Here's a little experiment I did to test my own advice to water succulents in more than one pass. I was kind of surprised by the results so I thought I would share. It's very much something you can try with your own plants. There are some useful conclusions from this, I think.
The idea is that bone-dry soil does not efficiently absorb water. Only after it has become moist (which takes some time) does it absorb efficiently. In order to test this I chose a cactus in soil I knew to be completely dry at the start.
I poured a measured amount of water (very carefully) onto the soil all around the plant until it started coming out the bottom, having passed through all the way. Then I let the plant sit for 15 minutes to make sure all the water had exited the container. At that point I collected the water in the saucer (very carefully) and measured how much was there using the same measuring cup. By subtracting this amount from the total input, you can arrive at a volume of water absorbed by the soil during each pass.
Here is the setup and the results.
The cactus is a stout Parodia from my balcony, where it grows in pretty much day-long full sun. It takes a week or less to dry out completely. When I set it in the saucer, I put in a couple of wood spacers to make sure the pot never ever sits in standing water.
This is what I recorded after the first pass, when I poured on 500ml and recovered 170ml.
And this is how much water was absorbed during each pass, separated by 10-15 minutes.
pass 1 - 330ml
pass 2 - 210ml
pass 3 - 100ml
pass 4 - 60ml
total after 4 passes - 700ml
Which tells you that if you stop watering after the first pass, the soil may achieve less than 50% saturation, even if you are watering until water comes out the hole in the bottom. That's why I recommend watering in more than one pass.
Try this yourself (no special tools required) if my result surprises you.
I recommend a largish pot to make the measuring easier.
