Soil Electrochemistry - Electro-Horticulture - page 48

the roots need access to these nutrients and ion transport across cellular membranes is the
means by which they extract them from soil (this will be covered later in Chapter 9 ) . As
mentioned before, the most significant feature of natural soils, especially clays, is the high
CationExchangeCapacity(CEC)whichprovidesplantswithgreateraccesstothenutrients
that are available.
Application of Electric Fields to Soil
As previously explained, osmosis occurs when water along with other particular
substances, are allowed to pass through a semi-permeable membrane. On the other hand,
electro-osmosis(EO) describes a totally different effect. When an externally applied
electrical field is applied to a porous medium such as soil, the pore-water begins to flow
fromtheareaaroundthepositiveelectrodetowardsthatofthenegativeelectrode!So,when
a direct current(DC)-based electric field is applied to an area of soil, it moves a portion of
the pore water toward the cathode(-) 2 . This can be visualized by looking at the image on
the left in the following illustration.
In a similar manner, through a process called electro-migration(EM), negative ions in the
soil will be electrically driven toward the positive electrode(anode), while at the same
time,positivelychargedsubstancesaredriventowardthenegativeelectrode(cathode) 3 . It's
effectively the transport of chemical ions without the help of electro-osmotic fluid flow.
This can be visualized by looking at the image in the center in the next illustration.
Sohowfastdothese ionsmove? Well, thespeedatwhichthese substances moveunderthe
soil is on the order of 5-20cm/da y 4 in fine sands and smaller grain structures like kaolinite
clay. The speed also depends on the polarity and strength of ionic charge. Since strong
cations have a strong positive charge, they have a tendency to be adsorbed to soil particles
becauseofthesoil'sinnatenegativecharge.Weakercationswillbeweaklyattractedtosoil
particles,makingthemmoremobile.Sinceanionshaveanegativechargeandtendtomove
away (via electrostatic repulsion) from the soil particles, they will flow through the soil
mass much more easily than cations 5 .
Moving up in the scale of particles that can be electrically-moved is the process of
electro-phoresis(EP) which is a similar mechanism where charged particles, colloids or
even cells or microbes can be moved by electric fields 6 . As mentioned above,
electrophoretic transport can include proteins and other biological substances including
microbial species like bacteria. This can be visualized by looking at the image on the right
in the following illustration.