"but it looks like the probes are intended for solutions only."

These fancier meters I believe work by putting some soil in some water and letting it settle then measuring the water. But first they calibrate the meter to take in account the properties of the water being used. Then the resulting readings would be on the soil properties. The meters come with directions for this or there are instructions on the web.

I can't help with the greenhouse tomatoes other than the blossom end rot is often a lack of calcium (gypsum) and the yellow on the leaves could be a lack of magnesium (Epsom Salt). But you have a lot of greenhouse tomato growers in here so I am sure they will have a proper solution for you.

My suggestion: Save your money. You'll drive yourself crazy trying to manipulate container soil pH - an exercise in futility. Most plants, tomatoes included, will tolerate a wide range of soil pH. Do you even know what the pH of the water you use on your plants is? That's more important & much easier to manipulate, BTW. If you live in an area that uses municipal water, call your water supplier - you'll be surprised I bet.

More important than soil pH:
Water pH?
What kind of soil are you using? What's in it?
How quickly does it drain?
What are you doing to make sure you are supplying the minor nutrients?
What are you doing to prevent excessively high root temps?
Are you allowing soil to dry sufficiently? (Wet feet can create artificial deficiencies and other problems by limiting root function - or rotting them).
Fungal issue - Are you wetting foliage when you water? If yes - stop.
What's a TDS meter? ;o)

Al

I've just started posting here, but I've been gardening strictly organically for 34 years. And IMO pH meters and soil tests are a complete waste of money. Why? Because no matter what they reveal, the solution is always the same: add more organic matter to the soil.
The original poster wrote
"From what I've read, the problem could be deficient N, excess N, deficient P, deficient K, deficient Ca, deficient Fe, pH, watering problems, or fungus."
If tomatoes get too much N, it inteferes with the uptake of phosphorous, which is crucial to fruit set. You can actually reduce your tomato yields by using the wrong commercial fertilizer. And the amount of calcium in soil is virtually never a problem, like with BER. It's not enough calcium reaching the fruit that causes it.
Being a Simple Person, I just use grass clippings for fertilizer. They will supply all the nutrients your plants need, in the right form, the righ "balance," and the right amounts at the right times. It's the way nature has been feding plants for millions of years.
Having been to San Francisco many times, I know grass clippings might not be the easiest thing to get in some areas. But given their incredible value, I'd make some effort to find some- like in parks, around hospitals, or commercial buildings. If the groundkeepers aren't using mulch mowers, they'll probably be happy to let you have some?

Tyrell - did you forget or not notice this is the container gardening forum?

Al

Assuming you are fertilizing your tomatos (with a low N fert hopefully) regularly the blossom end rot is almost certainly improper watering.

In almost any soil, container or ground, BER is lack of water, not calcium. Lack of water = lack of nutrient uptake.

OK, about the ph meter thing. I have one that cost around $30 and has 4 probes and a light meter. The light meter is garbage, the soil moisture is silly since not everything wants the same moisture level, the fertility is pointless since it doesn't break the levels down by nutrient, but the ph is right on.

HOWEVER, in a container ph measuring of soil is pointless. Virtually all potting mixes are ph adjusted to be 6.5-7.2 and the azalea or low ph mixes are adjusted to be lower. Because soil volume in a container is so small, the ph will change with watering quickly so it is better to buy a water ph test kit and ph up/down kits(if your water supply is out of line) so that the ph of your irrigation water matches what you want the ph of the soil to be.

If you really want a ph meter for container soil I would suggest sticking with the cheapies ($20-$30) and quit breaking those probes ;-) I have had mine for years and it still works although it doesn't get much use since I know my ground soil's ph and commercial container soil ph is always in the initial range I stated above.

Yup...just keep adding new material. You'll get it right sooner or later. I've never taken soil samples and i still grow great tasting maters.