Search

Many coarse‐textured surface soils in the southeastern Coastal Plain are prone to develop tillage pans which restrict root proliferation and moisture utilization in subsoils. The effect of tillage treatments on soybean [Glycine max(L.) Merr.] growth and soil moisture utilization was investigated on Norfolk sandy loam (Typic Paleudult) and Wagram loamy sand (Arenic Paleudult) soils. The three tillage treatments used were: conventional moldboard plowing and disking only before planting; or plowing and asking, followed by either chisel‐plowing; or subsoiling with bedding before planting. Measurements were made of top and root dry weight, leaf area, grain yield, soil water content and soil moisture tension. Breaking the tillage pan by chisel‐plowing or subsoiling increased top growth and leaf area of soybeans at full bloom. The percentage of roots below 30 cm was increased by subsoiling and chisel‐plowing when rainfall was below normal during the growth period. A higher proportion of roots was in the top 10 cm of soil with conventional tillage than with chisel‐plowing or subsoiling. Chisel‐plowing and subsoiling increased the amount of soil moisture utilized below a depth of 30 cm. Grain yields were increased by chisel‐plowing or subsoiling in those years when rainfall was below normal during lateflowering and the beginning of pod development. Subsoiling or chisel‐plowing of soils with compacted pans will increase soybean yields during years in which below‐average precipitation occurs during late‐flowering and the beginning of pod set. Yield increases are attributed to increased root proliferation below the pan and greater utilization of subsoil moisture.

Deep tillage and irrigation are two soil management techniques that have been used separately to alleviate water shortages for crops growing on soils having root restricting tillage pans. The purpose of this study was to evaluate the effects of in‐row‐subsoiling (S), irrigation (I), and in‐row‐subsoiling plus irrigation on corn (Zea maysL.) production on Wagram soil (fine‐loamy, siliceous, thermic Arenic Paleudults) having a dense tillage pan. Main plot treatments were: CD (conventional tillage [double disking], dryland); CI (conventional tillage, irrigated when soil water pressure [SWP] at the 0.25‐m depth reached −40 kPa); SD (in‐row‐subsoiled and bedded, dryland); and SI (in‐row‐subsoiled and bedded, irrigation when SWP < −40 kPa at 0.30‐m depth). Total growing season precipitation in 1979, although poorly distributed, was near normal; 1980 and 1981 were both drought years. The 3‐yr mean amounts of irrigation water applied were 179 mm for the CI and 145 mm for the SI treatments. The CI, SD, and SI treatments always had significantly higher yields of grain and stover and lower numbers of barren plants compared to treatment CD. Maximum corn height was greatest (2.68 m) for treatment SI and leaf area indices for treatments CI, SD, and SI were 23, 28, and 36% greater, respectively, than the 2.56 value for CD. Mean grain yields for treatments CI, SD, and SI were 215, 197, and 277% greater, respectively, than the 2.58 Mg ha−1CD yield. Root length and root mass in the 0.1 to 0.6‐m soil depth were greater for the dryland treatments. Apparent irrigation water use efficiencies were 0.031 and 0.014 Mg ha−1mm−1for treatments CI and SI, respectively. Estimates of subsoil water utilization using neutron attenuation and tensiometer data were 50 mm yr−1which resulted in an apparent subsoil water use efficiency of 0.10 Mg ha−1mm−1.

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−1) 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−3which 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.

Soil map units in the North Carolina Piedmont are a complex of interfluves, valley slopes and foot slopes. Specific landscape positions are commonly too small to delineate on 1:15 480 or larger scale maps and most contain areas with varying degress of erosion that cannot be separated at common map scales. Munsell color hues are a good indicator of clay content of the Ap horizon and can be used as a guide to erosion class. Within Cecil map units the clay content of the Ap horizons increases from about 10 to 30% as the Munsell hue shifts from 10YR to 5YR. Within Georgeville map units the clay content of the Ap horizons increase from about 30 to > 40% as the Munsell hues change from 7.5YR to 2.5YR. The distribution of erosion classes within a soil map unit delineation varies with landscape position. Within the Cecil map units there are about equal areas of slight and severe erosion on interfluves. Shoulders and linear valley slopes are moderately to severely eroded whereas head and foot slopes (depositional areas) are usually slightly to moderately eroded. The interrelationships between landscape position and erosion class, and among landscape positions, growing season moisture and crop yield, create difficulty for quantifying the effects of erosion on soil productivity. The close relationship between landscape position and erosion class also suggests that a large variation in the original thickness of the A plus E horizons may have contributed to present distribution of erosion classes. The variability in surface properties within a map unit is not necessarily random but is related to the range of intensities of the processes that have been operating within a local landscape.

Subsoiling and chisel plowing are deep tillage practices which have increased crop yields on soils containing tillage pans. The relationship between crop response to deep tillage practices as influenced by soil property variability is not well understood. This study was conducted to obtain information about the response of corn (Zea maysL.) to selected tillage practices as influenced by soil variability. The effects of selected tillage practices on soil physical properties and corn production were investigated during 1981 and 1982 in a field with a tillage pan. The surface textural class of the Norfolk series (fine‐loamy, siliceous, thermic Typic Paleudults) in the field ranged from loamy sand to sandy loam. Three distinct soil units were delineated on the landscape based on depth to the B horizon. Four tillage treatments were investigated: disking, bedding, subsoiling plus bedding, and chisel plowing. Soil and plant parameters monitored at various times during the study were in situ mechanical impedance, bulk density, corn height, leaf area, barren stalk population, and grain yield. Significant interactions between tillage and soil unit for grain yield indicate a variable response to tillage due to depth to the B horizon. Mechanical impedance and bulk density in the conventional disk and subsoil treatments were negatively correlated with depth to the B horizon, implying that relatively less root restriction occurs when the B horizon is deeper. Grain yield response to deep tillage (subsoiling and chisel plowing) was significant during 1981 but not in 1982; this behavior was attributed to differences in rainfall distribution between the two years. Barren stalk population was significantly decreased by deep tillage both years.

The spatial variability of Rhizobium japonicumpopulations was evaluated for two soils on the Coastal Plain of North Carolina. The Nahunta soil (fine‐silty, siliceous, thermic Arenic Paleudults) was somewhat poorly drained and had an organic matter content of 20 g kg−1whereas the Pocalla soil (loamy, siliceous, thermic, Arenic Paleudults) was well drained and had 10 g kg−1organic matter. One hundred and five soil samples from the Ap horizon were collected from an apparently uniform 18‐m by 18‐m area at each site and were subjected to analysis by the plant infectivity—most‐probable‐number (MPN) technique to determine the number of rhizobia. The log10MPN (log MPN) of R. japonicumvaried from 2.63 to 7.03 g−1soil; mean log MPN values were 4.10 and 6.07 g−1soil for Pocalla and Nahunta, respectively. Semivariograms for sample separation distances (h) < 2 m at each site were developed from soil samples collected on transects (fences). Semivariograms for h≥ 3 m were developed using data collected on a 3‐m grid. Analysis of the semivariograms constructed from the log MPN data indicated a different variance structure at each site. Moreover, the variance structure for log MPN was found to be directional, i.e., it was different in the direction “parallel to the row” compared to the direction “perpendicular to the row.” For the Pocalla soil the semivariance in the “down‐the‐row” direction was greater for values of h< 1.05 m than it was for h≥ 3 m. This apparent discrepancy is thought to be due to selection of the transect from which the 0.2‐m spaced samples were collected in a region of the field which is not typical of the field as a whole. This result demonstrates the importance of using a sampling regime which is not biased toward one particular area of a field. Root densities, inherent soil properties, seasonality, and management practices are possible contributors to the subtle changes in rhizobia populations that were observed.

Mechanical impedance (MI) is an important property of root‐restricting, tillage‐induced pans common to Ultisols in the Coastal Plain of North Carolina. The relationship between MI and other soil properties is not well understood. The purposes of this study were to quantify selected physical, chemical, and mineralogical properties of tillage pans and to correlate MI to these measurements. Mechanical impedance of tillage‐induced pans at 50 sites in the Coastal Plain of North Carolina was measured with a constant rate cone penetrometer on triplicate, undisturbed soil cores. Sites were selected to give a broad range of measured MI values. Impedance at a soil water pressure (SWP) of ‐0.10 bar ranged from 3.8 to 39.4 kg/cm2; for ‐1.00 bar, 19 to 98 kg/cm2. Other measured soil properties included particle size, the particle‐size coefficient of uniformity (CU), bulk density (Db), saturated hydraulic conductivity, dense soil angle of repose (DSAR), soil water characteristic, cation exchange capacity (CEC), external and total surface area, free iron oxides, and the amounts of organic matter, Ca, Mg, P, K, Mn, gibbsite, kaolinite, and amorphous silica and alumina. A stepwise regression procedure (maximum R‐square improvement technique) was used to select the best models relating MI to the measured soil properties. For this grouping of soils, no one physical, chemical, or mineralogical parameter correlated well with MI. The best single variable model was found for Pwas the independent variable but this relationship had an R2value of only 0.15. The best three‐variable model included Pw, DSAR, and Dband had an R2= 0.67. When MI data was subjected to a log10transformation, the R2for the three‐variable model increased to 0.74.

Interpretations and inferences drawn from soil water content measurements collected in agronomic field studies are dependent upon the particular method of data analysis. The purpose of this study was to examine alternative approaches for analyzing soil water content data measured by neutron attenuation. Data from tillage studies conducted at two locations on the Atlantic Coastal Plain were used to demonstrate and compare the methods of data analysis. Analysis of variance was performed on the field‐measured data (total soil water content measured by neutron attenuation) and on either six or eight relevant soil water variables derived from them. Derived variables included (i) the net change in total water content between successive measurement dates; (ii) a transformed soil water content; and (iii) six variables derived using bulk density and 15 bar soil water contents to provide estimates of plant‐available water. The total soil water content variable has a coefficient of variation (C.V.) of about 12%, and the cost of collecting data and analyzing it is relatively low. Differences in total water content between successive dates have C.V.s generally about 100%. Requiring the measurement of the 15 bar percentage and bulk density values, which allows plant‐available water to be calculated, increases the costs of data collection. Incorporation of this information into the water variables results in data which have a C.V. of about 30%. In field experiments, the investigator should select the appropriate soil water variable only after examining the relative costs, expected C.V.s, and the anticipated use of the data.