Determination Of Soil Bulk Density Research Articles

Investigation of impact of soil texture, depth and gamma ray energy on the mass attenuation coefficient and determination of soil bulk density using Gamma-Ray Spectrometry

The determination of the gamma-ray attenuation coefficient is a critical parameter in the characterization of the extent and diffusion of gamma radiation within intricate materials, such as soil. Soils exhibit a complex and diverse composition, characterized by the presence of several phases, including minerals, gasses, organic matter, water, and metals. The study demonstrates the utilization of gamma-ray spectrometry to determine the mass attenuation coefficient and soil bulk density. The impact of incident gamma-ray energy, soil texture, and soil depth on the mass attenuation coefficient is also investigated. The experimental measurements were conducted utilizing a gamma spectrometer equipped with a 2″ × 2″ NaI(Tl) detector at gamma energies of 0.356, 0.662, and 1.332 MeV using 133Ba, 137Cs, and 60Co sources, respectively. The soil samples corresponding to two textures and three depths were collected from the research farms of the Department of Soil Science, Punjab Agricultural University, Ludhiana. The study revealed that the attenuation coefficient depends on the energy of incident radiation and the nature as well as the composition of the soil. The mass attenuation coefficient was observed to decrease rapidly with the increase in gamma radiation energy. Mass attenuation coefficient was found to be higher for sandy loam compared to sandy clay loam textured soil. Moreover, the attenuation coefficient was higher for the lower depths than the upper depth. The comparison between the experimental and predicted values of the mass attenuation coefficient, utilizing the XCOM-NIST framework, demonstrated a satisfactory level of concordance within a 10% margin for gamma energies measuring 0.356 and 0.662 MeV. However, the agreement was slightly less favorable, with discrepancies of less than 12% for sandy clay loam samples and less than 15% for sandy loam textured soil samples, when examining a higher gamma energy of 1.332 MeV. The measured value of mass attenuation coefficient was used to calculate soil bulk density, which agreed reasonably well with the values obtained with the conventional method. Specifically, the sandy clay loam samples displayed a range of 7–14% deviation, while the sandy loam textured soil samples exhibited a range of 14–23% deviation. The investigation can be expanded to ascertain significant soil properties, such as porosity and moisture content.

Changes in soil physical and mechanical properties under different tillage and cropping systems in alfisol soil of southwestern Nigeria

Sustainable agricultural strategies such as conservation agriculture (CA) and integrated land management are required to mitigate land degradation and food insecurity. This study was conducted to investigate the effects of different cropping systems: sole sorghum (SOR), sole cowpea (COW), sole soybean (SOY), sorghum-cowpea intercrop (SC), and sorghum-soybean intercrop (SS); and tillage practices: conventional tillage (CT), no tillage (NT), and compacted no till (NTc) on physical and mechanical properties of an Alfisol in Southwestern Nigeria. The experimental layout comprised a split plot design accommodating the 3 tillage and 5 cropping systems in a randomized complete block design with three replications. Undisturbed soil samples were collected from 0 to 15 ​cm, and 15–30 ​cm soil layers for the determination of soil bulk density (BD), total porosity (TP), and unconfined compressive strength (quf). The results showed that bulk density was lower, while total porosity was higher under intercrops than monocrops in all the tillage treatments. Conventional tillage had the least BD compared to no tillage and compacted no till plots. Soil vane shear strength (Ʈ) and unconfined compressive strength (quf) were generally lower under the intercrops than the sole sorghum plots. Averaged over the two soil depths, the mean soil quf of SS intercrop was 1.28 times lower than the mean soil quf of SOR but was 1.06 times higher than the mean soil quf of SOY. SC intercrop had a 14.20% and a 9.15% lower average soil quf than SOR and COW in 2019. Unconfined compressive strength and vanes shear strength significantly positively correlated with BD and negatively with TP, organic carbon (OC), organic matter (OM), and total nitrogen (N) in both cropping years. The research demonstrates that farming approaches that integrate soil cover preservation and minimal soil disturbance with diverse cropping systems improve soil physical and mechanical behavior.

PHYSICAL AND CHEMICAL ATTRIBUTES AND CARBON STOCKS IN A YELLOW ACRISOL IN DISTINCT USE AND MANAGEMENT SYSTEMS IN WESTERN AMAZON

Understanding and measuring the impacts of soil use and management systems on the soil physical, chemical properties and carbon stocks is fundamental for the development of sustainable systems in Western Amazônia. The present study evaluated the alterations in the soil physical, chemical properties, and carbon stocks after the substitution of a native forest (NF) into a pasture (PAST) and agroforestry system (AS) in a yellow Acrisol. In each soil use system, we opened four pits to collect soil samples with preserved structure, for the determination of soil bulk density (BD). We also collected samples with altered structure, for the determination of soil total organic carbon, pH (H 2 O), Ca 2+ , Mg 2+ , K + , P, remaining P, Al 3+ and H+Al at the 0-5, 5-10, 10-15, 15-20, 20-30, 30-40 and 40-50 cm layers. The change in the soil use system from the NF, independently on the agroecosystem (PAST or AS), increased the soil bulk density values, mainly at the 5 to 20 cm layers, with higher values in the PAST. The nutrients evaluated and the soil total organic carbon presented low concentrations, being increased by the soil use change from the NF to PAST and AS. Thus, the basic cations and the available P are below the minimum thresholds and concentrated at the soil first layers. The PAST and AS increased the concentrations of these basic cations, and the soil carbon stock was not altered when compared to the NF, however, there was an increase in the soil bulk density, this increase was more profound in the PAST. Keywords : Acre, agroforestry system, native forest, pasture, soil conservation

Simultaneous determination of soil bulk density and water content: a heat pulse‐based method

SummarySoil bulk density (ρb) and volumetric water content (θ) determine the volume fractions of soil solids, water and air, and influence mass and energy transfer in soil. It is desirable to monitor ρb and θ concurrently and non‐destructively. We present a heat pulse‐based method for simultaneous determination of ρb and θ from soil thermal properties. The method uses equations that relate ρb and θ to soil volumetric heat capacity (C) and to soil thermal conductivity (λ). We developed a three‐step procedure to calculate ρb and θ from C and λ measured by a heat pulse sensor, with soil texture and specific heat of soil solids known a priori. Laboratory evaluation of soil samples with various textures showed that the three‐step method provided reliable estimates of ρb and θ at θ values greater than the critical water content (θc) when λ started to respond notably to increases in θ. This method provides a new way to determine ρb and θ simultaneously with heat pulse sensors.Highlights We developed an approach to determine soil bulk density (ρb) and water content (θ) simultaneously with a heat pulse sensor. We estimated ρb and θ from soil thermal properties based on heat capacity and thermal conductivity models. The new approach provided reliable ρb and θ values at water contents >θc, the critical value. At θ < θc, the approach gave unstable results because soil thermal conductivity was insensitive to ρb.

Improved thermo-time domain reflectometry method for continuous in-situ determination of soil bulk density

Quantifying the dynamics of surface soil bulk density (ρb) is important for characterizing water, heat, and gas exchanges in agricultural and environmental applications. Unfortunately, very few approaches are available for continuous in-situ monitoring of ρb. The soil heat capacity-based (C-based) thermo-time domain reflectometry (thermo-TDR) approach has been used to measure ρb in-situ, but this approach gives ρb estimates with relatively large errors. In this study, we present a new soil thermal conductivity-based (λ-based) thermo-TDR approach for continuous and automatic determination of ρb variation in-situ. An error analysis, literature data, and field experiments were used to evaluate the performance of the C-based and λ-based approaches. The error analysis undertaken on hypothetical soils indicated that the new λ-based approach was less sensitive to errors in the measurement inputs than was the C-based approach when the same relative errors occurred, except on very dry soils. Thermo-TDR measurements reported in the literature on seven soils showed that the new λ-based approach provided more accurate and precise ρb estimates, with coefficient of determination (R2) of 0.70 and root mean square error (RMSE) of 0.103 Mg m−3, than did the C-based approach which gave ρb with R2 of 0.32 and RMSE of 0.178 Mg m−3. Two field experiments were conducted to test the performance of the new λ-based thermo-TDR approach for monitoring ρb dynamics. The results showed that following tillage surface ρb increased by about 35% within 40 days. The ρb obtained by the λ-based thermo-TDR approach agreed well with independent core sampling measurements, with an average RMSE of 0.122 Mg m−3. The C-based approach failed to give acceptable ρb estimates in most cases because of probe deflection and environmental factors. We conclude that the new λ-based thermo-TDR approach is a promising method for continuous in situ measurements of ρb.

Variation in soil properties under different land use types managed by smallholder farmers along the toposequence in southern Ethiopia

Understanding soil properties under different land use types along toposequences is vital in determining the types of soil management practices that could be implemented by smallholder farmers for improving the soil health as well as soil productivity. It is also important in addressing the issues of agricultural sustainability. With the objectives of investigating variation in soil properties under different land use types managed by smallholder farmers along the toposequences, three land use types: agroforestry land (AG), cultivated land (CL) and grazing land (GL) under three slope categories (upper, middle and lower slope) were selected in southern Ethiopia, Deko watershed. A total of 108 composite soil samples [3 treatments (land use types)−3 replications−3 slope categories (upper, middle and lower)−4 soil depth layers: 0–20cm, 20–40cm, 40–60cm and >60cm] were collected for laboratory analyses. In addition, undisturbed soil samples were taken using core sampler from each soil depth for the determination of soil bulk density. To infer the biological properties of the soils naked eye soil macro-fauna (NEMF) was counted. The results showed that among the soil properties sand (p<0.001), clay (p=0.002), soil bulk density (p<0.001), soil reactio (pH, p<0.001), Electrical conductivity (EC, p<0.001), Soil organic carbon (SOC %, p<0.001), Total nitrogen (TN %, p=0.001) and NEMF (p<0.001) have shown a significant variation among the land use types while sand (p=0.027), silt (p=0.001), clay (p<0.001), SOC (p<0.001) and TN (p=0.002) exhibiting significant variation among the slope categories. Similarly, variation of soil bulk density (p<0.001), soil pH (p<0.001), SOC (p<0.001), TN (p=0.002) and NNEMF (p<0.001) were also statistically significant along the soil depth. Clay was the dominant soil textural fraction showing an increasing trend along the toposequence. Agroforestry land use type has the higher SOC, TN, pH values and EC while cultivated land use type having relatively low amount of SOC, TN, pH and EC. The lower values on cultivated land use types were due to the effects of continuous tillage practices by the smallholder farmers in the area. Slope had minimum effect on soil pH, EC, soil bulk density and the number of observable macro-faunas in the area. Moreover, SOC and TN were increased down the slope on the cultivated land use types of the area. The role of erosion is magnificent in removing SOC and TN from upper slope to lower slope as the area is more vulnerable to erosion due rugged topography. Thus, applying soil and water conservation measures on cultivated land use types play a vital role in minimizing the removal of soils down the slope.

Determining Soil Bulk Density for Carbon Stock Calculations: A Systematic Method Comparison

Core Ideas Little is known about the methodological errors of bulk density quantification. Soil probes are easy to handle, but systematic methods comparisons are lacking. We compared methods among three soil ring types and three soil probe types at four sites. Accurate and effective determination of soil bulk density (BD) is needed to monitor soil organic C (SOC) stocks and SOC stock changes. However, BD measurements are often lacking in soil inventories and BD is estimated by pedotransfer functions with substantial uncertainty. In a systematic method comparison, we evaluated different methods for BD determination in the field by comparing the performance of MINI (5 cm3) and BIG (250 cm3) sample rings and of three driving hammer probes differing in diameter, material, and extraction method. Bulk density determined with 100‐cm3 sample rings was defined as the reference method (REF). All methods were tested at five depth increments in nine subplots at four sites with differing soil texture and SOC content. All methods determined BD in the depth increments with low systematic error (8% for probes and 2% for sample rings). The random error of the probe samples was, on average, 50% higher than that of the ring samples when the cores of the probes were adequately corrected for compaction or stretching. The BD was significantly overestimated (by 2%) when determined with MINI rings, and the variation in BD was not reduced with BIG sample rings rather than the smaller REF sample rings. The performance of the driving hammer technique varied widely among probe types and sites. The sheath probe had the smallest systematic error of all probes tested and is recommended for soil inventories. All methods for estimating BD had smaller errors than pedotransfer functions.

Comparison of Soil Sampling Devices for Soil Bulk Density Determination for Carbon Sequestration Monitoring

Soil bulk density is often used to adjust soil carbon (C) values for changes in soil density because of management differences in C-sequestration studies. Often hand-sampling devices used for soil fertility monitoring may be used for sampling soil to a depth of up to 30 cm. Four sampling devices of differing characteristics that can be used with hand-sampling equipment across full tillage, reduced tillage, and no-tillage to determine soil bulk density were evaluated. There were no significant differences in soil bulk density values between three cylindrical devices, which measured from 19 to 31 mm in diameter. However, a 21 mm × 76 mm rectangular device gave significantly lower bulk density values than the cylindrical devices. Calculation of soil organic carbon (SOC) mass per unit area showed similar results for the cylindrical devices with the rectangular device giving a significantly lower value than the cylindrical devices. Using a cylindrical device on a hand probe in a consistent manner will give satisfactory results when sampling surface soils.

Soil organic carbon and influencing factors in different landscapes in an arid region of northwestern China

Knowledge of the spatial pattern of soil organic carbon (SOC) and the factors influencing it in various landscapes is essential for understanding carbon cycles. An arid region with an area of 100km2 in northwestern China consisted of desert, cropland and wetland was investigated. The vertical patterns of SOC density in the three different landscapes and the horizontal distribution of SOC density in the study area were evaluated. The differences in SOC density among different landscapes and soil layers were analyzed, and the primary factors influencing SOC density were determined. The density of SOC was low and remained homogeneous in the profiles of desert soil. The vertical distributions of SOC density in cropland and wetland were well described by logarithmic functions (R2=0.97 and 0.92, respectively, P<0.001). Geostatistical analysis showed that SOC density presented moderate spatial variability and strong spatial dependence across all depths. Wetland and desert were easily recognized by the highest and lowest SOC densities in the study area, respectively. The densities of SOC in the 3-m profiles were 59.35, 149.6 and 174.4Mgha−1 for desert, cropland and wetland, respectively. The SOC in the 1–3m layer represented 67.0, 52.7 and 58.0% of the total SOC stored in the 0–3m profiles of desert, cropland and wetland, respectively. Clay and silt particles were the major determinant of SOC in the study area. The variability in SOC density explained by clay+silt content increased with depth ranging from 46.0 to 82.2% in desert and from 45.3 to 76.7% in cropland. The variability in SOC density accounted for by clay+silt content decreased from 52.2% in the 0–0.3m layer to 43.3% in the 0–1m layer of wetland. The remaining SOC density variability could be attributed to factors not included in this study, such as geography, vegetation and the degree of erosion. Errors in the measurement of SOC concentration and the distribution of soil-particle size, however, may introduce uncertainty in the determination of soil bulk density and thus the estimation of SOC density. The concentration of SOC in the 0–0.3m layer increased by 196.3% after the reclamation of native desert less than 40years ago and decreased by 5.3% after the cultivation of wetland as cropland for less than 30years. Short-term cultivation is insufficient to significantly alter SOC concentration in the deeper layers of desert and wetland soils. The results of this study may be of further use in optimizing strategies for the protection of wetland, ecological restoration of desertified land and the sustainable management of cropland in arid regions of northwestern China.

Comparison of soil carbon stocks in Scottish soils between 1978 and 2009

SummaryThe change in soil carbon (C) stock over a 19–31‐year period (mean 25 years) has been measured at 179 sites on a 20‐km grid across Scotland. Sampling was by horizon from a profile pit. Although soil bulk density determinations were absent at the first sampling time, we used bulk density values from the second sampling time calibrated against NIR spectra to predict the missing values. There was no detectable change in overall total soil C stock (mean ± standard error, to a depth of 100 cm), which was 266 ± 15 and 270 ± 15 t C ha−1 for the first and second sampling times, respectively, or generally in C stock within specific vegetation or soil types. The exception was for soils under woodland, excluding those on deep peat, which exhibited a significant (P = 0.05) gain of 1.0 t C ha−1 year−1. Soils under woodland (mainly coniferous plantation) also showed a significant (P = 0.04) increase in C content (g kg−1), a significant decrease in bulk density (P = 0.006) and an increase in the thickness of the Litter‐Fermentation‐Humus (LFH) layer (P = 0.06). Recalculating the C stock to a depth of 15 cm showed a significant increase in overall C stock (when deep peat sites were excluded) as well as specifically in moorland and woodland soils, suggesting that had we sampled only to 15 cm, we would have reached a different conclusion. Both improved grassland soils and those initially under arable cultivation showed a significant decrease in C content. However, the mean thickness of Ap horizons increased from 29 to 32 cm, with a concomitant decrease in C content and a slight increase in bulk density; this we ascribe to deeper ploughing between the sample periods. In the context of possible soil C losses, we can be 95% confident that the mean loss does not exceed 0.2% year−1 and 99% confident that it does not exceed 0.4% year−1.

Dependência espacial dos atributos físicos de três classes de solos cultivados com cana-de-açúcar sob colheita mecanizada

Objetivou-se, neste trabalho, avaliar as propriedades físicas de três classes de solo cultivadas com cana-de-açúcar em sistema de colheita mecanizada na região de Mato Grosso do Sul. O local de estudo pertence à Usina Eldorado, Grupo ETH Bioenergia S/A, localizada no município de Rio Brilhante, MS. A amostragem em cada área experimental foi realizada a partir de uma malha, com 180 m de comprimento e 145,6 m de largura, totalizando 90 pontos em forma de grade, com 9 colunas e 10 linhas, onde foram coletadas amostras de estrutura indeformada nas camadas de 0-0,2 m e 0,2-0,4 m de profundidade nas entrelinhas da cultura. Após coleta, as amostras foram processadas para determinação da densidade do solo (DS), porosidade total (PT) e umidade do solo (q) para interpretação dos dados. A palhada deixada sobre superfície auxiliou na mantença de bom nível de matéria orgânica, corroborando na formação, agregação e estabilidade de poros, impedindo o aumento da densidade. A redução da porosidade total e consecutivo aumento na densidade do solo, principalmente na profundidade de 0,2-0,4 m, indicam compactação nesta camada, justificada pelo intenso tráfego de veículos e ausência de revolvimento do solo.

Illinois Soil Nitrogen Test Predicts Southeastern U.S. Corn Economic Optimum Nitrogen Rates

An accurate and quick soil N test is needed for N fertilizer recommendations for corn (Zea mays L.) for the humid southeastern USA. The Illinois soil N test (ISNT) has been used to distinguish fertilizer-responsive from unresponsive sites in Illinois. We determined relationships between economic optimum N rates (EONR) and ISNT levels in representative southeastern soils in 35 N-response trials in the Piedmont (n = 4) and Middle (n = 8) and Lower (n = 23) Coastal Plains of North Carolina from 2001 to 2004. The ISNT was strongly correlated with EONR for well or poorly drained sites (r2 = 0.87 [n = 20] and 0.78 [n = 10], respectively); data were insufficient for establishing correlations for very poorly drained or severely drought-stressed sites. Expressing ISNT on a mass per unit volume basis vs. EONR improved the correlations slightly (r2 = 0.88 and 0.79 for well and poorly drained sites, respectively), but these improvements would not justify the necessary soil bulk density determinations. Regressions of ISNT vs. minimum, average, and maximum EONR based on different N-fertilizer cost /corn price ratios (11.4:1, 7.6:1, and 5:1, respectively) showed strong correlations with EONR for well-drained sites (r2 = 0.77, 0.87, and 0.87, respectively) and poorly drained sites (r2 = 0.84, 0.78, 0.70, respectively). The ISNT–EONR correlations were different among the cost/price ratios for well-drained sites, but not different for poorly drained sites. Because ISNT predicted EONR robustly to different cost/price ratios, ISNT has the potential to modify or replace current N recommendation methods for corn.

Sampling equipament for soil bulk density determination tested in a Kandiudalfic Eutrudox and a Typic Hapludox

In spatial variability studies of soil physical properties the influence of different samplers on the results is seldomly taken into account. The objective of this work was to evaluate the performance of five different types of sampling equipment for soil bulk density determination, in two different soils a Kandiudalfic Eutrudox and a Typic Hapludox, both of Piracicaba, SP, Brazil. Equipment used for soil undisturbed sampling were: (i) Uhland soil sampler; (ii) Kopecky's Ring; (iii) soil core sampler, model "Soil Moisture"; (iv) soil core sampler Bravifer AI-50 and (v) soil core sampler Bravifer AI-100. The sampling was made in 4 grids of 1m², each with 25 sampling points, with five replications, resulting 100 samples for each soil. It was concluded that the sampling techniques can influence soil bulk density distributions, mainly in the case of clayey soils (Kandiudalfic Eutrudox). The Kopecky's Ring presented larger soil density values, overestimating this property for the two soils studied. The soil core sampler Bravifer AI-50 presented results closest to the overall average of the Typic Hapludox.

An Experiment to Demonstrate Density and Water-Holding Capacity of Organic Matter

An experiment is described that allows students to estimate particle and bulk density and water-holding capacity of organic matter. Using compressed disks that are readily available from nurseries, the particle density of organic matter can be approximated; and, after water absorption, the bulk density and water-holding capacity can be calculated. How students conducting this experiment can increase their understanding of the physical properties of organic matter is discussed in the paper. T HE physical properties of soil organic matter is an important topic in an introductory soil science course. The addition of organic matter to soils can increase their waterholding capacity and lower their bulk density. Determination of soil bulk density can be achieved in an introductory soil science lab using a core sampler in a straightforward manner (Blake and Hartge, 1986). However, organic matter does not lend itself to easy manipulation for bulk density calculations. To quantitatively appreciate the water-holding capacity of soil, students commonly calculate gravimetric water content. Gravimetric water content is commonly 1.0 Mg m -3 for organic soils. Mineral soils are readily available for lab experiments, but organic soils (Histosols) are more difficult to obtain in most parts of the world. Particle density of mineral soils is commonly determined through displacement in a water medium. Calculation of the particle density of organic matter is less common due to the difficulty of separating out the soil organic matter fraction and determining the volume of the extracted fraction. Organic matter disks are used as a medium for producing seedlings from seed. These disks are commonly composed of compressed sphaghnum peat moss. Upon the addition of water, these disks swell, predominately in the vertical dimension, to approximately 6 to 7 times their original height. A simple experiment using compressed organic matter disks was designed to help students: (i) appreciate the contribution of organic matter to the water-holding capacity and bulk density of soils, and (ii) estimate particle density of organic matter.

Comparison of clod and core methods for determination of soil bulk density

Abstract An interagency forest monitoring program has been initiated on a systematic network of forested plots often accessible only by foot traffic along a compass line. Extensive site classification and physiochemistry data are being collected for vegetation and soil indicators of forest health. In order to select a suitable technique for obtaining an estimate of mineral soil bulk density, conventional clod and core methods were compared across a wide range of forest soils within the Coastal Plain of southeastern Virginia. Replicate soil clods and cores were collected from two to four horizons within each of six pedons representing different soil series. Following analysis, the sample data were used to evaluate main and interaction effects due to differences in the method, series, horizon, and texture effects. Differences between the clod and core data were highly significant on average (P < 0.001) as were all of the main effects. Both methods exhibited high precision (average CV < 5%) within the indivi...

Measurement of Soil Bulk Density and Moisture

ABSTRACT BULK density and moisture content of soil influence the soil conditions created by tillage, vehicle trafficability, soil compaction caused by vehicles, water infiltration and drainage, and crop growth. Measurement and description of soil conditions are necessary for correct management decisions to create soil conditions that will maximize short- and long-term crop production. Techniques for measurement of soil bulk density and soil moisture are reviewed and advantages and disadvantages of each method discussed. Although the physical properties of the surface layer of soil are very important, properties in that layer are the most difficult to measure. There are no quick, easy, and accurate methods for determination of soil bulk density or soil moisture content. However, there are several methods for each that may be used successfully.

Characterizing Soil Deformation by Direct Measurement Within the Profile

ABSTRACT Aunique feature of the University of Kentucky soil bin enables deformation to be characterized by studying a cross-sectional soil profile grid pattern. Modular sections of the bin are laterally removed to expose the cross-section after passes of a pneumatic tire. The measured displacements of the grid points were converted to values of volumetric strain and then compared to soil density as measured by a dual probe gamma-ray density gauge following tests at various soil conditions. Final soil bulk density determinations using the two methods were not statistically different..

A Rapid, Low‐Cost Method for Determination of Soil Bulk Density

AbstractProper characterization of soil bulk density is essential for an accurate interpretation of chemical and microbiological parameters in the field. Reluctance to report soil analyses on a volumetric basis is usually related to requirements of additional time and specialized equipment for determination of soil bulk density. We developed a rapid method utilizing a low‐cost hand sampler (&lt; $125) for determination of soil bulk density over several intervals to a depth of 300 mm. The method appears accurate, using five cores per experimental unit, and enables collection of up to ten profiles in the same time required for taking one profile by the Uhland sampling techniques. The method is very precise and coefficients of variation over a range of tillage‐management practices, which include inherent soil variability, averaged below 5%. Soil samples collected with this method can also be used for other chemical and microbiological analyses.

Passive microwave sensing of soil moisture content: The effects of soil bulk density and surface roughness

Microwave radiometric measurements over bare fields of different surface roughness were made at frequencies of 1.4 GHz, 5 GHz, and 10.7 GHz to study the frequency dependence, as well as the possible time variation, of surface roughness. An increase in surface roughness was found to increase the brightness temperature af soils and reduce the slope of regression between brightness temperature and soil moisture content. The frequency dependence of the surface roughness effect was relatively weak when compared with that of the vegetation effect. Radiometric time-series observations over a given field indicate that field surface roughness might gradually diminish with time, especially after a rainfall or irrigation. The variation of surface roughness increases the uncertainty of remote soil moisture estimates by microwave radiometry. Three years of radiometric measurements over a test site revealed a possible inconsistency in the soil bulk density determination, which is an important factor in the interpretation of radiometric data.

Calibration and Field Testing of a Two‐probe Gamma Gauge

AbstractA series of laboratory and field calibration tests was carried out with a commercial double‐probe gamma gauge in an attempt to resolve difficulties encountered in routine field use of the equipment. The laboratory tests showed that transmitted count rate is affected by the extent of the medium surrounding the beam path, indicating that linearity between log of count rate and sample density is not sufficient proof of monochromatic radiation. The equipment was also found to be sufficiently sensitive to require the use of separate attenuation coefficients for soil solids and for water. A universal calibration equation for wet density can therefore not be used with this equipment. Due to the above two phenomena, the calibration stand supplied with the instrument does not yield a calibration curve adequate for accurate work in the field range of soil moisture. A valid calibration therefore requires measurements of moisture and density in a large soil volume over a range of conditions. In view of the technical difficulties in meeting these conditions in the laboratory, empirical field determination of a multipoint calibration curve appears to be the best choice. Once such a curve has been developed, simple two‐point calibration checks of unknown soils could be performed in the laboratory, provided the assumptions of linearity between log of count rate and medium density, and of the effective universality of the attenuation coefficient of soil may be taken as valid. When properly calibrated, the equipment tested is sensitive and reliable for the determination of soil bulk density in the field.