Recovery of 15N-labelled fertiliser by a perennial ryegrass seed crop and a subsequent wheat crop

Large amounts of nitrogen (N) fertiliser (150–200 kg N/ha) are currently being applied to perennial ryegrass (Lolium perenneL.) seed crops in New Zealand. Due to increasing requirements for efficient use of N fertilisers and minimising nitrate contamination of the environment, a field experiment was established using ¹⁵N-labelled fertiliser to follow the fate of applied N. Urea-¹⁵N was applied to a perennial ryegrass seed crop in April (30 kg N/ha), August (30 kg N/ha), September (60 kg N/ha) and October (60 kg N/ha). The urea-¹⁵N was applied in solution and watered in to minimise volatilisation loss. At the time of harvest (December), 9% of the applied ¹⁵N was in the seed, 29% in the straw, 19% in the roots and 39% in the soil organic matter. Losses of ¹⁵N were minimal as the N was applied in several applications, each one at a relatively low rate, and at times when leaching was unlikely to occur. Ryegrass plants used a greater proportion of the N applied in September and October (61–65%) compared with that applied in April (44%). Consequently more N was recovered from the soil in the autumn application (57%) than from the September and October applications (28–44%). The availability of the residual fertiliser N to a subsequent wheat (Triticum aestivum L.) crop was studied in a glasshouse experiment. The residual fertiliser N was present in the soil and ryegrass roots and stubble. The wheat plants only recovered 7–9% of this residual N. Most of the N taken up by the wheat came from the soil organic N pool. Overall, applying a total of 180 kg N/ha to the ryegrass appeared to have minimal direct impact on the environment. In the short term N not used by the ryegrass plants contributed to the soil organic N pool.     

Fate of urea applied to winter spinach in New Zealand

Field trials were conducted during three years (1996–1998) to follow the fate of N fertiliser applied to spinach (Spinacea oleracea L.) grown over winter (May–September) in New Zealand. Urea labeled with ¹⁵N was applied to the spinach at different rates (0, 200 or 400 kg N/ha), times (as two or three split applications) and placement methods (broadcast or banded). The results showed that the typical fertiliser practice of applying 350 kg N/ha near planting time followed by 50 kg N/ha 4 weeks later, resulted in leaching losses of 119–292 kg N/ha depending on the rainfall. Plants recovered only a small proportion of the fertiliser N (3–18% of the ¹⁵N). Better utilization of the fertiliser and lower leaching losses occurred when the fertiliser was applied strategically in split applications with only a small amount (10%) applied near sowing and the majority applied to match crop demand. For example, in 1998 applying 200 kg N/ha (20 kg N/ha initially+100 kg N/ha after 4 weeks and 80 kg N/ha after 8 weeks) resulted in twice the spinach yield (25 t/ha compared with 13 t/ha) and less than half the leaching loss (108 kg N/ha leached compared with 246 kg N/ha) compared with the typical fertiliser practice.     

Fate of fertiliser N applied to winter wheat growing on a Vertisol in a Mediterranean environment

A field study using 15N was conducted on a Vertisol in semi-arid Morocco to assess the fate and efficiency of fertiliser N split applied to winter wheat (Triticum aestivum L.). Splitting of fertiliser N is highly crucial in semi-arid regions, considering the increased moisture stress towards the end of the growing season. A N fertilisation rate of 100 kg N ha-1 was split according to two schemes: i) 25% at planting, 50% at tillering and 25% at stem elongation; or ii) 50% at tillering and 50% at stem elongation. The application of 100 kg N ha-1increased the vegetative dry matter production with more than 2000 kg dry matter ha-1 in comparison with the control treatment. Nitrogen fertilisation had no significant effect on the grain yield production. Moreover, the 1000 grain weight decreased from 32 to 26 g due to N fertilisation. Total N uptake was about 50 kg N ha-1 higher for the fertilised plants in comparison with the unfertilised plants, but it was not affected by the splitting pattern of the fertiliser N. Recoveries of 15N-labelled fertiliser by the plant (above-ground plant parts plus roots from the upper 20 cm layer) were low (31% and 24% for the 3-split and 2-split application, respectively). More N in the plant was derived from fertiliser when applied early in the growing season than when applied late in the season. About 13% of the N in the plants was derived from the 50 kg N ha-1 at tillering, while only 5% was derived from the N application (50 kg N ha-1) at stem elongation. At harvest, a high residual of fertiliser-derived N was found in the 0–90 cm profile (62% and 72%, for the 3-split and 2-split application, respectively). Less than 10% of the applied N could not be accounted for, the amount being highest for the application at tillering. This N not accounted for was mainly ascribed to denitrification after an important rainfall event. The application of fertiliser N led to an increase of about 20 kg N ha-1 in soil N uptake by the crop (positive ANI). The results suggested a dominant influence of moisture availability on the fertiliser N uptake by wheat.     

Response of winter wheat to N-fertiliser: Quantitative relations for components of growth

Measurements were made of yield of dry matter, plant-N content, and the distribution of mineral-N down the soil profile in 10 fertiliser-N experiments. In one of them detailed measurements were made throughout growth. Rate of N-uptake by the crop was unaffected by the amount of mineral-N in the upper 90 cm of soil when it was above about 30 kg N ha^–1. The %N in plants that received ample N-fertiliser declined with increase in plant mass according to a previously derived equation. During senescence there was an apparent loss of N from the crop.N-nutrition in the different experiments had little effect on the partition of assimilate between grain and straw. At harvest grain and straw weights were well related by a linear model which had the same gradient but different intercepts for each experiment. Grain %N was about four times greater than straw %N. Regression analysis supported the view that high evaporative conditions or temperatures during the growing period induced earlier harvest dates, less grain relative to straw, and a higher %N in the plant when ample N-fertiliser was applied but not when N-fertiliser was withheld.Other analyses indicated that cereal roots were generally unable to extract mineral nitrogen from the soil when the concentration was less than about 0.18 kg N ha^–1 cm^–1, that at low levels of N-nutrition the recovery of available inorganic-N from soil by the grain and straw was about 80%, and that the average mineralisation rates from early spring to shortly after harvest date varied between 0.22 and 0.88 kg N ha^–1 d^–1 from site to site.     

Integrated soil management for the savanna zone of W. Africa: legume rotation and fertilizer N

Integrated soil management with leguminous cover crops was studied at two sites in the northern Guinea savanna zone of northern Nigeria, Kaduna (190 day growing season) and Bauchi (150 days). One-year planted fallows of mucuna, lablab, and crotalaria were compared with natural grass fallow and cowpea controls. All treatments were followed by a maize test crop in the second year with 0, 30, or 60 kg N ha^–1 as urea. Above ground legume residues were not incorporated into the soil and most residues were burned early in the dry season at the Kaduna site. Legume rotation increased soil total N, maize growth in greenhouse pots, and dry matter and N accumulation of maize. Response of maize grain yield to 30 kg N ha^–1 as urea was highly significant at both sites and much greater than the response to legume rotation. The mean N fertilizer replacement value from legume rotation was 14 kg N ha^–1 at Kaduna and 6 kg N ha^–1 at Bauchi. W ith no N applied to the maize test crop, maize grain yield following legume fallow was 365 kg ha^–1 higher than natural fallow at Bauchi and 235 kg ha^–1 higher at Kaduna. The benefit of specific legume fallows to subsequent maize was mostly related to above ground N of the previous legume at Bauchi, where residues were protected from fire and grazing. At Kaduna, where fallow vegetation was burned, maize yield was related to estimated below ground N. The results show that legume rotation alone results in small maize yield increases in the dry savanna zone.     

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.     

Assessing agronomic and environmental implications of different N fertilisation strategies in subtropical grain cropping systems on Oxisols

A multi-season ¹⁵N tracer recovery experiment was conducted on an Oxisol cropped with wheat, maize and sorghum to compare crop N recoveries of different fertilisation strategies and determine the main pathways of N losses that limit N recovery in these agroecosystems. In the wheat and maize seasons, ¹⁵N-labelled fertiliser was applied as conventional urea (CONV) and urea coated with a nitrification inhibitor (DMPP). In sorghum, the fate of ¹⁵N-labelled urea was monitored in this crop following a legume ley pasture (L70) or a grass ley pasture (G100). The fertiliser N applied to sorghum in the legume-cereal rotation was reduced (70 kg N ha^?1) compared to the grass-cereal (100 kg N ha^?1) to assess the availability of the N residual from the legume ley pasture. Average crop N recoveries were 73 % (CONV) and 77 % (DMPP) in wheat and 50 % (CONV) and 51 % (DMPP) in maize, while in sorghum were 71 % (L70) and 53 % (G100). Data gathered in this study indicate that the intrinsic physical and chemical conditions of Oxisols can be extremely effective in limiting N losses via deep leaching or denitrification. Elevated crop ¹⁵N recoveries can be therefore obtained in subtropical Oxisols using conventional urea while in these agroecosystems DMPP urea has no significant scope to increase fertiliser N recovery in the crop. Overall, introducing a legume phase to limit the fertiliser N requirements of the following cereal crop proved to be the most effective strategy to reduce N losses and increase fertiliser 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.     

Response of sorghum to fertilizer phosphorus and its residual value in a Vertisol

The response of crops to added P in Vertisols is generally less predictable than in other soil types under similar agroclimatic conditions. Very few studies have considered the residual effects of P while studying responses to fresh P applications. Field experiments were conducted for three years to study the response of sorghum to fertilizer P applied at 0, 10, 20 and 40 kg P ha^–1, and its residual value in a Vertisol, very low in extractable P (0.4 mg P kg^–1 soil), at the ICRISAT Center, Patancheru (near Hyderbad), India. In order to compare the response to fresh and residual P directly in each season, a split-plot design was adopted. One crop of sorghum (cv CSH6) was grown each year during the rainy season (June-September).The phenology of the sorghum crop and its harvest index were greatly affected by P application. The days to 50% flowering and physiological maturity were significantly reduced by P application as well as by the residues of fertilizer P applied in the previous season. In the first year of the experiment, sorghum grain yield increased from 0.14 t (no P added) to 3.48 t ha^–1 with P added at the rate of 40 kg P ha^–1. Phosphorus applied in the previous year was 58% as effective as fresh P but P applied two years earlier was only 18% as effective as fresh P.     

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.     

Crop production,nitrogen recovery and water use efficiency in rice–wheat rotation as affected by non-flooded mulching cultivation (NFMC)

Non-flooded mulching cultivation (NFMC) for lowland rice, as a novel water-saving technique, has been practiced in many areas of China since the 1990s. However, the information on NFMC effects on crop production, nitrogen and water use in rice–wheat rotations is still limited. A field experiment using ¹⁵N-labeled urea was conducted to evaluate the impacts of NFMC on crop yield, fertilizer N recovery and water use efficiency in rice–wheat rotations. Plastic film mulching (PM), and wheat straw and plastic film double mulching (SPM) resulted in the same rice grain yield (7.2 t ha^–1) while wheat straw mulching (SM) and no mulching (NM) led to 5 and 10% yield reduction, compared with rice under traditional flooding (TF). In the rice–wheat rotation, crop productivity in PM, SM or SPM was comparable to that in TF but greater than in NM. Weed growth and its competition with rice for nitrogen were considered the main reason that led to yield decline in NM. Compared with TF, NFMC treatments did not obviously affect fertilizer N recoveries in plant and soil in both rice and wheat seasons. The total fertilizer N recoveries in crop, weed and soil in all treatments were only 39–44% in R–W rotations, suggesting that large N losses occurred following one basal N application for each growing season. Water use efficiency, however, was 56–75% greater in NFMC treatments than in TF treatment in the R–W rotation. The results revealed that NFMC (except NM) can produce comparable rice and wheat yields and obtain similar fertilizer N recovery as TF with much less water consumption.     

Impact of rice straw management practices on yield,nitrogen uptake and soil properties in a wheat-rice rotation in northern India

To study the long term effects of rice straw management practices in a wheat-rice rotation, experiments were started from the dry season (Nov–May) of 1984 to wet season (July–Nov) of 1989. Each year, six straw management practices, viz. control (C), straw incorporation (SI), straw mulch (SM), straw burning (SB), animal manure incorporation (AM), and straw and animal manure incorporation together (SI+AM) were imposed to wheat crop and their subsequent residual effect was studied on the following rice crop under three levels of N, viz. 0, 60 and 120 kg N ha^–1. The rate of straw and animal manure used was 5 t ha^–1 on dry weight basis.The wheat yield and N uptake did not vary significantly under control and SB throughout the experimental period. But, the production level of wheat and N uptake were consistently higher under AM and SM over these two treatments. The SI+AM which had significantly lower wheat yields and N uptake over the AM during the first crop, became equal to that of AM and SM during the second and third crops, and out yielded these two treatments from the fourth crop onward. Straw incorporation which produced wheat yield and N uptake even less than control and SB during the first two crops, resulted in wheat yield and N uptake equivalent to AM and SM from the fourth crop onward.None of the straw management practices had residual effects on the yields and N uptake during the first rice crop, except SM which reduced the rice yields and N uptake in the first two crops. However, AM and SI+AM in the second crop; AM, SI+AM and SI in the third crop; and AM, SI+AM, SI and SM from the fourth crop onward had significant and positive residual effects on rice yields and N uptake. Among these four treatments, SI+AM produced residual effects which were significantly higher than the remaining three treatments. Considering the production levels of wheat and rice, SI+AM treatment resulted in savings of 60 kg N ha^–1 each for wheat and rice.After five years of experimentation, the maximum soil build-up of organic carbon; available N, P and K; and DTPA-extractable Zn, Cu, Fe and Mn was observed under SI+AM, followed by AM and SM and it was minimal under SB and control treatments. The treatments of AM and SI+AM also resulted in a high percentage of water-stable aggregates of 70.25 mm in diameter (80.9%), larger mean weight diameter (0.82 mm), higher porosity (54.2%) and lower bulk density (1.19 Mg m^–3).     

Effect of green manure on the yield,N uptake and floodwater properties of a flooded rice,wheat rotation receiving15N urea on a highly permeable soil

Field studies were conducted for two years on a rapidly percolating loamy sand (Typic Ustochrept) to evaluate the effect of green manure (GM) on the yield,¹⁵N recovery from urea applied to flooded rice, the potential for ammonia loss and uptake of residual fertilizer N by succeeding crops. The GM crop ofSesbania aculeata was grownin situ and incorporated one day before transplanting rice. Urea was broadcast in 0.05 m deep floodwater, and incorporated with a harrow. Green manure significantly increased the yield and N uptake by rice and substituted for a minimum of 60 kg fertilizer N ha^–1. The recovery of fertilizer N as indicated by¹⁵N recovery was higher in the GM + urea treatments. The grain yield and N uptake by succeeding wheat in the rotation was slightly higher with GM. The recovery of residual fertilizer N as indicated by the¹⁵N recovery in the second, third and fourth crops of wheat, rice and wheat was only 3, 1 and 1 per cent of the urea fertilizer applied to the preceding rice crop. Floodwater chemistry parameters showed that the combined use of the GM and 40 kg N ha^–1 as urea applied at transplanting resulted in a comparatively higher potential for NH₃ loss immediately after fertilizer application. The actual ammonia loss as suggested by the¹⁵N recoveries in the rice crop, however, did not appear to be appreciably larger in the GM treatment. It appeared the ammonia loss was restricted by low ammoniacal-N concentration maintained in the floodwater after 2 to 3 days of fertilizer application.     

Residual effects of growing mungbean and uridbean on the yield and nitrogen uptake of a succeeding wheat crop

A two year field experiment was carried out at the Indian Agricutural Research Institute, New Delhi - 110012, India to assess the effect of mungbean (Vigna radiata L.) and uridbean (Vigna mungo L.) residues on the yield and N uptake of a succeeding wheat crop as compared to sorghum fodder. Sorghum produced 3.5–7.5 times more dry matter and removed 2–3 times more nitrogen than mungbean or uridbean during same duration (80 ± 10 days) of their growth. Without N application the grain yield of wheat following mungbean and uridbean (without residue incorporation) was 0.45 and 0.48 t ha^–1 more than the yield of wheat following sorghum fodder. These yields were equivalent to that predicted when 36 and 38 kg urea-N ha^–1, respectively, was directly applied to wheat. The residual effects of these grain legumes were higher when succeeding wheat was fertilized with 60 kg urea-N ha^–1; at this level mungbean and uridbean spared 52 and 43 kg urea-N ha^–1, respectively, in succeeding wheat. The residual effect of mungbean and uridbean further increased when their residue was incorporated in soil; with this practice they spared 94 and 115 kg urea-N ha^–1, respectively, without N application to wheat and 74 and 82 kg urea-N ha^–1, respectively, with an application of 60 kg urea-N ha^–1 to wheat.Mungbean and uridbean, without residue incorporation, increased aboveground plant-N uptake of succeeding wheat by 11.5–34.9 and 10.8–34.0 kg N ha^–1, respectively; whereas with residue incorporation, they increased aboveground plant-N content of succeeding wheat by 26.1–45.8 and 32.7–47.7 kg N ha^–1, respectively.The results of the present study indicate that there is both an indirect sparing effect and a direct residual effect of mungbean and uridbean on the nitrogen needs of succeeding wheat, more so when their residues are incorporated in soil.     

Use of nitrification inhibitors (neem and DCD) to increase N efficiency in maize-wheat cropping system

A 2-year field experiment was conducted to study the effects of the nitrification inhibitors dicyandiamide (DCD) and neem cake on the efficiency of applied prilled urea nitrogen in a maize-wheat cropping system. Prilled urea (PU), neem cake coated urea (NCU) and DCD blended urea (DCDU) were applied to maize at two levels (60 and 120 Kg N ha^–1) and two methods (all preplant and split) of N application along with a no-nitrogen control and their relative residual effect was studied on succeeding wheat grown with three levels of N as PU.In 1990 maize responded well to N up to 60 kg N ha^–1; at this level PU increased maize yield by 1.03 t ha^–1, whereas NCU and DCDU increased maize yield by 1.55 and 1.18 t ha^–1 over the control, which was equivalent to an application of 127 and 94 kg N ha^–1 as PU, respectively. Furthermore, when the results were averaged over two years of study, residual N from the application of NCU and DCDU at 60 kg N ha^–1 left after maize cropping increased the grain yield of the succeeding wheat crop grown with 60 kg N ha^–1 as PU by 1.97 and 1.68 t ha^–1, respectively, over a no nitrogen control or 60 kg N ha^–1 as PU applied to the maize. This was equal to an application of 96 and 82 kg N ha^–1 as PU to wheat.Thus, neem cake increased the efficiency of urea N applied to maize and benefits were also seen in the succeeding wheat yield in the maize-wheat cropping system.     

Influence of agronomic management of legume crops on soil accumulation with nitrate

Nitrate is known to accumulate under legume crops. The effects of legume crop, inoculation, row width, sowing rate, sowing date, and intra-cropping with wheat, on the amount and soil distribution of mineral N, residual soil water, crop biomass and crop N were studied at Wagga Wagga in south-east Australia. After removal of most of the above-ground plant material, the treatment effects on the biomass, N content, grain yield and grain N of wheat, established in the following season, were also measured. In a later experiment at Wagga, the recovery of 15N applied to the mid-row of lupin crops established at three row widths was estimated at crop maturity. At Condobolin, row width effects on the soil distribution of mineral N, biomass, N accumulation and N fixation of crop legumes and cereals, were determined. At physiological maturity, at Wagga Wagga, very little nitrate was left beneath cereals. Significantly more was left under legume crops, mostly below 30 cm of soil depth, and it was distributed differently depending on crop, inoculation, and sampling location. More nitrate was left under pea and faba than under lupin, and in response to inoculation. Mixing wheat with narrow-leaf lupin did not prevent nitrate accumulation in soil. For most of the legumes more nitrate was left in the mid-row than in the in-row; and more nitrate was left at the mid-row of lupin crops sown with wider rows. The additional nitrate left with wider rows increased the growth, N content, grain yield and protein of wheat established in the following season. 15N labelled nitrate applied mid-row was used less effectively by lupin as row width increased, in a dry season. At Condobolin, lupin established with wide rows used less soil nitrate than with narrower rows but maintained crop N by increased N fixation. In contrast, field pea maintained N demand by increasing nitrate uptake at intermediate row spacing. The study shows that the amount of nitrate accumulated in soil during legume cropping is susceptible to agronomic management, particularly crop selection, row width and inoculation; and that variation in the amount of this nitrate may carry forward to impact wheat production in the follow-on season.     

Changes in the nutrient status of soil caused by cropping and fertilization in a Typic Ustochrept

Field experiments were conducted during 1984–1986 on an alluvial (Typic Ustochrept) soil (pH 8.0, organic carbon 0.46%) at IARI farm, New Delhi to study the changes in available soil nutrients (N, P, K, Mn, Fe, Zn and Cu) at different production levels. Fertilizer was applied to wheat followed by maize, based on the Targetted yield concept, and mustard was grown after the sequence to estimate the residual effect of nutrients. Nutrient applications for the largest yield targets (6 t ha^–1 of wheat followed by 4 or 5 t ha^–1 of maize) resulted in a comparatively greater buildup of soil nutrients (N, P and K), the greatest yield of a succeeding mustard crop, and a better soil nutrient status than that at the start of the experiment, even after the mustard. When both crops were fertilized for the largest target yield with straight fertilizers (Urea, SSP and KCl), the additions of N, P and K and of micronutrient cations (Mn, Fe, Zn and Cu) maintained a favorable balance for major and trace nutrients and provided a sound basis for profitable crop production.Part of the Ph.D. thesis of the senior author submitted to the Indian Agricultural Research Institute (IARI), New Delhi.     

Nitrogen use efficiency of <Superscript>15</Superscript>N-labelled sheep manure and mineral fertiliser applied to microplots in long-term organic and conventional cropping systems

Nitrogen (N) utilisation by crops has to be improved to minimize losses to the environment. We investigated N use efficiency of animal manure and mineral fertiliser and fate of fertiliser N not taken up by crops in a conventional (CONMIN) and a bio-organic (BIOORG) cropping system of a long-term field experiment over three vegetation periods (winter wheat–soybean–maize). Microplots planted with wheat received a single application of ¹⁵N-labelled slurries (either urine or faeces labelled) or mineral fertiliser. At the end of each vegetation period we tested whether higher microbial activity and larger microbial biomass in BIOORG than CONMIN soils, and lower long-term N input level in BIOORG, affected use efficiency and fate of fertiliser N not taken up by crops. Recovery of ¹⁵N in wheat was 37%, 10% and 47% from urine, faeces and mineral fertiliser, respectively, and decreased strongly in the residual years. In total 41%, 15% and 50% of ¹⁵N applied as urine, faeces and mineral fertiliser was recovered by the three crops. ¹⁵N recovered from originally applied urine, faeces and mineral fertiliser in the topsoil (0–18 cm) at the end of the third vegetation period was 19%, 25% and 20%, respectively. Of urine-, faeces- and mineral fertiliser-¹⁵N, 40%, 61% and 29%, respectively, was not recovered by the three crops and in topsoil suggesting significant transport of ¹⁵N-labelled components to deeper soil layers. CONMIN and BIOORG differed neither in fertiliser N use efficiency by crops nor in ¹⁵N recovery in soil indicating insignificant difference in the turnover and utilization of the applied manure nitrogen in the conventional and the bio-organic cropping systems.     

Response of dryland wheat to fertilizer nitrogen in relation to stored water,rainfall and residual farm yard manure

Yield response of dryland wheat to fertilizer N application in relation to components of seasonal water (available soil moisture and rainfall) and residual farm yard manure (FYM) was studied for five years (1983–84 to 1987–88) on a maize-wheat sequence on sandy loam soils in Hoshiarpur district of Punjab, India. Four rates of N viz. 0, 40, 60 and 80 kg ha^–1 in wheat were superimposed on two residual FYM treatments viz. no FYM (F₀) and 15 t ha^–1 (F₁₅) to preceding maize. FYM application to maize increased the residual NO₃-N content by 19–30 kg ha^–1 in the 180 cm soil profile. For a given moisture distribution, F₁₅ increased attainable yields. Over the years, F₁₅ increased wheat yield by 230 to 520 kg ha^–1. Response to fertilizer N was lower in FYM amended plots than in unamended plots. Available soil moisture at wheat seeding and amount and distribution of rainfall during the vegetative and the reproductive phases of crop development affected N use efficiency by wheat. Available soil moisture at seeding alone accounted for 50% variation in yield. The residual effect of FYM on wheat yield could be accounted for by considering NO₃-N in 180 cm soil profile at seeding. The NO₃-N and available soil moisture at wheat seeding along with split rainfall for two main phases of crop development and fertilizer N accounted for 96% variation in wheat yield across years and FYM treatments.     

Determination of critical level of zinc in non-calcareous soils for predicting response of wheat to applied zinc

Studies were conducted in ten non-calcareous arid brown soils (India) to determine the critical level of soil Zn for predicting response of wheat to zinc fertilization. The per cent mean response at 5 mg kg^–1 added Zn varied from 1.3 to 51.4 with a mean value of 17.5 per cent over control in terms of grain yield (g pot^–1). Further, Zn application resulted in significant increase in Zn concentration in various plant parts in all the soils irrespective of the initial Zn status. The critical level of Zn in soil and plant below which response to applied Zn may be expected was found to be 1.75 mg kg^–1 for 0.1 N HC1 extractable soil Zn and 1.7 mg kg^–1 for plant tissue Zn.