Agronomic Effectiveness Research Articles

Application of Organic and Inorganic Fertilizer for Sustainable Maize and Cowpea Yields in Nigeria

A pot experiment evaluating the influence of five rates each of compost, inorganic fertilizer (MFert), and combination of both fertilizers (OBF) on maize (Zea mays L.), performance was conducted in the greenhouse using three cropping cycle. This was followed by a field trial lasting four cropping seasons at Ilora in the derived savanna and at Ibadan in the forest zones of Nigeria. Maize followed by cowpea was used as test crops on the field. Application of the fertilizers led to increases in maize and cowpea grain yields and improved the nutrient element concentrations in maize leaf tissue. Agronomic effectiveness of compost and OBF on maize relative to MFert was 88 and 118%, at Ilora and 81 and 97% at Ibadan. There was an increase of 56 and 89% Relative Agronomic Effectiveness (RAE), for compost and OBF over MFert in the residual study on cowpea. Also, the soil macro and micro nutrient status was highly improved in comparison with the MFert at postharvest period. The optimum rates of the organic fertilizers for sustainable maize and cowpea yield were between 5 and 10 t ha−1. The agronomic effectiveness of the residual organic fertilizers was higher than that of MFert. Optimum rate of OBF was 5 t ha−1. Application of compost significantly increased total microbial biomass in the soil. Also, compost and OBF application led to a fairly high population of Azotobacter (N-fixing bacteria) and thus its biomass. The MFert, however caused a sharp reduction in microbial biomass in the soil. The organic fertilizer improved aggregate stability index and permeability of the soil. On the whole, the positive effects obtained from OBF and compost in this study favor the recycling of farm wastes for sustainable crop production.

Greenhouse Evaluation of Phosphorus Sources Produced from a Low‐Reactive Brazilian Phosphate Rock

The Patos de Minas phosphate rock (PR) in Brazil is not suitable for direct application due to its low reactivity or for commercial production of P fertilizers because of its high Fe‐Al content. The objective of this study was to investigate the potential agronomic effectiveness of P sources with low water solubility produced from the PR. The P treatments were (i) compacted granular Patos PR with a single superphosphate (SSP) containing high water‐soluble P content at a P ratio of 50:50, (ii) powdered low‐grade single superphosphate (LG SSP) produced from Patos PR with low water solubility, (iii) powdered Patos PR, (iv) powdered mixture of PR and SSP at 50:50 P ratio, and (v) compacted granular SSP. The P sources were incorporated into an acid soil with pH 5.3 at 0, 10, 25, 50, and 100 mg P kg−1 to grow wheat (Triticum aestivum L.) and ryegrass (Lolium perenne L.) as test crops in a greenhouse study. We confirmed that Patos PR was low in agronomic effectiveness, only 1 and 30% as effective as SSP in producing dry matter yield of wheat and ryegrass, respectively. There were no significant differences between LG SSP or compacted PR+SSP [PR+SSP(C)] and SSP in dry matter yields of both crops, whereas mixed PR+SSP was less effective than PR+SSP(C) compared with granular SSP. Compacted PR+SSP and LG SSP can be potential P sources to be produced from the low‐grade PR that could not be used either for direct application or acidulated P fertilizers.

SHORT COMMUNICATION: Agronomic effectiveness of novel phosphate fertilisers derived from an igneous Zimbabwe phosphate rock

A review of literature shows that work on non-conventional phosphate fertilisers has been done exclusively on sedimentary phosphate rocks. The potential of using novel phosphate fertiliser materials derived from unreactive igneous Dorowa (Zimbabwe) phosphate rock was investigated in a greenhouse experiment. Three phosphate fertiliser materials; (1) finely ground (0.150-mm screen) Dorowa phosphate rock (DPR), (2) partially acidulated Dorowa phosphate rock (PADPR), and (3) a compacted mixture of DPR + triple superphosphate + urea + potassium chloride (DTUK) with half of P from DPR and half from triple superphosphate (TSP) were made from Dorowa rock and their relative agronomic effectiveness compared with that of single superphosphate (SSP) on an acid Hartsells silt loam (pH 4.8) with maize (Zea mays L.) as the indicator crop. Direct application of DPR was found to be ineffective in increasing phosphorus (P) uptake and dry matter yield of maize. The relative agronomic effectiveness of DPR partially acidulated with 50% of the sulfuric acid (H2SO4) required for complete acidulation, in increasing P uptake and dry matter yield was 60% and 75%, respectively. The compacted fertiliser product, DTUK, was equally effective in increasing P uptake and dry matter yield as SSP. Key Words: Acidulation, agronomic effectiveness, fertiliser compaction, Zea mays Résumé Une revue de la littérature montre que le travail sur engrais phosphatiques non conventionels a été fait exclusivement sur les roches (substances) phosphate sedimentaires. Le potentiel d'utilisation des nouveaux matériaux fertilisants phosphatiques dérivés de réactifs éruptifs Dorowa (Zimbabwe) ont été examiné dans une serre experimentale. Trois matériaux fertilisants phosphate ; finement moulu (0, 150 mm tamis) Dorowa roche phosphatique (DRP), et 2) DRP acidulé (DRPA), et 3) un melange compacte de DRP et le triple superphosphate+ urée+ chloride de potassium (DTUK) avec la moitié de P du DRP et la moitié de triple superphosphate (TSP) était faite à partir de la roche Dorowa et leur efficacité agronomique relatives comparées avec celle du superphosphate (SSP) sur un depôt d'acide Hartsells terreau (BH 4, 8) avec le maïs (Zea mays L.) comme indicateur de plante. Une application directe du DRP était trouvée être non effective dans la croissance de l'assimilation du phosphore (P) et production de matière sèche de maïs. L'efficacité relative agronomique du DRP partiellement acidulé, avec 50%d'acide sulfurique (H2SO4) demandée pour une complète acidulation, pour l'accroissement de l'assimilation et la production de matière sèche de P était respectivement de 60% et 75%. Le produit fertilisant compacté DTUK était également effectif dans accroissement de l'assimilation et la production de matière sèche de P comme le SSP. Mots Clés: Acidulation, efficacité agronomique, compaction de fertilisants, Zea mays (Af Crop Sci J 2003 Vol 11 No 3 pp235-243)

Changes in Chemical Properties and Temperature during the Composting of Tobacco Waste with Other Organic Materials, and Effects of Resulting Composts on Lettuce (Lactuca sativa L.) and Spinach (Spinacea oleracea L.)

ABSTRACT This study (i) investigated changes that take place during the composting of tobacco wastes with other organic materials, (ii) characterized the resulting composts, and (iii) evaluated their agronomic effectiveness. Four composts were made, all of which contained tobacco waste and sawdust/wood shavings but differed in the third ingredient, which was either cow dung, pig dung, poultry manure or cabbage waste. Changes in pH and electrical conductivity (EC) during composting were consistent with those generally observed, in spite of the presence of toxic tobacco wastes. Changes in temperature regime and the results of a bioassay (germination test) suggested that compost maturity was achieved after 45–59 days in all four composts. Mixing of tobacco waste with other organic wastes reduced the nicotine content of tobacco wastes from 12180 mg kg−1 to 4872 mg kg−1 by dilution while composting reduced it further to < 160 mg kg1 in the final composts. Tobacco waste had a depressing effect on microbial activity in soil but the composts stimulated it, possibly as a result of their lower EC and reduced nicotine and related alkaloid levels. All final composts had C:N ratios ≶ 17 and so were considered conducive to N mineralization and thus suitable for horticultural use. However, they had high electrical conductivity (6.0 to 9.3 mS cm−1) and may not be ideal for salt-sensitive crops or as sole components in horticultural growing media without modification. Application of the composts to soil (to supply 80 mg N kg−1) improved the growth of lettuce relative to the control and uncomposted tobacco waste but full growth potential was not attained due to inadequate nitrogen supply. A subsequent experiment with spinach designed to address the N limitation revealed that satisfactory yields could be obtained by the application of 20 t ha−1 cow dung compost or 10 t ha−1 compost in combination with NPK fertilizer supplying 50 kg N ha−1.

Yield and Biomass Performance of Corn (Zea mays L) to the Application of Agrofer Organic Fertilizer

A trial generally aimed to generate information on the agronomic effectiveness of 10 bags per hectare AGROFER to mm. Specifically, the objectives of the study was to evaluate the marketable yield response of com to different rates and combination of AGROFER and inorganic fertilizers and to determine the effects of AGROFER on the biomass production of com. This was laid-out in Randomize Complete Block Design at Catalunan Pequefio, Davao City, during the period July to October, 2000. The combination of full inorganic fertilizer (2 bags per ha Ammophos + 1 bag/ha Muriate of potash + Urea 1 1/2 bags/ha) plus 10 bags/ha AGROFER (T6) gave the highest biomass yield of com (20.050 t/ha). Marketable yield of com (5,950 kg/ha) treated with 10 bags/ha AGROFER alone (T5) was comparable with the marketable yield of corn applied with one half recommended rate of inorganic fertilizer (1 bag/ha Ammophos + 1/2 bag/ha Muriate of potash + % bag/ha Urea) (T3) (5,550 kg/ha). Yield in T5 was higher than the yield of com in Tl (no fertilizer applied) (5,000 kg/ha). Much better economic yield was observed when 10 bags/ha AGROFER was mixed with 2 bags/ha Ammophos + bag/ha Muriate of potash + 1 1/4 bags/ha Urea (T6) and applied to com (7,050 kg/ha).

Relative Agronomic Effectiveness of phosphate rocks and P adsorption characteristics of an Oxic Rhodustalf in Eastern India

AbstractThe Relative Agronomic Effectiveness (RAE) of rock phosphates as compared to water soluble Triple Super Phosphate was measured on direct, residual, and cumulative application of the P fertilizers in a field experiment with rice on an Oxic‐Rhodustalf in the eastern plateau region of India. The fertilizers were Morocco Rock Phosphate (MORP), Mussoorie Rock Phosphate (MRP), Partially Acidulated Rock Phosphate (PARP), and Triple Super Phosphate (TSP). The RAE of the rock phosphates were lower for direct application (54–80 %) and cumulative application (70–93 %) of P but roughly equal or larger for the residual effect (92–142 %) as compared to TSP. The P adsorption characteristic of the experimental soil conformed to the linear relationship of both Freundlich and Langmuir isotherm equation. The adsorption data when plotted according to Langmuir equation deviated from a single linear relationship at higher concentration (10 μg ml–1), thereby giving two adsorption maximum values ( 68.49 μg g–1 and 256.41 μg g–1) and binding energies ( 2.86 ml μg–1 and 0.089 ml μg–1) for the soil. Two populations of P adsorption site with widely different affinity for P probably existed in the soil.

PHOSPHORUS AVAILABILITY AS AFFECTED BY THE APPLICATION OF PHOSPHATE ROCK COMBINED WITH ORGANIC MATERIALS TO ACID SOILS IN WESTERN KENYA

Most of the agricultural lands in the highlands of western Kenya are depleted of plant nutrients, particularly phosphorus. This depletion has resulted in a continued decline in crop production in the area. Recent experiments, in which direct application of indigenous phosphate rocks were evaluated, have yielded variable results, depending on the relative reactivity of the rock phosphate tested. The effectiveness of rock phosphate was generally low compared with fertilizers such as triple super phosphate. This is attributed to the relatively low solubility of the rocks as opposed to the readily water soluble phosphorus fertilizers. Phosphate rocks are available locally and are cheaper than triple super phosphate. If the solubility of these phosphate rocks could be improved, the resource-poor smallholder farmers in western Kenya would have an affordable source of phosphorus for their crops. Greenhouse and field experiments were conducted to evaluate the effects of combining different on-farm organic materials with the Minjingu rock phosphate (from Tanzania) on the availability of phosphorus to maize (Zea mays) in western Kenya. The greenhouse results indicated that there were significant positive linear relationships between rock phosphate application rates and (i) the Olsen-extractable soil phosphorus in the soil samples taken four weeks and nine weeks respectively after the application of the treatments; (ii) the dry matter yield, and (iii) phosphorus uptake. Results of the field experiments showed that rock phosphate combined with farmyard manure or crop residues (maize stover) generally increased the Olsen-extractable soil phosphorus, maize yields and phosphorus uptake, particularly in the first season when both the Minjingu rock phosphate and organic materials were applied, but the effectiveness of the materials and their combinations varied between the two sites. Combining Minjingu rock phosphate with the organic materials improved its relative agronomic effectiveness for maize.

Cadmium availability from granulated and bulk-blended phosphate-potassium fertilizers.

Recent field experiments have shown that high chloride (Cl) in irrigation waters can increase soil cadmium (Cd) uptake by crops because of the formation of soluble ion-pair complexes of Cd with Cl in soil solution. The present study was aimed at testing a hypothesis that KCl in granulated PK fertilizers may enhance Cd uptake by crops from Cd-containing P fertilizers because of close contact between Cd and Cl in the same granules. Less effect would be expected if the same granular PK fertilizers were bulk-blended because of separation of Cd and Cl in different granules. A single superphosphate (SSP) containing 32 mg Cd kg(-1) was granulated by the compaction process with KCl at a P to K ratio = 1:1. Granular KCl was also bulk-blended with granular SSP or Cd-free monocalcium phosphate (MCP) at the same P to K ratio. An acid Ultisol (pH 5.2) was treated with PK fertilizers at 400 mg kg(-1) each for P and K. Upland rice (Oryza sativa L.) and soybean [Glycine max (L.) Merr.] were grown to maturity, and signalgrass (Brachiaria decumbens Stapf.) was cut four times during the study. The results showed that the agronomic effectiveness in increasing crop yield was the same with SSP and MCP whether granulated or bulk-blended with KCl. Concentrations of Cd in plant tissue samples of all crops were much lower for MCP than for SSP. In all the plant tissue samples, except grain samples of upland rice, Cd concentrations obtained with granulated (SSP + KCl) were significantly higher than that with bulk-blended (SSP) + (KCl).

Synthesis, Characterization, and Agronomic Evaluation of Iron Phosphate Impurities in Superphosphates

Two of the most common impurities found in superphosphates (single superphosphate [SSP] and triple superphosphate [TSP]) in the forms of Fe3KH8(PO4)6·6H2O and Fe3KH14(PO4)8·4H2O were synthesized (H8‐syn and H14‐syn, respectively), characterized, and agronomically evaluated to investigate cost‐effective means to optimize the utilization of phosphate rocks (PRs) containing Fe impurities. A solubility study showed that more P was released from both compounds as pH increased in the 0.01 M KCl solutions (pH 3.0–7.5) and more P was released from H14‐syn than H8‐syn. The two Fe‐K‐P compounds were mixed and compacted with monocalcium phosphate (MCP) at 0, 25, 50, 75, and 100% of total P as MCP. In a greenhouse study, rates of P were applied at 0, 10, 20, 40 and 80 mg P kg−1 from H8‐syn, H14‐syn, and MCP, while the compacted mixtures were applied only at 40 mg P kg−1 to an Ultisol (thermic Rhodic Kanhapludults, pH 5.3) cropped with upland and flooded rice (Oryza sativa L.) for 65 d. The results showed that P uptake and dry‐matter yield were greater with H14‐syn than H8‐syn for both crops and both compounds were more effective for flooded rice than upland rice. The calculated values of relative agronomic effectiveness (RAE) of H8‐syn and H14‐syn with respect to MCP were 32 and 72% in dry‐matter yield for upland rice and 55 and 102% for flooded rice, respectively. To reach 90% of maximum dry‐matter yield obtained with MCP, it required approximately 43 and 35% of total P as water‐soluble P (WSP) in the mixtures of H8‐syn and H14‐syn with MCP for upland rice and only 17 and 11% for flooded rice, respectively.

Evaluation of Rape Response to Different Sources of Phosphate Rock in an Alkaline Soil

A greenhouse experiment was conducted to evaluate the agronomic effectiveness of one unground, as received from Tunisia, and eight finely ground phosphate rock (PR) sources from Syria, Kazakhstan, Jordan, Russia, Mali, Burkina Faso, Tanzania, and China as compared with TSP for rape grown on an alkaline soil (pH 7.7). The P rates applied were 0, 5, 10, 25, and 50 mg P kg−1. The rape was grown to maturity. The yield of rapeseed obtained with each P source was fitted to a linear function with P rate. The relative agronomic effectiveness (RAE) of each PR with respect to TSP was calculated as the ratio of the slopes of the response function. The results show that RAE ranged from 0 to 88%, depending on the reactivity of PR. RAE correlated highly with the solubility of all PR sources as measured by 2% citric acid (CA) (R2=0.947) in a semi-log function. When the unground, highly reactive PR was excluded in the correlation, R2 was increased to 0.957 with 2% CA. The correlation was also very high (R2=0.963) when the RAE of the unground PR was plotted against its solubility measured in the finely ground form along with other finely ground PR sources. Low-reactivity PR with 2% CA-soluble P as low as 2.5% P can be 50% as effective as TSP in increasing yield of rapeseed even in alkaline soils. Use of indigenous PR sources, especially high reactive, for rapeseed production is feasible in those countries where agricultural soils are alkaline.

Modal Analysis to Estimate the Composition of Single Superphosphates

Better knowledge of the composition of acidulated phosphorus (P) fertilizers will provide useful information to properly manage these P sources. Single superphosphate is one of the major P sources utilized by some countries to increase the amounts of bioavailable P to commercial crops. This study was conducted to estimate the composition of single superphosphates produced from gross, fine and refloatable Araxá phosphate rock (SSP 1, SSP 2, and SSP 3, respectively) and also one produced from Patos de Minas phosphate rock (SSP 4). The original fertilizer and its water-insoluble residue (WIR) were analyzed for P, silicon (Si), iron (Fe), aluminum (Al), calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), titanium (Ti), barium (Ba), strontium (Sr), zinc (Zn), fluorine (F), free water, water of hydration and citrate insoluble P. Modal analysis was used to estimate the composition of the SSP products. This analysis was based on 1) qualitative information on the type of compounds provided by x-ray diffraction, 2) quantitative chemical information on total elemental content, 3) solubility index of the compounds, and 4) chemical composition of the compounds or minerals. The modal analysis, proposed in this study, was adequate to estimate the composition of the four SSP fertilizers. The results indicated that Fe3(K,Na,H)H8(PO4)6·6H2O was the main water insoluble phosphate impurity in the three SSP from Araxá PR, with an increase of this compound in the order SSP 1<SSP 2<SSP 3 and a decrease of Ca(H2PO4)2·H2O in the order SSP 1>SSP 2>SSP 3. CaAlH(HPO4)2-F2·2H2O was present in the SSP 4 in a significant amount. Results of the modal analysis can help to further understand the agronomic effectiveness of SSP fertilizers in experiments conducted in different crop systems.

Effectiveness of the water-insoluble component of triple superphosphate for yield and phosphorus uptake by plants

The water-insoluble phosphorus (P) component (‘residue source’) was separated from four commercial triple superphosphate (TSP) products and its agronomic effectiveness evaluated. Two of the TSP products were sourced from the USA and two from the UK. Effectiveness was measured using a pot trial technique with ryegrass as the test crop. The residue sources were evaluated on 13 soils varying in key properties: readily plant-available P, texture, pH and organic matter content. Four of the soils were from Germany, one from Spain, five from the UK and three from France. Grass dry-matter yield and P offtake were measured at every cut on every soil. For both of these variables, significant responses to applied P were obtained on all soils. Mono-calcium phosphate (MCP) was used as the comparative P source and effectiveness of the residue sources was estimated relative to that of MCP. Using dry-matter data, the residue sources were 44–87% as effective as MCP; P offtake data provided corresponding estimates of 35–79%. Both methods of estimation indicated the same ranking of residue sources in terms of relative effectiveness, those derived from USA products being more effective than those derived from UK products. The ranking was the same for all soils. The effectiveness of the residue sources relative to that of MCP appeared little affected by measured soil properties: texture, pH, available P or organic carbon content. The results indicate there is little difference in agronomic effectiveness of TSP products with water solubility greater than 85%.

Characterization and Agronomic Evaluation of Single Superphosphates Varying in Iron Phosphate Impurities

An increase in the concentrations of water‐insoluble iron phosphate (Fe‐P) compounds in acidulated P fertilizers has raised the concern that the agronomic effectiveness of these P fertilizers will decrease. This study was conducted to evaluate the agronomic effectiveness of three sources of single superphosphate (SSP) varying in Fe‐P impurities for upland and flooded rice (Oryza sativa L.) grown for 55 d. A modal chemical analysis and X‐ray diffraction were used to characterize the SSP samples and their water‐leached forms. A greenhouse study was conducted where rates of 0, 5, 15, 30, 50, and 100 mg P kg−1 as total available P (water plus citrate‐soluble P) were applied from each SSP source and monocalcium phosphate (MCP) to a Hiwassee clay loam (thermic Rhodic Kanhapludult). The water‐soluble P contents of the SSP sources were 46, 80, and 86% of the total available P (water + citrate) corresponding to an increase of Fe content (2, 4, and 7%) in the phosphate rock sources used for SSP production. The main Fe‐P impurities in the SSP samples were identified as Fe3NaH8(PO4)6·6H2O and Fe3H9(PO4)6·6H2O. Single superphosphate with only 46% of water solubility was 91% as effective as MCP in increasing dry matter yield and 76% as effective for P uptake by upland rice. The other two SSP sources were as good as MCP in effectiveness for upland rice. All of the SSP sources were equally as effective as MCP in producing dry matter yield and P uptake by flooded rice.

Beneficiation of rock phosphate fertilisers by mechano-milling

Mechano-milling is a process where materials are ball-milled at high energy to induce chemical and physical reactions. This study investigated the effect of milling on the properties and agronomic effectiveness of six apatite rock phosphates (RP). The effects of milling were evaluated by X-ray diffraction, BET-N2 surface area measurements, electron microscopy and solubility in 2% citric acid. Milling increased the solubility of RPs by increasing the proportion of X-ray amorphous material and reducing the size of remaining apatite crystals. However, milling also caused agglomeration of particles, which reduced the surface area. Milling increased the unit-cell a dimension of apatite, possibly due in part to the formation of low reactive fluorapatite such as occurs during calcination. These changes induced by mechano-milling might be expected to reduce the solubility of RPs; however, solubility increased because amorphisation was dominant. The fertiliser relative effectiveness of RPs based on phosphorus content of wheat plants was increased by a factor of up to three by milling. It is concluded that beneficiation of apatitic RPs by mechano-milling may greatly improve their agronomic effectiveness and milling may provide an economically and environmentally superior option for the manufacture of phosphate fertilisers and enable the utilisation of impure RPs.

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.

Effect of water and citrate solubility on agronomic effectiveness of acidulated phosphates in three consecutive corn crops

In the process of phosphate rock acidulation, several impure P compounds may be formed along with the desirable Ca and NH4 phosphates. Such compounds normally reduce the content of water-soluble P and thus the agronomic effectiveness of commercial fertilizers. In order to study this problem, a greenhouse experiment consisting of three consecutive corn crops was conducted in samples of a Red-Yellow Latosol (Typical Hapludox) in a completely randomized design (6 x 2 x 2), with four replicates. Six commercial fertilizers were added to 2 kg of soil at a rate of 70 mg kg-1 P, based on the content of soluble P in neutral ammonium citrate plus water (NAC + H2O) of the fertilizers. Fertilizer application occurred either in the original form or leached to remove the water-soluble fraction, either by mixing the fertilizer with the whole soil in the pots or with only 1 % of its volume. The corn plants were harvested 40 days after emergence to determine the shoot dry matter and accumulated P. For the first crop and localized application, the elimination of water-soluble P from the original fertilizers resulted in less bioavailable P for the plants. For the second and third crops, the effects of P source, leaching and application methods were not as evident as for the first, suggesting that the tested P sources may have similar efficiencies when considering successive cropping. The conclusion was drawn that the water-insoluble but NAC-soluble fractions of commercial P fertilizers are not necessarily inert because they can provide P in the long run.

Comparison of phosphate fertilizers for pasture and their effect on soil solution phosphate

A field experiment was conducted over a period of six years to evaluate the agronomic effectiveness on permanent pasture of (i) an unground North Carolina phosphate rock (PR), and (ii) the PR treated with elemental sulfur (S) and Thiobacillus spp. bacteria (PR/S). Single superphosphate (SSP) was used as the reference fertilizer. Changes in soil solution phosphate (Pi) with time at 0–20 mm, 20–40 mm, and 40–75 mm depths, as effected by fertilizer treatments at different rates, were also determined. Application of PR/S gave the same dry matter yield as SSP from the first year onwards whereas PR gave less dry matter yield than SSP in the first 3 years and then again in year 5. The differences between fertilizers were not significant in years 4 and 6. Phosphate concentration in soil solution in the control treatment was greater in summer than in spring as a result of low pasture growth in summer, which was due to low rainfall. On application of SSP, Pi concentration in solution increased with increasing rate of application and the values rapidly declined with time. At 30 kg P ha−1 rate, Pi in solution returned to similar concentration to that of control in about 3 to 6 months after application each year and there appeared to be no build up in soil labile phosphorus (P) (soil P in equilibration with solution P), as indicated by solution Pi, with time after 6 annual applications. At greater rates of SSP application there was evidence of accumulation of labile P as indicated by increased solution Pi concentrations. Application of PR increased solution Pi concentration up to 90 kg P ha−1 rate and the value marginally decreased at 120 kg P ha−1 rate. Maximum Pi concentration was observed every year about 6–10 months after PR application. Application of SSP at 60 kg P ha−1 or greater rate resulted in enhanced solution Pi up to 75 mm depth of soil sampling. Application of PR at equivalent rates of P did not significantly increase solution Pi below 20 mm depth. It was concluded that for permanent pastures PR/S is as effective as SSP from the first year of application whereas PR from the fourth to sixth year.

Comparison of greenhouse and 32 P 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 (CP 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.

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.

Field evaluation of partially acidulated phosphate rocks in a Ferralsol from Cuba

Phosphorus (P) is needed in large areas of developing countries toimprove soil fertility for crop production. The use of phosphate rock (PR) isan alternative to costly soluble P fertilizers, but it is ineffective usuallyin non-acid soils unless it is modified i.e. partially acidulated (PAPR). Alaboratory incubation study using the isotopic exchange kinetic method of32P and field experiments were undertaken on a neutral Ferralsol ofCuba to evaluate the effectiveness of PAPRs as fertilizers for common bean(Phaseolus vulgaris, L.). Sulfuric-acid based PAPR using40%, 50% and 60% of the acid required to produce singlesuperphosphate were studied. In the laboratory experiment Trinidad de GuedesPAPR was effective in providing P to the soil, through increases inisotopicallyexchangeable P and the percentage of P derived from fertilizer (%Pdff). In the three field experiments carried out to compare the P sources,yields of common bean were increased by PAPR, though the response was less thanwith triple superphosphate (TSP). The relative agronomic effectiveness (RAE) ofPAPR was greater than that of unacidulated PR. Taking into account the RAEvalues and the current cost of the P sources, the choice of Trinidad de GuedesPAPR instead of TSP could be economic, although the RAE value for PAPR waslowerthan that of TSP. This result indicates that PAPR could be used in thesoil understudy to obtain the best economic return. DM yield, P uptake and grain yield ofcommon bean were significantly increased by applying P as 50% PAPR. Lowcost improvement of the agronomic value of PR can be achieved by partialacidulation, so this modification of the phosphate rock show promise forutilization of PR reserves indigenous to developing countries.