Lime is to adjust the pH. What plant?

The purpose of the lime is to offset the pine bark's acidity. Al has said it's generally not a disaster if you forgot to add lime, but as mblan said, tell us what you're growing to make sure it's not highly sensitive to soil pH.

You can buy garden lime (also called dolomitic lime) at home improvement stores, garden centers, hardware stores, and other places that sell plants. There may be substitutes; consider using the search box at the top of the page to find other posts about it.

Thanks, I stand corrected.

Sorry for my delay. The plant is a monstera plant

@tropicofcancer I don't know about pine bark, but in the Western US we have mostly Douglas Fir bark. And I routinely test fir bark at between 4 and 4.5 pH, so it is quite acidic on its own with no peat added. I add bark to distilled water and wait 15 to 30 minutes and test the water with pH papers.

For 511 mix, are we aiming for a 5.5 pH, approximately?

I have found a sometimes significant difference in pH using that testing method if I wait overnight (8-12 hours) instead of just 1 hour (or less).

Ideal pH for soilless container mixes is supposed to be 5.6-6.2 or so. But I have seen many experienced growers here suggest that in containers the pH is not hugely important as long as it isn't far to either end of the spectrum. So who knows. After a couple weirdly variable bags of pine bark (4.0 pH vs. 6.0 pH) I have started testing each bag before using it, then adding lime based on the starting pH with 5.5-6.5 range being the end goal. This has worked well for me with a wide variety of woody plants in 511.

My statement that pine bark is close to neutral must be incorrect. A few years back I had done some some quick tests and perhaps (most likely) they were wrong. I should do some more tests since many here and published papers assert that pine bark can vary wildly in acidity.

These are my thoughts regarding container Ph: Let us say you start with a soil Ph of about 6. If neutral water is used then the soil Ph should remain at 6 for ever. However, bark will decompose and move towards neutral over time. If alkaline water is used then the soil will move towards neutral quicker and over time it will start becoming alkaline. Conversely, water Ph will drop in the beginning and over time starting water Ph will start dominating.

How quick is the process? Depends on the buffering capacity of the soil. Higher the buffering capacity, the better is its resistance to Ph change. I think, not entirely sure, bark/peat based soils do not have as good a buffering capacity of clay based soils. Sand/grit have the least. Composted matter also have a high buffering capacity. The addition of lime to 511 probably improves its initial buffering capacity too.

Nutrient availability is best around neutral or slightly acidic. Looking at the charts it seems 6.5 is ideal. Our tap water is nearly neutral although I should do that test more frequently. Being neutral I have stopped bothering with Ph altogether.

Rainwater will test a bit acidic and so will distilled water. Distilled water will absorb CO2 as soon as it is exposed to air and become tad bit acidic. Both distilled water and rainwater have no buffering capacity at all.