People spray smelly soap on fruit trees all the time as a pesticide, with no effect on taste. Nothing of note but a little sodium (from the hand soap, beneficial potassium from the insecticidal soap) should be added to the soil anyway, as the organic end of the molecule will decompose readily in a soil environment.

I note that the university experiment station near here uses very small ("motel" size) bars of Cashmere Bouquet® Bar Soap (if I recall correctly) without unwrapping them, just with a hole bored through them to pass through a tie. Thus they hardly dissolve off at all in rain.

Toad:

Here at the Jellyman experiment station, I hung dozens of bars of Cashmere Bouquet around the fruit trees and garden one year, and it seemed to have no effect at all on the deer. Damage was the same as usual. Thus ended my soap experiment. I was never concerned that the soap would harm the soil or the produce.

Don Yellman, Great Falls, VA

Thank you both, gonebananas and jellyman. Glad to know I'm not doing any damage myself.
jellyman, what do you use, since the soap didn't work? Let me just say that the deer come by and munch happily on the well-established apple trees. I just figure they can have what they can reach. I didn't plant them and don't much care for the variety of apple (no idea which) they produce. But the little frost peach is the first tree I've ever planted, so I'm feeling protective.

Toad,

Better get some 4 foot tall fencing and form a hoop with about 13 or 14 feet of it, set so the tree is centered in the middle or you are gonna be feeding the deer peach leaves!

Real fencing, like the 2 x 4 welded type is very good, albeit a little more expensive than field fencing. I get rolls of used fencing from fencers and just use that, as it is free.

Leave the fencing up in fall and winter too, as it prevents the bucks from destroying all your hard earned bark with their antlers!

Until you get your fencing up, you can spray after rains and weekly a mix of 1 or 2 tablespoons of fish emulsion in a gallon of water. Add some urine if it makes you happy. Deer do not like human smells, especially on the leaves they eat.
And fish emulsion stinks, so they don't care for that either.

I had Zest in nylons tied to small trees. Where the rain had "melted" it, the deer left alone. Sadly, that left a lot of leaves which tasted just fine to the deer.

I know some people will say it doesnt work, but I got a spray called Liquid Fence that I sprayed on my one year peach trees and the deer have left them alone. I planted some other trees non fruit, and I didnt spray them thinking the deer would leave them alone. They ate every leave off the trees, lucky they started regrowing thier leaves and this time I sprayed them with the liquid fence and they have not touched them sense. I think you have to apply it weekly until they leave the tree alone then you have to spray it monthly there after. I planted trees in three different spots and the deer ate ALL the new trees that I didnt spray. I guess now I swear by it. It has a nasty smell but it is safe on fruit and the trees. I would try it.

Totally agree with Jellyman.

Tried soaps, wasn't concerned that the soap will do any harm to the tree or fruits, but the soaps had no effect on deer.

I have tried all the common wisdom that I could found on internet, human hair, pepper spray, fertilizer with wolf urine, none of them worked. (Pepper spray might work, but can you wake up at 3am to spray the trees after a heavy rain? I couldn't, so I had to give up this idea.)

The only proven way is fencing.