Microwave Soil Treatment and Plant Growth

*Graham Brodie, Muhammad Jamal Khan and Dorin Gupta*

#### **Abstract**

Crop yield gaps can be partially overcome by soil sanitation strategies such as fumigation; however, there are fewer suitable fumigants available in the marketplace and growing concerns about chemical impacts in the environment and human food chain. Therefore, thermal soil sanitation has been considered for some time and microwave soil treatment has some important advantages over other thermal soil sanitation techniques, such as steam treatment. It is also apparent that microwave soil sanitation 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.

**Keywords:** microwave pasteurization, agriculture, pathogen control, nutrient, production response

#### **1. Introduction**

Crop yield gaps are a significant issue for food security and agricultural sustainability. Crop yield gaps are defined as the differences between optimal yield potential and actual crop yield [1]. Yield potential (Yp) is the yield of a crop cultivar when grown in an environment to which it is adapted, with non-limiting water and nutrient supplies, and with pests, weeds, and diseases being effectively controlled [1]. For example, the impact of weeds on crop yield potential has been widely demonstrated [2] and modeled [3–5]. Noling and Ferris [6] demonstrated that nematodes can reduce alfalfa yields by more than 70%. Similarly, fungi can significantly reduce crop yield potential [7, 8]. The impact of various pathogens on crop yield potential can be demonstrated with some simple models.

According to Noling and Ferris [6], the impact of nematode populations on perennial crops, such as alfalfa, can be described by:

$$Y\_{loss} = a \left(\mathbf{1} - e^{-bN}\right) \tag{1}$$

where Yloss is the yield loss, a is the maximum yield loss for the system, b is a population sensitivity parameter for the crop (i.e., damage rate), and N is the nematode population. Therefore, the potential crop yield is described by:

$$\mathbf{Y} = \mathbf{Y}\_o \left[ \mathbf{1} - \mathbf{a} \left( \mathbf{1} - \mathbf{e}^{-\text{bN}} \right) \right] \tag{2}$$

where Yo is the optimal yield.

In a resource limited environment, the rate of population growth is described by:

$$\frac{dN}{d^\circ D} = r \left(\frac{k-N}{k}\right) N \tag{3}$$

where °D is the degree days which are suitable for the growth of the pest or pathogen, k is the maximum sustainable population of the pest or pathogen (i.e., the carrying capacity), and r is the base population growth rate. One Degree Day is determined according to some basis temperature (Tb):

$$\mathbf{^\bullet D} \stackrel{\text{def}}{=} \frac{\mathbf{T\_{max}} - \mathbf{T\_{min}}}{2} - \mathbf{T\_b} > \mathbf{0}.\mathbf{0} \tag{4}$$

Equation (3) can be rearranged to become:

$$\frac{dN}{\left(\frac{k-N}{k}\right)N} = r \bullet d^{\circ}D \tag{5}$$

Integrating both sides of Eq. (5) gives:

$$\mathbf{2\tanh^{-1}(} \frac{2\mathbf{N}}{\mathbf{K}} - \mathbf{1} \mathbf{) = r\mathbf{\cdot} \mathbf{\cdot} \mathbf{\bullet} \mathbf{D} + \mathbf{C} \tag{6}$$

Therefore, Eq. (6) becomes:

$$N = \frac{K}{2} \left[ 1 + \tanh\left(\frac{r \bullet^\bullet D}{2} + \frac{C}{2}\right) \right] \tag{7}$$

To evaluate the constant of integration (C), it is appropriate to choose a boundary condition on the problem. It is noted that at the start of any study (i.e., when °D = 0 for this study period), the population will have some starting population value "No." Substituting this into Eq. (7) and setting °D = 0 gives:

$$\text{No} = \frac{\text{K}}{2} \left[ \mathbf{1} + \tanh\left(\frac{\text{C}}{2}\right) \right] \tag{8}$$

or:

$$\mathbf{C} = 2 \bullet \tanh^{-1} \left( \frac{2 \bullet \mathbf{No}}{\mathbf{K}} - 1 \right) \tag{9}$$

illustrate the importance of the impact of pathogens and pests on crop yield, if a crop requiring 1500 growing degree days to mature is exposed to an initial *Meloidogyne hapla* nematode population of 1085 individuals kg�<sup>1</sup> of soil, the yield potential would be 0.3 at the end of crop maturation; however, if the crop was exposed to an initial population of only 4 individuals kg�<sup>1</sup> of soil because of some pre-sowing soil sanitation strategy, the crop yield potential would be approximately 0.7. Therefore, pre-sowing soil sanitation could provide a crop yield increase (com-

*Crop yield potential for Alfalfa affected by* Meloidogyne hapla *nematodes as a function of degree days, based*

*Population growth in* Meloidogyne hapla *nematodes as a function of degree days, based on the initial inoculum*

<sup>0</sup>*:*<sup>3</sup> � 100 ¼ 133%. Although this may appear to be a significant crop yield increase, the pre-sowing soil sanitation is simply bridging a little more of the crop yield gap by treating the soil to remove crop inhibiting organisms before sowing the crop. In fact, the modeling suggests that the crop growing on the sanitized soil may still not have

pared with untreated soil) of: ð Þ <sup>0</sup>*:*7�0*:*<sup>3</sup>

*on the initial inoculum of the soil (calculated from Eq. (2)).*

**Figure 1.**

**Figure 2.**

**181**

*of the soil (calculated from Eq. (10)).*

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

reached its full crop yield potential.

Therefore,

$$N = \frac{K}{2} \left[ 1 + \tanh\left(\frac{r \bullet ^\bullet D}{2} + \tanh^{-1}\left(\frac{2 \bullet No}{K} - 1\right)\right) \right] \tag{10}$$

Using data from Noling and Ferris [6] as a guide, the population of *Meloidogyne hapla* nematodes in their study would increase as shown in **Figure 1**. When these population models are applied to the crop yield model in Eq. (2), the apparent crop yield decline is similar in form to that presented in Noling and Ferris [6], as shown in **Figure 2**.

Different crops require differing numbers of degree days to reach maturity. For example, maize requires between 800 and 2700 degree days while barley requires between 1290 and 1540 degree days. Using the data presented in **Figure 2** to

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

#### **Figure 1.**

In a resource limited environment, the rate of population growth is described by:

*k* � *N k* 

where °D is the degree days which are suitable for the growth of the pest or pathogen, k is the maximum sustainable population of the pest or pathogen (i.e., the carrying capacity), and r is the base population growth rate. One Degree Day is

*N* (3)

<sup>2</sup> � Tb>0*:*<sup>0</sup> (4)

*<sup>N</sup>* <sup>¼</sup> *<sup>r</sup>*<sup>∙</sup> *<sup>d</sup>*°*<sup>D</sup>* (5)

¼ r∙°D þ C (6)

(7)

(8)

(9)

(10)

*dN <sup>d</sup>*°*<sup>D</sup>* <sup>¼</sup> *<sup>r</sup>*

°D <sup>≝</sup> Tmax � Tmin

*dN k*�*N k*

> <sup>K</sup> � <sup>1</sup>

<sup>1</sup> <sup>þ</sup> tanh *<sup>r</sup>*∙°*<sup>D</sup>*

To evaluate the constant of integration (C), it is appropriate to choose a boundary condition on the problem. It is noted that at the start of any study (i.e., when °D = 0 for this study period), the population will have some starting population

<sup>1</sup> <sup>þ</sup> tanh <sup>C</sup>

2 þ

2

<sup>K</sup> � <sup>1</sup> 

<sup>2</sup> <sup>þ</sup> tanh �<sup>1</sup> <sup>2</sup> <sup>∙</sup> *No*

Using data from Noling and Ferris [6] as a guide, the population of *Meloidogyne hapla* nematodes in their study would increase as shown in **Figure 1**. When these population models are applied to the crop yield model in Eq. (2), the apparent crop yield decline is similar in form to that presented in Noling and Ferris [6], as shown in **Figure 2**.

Different crops require differing numbers of degree days to reach maturity. For example, maize requires between 800 and 2700 degree days while barley requires between 1290 and 1540 degree days. Using the data presented in **Figure 2** to

*<sup>K</sup>* � <sup>1</sup>

*C* 2

2 tanh �<sup>1</sup> 2N

value "No." Substituting this into Eq. (7) and setting °D = 0 gives:

No <sup>¼</sup> <sup>K</sup> 2

<sup>1</sup> <sup>þ</sup> tanh *<sup>r</sup>*∙°*<sup>D</sup>*

<sup>C</sup> <sup>¼</sup> <sup>2</sup> <sup>∙</sup> tanh �<sup>1</sup> <sup>2</sup> <sup>∙</sup> No

*<sup>N</sup>* <sup>¼</sup> *<sup>K</sup>* 2

determined according to some basis temperature (Tb):

Equation (3) can be rearranged to become:

Integrating both sides of Eq. (5) gives:

Therefore, Eq. (6) becomes:

*Sustainable Crop Production*

or:

**180**

Therefore,

*<sup>N</sup>* <sup>¼</sup> *<sup>K</sup>* 2

*Population growth in* Meloidogyne hapla *nematodes as a function of degree days, based on the initial inoculum of the soil (calculated from Eq. (10)).*

#### **Figure 2.**

*Crop yield potential for Alfalfa affected by* Meloidogyne hapla *nematodes as a function of degree days, based on the initial inoculum of the soil (calculated from Eq. (2)).*

illustrate the importance of the impact of pathogens and pests on crop yield, if a crop requiring 1500 growing degree days to mature is exposed to an initial *Meloidogyne hapla* nematode population of 1085 individuals kg�<sup>1</sup> of soil, the yield potential would be 0.3 at the end of crop maturation; however, if the crop was exposed to an initial population of only 4 individuals kg�<sup>1</sup> of soil because of some pre-sowing soil sanitation strategy, the crop yield potential would be approximately 0.7. Therefore, pre-sowing soil sanitation could provide a crop yield increase (compared with untreated soil) of: ð Þ <sup>0</sup>*:*7�0*:*<sup>3</sup> <sup>0</sup>*:*<sup>3</sup> � 100 ¼ 133%.

Although this may appear to be a significant crop yield increase, the pre-sowing soil sanitation is simply bridging a little more of the crop yield gap by treating the soil to remove crop inhibiting organisms before sowing the crop. In fact, the modeling suggests that the crop growing on the sanitized soil may still not have reached its full crop yield potential.

#### **2. Soil sanitation**

Many soilborne plant pathogens flourish during the crop growing season and survive between seasons, either in the soil or above-ground, by means of resting structures, such as propagules that are either free or embedded in infected plant debris. Soil sanitation aims to reduce or eliminate the pest population from all sources, thus breaking the continuity of survival in time and space between crops. Soil sanitation (e.g., by fumigation or heating) is a routine procedure in many agricultural systems [9].

#### **3. Fumigation**

Soilborne diseases, plant-parasitic nematodes, and weeds can be devastating, and preplant soil fumigation is commonly relied upon to mitigate the risk of crop loss [10]. Methyl bromide has been widely used for soil sanitation in the past; however, because of its ozone depleting impacts it has been included in the 1987 Montreal Protocol as a substance whose use should be reduced and eventually eliminated. Under the Montreal Protocol exemptions were granted for substances (like Methyl Bromide) where no economic alternative existed [11]. Even so, especially in the Strawberry runner industry, alternative treatments have been investigated and found to be wanting [12, 13]. Most alternative treatments involve other fumigants, such as Metam sodium or chloropicrin [14], or thermal processes, such as solarization or applying steam.

Klose et al. [14] showed that weed seeds and soil pathogens exhibit a logistic dose-response to a commercial soil fumigant formulation of 1,3-dichloropropane (1,3-D; 61%) and chloropicrin (33%). It has been shown elsewhere [15] that a more physically meaningful representation of logistic dose responses can be described by:

$$S = a \bullet \text{erfc}[b(D - c)] \tag{11}$$

demonstrated empirical relationship between lethal temperature and temperature

where T is the lethal temperature (°C), and Z is the lethal temperature holding time, in minutes [16]. Individual relationships for different species of plants and pathogens [9, 17, 18] have been developed over time (**Figure 3**). Ultimately, heat can provide similar lethal effects to chemicals and therefore has been used in soil

It has been demonstrated that steam soil treatment is as effective as some soil fumigants at reducing pre-sowing soil pathogen loads [19]; however, if the steam is applied to the surface of the soil (i.e., not injected), effective treatment is shallow compared with conventional soil fumigation techniques. This is due to limitations of heat being transferred from the steam into the soil. The governing equation for heat transfer from a hot fluid (air, water or steam) with a temperature of Tf into a solid,

*<sup>A</sup>* <sup>¼</sup> *h Ts* � *<sup>T</sup> <sup>f</sup>*

where q is the heat flow (W), A is the cross sectional area through which the

[20]. When studying thermodynamic processes, temperatures are usually expressed

The convective heat flow coefficient depends on a number of other parameters and conditions [21]. For example, the convective heat flow coefficient for a vertical surface where natural convection achieves turbulent fluid flow conditions over the

), and h is the convective heat flow coefficient of the soil's surface

such as soil, with an initial temperature of Ts, is expressed as:

*q*

*T* ¼ 79*:*8 � 12*:*8 ∙ log <sup>10</sup>*Z* (12)

(13)

holding time has been developed by Lepeschkin [17]:

*Lethal temperature/time functions for several important pathogenic organisms.*

sanitation processes for some time.

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

**5. Steam treatment**

**Figure 3.**

heat passes (m2

**183**

in absolute (Kelvin) values.

surface is given by [21]:

where S is the surviving portion of the population, erfc(x) is the Complementary Gaussian Error Function, D is the fumigant dose (μmol kg�<sup>1</sup> ), and a, b and c are constants that are determined experimentally. Equation (11) is based on an underlying normally distributed population susceptibility to some treatment; therefore, the cumulative effect (mortality) in the population becomes the integral of the normal distribution function, which is described by the Gaussian Error Function, and population survival, which is the whole population minus the mortality rate, is therefore described by the Complementary Gaussian Error Function. Therefore, it is anticipated that the crop yield response to varying doses of pre-sowing soil fumigation treatment should also have a Gaussian Error form, as a function of applied pre-sowing fumigant dose.

Growing concern over the use of excessive chemicals in agriculture, with adverse effects on on-farm and off-farm environments, has prompted a search for alternative soil sanitation options. Soil heating has provided some similar pest and pathogen control to chemicals.

#### **4. Soil heating**

The fatal impacts of high temperatures on botanical and zoological specimens have been studied in detail for over a century [16]. In particular, a thoroughly

**2. Soil sanitation**

*Sustainable Crop Production*

agricultural systems [9].

as solarization or applying steam.

applied pre-sowing fumigant dose.

pathogen control to chemicals.

**4. Soil heating**

**182**

**3. Fumigation**

Many soilborne plant pathogens flourish during the crop growing season and survive between seasons, either in the soil or above-ground, by means of resting structures, such as propagules that are either free or embedded in infected plant debris. Soil sanitation aims to reduce or eliminate the pest population from all sources, thus breaking the continuity of survival in time and space between crops. Soil sanitation (e.g., by fumigation or heating) is a routine procedure in many

Soilborne diseases, plant-parasitic nematodes, and weeds can be devastating, and preplant soil fumigation is commonly relied upon to mitigate the risk of crop loss [10]. Methyl bromide has been widely used for soil sanitation in the past; however, because of its ozone depleting impacts it has been included in the 1987 Montreal Protocol as a substance whose use should be reduced and eventually eliminated. Under the Montreal Protocol exemptions were granted for substances (like Methyl Bromide) where no economic alternative existed [11]. Even so, especially in the Strawberry runner industry, alternative treatments have been investigated and found to be wanting [12, 13]. Most alternative treatments involve other fumigants, such as Metam sodium or chloropicrin [14], or thermal processes, such

Klose et al. [14] showed that weed seeds and soil pathogens exhibit a logistic dose-response to a commercial soil fumigant formulation of 1,3-dichloropropane (1,3-D; 61%) and chloropicrin (33%). It has been shown elsewhere [15] that a more physically meaningful representation of logistic dose responses can be described by:

where S is the surviving portion of the population, erfc(x) is the Complementary

constants that are determined experimentally. Equation (11) is based on an underlying normally distributed population susceptibility to some treatment; therefore, the cumulative effect (mortality) in the population becomes the integral of the normal distribution function, which is described by the Gaussian Error Function, and population survival, which is the whole population minus the mortality rate, is therefore described by the Complementary Gaussian Error Function. Therefore, it is anticipated that the crop yield response to varying doses of pre-sowing soil fumigation treatment should also have a Gaussian Error form, as a function of

Growing concern over the use of excessive chemicals in agriculture, with adverse effects on on-farm and off-farm environments, has prompted a search for alternative soil sanitation options. Soil heating has provided some similar pest and

The fatal impacts of high temperatures on botanical and zoological specimens have been studied in detail for over a century [16]. In particular, a thoroughly

Gaussian Error Function, D is the fumigant dose (μmol kg�<sup>1</sup>

*S* ¼ *a* ∙*erfc b D* ½ � ð Þ � *c* (11)

), and a, b and c are

**Figure 3.** *Lethal temperature/time functions for several important pathogenic organisms.*

demonstrated empirical relationship between lethal temperature and temperature holding time has been developed by Lepeschkin [17]:

$$T = 79.8 - 12.8 \bullet \log\_{10} Z \tag{12}$$

where T is the lethal temperature (°C), and Z is the lethal temperature holding time, in minutes [16]. Individual relationships for different species of plants and pathogens [9, 17, 18] have been developed over time (**Figure 3**). Ultimately, heat can provide similar lethal effects to chemicals and therefore has been used in soil sanitation processes for some time.

#### **5. Steam treatment**

It has been demonstrated that steam soil treatment is as effective as some soil fumigants at reducing pre-sowing soil pathogen loads [19]; however, if the steam is applied to the surface of the soil (i.e., not injected), effective treatment is shallow compared with conventional soil fumigation techniques. This is due to limitations of heat being transferred from the steam into the soil. The governing equation for heat transfer from a hot fluid (air, water or steam) with a temperature of Tf into a solid, such as soil, with an initial temperature of Ts, is expressed as:

$$\frac{q}{A} = h\left(T\_s - T\_f\right) \tag{13}$$

where q is the heat flow (W), A is the cross sectional area through which the heat passes (m2 ), and h is the convective heat flow coefficient of the soil's surface [20]. When studying thermodynamic processes, temperatures are usually expressed in absolute (Kelvin) values.

The convective heat flow coefficient depends on a number of other parameters and conditions [21]. For example, the convective heat flow coefficient for a vertical surface where natural convection achieves turbulent fluid flow conditions over the surface is given by [21]:

*Sustainable Crop Production*

$$h = \frac{k}{L} \left\{ 0.825 + \frac{0.387 Ra\_L^{\prime \circ}}{\left[ 1 + \left( \frac{0.492}{Pr} \right)^{9\_{\text{f6}}} \right]^{8\_{\text{f7}}}} \right\} \tag{14}$$

**6. Microwave soil heating**

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

the material, which is heat.

tion issues.

[27, 36, 37].

**Figure 5.**

**185**

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

atomic, molecular, cellular and subcellular level [24].

[25, 26].

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

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

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

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 compac-

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

where k is the thermal conductivity of the heating fluid (W m�<sup>1</sup> K�<sup>1</sup> ), Pr is the 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]:

$$Ra\_L = \frac{\text{g}\beta}{\nu \alpha} (T\_s - T\_\infty) L^3 \tag{15}$$

where g is the acceleration due to gravity; β is the thermal expansion coefficient 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 viscous and thermal diffusion rates given by [21]:

$$Pr = \frac{\nu}{a} \tag{16}$$

where v is the kinematic viscosity (m<sup>2</sup> s �1 ) and α is the thermal diffusivity (m<sup>2</sup> s �1 ).

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 soil.

#### **Figure 4.**

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