Uptake Of Fertilizer Research Articles

Accumulation and retranslocation of foliar nitrogen in fertilised and irrigated Pinus radiata

The Biology of Forest Growth experiment was established in a 10-year-old Pinus radiata stand and comprised the following treatments: once-only fertilisation (400 kg N ha −1) with and without irrigation, a repetitive weekly application of liquid-soluble fertiliser applied with irrigation at rates ranging up to 20 kg N week −1, and unfertilised stands with and without irrigation. The treatment resulted in highly contrasting stands in terms of water and nutrient relations, rates of growth and allocation of biomass. Uptake of N from soil and litter, accumulation of N into foliage, subsequent retranslocation and resultant concentrations of N in each age class of foliage were measured in each stand over 4 years. Following fertilisation there was a rapid uptake of N into foliage and negligible net retranslocation within foliage during the subsequent 6 months. However, retranslocation of N out from ageing foliage became significant in the second and subsequent years and there was a positive linear correlation between the amount retranslocated and the total amount of N in the foliage of both irrigated and non-irrigated stands. Retranslocated N constituted 40–50% of N required for new foliage production. The sum of retranslocated N, and directly measured N taken up from soil, explained 98% of the variation in N accumulated into new foliage and 90% of the variation in new foliage production. However retranslocation was correlated with growth only if stands were irrigated. The peak foliage production of 8 t ha −1 year −1 resulting from fertilisation, required 130 kg N ha −1 year −1 which was derived from uptake from soil and retranslocation from older foliage. This was similar to the peak requirement of unfertilised 5- to 6-year-old stands on fertile sites in New Zealand. It was estimated that the duration of the growth response of trees associated with (a) initial uptake of fertiliser N from mineral soil, (b) re-mineralisation of N which was initially immobilised after fertilisation, (c) enhanced uptake from an N-enriched litter layer, and (d) retranslocation from older foliage, was likely to exceed 12 years.

Sources of nitrogen and yield advantages for monocropping and mixed cropping with cowpeas (Vigna unguiculata L.) and upland rice (Oryza sativa L.)

Sources of N used by cropped and intercropped cowpeas and rice were determined by the 15N isotope-dilution technique. The biological efficiency of intercropping cowpeas and rice was assessed by calculating the land equivalent ratio of dry matter yield, total N, and uptake of N. A reduced N uptake by both cowpeas and rice during mixed cropping was attributed to mutual competition, with both crops competing effectively for the scarce environmental resources. The lack of a significant difference in the uptake of fertilizer and soil N by mixed-crop rice and cowpeas is an indication that the soil N was sufficient and that the mixed cropping did not create any imbalance in soil and fertilizer N uptake. The land equivalent ratio ranged between 120% and 180% for shoot dry matter and total N, showing that biological efficiency was increased by intercropping cowpeas with rice. The proportion of N derived from the atmosphere by mono- and intercropped cowpeas was not significantly different, showing that the potential of cowpeas to fix N2 was independent of the cropping system, since the rice did not stimulate the cowpeas to fix more N2.

Fate and interaction with native soil N of ammonium N applied to wetland rice (Oryza sativa L.) grown under saline and non-saline conditions

The effect of salts on the balance of fertilizer N applied as 15N-labelled ammonium sulphate and its interaction with native soil N was studied in a pot experiment using rice (Oryza sativa L.) as a test crop. The rice crop used 26%–40% of the applied N, the level of applied N and salts showing no significant bearing on the uptake of fertilizer N. Losses of fertilizer N ranged between 54% and 68% and only 5%–8% of the N was immobilized in soil organic matter. Neither the salts nor the rate of N application had any significant effect on fertilizer N immobilization. The effective use of fertilizer N (fertilizer N in grain/fertilizer N in whole plant) was, however, better in the non-saline soil. The uptake of unlabelled N (N mineralized from soil organic matter and that originating from biological N2 fixation in thes rhizosphere) was inhibited in the presence of the salts. However, in fertilized soil, the uptake of unlabelled N was significantly enhanced, leading to increased A values [(1-% Ndff/% Ndff)x N fertilizer applied, where Ndff is N derived from fertilizer], an index of interaction with the added N. This added N interaction increased with increasing levels of added N. Since the extra unlabelled N taken up by fertilized plants was greater than the fertilizer N immobilized, and the root biomass increased with increasing levels of added N, a greater part of the added N interaction was considered to be real, any contribution by an apparent N interaction (pool substitution or isotopic displacement) to the total calculated N interaction being fairly small. Under saline conditions, for the same level of fertilizer N addition, the added N interaction was lower, and this was attributed to a lower level of microbial activity, including mineralization of native soil N, rootdriven immobilization of applied N, and N2 fixation.

Nitrogen and Phosphorus Release from Decomposing Needles of Southern Pine Plantations

AbstractFertilizer application and weed‐control treatments have increased biomass accumulation of loblolly (Pinus taeda L.) and slash pines (P. elliottii Engelm. var. elliottii) in north‐central Florida. While the effects of these treatments have been immediate, the long‐term influences on rates of nutrient cycling are unknown. This study measured rates of mass loss and changes in N and P contents in Oi horizon needles decomposing in the laboratory, and in Oi, Oe1, and Oe2 horizon needles decomposing in situ (litterbag study). The quality of organic‐matter substrates in needles (determined by laboratory incubation) was increased by both weed‐control and fertilizer treatments (P < 0.05), and litter quality of loblolly pine was greater than that of slash pine. However, these effects were not manifest in the field study. Across all treatments, an average of 31% of the original organic matter was lost after 1 yr of decomposition in situ. Generally, N was immobilized into Oi horizon needles but was released in modest amounts (<15% yr−1) from Oe1 and Oe2 horizon needles. In contrast, substantial release of P from Oi horizon needles (>35% yr−1) was followed by smaller release from Oe1 and Oe2 horizon needles (< 15% yr−1). More P was cycled per unit of organic matter in fertilized than nonfertilized plots, and loblolly pine cycled more P than slash pine. Changes in total P content of needles during decomposition were accounted for by changes in the pool of inorganic P. The rapid recycling of P in fertilized plots has the potential to enhance long‐term productivity beyond the immediately beneficial effects derived from fertilizer uptake.

Mineralization of Nitrogen and Phosphorus from Soil Organic Matter in Southern Pine Plantations

AbstractSustained weed control and/or annual applications of fertilizer have accelerated stand development of young slash pine (Pinus elliottii Engelm. var. elliottii) and loblolly pine (P. taeda L.) growing on Spodosols in Florida. Productivity of these plantations will depend largely on the extent to which nutrients are recycled through soil organic matter derived from decomposition of fresh residues. We attempted to sample this newly formed soil organic matter and measured the effects of weed control and fertilizer treatments on N and P mineralization under laboratory conditions (42‐d aerobic incubation). Sequential in situ containments using interbed soil were used to provide information on field rates of mineralization. The laboratory study demonstrated that specific N mineralization was little affected by cultural treatment. In contrast, specific P mineralization was consistently increased by fertilizer application, both in the laboratory and in situ. This suggests that, in organic residues, the general composition of P substrates was altered by fertilizer application, while organic‐N substrates remained unaffected. The effect of weed control inhibited specific N and P mineralization in the laboratory, indicating that organic residues of understory vegetation are a better supply of available N and P than are organic residues of pine. In fertilized plots and for P, annual rates of in situ mineralization were large relative to demand (uptake) by vegetation. The recycling of P through organic residues of fertilized plots has the potential to enhance long‐term productivity beyond the immediate benefits derived from fertilizer uptake.

Effect of annual weeds on water and nitrogen availability to Pinus radiata trees in a young plantation

The importance of competition between trees and annual weeds for nitrogen (N) and water was evaluated in a Pinus radiata D. Don (radiata pine) plantation between two and three years of age growing on a sandy podzol in the southeast of South Australia. Increasing the width of the weed-control strip spanning the tree row had little effect on tree water status, but increased the uptake of N by trees. Application of N fertilizer alleviated N deficiency and tree growth suppression induced by weeds, suggesting that when N supply is high (e.g., on fertile sites) intense weed control is unnecessary in plantations beyond two years of age. On the other hand, competition for N by weeds would seriously aggravate N deficiency in young pine trees grown on low-N soils. When N fertilizer was applied, weeds rapidly absorbed mineral N from the soil, and within three months, the N content of weeds was 10.2 g N m −2 greater than in the unfertilized control. This represents 68% of the amount of N applied in fertilizer. Weeds therefore increased the uptake of fertilizer N by plant biomass, and thereby improved N retention on site. Tree growth increased with N fertilizer at all levels of weed control and responses to weed control and N fertilizer were additive. The growth of weeds increased the soil organic carbon status and the rate of N mineralization. The presence of weeds reduced the incidence and severity of stem deformation, which was exacerbated by N fertilizer application and complete weed control. In general, strip weed control may be a better option than complete weed control in managing weeds in young P. radiata plantations.

Dinitrogen fixation and soil n uptake by soybean as affected by phosphorus availability

Abstract The effects of phosphorus supply (0, 30, and 90 mg P kg‐1) on growth, N2 fixation, and soil N uptake by soybean (Glycine max (L.) Merr.) were studied in a pot experiment using the 15N isotope technique. Phosphorus supply increased the top dry matter production at flowering and the dry matter production of seeds, straw, pod shells, and roots at late pod filling of inoculated soybeans. Phosphorus supply reduced the N concentration of plant tops at flowering, but increased the amount of N accumulated at both flowering and late pod filling. In inoculated soybeans total N accumulation paralleled the dry matter production. The P concentration in above‐ground plant parts of nodulated soybeans was not affected by P application. At flowering only 18 to 34% of total N was derived from N2 fixation, whereas as much as 74% was derived from N2 fixation at late pod filling. Only the addition of 90 mg P kg‐1 soil significantly increased the amount of N2 fixed at the late pod filling stage. Phosphorus supply did not influence the uptake of fertilizer or soil N in soybeans, even if the root mass was increased up to 60% by the P supply.

Flood irrigation of wheat on a transitional red-brown earth. II. Effect of duration of ponding on availability of soil and fertilizer nitrogen

Grain yield of wheat growing on a transitional red-brown earth was reduced by long periods of ponding during irrigation. To determine whether fertilizer N loss was a major cause of this yield decline, 15N-labelled urea was applied to microplots at the same time and rate as the crop was topdressed with urea (end of tillerfng, 100 kg N ha-1). In addition, 15N-labelled nitrate was used to assess denitrification potential during and after each irrigation. Prior to the first irrigation (19days after urea application), 76% of the urea N was immobilized in the plants (33%) and soil (43%), 15% was present as soil mineral N, and 9% was not accounted for. The majority (85%) of the urea-derived mineral N was present as ammonium in the 0-0.1 m soil layer. After the first irrigation, amounts of mineral N in the soil remained very low at 3-6 kg N ha-l in the 0-0.2 m layer, with only 5-15% of this in the nitrate form. There was no significant effect of duration of ponding on plant or soii recovery of urea N after any irrigation. Mean plant recovery increased to 52%, over the first 55 days following urea application, while soil recovery declined to 22%. Beyond this stage changes in plant and soil recoveries were negligible. Plant total N content increased throughout the season due to ongoing mineralization of native soil N; however, there was negligible net mineralization of recently immobilized 15N beyond day 55. At physiological maturity, 46% of the N acquired by the plants was derived from the fertilizer. Losses of urea N increased to 25% over the first 55 days, and appeared to be due to nitrification-denitrification. Field studies with labelled nitrate indicated that denitrification potential in the soil was high throughout the experimental period. After the final two irrigations there were some significant (P < 0.05) effects of duration of ponding on N loss from urea, with lowest losses in the sprinkler, 1 h and 12 h treatments. The data indicate that, on the transitional red-brown earth, the adverse effect of long periods of ponding on wheat yield was not due to decreased availability or uptake of fertilizer or soil N in the longer term.

Grain protein responses to nitrogen applied to wheat growing on a red earth

Dryland wheat was fertilized with ammonium nitrate or liquid urea-ammonium nitrate at the time of sowing or about 3 months later (generally at the terminal-spikelet stage) on a well-drained site near Harden on the south-west slopes of New South Wales. The experiments continued from the second to the fifth year (1981-1984) of the cropping phase of a crop-pasture rotation. The maximum agronomic efficiencies for yield in the four consecutive years were 19, 4, 23 and 25 kg grain per kg of applied nitrogen (N). The three large responses were obtained in wetter than average seasons and the small response was obtained during drought. In the last three years of the study the yield response to nitrogen at the terminal-spikelet stage was found to be close to but slightly less than that for N applied at sowing. In those years the agronomic efficiencies for the late-applied N were 0, 22 and 22. The apparent recovery of fertilizer N in the above-ground parts of the crop at maturity was up to 70% of the fertilizer applied in the year of sowing, and, after the drought during which there was little uptake of fertilizer N, up to 62% by the subsequent crop. The fertilizer efficiencies in the non-drought years were higher than generally reported in south-eastern Australia, and indicate potential for profitable delayed application of N fertilizer to wheat. Grain-protein responses were variable from year to year and are discussed against a simple theoretical background of the amount of N applied and grain-yield response.

FATE OF LABELLED FERTILIZER NITROGEN IN COMMERCIAL LOWBUSH BLUEBERRY STANDS

Labelled fertilizer nitrogen (15N) was applied to four commercial lowbush blueberry stands in order to assess the fate of fertilizer N. Thirteen weeks after application, 45 and 64% of the labelled fertilizer N was recovered from blueberry plants at Debert and Pigeon Hill, respectively. This suggests rapid uptake of fertilizer N by blueberry plants. Sixteen months after application, an average of 67% of the fertilizer N was recovered from the four sites. The results of this study support other evidence suggesting that lowbush blueberry systems conserve fertilizer N. Key words: Blueberry, 15N, fertilizer

FATE OF APPLIED NITROGEN FERTILIZER ON OREGON CRANBERRIES.

Little work has been done to establish the rate and timing of nitrogen fertilizer applications to optimize return from fertilizer expenditures and minimize potential for ground and surface water pollution in Oregon cranberries (Vaccinium macrocarpon Ait.). Predicting cranberry N requirements is difficult because cranberries require little N and soil tests for N are not helpful for perennial crops, especially when grown in shallow sandy soils. We used 15N-labeled ammonium sulfate to measure both plant uptake and movement of fertilizer N in a south coastal Oregon cranberry bed. A bed planted to the Stevens variety was fertilized with 15N-labelled ammonium sulfate at two rates (18 kg/ha and 36 kg/ha) applied at five phonological stages: popcorn, hook, flowering, early bud, and late bud. Plant N uptake and translocation were measured throughout the growing season in uprights, flowers, berries, and roots, Initial results indicate that when N was applied at popcorn stage approximately 12% of the N was present in the above-ground vegetative biomass at harvest. Incorporation of fertilizer N into the duff and mineral soil was measured. An estimate of fertilizer N leaching was made by trapping inorganic N below the root zone using ion exchange resin bags.

Fate and efficiency of N fertilizers applied to pearl millet in Niger

Field studies were conducted in Niger using 15N-labeled fertilizers to assess the fate and efficiency of fertilizer N in pearl millet (Pennisetum glaucum [L.] R.Br.) production. Total plant uptake of fertilizer N was low in all cases (20%–37%), and losses were severe (25%–53%). The majority of N remaining in the soil was found in the 0- to 15-cm layer though some enrichment at lower depths was found when the N fertilizer was calcium ammonium nitrate (CAN). In a comparison of urea placement methods (band, broadcast, or point placement), no significant differences in 15N uptake or yield were noted though point placement did exacerbate 15N loss. The mechanism of N loss is believed to have been ammonia volatilization. Yields were similar whether urea or CAN was used, but 15N uptake from CAN was higher. A statistical model was developed relating millet yield and N response to midseason rainfall. In drought years, no N response was found, whereas in years of good rainfall a response was found of 15 kg grain for each kilogram of N applied (at 30 kg N ha-1 rate).

RELATIVE EFFICIENCY OF POINT-INJECTION AND SURFACE APPLICATIONS FOR N FERTILIZATION OF WINTER WHEAT

Conventional methods of N application for winter wheat often exhibit low fertilizer use efficiency. The comparative effectiveness of a new method, point-injection of N solution, was evaluated in two similar microplot field experiments established in southern Alberta. The first experiment, conducted over three site-year combinations in 1985 and 1986, compared yield response and fertilizer uptake in four spring-applied fertilizer treatments: broadcast urea-ammonium nitrate (UAN), broadcast urea, broadcast ammonium nitrate, and point-injected UAN, all applied in solution form. The second experiment, conducted at five sites in 1987, compared four spring-applied fertilizer treatments: surface-banded UAN, broadcast urea (granular), broadcast ammonium nitrate (granular), and point-injected UAN. All fertilizers were labeled with 15N to permit direct estimation of fertilizer uptake. The experiments demonstrated significant increases in fertilizer efficiency with point-injection under some conditions. In five of eight comparisons conducted over a 3-yr period, point-injection treatments exhibited significantly higher fertilizer use efficiency than conventional broadcast methods of application. Average fertilizer-N recovery by the crop at all eight sites was 37% in the point-injection treatments compared with only 26% in the broadcast ammonium nitrate treatment, the next most effective method of N application. When one site was excluded, because of possible confounding effects of application time, average recoveries were 34 and 26%, respectively. The increased efficiency of point-injected fertilizers was attributed to the direct placement of fertilizer N into the active rooting zone of the crop. The advantage of point-injection over conventional methods of application was highly variable, ranging from approximately 0 to over 100%, in part because of variations in precipitation patterns. The results of these microplot studies suggest that point-injection has potential for significant enhancement of fertilizer use efficiency in winter wheat, particularly in semi-arid production regions. Key words: 15N, nitrogen, urea, ammonium nitrate, fertilizer placement

Water and fertilizer interrelations with irrigated maize

This study was designed to determine maize yield interactions with water ( W) and fertilizer ( F) using a line source sprinkler system. Generally, maize yields increased with increasing water and fertilizer applications. The maximum grain yield of 6.15 t/ha was produced with 523 mm of water and 300 kg/ha of NPK (15-15-15) fertilizer, while, the highest dry matter yield of 17.84 t/ha occurred with 495 mm of water and 300 kg/ha of NPK fertilizer. Maize yields were highly correlated with water and fertilizer with R 2 = 0.95 for grain yield and R 2 = 0.926 for dry matter yield. A highly significant W × F interaction (at 1% level) indicated high maize yields when fertilizer uptake is enhanced by high soil moisture level. Water productivity was increased by fertilizer application, but the relationship was not as definite with soil moisture. Production functions were generated to reliably predict maize yields.

Optimum Application Parameters for Point Injection of Nitrogen in Winter Wheat

AbstractPoint injection may enhance N fertilizer use efficiency in winter wheat but optimum application parameters have not been identified. Three field experiments, employing 15N tracer techniques and manual injections on small plots, were established at two sites in southern Alberta, Canada, to identify optimum injection intervals and injection depth for spring point‐injection of urea‐ammonium nitrate solution (UAN). The optimum lateral (across‐row) injection interval was found to be the equivalent of two row spacings (40 cm). Higher intervals resulted in unacceptable variability in N availability because wheat in rows not directly adjacent to injections assimilated comparatively little fertilizer‐derived N. The optimum longitudinal injection interval was also approximately 40 cm. Analysis of fertilizer N distribution from the injection point demonstrated unacceptable variability when this interval was exceeded. Crop uptake of fertilizer N was reduced when the injection interval was increased from 40 to 60 cm. Adoption of smaller injection intervals in both dimensions, while not jeopardizing fertilizer use efficiency, would increase the cost of fertilizer application. The optimum injection depth was observed to be approximately 10 cm. Grain yield response and fertilizer N recovery in the crop increased four‐ and three‐fold, respectively, when injection depth was increased from 2.5 to 10 cm. This effect was attributed to the inaccessibility of fertilizer N residing in dry surface soil. Injection of fertilizer at 15 cm rather than 10 cm demonstrated no additional advantage.

Seasonal Uptake Patterns of Fertilizer Nitrogen Applied in Split Applications to Rice

AbstractWhen rice (Oryza sativa L.) is grown in a direct‐seeded, delayed‐flood system as used in the southern USA, split topdress applications of N fertilizer have proven to be one of the most effective methods for achieving relatively high grain yields and efficient fertilizer‐N uptake. Knowledge concerning the seasonal uptake patterns of fertilizer N applied in split applications is limited. Consequently, a field study was conducted to measure the seasonal uptake of N fertilizer from split applications made at preflood, 1.3‐cm internode elongation (IE), and 14 d after IE (IE + 14 d). To evaluate each fertilizer application, 15N‐labeled urea (2.42 atom% 15N) was applied at preflood (10 g N m−2), IE (5 g N m−2), or IE + 14 d (5 g N m−2). Fertilizer N contained in the soil as NH4, NO3, and organic N were determined periodically after each 15N application, along with the fertilizer and total N in the rice roots, shoots, and panicles. Plant uptake of fertilizer N from the preflood application was essentially complete within 21 d after application (63.2% of the applied N). Nitrogen uptake was essentially complete within 3 d after application of the midseason treatments. Three days after IE, 63.3% of the applied N had been taken up by the plant whereas the plant had taken up 70.1% of the applied N 3 d after the IE + 14 d N application. Approximately 20% of the fertilizer N was recovered in the soil organic fraction at maturity regardless of the time of fertilizer application. Fertilizer N incorporated into the soil organic fraction reached a maximum within 21 d after the preflood treatment.

A two-year experience on 15N-urea fertilization of a corn field amended with fly ash

The effect of different rates of fly ash application on N balance and harvested fertilizer uptake efficiency was investigated in a 2 yr experiment in corn (Zea mays L.) plots, following the addition of 15N-urea (200 kg N ha−1). In both years, the amount of NH inf4 sup− -N, NO inf3 sup− -N, and organic N in soil at various times was not affected significantly (P = 0.05) by the addition of different rates of fly ash. During the first cropping season, the N losses (as determined by a N balance sheet) were 13.4, 13.9, and 11 % of the added N in plots treated with 0, 1.2, and 3.5 % fly ash, respectively. In the second cropping season the N losses were 13.3, 8.7, and 6.9 %. The data obtained in this study provide the type of information which should always be required before the agronomic use of fly ash can be recommended in a particular soil.

The effect of stubble management and N-fertilization practices on the nitrogen economy under intensive rice cropping

The fate of 15N-labelled fertilizer applied to rice (Oryza sativa L) was studied in microplots established within two field experiments comprising a range of stubble levels, stubble management techniques, N application rates and times. The first experiment investigated uptake of soil and fertilizer N in plots where application of 0 or 100 kg N ha-1 to the previous rice crop had produced 11.5 and 16.1 t ha-1 of stubble respectively. The stubble was then treated in one of four ways-burn (no till); burn then cultivated; incorporated in autumn or incorporated at sawing. Microplots within these large plots received 60 kg ha-1 of 5% 15N enriched urea at sowing, just prior to permanent flood (PF), or just after panicle initiation (PI) of the second crop. The second experiment was undertaken within a field in which half of the plots had stubble from the previous three rice crops burned, while the other plots had all stubble incorporated. In the fourth successive rice crop, the two stubble management systems were factorially combined with three N rates (0, 70 or 140 kg N ha-1) and three application times (PF, PI or a 50 : 50 split between PF and PI). Nitrogen uptake and retention in the soil were studied within 15N-labelled microplots established within each of these large plots. Only 4% of the 15N applied at sowing in the first experiment was recovered in the rice crop, while delaying N application to PF or PI increased this to an average of 20% and 44% respectively over the two experiments. The doubling of N application rate doubled fertilizer N uptake and also increased uptake of soil N at maturity by 12 kgN ha-1. Three years of stubble incorporation increased average uptake of fertilizer and soil N in the second experiment by 5 and 12 kg N ha-1 respectively. In both experiments, the soil was the major source of N, contributing 66-96% of total N uptake. On average, in the fourth crop, 20% of fertilizer N was in the grain, 12% in the straw and 3% in the roots, while 23% was located in the top 300 mm of soil. A further 3% was in the soil below 300 mm. The remaining 39% was lost, presumably by denitrification.

Greenhouse Study on the Effects of the Urease Inhibitors Phenyl Phosphorodiamidate and N‐(n‐butyl) Thiophosphoric Triamide on the Efficiency of Urea Applied to Flooded Rice

AbstractUrea was split applied to transplanted rice in a greenhouse experiment with two‐thirds applied as labeled 15N urea at 15 days after transplanting (DAT) and one‐third (not labeled) at 42 DAT to determine the effect of the urease inhibitors phenyl phosphorodiamidate (PPDA) and N‐(n‐butyl) thiophosphoric triamide (NBPT) on urea hydrolysis, plant uptake, yield, and loss of fertilizer N. An acidifying agent [Al2(SO4)3] and an algicide were used to reduce the floodwater pH and thus slow the degradation of PPDA, keeping it effective for a longer period. Algicide addition extended the effectiveness of PPDA inhibition by about 2 days and increased plant uptake and grain yield significantly over that with urea use alone. Al2(SO4)3 addition extended the effectiveness of PPDA only about 1 day, increased N uptake slightly, but failed to increase grain yield. NBPT effectively slowed urea hydrolysis, more than doubled plant uptake over that with urea alone, and increased grain yield by 38%. Percolation at 0.5 cm per day caused plant N uptake to increase by about 6% in all treatments but it was not essential for the inhibitors to have a beneficial effect. For the first split application, fertilizer losses of 50% from urea were decreased to about 10% by use of NBPT and to 28% with PPDA alone, and by combination of PPDA with the algicide losses were 22%.

Uptake of selenite fertilizer by subterranean clover pasture in Western Australia

. Subterranean clover (Trifolium subterraneum) based pastures were fertilised with sodium selenite at 9 rates from 0 to 800 g Se/ha on 2 sites in 1983. In order to measure the residual value in 1984 and 1985, further applications of sodium selenite were superimposed on the original 9 treatments. Green pasture was sampled annually, dry pasture was sampled once, only in 1984 and the concentration of selenium in the pasture was measured. The sampled pasture was sorted into 2 components: subterranean clover, and non-subterranean clover. Except for the third site that had a quadratic response for the non-subterranean clover component of the pasture, the concentration of selenium in plants increased linearly with application rate. The selenium concentration in subterranean clover was lower than that in the other species in the pasture. Differences between years were large: in 1985, the concentration in plant material was twice that in 1983 and 1984. The dry summer feed had higher concentrations of selenium than the green pasture. The residual value of selenite was 25% in the first year and 15% in the second year. Adequate dietary levels of selenium for sheep would require an annual application of about 200 g sodium selenite/ha to these soils.