Effect of organic and inorganic phosphorus sources on maize yields in an acid soil in western Kenya

Maize production in western Kenya is commonly limited by P deficiencies and aluminum phytotoxicity. Due to high costs of imported fertilizers and lime, focus is now shifting to solutions that utilize local resources. We tested the effect of three inorganic P sources i.e., triple superphosphate (TSP), Minjingu phosphate rock (MPR) and Busumbu phosphate rock (BPR), each applied in combination with two organic materials (OMs) i.e., farmyard manure (FYM) and Tithonia diversifolia green manure (tithonia), or with urea on soil chemical properties related to soil acidity, P availability and maize yields for three consecutive seasons in western Kenya. The OMs and inorganic P sources were applied to provide 20 and 40 kg P ha⁻¹ respectively in their combination. Where urea was used, the inorganic P sources were applied at 60 kg P ha⁻¹. Maize did not respond to application of TSP, MPR or BPR with urea in the first two seasons. However, after three seasons, maize significantly responded to application of MPR with urea. FYM was more effective than tithonia in increasing the labile inorganic P pools but it gave lower maize yields than tithonia which was more effective in reducing the exchangeable Al. It appears that the ability of an OM to lower the exchangeable Al is more important in increasing maize yields than its ability to increase P availability. The effectiveness of the inorganic P sources in increasing maize yields followed the order of their effectiveness in increasing available P, i.e., TSP > MPR > BPR, once Al phytotoxicity was reduced by application of tithonia but the difference between TSP and MPR was not significant. The extra maize yield obtained by the additional 40 kg P ha⁻¹ from the inorganic P sources was, however, in most cases not substantial enough to justify their use. Economic considerations may therefore favour the use of tithonia or FYM when applied alone at 20 kg P ha⁻¹ than when combined with any of the inorganic P sources used in this study at a total P rate of 60 kg ha⁻¹.     

Extractable Bray-1 phosphorus and crop yields as influenced by addition of phosphatic fertilizers of various solubilities integrated with manure in an acid soil

Soil extractable Bray-1 P (B1P) and response to phosphate (P) of Setaria anceps cv. Kazungula (Setaria grass) were monitored in a field trial bimonthly for 14 months in an acid soil fertilized with triple super phosphate (TSP), Gafsa phosphate rock (GPR) or Christmas Island phosphate rock (CIPR) integrated with or without manure. Extractable B1P from the same soil incubated with the same fertilizers in wet and dry 3-day cycles for 91 days was determined. Field experimental design was randomized complete block (RCB) with three replications. Results indicated that B1P magnitude for field and incubation trial were; TSP > GPR > CIPR, consistent with their solubility. An integration of manure and fertilizers resulted in much higher extractable B1P than sole fertilizers or manure. Over time, P availability decreased at a fast rate for the first 6 months and later was relatively constant. The dry matter yields (DMYs) exhibited quadratic relationships with P rates. Maximum DMYs (6–11 t ha⁻¹) were attained between 100 and 200 kg P ha⁻¹, above which they declined. Average DMYs were not significantly different for TSP, GPR and CIPR (6.1–6.6 t ha⁻¹). Maximum individual DMY were attained at 2–6 months and then declined to a minimum (2–4 t ha⁻¹) after 1 year. Cumulative yields (20–55 t ha⁻¹) also were not significantly different for the three fertilizers. Manure-CIPR integration increased DMY whilst in GPR and TSP/manure combinations DMYs were depressed. The PRs could supplement the expensive TSP without loss of yields but the non-reactive PR should be integrated with manure.     

Applications of poultry litter and triple superphosphate fertilizer to a sandy soil: effects on soil phosphorus status and profile distribution

To determine P loadings, added through poultry litter, sufficient to cause downward movement of P from the cultivated layer of a sandy soil, six rates of poultry litter were applied annually for four years to a site in central England. (total loading 0 – 1119 kg P ha^-1). A single extra plot also received an extra 1000 kg ha^-1 as triple superphosphate (TSP; total loading 2119 kg P ha^-1) and three other treatments received 200 – 800 kg ha^-1 P as TSP only. Annual soil sampling in 30-cm increments to 1.5-m depth provided information on P build-up in the topsoil and P movement to depth. There were strong linear trends between P balance (P applied – P removed in crops) and total P, Olsen bicarbonate extractable P and water-soluble P in the topsoil. Phosphorus from TSP and poultry litter fell on the same regression lines, suggesting that both would be equally effective as fertilizer sources. We calculated that 100 kg ha^-1 surplus total P would increase the Olsen extractable P content by c. 6 mg kg^-1 and the water-soluble P by c. 5 mg kg^-1. Thus, relatively large amounts of P would need to be applied to raise soil P status. We found some evidence of P movement into the soil layers immediately below cultivation depth. However, neither soil sampling nor soil solution extracted through Teflon water samplers showed evidence of movement into the deep subsoil (1 m) despite large P loadings.     

Biomass ashes and their phosphorus fertilizing effect on different crops

The reutilization of biomass ashes in agriculture is an important issue to create nutrient cycles and to save fertilizer. To analyse the P fertilization effect of crop biomass ashes (rape meal ash (RMA), straw ash (SA), and cereal ash (CA)) in interaction with different crops, two pot experiments with a poor loamy sand deficient in P were carried out. Besides the three ash treatments, other treatments included triple superphosphate (TSP) as a high soluble P source, potassium chloride (KCl) as a high soluble K source, and a control (CON) without P and K. The main crops (maize, lupin, summer barley, and oilseed rape) were cultivated in the first experiment from April to May and the catch crops (oil radish, phacelia, italian ryegrass, and buckwheat) were cultivated in the second experiment from August to September. Plant parameters (biomass and P uptake of shoots), soil pH, different P pools of the soil (total P (Pt), water soluble P (Pw), double lactate soluble P (Pdl), oxalate soluble P (Pox)), P sorption capacity (PSC), and the degree of P saturation (DPS) were investigated. The fertilization effect of biomass ashes was comparable with that of TSP. On average of all crops, the highest P uptake (86.7 mg pot⁻¹) was found after RMA application, and the lowest P uptake (66.6 mg pot⁻¹) for CON. The readily bio-available soil P contents (Pw and Pdl) were significantly increased when P was supplied, regardless of whether P was given with ash or with high soluble TSP. The P fertilization effects also depended on the cultivated crops. The ash treatments resulted in highest increases of soil Pw values when combined with buckwheat cultivation. After buckwheat harvest the Pw content in the control was 8.0 mg kg⁻¹, and in the ash treatments between 13.9 mg kg⁻¹ (CA) and 15.7 mg kg⁻¹ (RMA). From the results of this study we conclude, that crop biomass ashes can be an adequate P source comparable to that of highly soluble commercial P fertilizer.     

Combining Tithonia diversifolia and minjingu phosphate rock for improvement of P availability and maize grain yields on a chromic acrisol in Morogoro,Tanzania

A 2-year field experiment was conducted to evaluate the effects of Tithonia diversifolia green manure combined with either Minjingu phosphate rock (MPR) or triple super phosphate (TSP) on soil chemical properties that influence P availability, P pools and maize grain yields, on a Chromic Acrisol in Morogoro, Tanzania. Leafy biomass of tithonia was applied before maize planting for two consecutive growing seasons. Treatments compared were the control, MPR and TSP each at 80 kg P ha⁻¹; tithonia alone at 2.5, 5.0, and 7.5 Mg ha⁻¹ dry matter and tithonia combined with MPR or TSP at 40 kg P ha⁻¹. Tithonia led to significant increases in soil pH, exchangeable Ca, labile (resin and NaHCO₃-Pi), and moderately labile inorganic P (NaOH-Pi). It reduced exchangeable Al and P sorption. Application of MPR alone had liming effects and resulted in increase in labile P. Combining tithonia with MPR had similar but more intense effects. Triple superphosphate alone led to acidification and this was reversed when TSP was co-applied with tithonia. Increasing the application rates of tithonia either alone or in combination with TSP or MPR led to more pronounced liming effects but the differences between 2.5 and 5.0 Mg tithonia ha⁻¹ were not significant due to moisture stress that was experienced during the season. The P and Ca concentrations of the maize plants at tasselling increased with the application of tithonia alone or combined with MPR or TSP, and were significantly correlated with maize grain yields (r = 0.75 and 0.64 for MPR and TSP, respectively). Tithonia added consecutively for 2 years increased total maize grain yields by 70% compared to that in the control. The relative agronomic effectiveness (RAE) of MPR increased from 46% in the first year of application to > 142% in the second year, indicating that the initially slow dissolution of MPR improved by combined application of tithonia and MPR, attributed to reduction of P sorption. It is concluded that tithonia can enhance P availability from the Chromic Acrisol through modification of soil properties associated with P transformation and availability. In cases where tithonia is found within the farmers’ fields its combined application with MPR can increase maize yields at a much-reduced cost associated with tithonia procurement.     

Rape genotypic differences in P uptake and utilization from phosphate rocks in an Andisol of Chile

A main constraint to agricultural productivity in the southern regionsof Chile is the low available soil P exacerbated by the high P sorptioncapacityof the predominant Andisols. Therefore, substantial amounts of P fertilizersmust be applied to obtain optimum growth and crop yields. One cost-effectivestrategy followed to supply P to crops grown in these soils is the directapplication of the local Bahia Inglesa PR source. However, a more sustainablestrategy would be to combine the use of the local PR with the crop species andcultivars that are able to grow in these acid soils and can utilize efficientlyPR. Rape is reported to be very efficient in utilising P from PR sources due toits capacity to exude organic acids to the rhizosphere. Therefore, the presentstudy was conducted to evaluate the ability of five rape cultivars grown in anAndisol of southern Chile in utilising P from two PR sources (Bahia Inglesa andBayovar) and triple superphosphate, a water-soluble P fertilizer. It was foundthat rape was able to absorb significant amounts of P from the PR sources andmuch less from the TSP and soil P. Both Bahia Inglesa and Bayovar PRs werefoundto be as effective as TSP for the rape genotypes in the Andisol Pemehue. Theuseof the ³²P isotope technique enabled to assess the ability of thegenotypes tested to utilize P from the different P fertilizers applied. Thegenotypes G2 and G3 showed increased P acquisition from the PR than thegenotypeG5. Combined utilization of P efficient genotypes and direct application of theBahia Inglesa PR seems to be a promising technology for attaining sustainableagricultural productivity in the Andisols of Chile. Further field trials forvalidating these findings at the level of cropping systems are needed. Thisagronomic testing should be accompanied by in-depth studies to assess therelative importance of the morphological and physiological traits determining ahigher P efficiency.     

Hard Minjingu Phosphate Rock: an Alternative P Source for Maize Production on Acid Soils in Tanzania

The phosphate rock (PR) deposit at Minjingu in northern Tanzania consists of two forms, locally called hard Minjingu phosphate rock, MPR (4.8 × 10⁶ Mg with 10.6% P) and soft MPR (3.3 × 10⁶ Mg with 13.3% P). Extensive chemical and instrumental analyses have shown that the two MPRs differ mainly in consistency, while their reactivities are comparable. Soft MPR in direct application has been extensively evaluated with good results under greenhouse and field conditions whereas hard MPR has only been tested in a greenhouse experiment. The agronomic value of directly applied hard MPR on maize growth was therefore tested under field conditions on four acidic soils low in available Ca and P at Magadu (Ultisol), Mlingano (Oxisol), Nkundi (Ultisol) and Sasanda (Andisol). The treatments tested were hard MPR, soft MPR, triple superphosphate (TSP) and a control. Each P source was applied at a rate of 80 kg ha⁻¹ P at Magadu, Mlingano and Nkundi but 160 kg ha⁻¹ P at Sasanda. Other nutrient deficiencies were corrected in each soil with appropriate fertilizers that were applied in all three years of experimentation. The soils gave significant positive responses to application of the three P sources but TSP application resulted in significantly higher P concentrations in leaves and grain yields than MPR addition in the first year. However, in the second and third years the performance of MPRs approached that of TSP and the relative agronomic effectiveness (RAE) of MPRs increased from 50 – 70% in the first year to 80 – 95% in year three. Moreover, crop performances on hard MPR-treated plots and soft MPR-treated plots were not significantly different. This important result suggests that hard MPR can replace soft MPR and even TSP on acidic soils low in available Ca and P. However, more testing is needed to ensure confident delineation of soils that respond to direct application of hard MPR from non-responding soils. The rather poor performance of MPR for the first year must also be improved.     

Effect of organic and inorganic phosphate fertilizers and their combination on maize yield and phosphorus availability in a Yellow Earth in Myanmar

Phosphorus (P) deficiency is a major constraint for crop production in many parts of the world including Myanmar and field research into management of P fertilizers and P responsiveness of crops on infertile soils has been limited. The purpose of this study is to determine maize yield response to different forms of P fertilizers on an acidic (pH 4.9) P deficient (Olsen-P 8 mg kg⁻¹) Yellow Earth (Acrisol) in Southern Shan State, Myanmar and to establish relationships between soil Olsen-P test values (0.5 M sodium bicarbonate extracted P) and maize yield. Field experiments were conducted during two cropping seasons. There were 15 treatments in total: P was applied at seven rates of a soluble P fertilizer as Triple superphosphate (TSP) (0–120 kg P ha⁻¹) to establish a P response curve; one rate of a partially soluble P fertilizer (Chinese partially acidulated phosphate rock, CPAPR) and two organic P fertilizers (farmyard manure (FYM) and Tithonia diversifolia) at 20 kg P ha⁻¹; combination of TSP and CPAPR at 20 kg P ha⁻¹ with FYM and Tithonia at 20 kg P ha⁻¹; an additional treatment (TSP 20 kg P ha⁻¹ plus 2.5 t ha⁻¹ dolomite) for assessing the liming effect of a local dolomite. In Year 1, applications of TSP at 40–60 kg P ha⁻¹ produced near maximum grain yields, whereas in Year 2 this could be achieved with a reapplication of 20–30 kg P ha⁻¹ on top of the residual value of the Year 1 application. In both years, CPAPR, TSP and Tithonia at 20 kg P ha⁻¹ significantly increased maize grain yield, but FYM failed to increase grain yield. In Year 1, CPAPR and TSP effects on grain yield were higher than that of Tithonia but in Year 2 the effects were same for all these three treatments. In both years the combination of FYM (20 kg P ha⁻¹) with TSP (20 kg P ha⁻¹) produced significantly higher grain yield than TSP at 20 kg P ha⁻¹ whereas 40 kg P ha⁻¹ of TSP application did not significantly increase grain yield over the TSP application at 20 kg P ha⁻¹. Similar results were obtained when half the P applied as CPAPR was substituted with P from Tithonia and FMP during the first year. The combined data from the two years experiment suggests that 90% of maximum maize grain yields can be obtained by raising the Olsen-P to 30–35 mg P ha⁻¹ soil at the silking stage of growth. Olsen-P for the treatments at silking in Year 1 was: Control < FYM, Tithonia < TSP, CPAPR and in Year 2 was: Control < FYM < Tithonia < TSP, CPAPR. The results showed that for a long-term approach, repeated annual applications of Tithonia can be considered as a potential P source for improving soil P status in P deficient Yellow Earths.     

Cadmium concentration in vegetable crops grown in a sandy soil as affected by Cd levels in fertilizer and soil pH

Field trials were conducted over a three-year period with chinese cabbage (Brassica pekinensis Rupr.) and carrots (Daucus carota L.) grown in a sandy soil with pH adjusted to 5.5 and 6.5. The NPK fertilizers containing 1, 30, 90, and 400 mg Cd kg^–1 P were applied at the rate of 0.07, 2.1, 6.3 and 28 g Cd ha^–1 yr^–1. The amounts of Cd added through phosphate rock also ranged between 0.1 and 28 g ha^–1 yr^–1. The increased Cd application rates through NPK fertilizers increased the Cd concentration in both vegetables but the differences among treatments were not found to be significant. The Cd uptake by both crops was significantly (p<0.01) higher at pH 5.5 than at pH 6.5. Chinese cabbage exhibited lower Cd concentration than carrots. Carrot leaves contained higher Cd than its roots. Cadmium removals by chinese cabbage and carrot were about 0.7 and 1.3 g ha^–1 yr^–1, respectively. At pH 5.5, Cd concentrations in the two crops, based on a three-year average, were 23 and 46% higher than at pH 6.5. Cadmium uptake by chinese cabbage from different sources of phosphate rock was affected to a very limited extent. Cadmium concentration generally increased over the years. Cadmium extracted by ammonium nitrate after harvest of the crops was closely related with soil pH and Cd concentration in the plants.     

Potential losses of fertilizer phosphorus by animal transfer

Pasture production, phosphorus (P) concentration, and P uptake by mixed pasture following addition in the autumn of 50 and 100 kgP ha^–1 as single superphosphate (SSP), triple superphosphate (TSP) and Sechura phosphate rock (SPR), and of 50 kgP ha^–1 of Chatham Rise phosphorite (CRP) were measured for one year on a Wainui silt loam (Typic Dystrochrept) and Tokomaru silt loam (Typic Fragiaqualf). A sharp increase was measured in the P concentration of mixed pasture immediately following the application of 50 and 100 kg P ha^–1 as either SSP or TSP at both sites. However, this increase was not accompanied by an increase in pasture production. In contrast, the application of 50 kgP ha^–1 as either SPR or CRP resulted in only small initial increases in the P concentration of mixed pasture, as did the addition of 100 kgP ha^–1 as SPR at both sites. The potential P losses by animal transfer in dung, which could result from the use of these four P fertilisers, were calculated using a P cycle constructed for intensively grazed, steep hill country pasture. Potential losses of fertilizer P, calculated as a percentage of fertilizer P added, were 7–14% for SSP and TSP, and 4–5% for SPR and CRP in the first year at the two sites. The implications of these results to the efficiency of P fertilizer use are discussed.     

Comparison of efficiency of Mussoorie partially acidulated phosphate rock and single superphosphate in a shallow Alfisol of the Indian semiarid tropics

The agronomic effectiveness of a partially acidulated phosphate rock (PAPR) was measured in a field experiment with sorghum (Sorghum bicolor cv. CSH-6) in a shallow Alfisol at the ICRISAT farm, Patancheru (Hyderabad), India. The experiment compared PAPR with single superphosphate. The PAPR was made by acidulating an indigenous Indian phosphate rock (Mussoorie) with H₂SO₄ at 50% acidulation level. P response was evaluated at a single relatively high N rate (120 kg ha^–1) with five rates of P (0, 2.2, 4.4, 8.8, and 17.6 kg P ha^–1). A significant response to P was obtained at rates up to 17.6kg P ha^–1.There was no significant difference due to source of P in terms of sorghum grain yield or total P uptake. Both Olsen and Bray 1 soil tests underestimated P availability from PAPR with respect to that from SSP.A rapid rate of P uptake was observed during grain filling to maturity (81–102 days), when 40% of the total P was taken by the plant. The internal efficiency of both P sources was the same.     

Effect of phosphate rock sources on biological nitrogen fixation by soybean

Very little information is available concerning the effect of phosphate rock (PR) sources on biological nitrogen fixation (BNF) in legume crops. In a greenhouse study, the¹⁵N isotopic dilution technique was used to compare the effectiveness of three sources of PR (Hahotoe rock, Togo; Tilemsi rock, Mali; and Sechura rock, Peru) with that of triple superphosphate (TSP) in increasing soybean seed yield and the amounts of N fixed by the soybean crop. The acid Hartsells slit loam was limed to pH 5.2 and incubated with 8.5 mg N kg^–1 as K¹⁵NO₃ and sucrose for 2 months prior to planting. Then fertilizer P was incorporated into the soil at 12.5, 25, 50, and 100 mg P kg^–1 rates.The relative agronomic effectiveness (RAE) of the three PRs with respect to TSP (RAE = 100%) in terms of increasing seed yield was Hahotoe rock = 6.0%, Tilemsi rock = 45.9%, and Sechura rock = 75.2%; this trend followed the same trend as PR reactivity, i.e., Sechura rock > Tilemsi rock > Hahotoe rock. BNF was affected significantly by all the P treatments. Of the total N derived from the three N sources (atmosphere, N_dfa; fertilizer K¹⁵NO₃, N_dff; and soil, N_dfs), N_dfa was highest with TSP and lowest with Hahotoe rock, whereas the reverse was found with N_dfs. Among various plant parts, more N_dfa was translocated and stored in seeds than in stems + leaves and roots. The RAE values of the three PRs with respect to TSP (RAE = 100%) in terms of influencing the amount of BNF were Hahotoe rock = 3.0%, Tilemsi rock = 43.4%, and Sechura rock = 71.2%. A linear relationship was found between the amount of BNF by the whole soybean plant and the soybean seed yield.     

Influence of modified forms of urea and nitrogen levels on weed growth and grain yield of lowland rice

The growth of weeds and their subsequent reduction of rice yield as affected by N source neem cake coated urea (NCU), dicyandiamide coated urea (DCU), rock phosphate coated urea (RPCU), urea supergranules (USG) and prilled urea (PU) was studied on a clay loam soil at Coimbatore, India. Experiments were conducted in northeast monsoon (NEM) 1981, summer 1982, and southwest monsoon (SWM) 1982 seasons.The crop was associated with eleven weed species, and the dominant weeds wereEchinochloa crus-galli, Cyperus difformis andMarsilea quadrifolia. The weed flora varied between seasons. Deep placement of USG reduced the dry weight of weeds in NEM and summer seasons at 60, 90 and 120 Kg N ha^–1 whereas it increased the dry weight at 60 and 90 but not 120 Kg N ha^–1 in SWM season. The dry weight of weeds decreased with increased N rates for all N sources during NEM and summer seasons. In SWM season, dry weight of weeds increased with increased N rates for all N sources except USG. The grain yield of rice was drastically reduced with the deep placement of USG at 60 but not 120 Kg N ha^–1 in SWM season. The differential effect of the N sources between seasons was due to the change of the weed flora. Dominance ofE. crus-galli during SWM season had greater influence on weed dry weight and grain yield of rice.Nitrogen uptake by weeds was frequently greater in unfertilized plots, particularly in NEM and summer seasons. In SWM season, the apparent fertilizer N recovery by weeds was high for USG. It decreased from 53% for 60 Kg USG-N ha^–1 to 4% for 120 Kg USG-N ha^–1.Contribution from the part of Ph.D. work of the first author at Department of Agronomy, Tamil Nadu Agricultural University, Coimbatore-641 003, Tamil Nadu, India.     

Comparison of the agronomic effectiveness of a phosphate rock and triple superphosphate as phosphate fertilisers for tea (Camellia sinensis L.) on a strongly acidic Ultisol

Phosphorus deficiency is a major problem affecting tea production in the highly weathered acid soils of humid and sub-humid tropics which are known to have high P fixing capacities. As many of these soils are strongly acidic and receive high rainfall, low-cost phosphate rock (PR) may effectively supply the plant P needs and limited preliminary experiments suggest this is so. A long-term glasshouse trial was conducted on 8-month old tea seedlings to compare the agronomic effectiveness of a locally available PR (Eppawala phosphate rock, EPR) with triple superphosphate (TSP) applied to a strongly acidic (pH water 4.55) marginally P deficient Rhodustult from Sri Lanka at six rates ranging from 10 to 60 kg P ha⁻¹. The results showed that TSP or EPR fertiliser at a rate as low as 10 or 20 kg P ha⁻¹ was sufficient to obtain maximun tea yield. The agronomic effectiveness of EPR was equal to that of TSP at the 5- and 10-month samplings. The concentration of soil P extracted by a cation–anion exchange resin membrane (resin-P) was higher in the TSP treated soil at 5 months due to its greater solubility but at 10 months, the EPR produced higher resin-P due to its increased dissolution over time. In the presence of tea plants, 52% of P from the EPR applied at the rate of 10 kg ha⁻¹, was dissolved at 5 months compared to 75% of dissolution at the 10-month sampling. In the absence of plants, the corresponding dissolution figures were 40% at 5 months and 55% at 10 months. The concentration of inorganic P extracted by 0.1 M NaOH (NaOH-P_i) (loosely characterising Fe + Al bound P) was significantly higher in the TSP treated soil and concentration of P extracted by 0.5 M H₂SO₄ (Ca bound P) was higher in the EPR treated soil. The results suggest that the low-cost, locally available EPR may be used profitably as a maintenace P fertiliser for tea plantations in moderately P deficient soils, which need to be confirmed by field studies. This revised version was published online in June 2006 with corrections to the Cover Date.     

Enhancing the agronomic effectiveness of natural phosphate rock with poultry manure: a way forward to sustainable crop production

Phosphorus inputs are required in highly weathered tropical soils for sustainable crop production. However, high cost and limited access to mineral P fertilizers limit their use by resource-poor farmers in West Africa. Direct application of finely ground phosphate rock (PR) is a promising alternative but low solubility of PR hampers its use. Co-application of PR with manure could be a low cost means of improving the solubility of natural PR and improve their agronomic effectiveness. Our objective was to quantitatively estimate the enhancement effect of poultry manure on P availability from low reactive PR (Togo phosphate rock) applied to highly weathered soils. We utilized two highly weathered soils from Ghana and Brazil for this greenhouse study. Using ³²P isotopic tracers, the agronomic effectiveness of poultry-manure-amended Togo rock phosphate (TPR) was compared with partially acidulated Togo rock phosphate (PAPR) and triple superphosphate (TSP). Four rates of poultry manure: 0, low (30 mg P kg⁻¹ soil), high (60 mg P kg⁻¹ soil) and very high (120 mg P kg⁻¹ soil) were, respectively, added to a constant amendment (60 mg P kg⁻¹ soil) of the P sources and applied to each pot of 4 kg soil. A Randomized Complete Block Design was used for the greenhouse experiment and Maize (Zea mays L.) was used as a test crop. The plants were grown for 42 days after which the above ground biomass was harvested for analysis. Without poultry manure addition, the agronomic effectiveness, represented by the relative agronomic effectiveness (RAE) and proportion of P derived from fertilizer (% Pdff) was in the order TSP > PAPR > TPR = control (P₀). In the presence of low rate poultry manure addition, the agronomic effectiveness followed the order TSP > PAPR = PR > P₀. However, at the high and very high rates of poultry manure addition, no significant differences in agronomic effectiveness were observed among the P sources, suggesting that at this rate of poultry manure addition, PR was equally as effective as TSP. The study showed that direct application of PR co-applied with poultry manure at a 1:1 P ratio will be a viable option for P replenishment. Thus a combination of PR and poultry manure could be a cost-effective means of ensuring sustainable agricultural production in P-deficient, highly weathered tropical soils.     

Effect of phosphorus application in legume cover crop rotation on subsequent maize in the savanna zone of West Africa

The benefit of planted fallow with legume cover crops may be limited on P deficient soil. A trial was conducted at two P deficient sites in northern Nigeria to test the hypothesis that application of P to legume cover crop fallow can substitute for N application to subsequent maize. Mainplots consisted of leguminous fallows followed by unfertilized maize, or native (mostly grass) fallows followed by maize with 0 or 40 kg N ha⁻¹ (Kaduna) and 0, 30 or 60 kg N ha⁻¹ (Bauchi). Three rates of P (0, 9, and 18 kg ha⁻¹) were applied to fallow sub-plots as single superphosphate. In the first year, dry matter accumulation of lablab (Lablab purpureus) responded to P application, while mucuna (Mucuna cochinchinensis) dry matter did not. Lablab mulch dry matter during the dry season was significantly increased by previous season P application while mucuna was not. Previous fallow vegetation was a significant factor for maize growth in the second year but the interaction with P applied to the fallow was not significant at P < 0.05. Substantial and similar yield increases were achieved with application of N fertilizer to maize and from application of 9 kg P ha⁻¹ to previous lablab. Depending on local economic circumstances, a good use of expensive inorganic fertilizer might be to apply P sources to cover crop legumes to profit from additional N benefits as well as residual effects of applied P. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interaction of urea with triple superphosphate in a simulated fertilizer band

Fertilizer nutrient diffusion from fertilizer bands and transformations in soil can affect fertilizer nutrient availability to crops and knowledge of the transformations is necessary for proper management. The interaction of urea and triple superphosphate (TSP) on urea hydrolysis and P transformations during diffusion processes from a fertilizer band was evaluated in a laboratory incubation experiment with two eastern Canadian soils (Ste Rosalie clay, Modifiers Typic Humaquept, pH 5.0; Ormstown silty clay loam, Modifiers Typic Humaquept, pH 6.0). Two fertilizer sources (urea and TSP) and three N and P rates (0, 100 and 200 kg ha^–1) were combined in a factorial arrangement. Fertilizer combinations were placed on segmented soil columns, incubated and segments were analyzed for N and P content. Acidification from dissolution of TSP retarded urea hydrolysis, and curtailed the rise in soil pH surrounding the fertilizer band. Urea hydrolysis caused dissolution of organic matter in soils, which might inhibit precipitation of insoluble phosphates. Banding urea with TSP increased 1M KCl extractable soil P, soil solution P, sorbed P concentration and total P diffused away from the band. Urea decreased 0.01M CaCl₂ extractable P, indicating probable precipitation of calcium phosphates with CaCl₂ extraction. Banding urea with TSP could benefit P diffusion to plant roots in low Ca soils and increase fertilizer P availability.     

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.     

Meat and bone meal as nitrogen and phosphorus fertilizer to cereals and rye grass

Meat and bone meal (MBM) contains appreciable amounts of total nitrogen (~8%), phosphorus (~5%) and calcium (~10%). It may therefore be a useful fertilizer for various crops. This paper shows results from both pot and field experiments on the N and P effects of MBM. In two field experiments with spring wheat, increasing amounts of MBM (500, 1000, 2000 kg MBM ha⁻¹) showed a linear yield increase related to the N-supply. A similar experiment with barley gave positive yield increase for 500 kg MBM ha⁻¹ and no further yield increase for larger amounts of MBM. Supply of extra mineral P gave no yield increase when 500 kg MBM ha⁻¹ or more was applied. Meat and bone meal as P fertilizer was studied in greenhouse experiments using spring barley and rye grass as test crops. N applications were 100 N kg ha⁻¹ to barley and 200 kg N ha⁻¹ to rye grass, either from mineral fertilizer or assuming that 80% of total N in MBM was effective. Four different P deficient soils were given increasing doses of MBM and compared with compound NPK fertilizer 11-5-18, mineral N fertilizer (0 kg P ha⁻¹) and a control (0 kg N ha⁻¹, 0 kg P ha⁻¹). In barley there was no significant yield difference between the NPK treatment and MBM treatment with equal N supply, and both had significant higher yield than the treatment receiving the same amount of mineral N without P-supply. The positive yield response of MBM was even larger in rye grass. Both in barley and rye grass a significant residual effect of P from MBM applied the year before was found when the treatments received the same amount of mineral N fertilizer (0 kg P ha⁻¹). The pot experiments confirmed the assumed N effect of MBM. When MBM is used according to the N demand of the crops, the P supply will be more than sufficient and residual P will be left in the soil. Since a part of this residual P was utilized by the crops of the following year, it is not recommended to apply P-fertilizer the year after MBM application.     

Biological nitrogen fixation potential by soybeans in two low-P soils of southern Cameroon

Biological nitrogen fixation (BNF) potential of 12 soybean genotypes was evaluated in conditions of low and sufficient phosphorus (P) supply in two acid soils of southern Cameroon. The P sources were phosphate rock (PR) and triple superphosphate (TSP). The experiment was carried out during two consecutive years (2001 and 2002) at two locations with different soil types. Shoot dry matter, nodule dry matter, and nitrogen (N) and P uptake were assessed at flowering and the grain yield at maturity. Shoot dry matter, nodule dry matter, N and P uptake, and grain yield varied significantly with site and genotypes (P < 0.05). On Typic Kandiudult soil, nodule dry matter ranged from 0.3 to 99.3 mg plant^?1 and increased significantly with P application (P < 0.05). Total N uptake of soybean ranged from 38.3 to 60.1 kg N ha^?1 on Typic Kandiudult and from 18 to 33 kg N ha^?1 on Rhodic Kandiudult soil. Under P-limiting conditions, BNF ranged from ?5.8 to 16 kg N ha^?1 with significantly higher values for genotype TGm 1511 irrespective of soil type. Genotype TGm 1511 can be considered as an important companion crop for the development of smallholder agriculture in southern Cameroon.