Q:

Harvesting carbon free electricity from a field of plants

In the Matrix the movie machines used fields of humans to efficiently harvest electricity from them.
In the endeavor to find a renewable carbon free energy source I can only see using plants this way.
Would it be efficient to harvest electricity from a field plants like potatoes? 
Copper is said to be toxic to plants and is a main component in using a plant as an electrochemical cell. At least one of the two probes in the plant would have to be modified to not be toxic, be at an exposure not to be so toxic to prevent the plant from growing or healing or moved periodically.
Solar power alone would eventually blocks all sunlight from the ground. You would also have to clear land to provide it. I could imagine a semi-transparent cell that would allow healthy light to pass but that has not been invented. 
The plant would still provide oxygen as it normally does while providing clean electricity.
What would a kilometer of trees or potatoes provide in power?
In this link below the answers mostly addresses whether an individual plant could survive this process and provide the process in which electricity is made. I am interested in the large scale of this process. 
https://biology.stackexchange.com/questions/82692/living-potato-clock

A:

As many people above have pointed out, your most effective strategy will be some sort of biofuel, not directly harvesting electricity. I don't know the science as well as some others might, but I really don't think you need to get into the science to see why it won't work.
The number one problem is that plants make the amount of energy they need to live. This should be fairly self explanitory - if they made more, they would have more to use and then they would have evolved to use it. If you start leeching energy off the plant then they're not going to grow normally.
The number two problem is that plants don't use energy in a form readily convertable to electricity. The question you link has an answer which pretty succinctly points to the problem of the potato example - the potato isn't actually the source of power!
Plants store energy in chemical bonds. Vastly simplifying things, breaking these chemical bonds releases energy as heat, that's why plants burn so well. You can, and people have, engineered plants for making biodiesel. Biodiesel is not "dirty" energy, it has net zero carbon emissions.
Long story short, plants don't run on electricity [citation needed] so you're not going to be able to just hook some electrodes up to them. Burning plants explicitly for this purpose is a clean, carbon neutral, and relatively efficient way of producing energy naturally.

A:

Potato batteries work of the galvanic potential between two different metals. The potato only provides a weakly acidic environment to permit the flow of electrons.  There have been developments harvesting energy from trees to power sensors that monitor for drought and fire.  Reading between the lines on reports made by the company, they extract 1-2 microwatt from a tree.    So 1 million trees could generate 1 W. 
But, the energy stored in the sap of Maple trees and Pine trees. Is much greater.  So, if the trees or bushes or whatever, were genetically modified to store the energy in an efficient form, then they could be milked and the sap used to generate electricity.
  If the sap stored energy then the plants could be milked and the sap used to generate electricity.  
Annuals like trees produce more energy than they need, storing the excess in their roots.  This helps them survive and regrow leaves after winter.  And can help them survive if they experience a bad year -- too hot, rainy, cloudy -- and they can't store as much energy in their roots as they normally wood. So there is excess energy available without killing the trees.

