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The response of upland cotton (Gossypium hirsutum L.) to legume and small grain cover crop establishment, in-season nitrogen (N) rate, and fertilizer N placement was investigated in two experiments located in coastal plain Virginia and North Carolina. The first experiment examined 1) soil compaction and cotton yield response to strip-tillage compared to no-tillage with a precision planted tillage radish and 2) the influence of legume mix, rye, and legume mix/rye combination cover crops with four in-season nitrogen (N) rates applied to cotton on cover crop biomass, cover crop nutrient uptake, soil compaction, soil N cycling, petiole nitrate-N (NO3-N) during the first week of bloom, cotton lint yield, and fiber quality parameters over two years. Legume mix cover crops resulted in greater N uptake, soil NO3-N during the growing season, and lint yields compared to LMR, rye, and fallow treatments over both study years. Soil compaction and lint yields were not significantly different between strip-tilled and no-till with tillage radish treatments in either year. Relative lint yields after LM were maximized at 93% relative yield with 110 kg N ha-1 applied in-season while relative lint yields for cotton following LM with 0 kg N ha-1 applied reached 75%, measuring at least 9% higher than cotton following other cover crop treatments. The second experiment investigated the effect of five N rates (0, 45, 90, 135, and 180 kg N ha-1) and three placement methods (broadcast, surface banded, and injected) on lint yield, petiole nitrate-N (NO3-N), lint percent turnout, and fiber quality parameters. Nitrogen rate and placement had a significant effect on lint yield but only N rate affected petiole NO3-N concentration. It was estimated that injecting fertilizer N requires an N rate of 133 kg N ha-1 to achieve 95% relative yield while surface banded fertilizer N required a rate of 128 kg N ha-1 to produce 90% relative yield. A critical petiole NO3-N concentration threshold of 5,600 mg NO3-N kg-1 was calculated to reach 92% relative yield. Other agronomic management practices such as cover crop termination timing, cover crop species blends, and number of fertilizer N applications are of interest in order to develop better recommendations and promote conservation agricultural practices in coastal plain Virginia and North Carolina. / Master of Science / Upland cotton (Gossypium hirsutum L.) response to diverse species cover crop mixes, conservation tillage method, fertilizer N rate, and fertilizer N placement at side-dress was measured in two field studies conducted on the coastal plain soil in Virginia and North Carolina from 2016-2018. The objectives of the following research were to 1) examine the influence of two conservation tillage practices and four cover crop mixes on cover crop biomass production, soil compaction, cover crop nutrient uptake, soil N cycling, petiole nitrate (NO3-N) and cotton lint yield and 2) measure cotton performance in response to five N rate and three placement application methods. Legume mix (LM) cover crops contained more N in biomass, resulting in higher soil NO3-N during the growing season and higher lint yields at harvest compared to a legume mix and rye combination (LMR), rye, and fallow treatments. Soil compaction and lint yield were not significantly different between strip-tilled and no-till/tillage radish treatments in either year. Nitrogen rate and placement had a significant effect on lint yield but only N rate affected petiole NO3-N concentration. Injection of fertilizer N required an N rate of 133 kg N ha1 to achieve 95% relative yield while surface banded fertilizer N required a rate of 128 kg N ha-1 to produce 90% relative yield. A critical petiole NO3-N concentration threshold of 5,600 mg NO3-N kg-1 was also calculated to reach 92% relative yield. Future application of these results can include investigation of optimal N source for Virginia cotton production, best N placement method for cotton grown in high residue systems, and an economic analysis to determine optimum agronomic management for Virginia coastal plain cotton production.

Flue-cured tobacco (Nicotiana tabacum L.) is an intensively cultivated crop that typically receives four to eight primary tillage passes before being transplanted on a raised row-ridge. Strip-tillage, a conservation tillage system that only requires tilling a small strip before transplanting, has been shown to be effective for tobacco producers in southside Virginia. The cost of fertilizer in recent years and the loss of applied nutrients has brought new attention to the impact of cover crops used in conservation tillage on the nitrogen fertilization of tobacco. A two-year study conducted at the Southern Piedmont Agricultural Research and Extension Center evaluated a strip-tillage production system on agronomic performance of flue-cured tobacco and evaluated the impact of cover crop management on soil nitrogen cycling and nitrogen uptake by plants. Treatments evaluated whether a wheat cover crop was broadcast or strip killed, topdressing a wheat cover crop with 0, 22, or 45 kg ha-1, and tobacco fertilization rates. Additional treatments included a soybean residue treatment, and a conventional tillage control. Topdressing wheat with nitrogen resulted in nitrogen being released late in the growing season. Whether a wheat cover crop was strip or broadcast killed had no effect on yield or cured leaf quality. Soybean residue did not provide adequate soil cover, but was shown to be a suitable ground cover option for tobacco production. Wheat not topdressed with nitrogen and tobacco receiving normal fertilization had adequate soil surface residue cover, good cured leaf quality, and yields that were comparable to those of conventional tillage. / Master of Science

The effect of tillage practices on leaching and persistence of atrazine and metolachlor was evaluated in a field study in the Coastal Plain region of Virginia. Field data were also used to validate pesticide transport models, GLEAMS and PRZM. The study was conducted on two 18x27 m plots located in a field that was in the second year of a two-year no-till wheat-beaDs-com rotation. One plot was conventionally tilled using a moldboard plow and a disk harrow before planting of com and application of chemicals. Soil samples were collected on six sampling dates during the crop growing season at 20 randomly selected locations in each plot with the 0-150 cm sampling depth divided into eight increments. Bromide concentrations were analyzed to provide an estimate of solute movement. High rainfall following chemical application led to rapid leaching of bromide, with the chemical moving faster in the no-till profile. Pesticide concentrations also showed a greater potential for leaching in the no-till plot in the early stages of the study. Chemical concentrations were higher in the no-till profile initially, and were higher in the tilled profile toward the end of the season. Atrazine dissipation was higher in the no-till plot, but there was no marked difference in metolachlor dissipation between the two tillage treatments. Over 35% of atrazine mass remained in the soil profile in both plots at the end of the crop growing season. Pesticide concentrations were found to vary largely over the two plots. The field data were used to evaluate the ability of the pesticide transport models, GLEAMS and PRZM, to represent chemical concentration distribution, depth of solute center of mass, and pesticide mass in the no-till and the conventionally-tilled root zone. The models were evaluated in three sequential steps. The fast simulation was completely uncalibrated, using best available estimates for the input parameters. For the second simulation hydrology parameters were calibrated to minimize errors in the hydrology component so as to better evaluate the prediction of pesticide behavior in soil. The third stage of the evaluation used pesticide dissipation half-life calculated from the field data. Model performance was evaluated using both objective and subjective criteria. GLEAMS and PRZM predicted pesticide concentration in soils reasonably well when run without any calibration. Bromide concentrations were predicted closer to the observed values than pesticides. Overall predictions by both models were better in the conventional tillage plot than in the no-till plot. The comparative effect of tillage on observed chemical concentrations was represented better by GLEAMS than by PRZM. The models under-predicted leaching of pesticides in the early sampling dates. Predicted pesticide mass in the root zone were reasonably close to the field measured values. Calibration of the hydrology component of the models did not improve the prediction of pesticide behavior in soils. The use of field pesticide half-life resulted in better prediction of pesticide persistence but did not improve the overall prediction of pesticide behavior in the two plots. The study identifies selection of input parameters and correct interpretation of results as important factors in the effective use of GLEAMS and PRZM as management tools. / Master of Science

The objectives of this research were: 1) to quantify the amount of supplemental nitrogen (N) to maximize organic broccoli (Brassica olearcea var. italica) on transition soils, 2) to evaluate the ability of leguminous cover crops lablab (Dolichos lablab L.), soybean (Glycine max L.), sunn hemp (Crotalria juncea L.) and a sunn hemp and cowpea mixture (Vigna sinensis Endl.) to supply N and suppress weeds and, 3) to compare the effect on N availability and broccoli yield potential of incorporating cover crops with conventional tillage (CT) or mulching cover crops with no-tillage (NT) practices. Broccoli was grown during the third year of organic transition in the spring and fall of 2006 at the Kentland Agricultural Research Farm in Blacksburg, VA. Supplemental N significantly increased broccoli yield up until 112 kg ha-1 with a quadratic correlation with leaf N. The NT treatment yielded no difference during the spring, but in the fall CT surpassed NT. On the other hand, N uptake, measured by leaf N, under NT conditions increased with supplemental N, which suggests NT has equivalent yield potential as CT when N is not limiting. Yields from leguminous residues did not differ, even though quality and quantity of cover crop biomass did. This suggests that N availability from cover crop legumes may be impacted other ecological process such as soil microbial activity. Also, cover crop residues differed in their ability to suppress weeds. The results from this study give organic growers in transition tools to maximize productivity and sustainability. / Master of Science

Field experiments were conducted in 1984 and 1985 in Fluvanna and Montgomery Counties in Virginia, to evaluate HOE-O661 (glufosinate) [ammonium-(3-amino-3-carboxypropyl )-methyl phosphinate], cyanazine (2-[[4-chloro-6-(ethylamino )-s-triazin-2-yl]amino]-2-methyl propionitrile)), glyphosate [N-(phosphonomethyl)glycine] and paraquat (1,1'-dimethyl 4,4'-bipyridinium ion) for the control of vegetation existing at the time of no-till corn establishment. These herbicides were applied alone or in combination with 2,4-D [(2,4-dichlorophenoxy)acetic acid] or dicamba (3,6-dichloro-2-methoxybenzoic acid) at three timings of application. Both locations were heavily infested with annual, biennial, and perennial broadleaf weeds, including horseweed (Conyza canadensis), buckhorn plantain (Plantago lanceolata), field bindweed (Convolvulus arvensis), curly dock (Rumex crispus) and wild mustard (Sinapis arvensis). All four non-selective herbicides provided good overall weed control. The addition of 2,4-D or dicamba generally improved the control of broadleaf weeds in all four experiments. Control of horseweed with paraquat was variable, but improved significantly when dicamba was applied in combination with paraquat. Glyphosate, cyanazine and HOE-O661 alone or in combination with 2,4-D or dicamba provided excellent horseweed control. Wild mustard control with glyphosate was dependent on the weed size and growth stage of development. Control with paraquat, cyanazine and HOE-0661 was not affected by differences in wild mustard size or growth stage. Glyphosate and HOE-0661 provided acceptable field bindweed control alone, and all herbicides gave excellent field bindweed control when applied in combination with 2,4-D or dicamba. Cyanazine and HOE-0661 showed better control of shoot growth on the more difficult-to-control weeds than paraquat and glyphosate when applied alone. With the addition of either 2,4-D or dicamba, cyanazine and HOE-0661 provided better overall weed control, in most cases, than corresponding paraquat and glyphosate combinations. Timing of application and weed size did not significantly affect herbicide activity, even at later application timings when weeds were well established and exceeded 60 cm in height. / M.S.

In full-season-soybean weed management experiments, the addition of glyphosate [N-(phosphonomethyl)glycine], paraquat (1,1'-dimethyl-4,4'-bipyridinium ion), or HOE-0661 [ammonium (3-amino-3-carboxypropyl)methylphosphinate] to chlorimuron [2-[[[[(4-chloro-6-methoxy-2-pyrimidinyl)amino] carbonyl]amino]sulfonyl] benzoic acid] plus linuron [N'- (3,4-dichlorophenyl)-N-methoxy-N-methyl urea] was required for effective weed control, especially of eastern black nightshade (Solanum ptycanthum Dun.). In double-crop experiments over 4 years including 10 experimental sites and 8 different weeds, chlorimuron plus linuron provided good control of vegetation at planting and residual weed control without glyphosate, paraquat, or HOE-0661. Cyanazine [2-[[4-chloro-6-(ethylamino)-1,3,5-triazin- 2-yl]amino]-2-methylpropanenitrile] and 2,4-D [(2,4- dichlorophenoxy)acetic acid] were the most effective herbicides for horseweed (Conyza canadensis (L.)Cronq.) control prior to establishing full-season no-till soybeans (Glycine max (L.)Merr.). In greenhouse experiments, emergence of tobacco (Nicotiana tabacum L.) seedlings was severely inhibited by all chlorimuron-containing treatments and by the highest rates of imazaquin [2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid]. Growth of tobacco transplanted into a treated potting mixture was severely reduced by all rates of chlorimuron-containing herbicides, and to a lesser extent by imazaquin. Postemergence applications of chlorimuron alone or imazaquin did not significantly affect tobacco growth. In the greenhouse, 'Vance' and 'Forrest' soybean varieties were more sensitive to chlorimuron than were 'W-20' (a sulfonylurea-resistant variety), 'Essex', or 'Hutcheson'. Herbicide injury increased with increasing soil pH. Under simulated rainfall conditions, chlorimuron movement in soil increased with increasing pH and rainfall. Soybean injury may be more directly related to chlorimuron in the soil water solution than to the amount of chlorimuron present in the soil profile. Laboratory experiments indicated that tolerance of pitted (Ipomoea lacunosa L.) and entire leaf (Ipomoea hederacea var. integriuscula Gray) morningglories to chlorimuron may be due to reduced herbicide uptake. Sensitivity of 'Vance' soybeans may be associated with the inability to rapidly metabolize chlorimuron herbicide. / Ph. D.

Experiments to evaluate the effects of tillage systems and nitrogen fertilizer rates on fresh market 'Pik Red' tomato (Lycopersicon esculentum Mill.) production were carried out at Blacksburg, Virginia (1981 and 1982) and at Carrasco and Jackson, Uruguay (1981-2). Two tillage systems were used: (a) conventional tillage (CT) - plowed, disced and rototilled, and (b) no-tillage (NT) using paraquat at 0.5 kg/ha. Four different nitrogen (N) rates were used: 40, 80 and 120 kg N/ha broadcast at transplanting and a 40/40 kg N/ha split applications - 40 kg N at transplanting and another 40 kg N after the flowers of the first cluster had set. During 1982, at Blacksburg a 0 kg N/ha rate was added. For all locations and years, water availability was equal or higher in NT plots than with CT after transplanting. The largest differences were found at the beginning of the growing season. Soil under NT tended to be cooler than under CT; these differences in soil temperature are attributed to higher moisture content with NT than CT and the shading effect of the mulch in the NT plots. Soil Nitrate (NO3) content was equal or lower with NT than with CT at Carrasco and Jackson. Absorption of NO3 by the rye cover crop in NT plots probably was a major cause of NO3 differences between tillage systems. Nitrogen, P and K content in leaf tissue were equal or higher while Ca and Mg were equal or lower in NT plants than in CT plants. Nitrogen tissue content was increased by the N rate only at Carrasco; no effect of N rate was found on absorption of other nutrients at Carrasco or Jackson. Total yield was not affected by the tillage systems except for Blacksburg in 1982 when the NT plots outyielded CT plots. Marketable yield was consistently increased by the NT system at all locations and years. Unmarketable yield was equal or higher with CT than with NT. No effect of the N rate was found for any yield or yield components except for Blacksburg in 1981 where the total average weight was affected by the N rate. Water availability rather than N supply is believed to be the limiting factor to yield production at all locations and years. Blossom-end rot was consistently reduced by the NT system at all locations and years. Improved water availability with NT than with CT was probably responsible for a more steady Ca supply to growing fruit with NT than with CT, thus reducing the incidence of the disorder. / Ph. D.

Improving soil health in agricultural soils is vital as the effects of climate change and an increasingly affluent population are putting a strain on land resources. Conservation agricultural practices such as cover cropping and conservation tillage systems (e.g., strip tillage, minimal tillage, or no tillage) are implemented to improve soil properties, but soil health outcomes in Coastal Plain cropping systems have been inconsistent and may take decades or longer to occur. Improving soil health and agricultural prosperity in the region requires: a better understanding of the rate and magnitude of response in soil properties to different types of management; and a better understanding of the interaction between management types, soil health parameters, and crop yield. In this study, four tillage systems (in order of decreasing intensity: conventional tillage > strip tillage > minimal tillage > and no tillage) and three winter cover rotations (in order of decreasing expected biomass input: high-biomass cover crop > winter cash crop > and fallow) were tested in a split plot design. For the first objective of this study, bulk density, penetration resistance (as depth to root restrictive layer), total organic carbon, carbon stocks, saturated hydraulic conductivity, and yield were measured repeatedly over a seven-year period to better understand the longitudinal response of these properties to management. Bulk density and saturated hydraulic conductivity showed a greater temporal variation in the more intensive tillage practices (strip tillage and conventional tillage) in the top 5 cm. Depth to root restrictive layer was consistently highest in the deep tillage treatments (strip tillage and minimal tillage). The change in soil organic carbon concentration between 2018 and 2022 in the top 5 cm was numerically greater in the strip tillage (0.67%) and no tillage (0.68) than the conventional tillage was 0.18%. In 2022, the carbon stocks were significantly increased in all three conservation tillage treatments compared to conventional tillage. Yield was significantly increased by winter cover in the years that a legume/non-legume cover crop mix was implemented. For the second objective of this study, the effects of 6 years of management were evaluated for total organic carbon, active carbon, short-term carbon mineralization, aggregate stability, and crop yield. These parameters were then correlated to determine drivers in soil health outcomes. Total organic carbon was increased in the strip tillage treatment (1.13% in 2021 and 1.61 % in 2022) compared to conventional tillage (0.83% in 2021 and 1.09% in 2022) in the top 5 cm. In 2022, total organic carbon under cover crop (1.51 %) and cash crop (1.46 %) treatments were also significantly higher than the fallow (1.33 %) in the top 5 cm. Active carbon results followed a similar but weaker trend in both years. Percent aggregate stability was improved by minimal tillage (47%) in 2022 compared to conventional tillage (41%). The findings from the correlation analyses suggest that total organic carbon concentration, soil texture, and tillage were all prominent drivers in improving soil health outcomes. These results demonstrate the relatively slow response of many soil physical properties to management, highlight the need for consistent management to improve properties, and emphasize the need to take measurements at different times of the year. This information can be used to develop more sustainable and resilient cropping systems in the Coastal Plain. / M.S. / Centuries of intensive farming practices have degraded agricultural soils, which contributes to the challenge of feeding a growing and increasingly affluent global population. Therefore, understanding which farming practices reverse this degradation and improve soil health is vital for regions such as the southeastern Coastal Plain, where agriculture is a prominent industry. Soil health is defined as the ability of soil to support life sustainably. Conservation agricultural practices, such as planting crops over the winter and conservation tillage systems (where at least 30% of the soil surface is left undisturbed) are implemented to improve soil properties, but soil health outcomes in Coastal Plain farms have been inconsistent and may take decades or longer to occur. Therefore, the improvement of environmental health and agricultural prosperity in this region requires a better understanding of the patterns of change in soil properties in response to different types of management; and a better understanding of the factors that most influence soil health outcomes. This study evaluated the combination of four tillage systems of differing intensities, and three winter crop types on soil health outcomes. For the first objective of this study, soil physical properties, soil carbon, and crop yield were measured repeatedly over a seven-year period to quantify how these properties respond to management. The soil physical properties showed temporary improvement by more intensive tillage but reverted in the months after tillage. The soil carbon increased from 2018 to 2022 in the three least-intensive tillage treatments. Crop yield was significantly increased in years when there was a mix of crop species planted. For the second objective of this study, the effects of management on soil carbon, soil stability, and crop yield were evaluated after six years of management. These soil health parameters were then correlated to determine the most prominent factors influencing change in soil properties. Soil carbon and soil stability were both improved in least-intensive tillage treatments. The findings from the correlation analyses suggest that soil carbon concentration, soil texture, and tillage were all prominent factors in improving soil health outcomes. These results demonstrate the relatively slow response in many soil physical properties to management, highlight the need for consistent management to improve soil health, and emphasize the importance of taking measurements at different times throughout the year. Furthermore, this is information that can be used to develop more sustainable and resilient cropping systems in the Coastal Plain.

A permeabilidade ao ar (Ka) pode ser utilizada para avaliar alterações que ocorrem na estrutura do solo em virtude de diferentes sistemas de manejo. Este estudo testou a hipótese de que o menor revolvimento do solo sob sistema plantio direto resulta na formação de poros contínuos e melhor condição de aeração no solo. O objetivo do estudo foi avaliar, por meio da Ka, porosidade de aeração (a) e índices de continuidade de poros (K1 e N), a aeração em um Latossolo Vermelho submetido ao preparo do solo convencional (PC) e plantio direto (PD). Amostras de solo indeformadas foram coletadas em cada sistema nas profundidades de 0-0,10 e 0,10-0,20 m em área experimental do Instituto Agronômico do Paraná IAPAR, no município de Ponta Grossa, Paraná. A Ka foi medida pelo método da carga decrescente em seis potenciais mátricos, -2, -6, -10, -30, -70 e -100 kPa. A macroporosidade e a Ka foi maior no PC nas duas camadas avaliadas. A continuidade de poros avaliada pela razão entre Ka e a (K1) foi similar entre os sistemas na camada superficial e foi maior para o PC em subsuperfície. O índice de continuidade N, referente ao coeficiente angular da relação log-log entre Ka e a foi diferente quando determinado para cada amostra individualmente (n=6) e quando obtido com todas as observações (n=120). Os dois métodos revelaram poros mais contínuos no PC em superfície. Na camada de 0,10-0,20 m o primeiro (n=6) não indicou diferença de continuidade de poros entre os sistemas, enquanto o segundo (n=120) resultou em uma maior continuidade de poros nesta profundidade no PD. / Air permeability (Ka) can be used for assessing soil structure changes due tillage systems. This study tested the hypothesis that the less soil disturbance under no-tillage result in continuous pores and better soil aeration conditions. The objective of this study was to evaluate a Rhodic Hapludox aeration dynamics, by measuring Ka, air-filled porosity (ea) and pore continuity indices (K1 and N), under conventional tillage (CT) and no-tillage (NT). Undisturbed soil cores were collected in each systems at depths of 0-0,10 and 0,10-0,20 m in Parana Agronomic Institute research farm - IAPAR, in Ponta Grossa, Paraná. Ka was determined using falling pressure method in six soil water potentials, -2, -6, -10, -30, -70 e -100 kPa. The soil macroporosity and Ka was higher in CT in both depths. The pore continuity assessment by ratio between Ka and ea (K1) was similar between the systems in the top layer and was higher at CT at depth of 0,10-0,20 m. The continuity index N, relative to the slope from the log-log relationship between Ka and ea was different when calculated for each core individually (n=6) or obtained including all data for each tillage (n=120). The two methods revealed a more continuous pores in CT in the top layer. At depth of 0,10-0,20 m the first (n=6) did not indicate pore continuity difference between the systems, while the second resulted in better pore continuity in NT at this depth.