Microwave Treatment of Soil for Weed and Pathogen Control

Soil is the foundation of agricultural and horticultural endeavors. The soil solution is a heterogeneous mixture of solids, liquids, gasses, and biological entities. The general health and nutritional status of soil determines the quantity and quality of its biological components, especially the plants that grow in it. The presence of pests and pathogens can impact soil health, leading to significant crop yield losses [1]. Various strategies are used to control pests and pathogens; however, modern agriculture has largely resorted to chemical control strategies.
Chemical pest control has been a blessing; however, the development of chemical resistance in the target pests and pathogens has been inevitable [2,3]. Various alternative control strategies have been explored, including soil heating. Soil steaming [4-6] and surface heating through flaming [7] or solarization have been explored; however, soil is a relatively poor conductor of heat [8] and the efficacy of soil heating systems is limited because of this. Microwave heating has also been considered, because microwaves volumetrically interact with materials and generates heat throughout the interaction volume, thus avoiding thermal conduction limits.

Microwaves are a form of light (electromagnetic energy), with wavelengths between 1cm and 1m long [9]. These wavelengths are invisible to humans; however, they strongly interact with dipolar molecules and ionic solutions, generating internal heat due to molecular agitation of these molecules in the electromagnetic field [10]. The extent of interaction between an electromagnetic field and a material depends on the material’s dielectric properties. Water is of interest to microwave heating because it has quite high dielectric properties at microwave wavelengths [10]. This is enhanced because water is a strong solvent of salts, creating ionic solutions, which greatly increases the loss factor of the dielectric properties [10]; however, the water molecule itself has a strong dipolar moment [11]. In the case of microwave treatment of soil, the water in the soil solution and the biological entities in the soil strongly interreact with the microwave fields to generate heat in the soil.

The propagation of microwave energy through soil depends upon the gravimetric (θg) and volumetric (θv) moisture content [12], bulk density [13], organic matter content [14], soil texture [15], and specific heat of the soil [8]. Soil moisture has the most effect on microwave soil heating [16]. The greatest challenge when heating soil, in-situ, is that the microwave energy must be projected into the soil rather than the soil being placed into a microwave oven cavity. Several devices, called applicators [9,17], can be used to apply microwave energy to a semi-infinite solid, like soil. The simplest option is to use an antenna, pointing to the ground [18]. The horn antenna is a very simple structure that has been used for soil heating for several decades [18,19]. Horn antennas are effective, but often heat more soil than is necessary for disinfection.
Novel applicator structures have been developed to restrict the application of microwave energy to the surface layers of the soil [20,21]. Slow-wave structures [21] and applicators based on the principle of frustrated total internal reflection [22] create evanescent microwave fields that propagate along the surface of the applicators, but do not penetrate very far into the soil, because of the exponential decay of the evanescent microwave fields. These heat the surface layers of the soil without wasting energy heating deeper layers of soil.

Davis et al. [23] demonstrated the efficacy of microwave energy for weed management. They developed a prototype system, called the “Zapper” [24], which could treat soil in situ, using a variant on a horn antenna to apply the microwave energy to the soil’s surface. To obtain consistent pre-emergent control of both broadleaved weeds and grasses, it was necessary to apply at least 183J cm- 2. Brodie et al. [25] later confirmed that 185J cm-2 of microwave energy, when applied to moist soil, could effectively kill various Lolium spp. (ryegrasses) seeds to a depth of 5cm. Treating seeds in dry soil required over 550J cm-2 of microwave energy to kill seeds to a depth of only 2-3cm [25].
The energy required to control emerged weeds using a horn antenna is quite variable (77-500J cm-2) [23,26], depending in the species and the height of the horn antenna above the ground. Recent experiments using a 15 cm wide slow-wave applicator, connected to a 5-kW microwave source, and being towed at an equivalent speed of approximately 0.6km hr-1 (17cm s-1), has demonstrated that applying 20J cm-2 of microwave energy can kill most emerged weeds (unpublished).

It has been demonstrated that microwave soil heating has an immediate impact on soil microbial communities [27]. The populations of some species are significantly reduced [28]; however, other species, including nitrifying bacteria and archaea, are relatively unaffected, except at extremely high doses of microwave energy [29-31]. Soil bacterial and fungal community compositions change significantly due to microwave soil treatment and recovery of biological diversity takes more than 4-5 weeks [30]. Recent experiments have demonstrated that microwave soil treatments, with similar intensity necessary to kill weed seeds, significantly reduces a number of soils borne fungal pathogens, including Fusarium spp., Macrophomina phaseolina, and Thielaviopsis basicola (unpublished).
The combination of removal of weed competition and soil disinfection provided by microwave soil treatment results in significant crop yield increases. In field experiments, increases in crop yield of between 18 % and 84%, compared with the untreated or hand weeded controls, have been observed [1,32]. Pot experiments have demonstrated that a single microwave soil treatment can provide significant crop yield increases over several seasons, with the longest observations spanning three years, so far [33].

Sustained experimental work has demonstrated that microwave energy can be used to effectively control weeds. Soil heating can inactivate dormant weed seeds, thus preventing germination and emergence; however, soil heating requires high energy inputs to achieve seed lethality. An additional benefit of microwave soil treatment is it potential to sanitise soil by changing the soil bacterial and fungal community profile. Microwave treatment does not sterilise the soil; however, it can reduce the populations of several economically important soil borne pathogens and significantly increase crop yields. Soil sanitation may find its application niche in high value horticulture, where soil fumigation is routinely applied prior to crop establishment. Control of emerged weeds requires far less microwave energy, especially when novel microwave applicators such as the slow-wave and frustrated total internal reflection principles, are adopted in their design.

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