Some thoughts on SIPs, soil, watering, and fertigation...
I realized today that the 5 gal SIP bucket should have another drain hole, right at the very bottom of the side wall of the bucket, that can be plugged with a cork or rubber plug. Why? Because in the first 2 weeks, before fertigation, there's no benefit from having water sitting in the reservoir, so it should be able to drain out. It is indeed beneficial to top water until runoff, but this grow showed that having 2" of water (or more) in the reservoir didn't help the growth of the plant (as compared to the nursery pot with no reservoir).
Another way to say this... the wet/dry cycle is still important, even in the SIP, until which time the nutrients in the medium are used up and fertigation is started.
This is just based on the outcome I've seen with this comparison grow, of these two identical clones of my HI-BISCUS pheno, in the super soil mixture that I used (1/3 each of coco, compost soil, worm castings—plus perlite and nutrients). This pheno is not super vigorous, but the two plants were looking very healthy up until about day 17 or so, when the leaves started to take on a slightly more pale/yellow hue, indicating lack of nutrients including nitrogen.
Here they are on DAY 13 looking fabulous. You can really see the contrast in leaf color and size between day 13 and day 32.
On DAY 26, leaf color has faded toward yellow, and more so for the SIP plant.
DAY 32. The nursery pot plant (right) experienced a normal wet/dry cycle and did better than the SIP plant (left), from day 17 to day 32. The SIP is showing diminished growth... leaves are smaller.
By day 32, the SIP plant's medium was still heavy with water. This tells me the SIP should have been allowed to drain fully, and that there was no benefit from having water in the reservoir. However, if I had started fertigating the SIP, say on day 14, then water+nutrients in the reservoir would have been a good thing. Nutrients of course feed all of the growth processes, which in turn drives the uptake of water.
The other SIP I'm running right now,
growing my CBD #9 pheno, is a different story. That pheno is robust and fast-growing. She's been running a bit on the dry side, without a buildup of water in the reservoir. I started fertigating her today (day 17).
So, there's a lesson here...
water in the reservoir is a much different situation than
water+nutrients in the reservoir. I think it's fine for some plain water to be in the reservoir, as long as the plant is using it up quickly, and there are nutrients in the medium. When the medium begins to run out of nutrients, then it no longer makes sense for plain water to be sitting in the reservoir. At that point, the reservoir instead needs to collect both water and nutrients, as a result of either top-fertigating, or fertigating down the fill tube.
I also feel that it's probably always best to top-fertigate the SIP, to bring nutrients to all the roots, upper and lower. The benefit of the SIP is that excess water+nutrients collect in the reservoir where the deeper roots will have constant access to them. In addition, I feel that—depending on how vigorous the pheno is, i.e. how fast it absorbs water and nutrients—there may need to be a "dry" period, where the reservoir level goes to zero, and subsequently the roots have a chance to use up most of the available water in the medium. (This is easily checked by simply lifting the SIP bucket to get an idea of the weight.) After this dry period, the SIP can then be top-fertigated to runoff once again.
The way I see it, the purpose of a SIP is to boost fertigation efficiency.
