Norfolk Soil Research Articles

Field evaluation of commercial monoammonium phosphate fertilizers

A field study was conducted for two years to evaluate 9 MAP fertilizers produced from the major sources of PR deposits located in Florida, Idaho and North Carolina compared with reagent grade MAP. Sources varied in chemical composition and had 81 to 100% of the A.O.A.C. available P water soluble. Each source was applied to a Norfolk (Typic Paleudults) and a Hartsells(Typic Hapludults) soil at rates to supply 0, 29 and 59 kg P ha−1. Forage yields and P concentrations in harvested forage were increased by the application of P (P ⩽ 0.05) to both soils during the two year study. The concentration of P in harvested forage was affected by the source of MAP only on the Hartsells soil in 1988. Nitrogen was not affected by the source MAP in any of the forage samples tested. Yield differences among sources were observed only in 1987 on the Norfolk soil and in 1988 on the Hartsells soil. Performance of the fertilizers was not related to the percentage of water soluble P or the content of metallic elements.

Decomposition rate of dicyandiamide and nitrification inhibition

Abstract Degradation of dicyandiamide (DCD) was assayed in laboratory studies at 8, 15, and 22 C in a Decatur silt loam and in a Norfolk loamy sand. Dicyandiamide was very short lived at 22 C, with half‐lives of 7.4 and 14.7 days in the Decatur and Norfolk soils, respectively. In the Norfolk soil at 8 C, half‐life increased to 52.2 days. In a nitrificaton study of both soils at 22 C, 80 mg (NH4)2SO4‐N kg‐1 of soil was applied with 20 mg DCD‐N kg‐1 of soil and 100 mg kg‐1 (NH4)2S04‐N was added with 5% nitrapyrin. Distinct lag phases preceded zero order nitrification with the inhibitor treatments. Lag periods were 2 and 2.6 times the half life of DCD in the degradation study for Decatur and Norfolk soils, respectively. Like most nitrification inhibitors, the effectiveness of DCD decreases with increasing temperature. In the Norfolk loamy sand, nitrification inhibition by DCD was equal to nitrapyrin for up to 42 days, but in Decatur silt loam, DCD was less potent to nitrapyrin as a nitrification inhibitor.

Effect of Selected Soil Properties on Cadmium Content of Tobacco

AbstractIn a greenhouse study, limed and unlimed soil from the A horizon of six mineral soils and three organic soils of the southeastern USA was amended with 0, 1, 2, and 4 kg Cd ha−1 as CdCl2 (1.8, 3.6, and 7.2 mg Cd pot−1). Tobacco (Nicotiana tabacum L.) was grown until flower initiation (42 d). The 2‐ and 4‐kg Cd rates reduced dry weight on the Norfolk soil (Typic Paieudult), but Cd had no effect on dry weight on the other soils. Cadmium concentration was a maximum in the lower leaves and decreased with leaf position up the stalk. At the 2‐kg Cd rate, liming reduced leaf Cd concentration in only three soils. Lack of liming effect in the other soils was thought to be a result of high organic matter, high initial pH, or small pH changes effected by the lime. Lime reduced the effects of Cd loading rate and soil types on Cd concentration in tobacco. Stepwise multiple regression was used to relate soil properties to leaf Cd concentration and removal of Cd by tobacco harvest. When organic and mineral soils were considered together, leaf Cd and Cd removal were best related to the Cd sorptive capacity of the soils as determined in previous studies (Cd removed from a Cd solution or sorbed Cd not removed with 1 M KCl). When only the mineral soils were considered, inherent soil properties like sand content, ammonium oxalate‐extractable Fe (Feo), and pH dominated the regression models. With limed mineral soils (pH 5.7–7.0), models for leaf Cd and Cd removal contained pH, Feo, and/or ammonium oxalate‐extractable Cd. Model‐calculated values and actual values differed by only ±4% so the models should be useful in ranking soils from the southeastern USA for their capacity to retain salt sources of Cd. Soil cation exchange capacity, which is used to regulate Cd loading rates on agricultural soils, did not appear in any of the models.

Winter legume and tillage effects on cotton growth and soil ecology

Field studies were conducted at 2 locations in Alabama during 1984 and 1985 to identify cultural practices which would improve cotton ( Gossypium hirsutum L.) stands when plantned no-till into winter legumes. The soils were a Decatur silt loam (Rhodic Paleudult) and a Norfolk sandy loam (Typic Paleudult). The experimental design was a randomized complete block with 4 replications. Whole plots consisted of winter annual legumes ( Vicia villosa Roth or Trifolium incarnatum L.) and fallow areas. Split-plot treatments established at cotton planting included conventional and conservation tillage, and fungicide. Soil samples were collected at cotton planting for population determination of Collembola and Acari species, and for estimation of cotton-disease fungus ( Rhizoctonia solani) infestation. Collembola populations were greater in the Decatur than Norfolk soils, and higher in legume-mulched than fallow soils by 39 and 72% for the Decatur and Norfolk soil, respectively. Disease infestation in the Decatur soil was 10% higher in legume than fallow plots, and 18% higher in legume than fallow plots in the Norfolk soil. Cotton populations were 19% less in legume than fallow areas, and 25% less with conservation (no-till) than with standard tillage. Bedding improved cotton stands by 21% compared to conservation tillage. Seed-cotton yields from the Decatur soil were consistently high (3798 kg ha −1, and there was no yield response to treatments. Maximum seed-cotton yields at the Norfolk site were achieved with conservation tillage in the fallow area, and conventional tillage in the legume area (both receiving fungicide).

Using Regionalized Variables to Estimate Field Variability of Corn Yield for Four Tillage Regimes

AbstractIn selecting an appropriate tillage system for a specific crop and field, it is desirable to choose one that maximizes yield for the field as a whole; however, various areas in a field often give different responses to the same tillage system. The theory of regionalized variables was used to evaluate corn [Zea mays L.] grain yield response to the following four tillage systems (tandem disking, tandem disking followed by bedding, in‐row‐subsoiling and bedding, and chisel plowing). These treatments were randomized, replicated three times, and stripped across the 198‐m‐long field of Norfolk soil (Typic Paleudults), which had a tillage pan and variable depth to B horizon. Grain was harvested in adjoining 15.2‐m‐long plots in each strip. Analysis of directional semivariograms for relative corn grain yield in the along‐the‐row direction, showed a different variance structure for each tillage treatment. Because only three separation distances were available to compute grain yield semivariance values in the across‐the‐row direction, a composite directional semivariogram in this direction was developed by combining relative grain yield‐semivariance values for all four treatments. A global semivariogram was constructed for each tillage treatment by combining the specific along‐the‐row semivariogram specific for each treatment with the composite across‐the‐row semivariogram, and was used to krige the relative corn grain yield for the entire field for each tillage treatment. Patterns of the kriged yields were appreciably different throughout the field for each tillage treatment. Corn grain yields, estimated using jackknife kriging at each location in the field where grain was harvested, compared favorably with the measured yields for all treatments.

Tillage Systems for Wheat Production in the Southeastern Coastal Plains1

AbstractIncreased wheat (Triticum aestivum L.) production has raised numerous questions regarding tillage practices and N fertilizer rates for the Coastal Plain. Therefore, eight field studies conducted on Ardilla (Fragiaquic Paleudults), Dothan (Plinthic Paleudults), and Norfolk (Typic Paleudults) soils are summarized to report N fertilizer and tillage effects. Average grain yield with moldboard plowing was significantly higher (0.4 Mg ha−1) than with disking in four of five studies, but it was significantly higher than chisel plowing (3.09 vs. 2.48 Mg ha−1) only once. No‐till yield was significantly lower (0.5 Mg ha −1) than yield with disking in two of four studies. Nitrogen response was significant in five of six studies, but the tillage ✕ N interaction was significant only once. Head number and weight were increased by plowing or higher N rates. Poor soil‐seed contact presumably caused erratic stands and lower no‐till yields because seedling emergence was not reduced by the equivalent of 10 Mg ha−1 of corn (Zea mays L.) residue. Approximately 100 kg N ha−1 was sufficient for conventional tillage, but chisel or moldboard plowing should be used rather than disking. For no‐till wheat, drills must provide good soil‐seed contact and N rates may have to be increased.

Corn Irrigation Scheduling in Humid Regions on Sandy Soils with Tillage Pans1

AbstractSeveral droughts during the past decade in humid regions of the USA, especially the Southeast, have intensified the interest in supplemental irrigation. One objective of this study was to evaluate the effect of supplemental irrigation in a humid environment on corn (Zea maysL.) production on an Atlantic Coastal Plain soil having a tillage pan. A second objective was to evaluate two alternative techniques for scheduling supplemental irrigation in humid regions. The study, conducted on a Wagram soil (Arenic Paleudults)‐Norfolk soil (Typic Paleudults) from 1979 to 1981, employed a split plot design with water level (irrigation scheduling) as the main block. Water levels were control or dryland (W1), scheduled irrigation water applications based on a computerized water balance model (W2), and scheduled water applications based on tensiometer measurements (W3). Subplot treatments were time of N application in 1979 and 1980 and cultivation intensity in 1981. Compared with a normal rainfall of 447 mm during the growing season, 464, 262, and 220 mm fell during 1979, 1980, and 1981, respectively. The average amount of irrigation water applied over the 3‐yr period based on the water balance model was 40 mm greater than the 3‐yr average of 179 mm applied based on the tensiometers. Grain yield as affected by subplot treatment was not significantly different for any year. Mean grain yields for the 3‐yr period were 2.58, 7.73, and 8.14 Mg ha−1, for treatments W1, W2, and W3, respectively. Either irrigation scheduling method offers large yield improvements in drier years, but no significant yield difference occurred for the two scheduling techniques. In the wetter year 1979, irrigation increased yield by 2.45 Mg ha−1, whereas in the dryer years of 1980 and 1981 the increases were 6.53 and 7.06 Mg ha‐1, respectively.

Adsorption, Mobility, and Efficacy of Alachlor and Metolachlor as Influenced by Soil Properties

Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] adsorption was positively correlated with soil organic-matter content, clay content, and surface area as measured by ethylene glycol monoethyl ether (EGME) or benzyl ethyl ether (BEE) and inversely correlated with herbicidal activity. Alachlor was adsorbed in slightly greater amounts by soil than metolachlor. Metolachlor had slightly greater bioactivity than alachlor on grass weeds, but the herbicides had similar activity on broadleaf weeds. Slightly greater amounts of metolachlor than alachlor were leached through a Norfolk soil and slightly greater amounts of alachlor were retained in the upper soil zones, compared with metolachlor.

Sulfur Fertilization of Winter Wheat Grown on Deep Sandy Soils

AbstractSurface horizons of sandy Coastal Plain soils of the southeastern United States are low in available SO−24, and crops are often dependent upon adsorbed SO−24 in the B horizon. The objective of this study was to determine the response of winter wheat (Triticum aestivum L.) to sulfur (S) fertilization on three soils which varied in depth to the B horizon; a Wagram soil (Arenic Paleudults) > 50 cm to the B, and two Norfolk soils (Typic Paleudults) 31 and 48 cm to the B. Sulfur rates covered the range from 0 to 90 kg S ha−1. Sources used were CaSO4 · 2H2O, elemental S plus bentonite (Sol‐U‐Sul) and (NH4)2SO4. Measurements were made of S concentration and N/S ratio of plants, root distribution with depth at spike emergence and grain yield at harvest. Yield response to S fertilization was obtained on the Wagram soil and the Norfolk soil in which root growth was restricted by a tillage pan but not on the Norfolk soil with the B horizon at 31 cm. A rate of 20 kg S ha−1 as SO2−4 applied in late February was adequate for optimum response. Sol‐U‐Sul applied in February was not effective. Ammonium sulfate was equally effective as CaSO4. Plants responded to S fertilization where nonfertilized plants had S concentrations of 0.6 g kg−1 dry weight and N/S ratios of 21. Response of wheat to S fertilization depended upon the depth to the B horizon and root distribution in the soil profile. No response to S fertilization was obtained on a Norfolk soil in which there was appreciable root growth in the 30‐ to 45‐cm depth. That depth contained an accumulation of SO2−4. Where root growth was essentially restricted to the top 30 cm of a Norfolk soil in which SO2‐4 accumulation started at the 45‐cm depth there was a large response to S fertilization.

Control of Green Peach Aphids with Side-Dress Applications of Temik, 1984

Abstract On 4 May tobacco was transplanted into plots 30 ft (17 plants) x 15 ft (4 rows–42 inches on center) in a field of Norfolk and Appling loamy sand soil at the Central Crops Research Station, Clayton, NC. Plots were arranged in a randomized complete-block design with 3 reps. On 7 Jun treatments were applied with a Noble granular applicator mounted on a John Deere Hi-Cycle 600. The granule dispersing fan on each hopper was replaced with a 2 inch dia hose which extended to the soil surface. This resulted in a narrow band adjacent to the plant. Within 1 h after application, all treatments were incorporated with a rotary tiller and ridged. Once a week, the topmost 6-inch leaf on 5 consecutive plants in both center rows were examined; all green peach aphids were counted and recorded. All plants in both center rows were also counted and examined for aphid colonies. Prior to analysis of variance, the proportion of aphid colonized plants were subjected to the arcsin-square root transformation.

Fluridone Adsorption on Mineral Clays, Organic Matter, and Modified Norfolk Soil

Adsorption and desorption characteristics of fluridone (1-methyl-3-phenyl-5-[3-txifluoromethyl)phenyl]-4-(1H)-pyridinone} on prepared clays and organic matter were studied in unbuffered and buffered aqueous solutions. In unbuffered aqueous solution the adsorption of fluridone decreased in the order: H-saturated organic matter (H-OM) > Ca-saturated montmorillonite (Ca-mont) > Ca-saturated organic matter (Ca-OM) > kaolinite. Based on the chemistry of fluridone, the nature of the adsorbent, and previously reported studies, the major adsorption mechanisms appear to be pH-dependent adsorption of protonated fluridone and direct protonation of the herbicide at acidic surfaces, supplemented by physical adsorption forces such as van der Waals attractions and charge transfer bonds. The adsorption of fluridone on Norfolk sand (Typic Paleudult; fine-loamy, siliceous, thermic), unmodified (CK), or amended with montmorillonite (HC) or organic matter (HM) at pH 4.0, 5.2, and 7.0 was also studied. Over all adsorption was greatest on HC soil, least on CK soil, and intermediate on the HM soil. In each system adsorption was inversely related to pH. These results were interpreted by comparison with adsorption observed on prepared clay minerals and organic matter.

Influence of Topsoil and Subsoil Acidity Environments on Root Growth of ‘Curtis’ and ‘Stuart’ Pecan Seedlings in a Simulated Soil Profile1

Abstract Open-pollinated seedlings of ‘Stuart’ pecan [Carya illinoensis (Wang.) K. Koch] had larger taproots, greater stem weights, and produced higher yields of coarse lateral roots (>0.5 mm) than ‘Curtis’ open-pollinated seedlings when grown in a simulated Norfolk soil profile. However, rate of taproot growth and production of small feeder roots (<0.5 mm and <0.25 mm) in the subsoil was greater for the ‘Curtis’ than for the ‘Stuart’ seedlings. Taproot and feeder root growth of both cultivars increased markedly with increases in subsoil pH. Rate of taproot growth and production of small feeder roots were limited by the acid subsoils despite adequate liming of the surface soil. Direct place ment of lime in the subsoil was required to achieve maximum root growth.

Effects of Topsoil-subsoil Fertilizer and Lime Amendments on Top and Root Growth of ‘Stuart’ Pecan Seedlings in a Simulated Norfolk Soil Profile1

Abstract Fertilizer and lime topsoil treatments increased top and root growth of open-pollinated seedlings of ‘Stuart’ pecan [Carya illinoensis (Wang.) K. Koch] under simulated Norfolk profile conditions in the greenhouse. Positive response was greatest on root growth when lime was added to the subsoil. Rate of tap root growth in the subsoil was increased 64% by increasing subsoil pH from 5.1 to 6.5. Lime additions to the subsoil reduced weights of lateral roots in the topsoil and increased yields in the subsoil. Subsoil lime at 2 and 3 meq Ca/100 g soil (pH 6.0 and 6.5) increased yields of the small feeder roots 3 and 6 fold over the untreated check (subsoil pH 5.1). Acid subsoils can be an important factor limiting growth of pecan roots under soil conditions prevalent in much of the Southeast.

Nutrient concentration and yield variation within southern soybean germplasm grown on acid Norfolk soil

Abstract Seventy‐one soybean genotypes were grown in the field on an acid Norfolk loamy sand to evaluate growth, seed yield, and nutrient concentrations in aerial plant fractions. The pH values in the Ap, A2, and B horizons were 4.6, 4.4, and 4.4, respectively. Cation exchange capacities (CEC) were 2.1, 0.9, and 2.8 me/100 g with an Al saturation of 34, 49, and 40%, respectively. Very few visual toxicity or deficiency symptoms were found among the genotypes. Analyses of aerial plant fractions collected when the genotypes were in bloom or early pod fill showed some significant differences, but In general P, K, Ca, and Mg concentrations were adequate, while Al, Fe, Mn, and Zn concentrations were very high. Average yields of genotypes in maturity groups IV, V, VI, VII, and VIII were 2.7, 2.7, 2.6, 2.4, and 2.2 t/ha, respectively. There were significant differences in seed yield among genotypes in maturity groups IV and VIII, but differences among genotypes in the other maturity groups were nonsignificant. Co...

Nitrogen, phosphorus and carbon transformations in a coastal plain soil treated with animal manures

A laboratory study was conducted to evaluate nitrogen, phosphorus and carbon transformations in a coastal plain soil (Norfolk sandy loam) treated with animal manures. Three types of manure—beef, poultry and swine—were either surface applied or incorporated into the soil. The results indicate that losses of ammonia N through volatilisation were higher in the soil treated with poultry manure than in the soil treated with beef or swine manures. Poultry and swine manure applications resulted in the accumulation of approximately the same amount of nitrate N. Accumulation of nitrate N in the soil treated with beef manure was approximately half that in the soil treated with poultry or swine manure. Application of animal manures to the soil increased soluble organic N, soluble P and soluble organic C. Application of all the manures increased the denitrification potential of the soil. The denitrification potential of the Norfolk soil was higher when treated with beef manure than when treated with poultry or swine manures. Denitrification potential was highest in the treatment where animal manures underwent 30-day decomposition. The nitrate N loss decreased as the manure decomposition periods were increased to 60, 90 and 120 days.

Phosphorus Adsorption‐Desorption Characteristics of Two Soils Utilized for Disposal of Animal Wastes

AbstractThe purpose of this study was to determine the effect of animal waste loading rates on P adsorption‐desorption characteristics of two soils. In a laboratory incubation study, Norfolk soil was treated with beef, poultry, or swine wastes; and allowed to decompose under optimum moisture conditions for a period of 30 days. Phosphorus adsorption‐desorption characteristics of the soil were measured at the end of the incubation period. Application of beef, poultry, and swine wastes to a Norfolk soil decreased adsorption capacity of the soil and increased soluble P (in 0.01M CaCl2), acid‐extractable P (0.05N HCl + 0.025N H2SO4), equilibrium P concentration (EPC), and P desorption (after four 1‐hour extractions).In a field study, increased rates of swine lagoon effluent application over a period of 5 years to a Norfolk soil (Site 1) and for 3 years to a Cecil soil (Site 2) also increased soluble P (in 0.01M CaCl2), acid extractable P, P desorbed, and EPC values, and decreased the adsorption capacity. At high loading rates of swine lagoon effluent, soluble P movement occurred to a depth of 75 and 30 cm at Site 1 and 2, respectively. Phosphorus adsorption increased with depth, and EPC values decreased with increasing depths of soil profile. A significant relationship was observed between EPC values, and soluble and acid extractable P.

Nitrogen Release Characteristics of Isobutylidene Diurea and its Effectiveness as a Source of N for Flue‐Cured Tobacco1

AbstractLeaching in coarse‐textured soils contributes to inefficient N utilization by flue‐cured tobacco (Nicotiana tabacum L.). Slow‐release N materials reduce N leaching but those with rate of release dependent on biological degraduation have not been successful in furnishing the high proportion of total N required by tobacco during early growth. The objectives of this study were to investigate the release characteristics of isobutylidene diurea (IBDU), with N‐release characteristics dependent primarily upon the effects of particle size on chemical dissolution rather than the effects of environmental variation on biological activity, in laboratory and greenhouse studies and to evaluate it as a source of N for flue‐cured tobacco in the field. Urea, NaNO3, and four particle sizes of IBDU (1.05, 0.75, 0.55, and 0.40 mm) were applied at rates of 200 and 400 mg N per kg of soil to a Norfolk sandy loam (Typic paleudult) in lysimeters and subjected to leaching at 10 intervals over a 109‐day period. All applied N from NaNO3 and 60% from urea were removed by the first leaching (0 days) at both rates of application. Only 5% of the applied N from IBDU was removed in the first leaching. At the 200 mg N application rate, 50 to 60% of applied N from the 1.05 to 055 mm IBDU particle sizes were removed after 10 leachings (109 days) but only four leachings (32 days) were required to remove equal amounts from the 0.40 mm particle size. At the 400 mg N application rate, 10 leachings were required for 55 to 60% removal of applied N from all particle sizes. The effects of particle size on the ability of IBDU to release N fast enough for maximum growth, but not so fast as to be subject to leaching, were evaluated in a greenhouse with two successive. crops of corn (Zea mays L.) grown under leached and non‐leached conditions. IBDU particle sizes of 1.05, 0.55, and 0.40 mm were applied to Norfolk soil in pots at rates of 214, 321, and 428 mg per 5 kg of soil. Dry matter for the first crop was not affected by particle size, but it was greatest for the 1.05 mm IBDU in the second crop. Within a particle size, dry matter was not altered by leaching but total N uptake was greatest from 0.40 mm IBDU. This suggests that N release from 0.40 mm IBDU was rapid enough to support maximum growth but was not so rapid as to be subject to leaching under imposed conditions. In a field experiment on Norfolk soil, I BDU particle sizes of 1.05 and 0.40 mm were compared with NaNO3 and urea as N sources for flue‐cured tobacco. Materials were banded at 67 kg N/ha: (a) days after planting, (b) as a split application of a single source 12 and 26 days after planting, and (c) as a split application of one source at 12 days after planting and NaNO3 at 26 days after planting. NaNO~ gave the highest yield of cured leaves when applied 12 days after planting, followed by urea, 0.4 mm IBDU, and 1.05 mm IBDU, respectively. Yields from both IBDU materials were improved when one‐half of the total N was applied 2 weeks later as NaNO3. Chemical composition of cured leaf varied little with treatment but urea gave higher levels of N and total alkaloids and lower sugars than the other materials. IBDU was effective in preventing loss of N by leaching in laboratory and greenhouse studies. As with other slow‐release N sources, however, it was inferior to water‐soluble N sources for flue‐cured tobacco in the field.

Effect of Detergent Application on the Growth of Corn

AbstractA greenhouse experiment was conducted to determine the effects of two detergents on the growth of corn (Zea mays L.) on Davidson clay loam and Norfolk fine sandy loam. A heavy‐duty non‐enzyme detergent (Bz) and a heavy‐duty enzyme detergent (Tx) were applied to the soils in watering solutions at concentrations of 0, 20, 800, 1,600, 4,800, 8,000, 10,000, 12,000, and 14,000 ppm. Stimulations in growth occurred on the Davidson soil that received the 1,600 ppm Bz or the 800, 1,600, 4,800, and 8,000 ppm Tx solutions and on the Norfolk soil that received the 800 ppm Tx solution. These stimulations were attributed to responses to P contained in the detergents. Yield decreases occurred on the Davidson soil that received solutions containing 8,000 through 14,000 ppm Bz or 14,000 ppm Tx, and on the Norfolk soil that received solutions containing 1,600 through 14,000 ppm Bz or 4,800 through 14,000 ppm Tx. These yield decreases were attributed to soluble salt damage from the Na salts contained in the detergents on all treatments, except where Tx was applied to the Norfolk soil. In the latter case, specific Na salts effects could not be separated from general salinity effects. Plants grown on the Davidson soil were more tolerant to detergent applications than those grown on the Norfolk soil, probably because of the greater salt retention of the former soil. It was concluded that the changes in soil pH and B availability due to detergent applications were not responsible for the yield decreases in this study.

Weed Competition in Cotton. I. Sicklepod and Tall Morningglory

Cotton (Gossypium hirsutumL.) was grown at two locations in Alabama during 1966 to 1969 with specific densities of sicklepod (Cassia obtusifoliaL.) and tall morningglory (Ipomoea purpureaL. Roth). Competition from sicklepod and tall morningglory caused cotton yield reductions at densities as low as eight weeds per 7.31 m of row. Tall morningglory was usually more competitive than sicklepod in cotton. Both species were more competitive on Lucedale sandy clay loam than on Norfolk sandy loam. Sicklepod at eight weeds per 7.31 m of row reduced yields from 10 to 23% on the Norfolk soil and approximately 40% on the Lucedale soil. At the highest density, 48 sicklepod plants per 7.31 m of row, yields were reduced 45 to 65% on the Norfolk soil and approximately 80% on the Lucedale soil. Tall morningglory at a density of eight weeds per 7.31 m of row reduced cotton yields 10 to 40% on the Norfolk soil and 50 to 75% on the Lucedale soil. Vegetative growth, boll, and seed size were not affected as much as cotton yields by competition. Percentage lint and lint fiber properties were unaffected by competition.

Weed Competition in Cotton. II. Cocklebur and Redroot Pigweed

Competitive effects of cocklebur (Xanthium pensylvanicumWallr.) and redroot pigweed (Amaranthus retroflexusL.) on cotton (Gossypium hirsutumL.) were evaluated in four experiments at two locations in Alabama during the period 1966 to 1968. Cocklebur was more competitive with cotton than was pigweed on Norfolk sandy loam soil. A density of eight cocklebur plants per 7.31 m of row reduced cotton yields 20 to 40%. Higher densities of cocklebur resulted in greater yield reductions. Forty-eight cocklebur plants per 7.31 m of row caused cotton yields to be reduced more than 80%. Pigweed at a density of 48 weeds per 7.31 m of row reduced cotton yields less than 50% on the Norfolk soil. On Lucedale sandy clay loam, cocklebur and pigweed were equally competitive and both species were more competitive than on Norfolk soil. A density of eight cocklebur or pigweed plants per 7.31 m of row resulted in 60 to 70% cotton yield reductions. The highest density of pigweed studied, 48 plants per 7.31 m of row, reduced cotton yields 90% on the Lucedale soil. Vegetative growth, boll, and seed size were not affected as much as cotton yields by either weed species. Percentage lint and fiber properties were unaffected by plant competition.