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.     

An evaluation of controlled traffic with reduced tillage for irrigated cotton on a Vertisol

Where Vertisols are used for the mechanised production of irrigated cotton, the main pre-planting tillage options are ‘direct listing’ (a ‘controlled traffic-reduced tillage’ treatment with retained ridges), deep ripping (0.45 m deep) and chisel ploughing (0.25 m deep). An experiment was established, on a commercial scale under furrow irrigation, to compare the effects of these treatments on the physical properties (aeration, strength and water content) of a frequently irrigated Vertisol over 3 years. The growth and profitability of three cotton crops and one wheat crop were monitored during this period. The soil was well structured when the experiment commenced. Soil measurements showed that whilst the deep ripped treatment had lower resistance to penetration than the direct listed treatment, it provided a less favourable environment for root growth than the direct listed soil due to prolonged waterlogging after irrigation. An excess of water entered the ripped profile under the prevailing irrigation regime. All treatments had a sodic subsoil that was poorly drained. The chisel ploughed treatment generally behaved in a fashion that was intermediate between the direct listed and deep ripped treatments. The direct listed treatment resulted in higher cotton lint yields and lower land preparation costs, in comparison with deep ripping. Profitability increases were of the order of 11% when the lint value was 1.78 Australian dollars kg⁻¹. The deep ripped soil had more stored water than the other treatments throughout each irrigation cycle, but this potential advantage could not be expressed in terms of better crop yield and improved water use efficiency. A supplementary experiment is needed to evaluate the three tillage options where irrigation water is applied less frequently.     

Assessment of tillage strategies to decrease nitrate leaching in the Brimstone Farm Experiment, Oxfordshire, UK

Tillage may influence nitrate losses from agricultural soils. Losses of nitrate were measured in drainflow at 60 cm depth and in combined surface runoff and interflow in the A horizon (=surface layer flow) on hydrologically sealed plots with a two-year comparison (1988–1990) of shallow-tine cultivation vs. mouldboard ploughing. Ploughing increased concentrations and loadings of nitrate in drainflow and surface layer flow, especially in the first year. After these two years the shallow-tined plots were ploughed to plant winter beans (Vicia faba L.), and nitrate in drainflow then increased over the next three winters, slightly exceeding that from the plots which had been ploughed throughout for winter cereals. The composition of the surface layer flow did not show this effect, however. Calculations of net winter mineralisation of soil organic nitrogen showed that shallow-tine cultivation may have decreased mineralisation slightly compared with ploughing in the first two years. These calculations did not indicate any increase in mineralisation for two winters after the minimally cultivated plots were ploughed in autumn 1990, probably because the soil was then very dry. This increase was apparently delayed until the fifth winter (1992/1993), which was much wetter than any since autumn 1990. In the previous eight years (1980–1988) half of the plots had been ploughed and half had been direct drilled. Averaged over the five winters 1988/1989–1992/1993, the five measures of nitrate loss in drainflow from plots previously direct drilled were 6–57% more than from plots previously ploughed, and winter mineralisation was 20% more, with no evidence of any decline in either with time. The nitrate produced by mineralisation of organic matter conserved by the eight years of direct drilling was mainly lost by leaching or denitrification; it was of little or no benefit to the crops. The results suggest that in the long term more nitrate is leached from land subject to periods of minimal or zero tillage and ploughing than from land ploughed every year.     

Long-term effect of ploughing, and organic matter input on soil moisture characteristics of a Ferric Lixisol in Burkina Faso

Soil moisture characteristic curves were determined in long-term trials at the agronomic research center of Saria (latitude 12°16′ N, longitude 2°09′ W) in West-central Burkina Faso. The agronomic treatments combined soil tillage with organic and chemical fertilizers. The twin values for soil moisture and water potential showed that on ploughed plots, moisture content was higher at low suction and lower at high suction than the hand hoed plots. Moisture contents were higher for extreme suctions (pF < 1.5 and >3) on plots that received high dose of animal manure. The bush fallow plots behaved as a ploughed plot at low suction and like a hand hoed plot at the high suction. Field capacities were around 9.50% (g/g) and 8% (g/g), respectively, for hand hoed and ploughed plots, while the wilting points for both were of 5–6% (g/g). Organic matter input improved field capacity and soil water content at wilting point but not the useful available water (UAW). The UAW ≥10 mm on the fallow and the control, while it was <9 mm on the other treatments in 0–20 cm soil layer. Soil structural modifications induced by tillage and organic matter input explained these differences in soil hydrologic regime.     

PORE CHARACTERISTICS OF SOILS FROM TWO CULTIVATION EXPERIMENTS AS SHOWN BY GAS DIFFUSIVITIES AND PERMEABILITIES AND AIR-FILLED POROSITIES

Gas diffusivity and permeability, and air-filled porosity, were measured in undisturbed soil cores at six water potentials between -2 kPa and oven dryness. All increased as water potential fell. In silt loam at 30 to 80 mm depth, relative diffusivity and air permeability at -2 kPa were 0.0013 and 5 × 10^?8cm² after direct drilling, and were 6 and 15 times greater respectively after ploughing, presumably because of the larger volume of air-filled large pores in the ploughed soil. These pores may also have been more continuous or less tortuous than in the direct drilled soil. However, at equal air-filled porosities up to 0.18 v/v, the pores were apparently more continuous and less tortuous in the direct drilled than in the ploughed soil. In the direct drilled silt loam at any given matric potential, air-filled porosity, gas diffusivity and permeability within and below the previously ploughed layer were isotropic. In clay loam at 30 to 80 mm depth gas diffusivity and permeability at -2 kPa were greater than in the silt loam irrespective of tillage but increased less on oven drying.     

Seasonal changes in surface aggregate stability under different tillage and crops

Temporal changes in the surface aggregate stability of an Oxic Paleustalf under different tillage practices (direct drilled/stubble retained versus conventional cultivated/stubble burnt) and under different crops (wheat (Triticum aestivum L.) versus lupin (Lupinus angustifolius L.)) were monitored at a 10-year-old rotation site in Wagga Wagga, N.S.W., Australia.

Seasonal fluctuations in aggregate stability were observed under all treatments and were greater than the differences detected between the different tillage and cropping treatments. The seasonal variation was significantly related to the soil water content at the time of sampling and the lowest stability occurred during the autumn/winter period. Cropping under direct drilling and stubble retention resulted in significantly higher stability and lower seasonal fluctuations in stability than under conventional tillage and stubble burning. Despite the seasonal fluctuation, water stability over the season of both of the macroaggregate (more than 250 μm) and microaggregate (less than 50 μm) fractions increased significantly.

While the mean (temporal) stability of the different treatments was significantly related to the mean organic carbon content (r = 0.91) and polysaccharide content, the temporal changes were not related to the soil organic carbon content nor the living root length density.

Lupin had a more beneficial effect on promoting macroaggregate stability than wheat under the conventional tillage/stubble burnt treatment but no significant difference was found under the direct drilled/stubble retained treatment.     

Effects of mouldboard ploughing and direct planting on yield and nutrient uptake of potatoes in Norway

The use of conservation tillage systems is now widespread for cereals in erosion-prone areas of Norway. However, few growers are willing to adopt such practices for other crops. An experiment was conducted in southern Norway (60°46′N, 10°49′E) from 1987 to 1993 to compare the effects of two different tillage treatments on potato (Solanum tuberosum L.) yield and quality. The conventional labour-intensive treatment, with autumn mouldboard ploughing, levelling and two passes with a spring-tine harrow in spring was compared with planting directly into untilled barley (Hordeum vulgare L.) stubble, with straw removed. The soil type in the experiment was a morainic, stony loam (Typic Cryoboroll, Orthic Melanic Brunisol). Prior to the start of the experiment direct-planted plots had not been ploughed for 9 years. During the trial years (1987–1993) a pattern of two different yield curves emerged as functions of the date of harvesting. The yield curve for direct planting was steeper, crossing that of conventional tillage on approximately 10 September, thus predicting higher tuber yield for direct planting when harvesting occurred after this date, but lower tuber yield than with conventional tillage in the case of early harvesting. The reason for this is thought to be cooler soil and delayed growth and maturation with direct planting. Furthermore, direct planted potatoes gave higher haulm yield, and also had higher contents of N, P and K in both haulm and tubers. No significant differences in tuber quality were found between the two treatments. A strong negative correlation was found between the yield increase caused by direct planting and mean air temperature in August. The trial indicated that direct planting of potatoes in cereal stubble is a viable alternative to conventional plough tillage on loam soil in Norway, provided that the growing season is adequately long.     

The influence of soil management on drainage hydrographs

Abstract. Drainage hydrographs from mole-drained plots having different tillage treatments (tractor tined, gantry tined, tractor ploughed and gantry ploughed) were measured for different rain events over a growing season. In the autumn just after tillage, a large rainfall produced peaky drain flows on the tined plots but a rather flat response from the ploughed plots. In the winter, the drain response to a small rain event showed less differences in peak flows between the treatments. The recession time constant of the hydrographs was used as an index of the structural macropore development in the soil above the water table. Hydrographs from the gantry plots recessed more quickly than those on the tractor plots and those on the tined plots recessed more quickly than those on the ploughed plots. Lack of soil compaction on the gantry plots and continuous vertical fissuring created by the non-inverting tillage tines resulted in the gantry tined treatment having the fastest drainage response. In the ploughed plots compaction and smearing of the soil at the base of the plough layer restricted the rate of downward movemenl of water. The work indicates that soil management practices can play an important role in the drainage and leaching of aggregated soils.     

Movement of suspended matter and a bromide tracer to field drains in tilled and untilled soil

Abstract. Drainage water was sampled intensively during a four-year field experiment on a sandy loam soil subjected to four unreplicated tillage treatments: (1) harrowing with a springtine harrow, drilling; (2) direct drilling; (3) ploughing with light subsurface compaction, one pass with a PTO-driven rotary harrow, drilling; (4) ploughing, one pass with a springtine harrow, drilling. In all years, the losses of suspended matter with drainage water (0.1–4.3 kg ha⁻¹ yr⁻¹) were smaller by a factor of 1.9 or more from direct drilled plots than from plots subjected to the other tillage treatments, strongly suggesting that tillage increased the losses. Annual bromide losses were governed by the amount of drainage water rather than by the tillage treatments. However, after one drainage season, more bromide was left in the soil at 0–100 cm depth with ploughless tillage than with ploughing, thus indicating more bypass flow without ploughing. The study demonstrated very changeable patterns of suspended matter and bromide concentrations in drainage water sampled from large field plots, and questions the representativeness of drainage water samples for water reaching the subsoil or shallow groundwater.     

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.     

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.     

Effects of N‐fertilization on shoots and roots of rape (Brassica napus L.) and consequences for the soil matric potential

The underlying question of these investigations asked, how and to which extent rape plants react with transpiration and soil water uptake to different degrees of nitrogen fertilization. Therefore repeated campaigns with concurrent measurements of plant surfaces (leaves, stems, pods), diurnal courses of leaf transpiration and root length density of rape plants growing on heavily (240 kg ha^—1), moderately, (120 kg ha^—1), and nil N‐fertilized plots of an experimental field in northern Germany were performed during two growing seasons. Additionally, matric potentials at different soil depths were measured. In the first year (1994) investigations were concentrated primarily on shoot area development and transpiration, whereas in the subsequent year (1995) root measurements were mainly undertaken. Also, the influence of soil management (ploughing, conservation tillage) was taken into consideration. The plots where the shoot measurements were carried out were ploughed in 1994 and rotovated in 1995. Matric potentials were measured in both years in ploughed soil and, for comparison, also in soils with conservation tillage. Shoot area index, as measure of the transpiratory capacity of the canopy, increased on ploughed soil and reached a maximum before flowering. Thereafter it decreased until harvest when the relative amount of green stems and pods was increasing. Then, the measured transpiration rate per pod surface area was equal to, or higher than, the transpiration rate per leaf surface area. Plant surface area was smaller in plots with conservation tillage and decreased generally with decreasing N‐fertilization. Increasing plant surface area was joined by an increasing density of plant canopy. Light interception was thus highest in the plots receiving 240 kg N ha^—1. Although the shading effect may cause a reduction of transpiration per plant, the total plant mass per area generally resulted in a greater water loss from these plots. Roots reached at least 110 cm depth. Root length density was significantly higher in the upper 10—30 cm of soil than at greater depths. Root mass was smaller in soil with conservation tillage than in ploughed soil. Oscillations of soil matric potentials in the diurnal and long‐term periods were highest in the upper 10 cm of soil. Here, they corresponded well with the cumulative diurnal transpiratory water loss. It is concluded that the soil water dynamics depends largely on the distribution of plant roots. As a result, rape plants did not change their specific transpiration capacity as a response to increased nitrogen fertilization. However, the transpiring plant surface and root length density increased the turnover rate of water by a higher plant density per plot. This effect was more pronounced in ploughed than in rotovated plots.     

Effects of tillage depth on organic carbon content and physical properties in five Swedish soils

The soil tillage system affects incorporation of crop residues and may influence organic matter dynamics. A study was carried out in five 15–20 year old tillage experiments on soils with a clay content ranging from 72 to 521 g kg⁻¹. The main objective was to quantify the influence of tillage depth on total content of soil organic carbon and its distribution by depth. Some soil physical properties were also determined. The experiments were part of a series of field experiments all over Sweden with the objective of producing a basis to advise farmers on optimal depths and methods of primary tillage under various conditions. Before the experimental period, all sites had been mouldboard ploughed annually for many years to a depth of 23–25 cm. Treatments included primary tillage to 24–29 cm depth by mouldboard plough (deep tillage) and to 12–15 cm by field cultivator or mouldboard plough (shallow tillage). Dry bulk density, degree of compactness and penetration resistance profiles clearly reflected the depth of primary tillage and substantially increased below that depth. Compared to deep tillage, shallow tillage increased the concentration of organic carbon in the surface layer but decreased it in deeper layers. Total quantity of soil organic carbon and carbon–nitrogen ratio were unaffected by the tillage depth. Thus, a reduction of the tillage depth from about 25 cm to half of that depth would appear to have no significant effect on the global carbon cycle.     

Soil water behaviour in response to changes in soil structure

Surface horizons of two Australian alfisols which had been cropped for 3 years to wheat by zero, minimum and ploughed tillage were compared for differences in structure. Total porosities and pore size distributions differed between treatments, but values for water and air permeability, sorptivity, diffusivity and evaporation rate were not necessarily ranked in the same order. The stability of soil structure was usefully described by the ratio of water to air permeability (k_w/k_a), which indicated the relatively fragile nature of the ploughed structures, despite their initially greater proportion of coarse porosity. Time of sampling after seeding also influenced hydraulic properties which were found to vary significantly over a 10-week period. Variations in vertical distribution of organic matter between tillage treatments is postulated as influencing the differences in structural stability.     

Soil carbon fractions and relationship to soil quality under different tillage and stubble management

Effect of 19 years of different tillage (direct drilled vs. conventional tillage) and stubble management (stubble retained vs. burnt) on soil carbon fractions were studied in a red earth, an Oxic Paleustalf at Wagga Wagga, NSW. The changes in carbon fractions were related to observed changes in soil structural stability and nitrogen availability. Significant differences in total organic carbon (TOC) were detected to 0.20 m depth, but the largest differences existed in the top 0.05 m where a difference of 8.0 g/kg (equivalent to 5.2 t ha⁻¹) was found between the extreme treatments (direct drilled/stubble retained (DD/SR) vs. conventional cultivation/stubble burnt (CC/SB)). Tillage had a much greater effect in reducing total carbon than stubble burning accounting for 80% of the total difference between the extreme treatments in 0–0.05 m layer. Tillage and stubble burning resulted in lower levels of different organic carbon fractions with tillage preferentially reducing the particulate organic carbon (POC) (>53 μm) (both free and associated POCs), whereas stubble burning reduced the incorporated organic carbon (<53 μm). We also found that tillage and stubble burning both significantly lowered the water stability of aggregate >2 mm, whereas stubble burning was related to the reduction of water stability of aggregates <50 μm. Furthermore, tillage was related to the decline in mineralisable nitrogen (MN) due to the loss of POC, especially the free POC fraction. POC was a more sensitive indicator of soil quality changes under different tillage and stubble management than TOC.     

Wheel traffic impact on soil conditions as influenced by tillage system in Central Anatolia

The increased limiting effects of soil compaction on Central Anatolian soils in the recent years demonstrate the need for a detailed analysis of tillage system impacts. This study was undertaken to ascertain the effects of seven different tillage systems and subsequent wheel traffic on the physical and mechanical properties of typical Central Anatolian medium textured clay loam soil (Cambisol), south of Ankara, Turkey. Both tillage and field traffic influenced soil bulk density, porosity, air voids and strength significantly except the insignificant effect of traffic on moisture content. Traffic affected the soil properties mostly down to 20 cm. However, no excessive compaction was detected in 0–20 cm soil depth. The increases of bulk density following wheel traffic varied between 10–20% at 0–5 cm and 6–12% at 10–15 cm depth. In additions, traffic increased the penetration resistance by 30–74% at 0–10 cm and 7–33% at 10–20 cm. Less wheel traffic-induced effects were found on chisel tilled plots, compared to ploughed plots. Soil stress during wheel passage was highly correlated with soil strength. Also, both tillage and traffic-induced differences were observed in mean soil aggregate sizes, especially for mouldboard ploughed plots. The obtained data imply that chisel+cultivator-tooth harrow combination provides more desirable soil conditions for resisting further soil compaction.     

不同耕作方式下玉米农田土壤养分及土壤微生物活性变化

通过连续3年的野外调查与室内分析试验,研究了不同耕作方式（翻耕、旋耕、免耕）对玉米农田土壤养分及土壤微生物活性的影响。结果表明,不同耕作方式下土壤pH值略显酸性,土壤容重与土壤总孔隙度变化趋势相反;土壤容重基本表现为:免耕 > 翻耕 > 旋耕;土壤总孔隙度TSP基本表现为:翻耕 > 旋耕 > 免耕,不同耕作方式差异均显著（p<0.05）。不同耕作方式下土壤养分（有机碳、全氮含量）和有效养分（有效磷、铵态氮和硝态氮）均呈现出一致性规律,大致表现为翻耕 > 旋耕 > 免耕,不同耕作方式下土壤全磷含量差异均不显著（p > 0.05）;与免耕相比,土壤微生物量碳和氮、土壤微生物数量（细菌、真菌、放线菌、固氮菌和纤维素菌）均有明显的增加,大致表现为:翻耕 > 旋耕 > 免耕。土壤微生物活度的变化范围为0.38~0.69,依次表现为翻耕 > 旋耕 > 免耕,不同耕作方式下土壤微生物活度差异均显著（p<0.05）。不同耕作方式下土壤微生物量碳周转率高于氮周转率,说明微生物量碳更新比微生物量氮快,其中翻耕处理下土壤微生物量碳和氮更新较旋耕和免耕快。通径分析发现,不同土壤环境因子对土壤微生物活度产生直接和间接负作用,其中有机碳、全氮、硝态氮、铵态氮、细菌数量对土壤微生物活度产生直接效应;土壤微生物量碳、微生物量氮、真菌数量和固氮菌数量对土壤微生物活度产生间接效应。     

Effects of ploughless tillage and straw incorporation on evaporation

The effects of ploughness tillage (stubble cultivation with a disc tiller or a spring tine cultivator, or with both in combination, replacing mouldboard ploughing) on evaporation were investigated using undisturbed soil lysimeters (height = 350 mm, diameter = 300 mm) excavated after spring cultivation in May from field experiments in eastern Sweden. The evaporation process was measured on ploughed and unploughed soil lysimeters, both with or without precipitation and with or without a seedbed. The effects on evaporation of incorporating crop residues with or without precipitation into a ploughed seedbed were also measured. The results from a heavy clay and from a silty clay loam showed that both ploughless tillage and incorporated straw reduced cumulative evaporation. The water-conserving effect was greater in irrigated treatments. The positive effect on water conservation was also greater on the silty clay loam than on the heavy clay. The soil structural changes brought about by ploughless tillage in layers under the seedbed acted to reduce the rate of evaporation from soil.     

Tillage effects on plant extractable soil water in a silty clay vertisol in Southern Italy

This paper describes how the CERES-Wheat simulation model can be used to estimate tillage effects on soil water regimes of a silty clay soil in Foggia, Southern Italy. The four tillage treatments compared are traditional mouldboard ploughing, ripper subsoiling, surface disc-harrowing and minimum tillage with rotary hoeing under continuous durum wheat cropping. For each tillage treatment the CERES-Wheat model was used to calculate the water balance for several layers in the root zone.

The water balance routine of the model estimates the water content within saturation and the lower limit at any time. Inputs required by the model are some basic information about the site, weather, genetic parameters and management practices as well as some soil properties, such as albedo, bulk density, organic matter and N contents. The model was calibrated by estimating the genetic parameters for the minimum tillage treatment in the season 1984–1985. The same set of parameters was used for the subsequent validation procedure. Statistical tests proved that the match between measured and simulated soil water content values was quite good. The simulation results also showed some differences among different tillage treatments. The model predicted the lowest plant extractable soil water values and a different water content distribution along the soil profile of the ripper subsoiling in comparison with the other tillage treatments. The soil water content was lower until 20–40 cm depth and higher at 40–60 cm depth in the ripper treatment as compared with the others. In deeper layers differences became non-significant. This might be due to the cracks produced by the ripper through which rainfall infiltrated in deep layers.     

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.