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.     

Application strategies for Sechura phosphate rock use on permanent pasture

At two phosphate (P) responsive sites in hill country the effectiveness of Sechura phosphate rock (SPR) as a direct application P fertilizer for permanent pasture was evaluated. Sechura was applied at two rates, in three different application strategies. The treatments were 16.7 and 50 kgP ha^–1 annually, 25 and 75 kgP ha^–1 biennially, and 50 and 150 kgP ha^–1 triennially giving a total of 50 and 150 kgP ha^–1, respectively, over three years. Single superphosphate (SSP) served as the standard P fertilizer. A comparison was also made between SPR and Chatham Rise phosphorite (CRP), another reactive PR. Total pasture and legume production and P uptake by pasture was measured with all fertilizer treatments over a three year period.In the year of application, SPR was as effective as SSP in stimulating total pasture and legume production and P uptake by pasture. This reflects the very reactive nature of this PR. In the second and third years of measurement, SPR did not show superior residual efffects to SSP. The ability of CRP to stimulate legume growth more than SPR in the second year following application demonstrates the danger of generalizing about the residual effects of reactive PR materials. Of the application strategies evaluated, a biennial appplication of 25 kgP ha^–1 as SPR maintained legume growth at a higher level than a smaller (16.7 kgP ha^–1) annual dressing. The biennial strategy also increased total pasture yield, in addition to legume production to a greater extent in the second and third years than a single (50 kgP ha^–1) triennial application.     

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

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

Effect of fertilizer type,sampling depth,and years on Colwell soil test phosphorus for phosphorus leaching soils

The relationships between (i) soil test phosphorus (P) (Colwell sodium bicarbonate procedure) and the level of P applied (from 0 to 1000 kg total P ha^–1) (relationship 1), and (ii) yield and soil-test P (relationship 2, the soil P test calibration), were measured in two field experiments on very sandy, P-leaching soils in the high rainfall (> 800 mm annual average) areas of south-western Australia. The soils were humic sandy podzols, or haplohumods, comprising 97% sand (20 to 2000 m). The experiments started in April 1984 and were terminated at the end of 1990. Soil-test P, measured on soil samples collected to 5, 10 and 25 cm depth each January in the years after P application, was related to yields of dried clover (Trifolium subterraneum) herbage measured later in each year. The four P fertilizers studied were single superphosphate, coastal superphosphate (made by adding, just before granulation, extra rock phosphate together with elemental sulphur while manufacturing single superphosphate), apatite rock phosphate, and Calciphos.Relationship (1) was adequately described by a linear equation (R² > 0.80, most being > 0.90). The slope coefficient estimates the extractability of P from the soil by the Colwell procedure, and is called extractability. Relationship (2) was adequately described by the Mitscherlich equation (R² > 0.75, most being > 0.90). For relationship (2), use of percentage of the maximum (relative) yield eliminated differences due to different maximum yields and yield responses (maximum yield minus the yield for the nil-P treatment). Soil test P ranged from about 4 to 150 g Pg^–1 soil. Soil test P and extractability were generally higher for samples of the top 5 cm of the soil than the top 25 cm, and were largest for single superphosphate and lowest for apatite rock phosphate. Both extractability (relationship (1)) and the curvature coefficient of the Mitscherlich equation (relationship (2)), differed for different P fertilizers and different soil sample depths. The curvature coefficient also differed for different yield assessments (harvests) in the same or different years. Different soil P test calibrations were required for different P fertilizers, soil sample depths and harvest in the same or different years. It is concluded that soil P testing provides a crude estimate of the current P status of P-leaching soils in Western Australia.     

Tissue testing for phosphorus in dried tops and seed: Effect of fertilizer type,plant species,and time of sampling within a year and in different years

The apparent relationship between yield and phosphorus (P) concentration in dried whole tops or seed was studied in field experiments. Yield and P concentration in tissue were measured at the time of sampling for several harvests within each year to estimate the current P status of the plants. For each sampling time, there were close relationships between yield and P concentration in tissue that gave a good indication of the current P status of the plants. For the same site and plant species, the relationship did not differ for different fertilizer types (superphosphate or rock phosphate) or method of applying the fertilizer. However, the relationship did differ for different harvests within and between years, and differed for different plant species.Tissue test for P values are sometimes used to predict future plant yields. In the experiments, the P concentrations measured in tissue harvested earlier during the growth of the annual plants were related to subsequent plant yields measured later on within each year. The relationship between future plant yields and tissue test for P values differed for different P fertilizer types and for different sampling times, both within and between years. It also differed for different plant species.     

The influence of seasonal conditions,plant species and fertilizer type on the prediction of plant yield using the Colwell bicarbonate soil test for phosphate

Testing for soil phosphate (P) using the Colwell procedure is widely used in south-western Australia to estimate fertilizer applications required for crops and pastures. The relationship between plant yield, expressed as a percentage of the maximum yield, and soil test values is assumed to be constant in different years for the same soil type and plant species. Data from 11 long-term field experiments in south-western Australia show that regardless of whether percentage of maximum or absolute yield is used, the relationship between yield and soil test values is different (1)in different years, for the same site and where the same P fertilizer type has been used. This occurred irrespective of whether the same or different plant species were grown in different years; (2)where different types of P fertilizer had been used, for the same site, same year and same plant species; (3)for different plant species, for the same site, same year, and same type of P fertilizer. We conclude that considerable errors in the recommendation of fertilizer rates may result from the assumption that there is a constant relationship between soil test and yield.     

A comparison of triple superphosphate and Gafsa ground rock phosphate fertilisers as P-sources for grass-clover swards on a poorly-drained acid clay soil

Organic farming practice prohibits the use of triple superphosphate (TSP) as a source of phosphorus. As basic slag is not now generally available, interest is focused on the relative value of ground rock phosphate (GRP). A comparison of TSP and Gafsa GRP was made during 1988–92 as to their ability to increase DM production under cutting from newly sown grass/white clover swards established in 1987 on an acid clay soil in SW England. Averaged over the different P fertiliser inputs and years, the DM yield was 8.0 t ha-1 y-1 (range 6.93–9.81) compared to 6.3t ha-1y-1 (range 6.00–7.71) without added P. Lime was added at either 3 or 6t ha-1 in 1987, and at half these rates in 1990. Whereas the yield improved by 45% with P at the lower rate of lime, it improved only by 12% at the higher rate. When P fertiliser was applied annually at 30 kg ha-1, TSP was superior to GRP, but when applied in one initial dose of 120 kg ha-1, GRP was superior over the subsequent 4 year period. There was no consistent effect of the addition of either P or lime on the clover content of the sward. The alkaline bicarbonate soil test (Olsen P) was a good predictor of available P within a given year; there was a general reduction of P availability over the course of the experiment. Mechanisms to explain the longer term effectiveness of GRP are postulated and discussed. It is concluded that farmers who are limited to using GRP rather than TSP would suffer a yield penalty over the longer term of 11.5%, and that P fertilizer for the ley phase in a rotation should be incorporated in one dose at the outset.     

Investigations on interactions of phosphorus compounds in partially acidulated phosphate rock and fertilizer effectiveness

Khouribga phosphate rock was partially acidulated with 50 and 70% of the required H₂SO₄ for complete acidulation. The unreacted rock residue was isolated by subsequent extractions with water and alkaline ammonium citrate solution. P solubility in 2% formic acid of this residues was reduced as compared to the original Khouribga phosphate rock. This loss in reactivity consistently increased with the degree of acidulation. Plant response to fertilizer application emphasized the negative effect of partial acidulation in an acid soil. Mixtures of superphosphate and phosphate rock were more effective than partially acidulated phosphate rock.Applications of apatitic P did not affect P efficiency on a neutral soil. Differences between mixed and partially acidulated phosphate rock could therefore not be observed. The effectiveness of the products was due to their content of acidulated P.Hydrolysis of monocalciumphosphate caused a further acidulation of the residual apatite in moist incubated granules. The extent of these reactions, however, was too low to improve P efficiency significantly.     

The initial and residual value for subterranean clover of phosphorus from crandallite rock phosphates,apatite rock phosphates and superphosphate

The residual value of phosphorus from superphosphate, crandallite rock phosphate (Christmas Island C-grade ore), 500°C calcined crandallite rock phosphate (Calciphos) and apatite rock phosphate from Queensland, Australia, was measured in a 6 year field experiment sited on lateritic soil in south-western Australia. Different amounts of each fertilizer were applied at the commencement of the experiment, and either left on the soil surface or mixed through the soil by cultivating to a depth of about 10 cm. Dry matter production of subterranean clover measured in spring (August) and bicarbonate-extractable phosphorus determined from soil samples collected in summer (January–February) were used as indicators of fertilizer effectiveness.The effectiveness values calculated for each fertilizer each year were similar for the treatments that were left on the soil surface and those which were mixed through the soil. The effectiveness of both ordinary and triple superphosphate were similar each year. They were the most effective fertilizers for the duration of the experiment. Using pasture yield as an indicator, the effectiveness of the superphosphates decreased by about 50% from year 1 to year 2, and by a further 10% over the remaining 4 years. Using bicarbonate-extracted soil phosphorus the effectiveness of both superphosphates decreased in a more uniform fashion by about 60% from year 2 to year 6. The effectiveness of all the rock phosphate fertilizers was approximately constant through time. As calculated from yield and bicarbonate-soluble phosphorus values, C-grade ore, Calciphos and the Queensland apatite were respectively 5%, 20% and 7% as effective as freshly applied superphosphate.The proportion of the total phosphorus content present in the rock phosphates which was initially soluble in neutral ammonium citrate was a poor predictor of the effectiveness of the phosphorus from these fertilizers determined using herbage yield or the amount of bicarbonate — soluble phosphorus extracted from the soil.The bicarbonate soil test did not predict the same future production for superphosphate and some of the rock phosphates in years 2 and 3 of the experiment, indicating that different soil test calibration curves are needed for the different fertilizers.     

Evaluation of two rock phosphates and a calcined rock phosphate as maintenance fertilizers for crop — pasture rotations in Western Australia

Two long-term (11 and 12 y) field experiments in south-western Australia are described that measured the relative effectiveness of three rock phosphate fertilizers (C-grade ore, Calciphos and Queensland (Duchess) rock phosphate), single, double and triple superphosphate. The experiments were on established subterranean clover (Trifolium subterraneum) — based pasture that had received large, yearly, applications of single superphosphate for many years before the experiments began so that in the first year the nil phosphorus (P) treatment produced 80 to 90% of the maximum yield. The experiments were conducted using a rotation of one year cereal crop (oats,Avena sativa at one site, and barley,Hordeum vulgare, at the other): 2 y pasture, a typical rotation on farms in the region. Five levels of each P fertilizer were applied every third year with the crop. Grain yield of cereals, P content of grain, pasture yield, and bicarbonate-soluble P extracted from the soil (available P) were used to estimate fertilizer effectiveness values.The three superphosphate fertilizers had identical values of fertilizer effectiveness. Superphosphate was always the most effective fertilizer for producing grain. The rock phosphate fertilizers were one-seventh to one-half as effective per kg P as superphosphate when assessed on the yield or P content (P concentration × yield) of grain within each cropping year. Bicarbonate-extractable soil P values demonstrated that superphosphate was two to fifteen times as effective as the rock phosphate fertilizers. The relationship between grain yield and P content in grain (i.e. the internal efficiency of P use curve) was similar for the different P fertilizers. Thus for all P fertilizers yield was not limited by other factors as it varied solely in response to the P content, which in turn presumably depended on the P supply from the fertilizers.The relative agronomic effectiveness of rock phosphates is greater for marginally P deficient soils than for highly P deficient soils but rock phosphate remains less effective than superphosphate. We conclude that the rock phosphates studied should not be substituted for superphosphate as maintenance fertilizers for soils in Western Australia that are marginally deficient in P. This result is consistent with the results of many field experiments on highly P deficient soils in south-western Australia. These have shown that a wide variety of rock phosphate fertilizers are much less effective than superphosphate in both the short and long term.     

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

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

我国低浓度磷肥的发展策略和过磷酸钙生产的技改方向

从我国磷矿资源以中低品位磷矿居多,低浓度磷肥本身含有作物所需中微量元素,低浓度磷肥制造成本低及我国农民长期的使用习惯,论述低浓度磷肥在我国存在的必要性。过磷酸钙企业目前存在规模小、分散、生产技术落后、污染严重、废弃物利用率低等问题,提出过磷酸钙生产的技改方向,如无熟化造粒技术,采用浓酸矿浆法并同时采用集散微机自控技术,主要设备实行定型化、系列化等,以增强企业的竞争力。     

Effect of water supply on the response of wheat and triticale to applications of rock phosphate and superphosphate

The effect of water supply on the response of wheat (Triticum aestivum) and triticale (×Triticosecale) to levels of freshly-applied rock phosphate and superphosphate, and the residues of these fertilizers applied 9 years previously in the field, was studied in three glasshouse experiments. The < 2 mm fraction of the top 10 cm of soil was used (1.8 kg soil per pot), and in one experiment, freshly-applied fertilizer was also added to the more acidic subsoil (10 to 20 cm). There were two water treatments: the soil was returned to field capacity by watering to weight, either daily (W1, adequate water) or weekly (W2, water stress). Yield of dried tops was used to calculate fertilizer effectiveness. The phosphorus (P) concentration in dried tops was used to determine critical P, which is the P concentration related to 90% of the maximum yield. Just before sowing, soil samples were collected to measure bicarbonate-extractable (soil test) P which was related to plant yield.Water stress reduced yields and maximum yield plateaus by 20 to 40%. Water stress reduced the effectiveness of all P fertilizers by between 20 to 60%, largely because of a reduction in the maximum yield potentials. In the field, water supply is seasonally dependent and it can affect the yield response of plants to freshly-applied rock phosphate and superphosphate and the residues of these fertilizers applied to the field in previous years. Relative to placing fertilizer in the topsoil, placing fertilizer in the subsoil improved effectiveness by about 26% for rock phosphate and 12% for superphosphate.The relationship between yield and P concentration in dried tops, and critical P, differed for W1 and W2. The soil test P calibration, which relates yield to soil test P, and the soil test P required to produce the same yield also differed for W1 and W2. Consequently critical P and soil test P calibrations depend on water supply, which in the field varies within and between growing seasons. This is so for freshly- and previously-applied rock phosphate and superphosphate.     

冷冻法硝酸磷肥副产粗硝钙液用于磷矿脱镁的研究

针对冷冻法硝酸磷肥副产粗硝钙液中的磷进入滤渣难以回收利用导致硝酸磷肥装置产能低、胶磷矿富集技术浸取工段氨逃逸率低致使浸取液中硝酸铵含量高造成下游无法使用的问题,开展了粗硝钙液用于磷矿脱镁的研究。实验方法及最优反应条件：原矿经破碎、煅烧（煅烧温度为900~1 100 ℃,煅烧时间为1.5~3.0 h）,用粗硝钙液浸取,利用粗硝钙液的热能和化学反应热控制反应温度为60~70 ℃（不需加热,节省了蒸汽消耗）,反应pH为4.5~6.5,反应过程中无磷析出。在此条件下制取的磷精矿中五氧化二磷质量分数≥35%、氧化镁质量分数≤1.0%,磷回收率为100.49%~100.60%。将制取的磷精矿进行酸解,提高了酸不溶物的分离效率,可满足下游生产白色硝酸磷肥对原料的要求,副产的浸取液可用于生产氨基酸液体肥和中量元素营养母粒。该方法无尾矿产生,绿色环保,为磷矿的高效利用提供了新的研究思路。     

磷酸分解磷矿的化学过程及其模型

采用热法磷酸制成含 2 0 % P2 O5左右的磷酸 ,用以与云南上蒜磷矿进行分解反应。在实验条件下 ,研究了分解产物 Ca(H2 PO4) 2 在磷矿颗粒表面形成固体膜的特性。研究了磷矿粒度、反应时间、反应温度、初始磷酸浓度对磷矿分解率的影响。用扫描电镜观察了磷矿颗粒表面形成的固体膜的形貌特征。用粒径不变的缩芯模型来描述磷酸分解磷矿的过程。推导出了该过程的理论模型和实验数据的回归模型     

Cowpea varietal differences in uptake of phosphorus from Gafsa phosphate rock in a low-P Ultisol

A greenhouse experiment was conducted to evaluate varietal differences in the uptake and availability of P from Gafsa phosphate rock (PR) to five cowpea cultivars grown in a low-P Paleustult soil from Ghana, using the A value technique. The³²P radioisotope used as a tracer was³²P-labelled triple superphosphate (TSP). Each cultivar received sole or a combined application of the two fertilizers (TSP, PR). From these treatments it was possible to estimate for each cultivar, A_R + A_S, A_S and by difference A_R (A_R and A_S stand for A values for phosphate rock and soil, respectively). Using this approach we measured significant genotypic differences in P uptake from PR and A_R values. The ranking of the cultivars in P uptake from PR was the same as for A_R, i.e. Asontem > Vallenga > Soronko > IT81D-1137 > Amantin. Similarly, ranking for uptake from soil P was the same as A_S, i.e. Vallenga, Soronko, IT81D-1137, Asontem and Amantin. Thus although PR could not be labelled directly, using the A value approach it was possible to distinguish between P availability from PR and soil to the plant. The recoveries of applied TSP ranged from 8.0% to 9.4% and those of PR from 2.3% to 3%.The other advantage of the A value method is that it made it possible for the different genotypes tested to be compared directly in units of a standard fertilizer, TSP in this case. Thus for Vallenga in this soil 3.01 kg of P in Gafsa PR was capable of supplying the same amount of P that could be supplied by 1 kg P of TSP; whereas for Amantin a higher amount i.e. 3.5 kg P as Gafsa PR was needed. This information is useful for adjusting application rates to be recommended for different P fertilizer sources in field trials so as to achieve similar effects.     

Comparison of single and coastal superphosphate for subterranean clover on phosphorus leaching soils

Coastal superphosphate, a partially acidulated rock phosphate (PARP), is being considered as an alternative fertilizer to single superphosphate for pastures in high rainfall (> 800 mm annual average) areas of south-western Australia. The effectiveness of single and coastal superphosphate, as P fertilizers, was measured in two field experiments using dry herbage yield of subterranean clover (Trifolium subterraneum). The experiments were started in April 1990 and were terminated at the end of 1993. In the years after P applications, soil samples were collected each January to measure Colwell soil-test P, which was related to plant yields measured later on that year, to provide soil P test calibrations.Relative to freshly-applied single superphosphate, the effectiveness of freshly-applied coastal superphosphate and the residues of previously-applied single and coastal superphosphate were less effective in some years (from 3% as effective to equally effective), and up to 100% more effective in other years. This large range in effectiveness values in different years is attributed to different climatic conditions. Soil P test calibrations were different for soils treated with single or coastal superphosphate. The calibrations were also different for different yield assessments (harvests) in the same year, and in different years. Consequently soil P testing can only provide a very crude estimate of the current P status of the soils.     

Comparison of Olsen and Pi soil tests for evaluating phosphorus bioavailability in a calcareous soil treated with single superphosphate and partially acidulated phosphate rock

The Pi test for phosphorus (P) is a new method in which strips of iron oxide impregnated filter paper are used as a sink to sorb and extract P from a soil solution. In a greenhouse experiment, the Olsen and Pi tests were compared for their effectiveness in evaluating P availability to maize on calcareous soils. Phosphate rock from Togo, partially acidulated with H₂SO₄ at 50% acidulation level (PAPR 50% H₂SO₄) and single superphosphate (SSP) were applied at different rates to a calcareous soil (Vernon Clay, pH 8.2, CaCO₃ 18.9%) which was preincubated with KH₂PO₄ to raise plant-available P to different levels. In soils treated with SSP, dry-matter yield of maize correlated equally well with Pi-P and with Olsen-P (r = 0.96^***). P uptake correlated significantly with P_i-P (r = 0.94^***) as well as Olsen-P (r = 0.97^***). Likewise, in soils fertilized with PAPR, significant correlations were found between dry-matter yield and Pi-P (r = 0.97^***) and between dry-matter yield and Olsen-P (r = 0.94^***). When all the data were pooled, Pi-P and Olsen-P correlated equally well with both dry-matter weight (r = 0.97^***) and P uptake (r = 0.94^***). Phosphorus extracted by the Pi test correlated significantly with P extracted by the Olsen test (r = 0.99^***).     

An assessment of soil tests for phosphate for the prediction of cereal yields on a sandy soil in Western Australia

Nine soil tests for phosphate were evaluated for predicting the yield and P content of wheat, barley and oats grown on a sandy soil in Western Australia: Olsen, modified Olsen 1 (soil:solution ratio 1:5), modified Olsen 2 (soil:solution ratio 1:50), Colwell, Bray 1, Bray 2, modified Bray 2T (shaking time 10 minutes), modified Bray 2C (pH 3.7) and lactate. The soil had been fertilized 5 years previously with 20 levels each of superphosphate (OSP, range 0 to 400 kg P ha^–1) and Queensland rock phosphate (QRP, range 0 to 20 000 kg P ha^–1). For each species and fertilizer taken separately, all the tests, except for lactate, gave a good prediction of yield. When data for OSP and QRP were pooled, Bray 2 and modified Bray 2T tests were unsatisfactory predictors of both yield and P content.A linear relationship (P < 0.05) between mean soil tests value () and the standard deviation ( ) of the test value was observed for each soil test. For QRP, the results for lactate were the most variable (i.e./ was greatest) followed by modified Olsen 2 > Bray 1 > Bray 2 > Olsen > modified Bray 2C > modified Olsen 1 > modified Bray 2T > Colwell. The order for OSP fertilized soil was Bray 1 > modified Bray 2T > Bray 2 > Olsen > Colwell > modified Bray 2C > modified Olsen 1 > lactate > modified Olsen 2. For combined OSP and QRP data, the results of the Olsen 1 and Colwell extractions were the least variable.Errors in the prediction of yield ( _Y ) for all crops resulting from an error in soil test values () were calculated. For OSP-fertilized soil variability in values for the Bray-1 test provided the highest error (about 16%) in the prediction of the yield, followed by Bray 2 (12%) > Bray 2T (10%) > Olsen (8%) > Colwell (7%) > modified Bray 2C (6%) > lactate (4%). Maximum error was at yields of about 65% of maximum yield. For soil fertilized with QRP, lactate provided the highest error (about 10%) in the prediction of yield, followed by the other tests (< 6%). Maximum error was at yields of about 35% of maximum yield.The Colwell soil test gave the most accurate overall prediction of yield for both fertilizers.     

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.