A pot experiment with summer cover crops and soil amendments was conducted in two consecutive years to elucidate the effects of these cover crops and soil amendments on `Clemson Spineless 80' okra (Abelmoschus esculentus) yields and biomass production, and the uptake and distribution of soil nutrients and trace elements. The cover crops were sunn hemp (Crotalaria juncea), cowpea (Vigna unguiculata), velvetbean (Mucuna deeringiana), and sorghum sudangrass (Sorghum bicolor × S. bicolor var. sudanense) with fallow as the control. The organic soil amendments were biosolids (sediment from wastewater plants), N-Viro Soil (a mixture of biosolids and coal ash, coal ash (a combustion by-product from power plants), co-compost (a mixture of 3 biosolids: 7 yard waste), and yard waste compost (mainly from leaves and branches of trees and shrubs, and grass clippings) with a soil-incorporated cover crop as the control. As a subsequent vegetable crop, okra was grown after the cover crops, alone or together with the organic soil amendments, had been incorporated. All of the cover crops, except sorghum sudangrass in 2002-03, significantly improved okra fruit yields and the total biomass production (i.e., fruit yields were enhanced by 53% to 62% in 2002-03 and by 28% to 70% in 2003-04). Soil amendments enhanced okra fruit yields from 38.3 to 81.0 g/pot vs. 27.4 g/pot in the control in 2002-03, and from 59.9 to 124.3 g/pot vs. 52.3 g/pot in the control in 2003-04. Both cover crops and soil amendments can substantially improve nutrient uptake and distribution. Among cover crop treatments, sunn hemp showed promising improvement in concentrations of calcium (Ca), zinc (Zn), copper (Cu), iron (Fe), boron (B), and molybdenum (Mo) in fruit; magnesium (Mg), Zn, Cu, and Mo in shoots; and Mo in roots of okra. Among soil amendments, biosolids had a significant influence on most nutrients by increasing the concentrations of Zn, Cu, Fe, and Mo in the fruit; Mg, Zn, Cu, and Mo in the shoot; and Mg, Zn, and Mo in the root. Concentrations of the trace metal cadmium (Cd) were not increased significantly in either okra fruit, shoot, or root by application of these cover crops or soil amendments, but the lead (Pb) concentration was increased in the fruit by application of a high rate (205 g/pot) of biosolids. These results suggest that cover crops and appropriate amounts of soil amendments can be used to improve soil fertility and okra yield without adverse environmental effects or risk of contamination of the fruit. Further field studies will be required to confirm these findings.
Qingren Wang, Yuncong Li, and Waldemar Klassen
Teresa Olczyk, Kent Cushman, and Waldemar Klassen
Okra (Abelmoschus esculentus) is grown as a direct-seeded cash crop at high plant populations (>87,000 plants/acre) on calcareous soils in Homestead, south Florida. A study was established in a commercial field in May 2005 to evaluate if high populations translated to higher yields. Seedlings were thinned to within-row spacings of 2, 4, 6, 8, and 10 inches in rows set 3 ft apart (87120, 43560, 29040, 21780, and 17420 plants/acre). Harvest data was collected from 29 July to 30 Sept. 2005 (26 harvests) from 10 ft of the center row within plots 15 ft long and 3 rows wide. Decreasing plant density resulted in decreasing plant height early in the season and increasing height late in the season. Density affected stem caliper with a clear trend of decreasing density and increasing caliper. Early, mid-, and total yields by weight (boxes/acre) were not affected by density, but plants at the lowest density produced 55% more late yield than plants at the highest density. Plants at the lowest density produced 30% fewer early pods and 31% more late pods than plants at the highest density. Decreasing plant density resulted in increasing average pod weight for early, late, and total harvests by as much as 14% to 18%. With inexpensive open pollinated cultivars such as `Clemson Spineless 80', there seems little economic incentive to reduce plant populations below what is commonly used in the Homestead area. Growers should not be alarmed, however, if plant stands are reduced to some extent after seeding.
Guodong Liu, D. Marshall Porterfield, Yuncong Li, and Waldemar Klassen
Large and/or aged seeds are prone to hypoxic conditions during germination. Germination of selected vegetable seeds including corn (Zea mays L.), squash (Cucurbita pepo L.), and tomato (Solanum lycopersicum L.) was studied in water with different concentrations of hydrogen peroxide (H2O2) solution ranging from 0, 0.06% to 3.0% (v/v) or in aeroponics, all with 0.5 mm CaSO4. Imbibition, oxygen consumption, proton extrusion, and alcohol dehydrogenase (ADHase) activity of corn seeds were measured gravimetrically, electrochemically, and colorimetrically as appropriate. The results showed that 0.15% H2O2 provided the optimum oxygen concentration for seed germination. The germination percentage of aged corn seeds treated with H2O2 was significantly greater than those without H2O2 treatment. Corn embryo orientation in relation to a moist substrate also significantly impacted oxygen bioavailability to the embryo and hence ADHase activity. Corn seeds without H2O2 imbibed significantly more slowly than those with oxygen fortification by 0.15% H2O2. Increased oxygen bioavailability improved the metabolism of the seeds, which extruded 5-fold more protons from the embryos. Each treated embryo consumed twice the amount of oxygen as compared with the untreated one and likewise for treated and untreated endosperms. Increased oxygen bioavailability may be used to improve production of the tested crops. The results from this research imply that consideration should be given to including oxygen fortification in seed coatings for aged seeds and for large seeds regardless of age. The artificial provision of bioavailable oxygen might be effective in rescuing the germplasm in aged seeds in plant breeding and in crop production.
Teresa Olczyk, Yuncong Li, Waldemar Klassen, and Qingren Wang
Summer cover crops can improve soil fertility by adding organic matter, supplying nutrients through mineralization, reducing nutrient leaching, and improving soil water and nutrient holding capacity. Other benefits include weed suppression and reduction of soil parasitic nematodes. A series of field experiments have been conducted at the UF IFAS Tropical Research and Education Center in Homestead, Florida to evaluate several summer cover crops for use in vegetable production in South Florida followed by field demonstrations conducted in the growers' fields. Best performing cover crops were legumes: velvet bean (Macuna deeringiana) and sunn hemp (Crotalaria juncea L. `Tropic Sun') providing 13 and 11 Mt of dry matter/ha, respectively. Sunn hemp supplied 330 kg N/ha followed by velvet been with 310 kg N/ha. Traditional summer cover crop sorghum-Sudan produced 4 Mt of dry matter/ha and retained only 36 kg N/ha. In addition Sunn hemp significantly reduced soil parasitic nematodes for successive crops. Limitations in use of Sunn hemp by more vegetable growers in South Florida include cost and availability of seeds.
Qingren Wang, Waldemar Klassen, Edward A. Evans, Yungcong Li, and Merlyn Codallo
Mulching in vegetable cultivation has been widely used to conserve water and improve yield. Field experiments with four treatments, including yard waste compost combined with plastic mulches in raised beds for winter fresh market bell peppers (Capsicum annuum L.), were conducted at two experimental sites [Pine Island Farm (PIF) and Tropical Research and Education Center (TREC)] in Miami-Dade county for two different years each. The treatments were: 1) control (PM): plastic mulch alone; 2) MC33: fumigation of the soil with a mixture of methyl bromide and chloropicrin and covered with plastic mulch; 3) herbicide/OM: organic mulch sprayed with herbicides (S-metolachlor and napropamide) without plastic mulch; and 4) PM/OM: organic mulch covered by plastic mulch. The treatment of PM/OM at both experimental sites in 2 years each increased the total marketable yields of bell pepper by 1.5- to 3.2-fold, total extra large fruit yields by 2.0- to 5.7-fold, and total large fruit yields by 1.4- to 2.6-fold, respectively, on average compared with the control, although some exceptions occurred between the two years at the TREC site. Under most circumstances at both experimental sites in two different years each, the PM/OM treatment also improved the total marketable yield and fruit quality (such as extra large fruits) for the first two harvests, which shows a preference to provide winter fresh market vegetables to meet a high demand. The economic benefit by PM/OM was the greatest among all the treatments. The results suggest that the application of organic mulch combined with plastic mulch can improve bell pepper yield and quality as a result of the improvement of soil fertility, especially the early harvests of winter fresh market fruits, which has shown a potential in the development of sustainable agriculture.
Qingren Wang, Waldemar Klassen, Yuncong Li, Merlyn Codallo, and Aref A. Abdul-Baki
Intensive rainfall during summer causes substantial nutrient leaching in a subtropical region, where most vegetable lands lay fallow during this period. Also, an excessive amount of irrigation water supplied during the winter vegetable growing season leads to soil nutrient loss, which greatly impacts vegetable yields, especially in soils that possess a low capacity to retain soil water and nutrients. A 2-year field experiment was carried out to evaluate the effects of various summer cover crops and irrigation rates on tomato yields and quality, and on soil fertility in a subtropical region of Florida. The cover crops were sunn hemp [Crotalaria juncea (L.) `Tropic Sun'], cowpea [Vigna unguiculata (L.) Walp, `Iron Clay'], velvetbean [Mucuna deeringiana (Bort.) Merr.], and sorghum sudangrass [Sorghum bicolor × S. bicolor var. sudanense (Piper) Stapf.], with a weed-free fallow as a control. The cover crops were planted during late Spring 2001 and 2002, incorporated into the soil in the fall, and tomatoes [Lycopersicon esculentum (Mill.) `Sanibel'] were grown on raised beds during Winter 2001–02 and 2002–03, respectively. Irrigation in various treatments was controlled when tensiometer readings reached –5, –10, –20, or –30 kPa. The cover crops produced from 5.2 to 12.5 Mg·ha–1 of above ground dry biomass and 48 to 356 Mg·ha–1 of N during 2001–02 and from 3.6 to 9.7 Mg·ha–1 of dry biomass and 35 to 277 kg·ha–1 of N during 2002–03. The highest N contribution was made by sunn hemp and the lowest by sorghum sudangrass. Based on 2-year data, tomato marketable yields were increased from 14% to 27% (p ≤ 0.05) by growing cover crops, and the greatest increase occurred in the sunn hemp treatment followed by the cowpea treatment. Irrigation at –10, –20, and –30 kPa significantly improved marketable yields by 14%, 12%, and 25% (p ≤ 0.05) for 2001–02, and 18%, 31%, and 34% (p ≤ 0.05) for 2002–03, respectively, compared to yields at the commonly applied rate, –5 kPa (control). Irrigation at –30 kPa used about 85% less water than at –5 kPa. Yields of extra-large fruit in the sunn hemp and cowpea treatments from the first harvest in both years averaged 12.6 to 15.2 Mg·ha–1, and they were significantly higher than yields in the fallow treatment (10.2 to 11.3 Mg·ha–1). Likewise at –30 kPa yields of extra-large fruit from the first harvest for both years were 13.0 to 15.3 Mg·ha–1 compared to 9.8 to 10.7 Mg·ha–1 at –5 kPa. Soil NO3-N and total N contents in sunn hemp and cowpea treatments were significantly higher than those in fallow. The results indicate that growing legume summer cover crops in a subtropical region, especially sunn hemp and cowpea, and reducing irrigation rates are valuable approaches to conserve soil nutrients and water, and to improve soil fertility and tomato yields and quality.
Rafael Muñoz-Carpena, Yuncong C. Li, Waldemar Klassen, and Michael D. Dukes
A low-volume/high frequency (LVHF) soil moisture-based drip irrigation system was tested on a shallow sandy soil at a commercial tomato (Lycopersicon esculentum) farm in southern Florida. Six LVHF irrigation treatments were compared with the standard commercial practice on the farm (control), where a portable pump was used for manual drip irrigation twice each week. In the six LVHF treatments the system was continuously pressurized by means of an electrical pump and a pressure tank, and controlled by an irrigation timer set to irrigate a maximum of five times per day with the irrigation time (i.e., volume) set according to historical evapotranspiration (ET) demands in the area. Two treatments were based on timer schedules, one to supply 100% of the maximum recommended crop water needs in the area based on historical ET (ET-100%), and the other to supply 150% of those needs (ET-150%). The other four treatments were created by interfacing two types of soil moisture sensors (switching tensiometers and granular matrix sensors with control modules) set at two moisture points (wet = 10 kPa, optimal = 15 kPa) in a closed control loop with the irrigation timer programmed at the ET-100% schedule. Results showed that the six LVHF treatments reduced water use while not significantly affecting tomato yields. Switching tensiometers at the 15 kPa set point performed the best (up to 73% reduction in water use when compared to the control, 50% with respect to ET-100%). The results show that water use below historical ET levels can be obtained without sacrificing yield by keeping the root zone moisture at controlled levels with the soil-moisture based system. Routine maintenance was critical for reliable operation of the switching tensiometers. Granular matrix sensor based irrigation behaved erratically, and did not improve water savings compared to ET-100%, indicating that this system was not effective under the conditions of the area due to the sensor's slow response to frequent wetting-rewetting cycles and characteristics of the interface.
Gladis M. Zinati, Herbert H. Bryan, Waldemar Klassen, and Aref A. Abdul-Baki
In the quest to produce tomatoes without using methyl bromide, cover crops including sunnhemp, cowpea, hairy vetch, and sorghum sudan were planted on calcareous gravelly soils of southern Florida in Oct. 1998. These crops, singly or in mix, were grown on raised beds for 3 months before they were mowed down with no tillage. Sorghum sudan was plowed down and covered with plastic mulch, a conventional farming practice. In addition, uncropped plots fertilized with 6 N–2.6P–10K at 0 or 1124 kg·ha–1 were either treated with or without methyl bromide-chloropicrin and plowed down. `Sanibel' tomatoes (Lycopersicon esculentum Mill) were transplanted in two plant densities (one row vs. two rows on a bed) immediately after mowing. Tomatoes were fertigated with 112 N and 186 K kg·ha–1 during the growing season. Sunnhemp biomass alone or in mix with cowpea was higher than any other treatment. Biomass of sorghum sudan and hairy vetch were lowest. Canopy coverage, nutrient content of cover crops, and their effects on tomato growth, nutrient content, and yield will be discussed.