There's nothing wrong with slightly alkaline soil. There were native plants growing there before urbanization, after all, right?

I used to garden on soil with a pH of approx. 8. Most things grew fine, so long as you liked your hydrangeas pink and never ever remotely blue, and avoided rhododendrons and blueberries. Even camellias and azaleas did well with just using acid fertilizer. Pick your plants to work with what you've got.

Using acid fertilizer and sulfur should be enough. But the other thing you need to consider is the pH of your irrigation water, if you don't rely solely on rainfall. City water tends to be buffered at a very alkaline pH to protect the city's investment in pipes. Well water in alkaline soil areas tends to also be alkaline.

Just how alkaline is your soil and water, and what negative effects do you see on your plants?

"Soil" aka sand here is between 7-8. Yes, something thrived here before humans came, but it was swampgrass. The terrain is no longer a swamp, else we couldn't live here. The swamps were drained by digging artificial lakes and using the dredgings to build up the land so that it could support houses and golf courses.

So really, the land has been so altered from what it was in its natural state, even the native plants can't live here. What used to be a below sea-level swamp is now a golf course elevated significantly above sea-level - nothing at all like what the land's natural state is. Moreover, invasive exotic plants like the Mexican petunias, maleulucas, carrotwoods and Brazilian peppers have taken over and out compete the natural plants.

The unamended soil/sand here is typically at least pH 7 and usually higher. There is a lot of natural limestone here as I understand it.

The problem with picking the plants to thrive with the conditions we have here is that the ones that do the best are the invasive exotics that destroy the mangroves when the birds carry their seeds off. So I plant non-invasives that are able to survive here and still are attractive. While they can survive, they can't thrive here without some help.

My current concern is my Tibouchina that is showing signs of too high of a soil pH. It is an acid loving plant and its leaves are yellowing and turn a crisp brown on the edges. I have researched this and checked with the extension office and I believe the problem to be too alkaline of a soil. I have only recently realized this and just started with the sulphur/acid-loving fertilizer for this plant.

I don't believe that the problem is too much fertilizer as I haven't fertilized since early May and we have had 35 inches of rain here since I fertilized, so whatever fertilizer I did put around the plant has been significantly washed out by now.

The irrigation water is pumped from the artificial lakes that were dredged to build up the land. The golf course here uses fertilizers as required and I am sure those wash into the lakes and alter the pH of the water, but I have no control over that.

I am going to retest the soil pH this fall after the rains stop and after the sulfur has had a chance to work. But in the meantime, I was looking for a temporary fix and hoping I could use vinegar or some other acid mixed in water to get my tree through the summer looking nice.

A pH of 7.0 is neutral, a soil test with numbers above 7.0 indicates an alkaline soil while a soil test with numbers below 7.0 indicates it is acidic. Most all soil nutrients are most readily available if the soil pH is in the 6.2 to 6.8 range and since that is where most of the soil nutrients are most readily available most plants grow best with a soil pH in that range.
What is your soils pH?

Does anyone have any thoughts on what a safe but effective ratio of vinegar:water for a temporary fix to lower pH?

I went with 1/2 a cup of vinegar to 1 gallon of water the other day. So far, I haven't killed the plant but we have had a decent amount of rain since then so it was washed away.

I don't know about using the vinegar. I've used it to kill weeds, but with the deep-rooted ones (like dandelions), it just burned the tops and the plants grew back from the unaffected (by the vinegar) roots. It may be so temporary that it isn't worth the effort, but I can't say for sure.

Have you had a regular soil test, or just the pH? If you did have a test, what was the calcium and magnesium levels? I ask because the calcium is totally unrelated to the pH, despite the common fallacy. Just because you have a high pH doesn't mean you have enough calcium.

Areas of high rainfall and sandy soils tend to have low calcium and magnesium levels.

If your soil needs calcium, try adding gypsum (calcium sulfate). It won't raise the pH, and may even lower it. And if you can find some that is kind of chunky, it would be likely to hang around in the soil, as opposed to the very fine stuff that tends to wash through in the wet season.

As to raising the soil level, I used to live in the desert (sandy alkaline soil), and raising the soil level wasn't much of an issue because the heat burned it practically into oblivion. But if you have that much organic matter that you could add, would it be possible to remove some of the plain sandy soil and replace it with organic materials, just to keep the soil at the same level?

Sue

Soil pH is a measure of the free Hydrogen ions in the soil. If there are lots of those H+ ions floating around your soil will be acidic. Lime, Calcium Carbonate (CaCo3) is used to change the soils pH because the chemical reaction in the soil removes on of the 0 ions form the CaCo3 which combines with the H+ ion to create a CaCo2 and an HO ion.

Here is a link that might be useful: About soil pH

There is not enough vinegar on Earth to lower the pH of your soil. If you do not dissolve all the limestone in the sand, then the limestone remains and the limestone WINS! I live on a limestone dome about 700 feet deep and 100 miles in each direction. At some point you have to give up on the idea of dissolving it all with vinegar.

Your problem is you are trying to grow plants which are not adapted to your environment. That is a violation of the first rule of gardening. You may have utter success growing plants that are adapted but you are doomed to frustration followed by failure trying to grow plants which are not.

Tibouchina will do well here if you water it enough and amend the soil to lower the pH. The sulfur is not a permanent fix - you have to apply it every few months. It's not that it is frustrating for me to grow tropicals here; it is just that it takes a lot of work and is expensive.

Please review this link and see if there is anything in here
that might help you find tropical plants that can tolerate alkaline soils.
Gardening in zone 10 is not that difficult unless you try
to grow plants thay do not fit into existing conditions.
Right plant, right place.

Here is a link that might be useful: Low-maintenance landscape plants for South Florida

dchall posted wise words...if your soil is limestone it is going to be difficult. BUT does it actually have limestone left in it? In dry climates it will persist for ages, but in Florida, I would think a lot of leaching has occurred over the years. I live on limestone but it has decayed into calcium based clay that is neutral.

So I don't really know whether vinegar is going to work or not, but the only thing to lose is your plants if you use too much. Eventually it reacts with whatever alkaline material is around, and will disappear. The question is what happens to the roots in the hours or days in between. A couple of tablespoons will drop the pH of a bucket of water quite a bit. I kinda feel like 1/2 cup/gal is a little strong. I would go with half that. I have nothing to base that on but my gut feelings. If you can find a pH meter, test the pH of a couple of fresh made mixes using the water you actually use on the garden. If it's real low it could make the plant roots unhappy. I would think maybe anything above 5 is probably OK, as long is it dissipates after awhile.

South fla is limestone rock with a thin skin of sand on top. Central fla is the same with a lot more sand on top.

Another thought I had was to contact local gardening experts, either a garden club or the county Ag Extension office and see what they recommend. If it's that widespread others will have dealt with this problem too.