Have you ever had your soil tested by your local extension to see if you lack any nutrients?
Here is one link of interest that does not talk about Copper per say but ideas on how to avoid the problem.

Here is a link that might be useful: Fighting Blight

I'd probably spray the crop directly with Bordeaux mix and let the Cu level rise from the runoff. Humans should be fine, the at risk creatures are earthworms and other sensitive soil critters.

While Copper is an essential plant micro nutrient a bit too much can create major problems, excess Copper will interfer with a plants ability to utilize Iron. I would be less concerned with what too much copper in the soil would do to the Soil Food Web then to a plant.
Start with a good, reliable soil test for soil pH and other nutrient levels. The spores from early and late blight need plant tissue to survive in the soil so good clean up or infected plant material might be necessary. Crop rotation might also be necessary, if there is enough room to do that.

If you haven't gotten a soil test recently, do so. Pay particular attention to the calcium, copper and zinc levels. If you're deficient in any of them, your crops can be susceptible to blights and fungus, says bio-ag expert Gary Zimmer (The Biological Farmer).

You don't want toxic levels, you just want ENOUGH.

If you use sprays, you will usually have to use them every year, because foliar sprays just don't do much for the soil. So you might as well just add it to the soil and go from there.

Sue

Belgianpup, you wrote: "If you haven't gotten a soil test recently, do so. Pay particular attention to the calcium, copper and zinc levels. If you're deficient in any of them, your crops can be susceptible to blights and fungus, says bio-ag expert Gary Zimmer (The Biological Farmer)."

Well, in the last two years blight has been devastating certain crops in organic gardens and farms in my region. Weather conditions seem to be what has promoted blight. One small family farm in my neighborhood was not hit in 2009. But in 2010 it was hit badly - wiping out virtually all of their tomato crop (which was grown in a tunnel type greenhouse), and damaging 80% of the dozen potato varieties in their field crop. These people have soil tests done frequently by a professional lab. They also have 35 years experience in organic crop cultivation.

The question around here seems to be whether, in addition to keeping the soil adequate in terms of plant nutrients, the extra copper is a good idea. By "extra copper" we're thnking either possibly in a spray form or as boosted copper levels in the soil. Apparently, it has long been known that copper disrupts an enzyme the blight fungus needs to live and reproduce in plant tissues. The use of copper, as a spray, is said to have developed in the vineyards of Bordeaux, France.

I guess you are questioning whether it is a good idea to boost copper in the soil above the "adequate" level, or what would normally be considered to be balanced in relation to other nutrients.

Try cultural methods also. Obviously, select resistant cultivars.

Kimmsr brings up a good point, rotation...where does the causal organism reside, in the soil? How does the blight organism get to the plants, splashing water, therefore.... employ practices that will keep splashing water (and therefore the causal organisms) off the plants, thereby avoiding the blight.

Our dry years have very little to no blight, during the wet years I must mulch both under the plants and a strip extending at least 4' out from the center of the rows to greatly diminish the threat of blight. The mulch keeps soil splashing away from the plants.

Plant in a location where the plants dry quickly and don't plant them as densely as you might like to, there are a number of diseases that just love free water on the plant surfaces.

Of course, one can spray copper formulations and still be technically organic, I think.

Pardon me while I pull my head out of my posterior, duh, early and late blight are both fungi (not soilborne bacteria, what was I thinking?) that are transported by wind and insects, as well as people working in the crops. To be sure, keep the crop dry as much as possible, even high humidity can be a problem but can't do much about that.