Effectiveness of partially acidulated phosphate rock as a source to plants in calcareous soils

In a series of greenhouse experiments granulated phosphate fertilizers prepared by mixing triple superphosphate with phosphate rock and partially acidulated phosphate rock, ranging in their content of water souble P from 95 to 17 per cent of total P were applied to neutral and slightly alkaline (pH 6.9–7.8), sandy loam to clay soils ranging in calcium carbonate content from 2 to 35 percent. Dry matter yield of clover, alfalfa, millet or maize were obtained, P uptake determined and sodium bicarbonate extractable P in soil measured. In one field experiment triple superphosphate was compared to mixture of triple superphosphate and phosphate rock on maize. X ray difraction on one triple superphosphate — phosphate rock mixture and on one partially acidulated phosphate rock showed that both fertilizers contain mainly monocalcium phosphate and fluorapatite. After incubation in soil the dicalcium phosphate content rose and the monocalcium phosphate disappeared.Parameters received in greenhouse experiments and in the field indicate that phosphate fertilizers composed of superphosphate and up to 50 percent phosphate rock are as efficient source of P to plants on calcareous and slightly alkaline soils as superphosphate. If this indication would be proven in extensive field experimentation it would lead to savings in acid consumption and in fertilizer manufacturing plant capacity for calcareous soils.     

The agronomic effectiveness of reactive rock phosphate,partially acidulated rock phosphate and monocalcium phosphate in soils of different pH

The initial and residual fertilizer effectiveness of North Carolina RP (rock phosphate), monocalcium phosphate and partially acidulated RP (made from North Carolina RP at 30% acidulation), both granulated and non-granulated, were measured in a glasshouse experiment. Triticale (xTriticosecale) was grown for 30 days on a soil that had been adjusted to three pH values (4.2, 5.2 and 6.2). Two crops were grown with a six month interval between crops. The effectiveness of the different fertilizers was compared using relationships between (1) yield of dried tops and the amount of P applied and (2) P content (P concentration in tissue multiplied by yield) and the amount of P applied. For the first crop, relative effectiveness (RE) of the fertilizers was calculated relative to granulated monocalcium phosphate, the most effective fertilizer. Monocalcium phosphate was not applied to the second crop, so relative residual effectiveness (RRE) was estimated for each fertilizer relative to the residual effectiveness of granulated monocalcium phosphate.The relative effectiveness of granulated monocalcium phosphate (band application) was greater (RE = 1.00) than of North Carolina RP (0.01–0.02) and partially acidulated RP (0.45–0.76) for all three soil pH values for the first crop. Granulation and band application increased the effectiveness of monocalcium phosphate and partially acidulated RP, but reduced the effectiveness of North Carolina RP. Both non-granulated monocalcium phosphate and partially acidulated RP were less effective than granulated partially acidulated RP for both crops. For the second crop granulated monocalcium phosphate was most effective and the RRE of non-granulated partially acidulated RP (0.16–0.32) and North Carolina RP (0.19–0.28) was greater than for non-granulated monocalcium phosphate (0.12). For the more acidic soil the RE of non-granulated North Carolina RP was four times higher than for the high pH soil for the first crop and 60% higher for the second crop, but it was still poorly effective relative to granulated monocalcium phosphate. Granulated North Carolina RP was least effective among all the fertilizers for all soil pH values and for both crops.     

An evaluation of the agronomic potential of partially acidulated rock phosphates in calcareous soil

The agronomic potential of four partially acidulated rock phosphates (PARP) made from a moderate reactive phosphate rock at 30 or 60 percent acidulation either by sulfuric acid alone or by combination of sulfuric and phosphoric acids was compared with that of monocalcium phosphate (MCP) and ground rock phosphate (RP) on a calcareous soil (Typic Hapluquent, pH 8.5) in greenhouse. Dry weight and P accumulation of successive cuttings of ryegrass shoots were used to evaluate the relative agronomic potential of these fertilizers. Results indicated that PARPs of higher water-soluble P content had similar immediate effectiveness as MCP at two earlier cuttings, however, they produced significantly less total dry matter than MCP did in overall six successive cuttings. PARPs were constantly inferior to MCP in terms of P uptake by plant in all the six cuttings. When compared to RP, on the other hand, PARPs had markedly higher relative effectiveness. RP itself affected neither the dry matter production nor the P uptake by plant as compared to control treatment.Fractionation of residual inorganic P in the soil samples at two time intervals during plant growth indicated that MCP-P mainly transformed to dicalcium phosphate and octacalcium phosphate, and to a less extent to Fe and Al associated P. These forms of P had significant correlation with P accumulation by plant. Raw RP did not subject to transformation after applied to the soil regardless the duration of culture time. No obvious dissolution of unreacted RP in PARP materials was detected. Plant dry matter production and P uptake were mainly correlated with water-soluble P added with the fertilizers. It is suggested from the experiment that although partial acidulation could substantially improved the effectiveness of rock phosphate and the immediate effect of the fertilizer was competitive with MCP, application of PARP to calcareous soils is only of short-term benefits; in a long run this fertilizer is not considered as a desirable source of P in calcareous soils since the unacidulated part in the fertilizer was unable to be solubilized in the alkaline conditions.     

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.     

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

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

Partial acidulation of an ‘unground’ phosphate rock: II. Plant availability of phosphate

Four greenhouse experiments were conducted using three soils to determine the availability to plants of P from unground North Carolina phosphate rock (PR) treated with 20% to 50% of the H₃PO₄ required for complete acidulation. The influences of soil P retention, P status, the method of preparation of partially acidulated phosphate rocks (PAPRs) and the granule size of the products were investigated. Perennial ryegrass was grown as the test plant for up to 8 months. Triple superphosphate (TSP) was used as the standard fertilizer and unground North Carolina rock was included for comparison.The dry matter yield and P uptake response curves showed that in all experiments PAPRs were markedly superior to the PR. P status of soils appeared to influence the effectiveness of PAPRs to a greater extent than P retention. In soils of low P status the degree of acidulation required for PAPR to be nearly or as effective as TSP was 50% whereas in a soil of high P status even 30% PAPR applied as a maintenance fertilizer was effective. There was a significant positive correlation between water soluble P of fertilizers and P uptake by ryegrass. However, in general PAPRs were more effective per unit of water soluble P than TSP. Granule size (< 1 mm and 1–2 mm) and method of preparation of PAPRs did not alter the effectiveness of PAPRs.     

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.     

Partially acidulated phosphate rocks: Phosphorus release characteristics

Five phosphate rocks varying in formic acid P solubility from 18.9 to 52.7%, expressed as percentage of total P, were acidulated with phosphoric or sulphuric acids to 0, 20%, 33% and 50% of full acidulation and granulated. In an incubation experiment fertilizer granules of diameter 1–2 mm were mixed with two acid soils and after 1 week incubation soil samples were extracted with a 0.5 M NaOH solution. In a dissipation experiment single fertilizer granules of 4 mm diameter were implanted into soil, incubated for 1 and 4 weeks and inorganic P fractionation in the residual granules and the surrounding soil was performed. Dissipated P was greater than the water soluble P content of the partly acidulated phosphate rock fertilizers indicating the dissolution of the non-acidulated phosphate rocks. The amount of P dissipated was related to the initial water soluble P content and to the formic acid solubility of phosphate rocks used for manufacturing the fertilizers. The P dissipated increased with an increase in soil acidity.     

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.     

The agronomic value of co-granulated Christmas Island Grade C phosphate rock and elemental sulphur

Certain low grade phosphate rocks have low agronomic value as direct applied fertilizers and make poor quality superphosphates. With some rocks there is potential to increase their solubility in soils and hence the fertilizer value by mixing with finely divided elemental sulphur (S°). The agronomic value of a prototype low cost, granular P and S fertilizer, Christmas Island Grade C phosphate rock/elemental sulphur (Xmas C/S°), was compared against single superphosphate (SSP) and reactive phosphate rock (RPR)/S° dry blends, North Carolina phosphate rock/S° (NCPR/S°) and NCPR/50% S-super (granular SSP + 44% S°). An eight- month glasshouse trial using ryegrass (Lolium perenne L. cv. Nui), grown in a central yellow-grey earth (Aeric Fragiaqualf), indicated that fertilization with Xmas C/S° produced dry matter yields, between 54 to 73% and 10 to 40% lower than SSP and NCPR/S° blends respectively, and lower plant P and S uptake. Fertilization with Xmas C/S° however produced significantly higher yield and P and S uptake than unfertilized pots and pots receiving Xmas C PR and S° alone. The two RPR/S° dry blends, namely NCPR/S° and NCPR/50% S-super, produced significantly lower yield and P and S uptake than the same two fertilizers granulated with water. The difference in yields and P and S uptake between blends and granulated forms increased with time. The recovery of fertilizer P and S by plants ranged from 3 to 35% and 2 to 45% respectively with the lowest recoveries for Xmas C PR alone and S° alone and the highest for SSP and SSP + S°. During the period of plant growth the percentage of S° oxidised from the S°, Xmas C/S° co-granule, NCPR/S° physical blend and NCPR/S° granular treatments were 37, 32, 32 and 45% respectively. Field evaluations, or use, of the co-granule should consider it's slow P and S release rates.     

Effect of phosphate,calcium, and pH on the dissolution of a phosphate rock in soil

The effect of phosphate (P), calcium (Ca), and pH on the dissolution of Sechura phosphate rock (SPR) in a Typic Dystrochrept was investigated in an incubation study over 90 days. Increasing the P status of the soil had little effect on either the rate or extent of dissolution of SPR, as measured by a single extraction with 0.5M NaOH, or on the amount of plant-available P in the soil as measured by the Bray procedure. This suggests that a reactive PR such as SPR could be effective as a maintenance P fertilizer on soils of medium and high P status.The dissolution of SPR at pH 6.4 was only 4% of total P added, compared to 33.6% of total P added at pH 5.2. Addition of Ca as CaCl₂, at the same rate of Ca as that added in Ca(OH)₂ to raise soil pH from 5.2 to 6.4, reduced the dissolution of SPR to 11% of total P added. The large effect of Ca on the dissolution of SPR contrasts with the small effect of soil P status and probably reflects the fact that the concentration of Ca in the soil solution is generally several orders of magnitude higher than that of P. The results obtained suggest that pH, per se, is of less importance in the dissolution and subsequent plant availability of P from phosphate rock in soil than previously thought.     

Unacidulated and Partially acidulated phosphate rock: Agronomic effectiveness and the rates of dissolution of phosphate rock

The agronomic effectiveness of unground North Carolina phosphate rock (PR) and partially acidulated phosphate rocks (PAPR) prepared by acidulation of the PR with 30%, 40% and 50% of the phosphoric acid needed for complete acidulation, was determined in a 4 year field experiment on permanent pastures. The soil developed from volcanic ash, and was highly P retentive. The rate of dissolution in soil of the PR component in PAPR and of PR applied directly was measured, together with bicarbonate extractable P. The priming effect of the monocalcium phosphate (MCP) component of PAPR on root growth was also investigated.Pasture yields showed that even the 30% acidulated PAPR was as effective as fully acidulated triple superphosphate (TSP), mainly due to the high reactivity of the PR used. The 50% acidulated PAPR tended to be superior to TSP. Soluble P in PAPR caused a marked increase in root proliferation, and dry matter yields were greater than predicted from the amounts of MCP and PR in PAPR. Directly applied PR was inferior to TSP in years 1 and 2 but was equal in year 4. (There was no pasture response to application of P fertilizers in year 3.)Dissolution rates of the PRs were determined applying a cubic model to PR dissolution data. The rate of dissolution increased with increasing acidulation and this is tentatively ascribed to increased root proliferation around PAPR granules and acidification of the clover rhizosphere during nitrogen fixation.     

A comparison of seven soil P tests for plant species with different external P requirements grown on soils containing rock phosphate and superphosphate residues

Seven soil tests for phosphate (P) (Bray 1, Bray 2, Truog, ammonium oxalate, Colwell, iron oxide-strip (Pi) and resin-strip soil tests) were evaluated for predicting the yield of plant species which have very different external P requirements. Two acid, sandy soils that had been fertilized six years previously with superphosphate and three rock phosphates were used. A glasshouse pot experiment with lettuce, wheat and maize was used to calibrate the soil tests.For some soil P tests, different calibrations relating yield to soil P test values were required for each plant species, P fertilizer and soil combination. The Bray 2 and Truog soil P tests were the worst predictors of yield for both soils and all plant species. The Pi and ammonium oxalate tests were the most predictive tests for one soil when data for all fertilizers were considered. The Bray 1 and Colwell soil P tests were the most predictive for the other soil. The resin-strip P test was poorly predictive of yield of lettuce and wheat for both the soils. The accuracy in prediction of yield on the basis of P test value decreased in the sequence maize > wheat > lettuce. This rank is opposite to the increasing external P requirements of these species.     

Influence of pH,time and rate of application on phosphate rock dissolution and availability to pastures

Soil Samples were collected from a field experiment conducted to evaluate the agronomic effectiveness of a reactive phosphate rock (PR), Sechura sand, relative to that of monocalcium phosphate (MCP) at different soil pHs and rates of application. The samples were analysed for P soluble in the soil solution and bicarbonate extractable P. The rate of dissolution of PR was calculated from the data on the fractionation of inorganic P. In MCP plots P in the soil solution decreased sharply with time especially at low pHs and high rates of fertiliser application. In PR plots the concentration remained with time at the same as or a slightly higher level than that was found one month after application. Solution concentration of P was lower at very high rates of PR application than at intermediate rates. In both MCP and PR plots bicarbonate extractable P decreased with increasing pH. Bicarbonate extractable P was linearly related to MCP but not to PR applied. The rate of dissolution and the proportion of PR dissolved decreased with increasing rates of PR application but the amount dissolved increased. Phosphate dissolved at high level of PR application did not seem to enhance proportionately either the concentration of P in soil solution or bicarbonate extractable P.     

The role of soil pH in the dissolution of phosphate rock fertilizers

The influence of soil pH on the dissolution of phosphate rock fertilizers was investigated in laboratory experiments with reactive North Carolina phosphate rock (PR) in a lateritic soil adjusted to several pH values. Increased soil pH resulted in decreased dissolution as estimated by the increase in exchangeable calcium (Ca) method. The extent of PR dissolution was related to soil pH by an equation of the form Log Ca = a–b pH, and it increased with contact period and rate of PR application. Increased plant available P, as estimated by NaHCO₃ soluble-P (BicP) was about one third of the P dissolved from PR. BicP was related to soil pH by an equation of the form Log Bic P = c–d pH. Dissolution of PR in soil can be considered as a simple chemical reaction between apatite and hydrogen ions supplied by soil constituents.     

Phosphate rock and phosphate rock/sulphur granules as phosphate fertilizers and their dissolution in soil

A field trial was conducted for 3 years to evaluate phosphate rocks and phosphate rock/sulphur granules as fertilizers for permanent pastures. Two reactive phosphate rocks, North Carolina (USA) and Chatham Rise (New Zealand), and an unreactive Florida (USA) were used. The materials were applied to a highly phosphate retentive allophanic soil of medium to high phosphorus status. Single superphosphate was employed as the standard fertilizer. The fertilizers were applied at four rates including a control in the first year and again in the third year. The field design enabled measurement of residual effects as well. All the plots received blanket applications of sulphate. The rate of dissolution of phosphate rock was measured by determining soil inorganic phosphate fractions at the highest rate of fertilizer application.The reactive phosphate rocks applied with or without sulphur were as effective as superphosphate in the first and third year of the trial respectively under low and medium responsive conditions. The Florida rock was at the best only 55% as effective as superphosphate. When applied after granulating with sulphur the value increased to 72%. In the second year there was no greater residual effect from the phosphate rocks compared with superphosphate. However, in the third year reactive phosphate rocks gave a slightly greater residual effect; averaged over rates of application the yield increase was 23% over control compared with 18% for superphosphate.The reactive phosphate rocks, applied with or without sulphur, dissolved at the rate of 44% of that added in the first year and 62% of that remaining in the second year. The corresponding values for Florida rock were 27% and 30%, and for Florida with elemental sulphur 35% and 33%. Over 3 years about 96% of the reactive rocks dissolved compared with 56% and 78% in the case of Florida and Florida with sulphur respectively.     

Influence of pH,time and rate of application on phosphate rock dissolution and availability to pastures

The agronomic effectiveness of an unground reactive phosphate rock from Sechura, Peru, was compared with that of monocalcium phosphate in a severely P deficient and highly P retentive soil (vitrandept) over a period of three years. Soil pHs were adjusted to pH 5.1, 5.3, 5.6 and 6.4. The sward consisted mostly of ryegrass (Lolium perenne) and white clover (Trifolium repens). Fertilisers were applied at six rates at pH 5.3 and three rates at other pHs in the first year. For two of the rates fertilisers were reapplied in the second year. Dry matter yields, P uptake and ground cover of clover were determined during the experimental period. In phosphate rock treated plots a negative linear relationship was obtained between soil pH and the logarithm of yield. The agronomic effectiveness of phosphate rock relative to monocalcium phosphate increased with time at all pHs. Calculated at fertiliser rates which produced near maximum yields, relative agronomic effectiveness at soil pHs 5.1, 5.3, 5.6 and 6.4 were respectively 58, 60, 18, and 5 in year one; 118, 125, 77 and 38 in year three. At pH 5.3, as the rate of application increased the relative agronomic effectiveness of the phosphate rock generally decreased in year one but was enhanced in the intermediate rates in years two and three. The data for ground cover of clover gave a similar trend to that for herbage yield and P uptake.     

Effectiveness of different phosphatic fertilizers measured using labelled superphosphate and phosphorus taken up by plants

The agronomic effectiveness of three P fertilizers (diamonium phosphate, rock phosphate and compost) was studied in a greenhouse experiment using wheat. A radioisotopic method, using triple superphosphate labelled with³²P, was used to evaluate the P in dried tops that was derived from i) the soil, ii) labelled superphosphate and iii) the fertilizer being studied.The ratio between P uptake from each fertilizer and P uptake from the soil was used to compare the effectiveness of the different fertilizers. P derived from diammonium phosphate was greater than P derived from the soil, except in one soil. P derived from rock phosphate was always lower than P derived from the soil. The effectiveness of compost depended on soil type. Compost can produce two kind of effects: i) a direct P contribution and ii) an indirect effect improving P uptake from the soil. The radioisotopic method can be used to study the effectiveness of fertilizers even when there are no differences in yield.     

Evaluation of the fertilizer value of some indigenous rock phosphates and basic slag of India

Summary Field and pot culture experiments were conducted in terai acid soil (Haplaquoll) to evaluate the fertilizer value of one basic slag and two rock phosphates, such as Purulia rock phosphate (Igneous) and Mussoorie rock phosphate (sedimentary). In the field experiments two crop sequences were followed: (i) Rice — wheat — greengram (ii) Greengram — rice — wheat. In terms of crop yield and P uptake Purulia rock phosphate did not show any significant effect, except in case of greengram grown as the third crop after its application. Mussoorie rock phosphate increased the yield and P uptake through its direct and residual effect in all the crops, except in rice. Irrespective of crop species and crop sequences basic slag showed considerable direct and residual effect in increasing the crop yield and P uptake. Its effect was at par with that of superphosphate. By total yield increase of three consecutive crops due to added P the efficiencies of the fertilizers were graded as basic slag > superphosphate = Mussoorie rock phosphate > Purulia rock phosphate for rice — wheat — greengram rotation and superphosphate > basic slag > Mussoorie rock phosphate > Purulia rock phosphate for greengram — rice —wheat rotation. Composting improved the efficiency of all the insoluble phosphatic fertilizers.     

国际磷矿工业发展趋势

本文从磷矿资源、磷矿生产、磷矿石的进出口以及世界磷矿石和磷肥的贸易角度,阐明目前磷矿工业的矿肥结合发展方向,提出中国的对策应是限制并停止磷矿石的出口,就地发展自己的磷肥工业,以减少高价的高浓度磷复肥的进口。