Variability Of Soil Physical Properties Research Articles

Spatial Variability Analysis of Soil Physical Properties of Alluvial Soils

Analysis and interpretation of spatial variability of soils is a keystone in site‐specific farming. Soil survey maps may have up to 0.41‐ha inclusions of dissimilar soils within a mapping unit. The objectives of this study were to determine the degree of spatial variability of soil physical properties and variance structure, and to model the sampling interval of alluvial floodplain soils. Soil profiles (n= 209) from 18 parallel transects were sampled with a mean separation distance of 79.4 m. Each profile was classified into surface, subsurface, and deep horizons. Structural analysis of soil bulk density (ρb), sand, clay, saturated hydraulic conductivity (Ks), volumetric water content (θv) at seven pressure potentials (Ψa) (−1, −10, −33, −67, −100, −500, and −1500 kPa) were modeled for the three horizons. Variance of soil physical properties varied from as low as 0.01% (ρb) to as high as 1542% (Ks). The LSD test indicated significant (P< 0.05) differences in sand, clay, ρb,Ks, and θvat various ΨaGeostatistical analyses illustrated that the spatially dependent stochastic component was predominant over the nugget effect. Structured semivariogram functions of each variable were used in generating fine‐scale kriged contour maps. Overall autocorrelation, Moran'sI, indicated a 400‐m sampling range would be adequate for detection of spatial structure of sand, silt, clay, and a 100‐m sampling range for soil hydraulic properties and ρbThe magnitude and spatial patterns soil physical property variability have implications for variable rate applications and design of soil sampling strategies in alluvial floodplain soils.

Variabilidade espacial de propriedades físicas do solo em uma parcela experimental

Experimentos de solo realizados no campo necessitam de estudos que verifiquem a variabilidade espacial do solo. O objetivo deste trabalho foi estudar a variabilidade espacial de propriedades físico-hídricas do solo em uma parcela experimental, usando métodos geoestatísticos. O experimento foi realizado em 1982 no Centro Experimental do Instituto Agronômico em Campinas (SP), em um Latossolo Vermelho sob preparo convencional, numa parcela de 30 x 30 m, com grade de pontos a cada 5 m. As propriedades analisadas foram: teor de água do solo, porosidade, densidade do solo, resistência à penetração e retenção de água. Para analisar a variabilidade espacial, utilizou-se a geoestatística, por meio da análise de semivariogramas, interpolação dos dados por krigagem e construção de mapas de isolinhas. A dependência espacial ocorreu principalmente nas variáveis obtidas na camada superficial do solo (0-25 cm), apresentando dependência espacial moderada e forte. Houve correlação positiva significativa entre retenção de água e densidade do solo. A dependência espacial encontrada e a semelhança de comportamento entre as variáveis permitiram inferir que amostragem ao acaso seria falha, pois esconderia a variabilidade encontrada, interferindo nas respostas dos tratamentos, caso fosse instalado um experimento que exigisse independência entre amostras.

Effect of variability of soil physical properties on clutter and false alarms in land mine detection

The effects of variability of the ground on land mine detection and false alarm rates are analyzed within the framework of a viscoelastic‐layered model of the ground. A matrix technique was used to describe sound interaction with layered viscoelastic ground. The resonance method in combination with a global search method is used to estimate a set of parameters for a three‐layered viscoelastic ground model. Results of the estimation show good agreement between computed and experimental data. The effect of a finite size of sound source on the acoustic‐to‐seismic transfer function is discussed. The effect of variability of ground properties on the acoustic‐to‐seismic transfer function (admittance function) is analyzed. Analysis is performed on the plane of parameters of the layered ground in a wide frequency range for all angles of incidence. It is revealed that small variations in the shear speed in the top layer of ground will not cause variation in the acoustic‐to‐seismic transfer function at low frequencies but may cause strong variation at high frequencies. Results of outdoor measurements of the acoustic‐to‐seismic transfer function are presented, and a correlation between the high magnitude of the acoustic‐to‐seismic transfer function in certain frequency ranges and moisture content on the surface is revealed. A simple model explaining the correlation between moisture content in the upper layer, the acoustic‐to‐seismic transfer function, and ground properties is suggested.

Modeling root growth and the soil–plant–atmosphere continuum of cotton crops

A simple and functional model to simulate cotton root growth is coupled to models of water balance and leaf water potential to predict the behavior of the soil–plant–atmosphere continuum (SPAC) of cotton crops. The root-growth model takes into account the dry matter partitioning to roots, root depth increase as a function of thermal time, soil mechanical resistance and soil water stress, and allows the inclusion of spatial variability of soil physical properties. The water balance model uses a cascade approach to simulate drainage while soil and plant evaporation are calculated separately following the model of Ritchie. The leaf water potential model (LWPM) is based on Ohm’s Law analogy, where water flow resistance to the roots is calculated using an empirical equation and the resistance within the plant is calculated as a function of a threshold leaf water potential. The model simulated correctly the soil resistance and the temporal and spatial distribution of roots in the soil profile. The model simulated total soil water content and midday leaf water potential well, even when a compacted soil layer was present, but the agreement to observed data was poor for the vertical distribution of soil water content, mainly in the zone of greatest root concentration.

Measurements of Mouldboard Plow Draft: I. Spectrum Analysis and Filtering

Measurements of mouldboard plow draft can potentially provide a useful tool for continuously mapping, as a surrogate variable, crop limiting soil properties such as soil strength/compaction. Numerous researchers have shown that loading on tillage tools can be periodic. One source of higher frequency periodic loading has been related to failure modes in the soil. Lower frequency periodic loading on tillage tools, assuming tillage depth is relatively constant, more often reflects local spatial variability in soil physical properties such as soil moisture, soil texture, organic matter, and/or soil strength/compaction. This study presents a spectrum and cross-spectrum analysis of mouldboard draft recorded during normal fall plowing operations along adjacent transects. One objective of this study was to determine the dominant periodic components in order to elucidate meaningful frequencies which might be better related to variation in soil physical properties. Spectral densities were generally dominated by signals with a few cycles per 300-m transect. The cross-amplitude and squared coherency between draft recorded on adjacent transects provided further indication of the frequencies which characterized the signals because they provide an indication of correlation as a function of frequency. Spatial dependence in the data was quantified using spatial autocorrelation and cross-correlation. An additional objective was to use the spectral information to help design more physically meaningful filters for the raw draft data. Taking the Fast Fourier Transform of the raw draft data and filtering in the frequency domain using an optimal filter informed via the spectrum and cross-spectrum analysis, provided a convenient method for isolating a more physically meaningful signal.

Resonances of Acoustic Waves Interacting with an Elastic Seabed

Journal of Computational AcousticsVol. 09, No. 03, pp. 1079-1093 (2001) ICTCA'99 PapersNo AccessRESONANCES OF ACOUSTIC WAVES INTERACTING WITH AN ELASTIC SEABEDMARGARETE S. FOKINA and VLADIMIR N. FOKINMARGARETE S. FOKINAInstitute of Applied Physics, Russian Academy of Sciences, 46, Ulyanov Str., Nizhny Novgorod, 603950, Russia Search for more papers by this author and VLADIMIR N. FOKINInstitute of Applied Physics, Russian Academy of Sciences, 46, Ulyanov Str., Nizhny Novgorod, 603695, Russia Search for more papers by this author https://doi.org/10.1142/S0218396X01001352Cited by:5 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail AbstractAn exact expression for the reflection coefficient is obtained with the Thomson–Haskell technique for the geoacoustical model of an ocean bottom consisting of an elastic homogeneous sediment layer overlying an elastic half-space. Characteristic equations for explicit determination of the position of each individual resonance contribution to the reflection coefficient are derived. Analytical expressions for the angular and frequency resonance positions are found. The resonance expression for the reflection coefficient is written in the form of a sum of resonance terms. Comparison between resonance theory and exact calculations for the elastic layer covering the elastic half-space is presented. The results of resonance formalism show excellent agreement with exact theory in all the cases.Presented at ICTCA'99, the 4th International Conference on Theoretical and Computational Acoustics, May 1999, Trieste, Italy. FiguresReferencesRelatedDetailsCited By 5An Acoustic Hyperlens with Negative Direction Based on Double Split Hollow SphereJung Sik Choi and Gil Ho Yoon17 June 2019 | Journal of Theoretical and Computational Acoustics, Vol. 27, No. 02Analytical solutions for the resonance response of Goupillaud-type elastic media using z -transform methodsG.A. Gazonas and A.P. Velo1 Jan 2012 | Wave Motion, Vol. 49, No. 1Effect of ground variability on acoustic-to-seismic transfer function and false alarms in landmine detectionVladimir N. Fokin, Margarita S. Fokina, James M. Sabatier and Zhiqu Lu1 Aug 2006 | The Journal of the Acoustical Society of America, Vol. 120, No. 2Effect of variability of soil physical properties on clutter and false alarms in land mine detectionVladimir N. Fokin, Margarita S. Fokina, James M. Sabatier, Ning Xiang and Wheeler B. Howard3 August 2004 | Radio Science, Vol. 39, No. 4Estimation of sea bottom parameters from the angle-frequency resonances of the reflection coefficientV. N. Fokin and M. S. Fokina1 Sep 2003 | Acoustical Physics, Vol. 49, No. 5 Recommended Vol. 09, No. 03 Metrics History Received 8 July 1999 Revised 7 September 2000 PDF download

Temporal Stability of Spatially Measured Soil Matric Potential Probability Density Function

Estimation of mean water status in a field is crucial to effective irrigation water management. Problems encountered with the estimation of mean field soil water status may be attributed to spatial variability of soil physical properties. Several investigators have shown temporal stability of spatial patterns of field measured soil water content, but temporal stability of field measured soil matric potential (ψm), a measure of soil water status more appropriate for irrigation scheduling, has not previously been reported to last for more than a few days within one irrigation cycle. This study investigated the temporal stability of spatial patterns of ψm both within and between sequential irrigation cycles. Sixty locations in a 1‐ha field were outfitted with a 1‐m neutron probe access tube and three tensiometers placed at 0.15‐, 0.3‐, and 0.5‐m depths. The observations obtained from 14 d of soil water content measurements and 46 d of ψm measurements within eight irrigation cycles were analyzed with Spearman's rank correlation coefficients and a relative differencing technique. The results showed temporally stable soil water content spatial patterns and also indicated temporally stable ψm spatial patterns if assumptions of full soil wetting at the beginning of the cycle and uniform evapotranspiration among locations were satisfied. Several locations in the field estimated the field mean ψm to within 10% within a given range of potentials, and a few estimated the field mean to within 20% across the entire range of potentials tested. Other locations estimated the lower and higher percentiles of ψm with similar accuracy.

Elevation and infiltration in a level basin. II. Impact on soil water and corn yield

The spatial variability of irrigation water recharge and crop yield is affected by a number of factors. Soil surface elevation, infiltration and soil water MAD are the most relevant related to level-basin irrigation. Measurements of soil water recharge (using a neutron probe) were compared to estimates based on ring infiltrometers and observations of the opportunity time. Estimates of cumulative infiltration (ECI) were obtained, separating the variability of infiltration and opportunity time (largely determined by elevation). Soil surface elevation was correlated with measured recharge, grain yield and total dry matter. A correlation was found between infiltration and the measurements of water recharge. While soil surface elevation can be regarded as a management variable, little can be done to reduce the variability of infiltration. Distribution uniformities from ECI were about 20% higher than those obtained from measurements of water recharge. Seasonal uniformity was only marginally higher than average uniformity, confirming the low random component of water recharge in level-basin irrigation. Deep percolation was more intense in areas with low MAD. This finding emphasizes the relevance of characterizing the variability of soil physical properties in surface irrigation. Extrapolation of the results of this research to field-scale irrigation basins should take into account the methodology used: in particular, the reduced scale of the experimental level basin.

Quantifying variability of soil physical properties within soil series to address modern land-use issues on the Canterbury Plains, New Zealand

Lack of accurate data to estimate soil physical properties for soil types is limiting the wide application of simulation models to address modern environmental and land-use issues. In this study, systematic sampling of soil profiles for soil physical characteristics has provided an improved basis upon which to estimate a number of soil physical properties for 4 soil series. The selected soils form a soil drainage sequence on the post-glacial surface of the Canterbury Plains and vary from shallow sandy loam, well-drained soils to deep clay loam, poorly drained soils. Three profiles within 3 map units were sampled for each of 4 soil series. Three horizons in each soil profile were sampled for soil porosity values, particle size, and saturated and near-saturated hydraulic conductivity. Variability in all data, as shown by coefficient of variation, increased in the order: total porosity = field capacity < wilting point < total available water = clay content < readily available water < macroporosity < sand content < hydraulic conductivity. Hydraulic conductivity exhibited high variability within horizons, between profiles, and within soil series. Temuka subsoils had extremely high variability in saturated hydraulic conductivity and this could be explained by their coarse prismatic structure. Analysis of variance identified horizons that differed in soil physical properties between soil series. Horizons that do not differ between series may be given pooled soil property values for the pooled series. Total porosity, field capacity, wilting point, clay content, and near-saturated hydraulic conductivity had the greatest number of differences (60–70%) between series comparisons, while total available water had fewest differences (5%). The series with greatest differences in drainage class (Temuka compared with Eyre or Templeton soils) recorded the largest number of differences in water release characteristics and particle size. There were few differences between well-drained Eyre and moderately well-drained Templeton series. Subsoils of Eyre series differed in hydraulic conductivity from subsoils for the other 3 series, but few differences in hydraulic conductivity were found between horizons of Templeton, Wakanui, and Temuka series. Hydraulic conductivity estimates for these series can therefore be pooled.

Differences in gravity-dominated unsaturated flow during autumn rains and snowmelt

Five tracer experiments have been performed in a coarse-textured soil near the new main airport at Gardermoen, Norway. In two lysimeter walls, 30 and 40 measuring points form the basis for spatial moment calculations. Although experiments were performed under different meteorological conditions (autumn and snowmelt) and at two different sites, the ratios of centres of vertical mass over cumulative infiltration were of the same order of magnitude, indicating a gravity-dominated flow. Two-dimensional transport simulations with SUTRA (Voss, 1984), with a priori estimated input parameters and random fields of soil hydraulic properties revealed a relatively good agreement with the experimental results. Three possible sources of heterogeneity affecting the vertical displacement of solute during snowmelt were identified: variability of soil physical properties, soil surface elevations and variability of ground frost during the melting period. To obtain accurate predictions, soil heterogeneity was the most important factor to characterize for the coarse-textured soil under consideration.

Quantifying tillage translocation and deposition rates due to moldboard plowing in the Palouse region of the Pacific Northwest, USA

Most of the erosion research in the Palouse region of eastern Washington State, USA has focused on quantifying the rates and patterns of water erosion for purposes of conservation planing. Tillage translocation, however, has largely been overlooked as a significant geomorphic process on Palouse hillslopes. Tillage translocation and tillage deposition together have resulted in severe soil degradation in many steep croplands of the Palouse region. Few controlled experiments have heretofore been conducted to model these important geomorphic processes on Palouse hillslopes. The overarching purpose of this investigation, therefore, was to model tillage translocation and deposition due to moldboard plowing in the Palouse region. Soil movement by moldboard plowing was measured using 480-steel flat washers. Washers were buried in silt loam soils on convex–convex shoulder, linear-convex backslope, and linear-concave footslope landform components, and then displaced from their original burial locations by a moldboard plow pulled by a wheel tractor traveling parallel to the contour at ca. 1.0 m s −1. Displaced washers were located using a metal detector, and the distance and azimuth of the resultant displacement of each washer from its original burial location was measured using compass and tape. Resultant displacement distances were then resolved into their component vectors of displacement parallel and perpendicular to the contour. A linear regression equation was developed expressing mean soil displacement distance as a function of slope gradient. Tillage translocation and deposition were modeled as diffusion-type geomorphic processes, and their rates were described in terms of the diffusion constant ( k). A multivariate statistical model was developed expressing mean soil displacement distance as a function of gravimetric moisture content, soil bulk density, slope gradient, and direction of furrow slice displacement. Analysis of variance (ANOVA) revealed a weak correlation between soil displacement and both bulk density and moisture content. Soil displacement was, however, significantly correlated with direction of furrow slice displacement. Tillage translocation rates were expressed in terms of the diffusion constant ( k) and ranged from 105 to 113 kg m −1 per tillage operation. Tillage deposition rates ranged from 54 to 148 kg m −1 per tillage operation. With respect to tillage deposition, the diffusion constant calculated from volumetric measurements of tillage deposits equals ca. 150 kg/m. The rates of tillage translocation and deposition are not completely in balance; however, these rates do suggest that soil tillage is a significant geomorphic process on Palouse hillslopes and could account for the some of the variations in soil physical properties and crop yield potential at the hillslope and farm-field scale in the Palouse region.

Stochastic Simulation of Field‐Scale Pesticide Transport Using Opus and GLEAMS

AbstractIncorporating variability in soil and chemical properties into root zone leaching models should provide a better representation of pollutant distribution in natural field conditions. Our objective was to determine if a more mechanistic rate‐based model (Opus) would predict soil water and pesticide mass in the soil profile more accurately than a capacity‐based model (GLEAMS) when spatial variability and uncertainly in parameters are considered. Predictions of spatial variations of soil water content and movement of aldicarb [2‐methyl‐2‐(methylthio)‐propionaldehyde O‐(methylcarbamoyl) oxime] and metolachlor [2‐chloro‐N‐(2‐methoxy‐1‐methylethyl) acetamide] in the root zone were compared using 3 yr of observed data from a 3.9‐ha agricultural field in southwest Georgia. Spatial variability of soil physical properties, pesticide properties, and pesticide application were described using probability distributions fitted to measured field data, after removing spatial trends that were physically meaningful. There were significant differences in mean soil water content predicted by the two models, although variations around the mean were comparable. Pesticide mass predictions were different on most post‐application dates in both mean and spatial variation. The less rigorous GLEAMS predicted mean depth‐averaged soil water content and pesticide mass in the 1.2‐m profile at least as good as the more mechanistic Opus, although it did not simulate depth distributions of water or pesticide mass as well as Opus. GLEAMS simulated spatial variations of depth‐averaged soil water content and pesticide mass in the field with reasonable accuracy while employing fewer parameters that exhibit lower spatial variability.

Fate of15N Fertilizer Applied to Trickle-irrigated Grapevines

Information on fate of nitrogen applied to vines is needed to improve fertilizer management. Nitrogen-15 enriched ammonium and nitrate fertilizers were applied in the spring through a trickle irrigation system to six Thompson Seedless vines of a vineyard on the West Side of the San Joaquin Valley of California. At fruit harvest, all above-ground plant parts were removed and analyzed for ¹⁵N. Soil around each vine was also sampled and analyzed for ¹⁵N in the inorganic and organic N fractions. Spatial patterns of fertilizer N for soil inorganic and organic N were analyzed using a median polish technique which indicated large variability with respect to direction, distance, and depth. There was a tendency for the fertilizer N from NH₄ to be located directly beneath emitters than from the NO₃. Nitrogen from the NH₄ application penetrated to only the 150-cm depth, whereas some N from the NO₃ application reached 210 to 240 cm. Most of the organic fertilizer N for both NO₃ and NH₄ applications was in the top 60 cm of soil where the vine roots were likely of greatest density. Overall recovery of fertilizer N was also quite variable, probably due to variability in soil physical properties and uneven surface application of water and fertilizer due to local surface ponding. Although not statistically significant, uptake of fertilizer N by above-ground plant components was slightly higher for the NH₄ application (24.2% of applied N) than the NO₃ application (21.5%). Soil organic N had significantly (95% level) higher N from NH₄ (19% of applied N) than from NO₃ (13%). This probably occurred due to longer residence time of N from NH₄ within the top 60 cm, where the bulk of roots and microbial activity existed, than for NO₃. Overall, about 67% to 79% of the fertilizer N applied in spring remained in the soil at harvest, and the vines took up the rest. There was no indication of significant N leaching below 2.4 m or denitrification of fertilizer N for the trickle-irrigated vines during the growing season.

Implications of Spatial Variability of Soil Physical Properties for Simulation of Evaporation at the Field Scale

Soil water retention curves in eight soil profiles on a sandy soil were determined by field measurements of soil water content and soil water matric potential made during 19 days of continuous drying using time‐domain‐reflectometry and tensiometers. Evaporation from the different profiles was simulated using the retention curves and estimated unsaturated conductivity functions, meteorological data as driving variables, and measured soil water matric potentials at 40 cm depths as the lower boundary condition. Good agreement was obtained between simulated and measured evaporation from different locations in the field. Areal mean evaporation obtained as the arithmetic mean of individual simulations was 2% smaller than the evaporation obtained using mean physical properties in one simulation. This difference was small compared to the variation in simulated evaporation between individual locations (62–142% of the meanevaporation).

Influence of rainfall on properties of soils developed on granite in Portugal

A synthesis of the physical, chemical, and mineralogical properties of soils developed on granitic rocks under different climatic conditions in Portugal was performed, including soil classification. Mean annual rainfall, the climate parameter best correlated with the variability of soil properties, was analyzed in terms of relationships with several soil properties (organic C content, soil pH, nature of the exchange complex, SiO2/Al2O3 ratio, selective extractions of Fe and Al, P retention, content and nature of clay fraction). Organic matter was the soil property best correlated with the variability of soil physical properties (porosity, bulk density, and available water), exchange capacity, exchangeable acidity, and extractable Al and Fe. In wetter areas, amorphous Al and P retention occurred in large amounts and were well correlated with each other. The effect of rainfall was especially marked for the range 500–1,200 mm per year. The amounts of rainfall corresponding to 800 and 1,200 mm appeared to sep...

Short-term variations of soil physical properties as a function of the amounts and C/N ratio of decomposing cotton residues. II. Soil compressibility, water retention and hydraulic conductivity

There are conflicting reports on the role of soil microfauna and flora on the decomposition of incorporated crop residues and its effects on soil structure. This study examined the temporal changes in soil bulk density, compressibility and hydraulic properties as affected by the amounts of added cotton ( Gossypium hirsutum L.) residues and carbon/nitrogen (C/N) ratios. The work was carried out at constant temperature (25°C) and soil water content (0.18±0.01 g g −1). All the measured properties showed the same trend indicating a transient “flush” of activity between the first and sixth week of incubation, with a sharp maximum (or minimum) in the third week. After 6 weeks these properties again changed direction, and tended towards their initial value. The maxima (minima) are indicative of pore clogging or slime accumulation lowering the hydraulic conductivity and increasing water retention in the 20–75 cm suction, increasing compressibility and lowering tensile strength. These results are consistent with the model of Hadas et al. (1994, Soil Tillage RRes., 32: 183–198). Most of the effect dissipated by Week 6 and was followed by some recovery, which may be due to the activity of a microbial population that their acts more slowly than that responsible for the intial flush, or may have had its initiation inhibited by the first population. Differences between treatments were very minor compared to the above temporal changes found in al the treatments. The sharp changes in properties within a short period after the start of incubation may explain why they were not detected in the field by Rawitz et al. (1989, Final Rep. on BARD project 812, Hebrew Uni. Jerusalem; 1994, Soil Tillage Res., 32: 347–366).

Short-term variations of soil physical properties as a function of the amount and C/N ratio of decomposing cotton residues. I. Soil aggregation and aggregate tensile strength

Additionof plant residues to soil activates a heterogeneous microbial population which in turn affects soil physical properties. A model of residue decomposition rate that is hierarchical and sequential is proposed to explain qualitatively the changes in soil structural parameters which are the result of these decomposition processes. Cotton ( Gossypium hirsutum L.) stalks were incubated in soil and the changes in aggregate geometric mean diameter (GMD), geometric standard deviation, tensile strength and characteristic tensile strenght were determined for the following treatment combinations of residue amount added, carbon/nitrogen (C/N) rattio and time of incubation: 0, 4 and 8 Mg ha −1 of residue added; 0 and 150 kg ha −1of elemental N applied as urea; incubation periods of 1, 3, 6 and 12 weeks. The experiment contained treatments designated as: control (no additions); low residues, no N; low residues, + N; high residues, no N; high residues, + N. The C/N ratios were 12 in the control soil, and 350–400, 20–24, 350–400 and 40–50 respectively, in the admixtures. An increase in the GMD of aggregates was observed by the end of the first week after a normal amount of residue was applied. This was apparently caused by a flush of fungal activity. However, when a large amount of residue was added, an increase in GMD was observed only after the sixth week, and is attributed mainly to bacteril activity. The same patterns were observed for aggregate tensile strength. It is suggested that the size and strength of aggregates apparently caused by fungi increased during the first week due to external reinforcement by hyphaee, whereas the changes appearing only after the sixth week and attributed to bacteria are due to internal reinforcement by bacterial secretions. The temporal variations of GMD are controlled by the amount of residue added and N availability. An immediate effect on GMD and strength was observed upon addition of residues with a high C/N ratio, but this effect only persisted for a short period of 1–3 weeks. This response pattern may be developed into a short-term management option to mitigate soil crusting or sealing in a newly sown field by an anticipated irrigation or rainfall by the timely application of high C/N ratio residues. Conversely, large amounts with a moderate C/N ratio may help stabilize the GMD for longer periods. Proper management of residue amounts and C/N ratio can thus provide the farmer with some measure of control over soil structure as expressed by GMD.

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.

Corn Grain Yield and Nitrogen Utilization in Relation to Subsoiling and Nitrogen Rate on Paleudults

AbstractSubsoiling is used to loosen root‐restricting layers and increase rooting depth and water availability; however, little information is available regarding how to adjust the recommended fertilizer N rate. This study evaluated corn (Zea mays L.) grain yield response to subsoiling and N rate in 12 Typic and Arenic Paleudults from 1985 through 1987. Corn earleaf and grain N concentrations were also measured. Chisel plowing and subsoiling were factorially combined with either 0, 80, 120, 160, and 200 kg N ha−1 or 80, 120, and 160 kg N ha−1. The relative yield increase (RYI) due to subsoiling ranged from –2 to 145% during the 3‐yr period due to variations in soil physical properties and rainfall distribution during the growing seasons. Regression equations for corn yield response to fertilizer N, based on pooled data sets with a RYI < or > 15% each year, were characterized as having the same slope but only different intercepts for each site and/or tillage system. Earleaf and grain N concentrations tended to be lower with subsoiling while grain N uptake was higher at five sites with relatively high RYIs. Apparent recovery of fertilizer N was similar between tillage systems, averaging 36% for subsoiled and 33% for nonsubsoiled treatments. Results from this study indicated that to obtain optimum corn yields the recommended N rate should be the same for chisel plowed and subsoiled tillage systems for soils with root‐restrictive E horizons.

Variation in soil physical properties between and within morphologically defined series taxonomic units

Eyre, Templeton and Wakanui series are morphologically defined taxonomic units which are used to partition alluvial soil variation across the Canterbury Plains near Lincoln College in New Zealand. The wider significance of the classification criteria is here assessed by quantifying the variability of physical properties of hydraulic significance ['field-saturated' hydraulic conductivity (Kfs), moisture content and bulk density] between and within the three taxonomic units. The overall effectiveness of the morphologically based classification system in partitioning variation in soil physical properties is considered by using analysis of variance. The classification of soils according to texture and mottling criteria is generally effective in separating soils in terms of the selected soil physical properties. Although some topsoil physical properties do not differ significantly between pairs of taxonomic units, all subsoil properties are clearly distinguishable between Templeton and Wakanui series. The differences are mainly attributable to spatial changes in soil texture and pore characteristics. Different amounts of variation in physical properties, however, are still present within each taxonomic unit. The variation in physical properties amongst the combined series taxonomic units is reduced to differing extents by the classification. More than half of the variance in moisture content at both topsoil and subsoil depths amongst Templeton and Wakanui taxonomic units, for instance, is accounted for by the classification, and is thus due to differences between the two series. Little contribution is made by the classification in reducing the heterogeneity of Kfs in topsoils. The classification is particularly effective in separating Wakanui from Templeton taxonomic units in terms of subsoil Kfs, an important property controlling water movement and storage.