Sharpsburg Silty Clay Loam Research Articles

Phosphorus Translocation Between Small, Non‐Reproductive Tillers and the Main Plant of Maize1

AbstractTiller growth and development are generally limited under high plant populations used in modern maize (Zea maysL.) production. The physiological relationship between the main plant and small, barren tillers has not been clarified. Two experiments were conducted to investigate the existence, direction, and relative magnitude of P translocation during different phenological intervals, and to observe the effect of tiller senescence on P translocation. A foliar application of 32P was made at each of three growth stages to the tiller or to the most recently expanded leaf on the main stalk of several maize plants grown hydroponically in the greenhouse. Plants were later separated into tiller, main plant, and roots and were assayed for radioactivity. In a field experiment, foliar applications of 32P were made to tillers of maize grown on a Sharpsburg silty clay loam (Typic Argiudoll, fine, montmorillonitic, mesic) when the main plants were in the 11‐ to 12‐ or 15‐ to 16‐ leaf stage. Aboveground tissue was removed 32 or 33 days after treatment and assayed for radioactivity. Apparent translocation of 32P from main plant to tiller was very small or nonexistent in plants grown in the glasshouse, whereas translocation from tillers to the main plant and roots was substantial. More than 24% of the radioactivity found in the whole plants at sampling had been translocated from the tiller, even though tillers accounted for <0.3% of the total dry weight at the 16‐leaf stage. In the field study, movement of isotope to developing grain from a tagged tiller depended directly on total grain weight and was greater when the tiller did not senesce before sampling. The presence of active, barren tillers may therefore be an important characteristic for reducing maize yield losses where environmental stress limits nutrient absorption and assimilation during late vegetative and early reproductive growth.

Irrigation of Soybean Genotypes During Reproductive Ontogeny. I. Agronomic Responses1

Successful irrigation management for soybeans [Glycine max (L.) Merr.] requires a knowledge of the effect of irrigations applied during specific stages of reproductive ontogeny. Eight soybean cultivars, varying in stem growth habit and maturity, were subjected to a factorial set of eight irrigation treatments involving either no irrigation or one irrigation applied at three reproductive stages: R1 to R2 flowering (F), R3 to R5 pod elongation (P), or R5 to R6 seed enlargement (S). The experiment was conducted for 3 years on a Sharpsburg silty clay loam (Typic Argiudoll). An F irrigation had little effect on ultimate seed yield (+20 kg/ha) and for some cultivars actually depressed yield below that of the nonirrigated check. A P irrigation consistently increased seed yield (+379 kg/ha), irrespective of cultivar or year. An S irrigation also enhanced seed yield (+ 384 kg/ha), but the degree of enhancement was inconsistent across years, being influenced by the amount of natural rainfall occurring during the seed enlargement stage. Although a dual F‐P irrigation treatment enhanced seed yield above that for both the nonirrigated check and single F irrigation treatment, the seed yield increase was significantly smaller (+166 kg/ha) than that achieved with a single P irrigation treatment. Soybean maturity was delayed 2 to 6 days with irrigation but the delay was greatest when irrigation was applied late in reproductive ontogeny. This senescence delay lengthened the seed‐fill period and was responsible in part for the seed yield enhancement effected by irrigation. Increases in soybean plant height and lodging were greatest with an F irrigation, intermediate with a P irrigation, and minimal with an S irrigation. Although irrigation effects on seed quality were variable depending on the cnltivar, there was an overall tendency for a P irrigation to improve and an S irrigation to worsen seed quality. An F irrigation had no effect on seed quality. Of the eight cultlvars evaluated, the determinate ‘Elf’ was the most yield responsive to irrigation and was relatively resistant to changes induced by irrigation in plant height, lodging, and seed quality. Based on these data, optimal yield enhancement in irrigated soybean culture can be achieved with the use of short‐stature or lodging‐resistant cultivars irrigated during pod elongation (R3 to R4) and seed enlargement (R5 to R6).

Influence of Water Stress on the Diurnal Exchange of Mass and Energy between the Atmosphere and a Soybean Canopy1

AbstractA micrometeorological‐physiological study was conducted at Mead, Nebr., during the summer of 1980 to examine the diurnal exchanges of mass and energy of well‐watered and water‐stressed soybean (Glycine max (L.) Merr. cv. Harosoy) canopies and to relate these exchanges to environmental and physiological variables. Data are presented for 2 clear days when the canopy was fully‐developed. Measurements of CO2, latent heat and sensible heat flux were made using the Bowen‐ratio energy balance technique. The soil of the area is a Sharpsburg silty clay loam (a fine, montmorillonitic, mesic Typic Argiudoll). Water stress greatly influenced the partitioning of available energy between latent and sensible heat flux. When the crop was well‐watered, sensible heat (H) was directed toward the crop and caused latent heat exchange (LE) to exceed net radiation (Rn). When the crop was water stressed, only two‐thirds of Rn was consumed as LE; the remainder was converted into sensible heat. Since both Rn and vapor pressure deficit were greater on the day when the crop was water‐stressed, stomatal closure appears to have been the primary cause of the reduction in LE.Carbon dioxide exchange was not sensitive to water stress in the morning but was severely limited by such stress during midday. The midday reduction in CO2 exchange appears to have been caused by a combination of high stomatal resistance limiting CO2 diffusion to the cell chloroplasts and low leaf water potential coupled with high air temperature affecting the enzymatic reactions associated with photosynthesis. Water use efficiency (defined in terms of the CO2‐water flux ratio) was greater when the crop was well‐watered than when it was stressed for water. A combination of water stress, a large vapor pressure deficit, and high air temperature reduced the CO2‐water flux ratio.

Alfalfa Establishment and Production with Continuous Alfalfa and Following Soybeans1

AbstractAlfalfa (Medicago sativa L.) is commonly grown in rotation with other crops but land can be continuously cropped with alfalfa. The purpose of this study was to determine the stand establishment and production of alfalfa cultivars that differed in disease resistance in a continuous alfalfa cropping situation, with limited supplemental irrigation. Two alfalfa cultivar experiments were spring‐seeded in 1970 at the Nebraska Agric. Exp. Stn. Mead Field Lab. on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudoll) soil with no known prior alfalfa cropping. The area was plowed in late May 1975 after the first cutting and fallowed. Soybeans (Glycine max L. Merr.) were seeded in 1976 on subareas reserved for seeding alfalfa in 1977 and 1978, and in 1977 on the subarea reserved for seeding alfalfa in 1978. Two alfalfa experiments were spring‐seeded per year in 1976, 1977, and 1978, a foliar fungicide and a fungicide seed treatment experiment. Forage yields did not differ among foliar fungicide treatments or between ‘Arc’ and ‘Ranger’ in the year of seeding or in subsequent years. Anthracnose (caused by Colletotrichum trifolii Bain) and foliar diseases were at subeconomic levels. Stands in the year of seeding did not differ among fungicide seed treatments or between ‘Agate’ and ‘Dawson’. Anthracnose and phytophthora root rot (caused by Phytophthora megasperma Drechs) were factors in stand decline and yield reduction of susceptible cultivars after the year of seeding. Stands in the year of seeding were excellent on the same land area where alfalfa was seeded in 1970 with no known previous alfalfa cropping, after plowing the 5‐year‐old stand of alfalfa in 1975 and seeding in 1976, and after 1 or 2 years of soybeans and seeding alfalfa in 1977 and 1978, respectively. Average forage yields of the experiments seeded in 1970 were 26% higher in the year of seeding, and 40 and 35% higher in the 1st and 2nd year after seeding, respectively, than in the experiments seeded in 1976 to 1978.

Leaf Pubescence Effects on the Mass and Energy Exchange Between Soybean Canopies and the Atmosphere1

AbstractMass and energy exchanges with the atmosphere were compared in two soybean (Glycine max L. Merr. cv. Harosoy) isolines differing in pubescence density. The study was conducted in a field with a Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll) during the summer of 1980 at Mead, Nehr. Mass and energy exchanges were determined by means of micrometeorological techniques. Evapotranspiration (reported in terms of latent heat flux) was reduced in the densely pubescent isoline. Canopy CO2 exchange was unchanged on a per unit land area basis. Water use efficiency (reported in terms of the CO2‐water flux ratio) was, accordingly, greater in the densely pubescent isoline. The increase in pubescence did not significantly alter the net radiation balance, turbulent mixing, canopy CO2 exchange, or plant water status. Observed differences in the partitioning of net radiation into latent and sensible heat can be explained by greater penetration of solar radiation into the densely pubescent canopy. Leaf pubescence appears to alter the spectral characteristics of the leaf and, thus, to facilitate the penetration of solar radiation into the canopy.

Weed Control in Corn Planted into Untilled Winter Wheat Stubble1

AbstractAs reduced tillage production systems expand in the Great Plains, a critical need will be non‐tillage methods for controlling established weeds prior to planting the crop. This research dealt with finding selective weed control methods for corn (Zea mays L.) in an ecofarming system consisting of a winter wheat (Triticum aestivum L.)‐corn‐fallow rotation on a Sharpsburg silty clay loam (Typic Argiudolls) at Lincoln and a Holdrege silt loam (Typic Argiustolls) at North Platte, Nebraska. Both established and subsequently emerging weed species must be selectively controlled without tillage in order to better conserve the soil, water, energy, and labor requirements in this marginal corn production area. This research has as an objective the moving of dryland corn production further west in the Great Plains region. Adequate control of annual grass weeds as a group was more difficult than control of broadleaf weeds. The more difficult to control weed species were velvetleaf (Abutilon theophrasti Medic.), fall panicum (Panicum dichotomiflorum Michx.), large crabgrass [Digitaria sanguinalis (L.) Scop.], and clammy groundcherry (Physalis heterophylla Nees). Weed yields were the lowest and corn yields were the highest on plots receiving atrazine [2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino)‐s‐triazine] at 2.2 kg/ha, cyanazine (2‐[[4‐chloro‐6‐(ethylamino)‐s‐triazin‐2‐yl]amino]‐2‐methyl‐propionitrile) at 2.8 kg/ha, or a combination of the two at 0.9 + 1.9 kg/ha. Four foliar‐active herbicide treatments—2,4‐D [(2,4‐dichlorophenoxy)acetic acid] at 1.1 kg/ha, 2,4‐D at 1.1 kg/ha + 9.4 liters/ha Sunspray 11E oil, paraquat (1,1'‐dimethyl‐4,4'‐bipyridinium ion) at 0.3 kg/ha, and glyphosate [N‐(phosphonomethyl)glycine] at 0.6 kg/ha—were equally effective for controlling established weeds and maintaining high corn yields when combined with the s‐triazine herbicides. The no‐till farmer has a number of herbicides available which will selectively destroy established and subsequently germinating weed species in corn without tillage.

Soil Physical Properties as Affected by Soybean and Other Cropping Sequences

AbstractInfiltration, water stability of soil aggregates, and water retention characteristics were measured on a Sharpsburg silty clay loam (Typic Argiudolls) in a 4‐year cropping sequence. The cropping systems include soybean (Glycine max L. Merr.), sorghum (Sorghum bicolor L. Moench), corn (Zea mays L.), and fallow, in various sequential cropping combinations.The influence of cropping systems on size distribution of water‐stable aggregates is indicated by the values of geometric mean diameter (GMD). The rank order of GMD for the soybean sequences was: soybean after fallow > soybean after sorghum > soybean after corn > continuous soybean. The low GMD of continuous soybean reflected the negative effect of soybean roots in building a stable soil structure. Cumulative infiltration after 4 hours of water application was 6, 13, 29, and 41 cm for continuous soybean, sorghum after soybean, fallow after soybean, and corn after sorghum, respectively. The low infiltration was associated with low macroporosity and decreased aggregate stability. Both Kostiakov's and Philip's equations fitted the infiltration data reasonably well statistically, but Kostiakov's equation was a better fit for the early and late stages of infiltration.

Selective Control of Sunflower in Soybeans1

AbstractSunflower (Helianthus annuus L.) is a difficult weed to control in soybeans [Glycine max (L.) Merr.]. A variety of herbicide treatments and application techniques were investigated. Greenhouse and field research was conducted during 1978 through 1980 to investigate the efficacy of using bentazon [3‐isopropyl‐1H‐2,1,3‐benzothiadiazin‐4(3H)‐one 2,2‐dioxide] and bromoxynil(3,5‐dibromo‐4‐hydroxybenzonitrile) postemergence to selectively control sunflower in soybeans. Using these herbicides in combination at lower than recommended rates reduced sunflower control costs from bentazon alone and soybean injury caused by bromoxynil. Field experiments conducted on a Sharpsburg silty clay loam (Typic Argiudoll) over a 3‐year period with metribuzin [4‐amino‐6‐tert‐butyl‐3‐(methylthio)‐as‐triazin‐5(4H‐one] applied preplant incorporated or preemergence revealed more effective sunflower control from preplant incorporated treatments. The roller applicator, a selective herbicide applicator designed to eliminate tall‐growing weed escapes, was used in field experiments for 3 years with glyphosate [N‐(phosphonomethyl)glycine], dicamba (3,6‐dichloro‐o‐anisic acid), and 2,4‐D [(2,4‐dichlorophenoxy)acetic acid]. Use of the roller applicator was found to be inadequate as a primary method of sunflower control in soybeans, although it was effective against weed escapes since treatments containing glyphosate did control sunflower.

Applied N and Moisture Level Effects on Yield, Depth of Root Activity, and Nutrient Uptake by Soybeans1

AbstractThe soybean [Glycine max (L.) Merrill] has commonly been recognized as a rather non‐responsive crop to fertilizer treatments compared with cereals, reasons for which are quite obscure. For studying the moisture and nutritional aspects that might be involved in this anomaly, a field experiment was conducted on Sharpsburg silty clay loam, a Typic Argiudoll, at three moisture levels, with three depths of 32P tagged P placement (22.5, 45, 90 cm) during 1974 and 1976. Objectives were to evaluate effects of fertilizer N, residual soil N, and moisture level on yield, zones of root activity, water use efficiency, and the total uptake of N, P, K, Ca, Mg, Fe, Cu, and Zn by the crop.Seed yield of soybean was increased 675 and 1148 kg/ha by high soil moisture levels compared with low levels in 1974 and 1976, respectively. Total dry matter, percent plant P derived from fertilizer and uptake of all nutrients were also increased with increased soil moisture level. Although placement depth of 32P‐tagged P had no effect on total dry matter accumulation and seed yield of soybean there was essentially equivalent utilization of the tagged fertilizer whether placed at 22.5, 45, or 90 cm depth evidencing the deep nutrient extraction capabilities of the soybean plant. Nitrogen fertilizer significantly increased seed yield of soybean at all moisture levels in 1974 but only at low moisture in 1976 with amounts of residual soil NO3‐N at planting explaining the difference between the 2 years. Applied N also increased removal of all other soil nutrients measured except Fe. Consumptive water use was greater the higher the rate of N applied and the higher the moisture regime.

SOIL EFFECTS ON PATULIN DISAPPEARANCE AND THE EFFECT OF AMMONIA ON PATULIN PHYTOTOXICITY

Patulin was applied to Holdrege silt loam, Sharpsburg silty clay loam, Peorian loess subsoil, and fine quartz sand in concentrations of 200, 400, and 800 micrograms per gram of soil or sand. Patulin disappearance was determined by chemical analysis and bioassay techniques using winter and spring wheat (Triticum aestivum L.). After application of 400 micrograms of patulin per gram of soil, only 27 to 69 percent of the patulin was immediately extractable using ethyl acetate. The inability to extract patulin from soil or sand was related to physical and chemical properties. Patulin seemed to be inactivated biologically and nonbiologically in the soil. After 192 hours, patulin was detected in only one of the autoclaved soils treated with 400 micrograms of patulin. Ammonia vapors detoxified patulin and produced products that could be measured by fluorescence under ultraviolet light, and this method of analysis could be used to screen routinely for patulin in samples.

Growth Stages and Distribution of Dry Matter, N, P, and K in Winter Wheat1

AbstractStage of growth descriptions were developed for winter wheat (Triticum aestivum L.) which will apply to single plants or a community of plants. The growth stages are easily discernable and precise. Accumulations of dry matter, N, P, and K in leaves, culms, head, and grain were measured at each growth stage.Observations and plant samples were taken of fieldgrown winter wheat (cultivar ‘Centurk’) over the 3‐year period of 1975‐77. Plants were grown at the Univ. of Nebraska Agronomy Farm, Lincoln, on a Sharpsburg silty clay loam (Typic argiudoll). Plants for growth stage description and chemical analysis were randomly sampled from a field population and separated into leaves, culms, head, and grain as these plant parts developed.Growth stages are defined and illustrated. The 10 developmental stages are: O—Emergence, 1—Crown development and tillering, 2—Leaf sheath elongation, 3—Culm elongation (jointing), 4—Flag leaf emergence, 5—Peduncle elongation (heading), 6—Flowering (anthesis), 7—grain filling, 8—Stiff dough, 9—Hard dough (ripening), and 10—Maturity.Dry matter accumulation increased rapidly from stage 3 through stage 8. Translocation of dry matter from leaves to grain began at stage 6 and from culms and head to grain at stage 7. Nitrogen uptake was most rapid from stage 2 to stage 4 with 80% of the total accumulation occurring by stage 7. Over 70% of the total N uptake was translocated to the grain at maturity. Uptake of P and K was most rapid from stage 3 to stage 7. Seventy‐five percent of P uptake was translocated to the grain at maturity but only 15% of K present in the plant was found in the grain at maturity.The stage descriptions provide a standard for researchers, extension workers, producers, students, and agribusiness personnel to accurately define and determine the growth stages of winter wheat. The dry matter accumulation and nutrient uptake data provides part of the information necessary in making correct management decisions.

Control of Weeds in Non‐Cultivated, Narrow‐Row Sorghum1

AbstractRecent increased awareness of our finite energy supply and concern about our diminishing farm labor supply has stimulated the reexamination of production methods of sorghum [Sorghum bicolor (L.) Moench] in the Great Plains. Objectives of this research were to increase sorghum yields with the use of narrow‐row culture and improve weed control while reducing labor, energy requirements, and production costs. Improved weed control methods in sorghum developed during the past decade have facilitated reduced tillage methods of producing sorghum. Experimental results indicate the feasibility of producing grain sorghum in narrow rows without cultivation under non‐irrigated conditions on a Sharpsburg silty clay loam (Typic Argiudoll) at Lincoln, Neb. Selective control of annual broadleaf weeds was more readily accomplished than the control of grass weeds. Early weed control was most important in eliminating sorghum losses from weeds. Sorghum kept weed‐free the first 4 weeks after planting suffered no yield loss from subsequent weed growth. Herbicide treatments applied preplant incorporated, preemergence, or early postemergence selectively controlled annual weeds and resulted in sorghum yields equivalent to those on handweeded plots. The sorghum producer has the possibility of achieving higher sorghum yields when the crop is grown in narrow rows while reducing tillage, labor, and energy requirements in his production system.

Serial Measures of Dry Matter Accumulation and Forage Quality of Leaves, Stalks, and Ears of Three Corn Hybrids1

AbstractDry matter yields and forage quality of plant components (leaf, stalk, and ear) through maturity could vary among corn (Zea mays L.) hybrids. These potential differences could affect total forage quality yield and the hybrid choice for corn silage use. To compare plant composition, total plant dry matter, and forage quality among corn hybrids from flowering to physiological maturity, we studied leaf, stalk, and ear dry matter production and forage quality of three corn hybrids with maturity. Three corn hybrids, ‘Nebr. 611’, ‘Nebr. 612’, and ‘Nebr. 712’, were grown under field conditions with optimum agronomic conditions on a Sharpsburg silty clay loam (Typic Argiudolls) soil and harvested at six kernel development stages within 9 weeks following flowering during 1973 and 1974. All forage was separated for leaf, stalk, and ear dry matter production, and analyzed for crude protein, and in vitro dry matter disappearance (IVDMD). Total plant dry matter production increased at most harvests both years for all hybrids. Hybrid 712 yielded significantly more forage than 611 and 612 for most harvests. Stalk dry matter production declined over the harvests both years while leaf production increased during 1973. Leaf production remained the same over the harvests in 1974 for hybrids 611 and 612. Ear dry matter production increased at each harvest for all hybrids both years. Leaf crude protein was twice as large as stalk crude protein and declined over all harvests for the leaf component. Percent crude protein of ears declined through the first three harvests only. No hybrid differences were detected in percent crude protein of the three yield components. Leaf IVDMD was higher than that of stalk IVDMD. Rate of IVDMD decline was greater in stalks than leaves. Leaf IVDMD was consistently higher in 712 than 611 or 612. Little change occurred in ear IVDMD over the six harvests, however, the values were significantly greater in 1974 than 1973. The greatest plant protein and IVDMD yield occurred at the last harvest, physiological maturity, both years for all hybrids. Variation was detected among hybrids for total plant dry matter and plant component yields but less variation was detected among hybrids in forage quality for each plant component.

Control of Weeds in Narrow‐Row Soybeans1

AbstractImproved weed control systems in the absence of row cultivation were researched in order to take advantage of higher yields of soybeans [Glycine max (L.) Merr.] grown in narrow rows. Weed control systems, utilizing 13 pre‐emergence and two postemergence herbicides or herbicide combinations and the rotary hoe, were devised for the production of narrow‐row soybeans at two locations in eastern Nebraska during 1972–74. Experiments were conducted on a Sharpsburg silty clay loam (Typic Argiudoll) at Lincoln and on a Judson clay loam (Cumulic Hapludoll) at Concord, Neb. Research was undertaken to reduce labor, energy, and production costs while attempting to achieve better weed control, earlier ground cover, and higher soybean yields. Selective control of grass weeds in soybeans was more readily accomplished than the control of such large‐seeded broadleaf weeds as Pennsylvania smartweed (Polygonum pensylvanicum L.), sunflower (Helianthus annuus L.), and valvetleaf (Abutilon theophrasti Medic.). Reduced application rates of grass herbicides were used successfully in narrow‐row soybeans. Weeds were controlled in soybeans, with a number of herbicide treatments, with only minor early crop injury and no stand reduction. Average weed yields from all herbicide treatments were markedly lower than those on weedy check plots, but were not significantly different from the hand‐weeded plots. All herbicide treatments except one produced average soybean yields that were equivalent to those on the hand‐weeded plots. Available weed control systems can eliminate the need for cultivation in narrow‐row soybean production. Such systems of weed control should increase soybean yields, reduce production costs, require less labor and fuel, and improve ground cover needed to protect our soil from wind and water erosion.

Effects of Beef Cattle Manure on Water‐stability of Soil Aggregates

AbstractA field experiment was established to determine the effect of applying varying large amounts of manure on (i) the size distribution of water‐stable aggregates and (ii) their resistance to dispersion during prolonged periods of shaking in water. It was hypothesized that the formation of highly water‐stable aggregates as a result of organic matter amendments to the soil took a relatively long period of time to occur.Beef cattle manure was applied at rates of 0, 180, and 360 metric tons/ha for 3 consecutive years to a Sharpsburg silty clay loam soil in eastern Nebraska. One year after the third manure application, soil samples were collected from the 0 to 10 cm depth of soil and air dried. Aggregates of 4,760–9,440 µm in diameter were wetted under vacuum and then shaken in water for 2, 20, and 200 min. The size distribution of aggregates in water was measured using pipette, elutriators, and sieves.Increased rates of manure application increased the percentage of water‐stable aggregates >295 µm in diameter and decreased the percentage in size groups 18.5–295 µm and <18.5 µm in diameter after 2‐ and 20‐min shaking periods. Distribution of aggregates present after 200 minutes of shaking was not affected by levels of manure application to the field plots.

Longevity of shattercane seed in soil across Nebraska*

Summary:Longevity of shattercane [Sorghum bicolor (L.) Moench] seed buried 22 cm deep in soil at three locations across Nebraska, U.S.A. was followed for 14 years. Shattercane seed lost viability more quickly when buried in a Keith very fine sandy loam in western Nebraska (Alliance, average seed germination 21%) than in a Holdrege silt loam in central (North Platte, 37% germination) or in a Sharpsburg silty clay loam in eastern Nebraska (Lincoln, 38% germination) which were similar. The maximum survival of the open‐panicle shattercane biotype, with glumes tightly enclosing the seeds, used in this study was 11, 12, and 13 years at Alliance, North Platte, and Lincoln, Nebraska, respectively.

Physical and Chemical Properties of Soil Associated with Heavy Applications of Manure from Cattle Feedlots

AbstractCattle feedlot manure at 0, 90, 180, or 360 metric tons ha‐1 year‐1 was incorporated with Sharpsburg silty clay loam to depths of 10, 20, and 30 cm. For plots tilled 10‐cm deep, the heaviest applications increased the soil organic carbon content from 2 to 5% after 2 years. Particle density decreased from 2.63 to 2.50 g cm‐3 and bulk density decreased from 1.05 to 0.90 g cm‐3. Geometric mean diameter of water‐stable aggregates increased from 80 to 800 µm. Modulus of rupture of soil cores decreased from 0.60 to 0.08 bars. Hydraulic conductivity of undisturbed soil cores increased five fold. Manure applications reduced the hydraulic conductivity of disturbed soils sampled in the fall, but there was no effect on spring samples. The heaviest application of manure increased the electrical conductivity of the hydraulic conductivity leachates from 0.8 to 3.2 mmhos cm‐1 in the fall samples and from 0.4 to 1.2 mmhos cm‐1 in the spring samples. The effects of manure on soil properties decreased with increased depth of tillage.

Trifluralin Dissipation in Soil Following Repeated Annual Applications

Soil residues from annual trifluralin (α,α,α-trifluoro-2,6-dinitro-N,Ndipropyl-p-toluidine) applications were determined over a 4-year period on a Sharpsburg silty clay loam at Lincoln, Nebraska. A phytotoxic residue of trifluralin persisted in the soil for 1 or more years especially at above-normal rates of trifluralin. Repeated applications of trifluralin did not cause a significant buildup of trifluralin in the soil. There was little herbicide remaining in the soil during subsequent years from normal rates of trifluralin.

Degradation of 2,4‐D and Atrazine at Three Soil Depths in the Field

AbstractSoil from three depths was treated with (2,4‐dichlorophenoxy) acetic acid (2,4‐D) and 2‐chloro‐4‐(ethylamino)‐6‐(isopropylamino)‐s‐triazine (atrazine) and buried in the soil profile for 5, 17, 29 and 41 months at two locations in Nebraska. Degradation of 2,4‐D stored in field pits at 15‐, 40‐, and 90‐cm depths was rapid under aerobic soil conditions at both locations. Rates of atrazine degradation decreased with increasing soil depths. In Sharpsburg silty clay loam, phytotoxic amounts of atrazine dissipated during the first 5 months at the 15‐cm depth, and during the first 17 months at the 40‐cm depth. Atrazine treated samples buried at 90 cm in the same profile contained phytotoxic amounts of atrazine after 41 months as shown by a soybean bioassay and gas chromatographic analysis.

Germination and Seedling Development of Common Milkweed and Other Species

Common milkweed(Asclepias syriacaL.) seed did not survive storage in water for 1, 2, and 12 months at 40, 30, and 5 C, respectively; but under dry storage seed viability was destroyed only at 40 C for 5 months. Stratifying common milkweed seed in water at 5 C was more effective in breaking dormancy than at 22 C. Mechanical scarification also caused a significant increase in germination. Exogenous 6-furfurylamino purine (kinetin) and gibberellic acid-3 (GA) increased germination while indole-3-acetic acid (IAA) had no effect. The optimum germination temperature scheme for common milkweed was an alternating 20 to 30 C, the maximum between 35 and 40 C, and the minimum between 15 and 20 C. Common milkweed seedlings were more susceptible to moisture stress than kochia [Kochia scoparia(L.) Schrad.] but less susceptible than hemp dogbane(Apocynum cannabinumL.) and sunflower(Helianthus annuusL.). In a greenhouse study common milkweed was less drought tolerant than eight other weed species. Common milkweed seedlings tolerated wider pH ranges than honeyvine milkweed [Ampelamus albidus(Nutt.) Britt.], kochia, and sunflower. Common milkweed seedlings were less tolerant to salt solutions than four other species. Common milkweed emerged from 6-cm depths in Sharpsburg silty clay loam (sicl) at 20 or 30 C but only from the 3-cm depth at an alternating 20 to 30 C. Results from root box studies showed that common and honeyvine milkweed produced less shoot and root weight than kochia and sunflower and had a lower growth rate index of roots than sunflower. Common milkweed seedlings resprouted 21 days after planting when clipped at ground level. As days from planting to clipping increased, sprouting activity increased.