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?

It's tough figuring out what is the best solution would be. Personally, I would probably just get a ph testing kit at a home depot - it costs just a few dollars, and once your ph is set, it doesn't change very often (unless you add something to the soil). As far as as moisture, I use this soil moisture meter. It has worked well for me as it was in the Goldilocks zone for prices. Rather than costing $2000 as a professional piece of technology, it cost about 80. And it's not like the cheap ones, in that salinity/fertilizer doesn't affect it. As far as light, not sure what to say. It is obviously very crucial to a plant's growth and I know that certain colors of light are more essential than others, but I don't know of an affordable device that works well. Honestly, I think that the natural light of the sun makes the most sense...

A couple points that might be of interest. Temperature will affect readings. Since your units don't appear to have a temperature adjustment try to take all your readings at or near room temperature. Even your calibration fluids should be warmed to room temp if you store them refrigerated. Since these solutions are buffered it takes alot of contamination to change their pH. pH elotrodes rely on ion transfer accross a membrane. If that membrane material becomes blocked you may need to rejuvinate the electrode. There are various recommendations on the web but basically you need to soak the electrode in a stronger acid for a short interval to digest the gunk from the porous membrane. This method works about 50% of the time. If the readings frequently "Drift" or are slow to reach the end reading you should consider rejuvination. Storing the electrodes dry is one of biggest problems. I never used salt soln but the tip should be kept from drying by either keeping in a tap water vessel or putting tap water in the cover cap before reinserting it onto the electrode tip. Some units are junk out of the box. We spent a $400 for a stainless steel unit from IQ Scientific Instr that never would give reliable readings. If you check out a source like colepalmer.com you see that they carry hundreds of electrodes for various applications. If you want better results than you can get with litmus paper then you might want to look into a credible unit.

sillyboy Well something is definitely wrong. I did notice some foam floating in the nutrient solution, that makes me wonder what else is in it other than the nutrients. Also you mentioned that you were using tap water (I think), that may be a problem as well. I would do a couple of separate tests, one Just plain water, and another with nutrients in it. And keep checking the ph in those, with and without adding the pH adjuster. Although I have read that citric acid also does not last as long (not saying 2 hours is long though), but it may not be as effective to what ever the problem is. I'm not sure what that foam is, and that has me wondering if that is a sign of a problem. I would also clean and flush the system (just because), what can it cost? Then re-add nutrients. Take notes (at least mentally), and see if the problem continues. Is it possible some chemical got into the system (like from cleaning it). Also I mean no disrespect, but I have no idea what meter you are using, and if it's calibrated correctly or not. So I would simply need to say you may want to make sure there is no problems there, simply to eliminate it as a problem. I use drops myself (no calibrating needed). Lastly I would wonder what would happen if you don't bother to add any pH adjuster, would the pH skyrocket, or top off.