Sally D. Logsdon

Tillage and crop effects on ponded and tension infiltration rates

Abstract Tillage and residue management practices influence soil surface compaction and sealing, but the effects vary with time and weather history. The objective of this study was to compare surface ponded and tension infiltration rates for different tillages and crop rotations at various dates in a Kenyon loam. Four tillage systems (minimum tillage (no-till system with two cultivations), chisel, moldboard plow, and ridge-till) and two rotations (continuous corn and soybean-corn rotation with corn in 1991) were examined. Ponded and tension infiltration rates were measured in the row at four dates: 18 June 1991, 9 July 1991, 23 September 1991, and 11 May 1992. Soil was collected for measurement of aggregate stability and undisturbed soil cores were collected for measurements of saturated hydraulic conductivity and bulk density on the first measurement date to relate the infiltration measurements. Minimum tillage had significantly faster ponded infiltration than chisel or moldboard, as well as greater aggregate stability and less bulk density at that date because of reduced surface sealing. Tillage and crop rotation effects on infiltration for the other measurement dates were inconsistent. When the measurement dates were compared, ponded and tension infiltration rates for the first measurement date were less than for measurements at later dates because of a surface seal which was not present at the later dates. Temporal changes in infiltration were greater than tillage or rotation differences. To quantify management effects on surface soil properties we conclude that several well documented measurements are required. If possible, the measurements should be taken soon after rainfall events.

Persistence of subsoil compaction from heavy axle loads

Abstract As the persistence of subsoil compaction has a major impact on agricultural sustainability, responses of soil pore characteristics and plant roots to subsoil compaction were examined as related to heavy axle loads. A Ves clay loam was originally compacted with three axle loads of less than 4.5 (control), 9, and 18 Mg in the fall of 1981 (soil dry (D)) and in the fall of 1982 (soil wet (W)). The 9 Mg treatment was recompacted on both trials in the spring of 1988 with an 18 Mg axle load, and maize and soybean were grown in rotation. Field measurements in 1988 and 1989 included bulk density, biopore area, root number and location, ponded and negative water pressure infiltration, saturated hydraulic conductivity ( K sat ), and ped size distribution. Bulk densities measured under the row in 1989 for the W plots were increased in the 10–28 cm layer by the new 18 Mg load, whereas the original 18 Mg treatment had higher bulk densities in the 28–68 cm depth. The control had lower bulk densities than both compacted treatments for the 18–58 cm increment in 1989. The number and areas of biopores in the D control plots were at least ten times greater than in W control plots. Compaction at 35 cm reduced K sat and ponded infiltration comapred with the control. Mean ped diameter ranged from 1.3 to almost four times greater in the compacted treatments than in the control. In spite of increased soil density, a substantial reduction in root growth was not observed. Roots grew around dense clods in the compact treatments allowing continual vertical extension. There was no evidence of poor drainage hindering crop growth in the heavy axle load treatments.

FIELD CALIBRATION OF THE THETA PROBE FOR DES MOINES LOBE SOILS

Knowledge of soil moisture is needed to understand crop water use, hydrology, and microclimate. A reliable, rapid technique is needed, and recently an impedance soil moisture probe (Theta Probe) has been accepted by the scientific community. The purposes of this study were to calibrate the probe for soils of Central Iowa through field sampling, to determine the number of samples needed for calibration, and to determine the effect of temperature on calibration. Laboratory calibration was conducted on Des Moines lobe soils across a range of water contents and temperatures. Including a temperature term increased the R 2 from 0.85 to 0.87. Field calibration was based on Theta Probe measurements on similar soils combined with gravimetric sampling and soil temperature determination. Although some scatter existed, the field calibration was adequate for Iowa soils (R 2 = 0.77). Inclusion of temperature did not significantly improve the calibration for the field data. To determine the appropriate number of samples needed for the field calibration, regression equations were determined from sample numbers ranging from 2 to 89, and the standard error was determined for each. Based on the standard error analysis, 20 samples was an adequate number, with no further improvement for additional data points.

Comparisons of Evening and Morning SMOS Passes Over the Midwest United States

This study investigates differences in the soil moisture product and brightness temperatures between 6 p.m. and 6 a.m. local solar time Soil Moisture Ocean Salinity (SMOS) passes for a region in the Midwest United States. This region has uniform land cover, consisting largely of maize and soybean row crops. The comparison was restricted to periods with no rainfall. There were 19 days available for analysis of the soil moisture product. It was found that there was a significant difference in the soil moisture product for all 19 days, with lower soil moisture for most mornings. The difference between the soil moisture products on some days exceeded the allowable error of 0.04 m3 m-3. In-situ and model results indicate that there should be virtually no change in soil moisture between the evening and morning. In order to investigate this discrepancy, measured brightness temperature was converted to a polarization index (PI), and evening and morning values were compared. Investigation of the measured brightness temperature was limited to five days where a large range in incidence angles was available. Large differences between evening and morning passes were found for incidence angles less than 40° that could not be explained with radiative transfer theory but may be attributed to technical issues. There was also a difference in the PI values between the evening and morning passes for incidence angles greater than 40°. This can be caused by a decrease in soil moisture from evening to morning or could be attributed to an increase in the volumetric water content of the vegetation.

ELECTRICAL CONDUCTIVITY SPECTRA OF SMECTITES AS INFLUENCED BY SATURATING CATION AND HUMIDITY

Electrical conductivity is an important soil property related to salinity, and is often used for delineating other soil properties. The purpose of this study was to examine the influence of smectite properties on the complex electrical conductivity spectra of hydrated smectitic clays. Four smectites were saturated with Ca, Mg, Na or K and equilibrated at four relative humidities ranging from 56 to 99%. X-ray diffraction was used to determine fractions of the various smectite layer hydrates (0 to 4 layers of interlayer water molecules) in each sample. A vector network analyzer was used to determine the real component of the complex electrical conductivity spectra (σ′) for frequencies (f) ranging from 300 kHz to 3 GHz. Values of the dc electrical conductivity(σ0), the frequency where the slope changes in the spectra (fr), and the slope at the high-frequency end of the spectra (n) were determined by fitting σ′ to σ′(f) = σ0(1 + f/fr)n. Both σ0 and fr increased with the total amount of water, the amount of interlayer water, and, for saturating cations in the order K < Mg < Ca < Na. The opposite trends were observed for n. The values of these parameters were influenced by the type of smectite, but the trends were not consistent for the effect of layer charge. The results indicate that interlayer water in smectites contributes to the electrical conductivity of hydrated smectites, and that polarization of water by local electrical fields has a substantial influence on the complex electrical conductivity spectra of smectites. The accuracy of salinity estimates for soils and sediments that are based on conductivity measurements maybe adversely affected unless the effects of hydrated clays on electrical conductivity are considered.

Effect of corn or soybean row position on soil water.

Crop plants can funnel water to the soil and increase water content more in the row relative to the interrow. Because the row intercepts more soil water after rains and higher root density, the soil may also dry out more between rains than does soil in the interrow. The objectives of this study were to determine if there is a row position difference in soil wetting after rain and drying between rains, and to determine the seasonal nature of these differences. The first experiment examined soil water content 0 to 0.06 m in row, interrow, and quarter corn row positions for eight sites at specific times during a corn (Zea mays L.)-growing season. During the growing season, the second experiment examined automated soil water measurements at one site for two corn years and one soybean (Glycine max [L.] Merr.) year at row and interrow positions to 0.15-m depth. Soil water content changes were significantly greater in the row than the interrow for some mid-season dates. Temporal soil water changes showed that row wetting and drying dominated over interrow soil water changes for mid season. The mean ratio of row/(row + interrow) soil water changes for wetting was 0.76 and 0.77 for corn and 0.64 for soybean and for drying was 0.58 and 0.84 for corn and 0.60 for soybean. Soybean showed the row effect for a shorter time of the season (up to 71 days) compared with corn (up to 159 days).

Comparison of electrical and thermal conductivities for soils from five states.

The arrangement of soil particles, particle size, mineralogy, solute concentration, and bulk density affects electrical (&sgr;) and thermal (&lgr;) conductivities, which are key properties for estimating soil physical states, subsurface water and energy balances, and land-atmosphere interactions. The purpose of this study was to compare how &sgr; and &lgr; change as a function of water content for soils under different vegetation and with different properties. Soil samples were collected from selected field sites in Idaho, Texas, Colorado, Iowa, and Ohio and packed into cylinders at a density of 1.2 Mg m−3 and then wetted to predetermined water contents (&thgr;) between 0.10 and 0.45 m3 m−3. A thermo-time domain reflectometer was used to determine &sgr; and &lgr; at each &thgr; at room temperature. Soil and vegetation-influenced differences within a state were only occasionally statistically significant; however, differences between states were highly significant for both &sgr; and &lgr;. The &lgr; decreased as the amount of sorbed water (related to soil-specific surface area) increased. The &lgr; increased more rapidly at low water contents than did &sgr;, but &sgr; increased more rapidly at high water contents. Changes in &sgr; and &lgr; with water content are related to the changing tortuosity of the solid plus liquid phases versus only the liquid phase (including sorbed water). This study contributes toward improved understanding of soil thermal and electrical conductivities over a range of soils.

Soil nutrient variability and groundwater nitrate-N in agricultural fields

Landscape may result in uneven nutrient loads within a field. The objective of this study was to determine the influence of landscape on soil carbon and nutrient levels, and on levels of nitrate-N in groundwater. Soil samples were collected in three fields, two transects each, 30 sites in each field. The soil morphology was characterized for the profile, and soil organic carbon and nutrient levels were determined for 0-0.15 and 0.15-0.3 m depths. Each field had wells installed at three of the sites. One field showed a wide range of landscape variability, and significant effects of curvature on soil carbon and nutrient levels. Another field showed no significant effect of slope or curvature on soil carbon and nutrient levels because the nutrient levels were quite variable, including high spikes. The third field had less variable landscape trends but still showed a few significant effects on soil carbon and nutrient levels. Nitrate-N levels remained high in two of the nine wells (20 to 50 mg L-1), suggesting that additions of nitrate-N at the concave or converging sites replaced any losses. Median nitrate-N levels at the other seven well sites were lower, ranging from 8 to 17 mg L-1. Influence of landscape on soil carbon and nutrients was more detectable when the landscape factors were highly variable without excessive variability in soil nutrient properties.

Howdoes dew affect L-band backscatter? analysis of pals data at the Iowa validation site and implications for smap

NASAs Soil Moisture Active Passive satellite mission will use both an L-band radiometer and radar to produce global-scale measurements of soil moisture. L-band backscatter is also sensitive to the water content of vegetation. We found that a moderate dew increased the L-band backscatter of a soybean canopy by 1 dB. Dew thus has the potential to add error to satellite observations of soil moisture.