Can probiotics acidify soil?

[Lycopersica]

I've been using rock phosphate with probiotics as a soil amendment. However, it seems to cause some sort of root stress when ever I use it on top of the soil. I applied it during my recent bout with blossom end rot. Afterwards, the mulch got a white crust and the plants struggled to get enough water. I had to start watering them every day and, even then, the new growth would wilt slightly on hot, sunny days.

Thankfully, the plants bounced back after I removed the discolored mulch and gave them extra water for a couple weeks. I'm still scratching my head as to what exactly went wrong though. At first, I thought I overfed them but I haven't had any problems with plant food or other amendments. The only thing I can think of is the probiotic. Could the bacteria be overpopulating and producing a lot of acid? And would an increase in soil acidity make it harder for roots to take up water?

[RayR]

What probiotic did you use?

[Lycopersica]

Originally Posted by RayR

What probiotic did you use?

It's Dr. Earth's Soft Rock Phosphate. According to the back of the box, it includes these microbes as probiotics:

Bacillus subtilis
Bacillus cereus
Bacillus megaterium
Azotobacter vinelandi
Lactobacillus acidophilus
Rhizobium japonicum
Aspergillus orzae

The only one I've heard of is lactobacillus acidophilus which I know produces lactic acid.

[RayR]

Those are pretty common beneficial soil microbes in many inoculants I've used so I don't think that would lower your PH significantly to cause lock out of some nutrients.
Have you checked the PH of your soil or were you just guessing that might be a possibility?

Soft Rock Phosphate is a good amendment in moderation but it's better incorporated into the soil before planting instead of sitting on the top of the soil. Soft Rock Phosphate is largely insoluble and will be broken down by biological activity before the phosphate and calcium will be available to the plant over time. Just look at it as a slow release P and Ca fertilizer.
Just a thought but If you put too much on top of the soil it could be caking and causing problems with water penetration.
A better way to add it to the soil during the growing season is to mix a little with your water as colloidal suspension and water it in so the fine particles and the microbe spores too will penetrate deeper into the soil to the root zone. That's where the magic happens.

[RayR]

Oops, Double Post

[Lycopersica]

I was just speculating about the pH. I haven't checked since the beginning of the season (before I added anything) when I had a professional soil test. It was mildly acidic (6.2) at the time but the plants didn't have any trouble before I added the rock phosphate.

I don't think it was mechanically preventing water penetration because I didn't have any problem when I later applied gypsum with a similar texture. Is it possible that the rock phosphate itself could've burned the surface roots from being too concentrated there?

The reason I ask (aside from curiosity) is that I'm starting to build a 'lasagna' bed for next year. I want to make sure it's okay to work the rock phosphate into the middle compost layers even though it caused problems as a surface treatment.

[RayR]

PH of 6.2 is good:
Toxicity is highly unlikely. Dr. Earth Soft Rock Phosphate is only 2% P as plant available Phosphoric Acid, the rest of the P is probably around 25%-30% bound up in insoluble minerals.
It's a mystery. I've never applied Soft Rock Phosphate that way, I never put anything on top of the soil other than compost or other organic mulch.

[Lycopersica]

Thanks for the insight. A mystery indeed. I think you're probably right that the problem has more to do with the way I'm applying it than the product itself. I didn't see any trouble when I worked in a small amount deeper in the soil before planting. I'll give it another try mixed into the lasagna bed (but not on the surface!). Wish me luck!