Soil Physical Constraints Research Articles

Assessing soil quality for rehabilitation of salt-affected agroecosystem: A comprehensive review

One billion hectares of land worldwide is affected by several kinds of salinity and associated problems. The soil quality (SQ) in salt-affected soil (SAS) is impaired because of the presence of excess electrolytes, disproportionate Na and Ca in soil solution and exchange phase, rhythmic changes in the hydrological cycle, decreasing soil organic matter, poor vegetative cover, low soil biological activity, and crop residue return. Sodic and saline–sodic soils have the potential to provide alkaline reactions and soil physical constraints to regulate the soil attributes affecting SQ. Because of high spatial variability and rapid temporal changes, selection of simple, robust, low cost, and high-throughput master indicators for assessing SQ is very essential for monitoring the aggradation or degradation of SAS. Therefore, screening the master indicators for developing a minimum dataset for SQ assessment of SAS is an important issue for sustainable management of soil in these agro-ecologies. We captured the SQ indicators for SAS from several ecosystems of different countries and discussed the problems of parameterization for assessing SQ. Improved SQ for optimum soil functioning is needed for confirming agricultural productivity and food security around the globe. This review describes the causes and drivers for sodification/salinization and mechanism-oriented rehabilitation options such as the application of mineral gypsum, flue-gas–desulfurized gypsum, elemental S, acidified biochar, polymer, salt tolerance mechanisms, and other agro-techniques for improving the quality of SAS. Based on the SQ assessment, a suite of site-specific soil management practices are advocated for the greening of SAS and prosperity.

Impact of contrasting soils in a high-altitude vineyard of <i>Vitis vinifera</i> L. cv. Malbec: root morphology and distribution, vegetative and reproductive expressions, and berry skin phenolics

The high-altitude viticulture of Mendoza is being increasingly recognised among the top wine regions worldwide. High levels of solar ultraviolet-B (UV-B) radiation, alluvial heterogeneous soils and cool night temperatures are major influences on Vitis vinifera L. phenotypic characteristics, especially on Malbec, Argentina’s emblematic grapevine variety. In the present work, the effect of intra-vineyard contrasting soil depth on Malbec was evaluated in a high-altitude vineyard (1450 m a.s.l.), over two growing seasons for physiological traits and over three vintages for yield components and berry skin phenolic compounds. The experiment was carried out in two parcels of shallow (SS) and deep (DS) soils, with different physico-chemical characteristics, and submitted to equal irrigation and crop management. Results showed that root depth was not limited by any soil physical constraints and fine roots (< 1 mm) were patchily distributed in sites with higher contents of silt, clay and organic matter in SS. Higher root quantity in SS was related to lower vegetative and reproductive expressions, shorter ripening period and reduced berry size and fruit yield when compared to DS. When SS plants experienced mild water stress (ΨS, stem water potential, between -6 and -9 bar) increments of anthocyanins, total polyphenols and antioxidant capacity in berry skins were observed. However, when SS plants reached ΨS ≤ -10 bar, physiological harm occurred. The results presented contribute to understanding the effects of soil type on Malbec vines during different seasonal environmental conditions, especially in stressful situations, such as those expected by climate change scenarios.

Particle size distribution and clay minerals in dryland soils of Aceh Besar, Indonesia

Soil particle size distribution is a fundamental physical property affecting other soil properties. This research aims to determine the distribution of soil particles and the composition of clay minerals on each layer of the horizon in four soil profiles (P1, P2, P3, and P4) with different parent materials in the dryland of Aceh Besar district which includes Entisols Jantho, Andisols Saree, Inceptisols Cucum, and Oxisols Lembah Seulawah. Particle size distribution (or texture) and bulk density (BD) were analyzed in the laboratory. Soil structure and consistency were observed directly in the field. The type of clay minerals was identified by X-ray diffraction, while Fe, Al, and Si-oxide were extracted by dithionite-citrate solution. The results shown that the particle size distribution and the physical properties of dryland soils of Aceh Besar vary between soil orders. Andisols Saree has better physical properties than the physical properties of other soils and low bulk density. The Al, Fe, and Si fractions and clay mineral composition in the soil profiles also vary considerably between soil orders. The Andisols are dominated by allophane minerals (amorphous fractions) while Entisols, Inceptisols and Oxisols consist of mixed minerals of feldspar, quartz, halloysite, goethite, and other clay minerals. These soil orders have some soil physical constraints i.e. high soil bulk density, low water holding capacity and poor soil structures. Improvements in the physical properties of the soil on Aceh Besar dryland are indispensable to improve the quality of the soil.

Precrops alleviate soil physical limitations for soybean root growth in an Oxisol from southern Brazil

The impact of soil compaction on soybean root growth and grain yield can be alleviated by the presence of biopores and root channels in the soil profile. We hypothesize that cover crops (ruzigrass and oats) are better than grain crops (wheat and maize) to reduce the soil physical limitation to soybean root growth. We aimed to identify which precrops have higher potential to reduce the mechanical and water stresses resulting from soil compaction and soil desegregation, and to enhance soybean (Glycine max L) root growth and yield in an Oxisol (Rhodic Eutrudox), with clayey soil texture. Soybean was grown after four precrops (ruzigrass, oats, wheat, or maize), under four soil compaction levels [soil chiselling (MTC), no-tillage (NT), NT additionally compacted with four passes of a tractor (NTCT), and NT additionally compacted with eight passes of a grain harvester (NTCH)]. Soil physical attributes (bulk density, macroporosity, water-filled pore space and soil penentration resistance) in the soil profile (0−50 cm) and soybean components (grain yield, cumulative root length density and root dry mass) were investigated. Soil physical attributes were improved over time due to the combined effects of natural wetting-drying cycles and biopores created by the roots of precrops. Ruzigrass increased soybean root biomass and length density, mainly at deeper soil layers of compacted treatments (NTCT and NTCH). The rate of increase of soybean root length density in the soil profile was higher after ruzigrass cultivation in comparison with maize and oats. Soil compaction effects on grain yield were partially (NTCH) or totally (NTCT) alleviated after two years and ruzigrass intensified the mitigation process. Ruzigrass also resulted in higher soybean yields in comparison with maize, whereas NTCH and MTC reduced yields in approximately 400 kg ha−1 (∼13 %) compared to NT and NTCT. Soil strengthening was more evident after ruzigrass and oats cultivation than maize or wheat cropping. Soil compaction in clayey Oxisols can be alleviated over time as a result of root channels provided by precrops combined with natural wetting-drying cycles. Among the evaluated precrops, ruzigrass is of particular interest, as it provides the most suitable soil physical environment for soybean root growth and grain yield. In contrast, chiselling was demonstrated to be a non-viable strategy to mitigate soil physical constraints for root growth and grain yields.

Soil management and diverse crop rotation can mitigate early-stage no-till compaction and improve least limiting water range in a Ferralsol

No-till management systems tend to cause soil compaction in the early years of their establishment. Soil compaction reduces crop production due to restrictions on root development. Management strategies combining crop rotation and soil conservation practices still need to be researched as potential strategies to mitigate no-till soil physical constraints in tropical regions with prolonged drought periods and still promote agricultural sustainability. The objective of this study was to determine if soil specific management could mitigate early-stage no-till compaction and improve least limiting water range in a Rhodic Ferralsol. We hypothesized that no-till cropping systems with concomitant conservation practices –soil fertilization, crop rotation, and intercropped brachiaria grass– would improve soil physical quality (SPQ) and still achieve high crop yields in a tropical region with dry winters and frequent dry spells during wet season. Six no-till cropping systems were tested: soybean monoculture (T1); corn monoculture (T2); corn and soybean rotation (T3); corn and soybean rotation with intercropped brachiaria (T4); and two more with increased fertilization: corn and soybean rotation with intercropped brachiaria (T5), and corn and soybean rotation (T6). Least limiting water range (LLWR) was used as an indicator of SPQ. LLWR is computed as a function of bulk density (Bd), and it is defined as the range of soil water content in which physical constraints to plant growth are at minimal. Its critical limits are water contents associated with field capacity and air-filled porosity (upper limits), along with wilting point and soil resistance (lower limits). For each Bd, there is a LLWR value: the span between the upper limit and the lower limit. An adaptation of LLWR, in which we substituted the wilting point by the critical water content (θ*), was also tested (LLWR*). Critical Bd (BdC) value was 1.30 Mg m−3 for LLWR and LLWR*. In monoculture treatments (T1 and T2) the maximum Bd values exceeded the BdC (LLWR = LLWR * = 0) and negatively correlated with crop yield. Alternatively, cropping systems with diverse crop rotation (corn/soybean/brachiaria) showed greater values of LLWR and LLWR* and less soil compaction than monoculture systems. Usage of LLWR* evinced water stress was the main limiting plant growth factor; viz. θ* was more limiting than mechanical resistance and deficient aeration. These results support the hypothesis that the use of soil conservation practices and crop rotation during initial years of no-till farming contributes favorably to SPQ without compromising crop production.

Assessing water uptake in sugar beet (Beta vulgaris) under different watering regimes

Sugar beet yield worldwide is substantially reduced as a result of drought stress. Water uptake may be limited by the plant (e.g. low root density) or by soil physical constraints. An experiment was conducted to assess the ability of sugar beet to produce roots and take up water throughout the soil profile under contrasting water regimes. Sugar beet was grown in columns, 15cm in diameter and 1m height in a glasshouse. In situ soil moisture was monitored hourly, and stomatal conductance was measured weekly. Root length and diameter at different depths were assessed destructively at 78 and 94 DAS. Greater water availability resulted in a higher root length and lower water use efficiency. Water uptake was initially from the upper soil layers but, as demand for water increased, there was a strong increase in root length density at depth. However, it was a further 16days, after roots reached the deep layers, before significant water was taken up. A possible reason for the delay, between presence of roots and water uptake by roots, was the absence of secondary xylem early on, which was supported by a second root anatomy study. Sugar beet can grow roots up to 1m deep and take up water from depth, however this did not happen until the late stages of drought stress and thus storage root dry weight had already been severely reduced, indicating that prevention of drought is necessary, early on, to minimise yield losses.

Role of Soil Physical Properties in Soil Health Management and Crop Productivity in Rainfed Systems-I:Soil Physical Constraints and Scope

Role of Soil Physical Properties in Soil Health Management and Crop Productivity in Rainfed Systems-I:Soil Physical Constraints and Scope

Soil physical conditions limit palm and tree basal area in Amazonian forests

Background: Trees and arborescent palms adopt different rooting strategies and responses to physical limitations imposed by soil structure, depth and anoxia. However, the implications of these differences for understanding variation in the relative abundance of these groups have not been explored. Aims: We analysed the relationship between soil physical constraints and tree and palm basal area to understand how the physical properties of soil are directly or indirectly related to the structure and physiognomy of lowland Amazonian forests. Methods: We analysed inventory data from 74 forest plots across Amazonia, from the RAINFOR and PPBio networks for which basal area, stand turnover rates and soil data were available. We related patterns of basal area to environmental variables in ordinary least squares and quantile regression models. Results: Soil physical properties predicted the upper limit for basal area of both trees and palms. This relationship was direct for palms but mediated by forest turnover rates for trees. Soil physical constraints alone explained up to 24% of palm basal area and, together with rainfall, up to 18% of tree basal area. Tree basal area was greatest in forests with lower turnover rates on well-structured soils, while palm basal area was high in weakly structured soils. Conclusions: Our results show that palms and trees are associated with different soil physical conditions. We suggest that adaptations of these life-forms drive their responses to soil structure, and thus shape the overall forest physiognomy of Amazonian forest vegetation.

Height-diameter allometry of tropical forest trees

Abstract. Tropical tree height-diameter (H:D) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were: 1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap). 2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A). 3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass. Annual precipitation coefficient of variation (PV), dry season length (SD), and mean annual air temperature (TA) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D. After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike's information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller D. Pantropical and continental-level models provided less robust estimates of H, especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account.

Soil Physical Constraints on Intrinsic Biodegradation of Petroleum Vapors in a Layered Subsurface

Naturally occurring biodegradation of petroleum hydrocarbons in the vadose zone depends on the physical soil environment influencing field-scale gas exchange and pore-scale microbial metabolism. In this study, we evaluated the effect of soil physical heterogeneity on biodegradation of petroleum vapors in a 16-m-deep, layered vadose zone. Soil slurry experiments (soil/water ratio 10:30 w/w, 25°C) on benzene biodegradation under aerobic and well-mixed conditions indicated that the biodegradation potential in different textured soil samples was related to soil type rather than depth, in the order: sandy loam > fine sand > limestone. Similarly, O(2) consumption rates during in situ respiration tests performed at the site were higher in the sandy loam than in the fine sand, although the difference was less significant than in the slurries. Laboratory and field data generally agreed well and suggested a significant potential for aerobic biodegradation, even with nutrient-poor and deep subsurface conditions. In slurries of the sandy loam, the biodegradation potential declined with increasing in situ water saturation (i.e., decreasing air-filled porosity in the field). This showed a relation between antecedent undisturbed field conditions and the slurry biodegradation potential, and suggested airfilled porosity to be a key factor for the intrinsic biodegradation potential in the field.

Soil microaggregates stability under different land use types in southeastern Nigeria

Abstract Low water stability of soil microaggregates accentuated by increased intensity of cultivation, high soil erodibility and climatic erosivity has become a major cause of increased water erosion process on agricultural lands in Imo State, southeastern Nigeria. Consequently; it has remained one of the most serious soil physical constraints to increased and sustained high level crop production in the area. Thus, this study was conducted in 2006 to quantify the effects of different land use types (LUTs) on the water stability of their various soil microaggregates in the area. Six LUTs [natural forest (NF); oil palm plantation (OPP); plantain plantation (PP); rubber plantation (RP); bush fallow (BF) and continuous cassava cropping (CCC)] were chosen for the study. Under each LUT, traverses were cut at equal intervals to obtain three plots per LUT that served as replications. From each plot, bulk samples were collected from 15 sampling points at 0–20 cm soil depth for determination of aggregate stability and other soil properties. The soil microaggregate stability was measured by clay dispersion ratio (CDR) and aggregated silt and clay, ASC (%) indices. Results showed that the CDR obtained from soil under NF LUT was significantly (P ≤ 0.05) lower (implying higher microaggregate stability) than what were obtained from other LUT soils. Conversely, the CDR of soils under CCC was significantly higher (indicating lower microaggregate stability) than what were obtained under other LUT soils. The relative increase (%) in CDR or decrease in microaggregate stability over the control (NF soil) were 16.67, 54.76, 57.14 and 78.57 for soils under OP, PP, RP, BF and CCC LUTs respectively. Similarly, the ASC (%) under NF soil was significantly (P ≤ 0.05) higher (indicating higher microaggregate stability) than those of other LUT soils. Contrarily, the ASC (%) of CCC, soil was significantly lower (lower microaggregate stability) than other LUT soils. The relative reduction (%) in ASC or microaggregate stability of the soils over the control (NF soil) stood at 25.37, 44.03, 44.03, 44.03 and 62.69 for soils under OPP, PP, RP, BF and CCC LUTs respectively. Correlation of each of the microaggregate stability indices (CDR and ASC) with some soil properties suggested that organic matter (OM), calcium (Ca2+) and magnesium (Mg2+) might be the most important variables related to each of the indices. By running multiple regression analysis, using linear, exponential, semi logarithmic and double logarithmic functional forms, whereas the semi logarithmic form was indicated as the best fit equation relating CDR to clay, OM and Ca; the linear form was the best fit model relating ASC to clay, OM and Ca. Thus, they could be used for prediction purposes.

Topsoil Properties as Affected by Tillage Systems in the Rolling Pampa Region of Argentina

Improved topsoil structural quality is expected under no‐till farming, but soil physical constraints can develop under continuous no‐till. Our objectives were: to evaluate the properties of loam, silty loam, and silty clay loam soils under various management practices on a regional scale; to clarify the relationship between soil organic matter pools and soil physical properties; and to find a minimum set of topsoil properties to characterize trends established by tillage. Thirty‐nine loam, silty loam, and silty clay loam soils were sampled from cropped fields managed using conventional tillage (CT) and no‐till (NT) as well as six undisturbed soils (uncropped). The A horizon thickness did not differ among soil textural groups and was 4 cm thicker in uncropped soils. Total and particulate organic C were significantly affected by management (uncropped > CT = NT, P < 0.001). Soil structural instability of uncropped soils (0.317 mm) differed from CT soils (0.723 mm) but not from NT soils (0.573 mm). Soil structural instability was negatively related to total and particulate organic C, and no relation was found with the resistant organic C pool. Water infiltration rate was the only topsoil property affected by a significant texture × management interaction. Lower infiltration rates in NT silty soils were caused by platy structural forms with horizontal pores. Soil penetration resistance (0–5 cm) was 0.77 MPa higher in NT than in CT soils. A minimum set of topsoil properties to evaluate tillage management includes structural instability, total or particulate organic C, infiltration rate, and penetration resistance.

Soil physical constraints for materialization of the green revolution in Africa by dissemination of Sawah based rice farming

Soil physical constraints for materialization of the green revolution in Africa by dissemination of Sawah based rice farming

Least limiting water range indicators of soil quality and corn production, eastern Ontario, Canada

The least limiting water range (LLWR) attempts to incorporate crop-limiting values of soil strength, aeration, and water supply to plant roots into one effective parameter (on the basis of soil water content). The LLWR can be a useful indicator of soil quality and soil physical constraints on crop production. This study focused on assessing dynamic cultivation zone LLWR parameters between different cropping/tillage/trafficked clay loam plots at Winchester, Ont., to identify potential management impact on surficial soil physical conditions for contrasting growing seasons. This study also evaluated dynamic cultivation layer LLWR variables as indicators of corn ( Zea mays L.) plant establishment and corn yield. The results suggest that no-till soils had lower average air-filled porosities (AFP) and O 2 concentrations than respectively managed tilled plots for both years of study. Potential trafficking effects on aeration properties were most evident in no-till relative to till; preferentially trafficked no-tilled plots had lower AFP and O 2 concentrations than respective non-preferentially trafficked no-till plots for both years of study. Corn establishment and yield variability were principally explained by cumulative differences between daily AFP and aeration threshold values, and the cumulative number of days daily AFP was below an AFP aeration threshold for specific corn growth stage periods. Lower AFP was linked to lower yields and plant establishments. Soil strength, as measured by cone penetration resistance, was important over certain sites, but not as important globally as AFP in predicting crop properties. Overall, conventional tilled soils that were not preferentially trafficked had most favorable aeration properties, and subsequently, greatest corn populations and yields. No-till soils were at greater risk of aeration limiting conditions, especially those in continuous corn and preferentially trafficked.

On‐farm evaluation of ridging and residue management options in a Sahelian millet‐cowpea intercrop. 2. Crop development

Abstract. In the Sahel, promising technologies for agricultural intensification include millet stover mulching and ridging. A four year on‐farm experiment was set‐up in order to assess the effect of various combinations of these two technologies on crop development and yield in a millet (Pennisetum glaucum (L.) R. Br.) ‐ cowpea (Vigna unguiculata (L.) Walp.) intercropping system. Treatments included bare surface, ridging, a surface applied banded millet stover mulch (2 t ha–1) and a banded millet stover mulch (2 t ha–1) buried in ridges. The latter three treatments were implemented exclusively in the cowpea rows, with an annual rotation between the millet and cowpea rows. On bare and ridged plots, millet yields fell below 100 kg grain ha–1 after the first year. This was ascribed mainly to soil acidification and loss of soil organic matter rather than to soil physical constraints or water availability despite extensive surface crusting and high soil penetration resistance and bulk density. Compared to the bare plots, ridging increased cowpea hay production by 330% over the four years which was attributed to lower soil penetration resistance and bulk density but also to a reduction of 0.15 cmol+ kg–1 exchangeable acidity in the ridges. Except during the severe drought year of 1997, millet grain yield in the banded mulch treatment remained fairly stable over time at 526 ± 9 kg ha–1. However, a detailed analysis revealed yield compensation mechanisms between various yield components depending on the timing of occurrence of the abiotic stresses. Cowpea productivity was always higher in buried banded mulch plots than in surface applied banded mulch plots but the former treatment appeared unable to sustain millet yields. This decline was attributed to a greater nutrient uptake by cowpea and more rapid acidification in the buried mulch treatment compared to the banded mulch treatment.

Modificações morfológicas e físicas de um Latossolo argiloso sob pastagens

O estudo foi realizado no Triângulo Mineiro e teve como objetivo avaliar as modificações morfológicas e físicas de um Latossolo Vermelho-Amarelo argiloso, quando comparadas uma pastagem antiga de baixa produtividade (BCO) e duas pastagens renovadas (BRP e BRC), utilizando uma área de Cerrado (CER) como referência. As análises físicas e morfológicas permitem concluir que: (a) existem impedimentos estruturais para o desenvolvimento radicular na pastagem antiga (BCO); (b) a porosidade formada por fissuras nas pastagens renovadas compensa a menor porosidade encontrada nos torrões; (c) a macrofauna de invertebrados teve importante papel na formação e na estabilidade de agregados e na regeneração dos volumes estruturais compactos; (d) nas pastagens renovadas, formadas somente por gramíneas ou consorciadas com Stylosanthes guianensis cv. Mineirão, os impedimentos estruturais deixam de existir; (e) o método do perfil cultural é recomendável em avaliações morfológicas de solos cultivados com pastagens.

IMPACT OF EROSION AND RESTORATION ONWATER AND NUTRIENT USE EFFICIENCY IN A HAWAII OXISOL

Yields, water use efficiency, and nitrogen use efficiency for field corn, Irish potato, sweet corn, sweet potato, and soybean were evaluated on a highly weathered Oxisol subjected to three levels of simulated erosion and three levels of nutrient replenishment. Simulated erosion reduced crop yields, water use efficiency (WUE) and nitrogen use efficiency (NUE); the reductions were particulary severe with the highest erosion level. Elevating nutrient levels in the eroded soil generally improved crop yields, WUE, and NUE. However, the improvement was only partial at the highest erosion level. This, and direct observations of crop roots, indicated that soil physical constraints exacerbated nutritional constraints. In addition, erosion reduces the level of beneficial microorganisms in the soil. All these conditions must be improved for full recovery of productivity and rehabilitation of eroded Oxisols. Inefficient use of water and nutrient inputs can be detrimental to rainfed agriculture, especially as the dryland farming practiced by millions of resource-poor farmers of the tropics.

SOIL PHYSICAL CONSTRAINTS AND THEIR EFFECT ON MORPHOLOGICAL CHARACTERS OF COCONUT (Cocos nucifera L. ) ROOTS

The objective of the study was to (1) evaluate land suitability for coconut (cocos nucifera L.) production in relation to soil physical properties, (2) identify the soil physical constraints and (3) study their effect on the morphological adaptation of coconut roots. Soil physical properties were found to be significantly related to coconut yield (R2=81.37; p<0.01). Multiple regression with cluster analysis of soil physical properties vs coconut yield enabled classification of soil series into three major groups namely (a) highly (b) moderately and (c) less productive series. It was observed that the high soil compaction which limited the available water and aeration capacity of soils resulted in retardation of the activity of coconut roots. Water stress due to soil compaction was found to induce production of more inactive roots by suberization and dehydration processes. Scanning electron microscopic image showed that soil compaction and water stress, reduced the cell volume per unit area of the absorption zone and the number of pores in respiratory organs of coconut roots resulting in retardation of water and nutrient absorption, and air exchange processes. This in turn malfunctions of absorption cells and respiratory organs of roots resulted in retardation of growth of coconut seedlings. The practical importance of these findings in formulation of land suitability maps for coconuts is discussed.

Management of clay soils for rainfed lowland rice-based cropping systems: an overview

The problem of concern in this project is that in the dry season following a lowland rice ( Oryza sativa L.) crop, yields of post-rice crops are generally low, despite adequate water commonly being available in the soil profile to grow a potentially high yielding dry season (DS) crop without irrigation. Maize ( Zea mays L.) yields are as low as 1 Mg ha −1 or less, soybean ( Glycine max L. Merr.) and cowpea ( Vigna unguiculata L.) at 0.3 to 0.8 Mg ha −1 in Indonesia and mungbean ( Vigna radiata (L.) Wilzek) around 0.5 Mg ha −1 in the Philippines. These are all very much below the yield potential of these soils. For example, mungbean yields of 2.2 Mg ha −1 have been achieved by IRRI in the Philippines on these soils without irrigation or additional fertilisers. The causes of low yields of DS crops after rice are mainly poor crop establishment and poor root growth due to soil physical constraints. These result from the breakdown of soil structure during wet cultivation (puddling) for rice. Yields are also limited by biological and chemical constraints. As a result of these low yields, farmers are reluctant to invest in post-rice crops. Therefore, land after lowland rice (at least 51 million ha in Asia according to Huke [Huke, R.E., 1982. Rice Area by Type of Culture: South, Southeast, and East Asia. International Rice Research Institute, Los Baños, Philippines, 32 pp.]) represents an underutilised resource that can be used to meet the food requirement of the ever increasing population of the developing world. To increase the utilisation of these soils, improved management practices are required to enable dry season crops to use the stored water in the soil profile after the rice crop. This paper outlines a project which was established with the general objective of developing soundly based soil management technologies that can overcome soil physical limitations to DS crop production after lowland rice. The specific objectives of the program were 1. to test a range of soil management and agronomic practices that have the potential to overcome adverse soil physical conditions for DS crops after rice, across a range of soil and climates; 2. to evaluate these practices by 2.1. measuring the changes in soil physical conditions throughout the complete cropping cycle from rice to DS crops; 2.2. determining the performance of the DS crop (establishment and growth) and its ability to extract soil water. 3. to determine the mechanisms involved in dispersion due to puddling and in flocculation and structural development as the soil dries after draining surface water from rice fields. Relevant outcomes from this project are described in the following papers in this issue.

Soil physical constraints to establishment of mungbeans ( Vigna radiata L. Wilczek) in paddy rice ( Oryza sativa L.) soils

In several rice producing countries, production of upland crops, such as mungbeans ( Vigna radiata L. Wilczek), is an important means to save foreign exchange on food imports and to exploit the residual soil water following paddy rice ( Oryza sativa L.) harvest. Since the soil physical requirements for establishment and growth of upland crops are different from those for paddy rice, this study examined (1) soil physical properties that may limit mungbean emergence in rice soils, and (2) soil management techniques that will alleviate the soil physical constraints to mungbean establishment. Dry soil conditions and high soil resistance were the major soil physical constraints limiting mungbean seedling emergence in clay soils in a growth chamber experiment. Since seedzone water content is high after rice harvest, soil strength was viewed as the major constraint to mungbean emergence in paddy rice soils. This hypothesis was tested in a field experiment where tillage practices (moldboard plow, no-till), planting techniques (inverted-T, dibble, furrow), and delays to planting after rice harvest (1–3, 4–5, 7–8, 11–12, or 14–15 days after field drainage) were used to create varying soil strength conditions. The soil was Maahas silty clay (Vertic Tropaquept). Mungbean emergence and soil strength data showed that the maximum emergence corresponded to soil conditions where soil resistance to seedling emergence was lowest. There were few benefits from tillage on mungbean emergence. Inverted-T and furrow planting performed best in tilled and no-till plots, respectively. The optimum time for mungbean planting was from 1 to 8 days after field drainage for no-till, and from 3 to 5 days after field drainage for tilled soils.