The effect of soil compaction on germination and early growth of Eucalyptus albens and an exotic annual grass

Most agricultural land has been compacted to some degree by heavy machinery or livestock trampling. This legacy is expected to influence the success of tree seedling recruits in farmland areas where natural regeneration is being encouraged. We investigated the impact of soil compaction on seedlings of a woodland eucalypt (Eucalyptus albens) and an annual grass competitor (Vulpia myuros) in a laboratory experiment. Replicate soil cores were created at five bulk density levels; 1.0, 1.1, 1.2, 1.3 or 1.4 Mg m^?3 with a soil water content of 20%. The depth of root penetration declined linearly with increasing bulk density, resulting in a decrease in root depth of around 75% in the most compacted soil compared with the least compacted soil for both species. Shoot length and primary root length did not vary between soil bulk density levels for either species, but seedlings responded to increasing levels of compaction with oblique (non‐vertical) root growth. Results suggest that young seedlings of both E. albens and V. myuros will be more susceptible to surface drying in compacted than uncompacted soils and therefore face a greater risk of desiccation during the critical months following germination. Any competitive advantage that V. myuros may have over E. albens is not evident in differential response to soil compaction.     

Nutrient uptake and growth of barley as affected by soil compaction

A field experiment with different levels of compaction was carried out on a mouldboard ploughed silty clay, with the objective of studying the effects on plant nutrient uptake and growth. Soil from the field was also used in laboratory studies of carbon and nitrogen mineralization, and plant uptake of water and nutrients. In the field, low as well as high bulk densities reduced biomass production and nutrient uptake of barley (Hordeum vulgare L.) compared to intermediate bulk densities, where grain yield was approximately 20% higher. In the beginning of the growing season, the concentration of phosphorus and potassium was lowest in plants grown in the loosest and in the most compacted soil, and suboptimal for plant growth. The uptake of nutrients transported by diffusion was more affected by compaction than for nutrients transported by mass flow. The reasons for lowered uptake in loose compared to moderately compacted soil could be reduced root-to-soil contact, a low diffusion coefficient for nutrients and/or reduced mass transport of water to seed and roots. Differences in plant nutrient concentrations between treatments gradually declined until harvest. Immediately after compaction there was probably oxygen deficiency in the compacted soil since the air-filled porosity was critically low, but as the soil dried out, mechanical resistance to root growth may have become a more important growth-limiting factor. In the laboratory study, severe compaction reduced carbon mineralization and uptake of water and nutrients by roots, and caused denitrification. There were only small differences between loose and moderately compacted soil in carbon mineralization, nitrogen concentration in the soil, uptake of water and nutrients and dry matter yield. The large yield increase due to recompaction in the field was not reproduced in the laboratory. Possible reasons are differences in soil temperature between the field and laboratory, in the sowing and fertilizing methods, the pretreatment of the soil and in the spatial variability of bulk density. It is possible that recompaction is needed only in the uppermost part of the soil, which is the loosest, dries out first, and is where the seed as well as the fertilizer are placed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response of citronella Java (Cymbopogon winterianus Jowitt) to VA mycorrhizal fungi and soil compaction in relation to P supply

Nutrient acquisition and growth of citronella Java (Cymbopogon winterianus Jowitt) was studied in a P-deficient sandy soil to determine the effects of mycorrhizal symbiosis and soil compaction. A pasteurized sandy loam soil was inoculated either with rhizosphere microorganisms excluding VAM fungi (non-mycorrhizal) or with the VAM fungus, Glomus intraradices Schenck and Smith (mycorrhizal) and supplied with 0, 50 or 100 mg P kg^-1 soil. The soil was compacted to a bulk density of 1.2 and 1.4 Mg m^-3 (dry soil basis). G. intraradices substantially increased root and shoot biomass, root length, nutrient (P, Zn and Cu) uptake per unit root length and nutrient concentrations in the plant, compared to inoculation with rhizosphere microorganisms when the soil was at the low bulk density and not amended with P. Little or no plant response to the VAM fungus was observed when the soil was supplied with 50 or 100 mg P kg^-1 soil and/or compacted to the highest bulk density. At higher soil compaction and P supply the VAM fungus significantly reduced root length. Non-mycorrhizal plants at higher soil compaction produced relatively thinner roots and had higher concentrations and uptake of P, Zn and Cu than at lower soil compaction, particularly under conditions of P deficiency. The quality of citronella Java oil measured in terms citronellal and d-citronellol concentration did not vary appreciably due to various soil treatments.     

Plant nutrition and growth: Basic principles

Soil compaction may restrict shoot growth of sugar beet plants. Roots, however, are the plant organs directly exposed to soil compaction and should therefore be primarily affected. The aim of this study was to determine the influence of mechanical resistance and aeration of compacted soil on root and shoot growth and on phosphorus supply of sugar beet. For this purpose, a silt loam soil was adjusted to bulk densities of 1.30, 1.50 and 1.65 g cm^–3 and water tensions of 300 and 60 hPa. Sugar beet was grown in a growth chamber under constant climatic conditions for 4 weeks. Both, decrease of water tension and increase of bulk density impeded root and shoot growth. In contrast, the P supply of the plants was differently affected. At the same air-filled pore volume, the P concentration of the shoots was reduced by a decrease of soil water tension, but not by an increase of bulk density. Both factors also reduced root length and root hair formation, however, in compacted soil the plants partly substituted for the reduction of root size by increasing the P uptake efficiency per unit of root. Shoot growth decreased when root growth was restricted. Both characteristics were closely related irrespective of the cause of root growth limitation by either compaction or water saturation. It is therefore concluded that shoot growth in both the compacted and the wet soil was regulated by root growth. The main factor impeding root growth in compacted soil was penetration resistance, not soil aeration.FAX no corresponding author: +49551 5056299     

Influence of root diameter on the penetration of seminal roots into a compacted subsoil

A field experiment was conducted to evaluate the influence of root diameter on the ability of roots of eight plant species to penetrate a compacted subsoil below a tilled layer. The soil was a fine sandy loam red-brown earth with a soil strength of about 3.0 MPa (at water content of 0.13 kg kg^-1, corresponding to 0.81 plastic limit) at the base of a tilled layer. Relative root diameter (RRD), which was calculated as the ratio of the mean diameters of roots of plants grown in compacted soil to the mean diameters of those from uncompacted soil, was used to compare the sensitivity of roots to thicken under mechanical stress.Diameters of root tips of plants grown in soil with a compacted layer were consistently larger than those from uncompacted soil. Tap-rooted species generally had bigger diameters and RRDs than fibrous-rooted species. A higher proportion of thicker roots penetrated the strong layer at the interface than thinner roots. There were differences between plant species in the extent to which root diameter increased in response to the compaction. The roots which had larger RRD also tended to have higher penetration percentage.The results suggest that the size of a root has a significant influence on its ability to penetrate strong soil layers. It is suggested that this could be related to the effects which root diameter may have on root growth pressure and on the mode of soil deformation during penetration.     

Effect of soil compaction on root growth and uptake of phosphorus

Summary Zea mays L. andLolium rigidum Gaud. were grown for 18 and 33 days respectively in pots containing three layers of soil each weighing 1 kg. The top and bottom layers were 100 mm deep and they had a bulk density of 1200 kg m^–3, while the central layer of soil was compacted to one of 12 bulk densities between 1200 and 1750 kg m^–3. The soil was labelled with³²P and³³P so that the contribution of the different layers of soil to the phosphorus content of the plant tops could be determined. Soil water potential was maintained between –20 and –100 kPa.Total dry weight of the plant tops and total root length were slightly affected by compaction of the soil, but root distribution was greatly altered. Compaction decreased root length in the compacted soil but increased root length in the overlying soil. Where bulk density was 1550 kg m^–3, root length in the compacted soil was about 0.5 of the maximum. At that density, the penetrometer resistance of the soil was 1.25 and 5.0 MPa and air porosity was 0.05 and 0.14 at water potentials of –20 and –100 kPa respectively, and daytime oxygen concentrations in the soil atmosphere at time of harvest were about 0.1 m³m^–3. Roots failed to grow completely through the compacted layer of soil at bulk densities 1550 kg m^–3. No differences were detected in the abilities of the two species to penetrate compacted soil.Ryegrass absorbed about twice as much phosphorus from uncompacted soil per unit length of root as did maize. Uptake of phosphorus from each layer of soil was related to the length of root in that layer, but differences in uptake between layers existed. Phosphorus uptake per unit length of root was higher from compacted than from uncompacted soil, particularly in the case of ryegrass at bulk densities of 1300–1500 kg m^–3.     

Effects of soil compaction on the microbial populations of melon and maize rhizoplane

Effects of soil compaction on the microbial populations of melon and maize rhizoplane were investigated in quantity and quality. The numbers of culturable bacteria and fluorescent pseudomonads on the rhizoplane were higher when plants were grown in more compacted soil and the relative increase was larger in fluorescent pseudomonads. Total bacterial counts, however, did not appear to be affected by soil compaction, resulting in the increase in the culturable bacteria among total counts in more compacted soil. The determination of extracellular enzymatic properties (pectinase, -glucosidase, -glucosidase and -galactosidase) of each 100 isolates from bulk soil and root samples suggested that the microbial populations on the rhizoplane, especially when plants were grown in highly, compacted soil, were composed of high ratios of bacteria with abilities to utilize root exudates efficiently. The microbial community structure estimated from the colony forming curves of bulk soil and root samples suggested that the microbial populations on the rhizoplane, especially when plants were grown in compacted soil, were likely to be composed of more r-strategists which were defined as those who formed colonies within 2 days.     

Rhizosphere carboxylate concentrations of chickpea are affected by soil bulk density

We investigated whether carboxylate exudation by chickpea (Cicer arietinum L.) was affected by soil bulk density and if this effect was local or systemic. We hypothesised that concentrations of carboxylates would increase with distance from the root apex due to continuous and constitutive accumulation of carboxylates, and that exudate accumulation would be greater in a compacted soil than in a loose soil. Plants were grown in split-root or single cylinders containing loose (1400 kg m (-3)) or compacted (1800 kg m (-3)) soil. Rhizosphere carboxylate concentrations were measured of whole root systems as well as of sections along the root. The root diameter was greatest of plants grown in the compacted soil; however, root diameters were the same for both root halves in the split-root design, whether they grew in loose soil or in compacted soil. Similarly, carboxylate concentrations tended to be lower for the whole root system in the compacted soil, but were the same for both root halves in the split-root design, irrespective of whether the roots were in loose soil or in compacted soil. These results indicate that both root diameter and carboxylate exudation by roots in chickpea is regulated systemically via a signal from the shoot rather than by local signals in the roots. There was no accumulation of carboxylates with increasing distance from the apex, probably because microbial degradation occurred at similar rates as carboxylate exudation. Malonate, previously suggested as deterrent to microorganisms, is likely only a selective deterrent.     

The effect of soil compaction on growth and P uptake by Trifolium subterraneum: interactions with mycorrhizal colonisation

The effects of vesicular-arbuscular mycorrhizal (VAM) colonisation on phosphorus (P) uptake and growth of clover (Trifolium subterraneum L.) in response to soil compaction were studied in three pot experiments. P uptake and growth of the plants decreased as the bulk density of the soil increased from 1.0 to 1.6 Mg m^-3. The strongest effects of soil compaction on P uptake and plant growth were observed at the highest P application (60 mg kg^-1 soil). The main observation of this study was that at low P application (15 mg kg^-1 soil), P uptake and shoot dry weight of the plants colonised by Glomus intraradices were greater than those of non-mycorrhizal plants at similar levels of compaction of the soil. However, the mycorrhizal growth response decreased proportionately as soil compaction was increased. Decreased total P uptake and shoot dry weight of mycorrhizal clover in compacted soil were attributed to the reduction in the root length. Soil compaction had no significant effect on the percentage of root length colonised. However, total root length colonised was lower (6.6 m pot^-1) in highly compacted soil than in slightly compacted soil (27.8 m pot^-1). The oxygen content of the soil atmosphere measured shortly before the plants were harvested varied from 0.18 m³m^-3 in slightly compacted soil (1.0 Mg m^-3) to 0.10 m³m^-3 in highly compacted soil (1.6 Mg m^-3).     

Analysis of soil moisture variations in an irrigated orchard root zone

Soil moisture and suction head in an irrigated orchard were continuously monitored by time domain reflectometry (TDR) probes and gypsum blocks, respectively, during and between successive irrigation events. On each side of the trees in the plot, two 30-cm long probes were installed vertically 10 cm below the soil surface (denoted as shallow) and another two probes were installed vertically 40 cm below the soil surface (denoted as deep). The variation in moisture content measured by the TDR probes between successive irrigation events was qualitatively divided into four stages: the first was during water application; the second initiated when irrigation stopped and the moisture content in the layer sharply decreased, mainly due to free drainage. The succeeding moderate soil-moisture decrease, caused by the simultaneous diminishing free drainage and root uptake, was the third stage. During the fourth stage, moisture depletion from the layer was solely by root uptake. The slopes of moisture content variation with time throughout this stage enabled the monitoring of water availability to the plant. The range of moisture content variations and moisture depletion rates between subsequent irrigation events was higher in the shallow (10–40 cm) than in the deeper (40–70 cm) layer. Irrigation nonuniformity and spatial variability of soil hydraulic properties contributed to the unevenness of the moisture distribution in the soil profile. However, as soon as moisture content within a layer reached field capacity, namely the free drainage had stopped, irrigation uniformity had a negligible effect on water flux to the plant roots. The measured data indicate that soil moisture is fully available to the plant as long as the momentary moisture flux from the soil bulk to the soil–root interface can replenish the moisture being depleted to supply, under non-stressed conditions, the atmospheric water demand. This flux is dominated by the local momentary value of the soil's bulk hydraulic conductivity, K _r, and it stays constant for a certain range of K _r values, controlled by the increasing root suction. A decrease in water availability to the plant appears for longer irrigation intervals as a break in the constant soil-moisture depletion rate during stage 4. This break is better correlated to a threshold K _r value than to threshold values of moisture content or suction. Therefore, it is suggested that moisture content or suction used to measure water availability or to control irrigation first be alibrated by K _r() or K _r() curves, respectively.     

Quantifying the impact of soil compaction on root system architecture in tomato (Solanum lycopersicum) by X-ray micro-computed tomography

Background and Aims

We sought to explore the interactions between roots and soil without disturbance and in four dimensions (i.e. 3-D plus time) using X-ray micro-computed tomography.

Methods

The roots of tomato Solanum lycopersicum ‘Ailsa Craig’ plants were visualized in undisturbed soil columns for 10 consecutive days to measure the effect of soil compaction on selected root traits including elongation rate. Treatments included bulk density (1·2 vs. 1·6 g cm⁻³) and soil type (loamy sand vs. clay loam).

Key Results

Plants grown at the higher soil bulk density exploited smaller soil volumes (P < 0·05) and exhibited reductions in root surface area (P < 0·001), total root volume (P < 0·001) and total root length (P < 0·05), but had a greater mean root diameter (P < 0·05) than at low soil bulk density. Swelling of the root tip area was observed in compacted soil (P < 0·05) and the tortuosity of the root path was also greater (P < 0·01). Root elongation rates varied greatly during the 10-d observation period (P < 0·001), increasing to a maximum at day 2 before decreasing to a minimum at day 4. The emergence of lateral roots occurred later in plants grown in compacted soil (P < 0·01). Novel rooting characteristics (convex hull volume, centroid and maximum width), measured by image analysis, were successfully employed to discriminate treatment effects. The root systems of plants grown in compacted soil had smaller convex hull volumes (P < 0·05), a higher centre of mass (P < 0·05) and a smaller maximum width than roots grown in uncompacted soil.

Conclusions

Soil compaction adversely affects root system architecture, influencing resource capture by limiting the volume of soil explored. Lateral roots formed later in plants grown in compacted soil and total root length and surface area were reduced. Root diameter was increased and swelling of the root tip occurred in compacted soil.     

Responses of ectomycorrhizal American elm (Ulmus americana) seedlings to salinity and soil compaction

American elm (Ulmus americana) seedlings were either non-inoculated or inoculated with Hebeloma crustuliniforme, Laccaria bicolor and a mixture of the two fungi to study the effects of ectomycorrhizal associations on seedling responses to soil compaction and salinity. The seedlings were grown in the greenhouse in pots containing non-compacted (0.4 g cm^?3 bulk density) and compacted (0.6 g cm^?3 bulk density) soil and subjected to 60 mM NaCl or 0 mM NaCl (control) treatments for 3 weeks. All three fungal inocula had similar effects on the responses of elm seedlings to soil compaction and salt treatment. In non-compacted soil, ectomycorrhizal fungi reduced plant dry weights, root hydraulic conductance, but did not affect leaf hydraulic conductance and net photosynthesis. When treated with 60 mM NaCl, ectomycorrhizal seedlings had several-fold lower leaf concentrations of Na⁺ compared with the non-inoculated plants. Soil compaction reduced Na⁺ leaf concentrations in non-ectomycorrhizal plants and decreased dry weights, gas exchange and root hydraulic conductance. However, in ectomycorrhizal plants, soil compaction had little effect on the leaf Na⁺ concentrations and on other measured growth and physiological parameters. Our results demonstrated that ECM associations could be highly beneficial to plants growing in sites with compacted soil such as urban areas.     

Effect of irrigation frequency on root water uptake in sugar beet

A 2-year trial was performed on autumn-sown sugar beet grown in pots in order to study the influence of irrigation frequency on the water used by plants along the soil profile. The outdoor pots, containing one plant each, were 1.3 m high and had circular openings, through which Time Domain Reflectometry (TDR) apparatus wave guides could be inserted. Three irrigation intervals were compared and plants were watered whenever the soil layer explored by roots had lost 30% (SWD₁), 50% (SWD₂) and 70% (SWD₃) of the total available water (TAW). During the irrigation season, the water extracted by the plants from each layer along the soil profile (RWU) was determined by monitoring volumetric soil moisture content (), by TDR. At harvest time, root length density (RLD) along the soil profile was assessed using the Tennant method. The applied irrigation frequencies significantly affected the RWU. With the SWD₃ protocol, irrigation was at longer irrigation intervals (9 days) and watering volumes were as high as 84 mm. In this treatment, the plants lost almost 60% of total water from the lower soil layer (0.6–1.0 m). In treatment SWD₁, the irrigation interval was very short (3 days), and water extraction from 0.0–0.6 m soil depth was 92.0%. In the intermediate treatment, the irrigation interval was 5.5 days and a more uniform water depletion was observed along the root zone, approximately equal between the 0–0.6 and 0.6–1.0 m soil layer. Water extraction of sugar beet plants at the deeper soil layers in response to long irrigation intervals was related to an increase in water uptake efficiency of the deeper younger roots and not to an increase in root length density, which, on the contrary, decreased. This morpho-physiological acclimatization to progressive soil water deficit was coupled with an increase of the root/shoot ratio.     

Scaling of root length density of maize in the field profile

Root length density is an important parameter in crop growth simulation and in evaluating consequences of root pattern on crop water and nutrient uptake. In this study, a scaling model was presented for estimating the profile distribution of root length density of maize (Zea mays L.). The model inputs are root length data of a reference profile and bulk densities of soil layers, as well as root length data in the first soil layer of a field profile to be investigated. Using the root length data of 10 soil profiles investigated over 2 years, the model was examined. The results show that the proposed scaling approach is effective in estimating the root length density of each layer of soil in the field profile. The relative root mean square error (RRMSE) of the developed scaling model was 25.28%, while that of the traditional exponential model was 39.53%. The scaling approach would facilitate determination of heterogeneous distributions of root length densities in the field.     

Effects of Soil Structure on Pea (Pisum sativum L.) Root Development According to Sowing Date and Cultivar

Spring peas are known to be very sensitive to compaction, particularly when sowing takes place soon after winter. Winter peas, which are sown in autumn, should present an opportunity to sow the crop in better soil structural conditions than for spring peas, because of more favourable moisture conditions at that time. As environmental conditions have a big influence on root systems, it is important to determine the effects of soil structure on pea root systems for different cultivars and sowing dates. A spring pea cultivar and a winter pea cultivar were both sown at two dates (one in autumn and one in spring) on soils with different plough-layer structures (compacted and uncompacted) at two sites in 2002 and one site in 2003. Soil structure was characterised by bulk density and the percentage of highly compacted zones in the ploughed layer. Root distribution maps were produced every month, from February to maturity. Root development was described in terms of general root dynamics, root elongation rate (RER) in the subsoil, final maximum root depth (Dmax) and root distribution at maturity. Root depth dynamics depended on compaction and its interaction with climatic conditions. The effects of compaction on RER in the subsoil depended on the experimental conditions. Dmax was reduced by 0.10 m by compaction. Compaction also reduced root distribution between 10 and 40% in the ploughed layer only. Pea cultivars differed in sensitivity to soil compaction, with a direct effect on the final depth explored by roots. These results are discussed in terms of their relevance to water and nutrient uptake.     

Cultivation, nitrogen fertilization, and set-aside effects on methane uptake in a drained marsh soil in Northeast China

To evaluate the effect of cultivation, nitrogen fertilizer, and set aside on CH₄ uptake after drained marshland was converted into agricultural fields, CH₄ fluxes and CH₄ concentrations in soil gas were in situ measured in a drained marsh soil, a set‐aside cultivated soil, and cultivated soils in Sanjiang Plain of Northeast China in August 2001. Over the measuring period, the highest CH₄ uptake rate was 120.7±6.2 μg CH₄ m^?2 h^?1 in the drained marsh soil and the lowest was 29.5±4.9 μg CH₄ m^?2 h^?1 in the set‐aside cultivated soil, showing that there was no significant recovery of CH₄ uptake ability 5 years after cultivation activity was stopped. CH₄ uptake rates were significantly less in the cultivated soils than in the drained marsh soil by 30.1–74.6%, which resulted mainly from cultivation and partly from nitrogen addition. A significantly negative correlation between CH₄ flux and bulk density in the cultivated soils tilled by machine suggests that cultivation reduced CH₄ uptake through compaction, because of the enhanced diffusion resistance for CH₄ and O₂. Nitrogen fertilization slowly reduced but persistently affected CH₄ uptake even after long‐term application of nitrogen.     

Influence of root density on the critical soil water potential

Estimation of root water uptake in crops is important for making many other agricultural predictions. This estimation often involves two assumptions: (1) that a critical soil water potential exists which is constant for a given combination of soil and crop and which does not depend on root length density, and (2) that the local root water uptake at given soil water potential is proportional to root length density. Recent results of both mathematical modeling and computer tomography show that these assumptions may not be valid when the soil water potential is averaged over a volume of soil containing roots. We tested these assumptions for plants with distinctly different root systems. Root water uptake rates and the critical soil water potential values were determined in several adjacent soil layers for horse bean (Vicia faba) and oat (Avena sativa) grown in lysimeters, and for field-grown cotton (Gossypium L.), maize (Zea mays) and alfalfa (Medicago sativa L.) crops. Root water uptake was calculated from the water balance of each layer in lysimeters. Water uptake rate was proportional to root length density at high soil water potentials, for both horse bean and oat plants, but root water uptake did not depend on root density for horse bean at potentials lower than −25 kPa. We observed a linear dependency of a critical soil water potential on the logarithm of root length density for all plants studied. Soil texture modified the critical water potential values, but not the linearity of the relationship. B E Clothier Section editor     

Measured and modelled differences in nutrient concentrations between rhizosphere and bulk soil in a Norway spruce stand

The gradient in soil characteristics from the bulk soil to the root surface is important to roots and to the organisms that live in the rhizosphere. Our ability to measure ion concentrations at the root surface is extremely limited, and models are largely untested. We used data from a well studied Norway spruce stand in SW Sweden to compare the measured difference in nutrient concentrations between rhizosphere and bulk soil with the difference predicted by a steady-state simulation model based on ecosystem budgets of nutrient uptake. The simulation model predicted depletion of NH₄, Ca, Mg, K in the rhizosphere, which shows that budgeted uptake rates were greater than the mass flow of bulk solution towards the root. In plots treated with ammonium sulphate, the model predicted an accumulation of S in the rhizosphere. In contrast, the observed rhizosphere concentrations were generally enriched in nutrients, relative to bulk soil. Collecting rhizosphere soil adhering to root surfaces may not be an appropriate method for describing the concentration gradient around the root. In addition, the simulation model omits some processes affecting conditions in the rhizosphere that are important to explaining nutrient uptake.     

Early growth of Brazilian pine (Araucaria angustifolia[Bertol.] Kuntze) in response to soil compaction and drought

Soil compaction leads to changes in soil physical properties such as density, penetration resistance and porosity, and, by consequence, affects root and plant growth. The initial growth of Brazilian pine is considered as being more affected by soil physical than chemical conditions, and the presence of a well-developed tap root system has been associated with this fact. A greenhouse experiment was conducted in order to evaluate the impact of soil compaction on the growth of Brazilian pine seedlings and on their susceptibility to a simulated drought period. In the first phase of the experiment, the effects of three levels of soil compaction on root morphology and plant growth were examined. Soil cylinders were artificially compacted in PVC tubes. Pre-germinated seeds were planted, and 147 days later 10 plants from each treatment were harvested for analysis. Higher values of soil density were associated with a shorter and thicker tap root. Growth of lateral roots and shoots remained unaffected at this stage. In the second phase, half of the plants (12) in each compaction treatment were drought-stressed by withholding water for a period of 77 days. Increased soil compaction again resulted in reduced length and increased diameter of the main tap root. This time, the effects were also extended to the lateral roots. Shoot extension growth and overall plant mass, however, increased with soil compaction. This greater mass accumulation in plants growing under increased soil compaction may be attributed to a more intimate contact between roots and soil particles. Drought stress reduced both root and shoot growth, but root mass was more negatively affected by drought stress in plants growing under high levels of soil compaction. Future investigations on the effects of soil compaction on the initial growth of Brazilian pine should include a wider range of compaction levels to better establish the relationship between soil physical parameters and plant growth.     

Oat residue and soil compaction influences on common root rot (Aphanomyes euteiches) of peas in a fine-textured soil

Management of common root rot (Aphanomyces euteiches Drechs.) in peas (Pisum sativum L.) is sought primarily by host crop avoidance for several years. Soil compaction is known to aggravate A. euteiches disease in peas but effects on infection and subsequent symptom development are not sufficiently known to assist in cultural control. Several isolated observations have noted that oat crop residues may suppress A. euteiches infection and disease in pea roots. The individual and combined influence (a factorial combination of two factors each at two levels) of a prior oat crop and soil compaction were studied for their effects on common root rot severity in processing peas grown in an A. euteiches disease nursery on a fine-textured soil in the northern Corn Belt of the USA. A previous crop of summer oats relative to prior-year peas significantly suppressed common root rot and increased pea fresh vine weight 210% at peak bloom stage. Both fresh vine weight and green pea yield were reduced as much as 63% by soil compaction and increased as much as 48% by a prior oat crop. Greater soil bulk density at the 10 to 25-cm depth identified wheel traffic compaction patterns in each year. A 10-fold reduction of saturated hydraulic conductivity in the 10 to 25-cm compacted zone and high soil-water potentials within the upper 60 cm both confirmed an impaired water drainage, especially during infiltration events. These observations support the use of a previous full season or summer oat crop jointly with chisel plowing, plus the prevention of excessive traffic during secondary tillage and planting, to reduce common root rot in a field infested with A. euteiches. Shallow incorporation of oat shoot and root residue by chiseling could be a crucial component of the cultural control of the disease. R Rodriguez Kabana Section editor