Re: How do you create a magnetic field for electroculture +measure the force?

This is a good idea.

Generally electromagnetic fields change with both time and frequency.
In this case, I think you can use time-harmonic electromagnetic fields,
that is, electromagnetic fields whose dependence on time is sinusoidal
function. If you use some device to watch the field at any point, you'll 
find the waveform is a periodic and oscillatory sinusoidal curve.

If you want to study effects of electromagnetic fields with different
frequencies, you need a signal generator which can generate 
electromagnetic fields of different frequencies. If the signal is too 
weak, you may need an amplifier. Then you need some transmission 
structure to guide the electromagnetic fields to the seeds.

The complexity and price of the devices you need depend drastically
on the frequency range you plan to investigate.

If you only plan to study the static electromagnetic field, that's
very simple. In this case the electric and magnetic fields can exist
independently. You can fill a box with soil, then put two metal plates
at the top and bottom of the box. Then you connect the two plates to
batteries or DC power and a static electric field is generated between
the two plates. Actually it's just a parallel-plate capacitor. The
strength of the field is proportional to the voltage so you can easily
control it. You can build several such capacitors and observe the effects
of different field strength. Accurately measure or calculate the value
of the field strength is relatively difficult. If the capacitor is almost
full of soil, you can measure its capacitance first, and then calculate
the electric field strength.

To generate a static magnetic field you need a coil. Put the soil
and the seeds in the coil. The magnetic field is proportional to the
currents through the coil so you can control it by controlling the
voltages. If you can measuer its inductance and the coil is almost full
of soil, you can also calculate the magnetic field strength in it.

For frequencies lower than megahertz you can still use the above methods.
Only in this case you'll connect the parallel plates or the coil to a
signal generator.

For higher frequencies the radiation and interaction between electric
and magnetic fields become significant. You can not simply use wires
to connect the parallel plates or the coil. The electromagnetic fields
will radiate into space and cannot reach them. Guiding structures such
as coaxials or waveguides are needed, and you have to put the soil and
seeds in a close metal box to prevent leakage.

I think some books on electric circuits and electromagnetics may help,
especially those chapters on the fields in a parallel-plate capacitor
and a coil inductor.