S. Tessier

Nitrous oxide emission from an agricultural soil subjected to different freeze-thaw cycles

Nitrous oxide (N2O) emissions from soils have been implicated as the causes of potential greenhouse warming and important nitrogen (N) loss. Soil freeze-thaw cycles have a significant effect on N2O emission. In this study, potential rates of N2O emission from an agricultural soil were examined in the laboratory by a soil core method. Spring soil cores from a sandy loam under conventional tillage and inorganic fertilizer management practices were distributed to four different freeze-thaw treatment groups. One group of the soil cores was not refrozen during the experiment, with the others subjected to one, two and three freeze-thaw cycles, respectively. The N2O flux from the soil core increased with increasing numbers of freeze-thaw cycles. The means of the fluxes ranged from 0.24 to 0.65 kg ha−1 day−1 for non-frozen soil cores and about 60% higher value were found for one freeze-thaw cycle treatment. The results also showed that the first freeze-thaw cycle promoted N2O flux as a pulse and the same patterns were found for additional freeze-thaw cycles. The information in this study should allow estimation of the potential rates of N2O emission contributed by freeze-thaw events for specific locations based on the soil climate data for the location.

SOIL BULK DENSITY ESTIMATION FOR TILLAGE SYSTEMS AND SOIL TEXTURES

Soil bulk density ( .b) is a physical property related to many phenomena, such as plant root development and solute transport in soils. In this study, a comprehensive data search of literature from 1980 to 1997 was made to examine .b as influenced by soil organic matter (OM), soil texture, primary tillage and secondary tillage practices and tillage depth. The literature data showed that .b is principally related to clay content and OM for no-till soils, while any soil textural variable and OM can be predictors of .b for tilled soil conditions, depending on primary tillage types and soil strata. Based on the literature data, regression models that incorporate tillage effects were proposed to estimate .b for nine tillage types, including no-tillage. The models predict .b for a given tillage condition by estimating the bulk density of soil for no-tillage ( .NT) and adding a change in density ( ..) induced by tillage which depends on tillage type used and the tillage depth. Model results for three common tillage types (moldboard plow, chisel, and no-tillage) were validated with field data from Quebec, Canada and good agreements in .b values between the field data and the model results were obtained. Model inputs are soil textural variables and OM. Thus, the model is widely applicable, and can be coupled into other studies, such as those of soil erosion, soil compaction, tillage evaluation, crop growth or soil water and solute transport.

EFFECTS OF SIX TILLAGE METHODS ON RESIDUE INCORPORATION AND CROP PERFORMANCE IN A HEAVY CLAY SOIL

A two-year on-farm study was conducted in a poorly drained, heavy clay soil in the Red River Valley, Manitoba, Canada, to explore six tillage alternatives. The six tillage methods consisted of a field cultivator (conventional tillage), a tandem disk, chisel plows with reversible shovels, twist shovels and narrow sweeps, and no-tillage. No-tillage gave the overall best results for the studied condition as it produced better or comparable yield, speed of emergence, and plant population, while requiring no tillage operations and leaving the most crop residue cover to protect against erosion. Chisel plow with reverse shovels could also be an alternative for clay soils, as it required relatively lower drafts and smaller amounts of incorporated residue in the seedbed, while giving crop performance that was comparable to the conventional tillage. The tandem disk resulted in the least residue cover (40% to 45%) in the field, which originally had 67% to 70% residue cover. The tandem disk plots contained the highest amount of incorporated residue (1.13 Mg ha-1) in the seedbed. Furthermore, the tandem disk resulted in a lower speed of crop emergence and plant population than the conventional tillage. Therefore, producers in the region would be taking a high risk with disking tillage.

SOIL COMPACTION BY MANURE SPREADERS EQUIPPED WITH STANDARD AND OVERSIZED TIRES AND MULTIPLE AXLES

A study was conducted on a heavy clay soil to evaluate soil compaction induced by different traffic treatments associated with liquid manure spreader systems. Five spreader weights (from 96 to 218 kN), two running gears (tandem and tridem, i.e., three axles) and two types of tire (conventional low section tire and oversized tire) were combined to obtain six traffic treatments, representative of liquid manure spreading operations in Quebec. Soil dry bulk density ( nb) and cone index (CI) were measured to evaluate compaction. Tire rut depths and the lateral influence zone were also investigated in the study. For a single pass of a spreader, soil compaction was confined to the tilled layer (about 0-250 mm depth) regardless of traffic treatments, and this did not affect emergence rates and yields of soybean under the particular soil and climate conditions that prevailed at the time of the study. Neither tandem nor tridem running gears were found to adequately contain soil compaction within the tilled layer for total spreader weights exceeding 154 kN. The acceptable limitation for a tandem spreader with conventional 21.5L-16.1 tires would be a total load of about 96 kN, resulting in average ground pressures of 150 kPa or less. If medium capacity spreaders are required, oversized tires are recommended for manure spreading on prairies or post-seeding applications on small grain crops. Finally, the recourse to multiple-axle running gears for larger spreaders to maintain or reduce unit load per axle or ground pressure does not result in less soil compaction than lower capacity tandem spreaders with comparable axle loads and ground pressure.

Finite element modeling of soil compaction by liquid manure spreaders.

A three-dimensional (3-D) finite element model was developed to predict the soil compaction caused by heavy liquid manure spreaders. The model includes both material and geometric nonlinearities. The updated Lagrangian method was applied to solve the geometric nonlinearity caused by large displacements. The finite element analysis was conducted with two types of tires, two different loads, and various inflation pressures. The finite element model was verified with the field test data. The results showed that the finite element model is adequate for predicting soil compaction. In agreement with observed data, the finite element results indicated that reused airplane tires with high inflation pressure resulted in the worst soil compaction, while an agricultural high floatation tire with a smaller load and lower inflation pressure caused the least soil compaction. The finite element results also demonstrated that soil compaction can be reduced by using lower tire inflation pressure.

MOLDBOARD PLOW PERFORMANCE IN A CLAY SOIL: SIMULATIONS AND EXPERIMENT

The effects of moldboard plow components and adjustments on its performance when used on a clay soil were analyzed using both modeling (Finite element method, FEM) and experimental results. With the FEM simulations, the average longitudinal force (Fx) was 1.4 and 2.7 times the average vertical (Fz) and lateral (Fy) forces, respectively. The specific vertical force (Fzs) was found to decline with width because of both the share edge effect and the greater soil-tool surface in contact which increased friction. The effects of the sharepoint seemed only predominant at very shallow depths to initiate penetration. A decrease in friction reduced specific longitudinal force (Fxs) but increased Fzs to promote plow penetration. From simulations with the same plow bottom Fy, a longer landside is required to prevent lateral movement of the plow bottom. During the field experiment, only the specific draft force (Ds) and fuel consumption showed a statistical difference between plowing depths with reductions of 16% and 8%, respectively, with the shallowest setting. Knife coulter and short landside had a lower Ds of 8% and 3%, respectively. Depth SD and distance to obtain average depth were considered as a measure of plow stability throughout the plot. The long landside and the knife coulter had both an increased in distance of 74 and 293 mm, respectively over the short landside and disc coulter.

A SURVEY OF CROP RESIDUE BURNING PRACTICES IN MANITOBA

Crop residue burning has become a concern in the Prairies, Canada due to its adverse impact on human health, the environment, and soil quality. A telephone survey was conducted in 2001 to investigate crop residue burning situations on farms in four rural municipalities of Manitoba, Canada. The survey questionnaire included 45 questions developed to identify the types and the percentage of producers who used burning as a crop residue management practice. Of the 84 eligible respondents, 47% practiced or possibly practiced crop residue burning. The motivating factors included the timeliness of field operations, such as fall tillage, fall fertilizer application and spring seeding, lower cost for residue disposal, increased crop yield, and better control of weeds and crop diseases. The survey also selectively gathered information on producers and farms background, crop, field equipment and farming practices, as these factors were expected to have impacts on the choice of crop residue management practices. Data show likelihood of reducing burning when producers practice longer crop rotations, use low disturbance disc-type seeding tools, and apply fertilizer in the spring. Education and awareness that leads producers away from the traditional crop residue burning practices may be started for young producers and large farms.

Modeling Mechanical Behavior of Agricultural Soils

The stress-strain behaviors of a Mawcook gravel-sandy loam and a Ste-Rosalie clay were studied under triaxial compression and hydrostatic compression loading paths performed on undisturbed cylindrical samples. The experimental data were compared to two nonlinear elastic models, the hyperbolic model and Bailey’s model, as well as to two elasto-plastic models, a modified Cam clay model, and the simple cap model. The hyperbolic model accurately predicted stress and volumetric strain for the two testing procedures, including the unload-reload cycles. Bailey’s model accurately predicted volumetric strain at a given stress. The two elasto-plastic models were accurate in predicting volumetric strain for the hydrostatic tests and reasonably accurate in predicting stress in the triaxial tests. The main source of error in predicting volumetric strain under triaxial compression was attributed to the variability in soil physical properties among the samples. The Cam clay model used only five parameters and could account for the effects of pre-consolidation, elastic and plastic deformation. The experimental results also indicated that soils with high clay content experienced more volume change at given stress state than the coarse sandy soils, and that soil mechanical behavior within and below the plow layer differed significantly.

Finite Element Prediction of Soil Compaction Induced by Various Running Gears

A three-dimensional finite element model was used to predict soil compaction induced by different running gears used on heavy liquid manure spreaders on two different soils. The finite element analyses were conducted for three types of tires: conventional implement I-1 high flotation bias tires, radial tires, and low pressure R-3 tires; and three types of running gears: two-, three- and four-axle. The soil compaction under removable rubber tracks was also studied under the assumption of uniform stress distribution. The results showed that substituting radial tires or low pressure tires for high flotation bias tires reduced soil compaction levels and that increasing the number of axles of the running gears lowered soil compaction significantly without increasing the width of the compacted zone. The results also implied that track-based running gears would possibly reduce traffic induced soil compaction. The simulation confirmed that axle load and the number of wheel passes were two major factors influencing subsoil compaction. The study of compaction on two soils indicated that, with similar running gears, compaction in clay soils was far more severe than that in coarsely textured soils.

VALIDATIONS OF MOLDBOARD PLOW SIMULATIONS WITH FEM ON A CLAY SOIL

Moldboard plow simulations performed at different depths (100, 150 and 200 mm) and speeds (0.25, 1 and 2 m/s) with the Finite Element Method (FEM) were validated by means of soil bin and field experiments in a clay soil. The Critical State and Coulombic models were used to describe soil mechanical and soil-tool friction behaviors, respectively. No significant differences were found between calculated and simulated longitudinal (Fx) forces. Predicted values of Fx were also in agreement with field experiment results except at speeds less than 1 m/s. The vertical forces calculated were significantly smaller for the lower and higher speed treatments. Lateral forces were not compared as the plow bottom frog provided extra lateral support during the experiments. The Cam clay model (no cohesion term) may be used for soil-tool systems for which soil cohesion is negligible. When used with appropriate material behavior and soil-tool interaction models, FEM may be used to solve 3-D problems having complex geometry.