Losses of nitrate-nitrogen in water draining from under autumn-sown crops established by direct drilling or mouldboard ploughing

The leaching of nitrate-N under autumn-sown arable crops was measured using hydro-logically isolated plots, about 0.24 ha in area, from 1984–1988. Fluxes of water and nitrate moving over the soil surface (surface runoff), at the interface between topsoil and subsoil (interflow), and in the subsoil (drainflow) were monitored in plots with mole-and-pipe drain systems (drained plots); surface runoff and interflow only were monitored in ‘undrained’ plots. Half the drained and undrained plots were direct-drilled, and on the other half seedbeds were prepared by tillage to 200 mm. Tillage increased the total leaching loss of nitrate by 21 % compared with direct drilling in drained plots. About 95% or the nitrate moving from the soil was present in the water intercepted by the subsoil drains in these plots. In undrained plots less water and nitrate were collected in total; more of the nitrate was present in interflow on ploughed plots and in surface runoff in direct-drilled land. Losses of nitrate for the whole experiment from 1978-1988 were analysed. This showed that, between the harvest of one crop and the spring application of fertilizer to the next, loss of nitrate-N from ploughed land (L_p) was approximated by L_p=22+Fkg N ha^?1, where F was the autumn fertilizer-N applied. After fertilizer was applied in spring, loss of nitrate-N depended on rainfall such that for 100 mm rainfall about 30% of the fertilizer-N was lost by leaching. About 18% more nitrate-N was lost from direct-drilled land than from ploughed land in spring, but the total loss was generally small compared to that over winter. The apparent net mineralization of organic-N was measured in 1988. In autumn and winter there was little effect of tillage treatment (26 and 31 kg N ha^?1 on direct drilled and tilled plots respectively). However, over the year 83 kg N ha^?1 were mineralized in tilled plots, and 67 kg N ha^?1 in direct-drilled plots. Five factors governing the leaching of nitrate are assessed and this identified that fertilizer nitrogen application to the seedbed of winter sown crops and the mineralization of nitrogen from the residues of the previous crop are the most significant factors for nitrogen leaching in the UK.     

A field study of effects of contrasting methods of cultivation on soil nitrate content during autumn, winter and spring

The nitrate content of three heavy clay soils (Evesham, Denchworth and Lawford series) was examined after direct drilling or ploughing, and of another clay soil (Denchworth series) with or without mole drainage and after tine cultivation. Autumn-sown crops were grown at all these sites during the years 1973–80. Except in 1974–75, the nitrate concentration of the soil solution sampled from direct-drilled plots was less than from the ploughed plots, particularly in late autumn and early winter. By January the plots which had been ploughed contained 12–65 kg N ha^?1 more mineral nitrogen than direct-drilled plots, but differences between cultivation treatments disappeared rapidly and had gone by early March. After tine cultivation, the nitrate content of the soil profile was not affected by the contrasting drainage status of the plots with or without mole drainage. These results support the view that although denitrification can be greater in direct-drilled soil, the larger nitrate concentrations present in ploughed soil result from the soil disturbance exposing less accessible substrates to mineralization by microbes. By this means, considerable amounts of mineral nitrogen are made available at the beginning of the winter, during the establishment of autumn-sown crops.     

Nitrate leaching as influenced by soil tillage and catch crop

Because of public and political concern for the quality of surface and ground water, leaching of nitrate is of special concern in many countries. To evaluate the effects of tillage and growth of a catch crop on nitrate leaching, two field trials were conducted in spring barley (Hordeum vulgare L.) under temperate coastal climate conditions. On a coarse sand (1987–1992), ploughing in autumn or in spring in combination with perennial ryegrass (Lolium perenne L.) as a catch crop was evaluated. Furthermore, rotovating and direct drilling were included. The experiment was conducted on a 19-year-old field trial with continuous production of spring barley. On a sandy loam (1988–1992), ploughing in autumn or in spring in combination with stubble cultivation and perennial ryegrass, in addition to minimum tillage, was evaluated in a newly established field trial. For calculation of nitrate leaching, soil water isolates from depths of 0.8 or 1.0 m were taken using ceramic cups. No significant effect of tillage was found on the coarse sand; however, a significant effect of tillage was found on the sandy loam, where leaching from autumn ploughed plots without stubble cultivation was 16 kg N ha⁻¹ year⁻¹ higher than leaching from spring ploughed plots. Leaching was significantly less when stubble cultivation in autumn was omitted. Leaching on both soil types was significantly reduced by the growth of a catch crop which was ploughed under in autumn or in spring. It was concluded that soil cultivation increased leaching on the sandy loam but not on the coarse sand, and that the growth of perennial ryegrass as a catch crop reduced leaching on both soil types, particularly when ryegrass was ploughed under in spring.     

Yield and N uptake as affected by soil tillage and catch crop

Two field trials with spring barley (Hordeum vulgare L.) were conducted at two locations in Denmark in order to evaluate the effects of tillage and growth of a catch crop on yield parameters under temperate coastal climate conditions. Ploughing in autumn or spring in combination with perennial ryegrass (Lolium perenne L.) as a catch crop was evaluated on a coarse sand (Orthic Haplohumod) from 1987 to 1992 at three rates of N fertiliser application (60, 90 and 120 kg N ha⁻¹ year⁻¹). Rotovating and direct drilling were also included as additional tillage practices. The experiment was conducted on a 19-year-old field trial with continuous production of spring barley. Ploughing in autumn or spring in combination with stubble cultivation and a catch crop, in addition to minimum tillage, was evaluated in a newly established field trial on a sandy loam (Typic Agrudalf) from 1988 to 1992. Yield parameters and N concentrations in grain and straw were determined. On the coarse sand, N uptake in the grain in ploughed plots without a catch crop was significantly greater when spring ploughed as opposed to autumn ploughed, but grain and straw yields did not differ significantly. Grain yield, straw yield and total N uptake did not differ significantly between direct drilled and autumn ploughed plots, but the trend was for grain yield to be lower with direct drilling. After 19 years of catch crop use, yield parameters in ploughed plots were greater than in plots without catch crops. This was most pronounced in the autumn ploughed plots. Rotovating the catch crop in the spring decreased grain yield compared with underploughing the catch crop in autumn or spring. No significant interactions were found between tillage and N rates. On the sandy loam, grain as well as straw yield and total N uptake were not significantly affected by catch crop or time of ploughing. Grain yield was significantly lower with reduced tillage (stubble cultivation in autumn) than in all other treatments.     

Soil N dynamics in a natural calcareous grassland under a changing climate

 This paper reports the results from a medium-term field scale investigation into the effects of simulated climate change on soil N mineralisation in a semi-natural calcareous grassland in southern England. The experiment utilised soil warming cables, automatic rainshelters and a watering system to examine two climate change scenarios: warmer winters with summer drought and warmer winters with enhanced summer rainfall. Gross N mineralisation rates in treated plots were determined, using ¹⁵N pool dilution techniques, at 6-weekly intervals over a 3-year period. Results from control plots showed a strong seasonality of mineralisation with highest rates in autumn and winter and lowest rates in summer. They suggest that water availability is the main constraint on microbial processes and plant growth. Unexpectedly, additional summer rainfall had no direct effect on N mineralisation at the time of application (summer). The treatment did, however, significantly (<0.05%) reduce rates in subsequent autumn and winter months. In contrast, summer drought significantly increased N mineralisation rates in autumn and winter. Winter warming similarly had no direct effect on N mineralisation in winter but decreased rates in spring. We hypothesise that the observed treatment effects result from changes in organic C and N input, in plant litter, resulting from the direct impact of climatic manipulation on perennial plant growth, death and senescence. This paper compares and contrasts the response to climate manipulation in the grassland system with results from other ecosystem types such as northern forests. Received: 1 December 1997     

Measurement of nitrate leaching losses from arable plots under different nitrogen input regimes

Abstract. Leaching losses of nitrate-nitrogen were measured from a set of eight hydrologically isolated plots on a clay loam soil over the period from September 1987 to February 1990. Variable drainflow recovery from the plots hampered accurate estimation of nitrate loading, but results suggest that, when inorganic nitrogen fertilizer is applied up to the recommended amount, there is little influence of the amount applied on the amount leached. We did, however, observe the following effects on nitrate leaching: leguminous green manure incorporated in autumn increased leaching of nitrate-nitrogen by 10–15 kg per hectare during the winter; autumn cultivation caused some increase in leaching compared with no cultivation in one year; some systematic variations in nitrate leaching occurred between years and between plots, but were unrelated to treatments.
From the results we conclude that green manuring does not provide sufficient nitrogen for organically grown crops on this soil but contributes significantly to nitrate leaching, and that growing spring cereals, with the land remaining in stubble as long as possible in autumn, may be the best strategy to minimize nitrate leaching.     

Long-term effect of tillage and manure application on soil organic fractions and crop performance under Sudano-Sahelian conditions

Human-induced degradation of natural resources in general and of soil in particular, is a major problem in many regions, including the Sudano-Sahelian zone. The combined effects of tillage and manure application on Lixisol properties and on crop performance were investigated at Saria, Burkina Faso, to find efficient soil management practices to improve soil fertility. A randomized block design with four treatments (hand hoeing only, hand hoeing+manure, ploughing only, oxen ploughing+manure) in three replications was started in 1990. Ten years later, total soil organic (SOC), particulate organic matter and C mineralization were measured. Initial SOC concentration was 4 mg/g and dropped to 2.1 mg/g soil in ploughed plots without manure and to 2.5 mg/g soil in hoed plots without manure. Manure addition mitigated the decrease of SOC in ploughed plots and even built up SOC in hoed plots, where it increased to 5.8 mg/g soil. Manure had a large effect on the fractions in which SOC was stored. In ploughed plots, a large amount of SOC was stored in physical particles >0.25 mm, while in hand hoed plots the maximum SOC was stored in finer fractions. In the topsoil, hoeing and manure resulted in a higher SOC than ploughing with no manure. However, in the 15–25 cm layer, particularly in September, particulate organic matter was greater in ploughed plots with manure than in hoed plots with manure. Crop yields were highest on ploughed+manure plots and lowest on ploughed plots with no manure. We conclude that applying manure annually mitigates the negative effect of ploughing and hand hoeing on SOC and related properties and therefore can contribute to the sustainability of the agricultural system in the Sudano-Sahelian zone.     

Carbohydrate composition in relation to structural stability, compactibility and plasticity of two soils in a long-term experiment

The effects of tillage on soil organic carbon content, carbohydrate content, monosaccharide composition, aggregate stability, compactibility and plasticity were investigated in a field experiment on a gleysol and on a cambisol under winter barley in South-East Scotland. Two long-term treatments (direct drilling and conventional mouldboard ploughing for 22 years) were compared with short-term direct drilling and broadcast sowing plus rotavation for 5 years. Carbohydrate released sequentially to cold water, hot water, 1.0 M HCl and 0.5 M NaOH was determined after hydrolysis as reducing sugar equivalent to glucose in both fresh and air-dried samples. All other measurements were made on dry soils only. About 3% of the soluble carbohydrate was extracted by cold water, 10% by hot water, 12% by HCl and 75% by NaOH from both the dry and fresh soils. The total reducing sugars of the fractions were proportional to the total organic carbon determined by dichromate oxidation or C analysis. Organic carbon and carbohydrates were concentrated near the surface of the direct drilled soil, but were more uniformly distributed with depth in the ploughed soil. The surface soil under direct drilling was more stable, less compactible and had greater plasticity limits than under ploughing. However, particle size distributions were unaffected by tillage so that differences in soil properties were attributed to differences in the quantity and quality of organic matter. Differences in compactibility, structural stability and plasticity limits between depths and tillage treatments correlated with total carbon and with total carbohydrates. The hot water extractable carbohydrate fraction correlated best with aggregate stability and the NaOH fraction correlated best with compactibility and plastic limit. Both fractions were greatest in the long-term direct drilled soil. The hot water fraction had a galactose plus mannose over arabinose plus xylose ratio of 1.0–1.6 in comparison to 0.4–0.7 in the NaOH fraction indicating that the microbial contribution within the hot water-soluble fraction was the greater. The hot-water fraction was likely to contain more exocellular microbial polysaccharides involved in the stabilizing of soil aggregates. The hot-water and NaOH carbohydrate fractions may be good indicators of soil organic matter quality relevant to the preservation of good soil physical conditions.     

Prediction of surface runoff and soil loss in southeastern Norway using the WEPP Hillslope model

Yearly and daily surface runoff and soil loss simulated by the WEPP Hillslope model v. 2002.7 were compared with measurements from two different soil erosion plot sites in southeastern Norway. The soil at Bjørnebekk (Bj) was a levelled silty clay loam (2% organic matter) and the soil at Syverud (Sy) was a loam (5% organic matter). The plots at Bj were 21 m long and 8 m wide, while the plots at Sy were 30 m long and 7 m wide. Slope at both sites was 13%. Four management systems for grain production were used: autumn ploughing (AP), winter crop rotation (WCR), autumn harrowing (AH) and spring ploughing (SP). Hydraulic conductivity and soil erosion parameters were determined using WEPP-recommended equations based on measured soil parameters. In general, the WEPP Hillslope model simulated fewer runoff events than measured for all management systems during 1990–1998. Runoff events in winter were seldom predicted and small events (<5 mm) were mostly missed. To use the WEPP Hillslope model on Norwegian soils, where major soil loss events from arable land occur during late autumn, winter and early spring, improvements in winter hydrology calculations are needed. In particular calculations of soil frost development and infiltration into frozen soil need to be improved. The WEPP-recommended soil erosion parameter equations were unsuitable for these two Norwegian soils, especially for levelled soil.     

The effect of tillage on soil surface properties and the water balance of a xeralfic alfisol

Two long term tillage trials were established in 1977 on a xeralfic alfisolat a location receiving 210 mm of rain in the winter (growing) season. One trial was cropped continuously and the other one in alternate years, to spring wheat. Yields and soil physical conditions did not differ markedly in the first 3 years between tillage treatments although the ploughed soil had the lowest soil strengths and highest porosities. However, by the fifth and sixth years soil physical conditions were much poorer for crop growth on the ploughed treatments and those yields on both trials were only half that of the direct drilled crops. There was a 10-fold reduction in hydraulic conductivity and an increase in aggregate instability measured by the slaking and remould-dispersion test. These changes were paralleled by reductions in depth to wetting front and the amount of water stored in the top 120 cm of the profile. Because the site was nearly level no significant runoff occurred and the 40–70 mm less soil water stored in the ploughed treatments at seeding were ascribed to increased evaporation from wetter soil surfaces. In constant-flux infiltration measurements, time to ponding was found to be least on flattened, undisturbed surfaces and greatest on combine-drilled, direct-seeded soil.

A relative reduction in organic matter in the 0–2.5 cm zone of the ploughed soil was found by the third year, and by the sixth year this had become a significant reduction compared with the direct drilled treatments. Concentration of organic matter in this layer of the direct drilled soils afforded protection against remould-dispersion of wet soil by traffic, stock or raindrop action. As alfisols have inherently weak structural stability, direct drilling is seen as providing a stable system for continuous cropping for this soil order in semi-arid regions. The increase in water use efficiency in areas where drought is a major constraint to yield is of paramount significance in this context.     

A long-term comparison of ploughing, tine cultivation and direct drilling on the growth and yield of winter cereals and oilseed rape on clayey and silty soils

The effects of mouldboard ploughing, shallow tined cultivation and direct drilling on yields of winter wheat, barley, oats and oilseed rape were compared over 10 years. Three field experiments were conducted on two non-calcareous clays (stagnogleys) and a weakly structured silty soil (argillic brown earth). Two spring N levels were applied to the winter wheat plots on the clay soil in three years and to the winter barley plots on the silty soil in one year. This paper reports the soil bulk density and water content at sowing and the crop growth, yield components and yields obtained during the later years of the study: 1979–1984 on the clayey soils and 1981–1984 on the silty soil.

In the years when cereals were grown, differences in yield between cultivation treatments were small and inconsistent. Oilseed rape yielded significantly more after direct drilling than ploughing because of better establishment and uniformity of growth.

The success of continuous reduced tillage depended on both burning crop residues and good weed control.     

Ploughless tillage in Sweden. Results and experiences from 15 years of field trials

Results and experience from 140 field trials carried out during the period 1974–1988 are presented. The whole of Sweden was represented in the investigation, but the majority of sites were in the central and southern areas of the country. In the unploughed experimental plots, conventional autumn ploughing to 20–25 cm was compared with two or three stubble cultivations to approximately 10–12 cm depth. on average, use of ploughless tillage reduced yields of winter wheat, spring barley, winter and spring oilseed rape and sugarbeet and somewhat improved yields of oats and potatoes. The results from ploughless tillage were generally improved by the use of dual rear tractor wheels instead of single wheels, by the placement of artificial fertilizer instead of broadcasting and by the removal of straw. The incidence of annual weeds increased on average by 25% and stoloniferous weeds by 100%. The most suitable soils were peats, sandy loamy tills, silty loams or silty clay loams and heavy clays.     

OXYGEN CONCENTRATIONS IN A CLAY SOIL AFTER PLOUGHING OR DIRECT DRILLING

The concentration of oxygen at three depths (15, 30 and 60 cm) has been measured in a direct-drilled or ploughed clay soil of low hydraulic conductivity (Evesnam series) which was cropped with winter wheat in three consecutive growing seasons (1972/73 to 1974/75). The mean oxygen concentrations between January and March varied with the seasonal rainfall; in the relatively dry winter of 1972/73they were 20 per cent (v/v) at 15 cm and 12 per cent (v/v) at 60 cm, but in the wetter winters of 1973/74 and 1974/75 they were less than 11 and 6 per cent (v/v) at 15 and 60 cms respectively. Direct drilling resulted in higher oxygen concentrations at 15 cm than ploughing in both wetter winters, the mean concentration being 10.2 and 7.2 per cent (v/v) in direct-drilled and ploughed plots respectively. Frequency distributions of values obtained at individual sampling points showed that when the mean oxygen concentration of the soil at 15 cm decreased to its annual minimum (in January-March), a significantly higher proportion of the sampling points under direct drilling continued to show concentrations of oxygen greater than 10 per cent (v/v). The results are interpreted as indicating that the higher oxygen concentrations found in this soil after direct drilling are due to the development of a system of continuous large pores and channels which would otherwise be destroyed by annual ploughing.     

Effects of tillage and seeding methods on soil physical properties and yield of upland rice for an ultisol in southeast Nigeria

The yield of direct-seeded and transplanted upland rice was investigated for seven tillage methods for an ultisol in a high rainfall region of southeastern Nigeria. The tillage methods were: two compaction passes of a 6-t roller with and without residue mulch; six compaction passes with mulch; ploughing with and without mulch and no-till with and without mulch. Soil compaction decreased seedling emergence and shoot and root growth. Residue mulching decreased seedling emergence by 35.6% in direct-seeded rice. There were more leaves, productive tillers and dry matter in the ploughed plots. Root densities at 10–20- and 20–30-cm depths were higher by 157 and 47%, respectively, in ploughed treatments. The highest grain yields of 6.3 and 6.1 Mg ha⁻¹ in ploughed plots for the first and second seasons, respectively, were associated with greater uptake of P, Na, Fe and Zn at flowering and of N, Mg, K, Mn and Cu at both maximum tillering and flowering growth stages. The grain yield in the first season was 2.1 and 2.5 Mg ha⁻¹ for two- and six-compaction pass mulched plots, respectively. In the second season, the yields of no-till and compacted treatments were identical and ranged from 1.5 to 2.4 Mg ha⁻¹. Mulching decreased grain yield by 43, 27 and 12% on compacted, no-till and ploughed plots, respectively, due to transient flooding and mechanical impedance to seedling emergence by the mulch cover. Within the unmulched treatments, ploughing increased rice yield by 71 and 35% over two compaction passes and the no-till treatments, respectively. The least bulk density and penetrometer resistance were also observed in ploughed plots.     

Effects of alternative tillage systems on soil quality and yield of spring cereals on silty clay loam and sandy loam soils in the cool, wet climate of central Norway

Tillage trials were established on a poorly drained silty loam overlying silty clay loam and on a freely drained sandy loam overlying medium sand, in 1988 and 1989, respectively. Autumn and spring ploughing and two ploughless systems were compared for 12–13 years, with three replications at each site. The ploughless treatments comprised deep versus shallow spring harrowing until 1999, and thereafter autumn plus spring harrowing versus spring harrowing only. In 6 years, treatments with and without fungal spraying of the cereal crops were included. In other years, fungicides were not used. Perennial weeds were controlled by herbicides as necessary, on nine occasions up until 2001. Average spring barley (Hordeum vulgare L.) and spring oat (Avena sativa L.) yields were similar with spring ploughing as with autumn ploughing at both sites. In treatments without ploughing, average yields on the silty loam over clay were 93% of those obtained with ploughing, and on the sandy loam over sand they were 81%. Smaller and non-significant yield differences were found between spring harrowing versus deep spring harrowing, and between autumn plus spring harrowing versus spring harrowing only. Fungal spraying increased yields markedly at both sites (25%), but there was no significant interaction between this treatment and tillage system. Oat was compared with barley in 2 years, with oat performing better under ploughless tillage. At both sites increases in penetrometer resistance occurred in the topsoil of unploughed treatments. These were considered particularly limiting on the sandy loam. On the silty loam there was an increase in surface horizon porosity in the absence of ploughing, which was associated with an increase in topsoil organic matter content. On this soil there was also a tendency toward lower penetrometer resistance at >30 cm depth on autumn plus spring harrowed soil than on ploughed soil, indicating that the plough pan may have diminished. This was supported by observations of greater earthworm activity on unploughed soil. Soil chemical analyses revealed that mineral N and plant-available P and K accumulated in the upper horizon under ploughless tillage. The percentage yields obtained in individual years with autumn as opposed to spring ploughing, were positively correlated with air temperature during 0–4 weeks after planting on the silty loam, and with precipitation during 0–12 weeks after planting on the sandy loam. In the case of yields obtained with spring harrowing only, relative to spring ploughing, positive correlations were found with 0–4 week temperature on both soil types, suggesting that low early season temperatures may limit yields under ploughless tillage.     

Recent yield results and trends over time with conservation tillage on morainic loam soil in southeast Norway

Abstract

Results for 1998–2004 are reported from four long-term (25–28 years) tillage trials, comparing conventional autumn ploughing with reduced tillage, normally spring harrowing only. Plant residues were retained during the period studied. The weather was somewhat wetter than the 1961–1990 normal. Results with reduced tillage were mostly similar to those seen in earlier trial periods. In Trial 1, mean grain yield was 95% with spring harrowing only versus autumn ploughing, 96% when harrowing in autumn was performed as well and 97% when the soil was ploughed every third year. In Trial 2 positive crop rotation effects were found both with and without ploughing, and reduced tillage gave 5% lower grain yield also in this trial. In Trial 3, tillage system did not affect yields of cereals grown in rotation with potatoes, but reduced tillage gave 12% lower potato yield than ploughing. Little difference in response to N fertilizer was found. In Trial 4, reduced tillage on large-scale (0.7 ha) plots gave 11% lower grain yields than annual ploughing, partly due to shallow sowing depth. No long-term trend in yield responses to tillage was discernible in any trial, and between-year variability was similar with both ploughing and reduced tillage. Percentage yields with reduced tillage relative to annual ploughing correlated positively with rainfall in May and with mean air temperature in August. It is concluded that the reduced tillage systems studied are sustainable in terms of productivity, relative to labour, machinery and energy inputs. Likely benefits of such systems include higher levels of organic matter and aggregate stability in surface soil horizons, but a disadvantage is the need for frequent herbicide use to control perennial weeds.     

Seasonal changes in the structure of clay soils in relation to soil management and crop type. II. Effects of cultivation and cropping at Compton Beauchamp

The effects of direct drilling and ploughing on the structure and macroporosity of a clay soil at Compton Beauchamp, Oxfordshire, growing winter wheat, were studied using large thin sections (10 * 5 * 3.5 cm) and image analysis of photographs of fluorescent resin impregnated soil blocks.
Composite coarse angular to subangular blocky and medium to fine granular aggregates were present in the top 3 cm of the direct drilled soil and in the top 10 cm of the ploughed soil. Settling of the soil and coalescing of these aggregates took place throughout the year under both treatments, but was most marked in the ploughed soil. In the subsoil (20 to 30 cm) most of the structural changes could be attributed to changes in the soil water content. Swelling in the winter led to the closure of the most macropores and shrinking in the spring and summer led initially to the development of vertical planar macropores and then to large cracks. Freezing and thawing in the winter created many very small subcuboidial aggregates at the soil surface which later coalesced as a result of raindrop impact.     

Influence of spring preparation date and soil water content on seedbed physical conditions of a clayey soil in Sweden

Secondary tillage performed under inadequate soil water contents usually leads to a poor seedbed. Under normal Swedish weather conditions, clayey soils ploughed during autumn form a very dry top layer in spring, which acts as an evaporation barrier so that deeper layers remain wet. Thus, the conventional approach considering soil workability in relation to a single value of soil water content is difficult to apply. Hence, a field experiment was carried out to study the effect of seedbed preparation date, the associated soil water contents and traffic consequences on the physical properties of a spring seedbed. The field was autumn ploughed and the experiment started as soon as the field was trafficable after winter thawing. The seedbed preparation consisted of three harrowing operations on plots 8 m×8 m (three replications) with a spring tined harrow and a tractor mounted with dual tyres and was performed on 10 occasions from the beginning of April to the middle of May. With the exception of some short periods after rain, the soil had a clear water stratification during the experiment, with a very dry superficial layer (5–20 mm thick) contrasting to water contents over 300 g kg⁻¹ from only 40 mm depth. After the harrowing operation, the seedbed aggregate fraction less than 2 mm increased from about 40% at the beginning of April to about 60% for the last four treatments in May. Contributing factors to the rise were attributed to the lower water contents of the top layer (<40 mm) and the drying–wetting and freezing–thawing cycles that occurred in the surface layer during April. There were no significant differences in bulk density after harrowing between the treatments but an increase in penetration resistance up to a depth of 180 mm in the harrowed plots was statistically significant (P<0.001). In the non-harrowed soil, penetration resistance also increased, including in those soil layers where water contents kept nearly constant.

In conclusion, the seedbed preparation dates had only a minor effect on soil compaction, as measured by bulk density and penetration resistance, due to the slow drying beneath the dry top layer. The fraction of fine aggregates in the seedbed increased with time. Thus, the optimal time for seedbed preparation depended mainly on soil friability and not on the risk of compaction.     

Nitrate leaching after cut grass/clover leys as affected by time of ploughing

Abstract. Nitrate leaching after one year of a cut grass/clover ley was measured in two succeeding years to investigate how the postponing of ploughing leys from early to late autumn or spring, in combination with spring or winter cereals affected leaching of nitrate. The experiment was conducted as three field trials, two on a coarse sandy soil and one on a sandy loam soil. For calculation of nitrate leaching, soil water samples were taken using ceramic suction cups. The experiments started in spring in a first year ley and ended in spring three years later. Total nitrate leaching for the three year periods for each trial ranged between 160–254 and 189–254 kg N/ha on the coarse sand and 129–233 kg N/ha on the sandy loam. The results showed that winter wheat ( Triticum aestivum L.) did not have the potential for taking up the mineralized N in autumn after early autumn ploughing of grass/clover leys, and that the least leaching was generally found when ploughing was postponed until spring, and when winter rye ( Secale cereale L.) was grown as the second crop rather than spring barley ( Hordeum vulgare L.). Nevertheless, leaching was generally high in the winter period even when winter rye was grown. On these soil types ploughing out should be postponed, whenever possible, to spring. Crop systems that maximize the utilization of mineralized N and thereby minimize nitrate leaching need to be further developed. Based on N balances, the data were further used to estimate the biological N fixation by the clover.     

A study of mole drainage with simplified cultivation for autumn-sown crops on a clay soil. 5. Losses of nitrate-N in surface run-off and drain water

Direct drilling of autumn-sown cereal crops reduced the loss of nitrate in drainage. Losses of nitrate nitrogen in water draining from arable land have been measured for 4 seasons, 1980–1984. The field experiment was on a mole-drained clay soil in southern England. Autumn-sown cereal crops were established by direct drilling or after ploughing and traditional seed-bed preparation. Losses ranged from 3 to 75 kg N ha⁻¹ year⁻¹, with an average of 34 kg N ha⁻¹ year⁻¹. Most of the loss (about 90%) was removed via the mole-drain system. Measured loss of nitrate from the direct-drilled soil was 76% (range 48–89%) of that lost from the ploughed soil. Mole drains apparently increased loss of nitrate directly to the river system. In the absence of mole drains, nitrate loss in surface drainage averaged 6 kg N ha⁻¹, compared with 4 kg N ha⁻¹ in the presence of drains. However, in one year, exceptionally high amounts of nitrate (80 kg N ha⁻¹) were lost from undrained, direct-drilled land because of poor crop establishment; deep leaching of nitrate in the undrained soil was not measured. Approximate calculations show that up to half the autumn-applied fertiliser-N was lost by leaching and up to 15% of spring applications.