Squash yield, nutrient content and soil fertility parameters in response to methods of fertilizer application and rates of nitrogen fertigation

Two field experiments were conducted to evaluate squash yield and nutrient content in response to different fertigation nitrogen (N) rates and method of fertilizer N application. The following treatments were studied in a randomized complete block design with four replications: zero N (N0), 50 (N1), 100 (N2) and 150 (N3) mg l^–3 N concentration in the irrigation water (IW) (fertigation treatments) and a soil application treatment (NS) equivalent to the N2 treatment. Irrigation was applied to replenish 80% of the Class A pan evaporation twice a week. Compared to the control (N0), shoot dry matter and yield were increased by all fertigation N rates and by the soil application treatment. However, soil application gave a lower yield than the equivalent fertigation N rate, indicating the comparative advantage of fertigation. The lowest fertigation N rate was adequate to give the highest yield in the first season, while in the second season a higher rate was necessary to achieve the maximum yield. The growth and fruit yield were higher in the second season as a result of the more favorable climatic conditions. Regression relationships indicate that the yield and the shoot dry weight were related to the fertigation N rates by polynomial quadratic relationships. The response to N in the second season was greater, as indicated by the steeper positive slope. The fruit yield was linearly related to both fruit number and fruit size in both seasons. N contents in shoots increased with N addition and were higher in both fruit and shoot during fruiting with the fertigation method. Soil salinity slightly increased with N application, especially in the top 15 cm, but remained low and acceptable for normal plant growth. Soil P increased mainly in the top soil following phosphoric acid application to all plots. Restricted P movement to deeper soil is attributed to the expected precipitation and/or sorption reactions with Ca and Mg in calcareous soils. It can be concluded that fertigation is more effective than soil application in increasing the yield and with fertigation lower N rates would be adequate to produce higher yield, thus lowering fertilization cost and minimizing environmental impact of over-fertilization.     

Fate of Fertilizer15N applied as urea and ammonium sulphate in opium poppy (Papaver somniferum L.) grown under greenhouse conditions

Laboratory incubation and greenhouse experiments were conducted to investigate the comparative effectiveness of urea and ammonium sulphate in opium poppy (Papaver somniferum L.) using¹⁵N dilution techniques. Fertilizer treatments were control (no N), 600 mg N pot^–1 and 1200 mg N pot^–1 (12 kg oven dry soil) applied as aqueous solution of urea or ammonium sulphate. Fertilizer rates, under laboratory incubation study were similar to that under greenhouse conditions. A fertilizer¹⁵N balance sheet reveals that N recovery by plants was 28–39% with urea and 35–45% with ammonium sulphate. Total recovery of¹⁵N in soil-plant system was 77–82% in urea. The corresponding estimates for ammonium sulphate were 89–91%. Consequently the unaccounted fertilizer N was higher under urea (18–23%) as compared to that in ammonium sulphate (9–11%). The soil pH increased from 8.2 to 9.4 with urea whereas in ammonium sulphate treated soil pH decreased to 7.3 during 30 days after fertilizer application. The rate of NH₃ volatilization, measured under laboratory conditions, was higher with urea as compared to the same level of ammonium sulphate. The changes in pH of soil followed the identical trend both under laboratory and greenhouse conditions.     

Movement and transformations of fertigated nitrogen below trickle emitters and their effects on pH in the wetted soil volume

The movement and transformations of ammonium-, urea- and nitrate-N in the wetted volume of soil below the trickle emitter was studied in a field experiment following the fertigation of N as ammonium sulphate, urea and calcium nitrate. Effects on soil pH in the wetted volume were also investigated.During a fertigation cycle (emitter rate 2lh^–1) applied ammonium was concentrated in the surface 10 cm of soil immediately below the emitter and little lateral movement occurred. In contrast, because of their greater mobility in the soil, fertigated urea and nitrate were more evenly distributed down the soil profile below the emitter and had moved laterally in the profile to 15 cm radius from the emitter. The conversion of applied N to nitrate-N was more rapid when urea rather than ammonium-N was applied suggesting that the accumulation of large amounts of ammonium below the emitter in the ammonium sulphate treatment probably retarded nitrification.Following their conversion to nitrate-N, both fertigated ammonium sulphate and urea caused acidification in the wetted soil volume. Acidification was confined to the surface 20 cm of soil in the ammonium sulphate treatment, however because of its greater mobility, fertigation with urea (2lh^–1) resulted in acidification occurring down to a depth of 40 cm. Such subsoil acidity is likely to be very difficult to ameliorate. Increasing the trickle discharge rate from 2lh^–1 to 4lh^–1 reduced the downward movement of urea and encouraged its lateral spread in the surface soil. As a consequence, acidification was confined to the surface (0–20 cm) soil.     

Comparison of several nitrogen fertilisers applied in surface irrigation systems. I. Crop response

The effect of several N carriers applied in the surface irrigation water on the growth, yield and N status of maize was studied in 2 seasons. The carriers applied in the water included anhydrous ammonia, ammonium sulphate, ammonium nitrate, potassium nitrate and urea and they were compared with a preplant band application of anhydrous ammonia and a control treatment. All N treatments received 100 kg N ha^–1. The site used in the second experiment was less responsive to N fertiliser than the first site and the crop growth in the second season was affected by an attack of charcoal rot (Macrophomina phaseolina).Urea, as a N source for fertigation, was superior to the ammonium forms, while the nitrate carriers were the least efficient. Water-run urea increased the maize yield by 27% when compared with the band application in the first season but was 6% lower in the second season. Fertigation allowed N to be applied during the grand period of growth when N stress was most likely to occur. This technique for applying N fertiliser to surface irrigated crops has been adopted by commercial growers.     

Phosphorus fertigation of trickle-irrigated potato

A 3-year field study, on Pellic Vertisol, investigated the response of trickle-irrigated potato (Solanum tuberosum L.) to four P levels applied with the irrigation water. Waters supplied with 130 and 120 mg l^–1 of N and K, respectively, and with P levels of 0, 20, 40 and 60 mg l^–1, were applied when the soil water potential was between 0.03 and 0.04 MPa. The water applied at each irrigation was equivalent to 0.8 of pan evaporation from a screened USWB Class A pan. With the application of 40 mg P l^–1 no P accumulation deeper in the soil profile occurred, whereas P in petioles was at levels recommended for high yield of good quality. At this P concentration in irrigation water, removal of P from soil by tubers was 22 kg/ha/year. The highest yield of good quality was obtained at 40 mg P l^–1.     

Chemical fertilizer pollution control using drip fertigation for conservation of water quality in Danjiangkou Reservoir

The use of drip fertigation to reduce fertilizer pollution of the Danjiangkou Reservoir of China was explored. Specifically, a 4-year experiment was conducted to identify the optimum fertilizer rate for fertigation of a tea plantation. The treatments included five fertigation levels, 10 % (10 % NPK), 20 % (20 % NPK), 30 % (30 % NPK), 40 % (40 % NPK) and 50 % (50 % NPK) of the traditional fertilizer dose, and the traditional fertilizer dose under rain-fed conditions as a control (CK). Relative to CK, fertigation treatments decreased total nitrogen (TN), total phosphorus (TP) runoff loss (kg ha^?1) and TN and TP concentrations (mg kg^?1) in leaching water by 51.9–70.8, 51.7–67.5, 56.1–85.4, and 39.1–56.5 %, respectively. Total N, TP runoff loss and TN concentrations in leaching water generally increased with increasing fertilization rate among fertigation treatments; however, there was no significant difference in TP concentrations of leaching water. Yield increased with increasing rate of fertilization among fertigation treatments. The yields of the 30–50 % NPK treatments were similar (P > 0.05), but higher (P < 0.05) than those of the 10–20 % NPK treatments. There were no significant differences in yield among the 30 % NPK, 40 % NPK and CK treatments. The relative yields increased rapidly when the relative fertilizer rate was between 10 and 33 %; however, this increase slowed at fertilization rates above 33 %. Based on the yield and fertilizer pollution control, fertigation with 33 % of the traditional fertilizer dose should be considered for tea production in the water source area.     

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.     

Irrigation method and application timing effect on potato nitrogen fertilizer uptake efficiency

Establishment of proper guidelines for irrigation and nitrogen (N) fertilizer management may lead to higher crop fertilizer N use efficiency (FNUE), increasing water conservation and reducing nutrient losses from agricultural systems. The objective of this study was to determine FNUE of potato for three application timings: at planting, emergence and tuber initiation cultivated under three irrigation methods: seepage, subirrigation and sprinkler. A total of 168 kg ha^?1 of N was equally split into three applications of 56 kg ha^?1 as ammonium nitrate (NH₄NO₃). FNUE from each application timing in all irrigation methods was evaluated substituting the conventional N fertilizer by an isotope labeled-ammonium nitrate (¹⁵NH ₄ ¹⁵ NO₃) with 1.18% enrichment in excess. Irrigation method had no significant effects on tuber yield and FNUE. The average tuber yield was 32.1 Mg ha^?1 and overall FNUE was 41%. Across the N application timing treatments, the lowest FNUE was measured for the at-planting application (18%), followed by the emergence N application (44%) and tuber initiation N application (62%). Unaccounted N fertilizer during the potato season amounted to 98 kg ha^?1 from the total 168 kg ha^?1 of N applied. N applied at emergence and tuber initiation were important to increase FNUE and tuber yield, however, some N was required at planting, even with the high potential of N losses for that application.     

Efficiency of urea and ammonium sulfate for wetland rice grown on an acid sulfate soil as affected by rate and time of application

A pot experiment was conducted in a greenhouse to assess the effect of rate and time of N application on yield and N uptake of wetland rice grown on a Rangsit acid sulfate soil (Sulfic Tropaquepts). Response of rice at N rates of 800, 1600 and 2400 mg N/pot (5 kg of soil) was compared between urea and ammonium sulfate when applied at two times: (i) full-rate basal at transplanting and (ii) one half at transplanting and one half at the PI stage. In addition, labelled¹⁵N sources were applied either at transplanting or at the PI stage to determine the nitrogen balance sheet in the soil/plant system.No significant difference in grain and straw yields between urea and ammonium sulfate at low rate was observed. At the higher N rates, urea produced higher yields than did ammonium sulfate regardless of timing. The highest yields were obtained when urea at the high N rate was applied either in a single dose or a split dose while lowest yields were observed particularly when ammonium sulfate at the same rate was applied. Split application of N fertilizer was shown to be no better than a single basal application. The occurrence of nutritional disorder, a symptom likely reflected by high concentration of Fe (II) in combination with soluble Al, was induced with high rate of ammonium sulfate.In terms of fertilizer N recovery by using¹⁵N-labelling, ammonium sulfate was more efficient than urea when both were applied at transplanting. In contrast, application at the PI stage resulted in higher utilization of urea than of ammonium sulfate. The recovery of labelled N in the soil was higher with urea than with ammonium sulfate when the two sources were applied at transplanting, while the opposite result was obtained when the same fertilizers were applied at the PI stage. The losses from urea and ammonium sulfate were not different when these fertilizers were applied at transplanting but loss from urea was higher than that from ammonium sulfate when both were applied at the PI stage.     

Nitrogen use efficiency in different rice-based rotations in southern China

Experiments in fields and micro-plots were conducted to investigate the optimal cropping system and nitrogen (N) fertilizer application rate and timing. The treatments consisted of Chinese milk vetch–rice (CMV–R) rotation with five N fertilizer application rates (0, 120, 180, 240, 300 kg N ha^?1) during the rice-growing season, and fallow–rice (F–R) and wheat–rice (W–R) rotations with only one N application rate (240 kg N ha^?1) each. Rice yield increased with increasing N fertilizer application rate under CMV–R rotation, and achieved highest yield under CMV–R180. There is a decreasing trend when N application rate exceeded 180 kg N ha^?1. Rice yield was always higher under CMV–R240 compared to W–R240 and F–R240. During the 2012 rice season, the fertilizer N-use efficiency, residual N fertilizer in soil and N fertilizer recovery efficiency of CMV–R180 reached largest under CMV–R rotation with different N treatments. Furthermore, the fertilizer N-use and recovery efficiencies of CMV–R240 and F–R240 were far higher than those of W–R240. In 2013, fertilizer N-use efficiency was the highest (>?50%) at the heading stage, which was nearly twice as much as the efficiencies during the basal and tillering stages. The N fertilizer loss rate during the basal stage was significantly higher than that at the tillering and heading stages, which was up to 60%. CMV–R rotation with 180 kg N ha^?1 achieved the highest rice yield of 9454 kg ha^?1 and high fertilizer N-use efficiency (40.6%) under a relatively lower N application rate. Therefore, Chinese milk vetch–rice cropping system could be a promising approach for decreasing fertilizer inputs to prevent N pollution problems and increasing rice yield, especially for the intensive rice-based cropping systems in southern China.     

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.     

Nitrogen losses from labelled ammonium sulphate and Urea applied to a flooded rice soil

A field study was carried out to estimate volatilization and denitrification losses of¹⁵N applied as urea of ammonium sulphate to a wet land rice soil. Nitrapyrin (a nitrification inhibitor) was also applied to some treatments along with the two N sources.The N level in floodwater increased rapidly, soon after applying fertilizer N, but decreased to lower values within a few days. At 1 week after applying urea and ammonium sulphate, N losses were 37.6% and 60.6% respectively. The corresponding figures after 4 weeks were 55.7% and 61.9% while with nitrapyrin added the corresponding values were 37.2% and 57.2% after 1 week and 52.7 and 65.0% after 4 weeks respectively indicating that losses due to dentrification are negligible.     

Nitrogen fertilizer value of activated sewage derived protein: Effect of environment and nitrification inhibitor on NO 3 - release,soil microbial activity and yield of summer cabbage

Yield response of summer cabbage (Brassica oleracea varcapitata cv. Hispi F₁) to N applied as organic (activated sewage sludge derived protein [Protox] and dried blood) and inorganic (ammonium nitrate, ammonium sulphate, sodium nitrate and urea) fertilizers was compared in relation to the N availability characteristics of the materials. Effects of the nitrification inhibitor dicyandiamide (DCD) on N release, crop yield and N status were also assessed. In addition CO₂ efflux was measured from amended soil to determine effects of fertilizer application on soil microbial activity. The organic N sources were mineralized quickly on application to soil and exhibited similar patterns of NH₄-N depletion and NO₃-N accumulation as functions of thermal-time as with mineral fertilizers. However, the yield response to organic N was marginally smaller (though not significantly) compared with mineral forms; probably because less N was released to the crop. This was reflected in smaller total N concentrations and N recoveries in plants supplied with organic fertilizer. Applied DCD increased the thermal-time for complete nitrification of NH₄-N sources and raised the total N content of the crop, but had no overall effect on crop growth. In contrast to inorganic N sources which generally reduced CO₂ efflux from soil, application of protein-based fertilizers increased the rate of soil microbial activity directly by raising substrate availability. Sewage sludge derived protein provided an effective alternative to mineral fertilizers for the nutrition of summer cabbage whilst minimising stress of the soil environment which may occur following the application of conventional forms of inorganic N to the soil.     

Effect of the timing and rate of nitrogen fertilization on the growth and recovery of fertilizer nitrogen within the potato (Solanum tuberosum L.) crop

The effect of the timing of N fertilizer application on the uptake and partitioning of N within the crop and the yield of tubers has been studied in two experiments. In 1985 either none, 8 or 12 g N m^–2 was applied and in 1986 none, 12 or 18 g N m^–2. Fertilizer N was applied either at planting, around the time of tuber initiation or half at planting and the remainder in four foliar sprays of urea during tuber bulking.¹⁵N-labelled fertilizer was applied to measure the recovery of fertilizer N in the crops.There was an apparent pre-emergence loss of nitrate from the soil when N was applied at planting in 1986, thereby reducing the efficiency of fertilizer use. Applying the N at tuber initiation delayed and reduced the accumulation of N in the canopy compared with crops receiving all their fertilizer at planting. Foliar sprays of urea slightly increased both tuber yields and tuber N contents when compared to a single application at planting. The proportion of the fertilizer N recovered in the crop was little affected by the rate of N application, but a greater proportion of foliar-applied N was recovered than N broadcast at planting, due partly to pre-emergence losses of nitrate in 1986. It is suggested that late applications of N was foliar sprays can be of benefit to crops with a long growing season and reduce environmental losses of N.     

Synchronizing N Supply from Soil and Fertilizer and N Demand of Winter Wheat by an Improved Nmin Method

Excessive nitrogen (N) fertilizer application and poor timing of N fertilizer application to winter wheat are common problems on the North China Plain. To study the possibilities of optimizing the timing and rate of N application, a field experiment was conducted from 1999 to 2001 in a suburb of Beijing. A control (no nitrogen) and two N fertilization strategies (conventional N application and optimized N fertilization) were designed to compare their effects on wheat growth, N nutrient status, grain yield and N balance. The conventional N fertilization strategy was given a fixed N rate of 300 kg N ha⁻¹, which was split, half in autumn and half in spring as a top-dressing. The timing and rate of N and application of the optimized N fertilization strategy were determined by the target value of soil mineral nitrogen demand for three growth periods of wheat, which is related to the target yield, and soil mineral N (N_min) in the effective rooting depth at the beginning of these three periods. Based on the optimized N fertilization strategy, a total of 55 and 65 kg N ha⁻¹ had to be applied to winter wheat in the re-greening and shooting stages of the first and second experimental years, respectively. Compared with the high N rate before sowing in the conventional N fertilization treatment, the optimized N fertilization treatment did not require any N fertilizer before sowing of wheat. Despite a much lower N fertilization rate, no significant difference in N nutrient status, growth during the wheat growing period or grain yield was observed between optimized N and conventional N fertilization treatments. As a consequence of optimizing the rate and timing of the N fertilizer application to match wheat demand, a much lower residual N_min and calculated apparent N loss was found as compared to the conventional N treatment. N recovery for the optimized N fertilization treatment (67% in 1999/2000 and 66% in 2000/2001) was much higher than that of the conventional N fertilization treatment (19% in 1999/2000 and 18% in 2000/2001). In conclusion, the optimized N fertilization strategy can synchronize N demand of wheat and the N supply from soil and fertilizer, and therefore drastically reduce N application rates without any yield losses.     

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.     

Fate of nitrogen applied in different fertilizers to the surface of a calcareous soil in Syria

¹⁵N-labelled ammonium sulphate or¹⁵N-labelled urea were each applied in solutionat a rate of 30 kg N ha^-1 to the surface of 20soil cores (52 mm internal diameter × 100 mm deep)located on a field experiment at the ICARDA station,Tel Hadya, Syria. Recovery of ¹⁵N-label in theammonium, nitrate, organic and/or urea-N pools in thesoil was measured on days 0, 1, 2, 5 and 13 afterapplication. Total recovery of ¹⁵N was initially100%, but by day 13 after application it had declinedto 51% with urea and 73% with ammonium sulphate.Ammonium nitrate labelled either as ammonium or asnitrate was also applied to the soil surface of 8other cores at the same time. ¹⁵N recovery in thefour soil N pools was measured only on day 12 afterapplication. Total recovery of ¹⁵N-label was 75%with labelled ammonium and 57% with labelled nitrate.Volatilization of ammonia from this calcareous soil(pH 8.1) is one probable mechanism of N loss fromammonium and urea fertilizers: with nitrate bothleaching beyond the base of the core (i.e. 100 mm) and denitrification were responsible for Nlosses. These large losses of N immediately afterapplication have implications for fertilizermanagement practices.     

Effects of nitrogen fertilizer application methods on growth and acetylene reduction activity ofAzolla pinnata and yield of rice

The influence of different methods of N fertilizer application-sub-surface placement (SS) and broadcast application (BC) as a single basal dose of 40 kg N ha^–1 of urea (U) and ammonium sulphate (AS)-on fresh weight (FW) and acetylene reduction activity (ARA) ofAzolla pinnata (Bangkok) and yield of rice was studied for consecutive three seasons. The FW, ARA and N yield ofAzolla were significantly superior with SS placement than with BC application. Of the two N sources, AS was superior to U in recording higher FW, ARA and N yield of Azolla irrespective of methods of N application. Crop yield and crop N uptake were higher with SS application alone and in combination with Azolla as compared to that of surface application of N fertilizers. The combined use of AS and Azolla recorded significantly higher crop yield and crop N uptake than that of U combined with Azolla, irrespective of methods of application.     

Effects of soil fumigation and N source on soil inorganic N and tomato growth

Soil fumigation, commonly used in vegetable production, may alter the rate of nitrification, affecting availability of N for crop use. The objective of this research was to examine effects of soil fumigation and N fertilizer source on tomato growth and soil NO₃–N and NH₄–N in field production. Experiments 1 and 2 included application of methyl bromide at 420 kg ha^-1 to a Norfolk sandy loam (fine loamy siliceous thermic Typic Kandiudult) in combination with preplant applications of calcium nitrate, ammonium nitrate, and ammonium sulfate at 144 kg N ha^-1. An additional fumigant, metam-sodium, was included in the second experiment at 703 L ha^-1 (268 kg sodium methyldithiocarbamate ha^-1). Experiment 3 included methyl bromide and metam-sodium, with ammonium sulfate as the sole source of N applied at 144 kg N ha^-1. In the first two studies, fumigants had little or no effect on soil NH₄–N or NO₃–N concentration. Tomato plants were larger and fruit yield was greater in fumigated plots, but there were few growth or yield responses to N source. In the third experiment, fumigants increased concentration of soil NO₃–N and NH₄–N at 16 days after fumigation (DAF), however, there was no effect on nitrification owing to fumigants. It appears that N source selection to overcome inhibition of nitrification is not necessary in plant production systems that involve fumigation