Vinegar will help lower the pH. Peters will already aid in the lowering of pH, so unless you suspect your water has very high pH, vinegar would not make a big difference.

Why not try to repot instead ? This would effectively solve any build-up (whatever it is that is causing it). This way you do not have to go back and forth trying to find a new balance.

IF you are interested in finding the cause, then take a look at the link below. You would need to identify if the yellow appear on new or old leaves, evenly on the leaves or in between veins, if the leaves are cupped or purplish striped. etc

Here is a link that might be useful: nutrient deficincies article

" I'm starting to think that the growth changes are due to a buildup of mineral salts in the potting mix. "

That theory could be tested easy enough ... Do you monitor the salinity of your contaner mixes over the course of the year ??

Good Day ...

Perhaps if you leach all the salts with lots of water and stop adding any fertilizer for a while that could help. That should be easy since you say your media is just peat and perlite.

I would look first to cultural issues. What are you potting up in Oct, an unusual time for repotting? - and are they house plants or something temperate you're growing indoors? Most anything you you're trying to grow during the period Oct - now would likely be trying to rest & need little in the way of water or fertilizer. If your water is under pH 7.5, it's probably not a water pH issue, but if it's over that, it could be. Most likely problem is overwatering and/or over-feeding. Also, in a mostly peat mix, common is soil compaction = poor aeration = wet soil = root rot.

To fertilize at this time of the year is usually counterproductive (Sep - now). I usually withold nitrogen applications after mid Aug until spring, unless the plant shows distinct signs of vigorous growth, which they almost never do. The plant doesn't want the fertilizer during winter, it wants to rest. Too much nitrogen does stimulate new growth, but at a large net loss in energy reserves. Fertilizer buildup in the soil can make it difficult to impossible for the plant to take up water. The fertilizer actually pulls water from plant cells, tearing plasma away from cell walls & causing cellular collapse. This is termed plasmolysis, or fertilizer burn.

Al

Thanks for all the helpful responses. I guess I needed to put more background information on my initial listing, too. I have bananas and tropicals that I grow outdoors in my yard all summer,then I pot them up in October in order to save them for next year. I have a 9X20 greenhouse packed full of them, too many for repotting, it costs a small fortune just to get them all potted up the first time.
They all seem to do fine until just around this time of the year, then I have that problem with the new leaves being thinner than the old ones, lighter green, and generally not as robust. I have not had problems with spider mites or insects this year, so I have ruled that out as a cause. From the link you sent me, it sounds the most like iron. I don't have a meter or testing equipment to monitor the salinity levels in my soil.
That may be something I want to invest in. I am starting to see a lot of new growth in all my plants, starting around January, when the days started to get longer. I think I will get a soil ph meter and see where the ph of the potting mix is standing right now. They are readily available this time of the year around here.

I would also consider that low light may be the cause of poor color. I think Al is closest to the mark when he talks about reducing fertilizer and letting plants rest. In the tropics the daylength doesn't change very much and plants can grow year-round as long as moisture is sufficient. I doubt that any of the tropicals naturally grow where the days get as short as they do in Indiana. Without supplemental light, their color will look poor this time of year. If this is the cause, then I would expect to see color improve as the days get longer again.

Sandy I won't use vinegar to lower the ph either it losses it"s ability after a short time.You should use ph down (from a pet store_fish tank supply)to lower ph.And ya I would definitly flush plants once a month and tone down on feedings like everyone else is suggesting too.happy gardening.

Sandy,

Here's another link to a Nutrient Troublechart that can give you a start.

Nutrient Troublechart

I think that Al has given a pretty good diagnosis (without having seen the plants, of course) From October to February is a reasonable time for the soil to become impacted.

I think you are also right when you write about the nutrient uptake troubles that you are seeing not being nutrient troubles in and of themselves, but symptoms of another cause--or causes.

If you are having a nutrient salt buildup problem, you can try to add some kelp tea which will increase the microrganisms in your pots and sooner or later will help to break down the salts. If you let the top of the soil in the pots dry out somewhat and you see white crystals form on the top of the soil, this is a good guess.

If you flush the pots more and you have a compaction problem, it is very possible that you can make the problem worse.

You can try a 400 Watt Metal Halide over some of your bananas to see if the problem results from lack of light. If the plants under the light perk up while the other plants still suffer, then you will be getting some signs. If lack of light is the problem and you try adding light, then the plants under the light will be using more nutrients and both the ratio of the nutrients and the amount of the nutrients will change.

Cold can affect the transpiration and way that plants uptake nutrients as well. If the plants further toward the outside walls are affected more, this may play a role.

There are also other nutrient mixes that you can use on your plants that can allow you to fine tune their feeding a little more. You need to adjust both the ratios of the nutrients as well as the amounts.

For feeding your plants Nitrogen, there are two kinds of Nitrogen: Ammonia and a form like Calcium Nitrate. After the plant strips off the Nitrogen, the question is what is left in the soil? The residue of Ammonia will turn the soil acidic while the residue of compounds like Calcium Nitrate will turn the soil alkaline.