Changes in soil mineral nitrogen, nitrogen leached, and surface pH under annual and perennial pasture species

2009 ◽  
Vol 60 (10) ◽  
pp. 975 ◽  
Author(s):  
B. S. Dear ◽  
J. M. Virgona ◽  
G. A. Sandral ◽  
A. D. Swan ◽  
S. Morris
Keyword(s):  
Soil Ph ◽  
Perennial Grass ◽  
Subterranean Clover ◽  
Soil N ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Mineral N ◽  
Available N ◽  
Surface Ph ◽  
Ornithopus Compressus

Soil mineral nitrogen (N) profiles during the growing season and changes in total soil N and available N after 3–4 years were examined under 9 different pasture swards containing annual legumes, lucerne (Medicago sativa L.), or one of 4 perennial grasses at 2 sites representative of the low and medium rainfall belt of south-eastern Australia. The effect of the presence of phalaris (Phalaris aquatica L.) or lucerne on the spatial variation in surface pH was also measured. The 9 pastures were subterranean clover (Trifolium subterraneum L.), subterranean clover with annual weeds, yellow serradella (Ornithopus compressus L.), lucerne, phalaris, cocksfoot (Dactylis glomerata L.), lovegrass (Eragrostis curvula (Schrader) Nees), wallaby grass (Austrodanthonia richardsonii (Cashm.) H.P. Linder), and a mixture of lucerne, phalaris, and cocksfoot. All the perennial treatments were sown with subterranean clover. Available mineral N values in the surface 0.10 m of soil following summer rainfall were substantially higher in pure subterranean clover or serradella (Ornithopus compressus L.) swards (24–50 μg N/g) than those containing a mixture of subterranean clover and perennials (9–20 μg N/g). Apparent leaching of soil nitrate down the profile during winter was greatest in annual pasture treatments and least in swards containing perennials. Soil pH(CaCl2) at the 0–0.10 m depth varied with proximity to perennial plants and was significantly higher (+0.2–1.1 pH units) near the base of perennial plants than in gaps between the perennials or in annual-only swards. Available mineral N to 1.0 m before cropping at the end of the pasture phase was highest following subterranean clover (175–344 kg N/ha) and serradella (202–316 kg N/ha) at both sites. Available N was lowest (91–143 kg N/ha) following perennial grass–clover swards at the drier site where the annual legume content was lower, but perennial grass–clover swards produced larger soil N values (147–219 kg N/ha) at the higher rainfall site. Removal of the pasture in August–September compared with November in the year before cropping increased available N at the time of sowing by an average of 44% (51 kg N/ha) at the drier site and 43% (74 kg N/ha) at the wetter site. Incorporating perennial pasture species in swards was found to be advantageous in reducing nitrate leaching and preventing a decline in surface soil pH; however, available soil N to following crops could be lower if the annual legume content of perennial grass-based pastures declined due to competition from the perennial species.

Organic amendment effects on tuber yield, plant N uptake and soil mineral N under organic potato production

2008 ◽  
Vol 23 (03) ◽  
pp. 250-259 ◽  
Author(s):  
Derek H. Lynch ◽  
Zhiming Zheng ◽  
Bernie J. Zebarth ◽  
Ralph C. Martin
Keyword(s):  
N Uptake ◽  
N Availability ◽  
Residual Soil ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Mineral N ◽  
Total N ◽  
Available N ◽  
Organic Potato ◽  
Certified Organic

AbstractThe market for certified organic potatoes in Canada is growing rapidly, but the productivity and dynamics of soil N under commercial organic potato systems remain largely unknown. This study examined, at two sites in Atlantic Canada (Winslow, PEI, and Brookside, NS), the impacts of organic amendments on Shepody potato yield, quality and soil mineral nitrogen dynamics under organic management. Treatments included a commercial hog manure–sawdust compost (CP) and pelletized poultry manure (NW) applied at 300 and 600 kg total N ha−1, plus an un-amended control (CT). Wireworm damage reduced plant stands at Brookside in 2003 and those results are not presented. Relatively high tuber yields (~30 Mg ha−1) and crop N uptake (112 kg N ha−1) were achieved for un-amended soil in those site-years (Winslow 2003 and 2004) when soil moisture was non-limiting. Compost resulted in higher total yields than CT in one of three site-years. Apparent recovery of N from CP was negligible; therefore CP yield benefits were attributed to factors other than N availability. At Winslow, NW300, but not NW600, significantly increased total and marketable yields by an average of 5.8 and 7.0 Mg ha−1. Plant available N averaged 39 and 33% for NW300 and NW600, respectively. Soil (0–30 cm) NO3−-N at harvest was low (<25 kg N ha−1) for CT and CP, but increased substantially both in season and at harvest (61–141 kg N ha−1) when NW was applied. Most leaching losses of NO3−-N occur between seasons and excessive levels of residual soil NO3-N at harvest, as obtained for NW600, must be avoided. Given current premiums for certified organic potatoes, improving yields through application of amendments supplying moderate rates of N or organic matter appears warranted.

Gross rates of soil N2O emission and uptake and denitrification gene abundance in temperate cropland agroforestry and monoculture systems

2021 ◽  
Author(s):  
Jie Luo ◽  
Lukas Beule ◽  
Guodong Shao ◽  
Edzo Veldkamp ◽  
Marife D. Corre
Keyword(s):  
Greenhouse Gas ◽  
Management Systems ◽  
Soil N ◽  
Soil Mineral N ◽  
Soil Mineral ◽  
Mineral N ◽  
Total N ◽  
Soil P ◽  
Gene Abundance ◽  
Denitrification Gene

<p>Monoculture croplands are considered as major sources of the greenhouse gas, nitrous oxide (N<sub>2</sub>O). The conversion of monoculture croplands to agroforestry systems, e.g., integrating trees within croplands, is an essential climate-smart management system through extra C sequestration and can potentially mitigate N<sub>2</sub>O emissions. So far, no study has systematically compared gross rates of N<sub>2</sub>O emission and uptake between cropland agroforestry and monoculture. In this study, we used an in-situ <sup>15</sup>N<sub>2</sub>O pool dilution technique to simultaneously measure gross N<sub>2</sub>O emission and uptake over two consecutive growing seasons (2018 - 2019) at three sites in Germany: two sites were on Phaeozem and Cambisol soils with each site having a pair of cropland agroforestry and monoculture systems, and an additional site with only monoculture on an Arenosol soil prone to high nitrate leaching. Our results showed that cropland agroforestry had lower gross N<sub>2</sub>O emissions and higher gross N<sub>2</sub>O uptake than in monoculture at the site with Phaeozem soil (P ≤ 0.018 – 0.025) and did not differ in gross N<sub>2</sub>O emissions and uptake with cropland monoculture at the site with Cambisol soil (P ≥ 0.36). Gross N<sub>2</sub>O emissions were positively correlated with soil mineral N and heterotrophic respiration which, in turn, were correlated with soil temperature, and with water-filled pore space (WFPS) (r = 0.24 ‒ 0.54, P < 0.01). Gross N<sub>2</sub>O emissions were also negatively correlated with nosZ clade I gene abundance (involved in N<sub>2</sub>O-to-N<sub>2</sub> reduction, r = -0.20, P < 0.05). These findings showed that across sites and management systems changes in gross N<sub>2</sub>O emissions were driven by changes in substrate availability and aeration condition (i.e., soil mineral N, C availability, and WFPS), which also influenced denitrification gene abundance. The strong regression values between gross N<sub>2</sub>O emissions and net N<sub>2</sub>O emissions (R<sup>2 </sup>≥ 0.96, P < 0.001) indicated that gross N<sub>2</sub>O emissions largely drove net soil N<sub>2</sub>O emissions. Across sites and management systems, annual soil gross N<sub>2</sub>O emissions and uptake were controlled by clay contents which, in turn, correlated with indices of soil fertility (i.e., effective cation exchange capacity, total N, and C/N ratio) (Spearman rank’s rho = -0.76 – 0.86, P ≤ 0.05). The lower gross N<sub>2</sub>O emissions from the agroforestry tree rows at two sites indicated the potential of agroforestry in reducing soil N<sub>2</sub>O emissions, supporting the need for temperate cropland agroforestry to be considered in greenhouse gas mitigation policies.</p>

Computer simulation of changes in soil mineral nitrogen and crop nitrogen during autumn, winter and spring

1987 ◽  
Vol 109 (1) ◽  
pp. 141-157 ◽  
Author(s):  
T. M. Addiscott ◽  
A. P. Whitmore
Keyword(s):  
Soil Temperature ◽  
Dry Matter ◽  
N Uptake ◽  
Sowing Date ◽  
Mineralization Rate ◽  
Soil Mineral N ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Mineral N ◽  
Crop N Uptake

summaryThe computer model described simulates changes in soil mineral nitrogen and crop uptake of nitrogen by computing on a daily basis the amounts of N leached, mineralized, nitrified and taken up by the crop. Denitrification is not included at present. The leaching submodel divides the soil into layers, each of which contains mobile and immobile water. It needs points from the soil moisture characteristic, measured directly or derived from soil survey data; it also needs daily rainfall and evaporation. The mineralization and nitrification submodel assumes pseudo-zero order kinetics and depends on the net mineralization rate in the topsoil and the daily soil temperature and moisture content, the latter being computed in the leaching submodel. The crop N uptake and dry-matter production submodel is a simple function driven by degree days of soil temperature and needs in addition only the sowing date and the date the soil returns to field capacity, the latter again being computed in the leaching submodel. A sensitivity analysis was made, showing the effects of 30% changes in the input variables on the simulated amounts of soil mineral N and crop N present in spring when decisions on N fertilizer rates have to be made. Soil mineral N was influenced most by changes in rainfall, soil water content, mineralization rate and soil temperature, whilst crop N was affected most by changes in soil temperature, rainfall and sowing date. The model has so far been applied only to winter wheat growing through autumn, winter and spring but it should be adaptable to other crops and to a full season.The model was validated by comparing its simulations with measurements of soil mineral N, dry matter and the amounts of N taken up by winter wheat in experiments made at seven sites during 5 years. The simulations were assessed graphically and with the aid of several statistical summaries of the goodness of fit. The agreement was generally very good; over all years 72% of all simulations of soil mineral N to 90 cm depth were within 20 kg N/ha of the soil measurements; also 78% of the simulations of crop nitrogen uptake were within 15 kg N/ha and 63% of the simulated yields of dry matter were within 25 g/m2 of the amounts measured. All correlation coefficients were large, positive, and highly significant, and on average no statistically significant differences were found between simulation and measurement either for soil mineral N or for crop N uptake.

Effects of soil nitrogen status and rate of inoculation on the establishment of populations of Bradyrhizobium japonicum and on the nodulation of soybeans

1989 ◽  
Vol 40 (4) ◽  
pp. 753
Author(s):  
J Brockwell ◽  
RR Gault ◽  
LJ Morthorpe ◽  
MB Peoples ◽  
GL Turner ◽  
...  
Keyword(s):  
Glycine Max ◽  
Soil Nitrogen ◽  
Bradyrhizobium Japonicum ◽  
Clay Soil ◽  
Mineral Nitrogen ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Nitrogen Status ◽  
Mineral N ◽  
Glycine Max L

Soybeans (Glycine max [L.] Merrill cv. Forrest) were grown under irrigation on a well-structured grey clay soil, previously free of Bradyrhizobium japonicum and containing relatively high levels of mineral N, at Trangie, N.S.W. There were two soil pretreatments, pre-cropped (which had the effect of reducing the level of mineral nitrogen in the soil) and pre-fallowed, and four rates of inoculation (B. japonicum CB 1809 - nil, 0.01 X, 1.OX [=normal] and 100X).Mineral nitrogen (0-10 cm) initially was higher in pre-fallowed soil than in pre-cropped soil (37.6 v. 18.5 mg N per kg). Depletion of mineral nitrogen occurred more rapidly in pre-fallowed treatments, so that, 7 days after harvest, mineral-N in pre-cropped soil was significantly higher than in pre-fallowed soil (14.4 v. 10.6 mg per kg).With high levels of soil mineral nitrogen, colonization of seedling rhizospheres by rhizobia and plant nodulation were diminished. These effects were ameliorated but not eliminated by increased rates of inoculation. The development of the symbiosis was also impeded by lower rates of inoculation (0.01 X, 1.OX).

New ley legumes increase nitrogen fixation and availability and grain crop yields in subtropical cropping systems

2017 ◽  
Vol 68 (1) ◽  
pp. 11 ◽  
Author(s):  
Lindsay W. Bell ◽  
John Lawrence ◽  
Brian Johnson ◽  
Mark B. Peoples
Keyword(s):  
N2 Fixation ◽  
Farming Systems ◽  
Grain Sorghum ◽  
Soil N ◽  
Soil Mineral N ◽  
Soil Mineral ◽  
Forage Legumes ◽  
Mineral N ◽  
Forage Sorghum ◽  
Grain Crops

Several new and existing short-term forage legumes could be used to provide nitrogen (N) inputs for grain crops in subtropical farming systems. The fixed-N inputs from summer-growing forage legumes lablab (Lablab purpureus), burgundy bean (Macroptilium bracteatum) and lucerne (Medicago sativa) and winter-growing legume species snail medic (Medicago scutellata), sulla (Hedysarum coronarium) and purple vetch (Vicia benghalensis) were compared over several growing seasons at four locations in southern Queensland, Australia. Available soil mineral N and grain yield of a following cereal crop were compared among summer-growing legumes and forage sorghum (Sorghum spp. hybrid) and among winter-growing legumes and forage oats (Avena sativa). In the first year at all sites, legumes utilised the high initial soil mineral N, with <30% of the legume N estimated to have been derived from atmospheric N2 (%Ndfa) and legume-fixed N <30 kg/ha. In subsequent years, once soil mineral N had been depleted, %Ndfa increased to 50–70% in the summer-growing legumes and to 60–80% in winter-growing legumes. However, because forage shoot N was removed, rarely did fixed N provide a positive N balance. Both lablab and burgundy bean fixed up to 150 kg N/ha, which was more than lucerne in all seasons. Prior to sowing cereal grain crops, soil nitrate was 30–50 kg/ha higher after summer legumes than after forage sorghum. At one site, lablab and lucerne increased the growth and yield of a subsequent grain sorghum crop by 1.4 t/ha compared with growth after forage sorghum or burgundy bean. Of the winter-growing legumes, sulla had the highest total N2 fixation (up to 150 kg N/ha.year) and inputs of fixed N (up to 75 kg N/ha), and resulted in the highest concentrations of soil N (80–100 kg N/ha more than oats) before sowing of the following crop. Wheat protein was increased after winter legumes, but there was no observed yield benefit for wheat or grain sorghum crops. New forage legume options, lablab, burgundy bean and sulla, showed potential to increase N supply in crop rotations in subtropical farming systems, contributing significant fixed N (75–150 kg/ha) and increasing available soil N for subsequent crops compared to non-legume forage crops. However, high soil mineral N (>50 kg N/ha) greatly reduced N2 fixation by forage legumes, and significant N2 fixation only occurred once legume shoot N uptake exceeded soil mineral N at the start of the growing season. Further work is required to explore the impact of different management strategies, such as livestock grazing rather than harvesting for hay, on the long-term implications for nutrient supply for subsequent crops.

Effects of long-term straw management and fertilizer nitrogen additions on soil nitrogen supply and crop yields at two sites in eastern England

2002 ◽  
Vol 139 (2) ◽  
pp. 115-127 ◽  
Author(s):  
MARTYN SILGRAM ◽  
BRIAN J. CHAMBERS
Keyword(s):  
Soil Nitrogen ◽  
Nitrogen Supply ◽  
Fertilizer N ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Mineral N ◽  
Plough Layer ◽  
N Rates ◽  
Soil Nitrogen Supply

The effects of straw incorporation (early and late cultivation) and straw burning were contrasted in a split-plot study examining the impact of long-term straw residue management, and six fertilizer nitrogen (N) rates on soil mineral nitrogen, crop fertilizer N requirements and nitrate leaching losses. The experiments ran from 1984 to 1997 on light-textured soils at ADAS Gleadthorpe (Nottinghamshire, UK) and Morley Research Centre (Norfolk, UK).Soil incorporation of the straw residues returned an estimated 633 kg N/ha at Gleadthorpe and 429 kg N/ha at Morley on the treatment receiving 150 kg/ha per year fertilizer N since 1984. Straw disposal method had no consistent effect on grain and straw yields, crop N uptake, or optimal fertilizer N rates. In every year there was a positive response (P<0·001) to fertilizer N in straw/grain yields, N contents and crop N offtakes at both sites. Nitrate leaching losses were slightly reduced by less than 10 kg N/ha where straw residues had been incorporated, while fertilizer N additions increased nitrate leached at both sites.At both sites there was a consistent effect (P<0·001) of straw disposal method on autumn soil mineral N, with values following the pattern burn>early incorporate>late plough. The incorporation of straw residues induced temporary N immobilization compared with the treatment where straw was burnt, while the earlier timing of tillage on the incorporate treatment resulted in slightly more mineral N compared with the later ploughed treatment. Fertilizer N rate increased (P<0·001) soil mineral nitrogen at both sites. At Morley, there was more organic carbon in the plough layer where straw had been incorporated (mean 1·09 g/100 g) rather than burnt (mean 0·89 g/100 g), and a strong positive relationship between organic carbon and fertilizer N rate (r2=93·2%, P<0·01). There was a detectable effect of fertilizer N on readily mineralizable N in the plough layer at both Gleadthorpe (P<0·001) and Morley (P<0·05). At Morley, there was a consistent trend (P=0·06) for readily mineralizable N to be higher where straw had been incorporated rather than burnt, indicating that ploughing-in residues may contribute to soil nitrogen supply over the longer term.

Measurement of soil mineral nitrogen to predict the response of winter wheat to fertilizer nitrogen after applications of organic manures or after ploughed-out grass

1993 ◽  
Vol 121 (2) ◽  
pp. 223-231 ◽  
Author(s):  
M. A. Shepherd
Keyword(s):  
Winter Wheat ◽  
Inorganic Nitrogen ◽  
Poultry Manure ◽  
Mineral Nitrogen ◽  
Simple Relationship ◽  
Pig Slurry ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Available N ◽  
The Relationship

SUMMARYIn the autumns of 1985, 1986 and 1987, a total of 84 fields in England and Wales, which had received a recent dressing of organic manure or had recently been ploughed out of grass, was sampled to 90 cm depth to measure soil mineral nitrogen (Nmin i.e. NH4-N and N03-N). Amounts of Nmin varied widely between sites, indicating the difficulty in estimating soil supply following ploughing grass or manure application although type of manure and Nmin were related. Soils contained much inorganic nitrogen where manures containing a large proportion of readily available N (pig slurry or poultry manure) had been applied. Where manures containing straw were applied, Nmin was greater in the second year after application than the first. The relationship between Nmin and response of winter wheat to fertilizer was measured on 62 of the 84 sites sampled; there was no simple relationship between N min and response to N. For fields just ploughed out of grass, the relationship was particularly poor. However, for fields with a history of arable cropping, there was no response to N fertilizer when Nmin was > 300 kg/ha N in either autumn or spring. Below this amount, response to fertilizer N varied considerably even for sites with similar amounts of Nmin. Nmin measurements are of value in identifying soils containing sufficiently large N residues to allow fertilizer applications to be reduced, or even omitted, with confidence. Where residues are smaller, models need to be developed which take into account other factors, such as mineralization of organic nitrogen reserves, which modify nitrogen supply.

scholarly journals An analysis of the response of sugar beet and potatoes to fertilizer nitrogen and soil mineral nitrogen.

1989 ◽  
Vol 37 (2) ◽  
pp. 129-141 ◽  
Author(s):  
J.J. Neeteson ◽  
H.J.C. Zwetsloot
Keyword(s):  
Soil Type ◽  
Field Trials ◽  
Soil Mineral N ◽  
Fertilizer N ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Mineral N ◽  
Organic Manures ◽  
The Difference ◽  
Prior Application

A statistical analysis was performed to investigate if, and to what extent, the response of sugarbeet and potatoes to fertilizer N depended on the amount of mineral N already present in the soil, soil type, and prior application of organic manures. For this purpose the results of 150 field trials with sugarbeet and 98 with potatoes were used. The analysis was focussed on the within-block stratum of variation in yield, where regression models were fitted to describe the response to N. For both sugarbeet and potatoes the best fit was obtained when not only fertilizer N was taken into account, but also soil mineral N, soil type and prior application of organic manures. The response to fertilizer N was weaker as the amount of soil mineral N was larger. The optimum amount of fertilizer N plus soil mineral N required was larger on sandy soils than on loam and clay soils. The difference was about 20 kg N/ha for sugarbeet and 100 kg N/ha for potatoes. When organic manures were applied prior to the application of fertilizer N, the optimum for both sugarbeet and potatoes was 15-50 N/ha lower than without application of organic manures. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Nitrogen availability in a Darling Downs soil following cereal, oilseed and grain legume crops. 1. Soil nitrogen accumulation

1986 ◽  
Vol 26 (3) ◽  
pp. 347 ◽  
Author(s):  
WM Strong ◽  
J Harbison ◽  
RGH Nielsen ◽  
BD Hall ◽  
EK Best
Keyword(s):  
Soil Nitrogen ◽  
Nitrogen Availability ◽  
Grain Legumes ◽  
Mineral Nitrogen ◽  
Grain Legume ◽  
Nitrogen Accumulation ◽  
Soil Mineral N ◽  
Soil Mineral ◽  
Soil Mineral Nitrogen ◽  
Mineral N

Available soil mineral nitrogen (N) was determined in a Darling Downs clay at intervals of 4-6 weeks throughout summer and autumn after harvest of two cereals (wheat and oats), two oilseeds (rapeseed and linseed), and four grain legumes (chickpea, fieldpea, lupin and lathyrus). Soil mineral N (0-1.2 m) at 40,68, 107, 150 and 185 days after harvest was affected (P < 0.05) by the prior crop. At 40 days it was generally higher following grain legumes (34-76 kg/ha N) than following oilseeds or cereals (16-30 kg/ha N). Net increase during the next 145 days was in the order of cereals (2 1-27 kg/ha N) < oilseeds (40 kg/ha N) <grain legumes (53-85 kg/ha N). These differences are partly accounted for by differences in the quantities of N removed in the grain of these crops. However, a large quantity of mineral N accumulated following lupin even though a large quantity (80 kg/ha) was removed in the grain.

Dynamics of nitrogen capture without fertilizer: the baseline for fertilizing winter wheat in the UK

2001 ◽  
Vol 136 (1) ◽  
pp. 15-33 ◽  
Author(s):  
R. SYLVESTER-BRADLEY ◽  
D. T. STOKES ◽  
R. K. SCOTT
Keyword(s):  
Winter Wheat ◽  
N Uptake ◽  
Soil Horizon ◽  
Soil N ◽  
Soil Mineral N ◽  
Fertilizer N ◽  
Soil Mineral ◽  
Mineral N ◽  
Fertilizer Use ◽  
The Uk

Experiments at three sites in 1993, six sites in 1994 and eight sites in 1995, mostly after oilseed rape, tested effects of previous fertilizer N (differing by 200 kg/ha for 1993 and 1994 and 300 kg/ha for 1995) and date of sowing (differing by about 2 months) on soil mineral N and N uptake by winter wheat cv. Mercia which received no fertilizer N. Soil mineral N to 90 cm plus crop N (‘soil N supply’; SNS) in February was 103 and 76 kg/ha after large and small amounts of previous fertilizer N respectively but was not affected by date of sowing. Previous fertilizer N seldom affected crop N in spring because sowing was too late for N capture during autumn, but it did affect soil mineral N, particularly in the 60–90 cm soil horizon, presumably due to over-winter leaching. Tillering generally occurred in spring, and was delayed but not diminished by later sowing. Previous fertilizer N increased shoot survival more than it increased shoot production. Final shoot number was affected by previous fertilizer N, but not by date of sowing. Overall, there were 29 surviving tillers/g SNS.N uptakes at fortnightly intervals from spring to harvest at two core sites were described well by linear rates. The difference between sowings in the fitted date with 10 kg/ha crop N was 1 month; these dates were not significantly affected by previous fertilizer. N uptake rates were increased by both previous fertilizer N and late sowing. Rates of N uptake related closely to soil mineral N in February such that ‘equivalent recovery’ was achieved in late May or early June. At one site there was evidence that most of the residue from previous fertilizer N had moved below 90 cm by February, but N uptake was nevertheless increased. Two further ‘satellite’ sites behaved similarly. Thus at 14 out of 17 sites, N uptake until harvest related directly and with approximate parity to soil mineral N in February (R2 = 0·79), a significant intercept being in keeping with an atmospheric contribution of 20–40 kg/ha N at all sites.It is concluded that, on retentive soils in the UK, SNS in early spring was a good indicator of N availability throughout growth of unfertilized wheat, because the N residues arising from previous fertilizer mineralized before analysis, yet remained largely within root range. The steady rates of soil mineral N recovery were taken as being dependent on progressively deeper root development. Thus, even if soil mineral N equated with a crop's N requirement, fresh fertilizer applications might be needed before ‘equivalent recovery’ of soil N, to encourage the earlier processes of tiller production and canopy expansion. The later process of grain filling was sustained by continued N uptake (mean 41 kg/ha) coming apparently from N leached to the subsoil (relating to previous fertilizer use) as well as from sources not related to previous fertilizer use; significant net mineralization was apparent in some subsoils.

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