation of plant available P in the soil which increased from the initially low status to high-very high due to continuous application of P fertilizer. The application of FYM in conjunction with 100% NPK led to significantly greater accumulation of available P as compared to 100% NPK treatment alone despite the higher amount of total P removal in the former treatment than that from the latter. A trifle build-up in available K was observed in K amended plots notwithstanding the negative balance of K based on the approach of input-output relationship. The release of K from non-exchangeable form contributed towards K uptake by the crops. The results suggested the need for modifying the existing K fertilizer recommendations to compensate for gradual loss of native soil K fertility.     

Assessment of methods for studying the dissolution of phosphate fertilizers of differing solubility in soil

Relationships between plant response and rates of dissolution of ground (< 150µm) North Carolina phosphate rock (NCPR), NCPR 30% acidulated with phosphoric acid (NCPAPR) and monocalcium phosphate (MCP) were assessed in pot experiments. The three fertilizers were incubated for 1, 50 and 111 days, at the rates of 75, 150 and 750µg P g^–1 soil, using two soils with different P-retention capacity. After each period of incubation, four pots were set up from each treatment, and perennial ryegrass (Lolium perenne) was grown in a growth chamber for about six weeks to assess the agronomic effectiveness of the fertilizers. Results in dry matter yield and P uptake showed that immediately following application (1 day incubation), the MCP (solution) was supplying more P to plants than either the NCPR or the NCPAPR applied at the same rate. After 50 and 111 days of incubation, the NCPR and NCPAPR were just as effective in the lower P-retention Tekapo soil. The relative agronomic effectiveness (RAE) of the NCPR and NCPAPR compared with MCP was generally poorer in the higher P-retention Craigieburn soil than in the Tekapo soil shortly after application, but improved with time of incubation. Ryegrass responses to the application of the three fertilizers corresponded to the changing trends of exchangeable P in the soils, measured by the isotopic method.Regressions were made between plant P uptake and indices describing the intensity factor (water extractable P), quantity factor (Bray I P, Olsen P, 0.5M NaOH extractable P and isotopic exchangeable P) and the kinetic factor (F_in) of soil P supply to plants in the Tekapo soil. The percentage of variation in plant P uptake explained by individual indices was generally less than 80%, no matter which of the three single variable models, the Mitscherlich, the quadratic or the power function was fitted. However, more than 96% of the variation in plant P uptake in the Tekapo soil could be explained by the power function models involving two variables. The rate of P dissolution (F_in) determined by the isotopic dilution method was included in all the two variable models. The results suggest that assessment of soil P supply to plants should consider the kinetic factor in addition to the intensity and quantity factors, particularly where P fertilizers with differing solubility are applied.     

Variable grain legume yields,responses to phosphorus and rotational effects on maize across soil fertility gradients on African smallholder farms

Promiscuous soyabean varieties have potential to contribute significantly to income generation, food security and soil N budgets on smallholder farms. One of the major factors limiting this potential is farmers’ preference to allocate nutrient resources to food security cereal crops on the most fertile fields, leaving grain legumes to grow on residual fertility on infertile fields. Two experiments were conducted to: (i) compare the current farmer practice with targeting manure and single super phosphate (SSP) to soyabean in a three-year rotation cycle on two fields with different soil fertility: an infertile sandy soil and a more fertile clay soil; and (ii) assess the effects of variability of soil fertility within and across farms on productivity of soyabean and groundnut. In the first experiment, soyabean (<0.2 t ha⁻¹) and maize yields (<0.7 t ha⁻¹) without fertilizer were poor on a degraded sandy soil. Both crops responded poorly to SSP due to deficiency of other nutrients. Manure application significantly increased soyabean and maize yields, led to yield stabilization over three seasons and also significantly increased the proportion of N₂ fixed by soyabean (measured using ¹⁵N natural abundance) from 60% to 83%. On the sandy soil, P was used more efficiently and gross margins were greater when SSP and manure were applied to maize in a maize–soyabean rotation. Soyabean and maize yields without fertilizer inputs were larger on clay soil with moderate fertility (0.4–0.7 t ha⁻¹ and 2.0–2.3 t ha⁻¹ respectively) and were significantly increased by application of SSP and manure. Within rotations, P recovery was higher when manure and SSP were applied to maize (43 and 25%) than when applied to soyabean (20 and 19%). However, application of manure to soyabean on the clay was more profitable than application to maize for individual crops and within rotations. In the second experiment, soyabean and groundnut yields were largest (∼1 and ∼0.8 t ha⁻¹ respectively) on plots closest to homesteads on wealthy farms, which were more fertile due to good past management. Yields were poor (< 0.5 t ha⁻¹) on other fields which previously had received little nutrient inputs. Soyabean and groundnut yields correlated well with available P (R ² = 0.5–0.7) and soil organic C (SOC) contents (R ² = 0.4–0.6). For smallholder farmers to maximise benefits from legume production they need to focus attention on the more fertile plots, although production should be optimized in relation to maize. Targeting nutrients to maize as currently practiced by farmers was more efficient and economic under poor soil fertility conditions, whilst potential exists to increase income by targeting manure to soyabean on the more fertile soils.     

The comparative agronomic effectiveness of rock phosphate and superphosphate for banana

The agronomic effectiveness of three rock phosphates (Idaho, Florida and North Carolina) as influenced by mycorrhizal inoculation withGlomus aggregatum was evaluated using small banana (Musa paradisiaca L.) corms as planting material. The treatments included superphosphate and a no-P control. The soil was fumigated to eliminate mycorrhizal propagules. The amount of P added was based on the quantity of material needed as superphosphate to establish 0.2 mg P L^–1 in solution. Plants were grown in an Oxisol in 9-liter pots for 3 months after growth commenced. Plant dry weight, P percentage in the 3rd leaf, and total P uptake were increased when plants fertilized with insoluble rock phosphates were inoculated with mycorrhiza-producing fungi. Phosphorus uptake by plants fertilized with Idaho, Florida, and North Carolina rock phosphates was 0.18, 0.42, and 0.97 as much as by plants fertilized with superphosphate. The beneficial effect of mycorrhiza on phosphate uptake was 136, 30, 2 and 24% for plants fertilized with Idaho, Florida and North Carolina rock phosphate, and superphosphate, respectively.     

The significance of available nutrient fluxes in N and P budgets for maize cropping on a Rhodic Kandiustox: a study with compost,NP fertilizer and stubble removal

Nutrient budgets may be useful tools for nutrient management of crops especially if they estimate the nutrient fluxes available from a variety of sources including organic and inorganic fertilizer, crop residues and soil organic matter. The aim of the present study was to develop a budget of available nutrients by determining the contribution of mineralized nutrient fluxes and fertilizer input relative to nutrient losses and removal in harvested products in the overall N and P balances. N and P inputs and outputs and available N and P fluxes in the soil were estimated for 3 consecutive maize crops where inputs and outputs were altered by NP fertilizer, compost and stubble removal on a Rhodic Kandiustox. A sensitivity analysis of calculated and measured nutrient budget items was conducted to identify the main factors affecting the accuracy of the nutrient balance calculations. Mineral fertilizer rate was the major factor for maize nutrient budgets as shown by its contribution to N and P balances. Without mineral fertilizer application, soil organic matter (SOM) mineralization was the most important within-season nutrient input. In the case of N, shoot uptake was the main output followed by denitrification. Phosphorus adsorption by the soil was the major P output from the available pools followed by shoot uptake. SOM mineralization maintained the pools of available N and P if stubble of the previous crop was returned. Mineral fertilizer application, which produced surplus balances of N and P, would however, be needed to attain high yield, even with stubble return. The available N and P from compost were not significant inputs in the nutrient balances until year 3. Total N and resin extractable P in soil after five crops supported the calculated nutrient balances indicating the importance of available nutrient fluxes in calculating N and P balances.