To quote a source:
In the 1840s, W. Ross of New York reportedly obtained a severalfold increase in the yield of a field of potatoes when he buried a copper plate (5 ft x 14 ft) in the earth, and a zinc plate of the same dimensions 200 ft away. The two plates were connected by a wire above ground, thus forming a galvanic cell. In similar experiments by Holdenfleiss (1844) with battery-charged zinc and copper plates, yields increased up to 25%.(7)
The same source also claims a temporary benefit form electrified seeds and more interestingly from the application of a low-power direct current, to quote:
P.V. Kravtsov, et al., reported that the population of ammonifying bacteria (especially the sporogenous type) increases about 150% when soil or compost is exposed to continuous low-power DC.
And, as ammonifying bacteria convert nitrogen into ammonia, which in the presence of O2 and free electrons, some of the ammonia may be converted into beneficial nitrite/nitrate (see, for example, On the aqueous reactions of the aminyl radical with molecular oxygen and the superoxide anion).
The source also claims benefits due to magnetic fields as does this paper: The effects of magnetic fields on plant growth and health.
[EDIT] To quote from this controlled study:
A study was conducted to test the hypothesis that a magnetic field can affect plant growth and health. The study divided plants into three groups. The first group of plant seeds grew in a low magnetic field. The second group grew in a high magnetic field. The third group grew in the absence of a magnetic field, serving as a control group. Several growth parameters were measured, including the germination rate, plant height, and leaf size. In addition, the health status was measured by leaf color, spots, the stem curvature, and the death rate. Plant growth was observed continuously for four weeks. The results showed that magnetism had a significant positive effect on plant growth. Plant seeds under the influence of the magnetic field had a higher germination rate, and these plants grew taller, larger, and healthier than those in the control group. No adverse effects of magnetism on plant growth were noticed.
What I am trying to understanding are possible paths, that is, causation arguments. So far, my only thoughts relate to increased oxygen kinetics (see my comments and references here), galvanic cell surface wire induced magnetic field, which could promote a charge alignment leading to increased radical activity, and including, for example, associated bacteria leading to the formation of beneficial nitrite/nitrate.