Danielle D. Treadwell

Cover Crop Management Affects Weeds And Yield in Organically Managed Sweetpotato Systems

A 3-yr field experiment was initiated in 2001 to evaluate weed suppression and sweetpotato productivity in three organic sweetpotato production systems. Organic systems were (1) compost and no cover crop with tillage (Org-NC), (2) compost and a cover crop mixture of hairy vetch and rye incorporated before transplanting (Org-CI), and (3) compost and the same cover crop mixture with reduced tillage (Org-RT). A conventional system with tillage and chemical controls (Conv) was included for comparison. Suppression of monocot and dicot weed density and biomass was similar between Org-NC and Org-CI each year, and were frequently similar to Conv. Org-RT was as effective as Org-NC in controlling dicot weed density and biomass each year, but did not suppress monocot weeds. At sweetpotato harvest, an increase in cover crop surface residue biomass in Org-RT was associated with a decrease in cumulative total weed density (R2 = 0.43, P = 0.0001); however, the amount of that residue was insufficient to suppress late-emerging monocot weeds. Total sweetpotato yield in Org-RT was at least 45% lower than other systems in 2002 and was most likely due to an increase in monocot weed density and biomass concurrent with a decrease in sweetpotato vine biomass. Total sweetpotato yield was similar among all systems in 2001 and 2004; the exception was lowest yields obtained in the Org-RT system in 2002. Organically managed sweetpotato with or without an incorporated cover crop produced sweetpotato yields equal to conventionally managed systems despite difficulties controlling weeds that emerged later in the season. Nomenclature: EPTC; napropamide; hairy vetch, Vicia villosa Roth; rye, Secale cereale L. ‘Wrens Abruzzi’; sweetpotato, Ipomoea batatas (L.) Lam. ‘Beauregard’.

Effect of Nutrient Solution, Nitrate-Nitrogen Concentration, and pH on Nitrification Rate in Perlite Medium

ABSTRACT Reconciling water quality parameters in sustainable aquaponic (integrated hydroponic and recirculating aquaculture) systems requires balancing nutrients and pH for the optimal growth of three organisms: the plant, the fish, and the nitrifying bacteria. Nitrifying bacteria convert fish waste into nitrate (NO3 −)-nitrogen (N) that may be used by the plants. Fish waste rarely supplies nutrients in adequate amounts for plants without supplementation. Increasing nitrification rate and efficiency would allow greater stocking density for fish and increased nutrient loads for plants. The objective of this research was to determine the nitrification rate response in a perlite trickling biofilter (root growth medium) exposed to hydroponic nutrient solution, varying NO3 −-N concentrations, and to pH levels optimum for plants (6.5) and nitrification (8.5). The experiment used recirculating tank batch culture and was based on typical startup characteristics for bringing biological filters up to full capacity in aquaculture systems. No significant difference (P value < 0.05) in nitrification rate was found when recirculating system water contained no nutrient solution versus a complete hydroponic nutrient solution or NO3 −-N concentrations of 0, 100, or 200 mg/L. These results indicate that hydroponic plant nutrient supplementation to concentrations found in plant production systems do not significantly affect nitrification rate in perlite medium. Nitrification was significantly impacted by water pH. Ammonia (NH3) oxidation of initial total ammonia nitrogen (TAN = ammonium (NH4 +)-N + NH3-N= 8 mg/L) occurred at the rates of 231 and 300 μ g L− 1 d− 1 at pH 6.5 and 400 and 540 μ g L− 1 d− 1 at pH 8.5, for experiments 1 and 2, respectively. The rates proceeded 1.75 times faster at pH 8.5 than at pH 6.5. Nitrite (NO2 −) oxidation occurred at the rates of 231 and 375 μ g L− 1 d− 1 for pH 6.5 and 267 and 540 μ g L− 1 d− 1 for pH 8.5 and proceeded 1.2 and 1.4 times faster, respectively. The increased ammonia oxidation rate (1.75) compared to nitrite oxidation rate (1.3) at pH 8.5 resulted in accumulation of NO2 −-N to levels near those harmful to plants and fish (observed peaks of 4.2 and 3.8 mg/L NO2 −-N, respectively). The potential for increased levels of un-ionized ammonia, which are toxic to fish and reduced plant nutrient uptake from micronutrient precipitation, are additional problems associated with pH 8.5. The advantages of increased nitrification efficiency, which averaged 23% in the current trials at the higher pH, when weighed against the potential increased water quality risks to the fish and plant, justify a compromise between pH optima for nitrification and plant production to pH 7 for aquaponic system water. A more flexible management strategy for these systems would be to supplement with plant nutrients, which would permit less reliance on the fish and nitrification to provide optimal plant nutrient levels.

Effect of Water pH on Yield and Nutritional Status of Greenhouse Cucumber Grown in Recirculating Hydroponics

ABSTRACT Cucumbers are produced in integrated hydroponic and aquaculture systems (aquaponics). Aquaponics balances pH for plants, fish, and nitrifying bacteria. Nitrification prevents buildup of toxic waste ammonia by conversion to nitrate (NO3 −)- nitrogen (N). The pH for hydroponic cucumbers (5.5–6.0) and nitrification (7.5–9.0) requires reconciliation to improve systems integration. Cucumbers were grown at pH of 5.0, 6.0, 7.0, and 8.0 with additional foliar sprays at pH 7.0 and 8.0. Plant shoot dry weight, length, N, and phosphorus (P) content at 14 DAT were similar from pH 5.0 to 7.0, but reduced at pH 8.0. Nutrient solution and shoot dry matter Mn and Fe decreased as pH increased. Foliar sprays had no effect on cucumber fruit yield. Early yield was higher at pH 5.0 compared to pH 8.0 but total yield was unaffected by pH. Cucumbers in recirculating culture may be maintained at pH levels more optimum for nitrification (7.5–8.0) except during production for early season markets.

Effect of sunn hemp (Crotalaria juncea L.) cutting date and planting density on weed suppression in Georgia, USA.

A field study was conducted in 2008 and 2009 at the USDA, ARS, Plant Genetic Resources Conservation Unit in Griffin, GA, to investigate weed suppression by sunn hemp (Crotalaria juncea L). The objectives were to (1) evaluate the effects of apical meristem removal (AMR) at three dates [5, 6, and 7 wks after planting (WAP) on May 14, 2008 and May 21, 2009] and (2) assess the impact of seeding rates (11, 28, and 45 kg ha−1) on weed biomass reduction. Weed species were identified at 4, 8, and 12 wks after sunn hemp planting. Sunn hemp cutting date had no significant effect on weed suppression in 2008 but significant differences for grass weeds at 4, 8, and 12 WAP and for yellow nutsedge at 8 and 12 WAP did occur when compared to the control in 2009. In comparison to the sunn hemp-free control plot in 2009, all three seeding rates had reduced grass weed dry weights at 4, 8, and 12 WAP. The total mass of yellow nutsedge when grown with sunn hemp was reduced compared to the total mass of yellow nutsedge grown in the weedy check for all seeding rates at 8 and 12 WAP. Lower grass weed biomass was observed by 12 WAP for cutting dates and seeding rates during 2008 and 2009. Sunn hemp cutting date and seeding rate reduced branch numbers in both years. The reduction in sunn hemp seeding rates revealed a decrease in weed populations.