Nitrogen uptake by autumn sown sugar beet

The influence of N fertilizer rate on uptake and distribution of N in the plant,¹⁵N labelled fertilizer uptake and sugar yield were studied for 3 years on autumn sown sugar beet (Beta vulgaris L.) under Mediterranean (Southern Spain) rain-fed and irrigated conditions. Available average soil N prior to sowing was 69 kg N ha^–1, and net mineralisation in the soil during the growth period was 130 kg N ha^–1. Maximum N uptake occurred in the spring and increased with increasing fertilizer rates in the irrigated crop. There was no increase in N uptake in the sugar beet cropped under rain-fed conditions because of water shortage. Maximum average N uptake both by roots and tops was between 200 and 250 kg N ha^–1. When N fertilizer was not applied, average uptake from the soil was between 130 and 140 kg N ha^–1. At the end of the growth period there was a marked translocation of N from the leaves to the root which increased with the N fertilizer rate. The N ratio top/roots at harvest was 0.45–0.5 and 0.8- - 1 in the irrigated and rain-fed sugar beet, respectively. Maximum¹⁵N labelled fertilizer uptake took place in May-June, being larger in irrigated sugar beet or when spring rainfall was more abundant. Fertilizer use efficiency varied between 30% and 68%. Sugar yield response to N fertilizer rates depended on the N available in the soil and on the total water input to the crop, particularly in spring. The response was more constant in the irrigated crop, where optimum yield was obtained with a fertilizer rate of 160 kg N ha^–1. In the rain-fed crop, the optimum dose proved more erratic, with an estimated mean of 100 kg N ha^–1. The amount of N required to produce 1 t of root and of sugar ranged between 1.5 and 3.8 kg N and between 11.1 and 22.4 kg N respectively, and varied according to the N fertilizer rates applied.     

N application to winter wheat at tillering and shooting: N balance at different growth stages

At two sites, microplots under winter wheat were given 140 kg N ha^–1 as labelled ammonium nitrate split in 80 kg N ha^–1 at tillering and 60 kg N ha^–1 at shooting. Soil and plant samples were analyzed at shooting, after anthesis and at grain harvest and a¹⁵N balance was established. The average recovery rate of 95% indicates that there were no marked N losses due to leaching and denitrification, which is attributed to the low rainfall in the two months after fertilizer application. Between 19 and 23% of the fertilizer N remained in the 0–30 cm soil layer as organically bound soil N. Up to 64% was taken up by the above-ground crop. On the loamy sand, 4% of the fertilizer N at harvest remained in the roots in the 0–30 cm layer and only 3% was found as inorganic N in the 0–90 cm soil layer. The fertilizer N applied diminished plant uptake of soil N in the period between fertilizer application and harvest. As compared with the control, the fertilized plants extracted 25 and 28% less soil N from loamy sand and loess soil, respectively. The results show that application of mineral N fertilizer helps to maintain the mineralizable N content of the soil, which has been accumulated in the course of long-term intensive crop production, by adding N to the soil organic pool and simultaneously reducing the supply of soil N to the plants.     

Calculated rates of soil acidification of intensively used grassland in the Netherlands

The major processes involved in acidification of soils under intensively managed grassland are the transformation and subsequent leaching of applied nitrogen (N), assimilation of excess cations in herbage and acidic atmospheric deposition. Carbonates from fertilizers and excess cations in purchased concentrates are the most important proton (H⁺) neutralizing agents applied to grassland. In this study, the effects of grazing, cutting and N application on the net proton loading from each of the main processes were calculated, using a simple model.On mown swards, simulated excess cation uptake by the sward released 4.5–9.3 kmol_c H⁺ ha^–1 yr^–1. The total proton loading on mown grassland decreased from about 8.0 to 5.3 kmol_c ha^–1 yr^–1 when fertilizer N input as CAN-27 increased from 0 to about 400 kg ha^–1 yr^–1. Contributions from atmospheric deposition ranged from 2.2 kmol_c ha^–1 yr^–1 when herbage yield exceeded 10 Mg ha^–1 yr^–1 to 3.0 kmol_c ha^–1 yr^–1 when herbage production was only 5.5 Mg ha^–1 yr^–1.On grazed swards, transformation of organically bound N from urine and dung to nitrate (NO ₃ ^- ) and the subsequent leaching of excess NO ₃ ^- was the main source of protons. Application of 400 kg N ha^–1 yr^–1 to grazed swards increased the proton loading from transformed N from 3.9 to 16.9 kmol_c ha^–1 yr^–1. The total proton loading on grazed swards exceeded that of mown swards when the input of fertilizer N exceeded 150 kg ha^–1 yr^–1.Underestimation of the amount of N immobilized in the soil biomass and lost by denitrification may have resulted in a slight overestimation of the amount of N lost by leaching and thereby also the simulated total proton loading.     

Very high applications of nitrogen fertilizer on grassland and residual effects in the following season

At very high nitrogen applications (480 and more kg N ha^–1 yr^–1) in field trials on all-grass swards the amount of N applied exceeded the amount of N harvested. In the humid temperate climate of the Netherlands in the subsequent spring approximately 25, 40, and 50% of this excess nitrogen was recovered as accumulated mineral nitrogen in the 0–100 cm layer of sandy, clay and heavy clay soil, respectively. The effect of this excess nitrogen on growth during the subsequent season was measured through the increase in DM and N yield over a reference treatment. In this season all treatments received a uniform application (40 kg N ha^–1 cut^–1). Residual effects were absent on sandy soil but distinct on the clay soils. On the clay soils each accumulated kg soil mineral nitrogen produced 15 kg DM. Assuming a relatively small contribution of residual nitrogen carried over in stubble, roots and organic matter, the accumulated soil mineral nitrogen would seem to be as effective as applied fertilizer nitrogen.     

Management of nitrogen by fertigation of potato in Lebanon

Reports on soil and groundwater contamination with nitrates in Lebanon and other developing countries could be related to the mismanagement of water and fertilizer inputs. The objective of this work was to determine the N requirements and N-use efficiency of a main-crop potato in Lebanon, irrigated by a drip system, compared to the farmer's practice of macro-sprinkler. In the drip irrigation, fertilizers input was as soil application at the time of sowing or added continuously with the irrigation water (fertigation). Nitrogen-fertilizer recovery was determined using ¹⁵N-labeled ammonium sulfate. Fertigation with continuous N feeding based on actual N demands and available sources allowed for 55% N recovery. For a total N uptake of 197 kg ha^–1 per season in the lower N rate, the crop removed 66 kg N ha^–1 from fertilizers. The spring potato crop in this treatment covered 44.8% of its N need from the soil and 21.8% from irrigation water. Higher N input increased not only N derived from fertilizers, but also residual soil N. Buildup of N in the soil with the traditional potato fertilization practice reached 200 kg N ha^–1 per season. With increasing indications of deteriorating groundwater quality, we monitored the nitrate leaching in these two watering regimes using soil solution extractors (tensionics). Nitrate leaching increased significantly with the macro-sprinkler technique. But N remained within the root zone with the drip irrigation. The crop response to applied N requires a revision of the current fertilizer recommendation in semi-arid regions, with an improved management of fertilizer and water inputs using fertigation to enhance N recovery.     

The effect of fertilizer placement on nitrogen uptake and yield of wheat and maize in Chinese loess soils

Field trials were carried out to study the fate of¹⁵N-labelled urea applied to summer maize and winter wheat in loess soils in Shaanxi Province, north-west China. In the maize experiment, nitrogen was applied at rates of 0 or 210 kg N ha^–1, either as a surface application, mixed uniformly with the top 0.15 m of soil, or placed in holes 0.1 m deep adjacent to each plant and then covered with soil. In the wheat experiment, nitrogen was applied at rates of 0, 75 or 150 kg N ha^–1, either to the surface, or incorporated by mixing with the top 0.15 m, or placed in a band at 0.15 m depth. Measurements were made of crop N uptake, residual fertilizer N and soil mineral N. The total above-ground dry matter yield of maize varied between 7.6 and 11.9 t ha^–1. The crop recovery of fertilizer N following point placement was 25% of that applied, which was higher than that from the surface application (18%) or incorporation by mixing (18%). The total grain yield of wheat varied between 4.3 and 4.7 t ha^–1. In the surface applications, the recovery of fertilizer-derived nitrogen (25%) was considerably lower than that from the mixing treatments and banded placements (33 and 36%). The fertilizer N application rate had a significant effect on grain and total dry matter yield, as well as on total N uptake and grain N contents. The main mechanism for loss of N appeared to be by ammonia volatilization, rather than leaching. High mineral N concentrations remained in the soil at harvest, following both crops, demonstrating a potential for significant reductions in N application rates without associated loss in yield.     

Effects of deep application of urea on NO and N2O emissions from an Andisol

A modeling study revealed that the depth of nitric oxide (NO) production in soil is crucial for its flux, while that of nitrous oxide (N₂O) is not. To verify this result, laboratory experiments with soil columns classified as Andisol (Hydric Hapludand) were conducted, with changing the depth of urea application, at 0–0.1 or 0.1–0.2 m. All the NO concentration profiles in soil exhibited a sharp peak at each fertilized layer within 5 days of fertilizer application. NO concentration in soil decreased abruptly as the distance from the fertilized layer increased. These findings imply that NO is produced mainly within the fertilized layer, but does not diffuse widely in the soil columns, because of rapid NO uptake within the soil. As a result, the NO flux from soil columns fertilized at 0.1–0.2 m depth over the 48-day study period was reduced to almost the same rate as that of the unfertilized one. The total NO emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m depth were 0.02, 1.39 (± 0.05) and 0.05 (± 0.03) kg N ha^–1, respectively, suggesting that NO emission derived from N fertilizer could be reduced to 2% by shifting the depth of fertilizer application by 0.1 m. On the other hand, soil N₂O concentration profiles exhibited a gentler peak, because of the lower uptake by soil. N₂O fluxes were affected more by the soil conditions, e.g. soil water content, than the distance between fertilized depth and soil surface. The total N₂O emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m were 0.02, 0.16 (± 0.03) and 0.25 (± 0.04) kg N ha^–1, respectively.     

Effects of different levels of chemical Nitrogen (urea) onAzolla and Blue-green algae intercropping with rice

Application of higher levels (60 and 90 kg N ha^–1) of nitrogen fertilizer (Urea) inhibited the growth ofAzolla pinnata (Bangkok) and blue-green algae (BGA) though the reduction was more in BGA thanAzolla. Inoculation of 500 kg ha^–1 of freshAzolla 10 days after transplanting (DAT) in the rice fields receiving 30, 60 and 90 kg N ha^–1 as urea produced an average of 16.5, 15.0 and 13.0 t ha^–1 fresh biomass ofAzolla at 30 DAT, which contained 31, 31 and 27 kg N ha^–1, respectively. The dry mixture of BGA (60%Aulosira, 35%Gloeotrichia and 5% other BGA on fresh weight basis) inoculated in rice field 3 DAT at a rate of 10 kg ha^–1 showed a mat formation at 80 DAT with an average fresh biomass of 8.0, 5.8 and 4.2 t ha^–1 containing 22, 17 and 12 kg N ha^–1, respectively with those N fertilizer doses.Application ofAzolla showed positive responses to rice crop by increasing the panicle number and weight, grain and straw yields and nitrogen uptake in rice significantly at all the levels of chemical nitrogen. But, the BGA inoculation had a significant effect on the grain and straw yields only during the dry season in the treatment where 30 kg N was applied. During the wet season and in the other treatments performed during the dry season no significant increase in yields, yield components and N uptake were observed with BGA.The intercropping ofAzolla and rice in combination with 30, 60 and 90 kg N ha^–1 as urea showed the yields, yield attributes and nitrogen uptake in rice at par with those obtained by applying 60, 90 and 120 kg N ha^–1 as urea, respectively but, the BGA did not. The analysis of soil from rice field after harvest showed thatAzolla and BGA intercropping with rice in combination with chemical fertilizer significantly increased the organic carbon, available phosphorus and total nitrogen of soil.     

Nitrogen fertilizer in leucaena alley cropping. I. Maize response to nitrogen fertilizer and fate of fertilizer-15N

Field microplot experiments were conducted in the semi-arid tropics of northern Australia to evaluate the response of maize (Zea mays L.) growth to addition of N fertilizer and plant residues and to examine the fate of fertilizer¹⁵N in a leucaena (Leucaena leucocephala) alley cropping system, in which supplemental irrigation was used. Leucaena prunings, maize residues and N fertilizer were applied to alley-cropped maize grown in microplots which were installed in the alleys formed by leucaena hedgerows spaced 4.5 metres apart. The¹⁵N-labelled fertilizer was used to examine the fate of fertilizer N applied in the presence of mulched leucaena prunings and maize residues.Application of leucaena prunings increased maize yield while addition of N fertilizer in the presence of the prunings produced a further increase in maize production. There was a significant positive interaction between N fertilizer and leucaena prunings in increasing maize production. The addition of maize residues in the presence of N fertilizer and leucaena prunings decreased maize yield and N uptake and increased fertilizer¹⁵N loss from 38% to 47%. Maize recovered 24–79% of fertilizer¹⁵N in one cropping season, depending on application rate of N fertilizer and field management of plant residues. About 20–34% of fertilizer¹⁵N remained in the soil. More than 37% of fertilizer¹⁵N was apparently lost from the soil and plant system largely through denitrification when N fertilizer was applied at 40 kg N ha^–1 or more in the presence or absence of plant residues. Application of N fertilizer improved maize yield and increased the contribution of mulched leucaena prunings to crop production in the alley cropping system.     

The effect of plant residues on plant uptake and leaching of soil and fertilizer nitrogen in a tropical red earth soil

Two field experiments, in which differing amounts and types of plant residues were incorporated into a red earth soil, were conducted at Katherine, N.T., Australia. The aim of the work was to evaluate the effect of the residues on uptake of soil and fertilizer N by a subsequent sorghum crop, on the accumulation and leaching of nitrate, and on losses of N.Stubble of grain sorghum applied at an exceptionally high rate (~ 18 000 kg ha^–1) reduced uptake of N by sorghum by 13% and depressed the accumulation of nitrate under a crop and particularly under a fallow.Loss of fertilizer N, movement of nitrate down the profile, and uptake by the crop was studied in another experiment after application of N as¹⁵NH₄ ¹⁵NO₃ to field microplots. By four weeks after fertilizer application 14% had been lost from the soil-plant system and by crop maturity 36 per cent had been lost. The pattern of¹⁵N distribution in the profile suggested that losses below 150 cm had occurred during crop growth. The recovery of¹⁵N by the crop alone ranged from 16 to 32 per cent. There was an apparent loss of N from the crop between anthesis and maturity. Residue levels common to sorghum crops in the region (~ 2000 kg ha^–1) did not significantly affect uptake by a subsequent sorghum crop, N losses, or distribution of nitrate in the profile.     

Initial and residual effects of nitrogen fertilizers on grain yield of a maize/bean intercrop grown on a Humic Nitosol and the fate and efficiency of the applied nitrogen

Initial and residual effects of nitrogen (N) fertilizers on grain yield of a maize/bean intercrop grown on a deep, well-drained Humic Nitosol (66% clay, 3% organic carbon) were evaluated. Enriched (¹⁵N) N fertilizer was used to study the fate of applied N in two seasons: using urea (banded) at 50 kg N ha^–1 in one season, and¹⁵N-enriched urea (banded), calcium ammonium nitrate (CAN, banded), and urea supergranules (USG, point placement) were applied in the other season (different field) at 100 kg N ha^–1. Nitrogen fertilizer significantly (P = 0.05) increased equivalent maize grain yield in each season of application with no significant differences between N sources, i.e., urea, CAN, and USG. Profitmaximizing rates ranged from 75 to 97 kg N ha^–1 and value: cost ratios ranged from 3.0 to 4.8. Urea gave the highest value: cost ratio in each season. Most (lowest measurement 81%) of the applied N was accounted for by analyzing the soil (to 150 cm depth) and plant material. Measurements for urea, CAN, and USG were not significantly different. The high N measurements suggest low losses of applied N fertilizer under the conditions of the study. Maize plant recovery ranged from 35 to 55%; most of this N (51–65%) was in the grain. Bean plant recovery ranged from 8 to 20%. About 34–43% of the applied N fertilizer remained in the soil, and most of it (about 70%) was within the top soil layer (0–30 cm). However, there were no significant equivalent maize grain increases in seasons following N application indicating no beneficial residual effect of the applied fertilizers.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

A microplot study of the fate of15N-labelled ammonium nitrate and urea applied at two rates to ryegrass in spring

Confined microplots were used to study the fate of¹⁵N-labelled ammonium nitrate and urea when applied to ryegrass in spring at 3 lowland sites (S1, S2 and S3). Urea and differentially and doubly labelled ammonium nitrate were applied at 50 and 100 kg N ha^–1. The % utilization of the¹⁵N-labelled fertilizer was measured in 3 cuts of herbage and in soil to a depth of 15 cm (soil_0–15).Over all rates, forms and sites, the % utilization values for cuts 1, 2, 3 and soil_0–15 were 52.4, 5.3, 2.4 and 16.0% respectively. The % utilization of¹⁵N in herbage varied little as the rate of application increased but the % utilization in the soil_0–15 decreased as the rate of application increased. The total % utilization values in herbage plus soil_0–15 indicated that losses of N increased from 12 to 25 kg N ha^–1 as the rate of N application was increased from 50 to 100 kg N ha^–1.The total % utilization values in herbage plus soil_0–15 over both rates of fertilizer N application were 84.1, 80.8 and 81.0% for urea compared with 74.9, 72.5 and 74.4% for all ammonium nitrate forms at S1, S2 and S3 respectively. Within ammonium nitrate forms, the total % utilization values in herbage plus soil_0–15 over both rates and all sites were 76.7, 69.4 and 75.7% for¹⁵NH₄NO₃, NH₄ ¹⁵NO₃ and¹⁵NH₄ ¹⁵NO₃ respectively. The utilization of the nitrate moiety of ammonium nitrate was lower than the utilization of the ammonium moiety.The distribution of labelled fertilizer between herbage and soil_0–15 varied with soil type. As the total utilization of labelled fertilizer was similar at all sites the cumulative losses due to denitrification and downward movement appeared to account for approximately equal amounts of N at each site.     

A farm-level evaluation of nitrogen and phosphorus fertilizer use and planting density for pearl millet production in Niger

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers' fields. Farmers in this region plant at low density (average 5,000 pockets ha^–1, 3 plants pocket^–1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha^–1 and 30 kg P₂O₅ ha^–1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets ha^–1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets ha^–1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets ha^–1 had average yields of 317 kg grain ha^–1 while those planting at densities higher than 6,500 pockets ha^–1 showed average yields of 977 grain ha^–1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets ha^–1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets ha^–1) or no economic benefit from fertilizer use will be realized.     

Recovery of 15N-labelled fertilizer applied to winter wheat and perennial ryegrass crops and residual 15N recovery by succeeding wheat crops under different crop residue management practices

The recovery of ¹⁵N-labelled fertilizer applied to a winter wheat (120 kg N ha^–1) and also a perennial ryegrass (60 kg N ha^–1) crop grown for seed for 1 year in the Canterbury region of New Zealand in the 1993/94 season was studied in the field. After harvests, ryegrass and wheat residues were subjected to four different residue management practices (i.e. ploughed, rotary hoed, mulched and burned) and three subsequent wheat crops were grown, the first succeeding wheat crop sown in 1994/95 to examine the effects of different crop residue management practices on the residual ¹⁵N recovery by succeeding wheat crops. Total ¹⁵N recoveries by the winter wheat and ryegrass (seed, roots and tops) were 52% and 41%, respectively. Corresponding losses of ¹⁵N from the crop-soil systems represented by un-recovered ¹⁵N in crop and soil were 12% and 35%, respectively. These losses were attributed to leaching and denitrification. The proportions of ¹⁵N retained in the soil (0-400 mm depth) at the time of harvest of winter wheat and ryegrass were 36% and 24%, respectively. Although the soil functioned as a substantial sink for fertilizer N, the recovery of this residual fertilizer by subsequent three winter wheat crops was low (1-5%) and this was not affected by different crop residue management practices.     

Uptake of NPK and yield of rainfed groundnut (Arachis hypogaea L.)

Experiments were conducted on sandy loam soils of Tirupati campus of Andhra Pradesh Agricultural University for two rainy seaons of 1980 and 1981 to study the effect of split application of NPK fertilizers on Spanish bunch groundnut. The fertilizer doses were 40 N, 20 P and 40 K kg ha^–1 in 1980 and 30 N, 10 P and 25 K kg ha^–1 in 1981.In 1980, uptake of N (48 kg ha^–1), P (7 kg ha^–1) and K (37 kg ha^–1) was maximum with the application of 10 N, 5 P and entire 40 K kg ha^–1 as basal and 30 N and 15 P kg ha^–1 at 30 days after sowing, leading to highest pod yield (0.76 t ha^–1). In 1981, application of 20 N, 10 P and 25 K kg ha^–1 as basal dose and 20 N kg ha^–1 at 30 days after seeding resulted in highest uptake of N (114 kg ha^–1), P (17 kg ha^–1) and K (58 kg ha^–1) and hence the pod yield (2.36 t ha^–1).Differences in the uptake of NPK and pod yield in 1980 and 1981 was due to variation in total rainfall and its distribution during the crop period. Rainfall was equally distributed throughout the crop period in 1981, whereas there were two prolonged dry spells of more than 40 days in 1980.     

Nitrogen requirements of corn (Zea mays L.) as affected by monocropping and intercropping with Alfalfa (Medicago sativa)

Intercropping perennials with corn has the potential to improve utilization of the growing season over monocropping corn in regions where a substantial portion of the growing season is too cool for corn growth. The biomass potential and fertilizer N requirements of monocropped corn (Zea mays) grown using conventional tillage were compared with those of corn intercropped with alfalfa (Medicago sativa) in 1987 and 1988. The intercropped alfalfa was harvested once prior to planting the corn each spring. Rotation effects on and N fertilizer requirements for monocropped corn following these treatments and also following monocropped alfalfa, were evaluated in 1989 and 1990. During the two years of intercropping for which data is presented, the critical intercropped corn biomass (13.05 Mg ha^–1) estimated using a quadratic-plus plateau model, was close to the monocropped corn biomass (13.01 Mg ha^–1), but an estimated 83 kg ha^–1 more N was required for intercropped corn to reach the critical biomass. Total biomass (intercropped corn and alfalfa) was 25% greater than that of the monocropped corn, and the total N uptake was 55% greater than that by monocropped corn over the two- year period. After rotation to monocropped corn using conventional tillage in 1989, corn biomass averaged over N rates following intercropping or monocropped corn was lower (P=0.01) than following monocropped alfalfa. Critical corn biomass estimated was highest following alfalfa and lowest following monocropped corn, and more N fertilizer was required to attain the critical biomass under continuous monocropped corn in 1989. Corn yields and N uptake values in 1990 were not significantly different among the cropping systems. The N fertilizer replacement values due to intercropping decreased from above 90 kg N ha^–1 in the first year of rotation to less than 40 kg N ha^–1 in the second year of rotation. Considering the higher potential for total biomass production and rotation benefit, intercropping is a viable alternative to conventional corn monoculture for forage production.     

Residual N and P fertilizer effect and fertilizer recovery on intercropped and sole-cropped corn and bean in semiarid northeast Brazil

The effects of a single ¹⁵N and P fertilizer application (16 and 12 kg ha^–1) on intercropped and sole-cropped corn and beans was followed over three consecutive years. Grain (0.1–0.9 ton ha^–1 yr^–1) and straw productions (0.2–2.5 ton ha^–1 yr^–1) were limited by rainfall and showed small responses to fertilizer. In the first year, plant N uptake was more than twice the fertilizer amounts, while P uptake was less than half the fertilizer amounts. Plant N derived from fertilizer was low (9–19%). Sole corn took up more (34%) than beans (16%) and the combined intercrop (26%) and also had higher recovery of fertilizer in the soil than single beans (50% against 28%). The distribution of fertilizer N and P in the soil showed a similar pattern in all treatments, with a high concentration around the application spot and decreasing concentrations at greater distances and above and below this point. Total P increases in a soil volume 10 cm around the application spot corresponded to 60% of the amount applied. Fertilizer contributions to the second crop were < 3% of total plant N and represented <6% of the applied amount. Therefore, the residual fertilizer effect on production was attributable to P. The patterns of fertilizer N and P distribution in the soil remained similar but N recoveries decreased 14–18%. Despite low rainfall, low productivities and reasonable proportions of fertilizer N remaining in the soil, the residual effects of the applied fertilizer N were too low to justify a fertilizer recommendation based on economic returns on the investment.     

The effect of foliar supplements of potassium nitrate and urea on the yield of winter wheat

Winter wheat crops were grown with ostensibly adequate supplies of all soil nutrients in 1990 and 1991 with the aim of testing if late foliar supplements of K and N, applied at key development stages, could improve grain yield and grain N content. Foliar sprays of KNO₃ solution, supplying up to 40 kg K ha^–1 in total, at flag leaf unfolded, inflorescence completed and the watery-ripe stage of grain filling, had no effect on yield, yield components or grain N. Urea, supplying 40 kg N ha^–1 at flag leaf unfolded, had no effects on grain yield and grain N in 1990, but in 1991 grain N was increased by 0.14% whilst yield was reduced by up to 0.6 t ha^–1. Urea scorched flag leaf tips in both years. In 1990, the spring was very dry and foliar supplements might have been expected to have had an effect, but on this highly fertile soil all crop K and N requirements were met from the soil.     

Recovery of15N-labelled fertilizer by sugar beet—spring wheat and winter rye—sugar beet cropping sequences

The recovery of¹⁵N labelled ammonium fertilizer was studied during two cropping sequences: sugar beet—spring wheat and winter rye—sugar beet with the labelled N applied to the first crop of each sequence. The difference between fall and spring application was also investigated. For the first cropping sequence 100 kg N ha^–1 labelled with 11.4%¹⁵N atom excess (a.e.) was applied to the sugar beets. This labelled N was followed in the sugar beets, in the soil profile at harvesting time as well as in the spring wheat of the following year. The first crop of sugar beet recovered 43–46% of the applied N, with 26–29% remaining in the soil at harvesting time and 25–31% could not be accounted for. Of the residual N, less than 1% could be recovered by the next crop of spring wheat. For the second cropping sequence 50 kg N ha^–1 labelled with 11.5%¹⁵N a.e. was applied to the winter rye and followed in the winter rye and in the sugar beets of the following year. The recovery of the labelled fertilizer N applied to the winter rye of the second sequence was 20–27% and the sugar beets of the next year could only recover 2%. With respect to time of application, no difference in fertilizer N recovery was found between fall or spring application for the two sequences.