Long-term residual value of North Carolina and Queensland rock phosphates compared with triple superphosphate

The effectiveness of large single applications of North Carolina reactive rock phosphate, Queensland non-reactive rock phosphate, and Calciphos, were compared to the effectiveness of superphosphate in field experiments in south-western Australia for up to 11 years after application. As measured using plant yield, superphosphate was the most effective fertilizer in the year of application, and relative to freshly-applied superphosphate, the effectiveness of the superphosphate residues declined to be about 15 to 65% as effective in the year after application, and 5 to 20% as effective 9 to 10 years after application. Relative to freshly-applied superphosphate, all the rock phosphates were 10 to 30% as effective in the year of application, and the residues remained 2 to 20% as effective in the 10 years after application. The bicarbonate soil test reagent predicted a more gradual decrease in effectiveness of superphosphate of up to 70% 10 years after application. For rock phosphate, the reagent predicted effectiveness to be always lower than for superphosphate, being initially 2 to 11% as effective in the year after application, and from 10% to equally as effective 10 years later. Therefore rock phosphates are unlikely to be economic alternatives to superphosphate in the short or long term on most lateritic soils in south-western Australia.     

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.     

Field evaluation of commercial triple superphosphate fertilizers

Field studies were conducted for three years (1987–1989) at two locations to evaluate 4 commercial triple superphosphate (TSP) fertilizers containing various levels of water-soluble P. The fertilizers had been produced from phosphate rock deposits located in Florida, North Carolina and Morocco. AOAC available P was 81 to 94% water-soluble. Water-soluble P was inversely related to the level of Fe and Al in the fertilizers. Phosphorus from each source was applied to a Malbis soil (Plinthic Paleudults) and a Hartsells soil (Typic Hapludults) at rates of 0, 25, 49 and 99 kg ha^–1. Potato (Solanum tuberosum L.) yields were increased by the application of P, except for the Malbis soil in 1988. Yields were not affected by the source of added P on either soil during the three years of the study. Fertilizer performance was not affected by the level of water-soluble P or the content of Fe and Al when band applied to potatoes under field conditions in the Southeastern United States.     

Residual value of superphosphate for oat and barley grown on a very sandy,phosphorus leaching soil

In a field experiment in a Mediterranean climate (474 mm annual rainfall, 325 mm (69%) falling in the May to October growing season) on a deep sandy soil near Kojaneerup, south-western Australia, the residual value of superphosphate was measured relative to freshly-applied superphosphate. The grain yield of five successive crops (1988–1992) was used to measure the residual value: barley (Hordeum vulgare), barley, oat (Avena sativa), lupin (Lupinus angustifolius), and barley. There was no significant yield response to superphosphate applied to the first crop (barley, cv. Moondyne). There were no results for the second crop (barley) due to weeds or the fourth crop (lupin) due to severe wind erosion which damaged the crop. The residual value of superphosphate was measured using grain yields of the third crop (oat, cv. Mortlock) for superphosphate applied one and two years previously, and the fifth crop (barley, cv. Onslow) for superphosphate applied one, two, three and four years previously. In February 1992, before sowing the fifth crop, soil samples were collected to measure bicarbonate-extractable phosphorus (P) (soil test P) which was related to the subsequent grain yields of that crop. This relationship is the soil test P calibration used to estimate the current P status of soils when providing P fertilizer recommendations.The residual value of superphosphate declined markedly. For the third crop (oat), it was 6% as effective as freshly-applied superphosphate one year after application, and 2% as effective two years after application. For the fifth crop (barley), relative to freshly-applied superphosphate, the residual value of superphosphate in successive years after application was 46%, 6%, 3% and 2% as effective. The soil has a very low capacity to sorb P, and P was found to leach down the soil profile. The largest yield for P applied one and two years previously in 1990, and two, three and four years previously in 1992, was 35 to 50% lower than the maximum yield for freshly-applied P.Soil test P was very variable (coefficient of variation was 32%) and mostly less than 8µg P/g soil. The calibration relating yield (y axis) to soil test P (x axis) differed for soil treated with superphosphate one year previously compared with soil treated two, three and four years previously. The top 10 cm of soil was used for soil P testing, the standard depth. P was leached below this depth but some of the P leached below 10 cm may still have been taken up by plant roots. Consequently soil test P underestimated the P available to plants in the soil profile. The soil test P calibration therefore provided a very crude estimate of the current P status of the soil.     

Effect of phosphate rock and triple superphosphate on soil phosphorus fractions and their plant-availability and downward movement in two volcanic ash soils under <Emphasis Type="Italic">Pinus radiata</Emphasis> plantations in New Zealand

Changes in phosphorus (P) fractions and their plant-availability and downward movement in two strongly P fixing acidic Andosols (Allophanic and Pumice Soils) under Pinus radiata plantations in New Zealand were studied 2 years after triple superphosphate (TSP) and a phosphate rock (BGPR, origin Ben Guerir, Morocco) application, each at four rates, to determine the fate and plant availability of fertilizer-derived P in these soils. The rate of increase of the concentrations of the P fractions was highest for NaOH-P_i (inorganic P associated with Fe and Al oxides and allophane) when TSP was applied and highest for H₂SO₄-P_i (predominantly calcium phosphates or apatite-type P minerals) when BGPR was applied. The largest pool of soil P, the NaOH-P_o (labile organic P), was unaffected by the P fertilizer applications. The rate of NaOH-P_i concentration increase was higher in the higher P fixing Allophanic Soil than in the Pumice Soil. Both types of fertilizers increased resin-P_i (Inorganic P freely available to the plant) and Bray-2 P concentrations but only the TSP application increased Olsen P concentration. Phosphorus derived from TSP and BGPR applications moved down to 10–20 cm soil depth within 2 years of application in the Pumice Soil, but did not move below 10 cm depth in the higher P fixing and less porous Allophanic Soil. The fertilizers significantly increased needle P concentrations 2, 3 and 4 years after fertilizer application, but did not have any significant effect on tree growth.     

Response of lowland rice and common bean grown in rotation to soil fertility levels on a Varzea soil

Brazil has approximately 30 million hectares of lowland areas, known locally as Varzea, but very little is known about their fertility and crop production potential. A field experiment was conducted for three consecutive years to evaluate response of lowland rice (Oryza sativa L.) grown in rotation with common bean (Phaseolus vulgaris L.) on a Varzea (low, Humic Gley) soil. Rice was grown at low (no fertilizer), medium (100 kg N ha^–1, 44 kg P ha^–1, 50 kg K ha^–1, 40 kg FTE-BR 12 ha^–1), and high (200 kg N ha^–1, 88 kg P ha^–1, 100 kg K ha^–1, 80 kg FTE-BR 12 ha^–1 fritted trace element-Brazil 12 as a source of micronutrients) soil fertility levels. Green manure with medium fertility was also included as an additional treatment. Average dry matter and grain yields of rice and common bean were significantly (P < 0.01) increased with increasing fertilization. Across the three years, rice yield was 4327 kg ha^–1 at low fertility, 5523 kg ha^–1 at medium fertility, 5465 kg ha^–1 at high fertility, and 6332 kg ha^–1 at medium fertility with green manure treatment. Similarly, average common bean yield was 294 kg ha^–1 at low soil fertility, 663 kg ha^–1 at medium soil fertility, 851 kg ha^–1 at high fertility, and 823 kg ha^–1 at medium fertility with green manure treatment. Significant differences in nutrient uptake in bean were observed for fertility, year, and their interactions; however, these factors were invariably nonsignificant in rice.     

Nitrogen-15 balance as affected by rice straw management in a rice-wheat rotation in northwest India

The sustainability of the productive rice-wheat systems of Northwest India is being questioned due to the complete removal of straw for animal consumption and fuel, or the burning of straw which has reduced the soil organic matter contents. However, straw incorporation at planting can temporarily reduce the availability of fertilizer-N and reduce crop yields. In a field study on a loamy sand soil, the effect of 6 mg ha⁻¹ rice straw incorporated into the soil 20 or 40 days before sowing (DBS) the wheat was compared with removal or burning of rice straw on the fate and balance of 120 kg ha⁻¹ of 5 atom% ¹⁵N-urea applied to wheat and to a following crop of rice. Wheat grain yield and agronomic efficiency (AE) of applied N (kg grain/kg N applied) were not influenced by rice straw management. However, N uptake (NU), and recovery efficiency (RE) of N by the difference method were lower with rice straw incorporation than with burning. Nitrogen-15 recovery by wheat was highest (41%) when the rice straw was removed or burned and lowest (30.4%) when 30 of the 120 kg N ha⁻¹ was applied at the time of straw incorporation at 20 DBS of wheat. However, this strategy of adding 25% of the urea-N dose at the time of straw incorporation resulted in the highest ¹⁵N losses (45.2%). Inorganic N remaining at harvest in the 0 to 60 cm soil profile, mostly NO₃ ⁻, was 5.5% after wheat and 4.2% after rice. Rice grain yields, NU, and RE were not influenced by rice straw management. Nitrogen-15 losses were similar in rice and wheat (31% with straw removed) despite total irrigation and rainfall inputs of 340 and 32 cm to rice and wheat, respectively. These results suggest to the farmers of northwest India that straw incorporation does not necessarily hurt grain yields, and indicates to researchers that work is still needed to improve N use efficiency in rice and wheat. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of P Fertilizer Placement and Temperature on Root Hair Formation, Shoot Growth and P Content of Barley Grown on Soils with Varying P Status

Seed placed phosphorus (P) is hypothesized to increase P utilization in plants, particularly in cool climate. A greenhouse experiment was conducted to evaluate the effects of seed placed P at two temperature regimes (7 °C night/9 °C day or 13 °C night/15 °C day) within the first weeks of the growing season on root hair formation, shoot growth and P content of spring barley (Hordeum vulgare L.). Three soils (loam, clay loam and sandy silt texture) were used, each with two levels of plant available P (low and high P-AL). In half of the pots, all P (10 mg P kg⁻¹ soil) was applied 5 cm below the seed. In the other pots, half of the P was applied in the same way, and half was given together with the seeds as starter fertilizer. When plants reached growth stage (GS) 21, two replicates were harvested and temperature adjusted to 15 °C night/17 °C day for the remaining three replicates, which were harvested at GS 49. Phosphorus fertilizer placement did not interact with temperature, soil P level or soil type, and it had also only few and slight effects on shoot dry matter (DM) and P content. The lower temperature regime delayed growth rate by nine days until GS 21, but shoot DM and P content was not significantly affected. At GS 49, both shoot DM and P content increased by 20% from low to high temperature. The soil P level influenced shoot P content to a high degree, with an increase by 190% at high P-AL compared to low P-AL at GS 21, and by 170% at GS 49. The length of root hairs within treatments was very variable, and no significant differences were found between the treatments. So the ability to adapt morphologically to suboptimal conditions was not great enough to avoid reduced growth because of P deficiency. Placing some P together with the seeds could either inhibit limited P uptake at suboptimal growth conditions in this trial. Seed placement of P is recommended in Norway, especially on silty soils. However, the results from this study support what are observed in field, that the positive effects of seed placed fertilizer are variable. The subject indeed needs further investigation before the effect of seed placed P are fully understood.     

A comparison of five soil phosphorus tests for five crop species for soil previously fertilized with superphosphate and rock phosphate

The P_i, Colwell, Bray 1, calcium acetate lactate (CAL) and Truog phosphorus (P) soil test reagents were assessed in two field experiments on lateritic soils in Western Australia that had been fertilized four years previously (1984) with triple superphosphate, North Carolina rock phosphate, Queensland rock phosphate, and in one experiment, Calciphos. Soil samples to measure soil P test were collected February 1987. Soil P test was related to seed (grain) yields measured later in 1987. Different crop species were grown on different sections of the same plot at each site. The species were lupins (Lupinus angustifolius), barley (Hordeum vulgare) and oats (Avena sativa) at one site, and lupins, oats, triticale (×Triticosecale) and rapeseed (Brassica napus) at the other site. For each reagent, the soil P test calibration, which is the relationship between yield, expressed as a percentage of the maximum yield, and soil P test, generally differed for different plant species and for different fertilizer types. Variations in soil P test required to produce half the maximum yield of each species at each site was least for the CAL reagent followed by the Colwell reagent.     

A model of S,P and N uptake by a perennial pasture

An approach is presented for simulating the uptake of S, P and N by a perennial pasture, based upon diffusion theory and plant uptake kinetics. Plant uptake was limited by either (i) diffusion to the root, (ii) root uptake capacity, or (iii) plant demand. For S and P, uptake as limited by diffusion was calculated from the nutrient depletion pattern around the root, which is dependent upon soil diffusivity for the anion, and the length of time since the root grew into the new soil. To avoid time-consuming calculations at various uptake times and distances from the root, the nutrient depletion pattern was simplified into an effective rooting radius, whereby all plant-available nutrient within the radius was made available to the plant over a 30 day period, and none from outside the radius. Mycorrhizal enhancement of P uptake was simulated by increasing the radius by a factor of between 1 and 3 when P was the primary limiting nutrient. Limitations (ii) and (iii) were included for S and P for the rare occasions when nutrient concentrations were high either in a single layer, or in the profile overall. For N, uptake was limited by uptake capacity, adjusted for soil N concentration, root length density and soil moisture content.Because of a lack of data on diffusion coefficients close to the plant root, a calibration factor was applied to the effective rooting radius, which is described in the second paper in the series.     

Effect of superphosphate and nitrogen on yield and take-all of wheat

Wheat was grown continuously in soil amended with 5 levels of superphosphate and with 4 levels of urea at 3 sites. The incidence and severity of take-all, caused byGaeumannomyces graminis var.tritici, declined with increasing rates of application of both superphosphate and urea.In both years, the severity of take-all on plants receiving neither superphosphate nor urea was about 40% while at the highest level of superphosphate and urea supply the take-all severity was approximately halved at 22%.There was an increase in grain yield in response to applied superphosphate and urea to the highest level of each nutrient. There was also an increase in the 1,000-kernal weights with superphosphate and urea fertilizer application.     

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.     

Balance sheet of phosphorus,sulphur and potassium in a long-term grazed pasture supplied with superphosphate

A balance sheet of P, S and K was constructed for a long-term trial which investigates the effects of three rates of superphosphate (9% P, 11% S) on pasture production on border-strip irrigated land grazed with sheep. A balance sheet of the inputs and outputs of P, S and K to the trial over a 38 year period showed that of the nutrients applied in fertiliser, only 51–59% of the P and 15–31% of the S were retained in the soil. Small amounts were lost in animal products (4–19% of the applied nutrients) but major losses were attributed to runoff of P as particulate matter (dung and soil particles) during irrigation and leaching of sulphate-S during irrigation. Losses of K from the site were small and had no effect on total soil K content. The distribution of soil nutrients across the border-strips was also investigated. The results showed that the concentrations of total soil P and S and exchangeable K were significantly greater at the sides of the irrigation borders than in the main strip area of pasture. This was caused by deposition of a disproportionate amount of dung and urine (and therefore nutrients) on the levees where the sheep tended to camp. It was calculated that with increasing superphosphate rates greater amounts of P were transferred to the levees due to the increased amounts of P being recycled via the animals (as a result of increased herbage P concentration, pasture production and stocking rate).     

Residual value of superphosphate measured using yields of different pasture legume species and bicarbonate — extractable soil phosphorus

The residual value of superphosphate was measured in three glasshouse pot experiments using three different lateritic soils (pH CaCl₂: 4.8–5.3) from south-western Australia. The residual value was estimated relative to levels of freshly-applied superphosphate using yield of dried tops and bicarbonatesoluble P extracted from the soil (soil test values). Up to five successive crops were grown. In each experiment, four different pasture legume species fertilized with mineral nitrogen were grown in rotation with a cereal species. The legume species includedMedicago polymorpha, M. murex, Trifolium subterraneum, Ornithopus compressus, O. perpusillus andO. pinnatus. The cereal species includedTriticum aestivum, ×Triticosecale, andHordeum vulgare. The comparative phosphorus (P) requirement of the different pasture legumes was estimated from the amount of P required to produce 50 or 90% of the maximum yield measured for each species at each harvest. Soil samples for the soil test were collected just before sowing each crop, and were related to the plant yields of that crop.Relative to freshly-applied superphosphate, the residual value of superphosphate measured using plant yield was similar for all pasture legume species, and decreased markedly, by about 50 to 80% between the first and second crop, and by a further 5 to 30% for subsequent crops. The decrease in residual value estimated using soil test values was less marked. For freshly-applied superphosphate, and for the same plant species, the relationship between yield and the level of P applied differed for different crops.There was no consistent, systematic trend for the comparative P requirement of the different legume species within and between crops of the three experiments and soils.For all crops, the relationship between yield of dried tops and P concentration in dried tissue generally differed for the different legume species, indicating the different species usually have different internal efficiency of P use curves. However, for each experiment, when the same cereal species was grown in all the pots, the relationship between yield and P concentration in tissue was similar for previously- and freshly-applied superphosphate, regardless of the pasture legume species grown in previous crops.The relationship between yield and soil test values usually differed, within each crop, for different plant species and for previously- and freshly-applied superphosphate. For the same plant species, the relationship also differed between different crops.     

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

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

Response of promiscuously nodulating soybean to N and P fertilization andBradyrhizobium inoculation in a ferruginous tropical soil (Haplustalf)

Field trials were conducted at Samaru, Nigeria over a three-year period (1986–88) to study the effects of N and P fertilization on the response of promiscuously nodulating soybean toBradyrhizobium japonicum inoculation in a ferruginous tropical soil. Phosphorus fertilization enhanced nodulation, while N fertilization had no consistent effect on nodulation. Both N and P increased dry matter production. Seed yields were not influenced by the application of N. However, P increased seed yields in two out of three years. Response of seed yield to P fertilization was significant up to 26.4 kg P ha^–1.Bradyrhizobium inoculation consistently enhanced nodulation while it increased seed yield in only one out of three years. Results demonstrate that P is an important nutrient for soybean production in ferruginous tropical soils. The roles of promiscuously nodulating soybean in the maintenance of tropical soil fertility are discussed.     

Effectiveness of rock phosphate, coastal superphosphate and single superphosphate for pasture on deep sandy soils

The effectiveness of coastal superphosphate and two rock phosphate fertilizers was compared with the effectiveness of single superphosphate for pasture production on deep, humic, sandy podzols in high rainfall (> 800 mm annual average) areas of south-western Australia. The pastures were subterranean clover (Trifolium subterraneum) or mixed subterraneum clover and serradella (Ornithopus compressus). Coastal superphosphate was made by adding rock phosphate and elemental sulphur to superphosphate during manufacture, as it came out of the den before granulation. One rock phosphate was a 50% mixture of apatite rock phosphate from Nauru and Christmas Islands, and which was also used to make the single and coastal superphosphate used in this study, and superphosphate made in Western Australia at the time these experiments started. The other rock phosphate was Calciphos, the fertilizer produced by heating (calcining), at about 500 °C, Christmas Island C-grade ore, a calcium iron aluminium rock phosphate. There were two types of experiments. In the three Type 1 experiments, levels of each fertilizer were applied annually. In the two Type 2 experiments, levels of fertilizer were applied once only to new plots in different years. Coastal superphosphate was the most effective fertilizer in the Type 1 experiments, with both rock phosphates and single superphosphate being equally effective. All fertilizers were equally effective in the Type 2 experiments. There were large variations in fertiliser effectiveness values between yield measurements in the same or different years. It is known that P leaches from freshly-applied superphosphate in these soils. The extent of this leaching probably varies between yield measurements affecting effectiveness values determined for all fertilizers because the effectiveness values were calculated relative to the effectiveness of single superphosphate. The humic, sandy podzols remain wet during the growing season, are acidic, and are known from laboratory studies to possess adequate hydrogen ions to cause extensive dissolution of North Carolina rock phosphate so that rock phosphates are equally or more effective than single superphosphate in these soils. When elemental sulphur in coastal superphosphate is oxidized to SO₄ hydrogen ions are produced which in previous studies has been shown to enhance dissolution of rock phosphate in biosuper, a mixture of rock phosphate and elemental sulphur.     

C10-polyacetylenes as allelopathic substances in dominants in early stages of secondary succession

cis-Dehydromatricaria ester (cis-DME) inSolidago altissima, andcis-matricaria ester (cis-ME),trans-matricaria ester (trans-ME), andcis-lachnophyllum ester (cis-LE) inErigeron spp. show strong growth inhibitory effects on other plants. Thecis- andtrans-DMEs were found in soil at the border ofS. altissima communities in concentrations that were inhibitory to test plants. Among four species ofErigeron, the most dominant plant,E floribundus, showed the highest concentrations of the esters. From the results of our experiments, we conclude that these polyacetylenes are probably allelopathic substances with ecological importance.     

Effectiveness of two partially acidulated rock phosphates,coastal superphosphate and Ecophos,compared with North Carolina reactive rock phosphate and superphosphate

The effectiveness of coastal superphosphate, a partially acidulated rock phosphate (PARP) made from apatite, and Ecophos, a PARP made from calcium iron aluminium (crandallite millisite) rock phosphate, was compared in pot experiments with the effectiveness of ordinary superphosphate (OSP) and North Carolina reactive apatite rock phosphate (NCRP). There were three experiments using different lateritic soils collected in Western Australia. Fertilizer effectiveness was measured using yield of dried wheat (Triticum aestivum) tops grown for 28 days. Three successive crops were grown. The phosphorous (P) fertilizers were applied and mixed with the soils before sowing the first crop. In addition, OSP was added to extra pots before sowing crops 2 and 3 in order to measure the effectiveness of the original P fertilizers relative to freshly-applied OSP for these crops.As measured using plant yield, coastal superphosphate was the most effective P fertilizer for three crops on an acidic peaty sand (pH water 5.0). Relative to freshly-applied OSP, it was 154% as effective for crop 1, 75% as effective for crop 2, and 36% as effective for crop 3. Corresponding values for Ecophos were 44, 29 and 19%, and for NCRP, 77, 67 and 29%, with the original OSP treatment being 61 and 56% as effective for crops 2 and 3. For three crops on a lateritic gravel loam (pH 6.5), both coastal superphosphate and OSP were the most effective fertilizers, and were equally effective for crop 1, and relative to freshly-applied OSP, were about 31% as effective for crop 2, and 16 and 21 % as effective for crop 3. Corresponding values for Ecophos were 47,15 and 11%, and NCRP, 33,15 and 5%. For two crops in a loamy sand (pH 5.4), OSP was the most effective fertilizer, and, relative to fresh OSP, it was 36% as effective for crop 2. Relative to fresh OSP, the effectiveness for crops 1 and 2 of coastal superphosphate was 57 and 18%, for Ecophos 71 and 27%, and for NCRP 50 and 36%.     

The residual value of rock phosphate and superphosphate from field sites assessed by glasshouse bioassay using three plant species with different external P requirements

A pot experiment with two lateritic soils measured the relative residual effectiveness (RRE) of superphosphate and three rock phosphate (RP) fertilizers applied six years previously in the field. Three plant species (lettuce, wheat and maize) having very different external P requirements were grown as indicators of P availability. Superphosphate had the maximum RRE (1.0) and low reactive Queensland RP had the minimum RRE (0.04–0.45) for all plant species. For one soil the RRE of reactive North Carolina RP was similar to that of superphosphate (0.87–1.04), but ranged from 0.07 to 0.30 for the other soil. The RRE of Calciphos (one soil only) ranged from 0.60 to 0.98 for all plant species.The RRE of rock phosphate decreased for the three crops in sequence maize> wheat> lettuce for a 30 days growth period. This ranking follows the increasing external P requirement of the three plant species. Very high rates of application of RP may have induced micronutrient deficiencies.