Abstract: An investigation has been carried out to isolate the effects of the structure set up during compaction on the subsequent volumetric behaviour of a silty soil. Specimens compacted dry and wet of optimum heave have been brought to a common suction and dry density condition prior to testing in a suction-controlled oedometer apparatus. The test results demonstrate that the initial compaction structure has a significant influence on the collapse potential of the soil. The effects are, however, quite different from those that would normally be deduced from conventional tests in which the influence of structure and initial suction differences can not be easily distinguished. (A) For the covering abstract see IRRD 881647.

Abstract: Soil compaction models are reviewed and the problems of combining them with environmental impact and crop production models are discussed. The impacts of compaction on soil nitrogen and on energy use are considered in detail. It is concluded that mechanistic (i.e. process-based) models of compaction, based on the Boussinesq equations for stress propagation, are likely to be useful in modelling environmental impacts. The major problems likely to be faced when linking models are identified. These are the variability of soil, the different spatial scales of compaction and environmental impact models, and the prediction of the effects of compaction on soil structure. The use of empirical equations to estimate the effects of compaction on structure-dependent soil properties is discussed. Such equations, called pedotransfer functions, attempt to estimate values of properties, such as conductivity and strength, from bulk density, texture and organic matter content. They are likely to be useful but cannot account for either soil conditions at the time of compaction or the nature of the compacting stresses. Both of these factors influence structure. Mechanistic crop production models are considered to be more suitable than empirical models for predicting compaction effects. However, for use on a large scale, the detailed equations of soil water flow in such models will have to be replaced by simpler water balance methods. Procedures will also be needed to generate daily weather data. Mechanistic soil nitrogen models are also likely to be more suitable than empirical ones for predicting compaction effects. The draught required for cultivation may be estimated with reasonable precision but it is difficult to predict the effects of compaction on seedbed quality. Thus, it is not easy to forecast the type of implement or the number of passes needed and so the total impact of compaction on energy use is not easily modelled.

Abstract: The soil nematode fauna was assessed as a potential ecological index for the progressive stages of degradation, as well as the possibilities of restoration of a Calcic Haploxeralf in a semi-arid environment in Spain. With this aim in mind, soil characteristics and nematode populations were compared in a virgin ecosystem (evergreen oak forest) and in a closely associated area subjected to intensive cereal cultivation. In addition, in the latter area, the effects of different tillage systems, local soil compaction and crop rotations were evaluated over a 3 year period. Nematode populations were compared in experimental plots subjected to three contrasted situations: (i) no-tillage versus conventional tillage, (ii) soil compacted by tractor traffic versus undisturbed by traffic, and (iii) barley monoculture versus barley-vetch or barley-sunflower rotations. The soil with a virgin ecosystem had the greatest number and diversity of fungivorous ( Tylenchus ) and omnivorous predator (mononchids and dorilaimids) nematodes, whereas the values for endoparasites ( Heterodera avenae and Pratylenchus ) nematodes increased in tilled soil. The population of bacterial-feeding nematodes (rhabditids) was the same in virgin and cultivated areas. The greatest density and diversity in the no-tillage system occurred in the bacterial-feeding, fungivorous and omnivorous predator groups. A favorable effect of crop rotation was that the population of plant parasites (pathogenic) remained below crop damage concentrations. The effect of traffic on soil compaction was reflected conspicuously by the vertical distribution of soil nematodes within the soil profile. The population of plant parasites increased with depth, whereas the opposite occurred with the bacterial-feeding and omnivorous predator groups.

Abstract: Field experiments during the 1992 and 1993 wet seasons examined in situ water conservation in rainfed lowland rice ( Oryza sativa L.) grown on a loamy sand by comparing subsoil compaction (C) with puddling (PUD), a plastic sheet barrier at 40 cm depth (PLS), and dry tillage (DT). Soil was compacted with six (C6) or nine (C9) passes of a 12-ton road roller over the soil surface with (+V) or without (- V) roller vibration. After compaction, soil was tilled to 20 cm depth. All plots except DT were puddled manually to 15 cm depth. Rice seedlings (Cultivar KDML105) were transplanted in all plots. Degree and depth of compaction increased with number of passes and with vibration of the roller. Treatment C9 + V compacted soil to at least 60 cm depth. Percolation decreased from 11.8 mm d −1 in DT and 6.3 mm d −1 in PUD plots to 1.4 mm d −1 in C9 + V plots. Compaction increased total days with surface water accumulation from 44 days in DT and 68 days in PUD plots to 90 days in C9 + V. Rice in plots with subsoil compaction had nearly twice the root length and root mass densities in the 0–20 cm soil layer and significantly increased shoot growth as compared with rice in DT and PUD plots. As compared with PUD, the standard tillage practice of farmers, treatment C9 + V increased grain yield by 1.48 Mg ha −1 (59%) in 1992 and by 0.98 Mg ha −1 (88%) in 1993, with 1.6 times greater water use efficiency (WUE) in 1992 and 1.9 times greater WUE in 1993. Treatment PLS yielded more grain than PUD, but less than C9 + V. We conclude that subsoil compaction technology is applicable at a field scale for rainfed rice grown in coarse-textured soils.

Abstract: The tropical environment generally allows 2 crops/year to be grown. Controlled traffic has been suggested as a means of improving soil conditions, which may also lead to increased crop yield. A field trial at Millaroo Research Station, North Queensland, on a cracking clay (Entic Chromustert) studied the effect of controlled traffic (in conjunction with direct drilling and tillage) and conventional ridging on soil properties and crop yield. Maize (Zea mays L. cv. Hybrid 50) was grown as the winter crop and soybean [Glycine max (L.) Merr. cv. Canapolis] as the summer crop. With few exceptions, there was no significant difference between any pair of treatments in sowing line water content, bulk density, aggregate size distribution, seedling emergence, mean time of seedling emergence, and final yield. Differences that did occur between crop cycles were due to climatic variation. Cone index measurements indicated no lateral spread of compaction from the traffic lanes in the controlled traffic system to the soil in the plant growth area. Under the ridged area, however, it appeared that a plough pan began to develop just below the depth of tillage. Although no marked benefit in soil properties or plant yield resulted from controlled traffic, it was possible to grow 2 crops/year for the duration of the experiment. In one season, only the controlled traffic treatments could be planted, due to unsuitable conditions for seedbed preparation. Double cropping under conventional cultivation systems is unreliable, due to the limited opportunity for seedbed preparation at the beginning of the wet season and the large number (up to 8) of operations required to prepare a seedbed. Controlled traffic, restricting soil compaction to the traffic lanes, is a system that helps to maintain a zone more favourable for plant growth, as indicated by the cone index measurements.

Abstract: This experiment was carried out to improve understanding of sward and soil responses to cattle and/or sheep rotational grazing of low-input (no N fertilizer), natural swards dominated by smooth-stalked meadowgrass (Poa pratensis) syn. Kentucky bluegrass and based on white clover (Trifolium repens). Treatments during two grazing seasons (1989–90) were: cattle grazing alone (C); cattle grazing followed by topping (CT); cattle grazing followed by sheep grazing (CS); and sheep grazing alone (S), Mean target pre- and post-grazing herbage masses were 2200 and 1100 kg dry matter (DM) ha−1. Plants in sheep-grazed swards regrew more quickly and accumulated more herbage (8·28 compared to 5·35 t DM ha−1 for cattle-grazed swards, unadjusted for rejected area) than in other treatments. After 2 years, soil in sheep-grazed areas contained 0·25% N, 5·7 kg available P ha−1, and 379 kg K ha−1, compared to an average of 0·19, 3·9, and 179 respectively for the same soil nutrients in cattle-grazed treatments. Besides differences in manure distribution and sward rejection, differences in soil compaction among treatments also may have affected soil organisms and thus plant growth. After 2 years, soil bulk densities (g cc−1) were 1·37, 1·37, 1·27, and 1·12; soil penetrometer measurements (kg cm sec−1 sec−1) to 20-cm soil depth were 9·8, 9·3, 9·5, and 6·7; soil nematodes per 100 g of soil were: 5333, 8705, 2810, and 15208; soil rotifers per 100 g soil were: 288, 242, 715, and 33; and earthworms m−2 (and their biomass (g m−2)) were 262 (205), 157 (162), 344 (409), and 294 (343) for C, CT, CS, and S treatments, respectively.

Abstract: The experiment investigates effects of subsoil compaction, of three densities of simulated root channels through the compacted layer, and of fertiliser addition on growth, carbon gain and water relations of Pinus radiata. Treatments were designed to simulate subsoil compaction produced by a previous tree crop and access to depth through the compacted layer via old root channels. Comparisons were also made of diurnal carbon assimilation, micro-climate, water potential, and mineral nutrition of well-exposed foliage. Subsoil compaction was negatively associated with growth, plant water potential, foliar nutrient concentration, water uptake and diurnal carbon gain per unit foliage area. All treatments with compacted subsoil grew more slowly than controls, and an interaction between root access to depth and nutrition is indicated. Diurnal carbon gain per unit foliage area of trees growing on unperforated, compacted subsoil was about 0.78 that of controls, while stem growth was about 0.56 that of controls. Root growth of trees on compacted subsoil with the largest number of root channels showed an effect of fertilisation on growth. Fertilised trees in this compaction treatment had a significantly greater fine root biomass in the upper 0.3 m of the profile, greater access to water at depth, higher plant water potential, higher soil water content below the compacted subsoil layer, and greater growth than unfertilised trees in the same soil compaction treatment.

Abstract: A discussion of a paper with the aforementioned title by Fishman, Desai, and Sogge, published in this journal (Volume 119, Number 8, August 1993), is presented. The discussion focuses on soil vertical stress near the face of the wall. Day queries whether the low vertical soil pressure measured near the wall is related to poor compaction or vertical load transfer through the geogrid-wall connection. No closure is offered by the authors.

Abstract: A relationship between tire sinkage depth and depth at which traffic increased bulk density by 0.05 Mg/m3 was developed from data in published literature. Laboratory and field experiments were also conducted to verify the relationship. In the laboratory, a universal testing machine was used with the aid of dimensional analysis and modeling theory to physically simulate compaction by a tractor tire. An oval metal plate 100 mm wide and 122 mm long was used to apply stresses of 25, 50, 100, and 150 kPa to soil in containers. Three soil water contents were used. For field verification, a tractor was used to traffic soil. Different tire inflation pressures and loads were used to create different tire sinkage depths. Differences in bulk density were used to determine depth at which applied stress caused significant soil compaction. Based on previous research and on laboratory and field experiments, compaction depth, Y, was found to be related to sinkage depth, X, by the empirical equation, Y = bXm where b and m are regression constants.

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.

Abstract: Hybrid Populus was grown from cuttings for 5 years on two reclamation sites in southern Illinois. Plantings were made in two years and establishment varied with weather and site conditions including grasshopper damage. There were few later losses and growth was rapid with early canopy closure. Harvested trees re-grew rapidly. The root depth reached within 3 years, opening up substantial root channels, was nearly equivalent to that of 8-year old trees. Best performance was found on strongly-compacted, poorly-drained clay soils contrary to cited reports for Populus. Hybrid Populus has great potential in reclamation for quickly shading out competing herbaceous cover, mitigating soil compaction problems, and growing well with soil flooding that limits growth of most kinds of trees.

Abstract: Field trials were conducted in two commercial Chenin blanc vineyards, situated at Stellenbosch (annual rainfall ca 700 mm) and Rawsonville (annual rainfall ca 800 mm). On average, soils contained 38% and 6% clay, respectively. Both soils showed pH (KCl) values of ca 4.5 from 200 mm downwards. In the sandy soil (Rawsonville) a compacted plough sole was present at 200 mm depth, while no such barrier occurred in the Stellenbosch soil. Calcitic lime (CL), or equivalent amounts of dolomitic lime (DL) and slaked lime (SL) were applied to soil surfaces, either as a single increment or as three equal increments over a three year period. Total application of CL amounted 30 t ha-1 and 20 t ha-1 for Stellenbosch and Rawsonville, respectively. Four years after the start of the experiment the pH of the SL-treated soil at Stellenbosch was increased to 5.0 at a depth of 600 mm. In contrast, CL and DL did not move deeper than 300 mm. At Rawsonville downward movement of ameliorants was impeded by the compacted plough sole. A fraction of the SL moved across this physical barrier, but the soil pH at 600 mm was increased to 4.6 only. Single and incremental applications of SL gave comparable results. Results indicated that particle size distribution annot solely be used as a reliable criterion for estimating the potential mobility of surface-applied ameliorants. Leaf analyses reflected the superior mobility of SL on both soils.

Abstract: Improper densification of soil in an excavation often requires a crew to return to the site and remove and properly compact the backfill. Since U.S. gas utilities excavate more than 2 million bellholes annually, the cost of this process is substantial. The Compaction Meter is a low cost, easy-to-use, compact instrument that helps utility crews optimize backfill compaction. The meter consists of a small sensor (placed at the bottom of an excavation), a connecting wire, and an electronic readout box. It operates on a principle that compaction tool impact energy transmitted to the sensor increases. When each layer of fill becomes more compacted, energy transmitted to the sensor increases. When each layer is sufficiently compacted, the readout box alerts the operator to stop, without interrupting the backfilling process. Utilities have demonstrated the unit's performance in field trials and a mnufacturer is readying the technology for commercialization.

Abstract: In the rainfed lowlands, a hardpan may be considered beneficial if leaching loss is reduced, or detrimental if root access to resources deeper in the profile is denied. This paper contrasts research into the coarse-textured, permeable soils of northeast Thailand with research into the silty, compaction-prone soils of northwest Bangladesh. The former research has indicated a benefit to yield of rainfed lowland rice by compacting the subsoil, which reduces percolation losses and increases resource availability to the crop. In contrast, use of deep cultivation or a pre-rice legume to perforate the hardpan is resulting in higher yields of rainfed lowland rice in the latter. Recent research exploring variation among rice lines in their capacity to penetrate a hardpan is also reviewed. The results are discussed in relation to recent research which indicates that control of root development under the anaerobic-aerobic transitions of the rainfed lowlands is not clear. The paper concludes that further understanding is required of factors which control root system development under rainfed lowland conditions. Crop simulation would also assist definition of conditions in which making or breaking the hardpan may be beneficial, and in quantifying the associated probabilities for contrasting sites.