Showing papers in "Soil Science Society of America Journal in 1985"

Temporal Stability of Spatially Measured Soil Water Probability Density Function

Abstract: Soil water data collected from three different fields are analyzed by two techniques (temporal analysis of the differences between individual and spatial average values; and Spearman's rank correlation) to search if time-invariant characteristic statistical properties of the probability density functioins can be assigned to individual locations. A grass field was equipped with 17 neutron access tubes and surveyed 24 times during a 2 1/2 yr-period. In another field planted with olive trees, nine neutron access tubes were installed and quarterly measurements were performed during two consecutive years. The latter field cropped in wheat was gravimetrically sampled on a regular spatial pattern five different times and was routinely surveyed during a 1-yr period at four selected locations by using a neutron moisture meter. All data show the existence of a very significant time-stability of particular individual locations characterized by the same parameter in the statistical distribution of the observations taken over the field. It is shown that some locations conserve the property to represent the mean and extreme values of the field water content at any time along the year. This stability seems to be explained to a large extent by relationships between soil texture and water content. Additional Index Words: spatial variability, time series analysis, Spearman's test, scaling factor, data reduction, representative sites of measurements. View complete article To view this complete article, insert Disc 4 then click button8

Direct Measurement of Oxygen Profiles and Denitrification Rates in Soil Aggregates

Abstract: An oxygen microelectrode was modified to measure O concentrations in wet aggregates of a silt loam soil. The microelectrode tip had an O-permeable membrane opening 3 µm in diameter, and O measurements could be made in as little as 0.1-mm increments to a depth of 12 mm. When aggregates were incubated in air, steep O gradients usually occurred over very small distances from the aggregate surface. The smallest aggregate exhibiting an anaerobic center had a radius of 4 mm, although small aggregates (radius ≤ 6 mm) were generally oxic. Larger aggregates (radius ≥ 10 mm) often had measureable anaerobic centers, with the exception of those from a native prairie soil which exhibited irregular O profiles and had aerobic centers, apparently due to O intrusion caused by old root channels. Oxygen profiles obtained in 45 degree increments around an aggregate circumference were used to construct contour maps of O concentrations within the aggregate. Oxygen gradients were somewhat asymmetric, suggesting nonuniformly distributed sites of O consumption. An average intra-aggregate O diffusion coefficient of 8.5 × 10 cm · s was measured for water-saturated aggregates. The radii of anaerobic centers within several aggregates, measured directly with the electrode, correlated with those calculated from a model of radial diffusion using measured respiration rates and the intra-aggregate O diffusion coefficient. Anaerobic centers were present in all aggregates that denitrified, but not all aggregates with anaerobic zones denitrified. The denitrification rate did not correlate with the size of the anaerobic zone, indicating that factors other than anaerobic volume contributed to the observed rates.

Measurement of Soil Water Content using Time-domain Reflectrometry (TDR): A Field Evaluation1

Abstract: 3,331240 7/1967 Nilsson et al., 3,481,188 12/1969 Mori. 3,712,121 1/1973 Fletcher et al. . 3,797,301 3/1974 Hawes ......................................... 73/84 3,906,781 9/1975 Vlasblom .................................... 73/84 3,999,424 12/1976 Pelissier. 4,059,008 11/1977 Torstensson ................................ 73/84 4,061,021 12/1977 Baldwin et al. . 4,726,239 2/1988 Boggess et al. ............................ 73/84 4,929,885 5/1990 Dishman. 5,010,776 4/1991 Lucero et al.. 5,067,346 11/1991 Field. 5246,862 9/1993 Grey et al., 5,313,825 5/1994 Webster et al.. 5,316,950 5/1994 Apitz et al.. III US005663649A

Depth of Surface Soil-runoff Interaction as Affected by Rainfall, Soil Slope, and Management

Abstract: The effective depth of interaction (EDI) between surface soil and runoff (the thickness of surface soil in which the degree of interaction is equal to that at the soil surface) was determined for five soils of varying physical and chemical properties under simulated rainfall, in order to quantify the effect of rainfall and soil characteristics on EDI. For all soils EDI increased (1.30–37.43 mm) with an increase in rainfall intensity (50–160 mm h⁻¹) and soil slope (2–20%), although the magnitude differed between soils. The effect of rainfall intensity was attributed to increased runoff energy enhancing mixing in the surface soil and was also a function of soil aggregation. The magnitude of the EDI increase with increasing soil slope was independent of soil type being a function of runoff energy alone. An avg 73% reduction in EDI following the incorporation of 100 kg wheat straw (Triticum aestivum L. sp.) ha⁻¹ and 80% reduction with a 0.5-mm² mesh screen, simulating crop cover, was obtained compared to the control (3.36 mm). For all soils the logarithm (ln) of soil loss was linearly related to the ln EDI. This is to be expected since factors affecting EDI (rainfall intensity, runoff energy, and soil aggregation) also influence soil loss. Regression slope of the logarithmic relationship was similar (at the 5.0% level) for all soils, and regression intercept was related to soil aggregation. Thus, EDI and the effect of rainfall and soil management can be estimated from soil loss. This relationship will improve the prediction of adsorbed chemical (P and pesticides) transport in solution, since chemical transport models presently use a fixed EDI value.

Determining soil hydraulic properties from one-step outflow experiments by parameter estimation. I: Theory and numerical studies

Abstract: The numerical feasibility of determining water retention and hydraulic conductivity functions simultaneously from one-step pressure outflow experiments on soil cores by a parameter estimation method is evaluated. Soil hydraulic properties are assumed to be represented by van Genuchten's closed-form expressions involving three unknown parameters: residual moisture content 6, and coefficients a and n. These parameters are evaluated by nonlinear least-squares fitting of predicted to observed cumulative outflow with time. Numerical experiments were performed for two hypothetical soils to evaluate limitations of the method imposed by constraints of uniqueness and sensitivity to error. Results indicate that an accurate solution of the parameter identification problem may be obtained when (i) input data include cumulative outflow volumes with time corresponding to at least half of the final outflow and additionally the final outflow volume; (ii) final cumulative outflow corresponds to a sufficiently large fraction (e.g., >0.5) of the total water between saturated and residual water contents; (iii) experimental error in outflow measurements is low; and (iv) initial parameter estimates are reasonably close to their true values. Additional Index Words: unsaturated hydraulic conductivity measurement, water retention measurement, transient flow, inverse prob-

Temporal Response of Soil Denitrification Rates to Rainfall and Irrigation

Abstract: The response of soil denitrification to increased soil moisture was compared in a non-aggregated sandy loam soil and an aggregated clay loam soil using a soil core technique and the acetylene inhibition method. Elevated field denitrification rates were observed on 9 of 11 occasions on three sites following irrigation or rainfall of > 1 cm water. The denitrification rate in the sandy loam soil increased immediately after water addition and reached a maximum rate within 3-5 h and returned to preirrigation levels within 12 h. A similar, but slower denitrification response occurred in the clay loam soil, requiring 8-12 h before a maximum rate was observed and 48 h before the original background rate was restored. Maximum denitrification rates of 209 and 383 ng N g~' d~' occurred following water inputs of 7 and 2 cm in the sandy loam and clay loam soils, respectively. These water additions resulted in air-filled porosities of 0.37 m m~ in the two soils. Nitrogen losses from the clay loam soil were double that of the sandy loam although the sandy loam received almost twice the water input. This difference was apparently due to the longer duration of the enhanced denitrification rate in the clay loam soil following the increase in the soil moisture. In the two soils 38 and 55% of the total N loss in late spring occurred within 48 h after rainfalls greater than 1 cm. These studies confirm that significant denitrification losses can occur in bursts in response to rainfall, and illustrate that sampling schemes based on integration of denitrification rate measurements must include these episodes to obtain meaningful estimates of N loss. A denitrification response to 1 Contribution from the Dep. of Crop & Soil Sciences and of Microbiology & Public Health, Michigan State Univ., East Lansing, MI 48824-1114. Published as Journal Article no. 11212 of the Michigan Agric. Exp. Stn. This work was supported by National Science Foundation Grant DEB-80-12168, USDA Cooperative Agreement no. 58-32U4-1-329 and USDA Regional Research Project NE-39. Received 4 Nov. 1983. Approved 6 Aug. 1984. 2 Former Graduate Student, Research Associate, and Professor of Soil Microbiology, respectively. Present addresses: A.J.S., Division of Plant and Soil Sciences, West Virginia Univ., Morgantpwn, WV 26506 and T.B.P., Soil Nitrogen & Environmental Chemistry Laboratory, Beltsville Agric. Res. Center, Beltsville, MD 20705. rainfall was not always the case, however, suggesting that NO3 or carbon may also limit nitrogen loss. Additional Index Words: acetylene inhibition method, air-filled porosity, soil cores, soil moisture, nitrogen loss. Sexstone, A. J., T. B. Parkin, and J. M. Tiedje. 1985. Temporal response of soil denitrification rates to rainfall and irrigation. Soil Sci. Soc. Am. J. 49:99-103. T RATE OF NITROGEN lOSS from Soil due tO denitrification is increased by factors that increase the extent of anaerobic sites in soil (Firestone, 1982). Increasing soil moisture acts as one such factor by decreasing the rate of oxygen diffusion through the soil matrix, allowing the development of anaerobic microsites as oxygen consumption exceeds the rate of diffusive supply. Several laboratory studies have demonstrated increased nitrogen loss with increasing moisture content from soils incubated in the presence of an aerobic soil atmosphere (Nommik, 1956; Pilot and Patrick, 1972; Bailey and Beauchamp, 1973). These studies have generally involved the use of disturbed soils and long-term incubations of several weeks. Smith and Tiedje (1979) suggested that it was important to understand the short-term denitrification response to increasing soil moisture, in order to better characterize denitrification N losses from field soils following rainfall or irrigation. They demonstrated that when the soil moisture content of a sandy loam soil was increased from 51% to 94% of saturation the denitrification rate increased rapidly after a lag period of 6 h. McGarity and Rajartnam (1973) recognized that the denitrification rate in field soils would be controlled 100 SOIL SCI. SOC. AM. J., VOL. 49, 1985 by epidsodic factors such as increasing soil moisture, and suggested that techniques were necessary for accurate short-term measurements that preserved the physical control of biological rates imposed by soil structure. Of the techniques subsequently developed for measurement of denitrification in the field, the acetylene inhibition technique (Yoshinari et al., 1976) appears to be the most suitable for short-term measurements. Artificially irrigated soil cores have been used to compare denitrification as affected by tillage practice on several field sites (Rice and Smith, 1982). No-till soils showed significantly greater nitrogen flux than conventionally tilled soils, which was attributed to the higher soil moisture content maintained in notill soils. Aulakh et al., (1982) made once-weekly measurements at various field sites using a soil core technique and observed that the denitrification rate was greatly elevated when rainfall had decreased the soil air-filled porosity (AFP). Ryden et al., (1979), Ryden and Lund (1980), and Rolston et al. (1982) used a soil cover technique to measure denitrification and reported that the N2O flux was greatest immediately following irrigation. The studies all show the expected relationship between moisture and denitrification, but they do not characterize and quantify the temporal response of denitrification rate to rainfall, which can be an important factor in determining the quantity of N lost. In this paper, we report on the short-term response of field denitrification rates to rainfall and irrigation. Peak denitrification rates usually occurred for brief periods after a moisture addition, but the response time and duration were different for light and heavy textured soil. MATERIALS AND METHODS

Phosphorus Cycling in Unfertilized and Fertilized Agricultural Soils

Abstract: Surface soil samples (0–50 mm depth) were taken from several grassed and cropped, unfertilized and P-fertilized soils at monthly intervals for 2 yr, to investigate seasonal variations in amounts and forms of P and relative importance of inorganic and organic P as sources of plant available P. Although no consistent seasonal variation in inorganic P content was observed for the unfertilized soils, amounts increased after fertilizer P addition. Organic P content was higher in the winter (Oct.–Mar.) than spring months (May–June) for both unfertilized and fertilized soils. Consequently, mineralization of organic P during the growing season, which contributed similar amounts of P (20–74 kg P ha) as added in fertilizer (13–100 kgP ha), was not inhibited by fertilizer P addition. Organic P variation was mainly due to changes in moderately labile organic P, with more labile and resistant pools remaining constant. The most labile organic P pool was maintained at a constant level possibly by mineralization and formation from moderately labile organic P. Little change in P content of unfertilized subsurface soil (50–150 mm) was observed, although inorganic and available P contents increased slightly following fertilizer P application. Available P (Bray-I P) was closely correlated with organic P in unfertilized soils and with inorganic P in fertilized soils. Slopes of these relationships were related to phosphatase enzyme activity and P sorption maximum for organic and inorganic P, respectively. The importance of organic P as a source of available P in both unfertilized and fertilized soils was demonstrated. The need to include organic P in soil-P fertility tests, especially with the increasing use of reduced-tillage practices, is emphasized.

Field calibration and validation of solute transport models for the unsaturated zone

Abstract: Data from two solute transport field experiments, one obtained from soil solution samplers and the other from soil coring, are used to illustrate the parameter estimation problem for two models describing area-averaged vertical transport of mobile solutes. The two models, the lognormal transfer function model (TFM) and the convection dispersion equation (CDE), were selected because of their simplicity and because they represent different hypotheses for the governing mechanisms determining longitudinal solute spreading on the field scale. The two parameters of each model are estimated using three calibration procedures: sum of squares optimization, method of moments, and maximum likelihood. The three calibration procedures yield different estimates for the parameters using a given data set, and different trends in parameter variation at different depths or times in a given experiment. Calculated uncertainty in the parameter estimates was high enough that no conclusion could be drawn about which model better described the data from the solution sampler experiment, but the TFM provided a better description of the coring experimental data after calibration than the CDE. It was concluded that future experiments on field scale solute transport through the unsaturated zone will have to monitor movement below 5 m in order to yield a data set suitable for distinguishing between different models.

Estimating Soil Water Characteristics from Simpler Properties or Limited Data1

Abstract: Broad-based regression equations of Rawls and associates were investigated for estimating the spatially variable soil water contentmatric potential relationships in a 1.6-ha watershed (Udertic Paleustolls) from soil textural and structural properties, with and without one or two known values of the relationships. Also examined were a simple log-log line based on two known values and estimates obtained from one known value for each relationship and a complete relationship for one case using the similar-media scaling concept. The results were compared with measurements on 189 soil cores representing different sites and horizons. With the equations based on soil texture, bulk density and organic matter content, the soil water contents calculated at different matric potentials were generally larger than the measured values. The mean relative error ranged from 8 to 29%, with the standard deviation of errors ranging from 17 to 36%. The model which incorporated one measured soil water content (at — 1500 kPa potential) as an additional independent variable, did not improve the results much. The model which incorporated two measured soil water contents (at — 33 and — 1500 kPa potentials) as additional variables reduced the errors in calculated values considerably. A simple log-log line drawn through the two known points gave nearly the same accuracy. The estimates from the method of scaling were better than those from the model based on textural and structural variables alone; the mean relative error in the calculated water contents ranged from — 0.96 to 9.10%, and the standard deviation of errors was also reduced. Additional Index Words: soil water storage, spatial variability of soil water properties, watershed modeling. View complete article To view this complete article, insert Disc 4 then click button8

Determination of Nitrate in Soil Extracts by Dual-wavelength Ultraviolet Spectrophotometry1

Abstract: A rapid method is described for determining nitrate concentration in a soil extract solution based on its UV absorbance at 210 nm The interference of nonnitrate species is accounted for by subtracting an empirically-determined multiple of the absorbance of the extract solution at 270 nm from its absorbance at 210 nm The value of the multiplication factor, R, is calculated from the 210: 270 nm absorbance ratio of the extract solution after it has been treated with Raney-Nickel catalyst to remove nitrate The method was tested using 10 Illinois soils The composite value of R for these soils was 305 ± 022 This value, as well as each individual value of R for the respective soils, was used to calculate the nitrate concentration The results were then compared to one another and to results obtained by the steam distillation method Correlation among these various methods was very high, giving values of r of 09987 and 0999 Large errors occurred when no correction for nonnitrate species was made Additional Index Words: soil testing, Raney-Nickel NO3, analysis, UV spectroscopy View complete article To view this complete article, insert Disc 4 then click button8

Time domain reflectometry field measurements of soil water content and electrical conductivity

Abstract: Simultaneous measurements of volumetric water content and bulk electrical conductivity were made using time domain reflectometry (TDR) with a single parallel transmission line (PTL) imbedded in the soil. Sixty PTL's were installed at five depths in 12 existing lysimeter plots, irrigated with different amounts of water at two salt concentrations (1.3 and 3.1 dS/m). The water content measurements obtained with TDR showed a good relationship to gravimetric determinations (r = 0.84) and were comparable to neutron probe measurements. The TDR measurements of bulk soil electrical conductivity were similar to the measurements of the same soil physical property measured with the four-probe electrode technique. The relation between bulk soil electrical conductivity and the conductivity of the soil solution is discussed. The importance of measuring both soil water content and salinity in a nondestructive manner on the same sampling volume, enabling repeated in situ measurements is stressed. Additional Index Words: soil salinity, neutron scattering, spatial variability, soil solution conductivity, four-electrode probe. Dasberg, S., and F.N. Dalton. 1985. Time domain reflectometry field measurements of soil water content and electrical conductivity. Soil Sci. Soc. Am. J. 49:293-297. T CONVENTIONAL METHOD for measuring soil salinity is by taking soil samples and determining the electrical conductivity of the extract of a saturated soil paste (Rhpades, 1982). These measurements can be converted into soil solution salt concentration by correcting for the soil water content at the time of sampling. The soil solution can be sampled directly by porous suction cups. This method, however, is limited to a narrow range of soil moisture between (approximately) field capacity and saturation and the small sample volume tends to make the measurements variable (Broadbent, 1981). Soil water content can be measured by destructive sampling and gravimetric determination or by the in situ neutron scattering method which has the advantage of measuring soil water content on a volume basis. The limitations 1 Contribution from the U.S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501. 2 Visiting Soil Scientist and Soil Physicist. The permanent address of the first author is Institute of Soils and Water, ARO, The Volcani Center, Bet Dagan 50-250, Israel. of the neutron method are its relatively large sampling volume, the inability to measure close to the soil surface, the radiation hazard involved, and the need for individual soil calibration (Graecen, 1981). The fact that water content and salinity are usually obtained from separate samples with different geometry introduces an additional error in soil salinity assessment. Recently, Dalton et al. (1984) proposed the simultaneous measurement of both soil water content and salinity for studies on water and salt management, using time domain reflectometry (TDR). The relative dielectric constant of soil is primarily related to its water content. Measurement of the dielectric constant in the time domain, by measuring the propagation velocity of a voltage pulse was introduced by FellnerFeldegg (1969). Topp et al. (1980) showed a unique relationship between the relative dielectric constant« and the volumetric water content 6 for a large range of soils and soil-like materials. In later work, parallel transmission lines (PTL) constructed of metal rods were used to measure water content in the field with TDR (Topp et al., 1982). The relative dielectric constant t is calculated from the measured transit time / of a voltage pulse through the length C of soil material as given by the electrode length according to = [ct/29] [1] where c = velocity of light in free space (3 X 10 m/s). The empirical relationship between relative dielectric constant e and soil volumetric water content 6 was found to be (Topp et al., 1980) 0 = -5.3 X 1CT + 2.92 X I0~e 5.5 X 10-V + 4.3 X 10~ [2] It was found that this relationship was nearly independent of soil texture, soil density, temperature and salt content. It was noted, however, that soil salinity did influence the TDR signal in the soil medium (Topp et al., 1980). Dalton et al. (1984) in attempting to account for the signal attenuation which occurs in saline soils, used results from transmission line theory to calculate the electrical conductivity of moist soil. Figure 1 shows the assumed equivalent circuit of a differential length of the parallel transmission line. L and R are the inductance and series resistance of the electrodes. C and G are the equivalent shunting capacitance Published March, 1985

Effect of Liming on the Distribution of Manganese, Copper, Iron, and Zinc Among Soil Fractions

Abstract: The availability of micronutrients is affected by soil properties such as pH, organic matter content, and clay mineralogy. Some of these properties can be altered by tillage. Therefore, a study was conducted to determine the influence of tillage on the distribution of Mn, Cu, Fe, and Zn among soil fractions. Soil samples were taken at the 0 to 2 cm depth from a long-term (8 yr) tillage experiment. The treatments were no-tillage, minimum tillage (fall tillage only), and conventional tillage (spring and fall tillage). Soil samples were fractionated sequentially to determine Mn, Cu, Fe, and Zn in the following fractions: exchangeable, organic, Mn oxide, amorphous Fe oxide, crystalline Fe oxide, and residual. Double acid-extractable P and Zn, organic matter content, and cation exchange capacity were higher in no-tillage treatments than in the other tillage treatments. Tillage had more effect on the distribution of Mn and Fe among fractions than on the distribution of Zn and Cu. No-tillage lowered the Mn in the exchangeable and amorphous Fe oxide fractions and raised Mn in the organic fraction compared to minimum tillage or conventional tillage. Iron in the exchangeable and organic fractions was higher for no-tillage, while amorphous oxide Fe and residual Fe were lower for no-tillage compared to conventional tillage. Both Mn and Fe appeared to be shifted by no-tillage from the oxide and residual forms into the more plant-available exchangeable or organic forms. Additional Index Words: conservation tillage, double acid, micronutrient distribution. View complete article To view this complete article, insert Disc 4 then click button8

Boron Adsorption on Aluminum and Iron Oxide Minerals1

Abstract: Boron adsorption behavior was investigated on various crystalline and x-ray amorphous Al and Fe oxide minerals. Adsorption increased at low pH, exhibited a peak in the pH range 7 to 8, and decreased at high pH. The magnitude of B adsorption was much greater for the x-ray amorphous materials. Since B adsorbs specifically on Al and Fe oxide minerals, the constant capacitance model containing a ligand exchange mechanism was used to describe its adsorption behavior. The constant capacitance model was able to represent B adsorption on all minerals over the entire pH range studied (3-12) using the same set of surface complexation constants. With the exception of amorphous Al oxide, B adsorption on these oxide minerals could be successfully described by optimizing only the B surface complexation constant. Other nonexperimental parameters were held fixed at values identical to those previously used in modeling phosphate, silicate, and selenite adsorption on Al and Fe oxide minerals. Additional Index Words: anion adsorption, ligand exchange, surface chemistry, constant capacitance model. View complete article To view this complete article, insert Disc 4 then click button8

Soil pH Buffering Revisited

Abstract: The buffering relationships of a wide variety of Vermont soils were investigated by adding either acid (H2SO4) or base (CaCO3). Soils were found to be very well buffered above pH 7 and below pH 4. When amendment added is expressed on an organic matter (OM) basis [mmol (H or 1/2 CaCO3) g 1 OM] and the check treatment is transposed to a common curve, all soils appear to follow a similar relationship (a unified buffer curve). Thus, OM appears to strongly influence the degree of pH buffering of Vermont soils. For practical purposes, pH buffering of soils in the pH 4.5 to 6.5 range is linear with added amendment. However, the change in slope of the unified buffer curve indicates that of the entire pH range soils may be least well buffered between pH 5 and 5.6. The low degree of pH buffering in this range may be due to a high degree of cation exchange capacity (CEC) buffering. The pH vs. percent base saturation (%BS) relationship has been misinterpreted as being the same as a pH vs. lime addition titration curve. In a soil without appreciable exchangeable Al, the %BS is essentially the soil's CEC at its current pH expressed as a percent of the CEC at pH 8.2. Ranges in pH where the %BS increases only slightly may be zones of low pH buffering. Additional Index Words: soil acidity, organic matter, cation exchange capacity, lime requirement. View complete article To view this complete article, insert Disc 4 then click button8

Mathematical models for potassium release kinetics in calcareous soils

Abstract: Potassium release from the coarse medium and fine silt and the coarse and medium-fine clay fractions of six Great Plain soils was determined by successive extraction with Ca-saturated cation exchange resins. All soils contained primarily montmorillonite-mica minerals. Results indicated that 65 to 80% of the total K released in 7000 h of extraction time occurred in the clay fraction. Four mathematical models (first-order rate, parabolic diffusion, power function, and Elovich) were used to describe cumulative K release. Comparisons of coefficients of determination (r) and standard errors of the estimate (SE) indicated that the Elovich, power function, and parabolic diffusion equations adequately described cumulative K release, whereas the first-order rate equation did not. Rate constants for the three equations were highly correlated with mica content and relative alfalfa yield and K uptake. In the past, others have used complex equations containing three simultaneous first-order rate terms to describe K release; however, results reported herein show that simple one-term equations can be used. Additional Index Words: nonexchangeable K release, resin extractable K, diffusion, kinetics, Elovich equation View complete article To view this complete article, insert Disc 4 then click button8

Effect of Erosion and Landscape Position on the Productivity of Piedmont Soils

Abstract: The objective of this study was to. determine the effect of past accelerated erosion in the North Carolina Piedmont on soil physical and chemical properties and soil productivity, and to determine the relationship between soil productivity, soil erosion, and landscape position. Data from five Piedmont fields were collected over a 2-yr period. Within each field, plots were located on all erosion classes and landscape positions present. Definite relationships between erosion class and soil physical and chemical properties were observed. Clay content increased approximately 10% for each erosion class change from slight to moderate to severe. Organic matter content tended to be higher on the more eroded sites. Differences in topsoil depth among erosion classes in a given field ranged from 2.4 to 8.7 cm and the severely eroded areas were always significantly shallower. Available water-holding capacity of the A horizon (g g) always was slightly higher on the more eroded sites. Available P decreased sharply with erosion severity but all sites had levels 17 g P m 3 which is considered adequate. The highest corn grain yields were usually obtained on the moderately eroded sites as compared to the slightly and severely eroded areas. This was particularly true in 1981 which was a dry year. In a more favorable year (1982) with regard to amount and distribution of growing season rainfall, the yield differences among erosion classes were less. The differences in corn grain yield among landscape positions were much more consistent than yield differences among erosion classes. Those landscape positions that received water from higher elevations produced the highest yields. Because landscape position and erosion severity are not mutually exclusive, we believe that much of the published data dealing with the effects of erosion on soil productivity are confounded by the effect of landscape positions on soil erosion. Additional Index Words: Ap Munsell Hue, clay content, Ap depth. View complete article To view this complete article, insert Disc 4 then click button8

Effect of Raindrop Impact Energy and Water Salinity on Infiltration Rates of Sodic Soils

Abstract: The effect of water drop impact energy and water salinity on the infiltration rate (IR) of two sodic soils: Calcic Haploxeralf (loess) and Typic Chromoxerert was studied using a rainfall simulation. Rain was applied at two energy levels: high energy rain with kinetic energy of 22.9 J/mm m² and a low energy rain, with kinetic energy less than 0.01 J/mm m². In the low energy rain, the initial IR of the soils (44 mm/h) was maintained during a distilled water rain on soils with ESP 2.5, and during saline water rain on soils with ESP values of 6.5 and 17–21.0. In Haploxeralfs with ESP values of 6.5 and 21.0 the final IR (FIR) under low energy rain dropped to 28.8 and 11.5 mm/h respectively. When the same soil and ESP levels were subjected to high energy rain, the FIR during saline water storms dropped to 6–9 mm/h and during distilled water simulated storms the FIR dropped to 0.9–1.4 mm/h. It was concluded, that both the water drop impact energy and the electrolyte concentration in the applied rain have a decisive effect on the IR drop of soils. When the chemical dispersion processes are minimal (low ESP and high water salinity) the mechanical impact of the drops predominate and the crust formation is mainly due to the distintegration of the soil aggregates and their compaction to a thin skin seal. When the soil ESP is high (> 2.5) and the water salinity is low (rain water), chemical dispersion processes have an increasing role in determining the IR of the soils.

Diffusional Constraints on Denitrification in Soil

Abstract: A model for NO⁻₃ reduction and diffusion in aggregated soils is presented and used in conjunction with a derived constant, the Thiele modulus, to examine the conditions which could contribute to a NO⁻₃ diffusion limitation of denitrification. The Thiele modulus is a function of the anaerobic radius of an aggregate, the maximum rate of NO⁻₃ reduction for a given soil, the Kₘ value for NO⁻₃ reduction, and the intra-aggregate NO⁻₃ diffusion coefficient. Results from this theoretical exercise suggested that the anaerobic radius is the most important factor in determining whether denitrification is limited by NO⁻₃ diffusion. The model predicts that under anaerobic conditions, only soils with a mean aggregate radius greater than 2 mm will experience a NO⁻₃ diffusion limitation. This limitation may not be effective in practice if the bulk NO⁻₃ concentration is much greater (100 times) than the Kₘ for NO⁻₃ reduction; this appears to often be the case in fertilized soils but may not be the case in soils of natural ecosystems. Under aerobic conditions, a diffusion limitation will exist in most aggregated soils, since only large aggregates have anaerobic microsites (i.e., large anaerobic radii) where denitrification can occur. A carbon limitation, through its lowering of the maximum denitrification rate, lessens the magnitude of any existing NO⁻₃ diffusion limitation. Diffusive limitations were experimentally examined in two ways. First, the ratio of denitrification rates of anaerobic cores relative to anaerobic slurries were used to indicate the degree of any substrate supply limitation; the slurry rates were always greater suggesting either NO⁻₃ or carbon was limiting. Second, soil cores were preincubated with either NO⁻₃ or a diffusible carbon source (succinate) at 4°C to allow diffusion but to suppress biological responses. These results revealed that carbon, rather than NO⁻₃, was limiting denitrification rates in this clay loam soil. These experimental results were in agreement with model predictions.

Corn emergence as influenced by soil temperature, matric potential, and aggregate size distribution.

Abstract: The effects of soil physical parameters on corn (Zea mays L.) emergence were determined in a growth chamber experiment. The treatments included three different soils, four soil temperature regimes (5–15, 10–20, 15–25, and 20–30°C), four soil matric potentials (−10, −33, −100, and −500 kPa), and seven aggregate size distributions (geometric mean diameter = 0.5, 1.0, 1.9, 3.6, 5.6, 6.8, and 11.1 mm). Corn was planted at 50 mm depth in soil aggregate mixtures contained in aluminum cylinders 152 mm in height by 76 mm inside diameter. Cylinders were placed in a growth chamber with a 10°C diurnal fluctuation and time to corn emergence was determined. Based on the growth chamber study, guidelines are developed that define best seedbed conditions for corn emergence. Time to corn emergence was influenced by the independent variables in the following order: soil temperature » soil matric potential > soil aggregate size distribution. Emergence time was related to soil temperature by a power function. Corn emergence was most rapid at the warmest soil temperature (20–30°C) when soil moisture was at or above field capacity and when aggregate size distribution corresponded to a geometric mean diameter between 1.0 and 6.8 mm. Time to corn emergence was less than 21 d for all soils and matric potentials when mean soil temperature was at least 15°C. The negative effect of low soil temperatures on corn emergence was partially compensated by high soil matric potential. A corn emergence model was developed from the relationship between percent emergence and growing degree days at the seed zone depth. Relationships between percent emergence and seed zone growing degree days were approximately the same for three soils and varied with the soil matric potential. The corn emergence model indicated a requirement of 59, 62, 67, and 76 seed zone growing degree days for 75% corn emergence at soil matric potentials of −10, −33, −100 and −500 kPa, respectively. The model was tested using corn emergence data from a field study consisting of eight combinations of three tillage and three surface residue treatments. Field data on corn emergence corresponded well with the growth chamber results obtained with constant diurnal soil temperature cycles. Heat units needed for 75% corn emergence under various tillage and residue conditions at field moisture content (matric potential = 0 to − 10 kPa) corresponded to 52 seed zone growing degree days. This corn emergence model should be useful in predicting optimum planting dates based on measured or predicted historical seed zone temperatures.