Raffaelli et al., used a steaming machine that uses a negative pressure steam technique that involves applying steam to the top 10 cm of the soil underneath a tarp into the soil. A fan is also used creating suction through polypropene tubes buried in the soil . The negative pressure technique does a better job in achieving greater steam temperatures in the lower layers of the soil . In contrast, the sheet steaming technique does a better job in steaming the top layers of the soil but often overheats the soil killing beneficial microbes . New steam machines have incorporated steaming hoods that are moveable where pipes are put directly into the soil and treat smaller areas and do not involve covering the soil with sheets . Raffaelli et al., observed increased carrot yields after steaming and it reduced weed competition during the first 45 days based on weed dry biomass at harvest. Raffaelli et al., also observed 90% weed control at 30 DAT and 70% weed control at crop maturity. Another study done by Fennimore et al., cannabis drying system observed a 25% to 93% time reduction in hand weeding time in strawberry when a temperature of 70 °C was held for 30 minutes. After steam application the temperature peaks and then there is a steady decline in soil temperatures . Co-application of exothermic compounds with steam helps maintain the temperature at critical temperatures for a longer period with better pest efficacy .
Studies done in Sweden and Norway saw slow declines in steam temperatures that dropped from 100 °C to 60 °C in a matter of 2 hours and that is due to using an older steam machine . There needs to be greater steam penetration in the soil to ensure that soil aggregates are targeted, and the critical temperature dwell time are realized .Applying steam in bands centered on the seedline is an efficient method of placing steam only where it is needed. Melander et al., found that using band steaming lowers energy consumption by only steaming in bands where the seedline will be located. Banded steaming is the most favored application technique and has been used recently in strawberry and lettuce production . Weed emergence was evaluated after banded steam application and resulted in a 30% reduction in hand weeding time in the intra rows of the bed . Steam controls weeds in the band closest to the crop where it is most difficult to control weeds. The weeds that are in the intra row space outside the bands can then easily be removed by a cultivator .A study performed by Fennimore et al., found that mobile steaming is more favorable than stationary steaming because it is easier to control the soil temperature by adjusting the speed of applicator and the ability to move with minimal labor input. In addition, steam involves no worker exposure to fumigants . Melander et al., and Pinel et el., stress the point that there is not much research done on mobile steaming in field settings, given the great need.
The mobile steamer used in the Melander et al., study has been used commercially and is close to the type of unit used in my 2020 spinach trial: the Steamy . It uses a rototiller to incorporate steam through shanks into the soil at about 1.6 m wide and 20 cm deep .Moreover, there have been studies looking at the amount of energy that is used by steam machines and Van Loenen et al., controlled major soil pests at a lower energy state using aerated steaming at a lower temperature of 50-60 °C in 11 minutes but was done in a laboratory setting. More banded steam studies need to be conducted on a larger field scale that mimics a commercial production with a natural disease population in the soil. More research needs to bedone to improve steam generator systems and efficacy in soil steam incorporation while using less fuel and maintaining the proper dwell time . The goal for steam use is explained in high-value cropping systems as a technique used in integrated pest management programs to reduce the abundance of harmful soil pests with little effect on the biology of the soil by disinfesting the soil before planting the crop . The process uses water vapor to transfer the heat from a heat source to the soil . Furthermore, when the water vapor is injected into soil and comes into contact with the cooler soil particle, then steam condenses and transfers the heat energy to the soil with the goal of reaching critical temperatures. Pathogens on or near the soil particles are controlled by the lethal temperatures . In addition, steam disinfestation will benefit organic growers and help those that have fields near sensitive sites like schools where strict regulations are put in place and chemicals cannot be applied . This will ultimately help with net revenues, increase yields, and reduce risk of economic loss to soil borne diseases .
Vegetable crops have been grown commercially in the Coastal areas of California since the early 1900s. These areas have fertile soils and a Mediterranean climate favorable for high value vegetable production year-round . Various types of romaine lettuce, carrot, broccoli, and spinach are commonly grown vegetables in Coastal California . Farmers have traditionally managed weeds by hand weeding, but are currently facing a labor shortage. Workers are not always available when needed. Crops can have multiple flushes of weeds depending on the weed seed bank in a given field during the growing season . High density lettuce crops are often weedy in the intra row space close to the crop where the weeds are difficult to remove with a cultivator. This leaves hand weeding as the best option . Leafy greens are grown intensively and continuously, often on the same land after each harvest throughout the year . Weed management in organic vegetable fields is even more difficult than conventional fields. Organic producers have few good weed control tools and new weed control options are needed . It is essential to use physical, cultural, and chemical control practices to develop an integrated pest management plan to avoid pest resistance and ensure good weed and pathogen control . Current circumstances such as increased labor, hand weeding costs, and lack of new herbicides threaten profitability and clearly there is a need for more pest control options. Hand weeding costs are increasing, partly due to California legislation in 2016 that increase the minimum wage by a $1.00 per year until it reaches $15.00 in 2022 . The agricultural industry will suffer because farmers will pay their workers more especially inlabor intensive crop systems . Crop and weed competition are a constant struggle and the crop needs to be protected from weeds and the earlier it is done, there is less of a chance of crop loss to weeds . In the late nineteenth century, steam pasteurization was developed and used to inject steam into the soil to eliminate soil pathogens, insects, and weed seeds . Steam soil pasteurization is a physical method that controls some of the most troublesome pathogens like Pythium spp. and S. minor in lettuce production . Old methods like sheet steaming applied steam to the entire soil profile and resulted in high fuel consumption . To reduce costs, prototype band steamers were developed to target steam into narrow strips of the soil carefully positioned where the seedline will be located after steam application . Mobile band steaming may be the best option for pest control in the seedline . Amendments in combination with steam may improve steam efficacy by increasing temperature dwell time in the soil and result in better weed and pathogen control . Hydrogen peroxide is an organic-compliment antiseptic that can kill harmful fungi and bacteria . Hydrogen peroxide is a naturally occurring molecule commonly used to disinfect foods and is water-soluble and decomposes quick with no residue left after application. Importantly, growing tray this chemical can create an exothermic reaction in the soil, in theory enhancing steam by increasing soil temperature when co-applied . Hydrogen peroxide continues to be a unique and useful amendment used in food processing plants that has much potential to be studied more extensively in high-value production with different steam applicators.Two lettuce trials were conducted in 2020 and one in 2021.
A single spinach trial was conducted in 2020 and a single carrot trial in 2021. With the exception of the spinach trial, all trials utilized a band steam applicator. In the spinach trial, a self-propelled steam applicator was utilized . Thus, three field trials were conducted in 2020 and two were conducted in 2021. The 2021 lettuce and carrot trials were performed using a banded steam machine developed by the University of Arizona at the Yuma Agricultural Center. For all trials, the steam applicator used softened water that was obtained from a Culligan Water softener. Critical trial events and dates can be found in Table 1. The site location for all trials was at the UC ANR Hartnell Research Station in Salinas, California. The soil type is a loam composed of 53% sand, 32% silt, 15% clay and with 2.09% organic matter. The electric conductivity of the soil is 1.65 with a pH of 7.03. Soils in all five trials were checked for abundance of Pythium spp. colonies and S. minor sclerotia before starting the trials. Soil samples were taken from all plots before and after steaming to measure efficacy on pathogen populations. Honest Observer By Onset temperature recorders were used to monitor soil temperatures during the steaming process and were left in the soil for 24 hours after steaming.During the summer of 2021, lettuce trial 4 and carrot trial 5 consisted of 8 beds. The lettuce trial plots were 180 ft long by one single bed 3.33 ft wide, whereas the carrot trial was 120 ft long by one bed wide. A prototype field steam applicator from the University of Arizona with a 35 BHP Clayton Sigma Fire generator equipped with a bed shaper with shanks that injected steam in a band, 4 inches deep by 4 inches wide centered on where the crop seed line was to be located . The steam applicator was attached to a 5520 John Deere tractor and was set at 2,000 RPM, moving 8 ft per minute during steaming. The lettuce variety was direct-seeded 3 DAT on July 12th 2021 and the carrot variety was seeded 1 DAT on July 9th 2021 in all plots. Both trials used a Stanhay belt planter that utilized 2.25 in-line seed spacing for 2 seed lines for the lettuce trial and scatter shoe for the carrot trial that dispersed the carrot seeds in a 3-inch band. Treatments were steam alone and a no-steam control. Treatments were replicated 4 times and arranged in a randomized complete block design. Data recorded in both trials were weed densities at 2-week intervals, disease incidence, temperature, time of hand weeding, lettuce canopy diameters and crop harvest.Management of weeds in high-value vegetable production systems is challenged by a lack of herbicides and a shortage of labor for hand weeding. Farmers are put in a difficult spot in trying to implement appropriate management plans to control pests and avoid chemical resistance while growing quality produce. Thermal methods to eliminate or reduce pathogens and weed seed in the soil may be a way to supplement the pest control needs of vegetable growers. Traditional steam pasteurization techniques used stationary methods of soil steaming commonly used in greenhouse production and is not suitable for in-field applications . Melander et al. and Pinel et al. suggested the need for a mobile steamer band application method which may provide an effective non-chemical alternative to soil fumigants. Steam pasteurization is especially attractive for organic farmers who have few tools to tackle pest management or conventional farmers who need to treat buffer zones in their fields when fumigants may not be applied. I learned that the ability to adjust the speed of application to control the soil temperature was an advantage in that it was possible to avoid over application of steam. We evaluated three steam applicators, two of which were commercial models that worked really well in field production sites. However, there is a need for improvements to consider in terms of design and speed. Steam was injected into the soil with a target temperature of 70oC, which was accomplished and maintained for 20 min dwell time in all trials.