Fundamental Soil Properties Research Articles

Synthesizing Bulk Density for Soils with Abundant Rock Fragments

AbstractBulk density is a fundamental soil property that is difficult to determine for gravelly to extremely gravelly soils because results vary significantly with sample volume. For such coarse soils, the representative volume (for whole‐soil bulk density) should be large, but guidelines for selecting an appropriate sample volume do not exist. We evaluated the representative volume for a soil with abundant rock fragments, by comparing measured properties of samples ranging in volume from 0.03 to 410 L. For whole‐soil bulk density determination, the representative volume is 4 L or larger for a soil horizon containing 34% (v/v) gravel and is between 5 and 50 L for a soil horizon containing 54% gravel. Intact samples of that size are prohibitively large, so we developed an alternative approach that starts with measurement of fine‐earth bulk density. For fine‐earth bulk density, the sample volume needed for representative results is between 0.2 and 1 L for gravelly to extremely gravelly soils. The alternative approach reliably synthesizes whole‐soil bulk density using (i) fine‐earth bulk density from modest‐sized samples, (ii) mass‐size distribution from large (>40 kg) representative disturbed samples, and (iii) rock fragment bulk densities. The mass and volume of rock fragments that should be in a sample are added to the mass and volume used to calculate fine‐earth bulk density. This method allows integration of lateral variability in the soil without the consequence of averaging properties across a large depth range.

Discrete ice lens theory for frost heave in soils

The existing segregation potential (SP) method for frost heave prediction in soils is semiempirical in nature and does not explicitly predict the relationship between heave rate, temperature gradient, and other more fundamental soil properties. The SP method assumes that the heave rate is directly related to the temperature gradient at the frost front but acknowledges that the SP parameter is dependent on pressure, suction at the frost front, cooling rate, soil type, and so forth. This paper extends and modifies an approximate analytical technique of Gilpin and accounts for the effects of distributed phase change within the freezing fringe in both the head- and mass-transfer components of the formulation. The approach requires as input a relationship between frozen hydraulic conductivity and temperature and predicts the discrete location of each ice lens within the freezing soil. The solution can be carried out quickly on a microcomputer to obtain the heave, suction at the frost front, ice lens temperature, and other results of interest with time. Furthermore, the discrete ice lens method predicts the effects of changing overburden pressure on the predicted heave rate. A method of extracting input parameters for the discrete ice lens procedure from a series of frost heave tests is proposed. The discrete ice theory has been tested and calibrated against well-documented frost heave test results in the literature, and very encouraging agreement between prediction and observation has been obtained. Key words: frost heave, discrete ice lens, segregation potential, hydraulic conductivity of frozen soil, freezing soil.

RELATIONSHIP OF SOIL DISPERSIBILITY TO INFILTRATION AND EROSION OF SOUTHEASTERN SOILS

Fifteen agricultural topsoils from Georgia were subjected to simulated rainfall at high intensity in runoff pan studies. Infiltration over time and total soil loss were not related to soil texture, but were correlated with each other. Both infiltration and soil loss were highly correlated with several measures of soil dispersibility. Dispersible clay measured after 36 h of shaking at an 8:1 water:soil ratio and a dispersible clay index weighting clay dispersion by time were significantly related to both soil loss (r = 0.88 to 0.89) and infiltration (r = -0.5 to -0.6). Dispersion expressions derived from plots of log (undispersed clay) versus time had lower correlation coefficients, as did dispersion variables that included both silt and clay. The high percentage of clay that is dispersible in these soils appears to seal water transmission pores and reduce infiltration, as observed in sodic soils. Therefore, dispersion may be a fundamental soil property to be considered in erosion prediction and control.

Close-Range Sensing of Soil Organic Matter

ABSTRACT IN the past few years the use of herbicides in crop pro-duction has increased tremendously. Using herbicides at the recommended rates is very important. Applying too much herbicide may damage crops or cause excessive residues, and it is very costly. Using too little herbicide can result in poor weed control. Researchers have found that the amount of certain soil-applied herbicides needed depends upon the organic matter content of the soil. Table 1 shows that as the percentage of organic matter increases the recommended herbicide rate also increases. Alexander (1969) reported that soil color can be used to estimate organic matter content well enough to determine suitable herbicide ap-plication rates. In fields with varying organic matter content (such fields are quite common), that content must be deter-mined at several locations in the field and the herbicide application rate must be adjusted accordingly as the sprayer traverses the field. Hence an application system that would automatically react to changes in the soil organic matter content would be useful. Kunz (1970), Philippi (1976), and Bocksnick (1977) investigated techniques for sensing soil color by the use of light reflec-tance. Their results showed promise for application to a practical sensor of soil organic-matter content that would be automated and which would be a key component of an automatic herbicide application system. In this study a method based on direct spectral absorp-tion measurements was used. The method is unique in that it provides an electronic measurement based on a fundamental soil property that correlates with organic matter content. Using this method we analyzed the requirements for the determination of organic-matter content and developed instrumentation to make the necessary measurements.

A method for soil compactibility determination

Soil compactibility, which is a valuable aid in research on soil physics and soil tillage, can easily be determined by measuring the force that is needed to compress the soil under a plunger in a cylinder. This measurement compares favourably with tests using more general states of stress. The test provides a relationship between compaction pressure p and void ratio e. This relationship is a fundamental soil property if there is no significant influence of friction between the soil and the cylinder wall. A theoretical relationship between the interfering friction and the sample dimensions is presented, together with experimental data that support the theory. In general, the basic hyperbolic shape of an experimental p- e relationship may be altered at low pressures by sample preparation procedure and at high pressures by saturation effects. After eliminating these effects the hyperbola gives a theoretical initial void ratio e i for p = 0 and a theoretical final void ratio e f for p = ∞. The pressure giving a void ratio half-way between e i to e f is called the median stress c. e i and e f are dependent on soil type and structure. The influence of moisture content is only reflected in c.

A PHOSPHATE SORPTION INDEX FOR SOILS

SummaryPhosphate sorption isotherms covering a wide concentration range (10−6 to 5 × 10−3m phosphate) were determined for 42 soil samples at 20 °C by a standardized technique. The slope of a plot of the sorption, x, against the logarithm of the equilibrium solution phosphate concentration, log c, measured at c= 10−4M, proved a suitable reference index to characterize the phosphate sorbing properties of the soils. Several single‐point methods were tested by statistical correlation against this reference index. Of these, the sorption, x, from one addition of 150 mg P/100 g soil gave r= 0.951, but r= 0.974 when the equilibrium concentration was also taken into account in the quotient x/log c. This quotient is therefore suggested as a simple yet adequate way of indicating a fundamental soil property, its phosphate sorption isotherm.

The efficiency of a rotary cultivator

The performance of a fully instrumented rotary cultivator operating under field conditions in a sandy clay loam is described. The aim of the work was to check the overall efficiency of power transmission from the tractor to the cultivator and to investigate the effect of rotor speed, depth of working and forward speed on power requirement. A set of values was obtained for the efficiency of the mechanical drive to the rotor. The theory that high rotor speeds are responsible for the high specific work requirements of rotary cultivators, compared with rotary diggers, was also examined. The experimental results apply only to conditions at the time of the experiment, no attempt being made to relate them to fundamental soil properties. They suggest that the lowest values of specific work requirement are obtained within a small rang of volumes of soil worked per rotor blade cut and that these volumes are dependent only on the combination of depth and bite length of the blade over the range of values measured.

“Single Value” Soil Properties: A Study of the Significance of Certain Soil Constants. V. On the Changes Produced in a Soil by Oven Drying (With One Text-figure.)

1. It is shown that results for the loss in weight of a soil on oven heating can be obtained to a very satisfactory degree of accuracy when a Hearson electrically controlled oven is used.2. Results obtained by heating soils to temperatures ranging from 5° to 250° give smooth curves connecting loss in weight with rise in temperature; from which it is concluded that there is no sudden alteration in the structure of a soil when it is heated to 100°, and that the airdry moisture of a soil, as determined with sufficient accuracy by the usual methods, is a convenient empirical factor, but not a representation of any fundamental soil property.3. An examination is made of the factors contributing to the observed total loss in weight when the soil is heated, and an explanation offered of the contributions made by the different types of soil water and by the soil colloids. It is found that the conclusions drawn from this discussion confirm views developed earlier with regard to the behaviour of the water in the soil, and the absence of any sharp dividing line between the different classes into which the soil water is usually divided.