**6. Microwave soil heating**

*<sup>h</sup>* <sup>¼</sup> *<sup>k</sup>*

*Sustainable Crop Production*

viscous and thermal diffusion rates given by [21]:

where v is the kinematic viscosity (m<sup>2</sup> s

(m<sup>2</sup> s �1 ).

soil.

**Figure 4.**

*the air.*

**184**

*<sup>L</sup>* <sup>0</sup>*:*<sup>825</sup> <sup>þ</sup>

where k is the thermal conductivity of the heating fluid (W m�<sup>1</sup> K�<sup>1</sup>

*RaL* <sup>¼</sup> *<sup>g</sup><sup>β</sup>*

of the fluid; υ is the kinematic viscosity of the fluid medium; α is the thermal diffusivity of the fluid medium; and L is the characteristic length of the surface. Finally, the Prandtl number used in Eq. (14) is a relationship between the fluid's

Prandtl number, and L is the characteristic length of the object being heated (m). The Rayleigh number (RaL) in Eq. (14) is also based on a complex relationship between temperature and the physical properties of the fluid. It is given by [21]:

where g is the acceleration due to gravity; β is the thermal expansion coefficient

*Pr* <sup>¼</sup> *<sup>ν</sup>*

Close examination of these equations shows that the convective heat transfer coefficient is dependent on the temperature differential between the fluid and the surface of the soil (see **Figure 4**) and the apparent surface area of the heat transfer interface. Injecting the steam into the soil through hollow tines effectively increases the surface area of the heat transfer interface between the cool soil and hot steam. Semi-commercial steam soil sanitation systems have been in operation for some time [13, 19]. They are functional, though their application is limited, because they are energy expensive and difficult to use due to their large and heavy operation systems. Soil heat treatment may be better achieved through direct heating of the

*Convective heat transfer coefficient (h) for air as a function of temperature differential between an object and*

�1

8 ><

>:

0*:*387*Ra*<sup>1</sup>*=*6 *L*

h i<sup>8</sup>

*=*27 9 >=

>;

*να* ð Þ *Ts* � *<sup>T</sup>*<sup>∞</sup> *<sup>L</sup>*<sup>3</sup> (15)

*<sup>α</sup>* (16)

) and α is the thermal diffusivity

(14)

), Pr is the

<sup>1</sup> <sup>þ</sup> <sup>0</sup>*:*<sup>492</sup> *Pr* � �<sup>9</sup>*=*16

Microwaves are non-ionizing electromagnetic waves (**Figure 5**) with a frequency of about 300 MHz to 300 GHz and the wavelength range of 1 m to 1 mm [23]. Biological and agricultural systems are electro-chemical in nature [24] and a mixture of organic and dipole molecules, i.e., H2O, arranged in different geometries [25, 26].

Interest in the study of the interactions of ultra-high frequency electromagnetic energy with complex biological system dates back to the nineteenth century [27]. The interactions of microwave energy with living systems are characterized at atomic, molecular, cellular and subcellular level [24].

The basic consideration in measuring the influence of microwave irradiation on living systems is the determination of the induced electromagnetic field and its spatial distribution. The bio-effects of microwave treatments can be described solely by differences in temperature profile between microwave and conventionally heated systems [28]. The energy of microwave photon at 2.45 GHz is 0.0016 eV [29]. This is not enough energy to break the structure of organic molecules [30]. The basic interactive mechanism of microwave energy with biological system/ materials is inducing torsion on polar molecules, i.e., H2O, Proteins and DNA, by induced electric field [31]. Oscillations in this torsion occur 2.45 billion times/ second for 2.45 GHz waves. These oscillations manifest as internal kinetic energy in the material, which is heat.

Microwave (electromagnetic) heating has major advantages over conventional heating techniques. Some of these include: rapid volumetric heating as opposed to surface heating only, precise control, rapid start up and shut down [32], and in the case of soil, having a lighter apparatus than a steam generator to avoid soil compaction issues.

Many of the earlier experiments on plant material focused on the effect of radio frequencies [33] on seeds [27]. In many cases, exposure to low energy densities resulted in increased germination and vigor of the emerging seedlings [34, 35]; however, exposure to higher energy densities usually resulted in seed death [27, 36, 37].

**Figure 5.** *The electromagnetic spectrum (adapted from [22]).*

Davis et al. [38, 39] were among the first to study the lethal effects of microwave heating on weed seeds. They treated seeds, with and without any soil, in a microwave oven and showed that seed damage was mostly influenced by a combination of seed moisture content and the energy absorbed in each seed. In addition, they suggested that both the specific mass and specific volume of the seeds were strongly related to a seed's susceptibility to damage by microwave fields. The association between the seed's volume and its susceptibility to microwave treatment may be linked to the "*radar cross-section*" [40] presented by seeds to propagating microwaves. Large radar cross-sections allow the seeds to intercept, and therefore absorb, more microwave energy.

Ferriss [8] conducted experiments on soil samples with moisture contents between 7 and 37% (wet/dry-weight) and showed that treatment in a microwave oven for 150 seconds eliminated populations of *Pythium*, *Fusarium* and all nematode species, except *Heterodera glycines* in the soil samples. Compared with autoclaving or Methyl bromide (MB) treatment, he found that microwave treatments released less nutrient into the soil solution but had less effect on soil *prokaryotes* and resulted in less recolonization of the soil by *Fusarium* and other fungi after treatment. Similar observations were made by Mattner and Brodie [41] during a preliminary experiment in soils growing strawberry runners at Toolangi, Victoria.

Speir et al. [42] examined the effect of microwave energy on low fertility soil (100 randomly selected cores at a depth of 50 mm), microbial biomass, nitrogen, phosphorus, and phosphatase activity. They reported that an increase in microwave treatment duration (90 seconds) dramatically increased the nitrogen level in the soil by a factor of approximately 10 times (106 μgNg–<sup>1</sup> ) compared with untreated soil (9– 10 μgNg–<sup>1</sup> ), but available phosphorus concentration declined as treatment time increased. Furthermore, relevant to soil productivity, Gibson et al. [43], demonstrated that shoot and root growth of birch (*Betula pendula*) significantly increased in microwave irradiated soil. Their experiment evaluated the effect of microwave treatment of soil supplemented with two mycorrhizas on birch seedlings. Shoot growth progressively increased with irradiation duration, with the highest dry shoot weight of 84 mg coinciding with the highest irradiation duration (of 120 seconds) compared to non-irradiated soil which resulted in 25 mg of growth. This result was achieved with no mycorrhizal supplementation. In addition, a recent study reported that microwave (915 MHz; different power � duration) soil treatment increased the dissolved organic carbon (+1.6-fold compared with the control), inorganic phosphorus (+1.2-fold compared with the control), and nitrate content in soil [44]. In addition, they grew the pregerminated seeds of *Medicago truncatula* Gaertn. in microwave treated soil and found that its dry biomass accumulation significantly increased in response to soil heating (75–80°C), compared with the untreated control soils.

Since then there has been ongoing research interest in microwave soil treatment and weed management. **Table 1** lists a subset of the papers that have been published on these and related topics. The consensus from these studies is that: microwave treatment can kill plants; moderate microwave treatment can break dormancy in some hard-seeded species; and high energy microwave treatment can sanitize soil.

Typically, responses of weed seeds and soil biota are both energy and depth dependent, because of the absorption of microwave energy with soil depth. The relationships between applied microwave energy and seed or biota survival at different depths are given by:

$$\mathbf{S} = \mathbf{a} \bullet \text{erfc} \left[ \mathbf{b} \bullet \left( \Psi \bullet \mathbf{e}^{-2 \text{cd}} - \mathbf{f} \right) \right] \tag{17}$$

experimentally. This is illustrated by the relationships for weed seeds and bacteria

**Paper title Reference** Douglas- fir tree seed germination enhancement using microwave energy [45] Microwave processing of tree seeds [46] Increasing legume seed-germination by VHF and microwave dielectric heating [47] Effects of low-level microwave radiation on germination and growth rate in corn seeds [48] Effects of microwave energy on the strophiole, seed coat and germination of acacia seeds [35] The effect of microwave-energy on germination and dormancy of wild oat seeds [49]

[50]

[55]

[58]

[36]

[66]

[67]

[70]

The effect of externally applied electrostatic fields, microwave radiation and electric currents on plants and other organisms, with special reference to weed control

treatments on barley seed germination and vigor

*Microwave Soil Treatment and Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.89684*

feasibility

*gossypiifolia* L.)

a field

**Table 1.**

**187**

metabolites of wheat

Control of field weeds by microwave radiation [51] Effect of microwave irradiation on germination and initial growth of mustard seeds [52] Inhibition of weed seed germination by microwaves [53] A possibility of correction of vital processes in plant cell with microwave radiation [54] Microwave irradiation of seeds and selected fungal spores [7] Response surface models to describe the effects and phytotoxic thresholds of microwave

Energy efficient soil disinfestation by microwaves [56] Microwave effects on germination and growth of radish (*Raphanus sativus* L.*)* seedlings [57] Report on the development of microwave system for sterilization of weed seeds: stage I –

Design, construction and preliminary tests of a microwave prototype for weed control [59] Thermal effects of microwave energy in agricultural soil radiation [60] Influence of low-frequency and microwave electromagnetic fields on seeds [61] An improved microwave weed killer [62] Observations on the potential of microwaves for weed control [63] Plant response to microwaves at 2.45 GHz. [64] Germination inhibition of undesirable seed in the soil using microwave radiation [65] Effect of microwave radiation on seed mortality of rubber vine (*Cryptostegia grandiflora* R.

Br.), parthenium (*Parthenium hysterophorus* L.) and bellyache bush (*Jatropha*

Effects of microwave treatment on growth, photosynthetic pigments and some

redistribution of cellular water as studied by NMR relaxation measurements

Microwave seed treatment reduces hardseededness in *Stylosanthes seabrana* and promotes

Effect of microwave fields on the germination period and shoot growth rate of some seeds [68] Germination of *Chenopodium album* in Response to Microwave Plasma Treatment [69] Work conditions for microwave applicators designed to eliminate undesired vegetation in

sterilize the soil. Although there is a general reduction in soil bacteria after

*Literature addressing the application of microwave technology to seed and weed treatment.*

Unlike in the case of chemical soil fumigants, microwave soil treatment does not

in (**Figures 6** and **7**).

where Ψ is the microwave energy density at the soil surface (J cm�<sup>2</sup> ), d is the depth in the soil (m) and a, b, c, and f are constants to be determined

Davis et al. [38, 39] were among the first to study the lethal effects of microwave heating on weed seeds. They treated seeds, with and without any soil, in a microwave oven and showed that seed damage was mostly influenced by a combination of seed moisture content and the energy absorbed in each seed. In addition, they suggested that both the specific mass and specific volume of the seeds were strongly related to a seed's susceptibility to damage by microwave fields. The association between the seed's volume and its susceptibility to microwave treatment may be linked to the "*radar cross-section*" [40] presented by seeds to propagating microwaves. Large radar cross-sections allow the seeds to intercept, and therefore absorb,

Ferriss [8] conducted experiments on soil samples with moisture contents between 7 and 37% (wet/dry-weight) and showed that treatment in a microwave oven for 150 seconds eliminated populations of *Pythium*, *Fusarium* and all nematode species, except *Heterodera glycines* in the soil samples. Compared with autoclaving or Methyl bromide (MB) treatment, he found that microwave treatments released less nutrient into the soil solution but had less effect on soil *prokaryotes* and resulted in less recolonization of the soil by *Fusarium* and other fungi after treatment. Similar observations were made by Mattner and Brodie [41] during a preliminary experi-

Speir et al. [42] examined the effect of microwave energy on low fertility soil (100 randomly selected cores at a depth of 50 mm), microbial biomass, nitrogen, phosphorus, and phosphatase activity. They reported that an increase in microwave treatment duration (90 seconds) dramatically increased the nitrogen level in the soil by a

), but available phosphorus concentration declined as treatment time

Since then there has been ongoing research interest in microwave soil treatment and weed management. **Table 1** lists a subset of the papers that have been published on these and related topics. The consensus from these studies is that: microwave treatment can kill plants; moderate microwave treatment can break dormancy in some hard-seeded species; and high energy microwave treatment can sanitize soil. Typically, responses of weed seeds and soil biota are both energy and depth dependent, because of the absorption of microwave energy with soil depth. The relationships between applied microwave energy and seed or biota survival at

where Ψ is the microwave energy density at the soil surface (J cm�<sup>2</sup>

depth in the soil (m) and a, b, c, and f are constants to be determined

<sup>S</sup> <sup>¼</sup> <sup>a</sup> <sup>∙</sup> erfc b <sup>∙</sup> <sup>Ψ</sup> <sup>∙</sup> <sup>e</sup>�2cd � <sup>f</sup> (17)

), d is the

increased. Furthermore, relevant to soil productivity, Gibson et al. [43], demonstrated that shoot and root growth of birch (*Betula pendula*) significantly increased in microwave irradiated soil. Their experiment evaluated the effect of microwave treatment of soil supplemented with two mycorrhizas on birch seedlings. Shoot growth progressively increased with irradiation duration, with the highest dry shoot weight of 84 mg coinciding with the highest irradiation duration (of 120 seconds) compared to non-irradiated soil which resulted in 25 mg of growth. This result was achieved with no mycorrhizal supplementation. In addition, a recent study reported that microwave (915 MHz; different power � duration) soil treatment increased the dissolved organic carbon (+1.6-fold compared with the control), inorganic phosphorus (+1.2-fold compared with the control), and nitrate content in soil [44]. In addition, they grew the pregerminated seeds of *Medicago truncatula* Gaertn. in microwave treated soil and found that its dry biomass accumulation significantly increased in response to soil heating (75–80°C), compared with the untreated control soils.

) compared with untreated soil (9–

ment in soils growing strawberry runners at Toolangi, Victoria.

factor of approximately 10 times (106 μgNg–<sup>1</sup>

different depths are given by:

**186**

more microwave energy.

*Sustainable Crop Production*

10 μgNg–<sup>1</sup>


#### **Table 1.**

*Literature addressing the application of microwave technology to seed and weed treatment.*

experimentally. This is illustrated by the relationships for weed seeds and bacteria in (**Figures 6** and **7**).

Unlike in the case of chemical soil fumigants, microwave soil treatment does not sterilize the soil. Although there is a general reduction in soil bacteria after

There is also considerable evidence that microwave soil treatment releases more

Fully replicated pot and field plot experiments have been undertaken over an extended period of time by the authors to better understand the impact of presowing microwave soil treatment on crop growth. In all cases, the experiments had at least 5 experimental replicates and in many cases, they used 10 experimental replicates. Experiments were undertaken to explore the effect of pre-sowing microwave soil treatments on plant growth and yield of wheat (*Triticum* spp*.*), rice (*Oryza sativa*), maize (*Zea mays*), canola (*Brassica napus*), processing tomatoes and strawberry runners. In most cases the potted experiments were repeated two or three times and in some cases the field experiments were also repeated. Microwave energy was applied to the soil in pots or in situ using a trailer mounted microwave prototype system with 4 individual 2 kW microwave generators (see **Figure 8**). The crops were planted within hours of the microwave treatment, once the soil had returned to ambient temperature. Plant growth rate, final plant height, and crop yield showed significant increases with increasing microwave energy (**Table 2**). In the potted trials and in one wheat field trial, hand weeded controls were included in the experiments to determine whether crop growth response was

Pre-sowing microwave soil treatment was found to have significant beneficial effects on subsequent crop growth. Most crops showed a typical Gaussian Error Function response to increasing microwave soil treatment dosage (**Figure 6**), as would be expected if the pre-sowing soil treatment were acting as a soil fumigant

*Prototype 4 by 2 kW microwave weed killer in a strawberry runner field at Toolangi, Victoria.*

nitrogen sources in the soil for the crop growth [73]. This may be due to the resilience of nitrifying bacteria and archaea to microwave soil heating. Khan et al. [72] showed that microwave soil treatment did not significantly affect ammonia oxidizing bacteria or ammonia oxidizing archaea. Vela et al. [74] also demonstrated that nitrifying bacteria in the soil were resilient to 40 kJ cm<sup>2</sup> of microwave energy at the soil surface; which is 70 times higher than the energy densities used during

experimental work undertaken by the current authors.

*Microwave Soil Treatment and Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.89684*

**7. Crop responses**

simply due to less weed competition.

(**Figure 9**).

**Figure 8.**

**189**

**Figure 6.** *Response of multiple species of weed seeds as a function of applied microwave energy and soil depth [71].*

**Figure 7.**

*Response of soil bacteria as a function of applied microwave energy and soil depth [71].*

microwave treatment (**Figure 7**), Khan et al. [72] demonstrated that immediately after microwave soil treatments, the relative abundance of *Firmicutes* increased while the relative abundance of *Proteobacteria* decreased significantly. They also showed that the relative abundances of beneficial soil microbes (*Micromonosporaceae*, *Kaistobacter* and *Bacillus*) were significantly higher, as soils recovered from high heating intensities induced by microwave soil treatment, compared with untreated soils.

*Microwave Soil Treatment and Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.89684*

There is also considerable evidence that microwave soil treatment releases more nitrogen sources in the soil for the crop growth [73]. This may be due to the resilience of nitrifying bacteria and archaea to microwave soil heating. Khan et al. [72] showed that microwave soil treatment did not significantly affect ammonia oxidizing bacteria or ammonia oxidizing archaea. Vela et al. [74] also demonstrated that nitrifying bacteria in the soil were resilient to 40 kJ cm<sup>2</sup> of microwave energy at the soil surface; which is 70 times higher than the energy densities used during experimental work undertaken by the current authors.

#### **7. Crop responses**

Fully replicated pot and field plot experiments have been undertaken over an extended period of time by the authors to better understand the impact of presowing microwave soil treatment on crop growth. In all cases, the experiments had at least 5 experimental replicates and in many cases, they used 10 experimental replicates. Experiments were undertaken to explore the effect of pre-sowing microwave soil treatments on plant growth and yield of wheat (*Triticum* spp*.*), rice (*Oryza sativa*), maize (*Zea mays*), canola (*Brassica napus*), processing tomatoes and strawberry runners. In most cases the potted experiments were repeated two or three times and in some cases the field experiments were also repeated. Microwave energy was applied to the soil in pots or in situ using a trailer mounted microwave prototype system with 4 individual 2 kW microwave generators (see **Figure 8**).

The crops were planted within hours of the microwave treatment, once the soil had returned to ambient temperature. Plant growth rate, final plant height, and crop yield showed significant increases with increasing microwave energy (**Table 2**). In the potted trials and in one wheat field trial, hand weeded controls were included in the experiments to determine whether crop growth response was simply due to less weed competition.

Pre-sowing microwave soil treatment was found to have significant beneficial effects on subsequent crop growth. Most crops showed a typical Gaussian Error Function response to increasing microwave soil treatment dosage (**Figure 6**), as would be expected if the pre-sowing soil treatment were acting as a soil fumigant (**Figure 9**).

microwave treatment (**Figure 7**), Khan et al. [72] demonstrated that immediately after microwave soil treatments, the relative abundance of *Firmicutes* increased while the relative abundance of *Proteobacteria* decreased significantly. They also showed that the relative abundances of beneficial soil microbes (*Micromonosporaceae*, *Kaistobacter* and *Bacillus*) were significantly higher, as soils recovered from high heating intensities induced by microwave soil treatment, compared with

*Response of soil bacteria as a function of applied microwave energy and soil depth [71].*

*Response of multiple species of weed seeds as a function of applied microwave energy and soil depth [71].*

untreated soils.

**188**

**Figure 7.**

**Figure 6.**

*Sustainable Crop Production*


access to the site once the soil has cooled to ambient temperatures. Unlike, other thermal treatment systems, such as steam treatment, microwave systems can be light and highly controllable, reducing other impacts on the soil such as compaction. Also, unlike other soil sanitation techniques, it is evident that microwave treatment does not sterilize the soil, but favors beneficial species of soil biota making more nutrients available for better plant growth. From these perspectives, microwave soil treatment may become an important pre-sowing soil sanitation technology for high-value cropping systems, allowing agricultural systems to better bridge

the crop yield gap.

*Microwave Soil Treatment and Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.89684*

**Author details**

**191**

Graham Brodie\*, Muhammad Jamal Khan and Dorin Gupta

\*Address all correspondence to: grahamb@unimelb.edu.au

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

The University of Melbourne, Victoria, Australia

provided the original work is properly cited.

*Means with different superscript letters (i.e. a, b, c etc) are statistically different from one another at a probability of 0.05.*

#### **Table 2.**

*Summary of pot and field trial crop yields in response to microwave soil treatment.*

**Figure 9.**

*Canola pod yield response to increasing microwave treatment.*

#### **8. Conclusions**

Pre-sowing microwave soil treatment acts as a soil sanitation technology and results in significant increases in crop yield, as would be expected from other soil sanitation techniques. Microwave treatment has some major advantages over other soil sanitation techniques in that it is purely thermal in nature and allows immediate

#### *Microwave Soil Treatment and Plant Growth DOI: http://dx.doi.org/10.5772/intechopen.89684*

access to the site once the soil has cooled to ambient temperatures. Unlike, other thermal treatment systems, such as steam treatment, microwave systems can be light and highly controllable, reducing other impacts on the soil such as compaction.

Also, unlike other soil sanitation techniques, it is evident that microwave treatment does not sterilize the soil, but favors beneficial species of soil biota making more nutrients available for better plant growth. From these perspectives, microwave soil treatment may become an important pre-sowing soil sanitation technology for high-value cropping systems, allowing agricultural systems to better bridge the crop yield gap.
