Tillage systems and soils in the semi-arid tropics

Even though conservation tillage may be ideal for the semi-arid tropics (SAT) in view of results from studies and tillage practices in the U.S.A. and Australia, studies conducted in semi-arid regions of Africa appear to support the use of conventional tillage systems. Some of the reasons for this apparent discrepancy are because of the physical properties of the soils in semi-arid Africa, particularly the Sahelian zones where the soils are sandy, have high bulk densities and therefore low total porosities and form crusts upon wetting and drying. Consequently, no-till or reduced tillage systems that do not have the soil surface covered by residue in irder to prevent formation of crust as a result of raindrop impact, tend to lose water through runoff in a region where water economy is essential. Also, because these soils have inherently high bulk densities, conventional tillage systems appear to be suitable since they increase the macropores, reduce both bulk density and strength and thus ensure prolific root distribution and the resultant exploration of water and nutrients at greater soil depths. Notwithstanding, it seems that since most of the SAT soils are structurally unstable, further conventional tillage even though it has ephemeral advantages, may in the long term be exacerbating the problems of structural instability and their deleterious effect on water and soil conservation and therefore on crop production. We suggest that at this stage soil tillage research in the semi-arid regions of Africa and Asia should re-examine some of the concepts of conservation tillage in relation to soil physical properties and processes in order to obtain a tillage system that ensures high crop yields without destruction of the soil resource.     

Tillage systems and soil properties in West Africa

West African soil resources have high potentials for enhancing agricultural productivity, if well-managed and restored. In this context, the importance of tillage systems have not been fully appreciated as an integral part of good farming systems in order to tally with the peculiarities of the soil, crops and the environment. Most improved tillage systems are not widely used, although the relatively small-scale uncontrolled application of mechanical tillage has had untold adverse effects on properties and productivity of soils in the humid and subhumid regions.

In contrast, mechanical soil tillage involving deep plow-till and soil inversion has proven beneficial on compact soils of arid and semi-arid regions. The plow-based systems not only reduce soil bulk density and soil strength but also improve the efficiency of water and nutrient use.

The exposure of structurally unstable Alfisols and Ultisols predominant in the humid and sub-humid regions by mechanical tillage can cause more adverse effects than beneficial effects on soil properties and crop yields, especially on a long-term basis. On the other hand, the no-till system with crop residue mulch can maintain favorable soil properties. The conservation tillage system, however, requires more research to make it applicable to diverse soil types, crops and ecoregions.

Apart from the long-term effects of tillage on the level of soil organic matter and the attendant release of nutrients, the effects of tillage systems on the chemical properties of soil are often contradictory and are confounded by many other factors so that clear-cut cause and effect relationships are not obvious. The interactions between fertilizer application, liming, soil organic matter content and tillage systems, especially on acid soils, are such examples. More detailed studies on nutrient dynamics under different tillage systems are necessary. The interactions between the relatively new technologies of alley cropping and agroforestry which allow a more continuous use of the land should be investigated vis-à-vis tillage systems.

Long-term, well-designed, adequately equipped experiments (which are scanty in West Africa) should be encouraged to elucidate and confirm results of many short-term experiments.     

科学耕作与留茬改良小麦-玉米两熟农田土壤物理性状及增产效果

为了探讨不同覆盖耕作方式对农田土壤物理性状及作物产量的影响,该试验研究了免耕、常规2种耕作方式和4种留茬高度的玉米秸秆还田处理,对麦-玉两熟农田土壤含水率、容重、孔隙度以及作物产量的影响。结果表明:在0～40cm土层内,秸秆还田的集雨和保水效果显著,免耕留茬0.5m还田处理的含水率比免耕无覆盖处理增加了15.95%。秸秆还田量对0～40cm内土壤贮水量的影响不同。耕作措施显著影响了土壤容重,小麦播种前常规留茬1m还田、常规全量还田处理容重低至1.0g/cm3左右。秸秆还田能增加土壤总孔隙度、降低毛管与非毛管孔隙度的比值。单一免耕处理降低了作物产量,而免耕覆盖能增产,其留茬1m还田处理比无还田处理增产22.44%,比常规留茬0.5m还田处理高3.64%。因此,免耕留茬1m还田处理在改善农田土壤物理性状和增加作物产量方面显著,该研究可为农田管理过程中耕作措施和秸秆还田量的选择提供参考依据。     

Effects of tillage systems on soil physical properties,root distribution and maize yield on a Colombian acid-savanna Oxisol

Abstract

Tillage systems may affect many soil properties, which in turn may alter the soil environment and consequently impact on root growth and distribution, and crop yield. In 1993, a long-term field experiment on sustainable crop rotation and ley farming systems was initiated on a Colombian acid-savanna oxisol to test the effects of grain legumes, green manures, intercrops and leys as possible components that could increase the stability of systems involving annual crops. In the present study, five agropastoral treatments (maize monoculture, maize-soybean rotation, maize-soybean green manure rotation, native savanna, maize-agropastoral rotation) under two tillage systems (no tillage and minimum tillage) were investigated. Lower bulk density and higher total porosity for all treatments and soil layers were found in no-tillage compared to the minimum tillage system. Between the two tillage systems, significantly higher maize grain yields (p<0.1) were obtained under no-tillage agropastoral treatments compared to the same treatments under minimum tillage. Maize yields on native savanna soils were markedly lower than in the rest of the treatments, indicating the need for improved soil conditions in subsoil layers for root growth of maize.     

耕作方式和秸秆还田对直播稻田土壤理化性质及其产量的影响

通过田间试验和土壤理化性质的分析,探讨秸秆还田和土壤耕作方式对直播稻田土壤理化性质和产量的影响。结果表明,秸秆还田使土壤的容重和坚实度降低,总孔隙度和非毛管孔隙度升高,同时提高土壤各层的有机质、全氮、速效磷、速效钾含量;深耕有利于培肥地力,增加土壤养分含量。不同土壤耕作深度和秸秆还田量对产量影响差异不显著,但农田的可持续发展能力得到了一定提高,同时,秸秆还田还能减少焚烧事件的发生,改善农村生态环境,实现农业清洁生产。     

Tillage methods and soil and water conservation in West Africa

A review is made of appropriate tillage methods for West African soils. Soil and ecological constraints to crop production and soil and water conservation measures in West Africa are discussed. Experimental results relating to the effects of different tillage methods on soil productivity and crop responses are described for different eco-regions in relation to the potentials of different tillage methods for soil and water conservation. The review shows that limited experimental data and diverse research methodologies make generalizations from the available information tenuous at best. Serious gaps in our knowledge of the ecological suitability of alternative tillage methods and soil and water conservation technologies are identified. The future course for regionally coordinated research in soil tillage and soil and water conservation is suggested.     

Trends in tillage practices in relation to sustainable crop production with special reference to temperate climates

The concept and some definitions of sustainable agriculture are reviewed. Most of these definitions include economic, environmental and sociological aspects. The finite area of land emphasizes the need for consideration of soil conservation and of soil quality in relation to sustainability. An important element of soil quality is rooting depth. Therefore loss of soil by erosion is a dominant factor in long-term sustainability. The effects of tillage on soil parameters in minimum data sets that have been proposed to describe soil quality are reviewed. Soil organic matter may be one of the most important soil quality characteristics in relation to tillage because of its influence on other soil physical, chemical and biological properties. Conservation tillage practices can increase the organic matter content, aggregate stability and cation exchange capacity (CEC) of the topsoil. However, bulk density and penetrometer resistance are also increased, especially with zero tillage. Although such soil quality parameters may form a basis for describing some of the consequences of particular tillage practices, they do not provide a basis for predicting the outcome in terms of crop growth and yield. This is both because critical values of soil quality parameters have not been defined and because in some soils biopore formation in zero or minimally tilled land can modify the soil for water movement and for root growth and function.

The effects of tillage on crop growth and yield in long-term experiments are reviewed. The review only includes experiments in North America, Europe and New Zealand that have lasted 10 years or more to allow for seasonal variation in weather, possible progressive changes in soil conditions and the learning phase often experienced when new tillage methods are used. While there is a good deal of variation in the results of these tillage experiments some patterns have emerged. In long-term experiments, yields of maize in Europe and the US and soybeans in the US have been similar after ploughing and no-tillage, especially on well-drained soils. In Europe, yields of winter cereals have also been similar after traditional and simplified tillage but yields of spring cereals have sometimes been less after direct drilling than ploughing.

Trends in tillage practices are reviewed. Conservation tillage in the US is increasing and is used on about 30% of cropland, including no-till on about 10% of cropland. This increase in use of conservation tillage is mainly attributed to the legal requirement for farmers who are in government price support programs to adopt conservation plans which may involve conservation tillage. However, the allowable rates of erosion in these plans are likely to be in excess of rates of erosion for long-term sustainability. Survey information on tillage practices needs to be considered in relation to predictions on suitability of conservation tillage based on experimental results. In the semi-arid prairies of Canada there is a trend toward fewer cultivation operations, but in eastern Canada the mouldboard plough is still the dominant tillage method. In Europe although erosion is less obvious it is believed to be increasing, but minimum tillage is not widely used. This is because of the need to remove at least some straw for successful minimum tillage in sequential winter wheat and barley crops, but there are few economic uses for straw, and burning is illegal in many countries. In the more moist cooler conditions of Europe grass weed proliferation is another constraint, at least with present technology. So far, the overall success of conservation tillage has not been limited by the growing problem of genetic resistance of weeds to herbicides. Societal attitudes to the continued use of herbicides may pose longer-term problems for some conservation tillage practices.     

Effects of soil water content during primary tillage – laser measurements of soil surface changes

Soil water content during tillage can have a large impact on soil properties and tillage outcome. Measurement of soil relief in relation to fixed elevation points provides a non-destructive method of monitoring loosening/compacting processes during the year. The main objective of this study was to determine the effect of soil water content during primary tillage on soil physical properties.

The treatments included mouldboard and chisel ploughing of a clay soil on three occasions in the autumn, with gradually increasing water content (0.76, 0.91 and 1.01 × plastic limit). Soil surface height was measured by laser within a 0.64 m² area from fixed steel plates after each tillage occasion, and before and after seedbed preparation in the following spring. The measurements of surface height were compared with measurements of other soil physical properties, such as bulk density, saturated hydraulic conductivity and seedbed properties.

Tillage at the lowest water content (0.76 × plastic limit) produced the greatest proportion of small aggregates, and generally the most favourable soil conditions for crop growth. Soil loosening, as measured by increase in soil height during primary tillage, was highest for mouldboard ploughing and for tillage at the lowest water content. Differences between tillage treatments decreased with time, but were still significant after sowing in the spring. Natural consolidation during winter was smaller than the compaction during seedbed preparation in the spring. No significant differences in bulk density were found between treatments, and thus soil surface height was a more sensitive parameter than bulk density determined by core sampling to detect differences between treatments.

Late tillage under wet conditions caused a greater roughness of the soil surface and the seedbed base, which was also found in the traditional seedbed investigation. The effect of tillage time on seedbed properties also resulted in a lower number of emerged plants in later tillage treatments.

The laser measurements were effective for studying changes in soil structure over time. The results emphasize the need to determine changes in soil physical properties for different tillage systems over time in order to model soil processes.     

Tillage and agricultural sustainability

Agricultural sustainability implies an increasing trend in per capita productivity to meet the present needs without jeopardizing the future potential. Soil tillage, soil surface management to alleviate soil-related constraints to crop production, is a basic and an important input with short- and long-term effects on sustainability. An important effect of soil tillage on sustainability is through its impact on the environment e.g. soil degradation, water quality, emission of greenhouse gases from soil-related processes, etc. The need to attain agricultural sustainability is particularly urgent in several tropical eco-regions and soils of low-carrying capacity in the tropics.

Soil tillage influences atricultural sustainability through its effects on soil processes, soil properties, and crop growth. However, there is no one blueprint of a universally applicable sustainable tillage system. Appropriate tillage systems are soil- and crop-specific and their adaptation is governed by both biophysical and socio-economic factors. In addition to increasing crop yields, tillage methods must also facilitate soil and water conservation, improve root system development, maintain a favorable level of soil organic matter content, and reverse degradative in the soil's life-support processes.

Important components or sub-systems of conservation-effective tillage systems include mulch farming, no-till or reduced tillage systems, use of cover crops and planted fallows, agroforestry, raised beds or ridge-tillage, and soil inversion or deep plowing. The ecological limits for the applicability of these components or sub-systems differ widely. The efforts of a multi-disciplinary team (comprising soil scientists, agricultural engineers, agronomists, economists and social scientists) are needed to develop site-specific tillage methods to achieve both short- and long-term goals of agricultural sustainability.     

Tillage systems and soil compaction in Africa

Introduction of mechanized agriculture induces profound changes in soil characteristics. Soil compaction originating from mechanical land clearing, mechanized cultivation, and continuous cropping is aggravated by crusting and hard-setting phenomena of soils, and widespread occurrence of naturally compacted upland soils and subsoil gravel horizons. Natural and anthropogenically induced soil compaction has detrimental effects on growth and yields of a wide range of crops. Furthermore, compaction can persist for a long time if no adequate measures are taken to minimize or alleviate it.

In humid and subhumid regions of Africa, the no-tillage system with crop residue mulch is an important method of controlling soil compaction, followed in significance, by biological and mechanical loosening where motorized land clearing is the causative agent. Biological methods involve cover crops and alley cropping or agroforestry. Where new land areas need to be opened up, land clearing should be done by the slash-and-burn method, so that most of the nutrients in the vegetation are returned to the soil. Where mechanical land clearing is inevitable, forest removal should be done by the use of shear blade, whereby most of the roots and stumps are left in the ground intact, and the forest litter is not removed.

In semi-arid and arid regions of Africa, alleviation of soil compaction can be done by two methods. One method is to use the controlled traffic tillage system. Controlled traffic results in both a loose-rooting zone and a firmed traffic lane, thereby providing good plant growth and trafficability for timely field operations. The second method is to use mechanical loosening techniques, i.e. ploughing by animal traction or tractor power, chiseling, deep ripping, subsoiling, and tied-ride system. The effect of mechanical loosening, however, tends to be of short duration if the ensuing field traffic is not controlled.     

Tillage effects on selected physical properties of Grantsburg silt loam

Abstract

The objective of the project was to determine the effects of tillage on soil physical properties. A tillage project, involving three treatments with eight replications [no‐tillage (NT), chisel plowing (CP), and moldboard plowing (MP)], was initiated in the spring of 1989 in southern Illinois. The soil on which the work was conducted was a Grantsburg silt loam (fine‐silty, mixed, mesic Typic Fragiudalf), with a root‐restricting fragipan found at an average depth of 64 + 14 cm from the soil surface. Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] were grown on the plot area on a yearly rotation. The soil physical properties evaluated were: penetration resistance; bulk density; aggregate stability; and pore size distribution by water‐release. Tillage effects on soil penetration resistance were mainly confined to the plow‐layer (i.e. top 23 cm of soil). Generally, the cone index (CI) values for the top 23 cm of soil for all treatments were below 2MPa, except at midseason in 1991, a dry year. Penetration resistance differences due to tillage treatments were not caused by differences in soil water content. Soil bulk density was generally highest for NT at planting, however, the bulk density for CP and MP increased later in the season attaining values comparable to those of NT treatment. Chiseling and moldboard plowing reduced soil aggregate stability. Soil temperature at planting was lower for no‐tillage compared to the moldboard plowed system. Effects of tillage on pore size distribution, for the first two years of the experiment, were significant only at planting. Total porosity was higher for MP than CP and NT in both years. At midseason, 1991, total porosity was lower with MP than with NT and CP. The improved NT crop performance relative to the CP and MP treatments could also be related to better seed bed and root bed conditions following soybean (third year) than sod (first year) and better weed control. Initial crop yield advantages of MP over the conservation tillage systems (NT and CP) deteriorated over time, resulting in decreased soil aggregation, total porosity and soil productivity.     

Soil properties, and cotton growth, yield and fibre quality in three cotton-based cropping systems

The effects of three cotton-based cropping systems on soil properties, black root rot severity, and growth of cotton in a Vertisol were evaluated after a series of floods in eastern Australia. The experimental treatments, which had been imposed since 1985, were conventionally and minimum-tilled continuous cotton, and minimum-tilled cotton–wheat rotation. Frequent rainfall and flooding during the winter of 1998 resulted in near saturated soil at spring sowing in October. Although conventional tillage operations were completed before flooding, minimum tillage operations were not possible due to excessive moisture and cotton was sown onto the old beds with no-tillage. Soil specific volume (electrical conductivity of a 1:5 soil:water suspension) EC_1:5, exchangeable Na content, pH and organic C were determined for the top 0.6 m of the profile in summer 1998 and again in 1999. Organic C in the surface 0.10 m was also evaluated during 1998–2000. Black root rot severity and mycorrhizal fungal colonisation were evaluated at 6 weeks after sowing. Tissue nutrient concentrations were measured in mature cotton plants. Cotton lint yield and fibre quality were evaluated after picking and ginning.

In comparison with either minimum- or conventionally tilled continuous cotton, minimum-tilled cotton–wheat rotation had the lowest exchangeable Na content and severity of bacterial black root rot, best surface structure and the highest crop growth, nutrient uptake and lint yields. Subsoil structure was the best with conventionally tilled continuous cotton. The 1998 floods appear to have decreased exchangeable Na and increased soil pH in all treatments. Surface organic C also decreased between 1998 and 2000. Soil structural damage was minimised by avoiding tillage and trafficking in wet conditions. Compared with 1998, average yield decreases in 1999 were of the order of 43%. Cotton lint fibre quality was also poorer in 1999.     

华北典型区域土壤耕作方式对土壤特性和作物产量的影响

华北平原是我国重要的小麦玉米种植区,长期土壤旋耕免耕和秸秆全量还田带来耕层变浅、犁底层变厚和上移、土壤养分表聚等现象,通过耕作方式改变,解决上述问题对维持区域粮食生产有重要意义。试验以冬小麦-夏玉米轮作系统为研究对象,分别在代表华北平原高产区的栾城试验区和代表中低产区的南皮试验区进行,设置冬小麦播种前进行土壤深耕、深松、窄深松3种处理,以生产上常用的旋耕为对照。所有处理夏玉米季均采用土壤免耕播种,测定项目包括土壤容重、作物根系、作物产量和水分利用效率。结果表明,不同耕作方式对土壤特性和作物产量的影响具有区域差异。南皮试验区土壤深耕(松)显著地(P0.05)提高了作物产量,深耕、深松和窄深松处理的冬小麦产量比旋耕分别增加16.5%、19.3%和13.1%,夏玉米产量分别增加17.3%、16.2%和21.9%,周年产量分别增加16.9%、17.6%和17.8%;深耕、深松和窄深松处理间作物产量差异不显著。栾城试验区冬小麦、夏玉米产量和周年产量各处理之间差异不显著。土壤深耕、深松、窄深松和旋耕均能降低0～20 cm土层土壤紧实度和土壤容重。冬小麦播种后,与土壤耕作前比较,土壤深耕、深松和旋耕处理土壤紧实度南皮试验区分别平均降低71.6%和68.2%,栾城试验区分别降低88.8%和?7.7%,常用的旋耕模式在栾城试区没有降低土壤紧实度。小麦收获时不同耕作方式0～40cm土层的土壤容重均低于土壤耕作前的土壤容重,至夏玉米收获时不同耕作处理的土壤容重与耕作前基本一致,不同耕作处理对土壤容重的影响差异不显著。在南皮试验区, 3种耕作方式与旋耕相比,均显著提高了冬小麦和夏玉米水分利用效率;在栾城试验区,各处理冬小麦和夏玉米水分利用效率差异不显著。本研究结果显示在华北平原高产区连续实施土壤旋耕模式没有影响作物产量,而在中低产区实施土壤深耕或者深松模式更利于作物产量提高。     

Comparative evaluation of the contributions of soil physicochemical properties to variations in the yields of four major staple food crops in eastern Nigeria

The contributions of soil variables to the variations in the yields of cassava (Manihot esculenta), yam (Dioscorea rotundata), maize (Zea mays) and pigeon pea (Cajanus cajan) were evaluated over 2 years in this study. The data were from three replicates of two randomized complete block design experiments sited in a newly cleared forest and on previously cultivated land both in Nsukka, eastern Nigeria. The 28 soil physicochemical properties and six crop yield parameters measured were partitioned between location and year before applying a stepwise regression procedure to analyze them.

The study showed that soil variables accounted for >70% of the variation in cassava root yields and harvest index. Both soil physical and chemical properties contributed but the former (particularly macroporosity, microporosity, total porosity and bulk density) contributed most. Selected soil variables also accounted for >70% of the variation in yam tuber yield and shape index of tubers especially in 1998. In both crop years chemical properties appeared to dominate over the physical ones. Soil variables accounted for between 51 and 99% of the variation in maize grain and stover yields. The only exception was the figure of 44% obtained at the forest location in 1998. Soil pH, total exchangeable acidity and microporosity were particularly important contributors to the variations in both maize yield parameters. The contributions of soil variables to pigeon pea yield parameters were low (<50%) except in 1999 at the forest location where seven soil variables accounted for over 85% variation in seed yield.

It was obvious generally from the study that soil variables were important determinants of yield variations in the four crops. It was also shown that physical properties should always be included in this kind of analysis. Also the number of soil variables which were of significance generally increased when the level of soil properties was low, as was the case with the cultivated site versus forest site, and 1999 versus 1998 analysis. Thus increasing the number of soil variables used and partitioning them into more homogeneous units helped to improve the results obtained using the procedure.     

Deep tillage and traffic effects on subsoil compaction and sunflower (Helianthus annus L.) yields

The main function of deep tillage is to alleviate subsoil compaction, but how long do the benefits of this technique remain? Traffic on loose soil causes a significant increase in soil compaction. Subsoiling and chisel plowing were carried out at 450 and 280 mm depth, respectively on a compacted soil in the west Rolling Pampas region of Argentina. The draft required, physical soil properties, root growth, sunflower (Helianthus annus L. Merr.) yield and traffic compaction over the subsequent two growing seasons were measured. Cone penetrometer resistance was reduced and sunflower yields increased following deep tillage operations. Subsoil compaction caused changes to the root system of sunflower that affected shoot growth and crop yields. Although subsoiling and chiseling had an immediate loosening effect, it was evident that after just 2 years, when traffic intensity was >95 mg km ha⁻¹, re-compaction and settling had occurred in the 300–600 mm depth range.     

Tillage effects on water storage during fallow, and on barley root growth and yield in two contrasting soils of the semi-arid Segarra region in Spain

In semi-arid areas under rainfed agriculture water is the most limiting factor of crop production. To investigate the best way to perform fallow and its effect on soil water content (SWC) and root growth in a barley (Hordeum vulgare L.) crop after fallow, an experiment was conducted on two soils in La Segarra, a semi-arid area in the Ebro Valley (Spain). Fallow was a traditional system used in these areas to capture out-of-season rainfall to supplement that of the growing period, usually lasting 16 months, from July to October of the next year. Soil A was a loamy fine Fluventic Xerochrept (Haplic Calcisol, FAO) of 120 cm depth and Soil B was a loamy Lithic Xeric Torriorthent (Calcaric Regosol, FAO) of only 30 cm depth. The experiment was continued for four fallow-crop cycles in Soil A and for two in Soil B. In Soil A, three tillage systems were compared: subsoil tillage (ST), minimum tillage (MT) and no-tillage (NT). In Soil B, only MT and NT were compared. In the fields cropped to barley, SWC and root length density (LV) were measured at important developmental stages during the season, lasting from October to June. In the fallow fields SWC was also monitored. Here, evaporation (EV), water storage (WS) and water storage efficiency (WSE) were calculated using a simplified balance approach. The fallow period was split in two 8-month sub-periods: July–February (infiltration) and March–October evaporation (EV). In Soil A, values of WSE were in the range 10–18% in 1992–1993, 1993–1994 and 1994–1995 fallow, but fell to 3% in 1995–1996. Among tillage systems, NT showed significantly greater WSE in the July–February sub-period of 1992–1993 and 1993–1994 fallow, but significantly lower WSE in the March–October sub-period, due to greater EV under NT. Consequently, no differences in total WSE were found between tillage systems. In Soil B, WSE was low, about 3–7%, and there were no difference between tillage systems. During the crop period, the differences in SWC and LV between tillage systems were small. Regarding yields, the best tillage system depended on the year. NT is potentially the best system for executing fallow, but residues of the preceding crop must be left spread over the soil.     

Nutrient availability and cocoyam yield under different tillage practices

Research information on the effect of tillage systems on cocoyam (Xanthosoma sagittifolium (L.) Schott) growth, nutrient status and yield is lacking in Africa. The effects of zero tillage with mulch, zero tillage without mulch, manual mounding, manual ridging and conventional tillage on cocoyam yield, growth and nutrient availability were compared during 2 years on an Alfisol (Oxic Tropuldaf) at Owo in the rainforest zone of Nigeria. The surface soil (0–20 cm) was chemically analyzed before and after crop harvest and selected soil physical properties were determined. Concentration of soil organic C, N, P, K and Mg and the leaf N, P and K were significantly influenced (p = 0.05) by tillage, with zero tillage with mulch being the most effective treatment in conserving the fertility of the surface soil (0–20 cm). Soil fertility, as indicated by organic C, N, P, K, Ca and Mg, declined significantly (p = 0.05) over time in all tillage systems, but this decline was more pronounced in the conventional tillage. Zero tillage with mulch, zero tillage without mulch, manual mounding, manual ridging and conventional tillage reduced the soil organic C concentration by 20, 23, 23, 24 and 33%, respectively over the 2-year period. The decreases in soil N concentration were 25, 31, 31, 38 and 56%, soil P concentration were 13, 15, 17, 16 and 26%, and soil K concentration were 16, 26, 31, 37 and 53%, respectively. Tillage did not affect corm and cormel yields in the first year. In the second year, due to the elimination of ploughing, significant differences were obtained in the cormel yield but not of corm yield. In 2005, zero tillage with mulch produced the highest cormel yield (13.5 mg ha⁻¹) of cocoyam followed by zero tillage without mulch (13.2 mg ha⁻¹), manual mounding (12.7 mg ha⁻¹) and manual ridging (12.5 mg ha⁻¹). The lowest cormel yield (9.5 mg ha⁻¹) of cocoyam was produced by conventional tillage. Soil water contents in zero tillage with mulch and zero tillage without mulch were significantly higher (p = 0.05) than in the other tillage systems. Soil bulk density ranging from 1.21 to 1.40 mg m⁻³ correlated positively with leaf nutrient concentration and yield. Cocoyam can be grown successfully on zero tillage, with mulch and without mulch or minimum tillage systems on an Alfisol of the humid tropics.     

Soil quality attributes of conservation management regimes in a semi-arid region of south western Spain

Soil quality is essential for plant growth and terrestrial ecosystem maintenance. Although soil properties can be influenced by the agricultural production system, this influence has seldom been studied under semi-arid Mediterranean conditions. We analyzed the effect of the management system on soil physical and chemical parameters and soil microbial communities over three consecutive years under different conventional and conservation management regimes: conventional tillage (CT), direct seeding (DS), direct seeding with a winter crop cover (DSC), and long-term conservation management after nine consecutive years of direct seeding with winter cover (DSC^LT). The study was conducted on a maize (Zea mays L.) crop under irrigation in south western Spain. An improvement of the physical, chemical and biological parameters of the DS and DSC soils with respect to the CT soil was observed after two years management. Soil water content increased around 30% during the three years in the DS and DSC soils; organic C, nitrogen, and aggregate stability increased after the second year; total culturable microorganisms were twice as numerous in DSC^LT as in the CT soil; and soil penetration resistance was 50% less in all soils under any of the conservation management regimes. Hence, there was a major improvement in soil quality related to a potential increase of crop yields, and a reduced environmental impact, after short-term as well as after long-term conservation management.     

Conventional- and no-tillage effects on upper root zone soil conditions

One major objective of tillage is to loosen a soil and, thereby, create an improved soil condition for water infiltration, crop establishment, and plant growth. This implies that where tillage is not performed, as with no-tillage, soil conditions might be inferior to those of a tilled soil. However, no major adverse effects of no-tillage on soil conditions have been noted in the semiarid region of Texas. Also, crop yields on dryland have been favorable. This study was conducted to determine the effects of conventional- and no-tillage crop production methods on water retention, organic matter concentration, mean weight diameter of water-stable aggregates, bulk density, and penetrometer resistance of Pullman clay loam (Torrertic Paleustoll) at Bushland, Texas. These factors were significantly affected, but there was no consistent advantage or disadvantage for either tillage method. Wheat (Triticum aestivum L.) and grain sorghum (Sorghum bicolor (L.) Moench) yields have been favorable in the dryland rotation fields that were sampled for the soil measurements. Grain sorghum yielded 5.10 Mg ha⁻¹ of grain with conventional tillage and an average of 5.32 Mg ha⁻¹ of grain with no-tillage in 1987, the year in which the soil conditions were evaluated. The favorable yields indicate that no-tillage management does not adversely affect any of the measured Pullman (Torrertic Paleustoll) soil physical conditions to the point that crop yields are adversely affected.     

Crop residue management and tillage methods for conserving soil and water in semi-arid regions

Soil degradation reduces soil productivity and is a serious problem on much of the land in semi-arid regions. To avert continued degradation, the soil productivity balance must be shifted from degrading processes to conservation practices. Crop residue management and conservation tillage are on the positive side of the balance. When adequate residues are available and conservation tillage is used, soil erosion is greatly reduced and water conservation is enhanced. Water conservation is important for improving crop yields in semi-arid regions, especially where irrigations is not used. A major constraint to residue management in many countries is low production and widespread use for other purposes. In such cases, clean tillage and appropriate support practices such as contouring, furrow diking, strip cropping and terracing may provide adequate soil and water conservation benefits. Where these are not adequate, alternative management practices should be implemented to ease the demand for residues, thus permitting more of them to be retained on the land for soil and water conservation purposes. Some alternative practices include limited or selective residue removal, substituting high quality forages for residues as animal feed, alley cropping, using wasteland areas more effectively, improving the balance between feed supplies and animal populations, and using alternative fuel sources.