D. E. Evans

Influence of Pecan Biochar on Physical Properties of a Norfolk Loamy Sand

Because the southeastern US Coastal Plain has high temperatures and abundant rainfall, its sandy soils have poor physical characteristics and low carbon (C) contents. To increase soil C, we added switchgrass (Panicum virgatum) and nonactivated recalcitrant pecan biochar. Biochar was developed by pyrolyzing ground pecan shells at 700 °C. Biochar had 88% C, 0.4% N (C:N ratio, 220:1); 58% of its C resided in polymerized aromatic ring structures. Biochar treatments were 0, 5, 10, or 20 g kg−1 of soil, which was the Ap horizon of a Norfolk loamy sand, a thermic Typic Kandiudult. Switchgrass was ground to a fine powder and added to the biochar treatments at rates of 0 or 10 g kg−1. Treatments were incubated in 750-g columns for 70 days at 10% (wt wt−1) water content. Biochar decreased soil penetration resistance; adding switchgrass also decreased it by the end of the experiment. Biochar and switchgrass affected aggregation, infiltration, and water-holding capacity; but results were mixed. Although the nonactivated biochar (and switchgrass) improved some soil physical characteristics, other biochar formulations may have more of an effect on soil properties.

Site-specific modeling of corn yield in the SE coastal plain

Abstract When site-specific agriculture became technologically feasible, existing crop models made computer simulation a natural choice for predicting yield under various combinations of soil, weather, and management. However, modeling for site-specific farming may require both greater accuracy and sensitivity to more parameters than current models allow. The objective of this paper was to evaluate the DSSAT V3.5 corn model, CERES-Maize, for sensitivity to parameters important to site-specific farming. The model was unexpectedly insensitive to inputs for soil type, depth to clay, nitrogen, and plant population, suggesting areas for attention. Although it was appropriately sensitive to rainfall, indicating sensitivity to soil water content is generally correct, there are known problems with the curve number procedure that calculates runoff. The runoff routine needs improvement, and a separate routine may be needed to accommodate within-field redistribution of runoff. The model also responded to maximum air temperature, but since crop temperature varies more than air temperature, perhaps crop temperature should be calculated from air temperature and water stress. Model accuracy issues aside, accommodating spatial inputs and model runs requires enhanced interfaces. These and other suggested enhancements to the model would improve its applicability for site-specific agriculture.

MODIFIED CENTER PIVOT SYSTEM FOR PRECISION MANAGEMENT OF WATER AND NUTRIENTS

Spatial yields since 1985 in a corn-wheat-soybean rotation at Florence, S.C., show little correlation of yield data with expected yields for soil map units. Research suggests that spatial yield variability for the southeastern Coastal Plain may be caused primarily by water relations. This causes difficulties in scheduling irrigation for conventional center pivot irrigation systems, which are not capable of applying variable depths of water to small areas of variation within the total system. Thus, the objectives of this work were to design and construct a site-specific center pivot irrigation system that could independently apply variable rates of water and chemicals to 100-m2 areas within the irrigation system. A commercial center pivot system was modified by adding three 9.1-m manifolds in each of 13 segments along the truss. Nozzles were spaced 1.5 m apart along each manifold, and both manifolds and nozzles were sized to provide 1x, 2x, and 4x nominal application rate at a given tower speed. All combinations of the three manifolds provided up to 7x nominal depth, which was 12.7 mm, in 1.8-mm increments when the outer tower traveled at 50% of full speed. A programmable, computer-controlled management system was installed near the pivot on the moving portion of the center pivot system. This controller obtained the position from the center pivot controller via a radio frequency modem and switched on the appropriate valves to obtain the application rate for a specific area. During 1995 and 1996, the system applied water and N fertilizer in a fixed-boundary field experiment. Measurements and observations of water and N application uniformities were acceptable; however, more extensive evaluation will be required before definitive conclusions can be reached regarding N application. Surface temperatures measured with an integral infrared thermometer system produced encouraging results that may be useful in management of water and nutrients. Using experience gained with this system, a second commercial center pivot system is being modified for site-specific water, nutrient, and pesticide management on a field with soil variation (irregular boundaries) typical of the Coastal Plain.

SPATIAL VARIATION OF CORN RESPONSE TO IRRIGATION

Lack of basic knowledge about spatially varying crop response to irrigation hinders optimal irrigation management and economic analysis for site–specific agriculture. The objectives of this research were to measure the mean response of corn to irrigation amounts on 12 soil map units and compare variation in the response within and among soil map units. This experiment was implemented from 1999 through 2001 with a center–pivot irrigation machine that had been modified to enable site–specific irrigation on small plots within a representative, highly variable Coastal Plain field. Four irrigation treatments (0%, 50%, 100%, and 150% of a base rate designed to hold soil water constant) and two N treatments (135 and 225 kg/ha, the recommended rainfed and irrigated rates) were imposed in 2 U 4 factorial randomized complete blocks on eight soil map units, plus randomized incomplete blocks on four additional map units. The water treatments had consistently significant main effects in the analysis of variance (ANOVA) in both linear and quadratic forms at the 1% level, and the variation within soil map units was significant at the 5% level in the latter two years and at the 1% level in 1999. Variation in yield among soil map units at any point on the response curves approximated 25% of the maximum yield in all three years. Variation in mean irrigation amounts to produce maximum yield in the eight most common map units was 61%, 61%, and 120% of the base rate amount in the three years. These data, the first such known for any soil, crop, or location in the world, have significant implications for the design, management, and economic profitability of irrigation on spatially varying soils. While the obvious design and management change would be from whole–field to site–specific approaches, even under whole–field situations, designers should consider more strongly the management zone size, range of application rates, and need for documentation.

Corn canopy temperatures measured with a moving infrared thermometer array

Measurement of water stress and scheduling of irrigation are both enabled by non–contact infrared thermometers (IRTs). Technological advances have miniaturized IRTs and reduced power requirements so that inexpensive self–powered units are now commercially available. The objective of this work was to test a linear array of IRT sensors mounted on a center–pivot irrigation machine, and to use this IRT array to examine spatial variation in water stress of corn under four irrigation treatments imposed on a highly variable field with a center pivot equipped for site–specific irrigation and agrochemical application. An array of 26 IRTs was mounted on the pivot, which was run dry for a full circle on 7 days during the 1999 corn growing season. Procedures were developed to adjust for time lag during the 3.5–hr measurement period. Significant differences were obtained among the varying water treatments, as expected, but also among plots within the same soil map unit and among soil map unit means. Distinct spatial patterns, not necessarily related to the 1:1200–scale soil map, were observed. These results emphasize the necessity to consider soil water relations during the development of management recommendations for site–specific agriculture.

Vapor pressure deficit calculations and their effect on the combination equation

Abstract Of the several models used to calculate potential evapotranspiration (PET), many researchers use the combination method because of its theoretical basis. This model can be affected by random errors in the input parameters (net radiation, air temperature, wind speed, and daily average vapor pressure deficit, ▿) and sensitivity analyses have described the impact of these errors. However, a more subtle non-random error may be introduced in PET estimates by changing the form by which the ▿ term is specified. At least 12 different ways to present ▿ have been published; the primary differences among them are the measured humidity parameter and the algebra used to compute ▿. The effect of all applicable published computational methods on monthly and seasonal PET values for a range of locations differing in evaporative demand was examined in this study. Related methods of computing ▿ resulted in little difference between PET values. The range of summer PET means obtained from the extreme methods was 8–17% of the best estimate method over all locations. Although this range approximates the expected accuracy of the combination method, it must be stressed that the net effect of the systematic and random errors may constitute a bias and, therefore, should be evaluated as such. Apparently innocuous computational differences can significantly affect PET results and, therefore, degrade confidence in the resulting values.

Corn Yield Response to Nitrogen Fertilizer and Irrigation in the Southeastern Coastal Plain

Availability of spatially-indexed data and crop yield maps has caused increased interest in site-specific management of crop inputs, especially water and fertilizer. As commercial equipment to implement site-specific applications of water and nutrients becomes available, crop response to variable inputs and decision support systems will be required to ensure profitable crop production while conserving natural resources and protecting the environment. The objective of this research was to determine corn yield response to a range of nitrogen fertilizer and irrigation amounts on a relatively uniform southeastern Coastal Plain soil under conservation tillage. Corn was grown in a field experiment using a center pivot irrigation system that had been modified to make site-specific applications of water and fertilizer during the period 1999-2001 on a site near Florence, South Carolina. Treatments included three antecedent crop rotations (prior four years), three irrigation regimes (0, 75%, and 150% of a base rate, IBR), and four nitrogen fertilizer amounts (50%, 75%, 100%, and 125% of a base rate, NBR), and with four replications. As expected, corn grain yields increased with irrigation and N fertilizer. Mean corn grain yields for the three-year study ranged from 6.3 to 8.9 Mg/ha for the 0% IBR treatment, 9.4 to 10.5Mg/ha for the 75% IBR treatment, and 10.0 to 10.6 Mg/ha for the 150% IBR treatment. The mean corn grain yields in response to N applications ranged from 6.4 to 8.0 Mg/ha for the 50% NBR treatment, 8.6 to 9.4 Mg/ha for the 75% NBR treatment, 9.1 to 10.9 Mg/ha for the 100% NBR treatment, and 8.8 to 11.7 for the 125% NBR treatment. However, the nature of the response varied among the three years, mainly because of differences in rainfall and rainfall distribution during the growing season. Also, during the first year, there was less response to N fertilizer (7.9 to 9.1 Mg/ha) possibly because of residual soil N from antecedent soybean crop. A regression analysis indicated that the slopes of the corn yield response to increased N fertilizer application were low for both irrigated and rainfed treatments in 1999. In both 2000 and 2001, the slopes were greater for the corn yield response to increased N fertilizer. In 2000, the irrigated treatments had a greater slope of the yield response for additional N fertilizer than did the rainfed treatments. Using an orthogonal contrast analysis, the overall yield response for the combined irrigation treatments to N fertilizer was quadratic in 1999 and 2000, and linear in 2001. These quadratic yield responses indicated that, for these conditions, a potential upper limit on production for the applied N-fertilizer and water (rainfall and irrigation) was approached. For the rainfed treatment, yield response to N fertilizer was linear in all three years. These results provide useful information that should be helpful in developing management strategies and decision support systems for profitable management of both water and N fertilizer on spatially-variable soils in the southeastern Coastal Plain while conserving natural resources and protecting the environment.

Water Flow Rates from a Site-Specific Irrigation System

Site-specific irrigation is defined as delivering different prescribed depths of water to specific areas in irrigated fields. Since the 1990s, site-specific irrigation research has been expanded to include the delivery of water and nutrients to specific field areas based on soil type, soil moisture status, crop needs, and other user-defined objectives. A site-specific center pivot irrigation system was designed and installed in a field with highly variable soils of the U.S. eastern coastal plain. The system consisted of 13 segments along the 140-m length of the three-tower center pivot with three delivery manifolds in each segment. The system was designed to apply approximately 12.5 mm of water in any selected segment when operated at 50% travel velocity. Quantifying water application depth and uniformity from the site-specific irrigation system is essential to documenting the system’s performance and interpreting experimental results. We developed a measurement system to evaluate the water delivery rates of the irrigation system. We compared the measured water delivery from each segment of the site-specific irrigation system to the design parameters. We found that the irrigation system was delivering water to the control areas at rates approximately as it was designed. A total of 77 segment and manifold combinations were tested. Of these 77 combinations, we found that 7 had flow rates greater than 10% different from the design flows. The manifolds with the lower flow rates typically were more likely to differ significantly from their design values. This was most likely related to potential clogging of the low flow nozzles that have smaller orifices. When the manifolds were used in combination, they compensated for each other and produced application depths near the design depths.

VARIABLE-RATE, DIGITALLY CONTROLLED METERING DEVICE

During the past decade, there has been increasing interest in applying water and chemicals to crops based on need or yield potential rather than applying uniformly to the entire field. While ground-driven variable-rate chemical application equipment is now being used, most irrigation systems continue to apply nominally uniform water depths. Our objective was to make variable-rate irrigation applications possible by developing a digitally controlled metering device. The device consists of a reservoir that is alternately filled and emptied at a rate determined by a digital pulse from an external source and requires pressurized sources of water and air. The flow rate can be altered by changing the cycle duration and frequency, by changing air and water pressure, or by exchanging the reservoir with one of different volume. Tests with prototypes indicate reproducible flow rates for a range of operating pressures and discharge cycle durations. Various sprinklers or nozzles may be attached to the outlet if specific distribution patterns are desired. Additionally, the metering device can be used in a wide variety of applications with a variety of fluids or gases for variable-rate flow or injection of a fluid into either another fluid or gas.

Irrigation and Nitrogen Impact on Bermudagrass Yield Response in the Southeastern Coastal Plain

In the southeastern region of the U.S., the cattle industry has a critical need for sustainable hay production. Yet this production is threatened by frequent short-term regional drought. This drought threat can be mitigated by properly managed irrigation. In this study on Tifton 85 bermudagrass, irrigation management, nitrogen fertility levels, and harvest interval were evaluated for their impact on hay quality and yield. The experimental treatments were arrayed in a split-plot design with harvest interval as the main treatment; irrigation by nitrogen (N) levels were the subplots. Treatments had four replicates and were repeated for two years. The optimal irrigation rate was set to maintain soil water potentials below -30 kPa. When needed, the full irrigation treatment received a 12.5 mm irrigation application. The reduced irrigation treatments received water at rates of 0%, 33%, and 66% of the full irrigation rate. In addition, each irrigation treatment had nitrogen rates of 168, 336, and 504 kg N ha-1. The irrigation and nitrogen treatments were harvested at four-week or eight-week intervals. Total harvests per year ranged from three to six. Over both years and for all harvests, there was no irrigation-nitrogen interaction for hay yield. Over all harvests, nitrogen significantly increased bermudagrass hay yield, nutrient concentrations, and forage quality. Forage quality was higher for the four-week harvest interval. Throughout the study, forage quality was maintained within desired industry standards. When irrigation was required, it significantly increased hay yield. During these periods, the four-week and eight-week 100% irrigation treatments yielded 612 and 1600 kg ha-1 greater, respectively, than the non-irrigated treatments. The four-week harvest interval was more sensitive to irrigation. Additionally, we observed a linear relationship between non-irrigated bermudagrass hay yields and average soil water potential. As soil water was depleted, non-irrigated hay yields decreased 31 kg ha-1 per kPa. Timely supplemental irrigation to maintain soil water potentials above -30 kPa can increase bermudagrass yields. Thus, irrigation management should be critically assessed for its potential role in sustaining hay production in the southeastern Coastal Plain.