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.     

Factors influencing the variation of some properties of soils in relation to their suitability for direct drilling

Soils from fifteen field experiments in the United Kingdom and one in France that compared direct drilling with mouldboard ploughing were examined with respect to their composition and physical properties. Indices of the stability and shrinkage of soil aggregates were obtained by measurement and an index of compactability was derived from an established relationship. It was hypothesized that these properties were possible determinants of soil responses to zero-tillage.Aggregate stability and shrinkage were correlated with organic carbon and clay content, respectively. Organic matter contents were greater at the surface of direct-drilled soils than in the complete topsoil layer after direct drilling or ploughing. In some cases the increase in organic matter significantly improved the physical properties of the soils.The three indices, of stability, shrinkage and compactability, were each ranked in high intermediate or low groups, and the soils classified according to their ratings in these groups. The resulting classification broadly agreed with a previous one of soil suitability for sequential direct drilling mainly based on experimental agronomy.     

Humic substances in soil and crop sciences: P. MacCarthy, C.E. Clapp, R.L. Malcolm and P.R. Bloom (Editors), American Society of Agronomy and Soil Science Society of America, Madison, WI, 1990, 281 pp., price US$ 30.00, ISBN: 0-89118-104-0

Studies were conducted to determine changes in organic matter and microbial biomass carbon in comparison with structural stability at the surface soil (0–5 cm) of a Charlottetown fine sandy loam, an Orthic Podzol, at three tillage and grassland sites situated in Prince Edward Island. The tillage experiments, established for 3–5 years, included comparisons of mouldboard ploughing, direct drilling, shallow tillage, and chisel ploughing. Two indices of soil structural stability were used: mean weight diameter (MWD) and aggregation index (AI). The latter index assigns a weight factor to aggregate size ranges based on their value for plant germination and root growth.Direct drilling and reduced tillage increased the level of soil organic carbon by 10–17%, relative to mouldboard ploughing. Organic carbon was more enriched in 1–2 mm and 4.75–9.00 mm macroaggregates, especially the former, compared with whole soil. The MWD of aggregates after wet sieving was 33% and 55%, relative to the grassland sites, for mouldboard ploughing and direct drilling, respectively. On these soils of similar mineralogy and particle size, a close linear relationship (r=0.942) was observed between organic carbon (r=0.947) and nitrogen (r=0.923). The AI was significantly correlated to both organic carbon and microbial biomass carbon using power regression. In contrast to MWD, the AI under direct drilling and reduced tillage, associated with an organic carbon level of 2.5%, approximated the AI under grass. Overall, the study showed that minimum tillage systems in humid climates can improve structural stability at the soil surface of fine sandy loams over a relatively short time frame.     

Soil organic carbon and nitrogen fractions and water‐stable aggregation as affected by cropping and grassland reclamation in an arid sub‐alpine soil

This paper investigates effects of cropping abandonment and perennial grass growing on soil organic C and N pools and aggregate stability, by comparing soils under native grassland, crop cultivation, perennial grass growing and cropping abandonment, in degraded cropland at a sub‐alpine site in north‐western China. The pools of total and particulate organic C (115 and 37 Mg ha⁻¹) in the 0–30 cm soil layer of native grassland were reduced by 31 and 54% after 30 years of crop cultivation. After 4 years of conversion from cropland to perennial grass growing total and particulate organic C pools were increased by 29 and 56%, whereas 4 year cropping abandonment increased particulate organic C by 36%. Rapid increases in total and particulate N were also found in perennial grass growing and cropping abandonment soils. The native grassland soil and soils of cropping abandonment and perennial grass growing had higher carbohydrate C concentrations in the 0–10 cm layer than the cropped soil. The rapid recovery of particulate organic fraction and carbohydrates in the re‐vegetated soils were probably due to higher plant biomass inputs and lower organic matter decomposition compared with those in the cropped soil. Aggregate stability of the 0–30 cm soil layer was significantly decreased by crop cultivation but showed a good recovery after 4 year re‐vegetations. This study suggests that reduction of soil organic matter and aggregate stability under crop cultivation may be remedied by cropping abandonment or perennial grass growing. Copyright © 2008 John Wiley & Sons, Ltd.     

An experimental study of crust development on chalk downland soils and their impact on runoff and erosion

The sealing of soil surfaces by rainfall, the development of soil crusts and their impact on runoff and erosion was investigated in the laboratory by means of simulated rainfall. The soils investigated were stone-free samples of chalk soils from southeast England, and soils with a 25% cover of stones. Vertical change to the surface and immediate subsurface of the soils was assessed through the examination of thin sections scanned into a computer and analysed with image processing software. Changes in roughness and microtopography of the soil surface were measured by use of a laser micromapper. Crusting occurred both in the presence and absence of stones and was inversely related to the organic matter content and aggregate stability of the soils. Crusting of stone-free soils was accompanied by a reduction in roughness of the soil surface, but roughness of the stone-covered surfaces increased as crusting developed. Increases in the particle density of the crust were related to silt content. Organic-rich soil from under permanent grass and from a soil recently brought into arable cultivation crusted less than soils used for arable cultivation for longer periods with lower organic content. The inwashing of silt into the pores of the soil during crusting reduces infiltration, and increases runoff and erosion. In the presence of a 25% cover of surface stones, the reduction in infiltration was 25% less than for stone-free soils and erosion ∼50% less.     

Soil organic matter,microbial properties,and aggregate stability under annual and perennial pastures

The use of annually sown pastures to provide winter forage is common in dairy farming in many regions of the world. Loss of organic matter and soil structural stability due to annual tillage under this management may be contributing to soil degradation. The comparative effects of annual ryegrass pastures (conventionally tilled and resown each year), permanent kikuyu pastures and undisturbed native vegetation on soil organic matter content, microbial size and activity, and aggregate stability were investigated on commercial dairy farms in the Tsitsikamma region of the Eastern Cape, South Africa. In comparison with soils under sparse, native grassy vegetation, those under both annual ryegrass and permanent kikuyu pasture had higher soil organic matter content on the very sandy soils of the eastern end of the region. By contrast, in the higher rainfall, western side, where the native vegetation was coastal forest, there was a loss of organic matter under both types of pasture. Nonetheless, soil organic C, K₂SO₄-extractable C, microbial biomass C, basal respiration, arginine ammonification and fluorescein diacetate hydrolysis rates and aggregate stability were less under annual than permanent pastures at all the sites. These results reflect the degrading effect of annual tillage on soil organic matter and the positive effect of grazed permanent pasture on soil microbial activity and aggregation. Soil organic C, microbial biomass C, K₂SO₄-extractable C, basal respiration and aggregate stability were significantly correlated with each other. The metabolic quotient and percentage of organic C present as microbial biomass C were generally poorly correlated with other measured properties but negatively correlated with one another. It was concluded that annual pasture involving conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and soil physical condition under dairy pastures and that a system that avoids tillage needs to be developed.     

Soil physical fertility, soil structure and rooting conditions after ploughing organically managed grass/clover swards

Effective management of soil structure and organic matter are essential in organic cropping to ensure good rooting conditions and to optimize the production of mineralized N and thus minimize greenhouse gas emissions. We investigated how mid‐winter or early spring ploughing and three grazing duration treatments prior to ploughing influenced soil structure, soil organic matter and plant root growth under the first spring barley crop after a grass–clover ley. The experiment was carried out over two seasons. We also studied the soil under first‐year oats in a long‐term rotations experiment where 2 or 3 years of arable crops followed 3 or 4 years of grass. Pore size distribution and pore continuity, bulk density, particulate (light fraction) organic matter, readily oxidizable organic matter (ROM), aggregate size distribution and root length densities were measured. Macroporosity appeared to be the best indicator of soil physical fertility; it was sensitive to changes in soil structure arising from compaction and root growth. This, along with visual examination, revealed the loosening resulting from ploughing. The generally favourable macroporosity amongst small, stable aggregates reduced the likelihood of development of anaerobism. Macroporosity and aggregate size can be estimated from visual examination of the soil, a method that offers the advantage of being quick and of sampling a large volume. The content of ROM was high 6.1–6.4 g 100 g^?1 whole soil. However, particulates formed only a small fraction (6–9%) of the ROM. Despite the favourable ROM and structure, the soil was susceptible to compaction damage during seedbed preparation in wet soil after ploughing which reduced grain yield in some plots. Grazing by sheep before ploughing and date of ploughing had minor effects on soil quality. Grazing for 2 months prior to ploughing increased root length density in the upper topsoil in the following arable crop, possibly because of the higher quality of the animal and grass–clover residues. Conservation of soil quality was related more to secondary tillage and sowing operations after ploughing than to duration and timing of grazing.     

The influence of straw ash on some soil properties that can affect herbicide performance

During autumn 1982, soil samples were collected from 15 winter cereal fields in central and southern England. The soils either contained ash resulting from the burning of straw in the field or were ash free. Ash containing soils had much higher adsorption levels (measured as Kd values for chlortoluron) than ash-free soils. There were smaller differences in organic matter, pH and nutrient content between the 2 soil groups. There was a better correlation between adsorption and organic matter content for ash-free soils than for soils containing ash. The ratio of adsorption level to organic matter content for ash soils was almost twice that for ash-free soils. It was concluded that ash can make a substantial contribution to herbicide adsorption on minimally cultivated soils and that this could affect herbicide performance. In a tillage experiment where ploughing was followed by direct drilling, adsorption levels increased much more rapidly than organic matter.     

THE EFFECT OF CULTIVATION ON THE STRUCTURE AND OTHER PHYSICAL CHARACTERISTICS OF GRASSLAND AND ARABLE SOILS (194–970)

Cultivating grassland changes the physical state of the soil. The paper describes observations and measurements made in the field and the laboratory on a wide range of soils (with series names) in England over a period of 25 years. Dry sieving showed that natural weathering of grassland aggregates after three months'exposure was very considerable. The drawbar-pull on ploughing soil recently out of grass, as compared with old arable, increased with the number of years out of grass. The number of earthworms per hectare on old grassland was 6 to 9 times that on old arable of the same soil series: after three years'cultivation it was reduced by about a half. The possibility of erosion by wind increased with the number of years out of grass. Old arable land on gentle slopes had its resistance to flowing water (sheet erosion) much increased by as little as 2 years under grass. The apparent densities of soils were at a minimum under old grass and increased under cultivation. On some soils arable for many decades, the apparent densities were near those considered limiting for root penetration. The volurne of water per cent in the soil at the sticky point decreased as the years out of grass increased, suggesting a decrease in the number of days the soil could be cultivated without smearing. The water-stability of soil aggregates decreased as the number of years after ploughing old grassland increased. Whatever the texture, on ploughing old grassland, the water stability of air-dry aggregates feil sharply in the first – years and then approached the value corresponding to that of old arable soils more slowly, apart from soils of low clay content in which it differed little from old arable land after 2 years. Measurement of changes in water stability and field observations in a 6-course rotation (3 years ley, 3 years cereal) on a loam which had been in a similar rotation for about a decade after old grassland, suggested that this balance was probably right for maintaining such a soil at a good cropping level. Puddling (poaching) by animals of wet arable loam soils overlying clay resulted in gleying to the surface: ferrous iron was detected chemically. On putting down to grass there was little increase in water stability of the aggregates after 4 years. The reduction in the total pore space caused by harvesting machinery and also as a result of some years'cultivation of grassland was considerable. The ease of penetration of soils when taking cores was compared by counting the number of blows to drive in a corer: the number was much less in a soil recently out of old grass compared with old arable. The root development of cereals was poor in old arable silt soils very unstable to wetting and drying and with few visible biopores (> 10–00 μm). On soils with many visible pores, root development was much greater and yields with similar rates of nitrogen were double. The total N in the soils feil when old grassland was ploughed, sometimes by as much as 75 per cent in about 20 years. There was sometimes a significant corre-lation between total N and the reduction in water-stable aggregation during the years following ploughing grassland: the correlation coefficient was much higher on ploughing old grassland rather than leys. The N level dropped much more quickly in the first few years after ploughing old grassland than later. All single-property measurements give only a limited assessment of the physical state of a soil. Soils can be assessed satisfactorily only by making a range of measurements appropriate to the farming System.     

Modification of porespace by tillage in two stagnogley soils with contrasting management histories

The soil porespace was studied in two long-term tillage experiments on two clayey stagnogleys in Southern England. The soils differed in respect of mineral and organic composition and previous management history. In both soils the total volume of pores and the volume fraction of macropores in the topsoil horizon declined with direct drilling compared with annual ploughing. This difference between tillage treatments appeared to develop more slowly in the soil that was formerly under continuous arable cultivation than in the soil that was previously in long-term grassland. Fluid transport coefficients were greater in ploughed topsoil in both soils; however, at the boundaries between topsoil and subsoil, and in the upper subsoil, permeability and gaseous diffusivity were greater after direct drilling. At a long-term arable site, soil was more consolidated below the depth of ploughing or shallow tillage, whereas in a former grassland soil ploughing disrupted the continuity of channel-type macropores.     

Structure of a silty soil in relation to management

In the United Kingdom silty soils have been classified as unsuitable for direct drilling since these soils may produce seed-beds that are inadequate for satisfactory crop emergence. This judgement was based on problems encountered in the first 2–3 yr of direct drilling as well as on an appreciation of soil physical properties. Longer-term experimentation (10 yr) has indicated that, subsequent to that initial period, soil conditions after zero-tillage are not a major limitation to growth of autumn-sown cereals. This eventual parity between simplified and conventional tillage can be attributed largely to improvement in the surface of direct-drilled land. In the tenth experimental year this layer comprised more strongly developed aggregates, of greater stability, than those from the annually ploughed soil. Porosity and soil strength measurements did not indicate that root growth would be severely restricted below this depth. When this soil was wet in winter it was weak and relatively unstable and so it would most likely be unsuitable for sequential direct drilling of spring-sown crops. At the same site soil that had been continuously under grass throughout the same 10 yr period developed better structure than that in the arable area, in part because of a greater earthworm population and an increased organic matter accumulation. In any subsequent change to arable usage this improved structure would best be conserved by avoiding soil inversion and using direct drilling or some other simplified method of cultivation.     

Spring barley growth, grain quality and soil physical conditions in a cultivations experiment on a sandy loam in Scotland

Widely differing cultivations for spring barley, ranging from conventional ploughing to direct drilling and including broadcasting on to undisturbed soil, were examined over two seasons (1979 and 1980), conventional ploughing yielding highest and either chisel ploughing or direct drilling lowest. Yield differences were associated with plant population differences; they were significant only in the abnormally wet season of 1980. Broadcasting before tine cultivation to 50 mm resulted in lower seedbed strength and higher yield than drilling after a similar cultivation. Compaction by the same level of seedbed traffic was greater in a treatment cultivated to 50 mm depth than in the conventional ploughing treatment. Grain milling energy, which is inversely related to endosperm suitability for malting, and grain nitrogen content both decreased with increasing yield. Total nitrogen removed per ha in the grain in 1979 was similar in all treatments (about 72% of applied). In 1980, it was generally lower (about 62% of applied). After allowing for differences in plant populations it was lowest after direct drilling, broadcasting and very shallow tine cultivation (60% of applied), highest after conventional ploughing (70% of applied) and intermediate after rotary cultivating and shallow tine cultivation (65% of applied). Soil water release characteristics were described by logistic equations. Treatment differences were greatest at matric potentials greater than -3 kPa at 10 and 60 mm depth. The porosities of the cultivated soils were similar to each other, and were greater than those of the direct drilled treatments.     

Direct drilling,shallow tine-cultivation and ploughing on a silt loam soil, 1974–1980

Results are given for an experiment in which direct drilling, shallow tine-cultivation and ploughing were compared on a silt loam soil over 7 years. A rotation of cereal and oilseed rape crops was established to minimise a possible interaction between disease, particularly take-all (Gaeumannomyces graminis), and cultivation treatment. Over the six harvest years shallow tillage produced on average a 6 and 9% heavier yield than ploughing and direct drilling, respectively. The importance of surface soil conditions at the time of seedling establishment was clearly evident. In years when the passage of the direct drill caused little soil disturbance and seeds were placed in a smeared slot, and particularly when crop residues were pressed into the slot, plant populations were diminished and so were yields. The degree of soil shattering by the passage of the drill and the consequent friability of the tilth varied between the extremes of water content of the top soil (0–5 cm). Direct drilling produced larger yields relative to ploughing after the first three seasons and this may reflect the increased organic matter content and stability of the soil aggregates in the surface layer (0–2.5 cm) which have already been reported for this soil. These changes may have facilitated the greater friability of the soil and the creation of tilth by the passage of the drill which ensured uniform germination and rapid establishment of the seedlings.The site was characterised by variations in the depth of topsoil over gravel (< 50 to > 100 cm), and the deepest soil gave the heaviest yield. This effect was relatively greater in dry seasons but it never interacted with the effect of tillage method.     

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.     

Cultivation and nitrogen requirement for winter barley as assessed from a reduced-tillage experiment on a brown forest soil

Direct drilling, spring-time cultivation to 60 mm and shallow mouldboard ploughing to 100 mm depth were examined as possible quick and cheap alternatives to mouldboard ploughing to 250 mm depth. Nitrogen was top-dressed either at the recommended rate or at 40% greater than the recommended rate. The experiment was on a well-drained sandy loam containing 3.8% easily oxidisable organic matter, representative of many coarsetextured Scottish soils, and ran for three seasons during 1980–1983. Crop and soil conditions were measured frequently throughout the duration of the experiment.No significant differences in yield were detected between the cultivation treatments at either nitrogen level in any year. The mean yield for the three seasons at the recommended rate of nitrogen top dressing was 5.8 t ha^?1 which was increased, on average, by 0.72 t ha^?1 by the higher rate of nitrogen top dressing, a profitable response. Grain water content, weight, specific weight and nitrogen content were influenced little by cultivation treatment. The similar grain yields and qualities were consistent with the similarly satisfactory seedbeds produced by the cultivation treatments and the stable structure of the soil which, though moderately compact, was relatively weak and ensured that soil physical properties were similar in all cultivation treatments. Both primary and secondary cultivation requirements are low. Shallow ploughing to 100 mm gave a small yield benefit and may be the most suitable cultivation treatment. Soil disturbance below this depth is probably not necessary for winter barley on such coarse-textured brown forest soils.     

Stability of soil aggregates in relation to organic constituents and soil water content

The effects of soil organic matter content, soil water content and duration of wet-sieving on aggregate stability of soils with contrasting cropping histories were investigated. Long-term pasture samples had a greater aggregate stability than long-term arable samples. However, air-drying aggregates before wet-sieving increased the aggregate stability of long-term pasture samples, but decreased that of long-term arable samples. With increasing duration of wet-sieving, the proportion of water-stable aggregates declined until a near-constant value was reached for each sample. Thus, within a sample there are aggregates possessing a wide range of stabilities; with increasing time under arable cropping there is an increase in the proportion of unstable aggregates present, and the measured aggregate stability, therefore, declines. Unstable aggregates (defined as those dispersed after wet-sieving for 1 min) generally had lower organic matter content than stable ones (those still intact after sieving for 15 min). The aggregate stability of a regrassed site (13 years of arable plus 2 years of pasture) was markedly higher than that of a corresponding site from 15 years of arable cropping. Nonetheless, levels of organic matter (organic C, total N and hydrolysable carbohydrate) were almost identical at the two sites. However, aggregates from the regrassed site did have a higher biomass C and water-extractable carbohydrate content than those from the 15-year arable site. For a group of soils with varying cropping histories, aggregate stability was significantly more closely correlated with hot water-extractable carbohydrate content than with organic C or hydrolysable carbohydrate content. It is suggested that the hot water-extractable carbohydrate fraction may represent a pool of carbohydrate involved in the formation of stable aggregates.     

Mechanisms of aggregate stabilization in Ultisols from subtropical China

The quantification and interpretation of aggregate stability depend on internal soil properties and external factors such as measurement method and aggregate size. The objectives of this study were to: (i) determine the aggregate stability in Ultisols from subtropical China applying the Le Bisssonais Method; (ii) determine the effect of initial aggregate size on its stability, and (iii) interpret mechanisms of aggregate stabilization in the soils. Three aggregate-size ranges (5–3, 3–2 and 2–1 mm) were obtained by dry sieving. After the wetting treatments, the dominant fraction of fragments for each soil was 2–1 mm or 0.63–0.2 mm. The mechanisms of aggregate breakdown was in the order, slaking>mechanical breakdown>micro-cracking. They differed with soil type and composition. The normalized mean weight diameter (NMWD) of the aggregates after fast wetting and wet stirring were more correlated with soil properties, such as degree of micro-aggregation (DOA), cation exchange capacity (CEC), K₂O, Fe₂O₃ or Al₂O₃ rather than clay and soil organic carbon (SOC) content. The binding force by soil organic matter was smaller than the force caused by entrapped air or the force of combination of mechanical stress by stirring and differential swelling of minerals.The smaller the aggregate, the larger was the aggregate stability according to NMWD. The rankings of the soils differed with the soil aggregate sizes and the wetting treatments. Sandy loams from sandstone (Sc and Sw) were the weakest soils while the purple mudstone (Pp) was the strongest. All the cultivated soils decreased in aggregate stability compared with the comparable uncultivated soils or parent materials irrespective of the cultivation time and the changes in SOC content after cultivation.     

Long-term experiments with different depths of mouldboard ploughing in Sweden

With the main objective to produce a basis for advice to farmers concerning optimal ploughing depth under various conditions, a series of field experiments were initiated throughout Sweden. At 19 sites on various soils (clay content 72–521 g kg⁻¹, organic matter content 21–89 g kg⁻¹) mouldboard ploughing to about 15, 22 and 28 cm depth was repeated annually for up to 17 years. The total number of location-years was 241. Traditional farming had previously been practised at the sites, including annual mouldboard ploughing to 20–25 cm depth. Spring-sown barley (Hordeum vulgare L.) and oats (Avena sativa L.) were the most frequent crops but many other crops were grown less frequently. Crop residues were generally returned to the soil; straw was chopped at harvest. Post-emergence herbicides were regularly used, generally resulting in an adequate control of annual weeds. However, the control of perennial weeds, particularly couch grass (Elymus repens L. Gould) was often inadequate. At ploughing depths of 22 and 28 cm, the mean crop yields were 2% and 3%, respectively, higher than at 15 cm. However, the results varied considerably between sites. In soils with a high silt content, the shallowest ploughing resulted in up to 10% higher yield than deeper ploughing, provided the control of perennial weeds was adequate. The main reason seemed to be improved structural stability in the surface soil because the concentration of organic matter in this layer became higher the shallower the ploughing. In clay soils with relatively stable structure, as well as in sandy soils, the deepest ploughing resulted in the highest yields, probably because of the deeper loosening. At sites where perennial weeds imposed problems, the weed control was better the deeper the ploughing, sometimes increasing the relative yield after deeper ploughing by several percent as compared with shallow ploughing. Most of this effect was obtained already at the intermediate ploughing depth. The results led to the following conclusions for Swedish agriculture. It may be profitable to plough sandy soils annually as deep as 30 cm, coarse sandy soils perhaps even deeper. In clay and clay loam soils, ploughing deeper than 20–25 cm generally cannot be recommended. In silty soils with an unstable structure, mouldboard ploughing, if any, should be shallow (≤15 cm), and perennial weeds should be controlled by other methods.     

EFFECTS OF CULTIVATION ON SOIL WATER RETENTION AND WATER USE BY CEREALS IN CLAY SOILS

The effects are repotted of direct drilling and mouldboard ploughing on soil water retention and extraction in three soils over two to four years. In the years 1972–75 when winter rainfall was close to or greater than the long-term average the maximum soil water content of the upper 100 cm differed little between the cultivation treatments. In contrast, after the dry winter of 1975–76, about 10 per cent more water was stored in the direct-drilled soil, especially below 50 cm. In the following dry summer, winter wheat extracted up to 22 mm more water from uncultivated soil and this was associated with better growth of the crop than after ploughing. When simulated rain was applied, water infiltrated rapidly to a depth of 50–100 cm in direct-drilled soil, whereas on ploughed land its movement was retarded below the plough sole even when cultivation had been carried out under conditions which minimized smearing. Water storage at depth was consequently greater under dry conditions in the direct-drilled soil while excess water reached drain depth more rapidly in wet conditions.     

Direct drilling and soil loosening for spring cereals on a fine sandy loam in Atlantic Canada

Changes in soil structure and properties, plant growth and diseases and agronomic aspects were determined, after 3 years, on a Charlottetown fine sandy loam, an Orthic Podzol with a perhumid soil moisture regime, subjected to three tillage systems for spring cereals. The tillage systems consisted of direct drilling, soil loosening with a “paraplow” followed by direct drilling and mouldboard ploughing.

Rate of plant growth and other crop measurements were not changed by the tillage systems, except for the depth of seeding. Direct drilling reduced the accumulation of N and K in the plant, and reduced grain N, compared with mouldboard ploughing. Soil loosening prior to direct drilling prevented the decline in N and K accumulation, and increased grain yield and N content, in comparison with mouldboard ploughing. Direct drilling caused changes in soil macro-aggregation and reduced the evaporation rate, and increased microbial biomass C and N, total organic C and N and extractable ions at the soil surface (0–5 cm), compared with mouldboard ploughing. In addition, earthworm numbers were increased under direct drilling. Root lesion and spiral nematodes were not influenced by tillage differences.

Soil loosening prior to direct drilling alleviated the significant reductions in soil macroporosity, and prevented the increase in soil bulk density, soil strength and percentage water-filled pore space (%WFPS) associated with direct drilling alone. Although soil permeability was optimum under direct drilling alone, the relative increase in %WFPS and reduction in soil aeration were associated with a concomitant increase in common root rot.