The fertiliser nitrogen requirement of cereals grown on sandy soils

The responses to fertiliser‐N of winter wheat and winter barley grown on sandy soils were measured in 72 experiments in England from 1990 to 1994. Yield without fertiliser‐N (Y₀) was c 1.1 t ha⁻¹ greater following root crops than following cereals. Following potato crops given organic manures, Y₀ was c 1.2 t ha⁻¹ greater than following unmanured potato crops, but Y₀ was no greater following sugarbeet to which organic manures had been applied. Only after the two driest winters was there sufficient variation in soil N supply in spring (SNS_s) for this to show a relationship with Y₀. However, Y₀ increased with increasing N mineralisation during the growing season (AM) in the three years it was measured. There was no consistent effect of sowing date on Y₀. Following potatoes, yield at optimum fertiliser‐N (Y_opt) decreased as sowing date was delayed, but this was not so after cereals, sugarbeet or overall. There was no increase in Y_opt with SNS_S or AM, but Y_opt decreased with increasing moisture stress (S) in June. The mean yield response to N_opt (Δ_Y) was c 0.4 and 0.8 t ha⁻¹ smaller following potatoes and sugarbeet respectively than following cereals, but not consistently so as there were large interactions between site, year and previous crop. Following root crops, Δ_Y was c 0.6 and 1.4 t ha⁻¹ less after sugarbeet and potatoes respectively that had been given organic manures. Without the addition of organic manures, Δ_Y following potatoes was similar to that following cereals. Regression on SNS_S and AM accounted for 28 and 15% respectively of the variance in Δ_Y. The optimum economic fertiliser‐N application (N_opt) was similar, at c 140 kg ha⁻¹, following cereals and potatoes. Following sugarbeet, cereal N_opt was only c 110 kg ha⁻¹. The differences according to previous crop reported here are consistent with mineralisation of crop residues on sandy soils being more rapid than on other soils; the potato residues were rapidly mineralised in autumn and lost by leaching over winter. Residues from later‐harvested sugarbeet were mineralised during the growing season of the subsequent cereal crop. Fertiliser‐N requirements were, at c 110–140 kg ha⁻¹, smaller than has been found on other soil types, and less than current recommendations for wheat. Requirements were significantly reduced in years of drought stress. No differences were found in N_opt between wheat and barley. These data do not justify the current advice to invariably reduce fertiliser‐N to cereals following potatoes by 20–25 kg ha⁻¹ on these sandy soils. On average a reduction of c 20 kg ha⁻¹ could be made following sugarbeet, with a further reduction of c 40 kg ha⁻¹ N if manures had been applied to the previous sugarbeet crop. A reduction of 40 kg ha⁻¹ N could also be made where cereals followed a potato crop to which manures had been applied. Further refinements on the basis of measurements of soil mineral N could not be justified. Seasonal variation in N response due to drought stress makes recommendations difficult on these soils. Adopting the fertiliser‐N recommendations proposed here would produce N surpluses to the soil of c 37, 10 and 27 kg ha⁻¹ respectively following cereals, sugarbeet and potatoes when cereal grain is removed but straw incorporated. On farms where straw is removed, N surplus would be largely eliminated. Our recommendation that no reduction in fertiliser‐N application to cereal crops grown on sandy soils should be made following potatoes will not increase fertiliser‐N use and is not expected to increase nitrate leaching. Some reduction in nitrate leaching may be achieved if recommendations following cereal crops and sugarbeet are made in accordance with the results reported here. © 2000 Society of Chemical Industry     

Responses of Cereals to Residual Fertiliser Nitrogen Applied to the Preceding Potato Crop

The effects of N fertiliser and poultry manure, applied to potato crops, on soil mineral N (SMN) after harvest and on the grain N offtake (N_off) and yield of subsequent unfertilised cereal crops, were measured at six sites in England during 1989–1994. At three sites of medium textured soil N_off increased by between 0 and 49 kg ha^-1 and grain yield by between 0 and 2·1 t ha^-1, with increasing potato fertiliser N (PFN). The increases were greatest at applications of fertiliser N in excess of the optimum requirement for potatoes. No responses were found on two sandy and one shallow soil. Poultry manure applied in autumn or spring before the potatoes were planted increased N_off by between 0 and 52 kg ha^-1 and yield by between 0 and 2·1 t ha^-1 at one site on medium textured soil but not on one of the two sandy soils. These results suggest that, at the rates of fertiliser N currently recommended for potatoes, the N requirement of subsequent cereal crops may be reduced by between 20 and 40 kg ha^-1 on retentive soils but not on sandy and shallow soils. These reductions are less than currently recommended. However, the results also suggest that adjusting cereal fertiliser N according only to previous crop is unsatisfactory. The range of fertiliser N applied to potato crops in commercial practice is sufficiently great to significantly affect N_off, yield and hence the fertiliser N requirement of the subsequent cereal crop. © 1997 SCI.     

Effects of husbandry treatments on nitrogen concentration of grain and related yields in winter-wheat experiments made in South-East England

Experiments by Rothamsted staff over 20 years show that N (protein) concentration in wheat grain is influenced considerably by several husbandry treatments other than total fertiliser N. Even small differences can be of practical importance if values lie close to the minimum standard for bread wheat. In 15 experiments which tested timing of fertiliser N, an extra 60 kg ha^?1 would have been needed in autumn to increase grain-N concentration by 0.1% on average, but only 43 kg in early and 30 kg in late spring. The response to autumn N was similar in a ley-arable rotation experiment Fertiliser N applied to a previous potato crop gave a grain-N% increase equivalent to a quarter of the fresh application on a silty clay loam soil but none on a sandy loam. Cumulative annual dressings of farmyard manure benefited grain-N% as did residues from FYM applied to a previous potato crop, which gave increases equivalent to those from 16 kg ha^?1 of fresh fertiliser N. In ley-arable rotation experiments, wheat after arable cropping did not reach bread-quality standard with the largest amount of fertiliser N (150 kg ha^?1), but after lucerne N% values exceeded the threshold value of 2.14% N with all rates. Benefits from lucerne and a grass-clover ley were still considerable when wheat was grown as a second test crop after potatoes. Yield responses to these husbandry treatments tended to be small and positive, except that in the presence of larger dosager of fertiliser N farmyard manure sometimes caused a depression.     

Effect of drought and irrigation on the fate of nitrogen applied to cut permanent grass swards in lysimeters: Nitrogen balance sheet and the effect of sward destruction and ploughing on nitrogen mineralisation

Fuor years after ¹⁵N labelled fertiliser nitrogen (as Ca(NO₃)₂ and equivalent to 400 kg N ha^?1) was applied to permanent grass swards growing in lysimeter monoliths, approximately one-quarter remained immobilised in soil organic matter. In the intervening years similar but non-labelled applications were made. Although differing rainfall regimes applied during the experiment had significantly affected nitrogen uptake by plants and nitrate loss in drainage, they caused no significant effect on the tracer nitrogen remaining in the soil, the ranges were 85–97 kg N ha^?1 and 79–94 kg N ha^?1 respectively for the 135 cm deep clay and silt loam soil monoliths. Labelled nitrogen unaccounted for in crop, drainage or soil was presumed to have been denitrified. These losses averaged 62 and 49 kg N ha^?1 on the clay and silt loam soils respectively; again the differing rainfall regimes caused no significant differences. The ratio between estimates of labelled nitrogen denitrified and of annual nitroux oxide loss was approximately 9:1 for both soils. The conversion from permanent grass to winter wheat in autumn 1981, involving killing and then burying the sward, resulted in no pronounced increase in net mineralisation of labelled nitrogen. However, the balance between crop uptake and the quantity leached did change. Labelled nitrogen assimilated was less for the wheat (growing without addition of fertiliser nitrogen) than for the grass in its last year, and the quantity leached was considerably greater than under grass swards that were supplied with an average rainfall distribution. Following the first wheat harvest total nitrogen leached averaged 51 and 44 kg N ha^?1 on the clay and silt loam soils respectively. Rates of nitrous oxide emissions during the autumn following sward destruction were greater than in earlier years, but this enhanced loss was of short duration. The crop clearly benefited from the succession of nitrogen applications made to grass, as grain yield and total nitrogen uptake exceeded 7 t ha^?1 and 120 kg N ha^?1 respectively on both soils. These quantities exceed the national averages for winter wheat and are also considerably greater than for crops from lysimeters which received no nitrogen fertiliser throughout the experiment.     

Uptake of foliar-applied urea by winter wheat (Triticum aestivum): The influence of application time and the use of a new 15N technique

A new ¹⁵N technique (termed the negative discard method) for measuring recovery of foliar-applied N by crops in the field is described. ¹⁵N-labelled fertiliser solution is sprayed on to a small area of crop, using a hand sprayer, while the surrounding area is sprayed with unlabelled N at the same rate. An area considerably larger than that given ¹⁵N is harvested with a small-plot combine-harvester, and crop recovery of foliar-applied N is calculated from the ¹⁵N enrichment of the resulting sample containing a mixture of labelled and unlabelled material. The technique was used to measure recovery of N from ¹⁵N-labelled urea solution sprayed on to winter wheat (Triticum aestivum L cv Avalon) at six different times from growth stage 39 (3 weeks before anthesis) to growth stage 73 (2 weeks after anthesis). Each treatment of 40 kg N ha^?1 was divided into two equal portions, the second being applied 1–2 days after the first, to minimise the risk of leaf damage. The crop had earlier received 210 kg N ha^?1, as ‘Nitro-Chalk’, in spring (50 kg ha^?1 at growth stage 22 and 160 kg ha^?1 at growth stage 31) which was more than sufficient to achieve maximum grain yield. At harvest, 70% of the foliar-applied N given at anthesis (growth stage 65) was recovered in the above-ground crop, including 64 % in grain. The proportion of labelled N recovered in the grain (92% of that in the above-ground crop) was slightly greater than with soil-applied N given earlier in the growing season. Recovery of foliar-applied N was slightly less for the earliest (growth stage 39) and latest (growth stage 73) times of spraying: 64% and 58% in above-ground crop, and 56% and 54% in grain, respectively. All of the foliar applications of 40 kg N ha^?1 increased %N in grain to the same extent as an additional 40 kg N ha^?1 applied to soil in spring.     

Unused fertiliser nitrogen in arable soils—its contribution to nitrate leaching

Nitrate present in arable soils in autumn is at risk to leaching during the following winter. To see whether unused nitrogen fertiliser was a major source of this nitrate, ¹⁵N-labelled fertiliser was applied to 11 winter wheat crops at rates of between 47 and 234 kg N ha^?1in spring. The experiments were on three contrasting soil types in south-east England. On average, 17′% of the N from spring-applied labelled fertiliser remained in the 0–23 cm soil layer at harvest (range, 7–36%) but only a small proportion was in inorganic forms (ammonium + nitrate). This was never more than 5 kg N ha^?1and averaged only 1·3% of the fertiliser N applied (range, 0·4–3·6 %). Between 79 and 98% of the inorganic N in soils at harvest was unlabelled, being derived from the mineralisation of organic N rather than from unused fertiliser. The amount of unlabelled N was much greater where wheat was grown after ploughing up grass or grass/clover leys than where it was grown in all-arable rotations. When wheat was grown without N fertiliser, soil inorganic N content at harvest was no lower than in plots given fertiliser at rates up to 234 kg N ha^?1. This work indicates that, for soil growing winter wheat, almost all of the nitrate at risk to leaching over the winter period comes from mineralisation of organic N, not from unused fertiliser applied in spring. Consequently, even a drastic reduction in N fertiliser use would have little effect on nitrate leaching.     

Effects of Interposing Sugar Beet on the Nitrogen Response of the Following Wheat Crop

Nitrogen response experiments were conducted on pairs of wheat fields at ten farms in East Anglia, UK, in 1984–1985 and 1985–1986, such that the previous crop of one of the pair was sugar beet (with tops ploughed in) and the other was wheat. Topsoil organic matter contents ranged from 1·4 to 4·3% and were associated with significant differences in soil mineral N (NH₄-N and NO₃-N) between sites. Average soil mineral nitrogen (N_min) to 90 cm in October after sugar beet was 34 kg N ha^-1, significantly less (P=0·03) than the 58 kg N ha^-1 after wheat. However, the net difference between N_min in October and N_min plus crop N in April was +25 kg N ha^-1 after beet, significantly more (P=0·002) than the +2 kg ha^-1 after wheat, suggesting some mineralisation from the tops over winter. There were no evident differences in net N mineralisation during spring and summer, and by harvest there was no significant difference due to previous crop in wheat yield with nil N or with ample N; mean optimum yields were 8·2 t ha^-1 after beet and 7·8 t ha^-1 after wheat. The mean optimum amount of fertiliser N (determined from a fitted linear plus exponential function) was 188 kg ha^-1 after beet, not significantly different from the optimum of 197 kg N ha^-1 after wheat. It is concluded that amounts of fertiliser N should be similar following sugar beet and following wheat. © 1997 SCI     

Nitrogen top-dressing requirements of winter oilseed rape

Forty-one field experiments were carried out on the spring N top-dressing requirements of winter oilseed rape in the years 1973 to 1977. The experiments were in the main rape-growing areas of England and Scotland, and almost all were on rape crops following one or more cereals. Seed and oil yield were increased by N top-dressing in 38 of the 41 experiments, with a mean requirement by satisfactory crops for about 230 kg ha^?1 of N. Those experiments with no response to N or no response beyond 90 kg ha^?1 of N were mostly low yielding and suffered from lodging, drought or bird damage. N reduced oil content by about 2% on average, but there was a considerably larger depression in 10 experiments, almost always associated with low yield, drought or lodging. N fertiliser increased the N content of the seed by about 0.5%. Timing of N application within the period mid-February to late March had little influence on seed yield or oil content but applying all or half the N in April tended to give a lower yield. There was no advantage from splitting the N application.     

Sulphur-coated urea as a fertiliser for potatoes

In a field experiment comparing sulphur-coated urea (SCU) and “Nitro-Chalk” at rates up to 400 kg N ha^?1, potato tuber yields were greater with “Nitro-Chalk” at rates up to 200 kg N ha^?1, but there was no consistent difference between forms of N at larger rates. The crop recovered more N from “Nitro-Chalk” at all rates except 350 kg N ha^?1. The largest measured yield was at 200 kg N ha^?1 with “Nitro-Chalk” and 250 kg N ha^?1 with SCU, and the optimum N rate inferred from split-line regressions of yield on N rate was also at a lower N rate with “Nitro-Chalk”. Yields of winter wheat following the potatoes increased significantly with the N rates given to the potatoes and, at rates above 200 kg N ha^?1, were larger where SCU had been used. SCU residues also lessened the response of the wheat to a spring top-dressing of new N slightly more than those of “Nitro-Chalk”. The sum of the apparent recoveries by the two crops of N given to the potatoes was greater from “Nitro-Chalk” at rates up to 200 kg N ha^?1. In the glasshouse, potato plants grown in 3 kg soil took 71 days to initiate tubers when given 2 g N as ammonium nitrate but this time was much decreased as increasing proportions of SCU were used. When 1 g N was given, the time was only 47 days with ammonium nitrate and the proportion of SCU had little effect. In the field, the number of tubers per plant by mid-June decreased with increasing N rate with “Nitro-Chalk” but not with SCU.     

Effects of nitrogen fertiliser on yield,dry matter content and flouriness of potatoes

The effects of incremental amounts of nitrogen fertiliser in the range 0–200 kg ha^?1 on yield ha^?1, tuber dry matter (DM), DM ha^?1and flouriness were evaluated in five potato cultivars over three seasons. In general, yield increased with the use of up to 100–150 kg N ha^?1and remained constant thereafter; % DM of tubers was significantly diminished by amounts of nitrogen > 150 kg ha^?1. In a single poor growing season, yields were small and both yield and% DM were less affected by nitrogen. The mean flouriness score of cooked tubers was highly correlated with specific gravity (SG) class (r = 0.94); tubers with SG > 1.08 were scored floury to very floury. Moderate nitrogen fertiliser use (? 100 kg ha^?1) had little effect on weight per cent of crop > SG 1.08; large amounts of N (200 kg ha^?1) resulted in a substantial decline in the size of this fraction; the effects of intermediate amounts varied with season and cultivar.     

Entwicklungstendenzen in der Stärke- und Zuckerindustrie in Abhängigkeit vom Rohstoff. Teil 4. Kartoffel

Development Trends in Starch and Sugar Industries Depending on the Source Material. Part 4. Potato. In Western Europe (52°N. Latitude) the potential starch yield is estimated to be about 17 t ha⁻¹. For potato crops that can grow until October the average starch yield is estimated to be about 8 t ha⁻¹. The big gap between these two yields must be explained mainly by insufficient water supply to the potato crop. Also a more optimal quantity of foliage and possibly an improved type of haulm may help to bridge the gap. It is assumed that the starch yield will increase annually by about 100 kg ha⁻¹. The yield of coagulable protein is approximately 500 kg ha⁻¹. About the same quantity of N-compounds is processed with the cellwall material for fodder. It is shown that selection on starch yield is in general also a selection on protein yield. The energy output/input ratio for the production of 1 ha potatoes for the starch industry is about 2.5. It is likely that this ratio will increase.     

Effect of nitrogen fertiliser on the fatty acid composition of oil from low erucic acid rape varieties

The effect of N fertiliser on the fatty acid composition of two low erucic acid varieties of winter oilseed rape (Primor and Rapora) was investigated in two field experiments in eastern England in 1976. Rates of N from 0 to 270 kg ha^?1 and application times from February to late April were compared. N increased growth and yield and reduced oil content but timing had little influence. Neither rate nor time of N fertiliser had any detectable effect on fatty acid composition in either experiment; the composition was as expected for these varieties, with over 60% oleic acid and 3% or less of erucic acid. It appears that the fatty acid composition of low erucic varieties is little affected by N fertiliser.     

Effects of sodium and potassium fertilisers on the composition of herbage and its acceptability to dairy cows

Fertilisation of herbage with Na can increase acceptability to cows, but the influence of fertiliser rate and fertilisation by K is unknown. In experiment 1, ten cows were grazed on pasture plots that had just been fertilised with 0–132 kg-Na ha⁻¹ (current Na) and had received 0–64 kg-Na ha⁻¹ in the previous grazing season (residual Na). Herbage Na concentration increased in proportion to current Na from 2·7 to 4·9 g-Na kg⁻¹ dry matter (DM) and also increased with increasing residual Na from 2·2 to 4·5 g-Na kg⁻¹ DM. Herbage K concentrations were low (10 g kg⁻¹ DM at 0 kg-Na ha⁻¹) and were only slightly reduced by Na fertiliser. Herbage Mg and Ca concentrations and DM digestibility were maximum at 66–99 kg-current-Na ha⁻¹. Cows grazed current-Na-fertilised plots to a lower height and spent more time grazing them. In experiment 2, pasture plots received no fertiliser, low and high isomolar and independent applications of Na and K or a combination of the two. The herbage was more mature than in experiment 1 and Na concentration of the herbage without Na fertilizer was high (5 g kg⁻¹ DM). Na fertiliser, therefore, only slightly increased Na concentration, more in clover than in grass, and had little effect on K concentration. K fertiliser increased K concentration from 16 to 20 g kg⁻¹ DM and reduced Na concentration to 3·5 g kg⁻¹ DM. Sodium fertiliser, therefore, only increased the acceptability of herbage to cattle when herbage Na concentrations were initially low (less than 5 g kg⁻¹ DM) and were increased substantially by the application of the fertiliser. © 1998 SCI.     

Effect of residual fertility and direct fertilisation on kernel,protein and oil yield of peanut (Arachis hypogaea L) grown in rice fallows

Kernel (2814–3467 kg ha^?1), protein (555–759 kg ha^?1) and oil (124–1556 kg ha^?1) yields of peanut (Arachis hypogaea L) varied significantly due to the residual effect of organic manures and inorganic fertilisers together. It was also observed that direct application of recommended fertilisers (25 N, 72 P₂O₅ and 37-5 K₂O kg ha^?1) gave the highest kernel (3669 kg ha^?1), protein (786 kg ha^?1) and oil (1606 kg ha^?1) yields. The reduction in kernel, protein and oil yield from the recommended level of fertilisers to half the recommended level of fertilisers was about 16, 15 and 12% while it was about 25, 33 and 27% with no fertiliser. Thus, the results revealed that the nutrients applied partly through organic manures and inorganic fertilisers to Kharif rice exhibits significant residual effects on the succeeding upland crop and hence the fertilisation must be considered not only for individual crops but also for the cropping system as a whole.     

Effects of nitrification inhibitors on uptakes of mineralised nitrogen and on yields of winter cereals grown on sandy soil after ploughing old grassland

An old grass sward on a sandy loam soil (Cottenham series) was ploughed-down in summer 1981 and winter wheat, winter oats and winter wheat respectively were grown on the site for the next 3 years. Nitrification inhibitors (dicyandiamide (DCD), nitrapyrin or etridiazole) were applied to the seedbed in all 3 years. In spring, the cereals were given 0, 35 or 70kg N ha^?1 as “Nitro-Chalk”. Inhibitors had little effect on the amounts or distribution of mineralised nitrogen in the soil profile or on the uptake of mineralised nitrogen during autumn and winter. Much mineralised nitrogen was leached during the autumn and winter 1981/82 and 1982/83, but amounts of available mineralised nitrogen were sufficient to meet the crop requirements. In these 2 years nitrogen fertiliser decreased yields and inhibitors had no consistent effect on yields or nitrogen uptakes. In 1984, winter wheat responded to spring-applied nitrogen fertiliser, while DCD or nitrapyrin increased yields and nitrogen uptakes. There was no evidence that yield increases were due solely to the increased availability of mineralised nitrogen caused by the inhibition of nitrification.     

Regular applications of poultry litter to a sandy arable soil: effects on nitrate leaching and nitrogen balance

Intensive poultry units often have insufficient land for spreading manure at agronomically and environmentally acceptable rates. This experiment measured the effects of annual applications, at several rates, on nitrate-N leaching and the soil–crop N balance on a sandy soil. Poultry litter from a broiler unit was applied each autumn 1992–1995. Total loadings on the main experiment area (instrumented with ceramic and Teflon water samplers at 1·0 and 1·5 m, and monolith lysimeters, 1·5 m deep) were 0, 60 and 150 t ha⁻¹. Additional plots (not instrumented) received 30, 90 or 120 t ha⁻¹. There was good agreement in the nitrate-N concentrations measured by the Teflon and ceramic water samplers and the lysimeters; all three methods gave acceptable measurements of nitrate leaching on structureless sandy soils. Autumn applications of poultry manure should be avoided: leaching was much greater than when delayed into December. At rates of broiler litter which supplied more N than the crop required (generally above 10 t ha⁻¹ each year), nitrate-N leaching losses were large; at the largest application rate (akin to a disposal, rather than a planned fertiliser strategy), concentrations peaked at c 500 mg litre⁻¹ N. Despite the movement of dissolved organic carbon to 1 m depth, the N concentration profiles measured by the water samplers did not provide clear evidence of subsoil denitrification. A nitrogen balance sheet, based on available N applied (as either fertiliser or manure) with some adjustment for mineralisation of the manure's organic fraction (10% annually) and for volatilisation (15%) was strongly correlated with soil mineral N each spring. © 1998 Society of Chemical Industry.     

The potassium requirement of crops grown in rotation

A 5-year trial measured the effect of varying rates of potassium fertiliser applied in combination with two rates of nitrogen to a rotation: potatoes, kale, barley, grass cut for conservation and wheat. Residual values of potassium applied during the rotation were measured on a crop of spring wheat planted in the sixth year. Crops grown in the rotation responded increasingly to potassium as follows: kale, barley, grass, wheat, potatoes. The higher rate of potassium applied (706 lb/acre^a K in 5 years) was justified by the total value of crops harvested; this return was derived mainly from potatoes and wheat. The higher rate of potassium applied was insufficient to maintain the soil in K balance (applications less removal in crops) when the higher rate of nitrogen was used. Soil analysis indicated no residues from the lower rate of K applied but the higher rate raised available K levels appreciably. The wheat crop grown after 5 years under the rotation showed residues from the lower rate of K applied equivalent to 55 lb/acre fresh K but no additional residue from the higher rate.     

Seedbed fertilizer requirements of winter oilseed rape

Thirty-four field experiments were carried out on the seedbed fertilizer requirements of winter oilseed rape in the main rape-growing areas of England and Scotland. Average seed yield was increased by seedbed N from 2.711 ha^?1 to 2.881 ha^?1 (when 200 kg ha^?1 N was given in spring). Economic analysis showed 60 kg ha^?1 as likely to be the most profitable rate of seedbed N for most situations. P fertilizer increased yield significantly on soils with low or moderate levels of available soil P but there was little response to K fertilizer on soils of high or medium K status and too few experiments on low K status soils to quantify requirements thereon. S application to the seedbed (as calcium sulphate) did not increase yield. Seedbed N decreased seed oil content slightly and not always significantly while P, K and S had no effect. Nutrient removal in the seed of an average 3 t ha^?1 crop would be 90 kg ha^?1 of N, 17 kg ha^?1 of P (40 kg ha^?1 P₂O₅) and 17 kg ha^?1 of K (20 kg ha^?1 K₂O).     

Nitrogenase Activity and Nitrogen-Fixing Bacteria Associated with Rhizosphere of Rice Cultivars with Varying N Absorption Efficiency

In a field investigation, nitrogenase activity and nitrogen-fixing bacteria associated with rhizosphere of rice cultivars with varying N absorption efficiencies were examined at three N-fertiliser regimes. A significant difference among the cultivars with regard to the nitrogenase activity of rhizosphere bacteria was observed. Under no or low (30 kg N ha^-1) fertiliser application, the cultivar with high boot leaf-N and high apparent N recovery had a higher nitrogenase activity. At 60 kg N ha^-1 fertiliser application, though the nitrogenase activity was stimulated, the differences with regard to nitrogenase activity among the cultivars were not significant. Microbiological analysis indicated that the cultivar with high N absorption efficiency harboured higher populations of nitrogen-fixing Azospirillum sp, Azotobacter sp and anaerobic bacteria. Under 60 kg N ha^-1 fertiliser application, the rhizosphere of both cultivars, which differed in their N use pattern, harboured almost identical population densities of nitrogen-fixing bacteria. Interestingly, the anaerobic nitrogen-fixing bacteria were greatly stimulated following the application of fertiliser. Notwithstanding the effect of N fertiliser management on N₂ fixation, N absorption efficiency of rice crop and the rhizosphere associated nitrogenase activity and nitrogen-fixing bacteria were found to be interrelated. Further investigations are required to evaluate the exact relationships including higher number of cultivars. © 1997 SCI.     

Nitrogen use efficiency in grain production and the estimated nitrogen input/output balance in China agriculture

BACKGROUND Understanding the nitrogen (N) use efficiency and N input/output balance in the agricultural system is crucial for best management of N fertilisers in China. RESULTS In the last 60 years, N fertiliser consumption correlated positively with grain production. During that period the partial factor productivity of N (PFP_N) declined greatly from more than 1000 kg grain kg^?1 N in the 1950s to nearly 30 kg grain kg^?1 N in 2008. This change in PFP_N could be largely explained by the increase in N rate. The average agronomic efficiency of fertiliser N (AE_N) for rice, wheat and maize during 2000‐2010 was 12.6, 8.3 and 11.5 kg kg^?1 respectively, which was similar to that in the early 1980s but lower than that in the early 1960s. Estimation based on statistical data showed that a total of 49.16 × 10⁶ t of N was input into Chinese agriculture, of which chemical N, organic fertiliser N, biological fixed N and other sources accounted for 58.2, 24.3, 10.5 and 7.0% respectively. Nitrogen was surplus in all regions, the total N surplus being 10.6 × 10⁶ t (60.6 kg ha^?1). CONCLUSION The great challenge is to balance the use of current N fertilisers between regions and crops to improve N use efficiency while maintaining or increasing crop production under the high‐intensity agricultural system of China. © 2012 Society of Chemical Industry