Phosphorus supplying capacity of heavily fertilized soils I. Phosphorus adsorption characteristics and phosphorus fractionation

Studies were conducted to investigate the P sorption characteristics and P fractions in eight intensively fertilized soils collected from southern and central Norway. Adsorption of P at the initial P concentrations in the soil solution was very high in the Særheim clay loam soil which contained high amounts of organic C and clay. Adsorption data were fitted well to the classical Langmuir equation. The P affinity constant (k), adsorption maximum (b) and maximum buffer capacity (mbc) calculated from this equation differed considerably among soils. The P affinity constant (r=0.96,p=0.01) and maximum buffer capacity (r=0.97,p=0.01) were highly and positively correlated to organic C. None of the soil parameters were related to adsorption maximum. Phosphorus desorption from the heavily fertilized soils varied widely and depended on the initial P status of the soil and soil texture. The ratio between desorbed P and total P was significantly correlated to sorption parameters. Multiple regression analysis showed that total P positively and organic C negatively affected P desorption in the soils. Iron-P was a major P sink in these soils and it was related to clay content (r=0.69,p=0.1) and organic P (r=0.76,p=0.0.5), but it did not relate to average P removed per harvest (RPH). Calcium-P and Al-P were not related to any of the soil parameters but these fractions were the major contributors to RPH as expressed by a multiple regression equation: RPH=0.397+0.0016 × Ca-P + 0.0012 × Al-P (r=0.84,p=0.05). High content of inorganic fractions shows that most of the residual P may be plant available, albeit at reduced rate with time, in these soils but the availability will depend on soil types.     

Fertilizer inputs,nutrient balance and soil nutrient supplying power in intensive,irrigated rice system. III. Phosphorus

Data from long-term experiments at 11 sites in Asia with a wide range of nutrient input treatments and yield levels were used to quantify crop P requirements of rice (Oryza sativa L.) and the P balance in intensive, irrigated rice systems. Uptake of 1.8–4.2 kg P was required to produce one ton of grain yield. Physiological P use efficiency varied between 220 to 900 kg grain kg P^-1. Without added P, there was a net loss of 7 to 8 kg P ha^-1 per crop; with added P there was a net gain of 4 to 5 kg P ha^-1 per crop. Phosphorus adsorption kinetics on mixed-bed ion-exchange resin capsules provided an integrative measure of soil P status, P diffusion, and acid-induced P solubilization. The resin capsule was a sensitive tool to characterize buildup or depletion of soil P as a result of different P balances. Both Olsen-P and the resin capsule were suitable methods to predict P uptake of tropical lowland rice. It is hypothesized that both methods measure a similar soil P pool which is soluble under alkaline, aerobic conditions but transformed into acid-soluble P froms as a result of submergence and reduction. Present recommendations for P fertilizer use on rice of 20–25 kg P ha^-1 are adequate to maintain yields of 5–6 t ha^-1, but sustaining higher yields of 7–8 t ha^-1 will require farm-specific management strategies based on knowledge of the long-term P balance and soil P-supplying capacity.     

Evaluation of antioxidant capacity of sesamol and free radical scavengers at different heating temperatures

Sesamol is a natural antioxidant found in sesame oil from roasted sesame seeds. Activation energy and antioxidant capacity of sesamol were determined and compared with other free radical scavengers (FRSs) including tert‐butylated‐hydroxyquinone (TBHQ), butylated‐hydroxyanisol (BHA), or α‐tocopherol in a lard model system treated with different heating temperature. Each FRS was added in lard and heated at 90, 120, 150, and 180°C for 48, 24, 8, or 2 h, respectively and antioxidant capacity was evaluated by conjugated dienoic acid (CDA) value, conjugated diene hydroperoxides, p‐anisidine value (p‐AV), and a modified 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) method. Apparent activation energy of sesamol was determined as 12.98 kcal/mol in a lard model system. Antioxidant capacity of sesamol was as good as that of TBHQ and was higher than those of BHA and α‐tocopherol at 90, 120, and 150°C based on CDA, conjugated diene hydroperoxides, and p‐AV assays. The results of a modified DPPH method showed that each FRS showed different distribution of radical scavenging compounds from oxidized lipids (RSOLs) during oxidation. Sesamol may replace synthetic FRSs like TBHQ and BHA in processed foods treated with high temperature. Practical application: Processed foods are frequently treated with high temperature during oven‐drying, roasting, baking, and deep‐fat frying. This study showed that sesamol, one of natural antioxidants, had stronger antioxidant capacities than other synthetic FRSs at the temperature ranges from 90 to 180°C. The results of this study can be applied in food industries producing deep‐fat fried foods including snacks, chips, and French fries to extend the shelf‐life of final foods with high temperature treatment.     

Phosphorus seed coating increases phosphorus uptake,early growth and yield of pearl millet (Pennisetum glaucum (L.) R. Br.) grown on an acid sandy soil in Niger,West Africa

In pot and field experiments conducted in 1990 and 1991 on an acid sandy, phosphorus (P) deficient soil in Niger, West Africa, the effect of seed coating on seedling emergence, early growth and grain yield of pearl millet (Pennisetum glaucum (L.) R. Br.) was studied. Seeds of pearl millet were coated with different rates (0; 0.5; 1.0; 2.0; 5.0; 10.0 mg P seed^–1) and types of P fertilizers (single superphosphate, ammonium dihydrogen phosphate; monocalcium phosphate, sodium dihydrogen phosphate and sodium triphosphate). Seedling emergence was generally reduced at coating rates higher than 0.5 mg P seed^–1 and prevented with single superphosphate and sodium triphosphate at rates higher than 5 mg P seed^–1. No correlation was found between the pH and osmomolity of the coatings and final emergence of millet seedlings. The most favourable effect on plant growth and P content was achieved with ammonium dihydrogen phosphate (AHP) as seed coating. This was attributed to the enhancement effect of ammonium on P uptake. Compared to the untreated control dry matter production at 20 days after planting (DAP) was increased by 280%, P content per plant by 330%, total biomass at maturity by 30% and grain yield by 45%. Although seed coating with AHP may be harmful to seedlings emergence, it represents a suitable method to enhance early growth and increase yield of pearl millet.     

Development and evaluation of an improved soil test for phosphorus. 2. Comparison of the Olsen and mixed cation-anion exchange resin tests for predicting the yield of ryegrass grown in pots

A glasshouse experiment was conducted on four soils contrasting in P sorption capacity and exchangeable Ca content with perennial ryegrass using six phosphate rock (PR) sources and a soluble P source applied at four rates (including a control). After three harvests (11 weeks) replicate pots of each treatment were destructively sampled and Olsen P and mixed cation-anion exchange resin (Resin P) extractions carried out. The remaining replicated treatments were harvested another seven times (during 41 weeks). Yields (for the last seven harvests) were expressed as percentages of the maximum yield attainable with MCP.In general, the Resin P test extracted more than twice as much P as the Olsen test. There was a significant increase in Resin P with an increase in the amount of each P source in all four soils, but Olsen P values were not significantly different for soils treated with different rates of each phosphate rock. The abilities of the Olsen and mixed resin soil P tests to predict the cumulative dry matter yield from 7 harvests and the relative yield of ryegrass were compared. Correlations between measured yield (for the last 7 harvests) and soil test for each soil, and relative yield and soil test for all four soils were assessed by regression analysis using Mitscherlich-type models.When dry matter yields were regressed separately against soil test values for each soil, the Resin P consistently accounted for 18–28% more of the variation in yield than did Olsen P. For Resin P a single function was not significantly different from the separate functions fitted to MCP and PR treatments. However, for Olsen P the separate functions for the MCP and PR treatments varied significantly from the single fitted function. The Resin P test (R² = 0.84) was a better predictor of relative yields over this range of soils than the Olsen test (R² = 0.75). Two regression models based on the regression of relative yield for MCP treatments against either Olsen or Resin were developed. These models were then fitted to the relative yield data on soils fertilized with PRs only. The Olsen P model was found to be a poorer predictor (R² = 0.41) than the Resin P model (R² = 0.73) because it underestimated the observed yield of the PR treatments.     

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.     

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.