D. Wayne Reeves

Assessing Indices for Predicting Potential Nitrogen Mineralization in Soils under Different Management Systems

A reliable laboratory index ofN availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N 0 ). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a first-order exponential equation to determine N 0 , k (mineralization rate), and N 0 * (N 0 estimated with a fixed k equal to 0.054 wk -1 ). Out of 13 indices, five [total C (TC), total N (TN), N mineralized by hot KCI (Hot_N), anaerobic N (Ana_N), and N mineralized in 24 d (Nmin_24)] were strongly correlated to N 0 (r > 0.85) and had linear regressions with r 2 > 0.60. None of the indices were good predictors ofk. Correlations between indices and N 0 * improved compared with N 0 , ranging from r = 0.90 to 0.95. Total N and Hush of CO 2 determined after 3 d (Fl_CO2) produced the best multiple regression for predicting N 0 (R 2 = 0.85) while the best combination for predicting N 0 * (R 2 = 0.94) included TN, Fl_CO2 Cold_N, and NaOH_N. Combining indices appears promising for predicting potentially mineralizable N, and because TN and Fl_CO2 are rapid and simple, this approach could be easily adopted by soil testing laboratories.

Evaluation of Three Winter Cereals for Weed Control in Conservation-Tillage Nontransgenic Cotton 1

The increased use of conservation tillage in cotton production requires that information be developed on the role of cover crops in weed control. Field experiments were conducted from fall 1994 through fall 1997 in Alabama to evaluate three winter cereal cover crops in a high-residue, conservation-tillage, nontransgenic cotton production system. Black oat, rye, and wheat were evaluated for their weed-suppressive characteristics compared to a winter fallow system. Three herbicide systems were used: no herbicide, preemergence (PRE) herbicides alone, and PRE plus postemergence (POST) herbicides. The PRE system consisted of pendimethalin at 1.12 kg ai/ha plus fluometuron at 1.7 kg ai/ha. The PRE plus POST system contained an additional application of fluometuron at 1.12 kg/ha plus DSMA at 1.7 kg ai/ha early POST directed (PDS) and lactofen at 0.2 kg ai/ha plus cyanazine at 0.84 kg ai/ha late PDS. No cover crop was effective in controlling weeds without a herbicide. However, when black oat or rye was used with PRE herbicides, weed control was similar to the PRE plus POST system. Rye and black oat provided more effective weed control than wheat in conservation-tillage cotton. The winter fallow, PRE plus POST input system yielded significantly less cotton in 2 of 3 yr compared to systems that included a winter cover crop. Use of black oat or rye cover crops has the potential to increase cotton productivity and reduce herbicide inputs for nontransgenic cotton grown in the Southeast. Nomenclature: Black oat, Avena strigosa Schreb. ‘SoilSaver’; rye, Secale cereale L. ‘Elbon’; wheat, Triticum aestivum L. ‘Pioneer P26 J61’; cotton, Gossypium hirsutum L. ‘Deltapine DP 5690’, ‘Deltapine NuCotn 35B’. Additional index words: Allelopathy, cover crops. Abbreviations: DAP, days after planting; PDS, postemergence-directed spray; POST, postemergence; PRE, preemergence.

Evaluation of weed control provided by three winter cereals in conservation-tillage soybean

Information is needed on the role of cover crops as a weed control alternative due to the high adoption of conservation tillage in soybean [Glycine max (L.) Merr.] production. Field experiments were conducted from fall 1994 through fall 1997 in Alabama to evaluate three winter cereal cover crops in a high-residue conservation-tillage, soybean production system. Black oat (Avena strigosa Schreb.), rye (Secale cereale L.), and wheat (Triticum aestivum L.) were evaluated for their weed-suppressive characteristics compared to a winter fallow system. Three herbicide systems were utilized: no herbicide, a mixture of two pre-emergence (PRE) herbicides, or PRE plus post-emergence (POST) herbicides. The PRE system contained pendimethalin plus metribuzin. The PRE plus POST system contained pendimethalin plus a prepackage of metribuzin and chlorimuron ethyl applied PRE, followed by an additional chlorimuron ethyl POST application. No cover crop was effective in controlling weeds without a herbicide. However, when black oat or rye was utilized with only PRE herbicides, weed control was similar to the PRE plus POST input system. Thus, herbicide reductions may be attained by utilizing cover crops that provide weed suppression. Rye and black oat provided more effective weed control in the PRE only herbicide input system than wheat in conservation-tillage soybean. The winter fallow, PRE plus POST herbicide input system yielded significantly less soybean one out of three years when compared to systems that included a winter cover crop.

Conservation Tillage, Rotations, and Cover Crop Affecting Soil Quality in the Tennessee Valley: Particulate Organic Matter, Organic Matter, and Microbial Biomass

Abstract The impact of conservation tillage, crop rotation, and cover cropping on soil‐quality indicators was evaluated in a long‐term experiment for cotton. Compared to conventional‐tillage cotton, other treatments had 3.4 to 7.7 Mg ha−1 more carbon (C) over all soil depths. The particulate organic matter C (POMc) accounts for 29 to 48 and 16 to 22% of soil organic C (SOC) for the 0‐ to 3‐and 3‐ to 6‐cm depths, respectively. Tillage had a strongth influence on POMc within the 0‐ to 3‐cm depth, but cropping intensity and cover crop did not affect POMc. A large stratification for microbial biomass was observed varing from 221 to 434 and 63 to 110 mg kg−1 within depth of 0–3 and 12–24 cm respectively. The microbial biomass is a more sensitive indicator (compared to SOC) of management impacts, showing clear effect of tillage, rotation, and cropping intensity. The no‐tillage cotton double‐cropped wheat/soybean system that combined high cropping intensity and crop rotation provided the best soil quality.

Influence of wheel traffic and tillage on microbial biomass, residue decomposition and extractable nutrients in a Coastal Plain Soil

The interactive effects of tillage and compaction from wheel traffic were tested on active bacterial and fungal biomass and organic matter decomposition in the planting row at the surface and within the plow layer of a Norfolk loamy sand (fine-loamy, siliceous, thermic Typic Kandiudult). This experiment was arranged in a split plot design with four replications. Main plots were compaction: 1) compaction from wheel traffic and 2) no compaction from wheel traffic; subplots were tilalge system: 1) conventional tillage and 2) no-tillage. Despite a significant increase in bulk density, compaction from wheel traffic and tillage system had no consistent effects on active bacterial or active fungal biomass either in the top 7.5 cm of soil or in the 15–20 cm depth of soil. Active bacteria and fungal biomass at both depths were usually lower in the winter months than the spring, summer or autumn months. Organic matter decomposition, nutrient mineralization and nutrient availability did not differ among soils that received tillage or compaction from wheel traffic. Organic matter decomposition was greater in all treatments when decomposition bags were buried at 15–20 cm than when they were placed on the surface of the soil. The soil that was sampled was an extremely sandy soil so there was probably not a significant effect of compaction on soil aeration and structure.

Cover Crop, Tillage, and N Rate Effects on Cotton Grown in Ultra-Narrow Rows

There has been recent renewed interest in growing cotton (Gossypium hirsutum L.) in ultra-narrow rows (<10 inch wide row spacing). Residue management information is needed for this cotton production system. The objective of this field study was to determine the effect of winter cover, tillage, and N rate on plant stands, yield, and fiber properties of cotton grown in 7.5-inch row widths. Field experiments were conducted near Auburn, AL [Compass loamy sand (coarseloamy, siliceous, subactive, thermic Plinthic Paleudult)] and near Florence, SC [Wagram sand (loamy, siliceous, thermic Arenic Paleudult)] in 1996 and 1997. Treatments were winter cover (cereal, legume, and fallow), tillage (conventional and conservation) and N rate (0, 40, 80, and 120 lb N per acre). Averaged over tillage and N rates, cotton stands following the legumes were 10% and 25% lower than following the fallow and cereal winter covers at Auburn and Florence, respectively. Stands tended to be lower for conservation tillage than for conventional, though differences between tillage systems were only significant in 1997 at Auburn. Tillage did not have a big impact on yield at either location. The response of cotton to N was cover crop specific. The greatest response to N occurred following the cereal cover crop at both locations and the smallest response occurred following the legume. Treatments did not substantially affect fiber quality. These data suggest that seeding rates for UNR cotton on these soils should be 15 to 20% above desired stands with conventional tillage and 20 to 25% above desired stands with conservation tillage. Nitrogen fertilizer application rates should be those recommended for wide-row production systems.