I’ll comment on peaches. I know you asked specifically about fertilization, but as mentioned there are many other factors which affect peach size.
I remember seeing pics of your orchard and it’s stunning well kept appearance. But around here I see size suffer significantly because people allow sod to grow too close to the trees. Peach trees compete very poorly with sod, with small fruit being one of the symptoms to show up.
I think this could also be true in higher density plantings (I think I recall your orchard may be fairly dense). The article I’ll link in a moment talks about a decrease in fruit size with an increase in density.
One big big factor is the maturity of the peach tree. Almost no one talks about this in the industry or research papers, but it is a very significant issue. I have seen this over and over, since I graft a lot of my own trees. I have mature peach trees along with younger peach trees of the same variety, and there is generally fairly stark difference, although it seems to vary with variety.
This year I harvested Redhavens off of mature trees along with a half a row of younger Redhavens. The younger Redhavens produced much smaller fruit over all. Glengo produces a little more consistent fruit, but still definite size difference in the age of the tree.
I don’t think most people would notice this because as their trees grow in maturity, they forget the average fruit size they picked when the trees were younger. I know this was the case when I just had a backyard orchard. I never really noticed the fruit size increasing as the trees matured.
I agree with Alan that early fertilization (or perhaps late fall the prior season) is going to add the most to peach fruit size. How this affects brix, I have no idea. My gut feeling is that early fertilization would not harm brix, and could perhaps improve brix due to more leaf tissue to “feed” the fruits.
I know we’ve talked about this a million times on the forum, but I think someone once posted some research which indicated N improved brix, as long as there wasn’t so much N that actually shaded the shoots with fruit on them.
That said, I would use the amount of shoot extension (i.e. vigor) as a gauge for how much N. I have some mature trees which didn’t put on enough shoot extension and should have been fertilized. Others, we’ve pruned twice this growing season to keep light coming in the tree (i.e. too much vigor).
Of course variety is a big deal. Some varieties are smaller and will “punish” you much harder if you leave too much fruit. Others don’t care how much fruit you leave, the peaches will still get huge (However, even these varieties will punish you by breaking off their scaffolds and/or secondaries.)
This summer I had some crop adjusters out to the orchard to estimate our bu./ac for insurance purposes. They counted some mature Redhavens. I think the average number of fruit was in the low 200s per tree, but on Redhaven had over 350 fruit. On that Redhaven, although some of the fruit was big, a lot of the fruit didn’t size as well as I’d liked.
Compare that to Baby Crawford. That tree probably also had 350 or more fruit on the same sized tree. I’ve picked at least 175 lbs. off that one tree, and there is still fruit on it. The fruit was all very uniform in size, and almost all large.
Small sized varieties like Harken can size fruit, but they have to be thinned hard and early.
I’ve posted this before, but here is an article on how to grow big peaches.