sip/swcs: what medium to use and why?

viethtoFebruary 25, 2011

I've been lurking these forums for about a week now, and have been researching subirrigation planters (sips), or self watering containers (swc).

I'm still in the planning/research phase, and haven't purchased anything other than an 18 gallon tote from Lowe's that says weather and crack proof with lifetime warranty, and a collection of plastic 500mL or/and 20oz water bottles from Nestle/Zypherhills. I plan on using acryllic yarn as wicks. Reason why I haven't started is because I've been reading about issues with SWCs and they either haven't been answered, or their answers raise more questions.

#1. I read somewhere that for container plants, you don't need the micro/bacterial ecosystem (in a soil environment)? I thought the bacteria was required for a nice relationship to continue providing food to the plant, as well as to help break down "slow release" fertilizers and any other materials in the pot.

#2. Assuming I could get the potting mix to not be too wet, are there any advantages of using potting mix other than being able to use any type of fertilizer (slow release I'm assuming)? Are any other SWC mediums (nonsoil mixtures, etc) able to use slow release fertilizers? NOTE: MiracleGro Continue Release instructions says to use their water soluble product for containers... I ask because that's something I already have on hand. I know that water soluble is the way to go.

#3. SWCs and potting mix have issues with being too wet. I've read that adding perlite to potting mix is like adding perlite to pudding. I've also read that it's possible to grow things in pure perlite, although I haven't found any instructions on how that works. I assume to just have the plant growing in it as if it is soil, and only use water soluble fertilizer?

#4. I read a thread about PWT (Perched Water Table) in SWCs and normal containers, and that a wick/toothpick/etc could lower PWT. If my SWC is up a decent height from the reservoir, couldn't I use 2 wicks? One long that reaches to the reservoir, and a shorter one half height to drain the water from the SWC back to the reservoir? It sounds plausible, but I'm sure I'm missing something.

#5. Why do people use soilless mixtures rather than pure perlite? Other than the cost.

#6. How well would a SWC of nonsoil (or potting mix if it does happen to matter. I assume potting mix would do well) live/react/whatever if it was then transplanted into the ground. Or converted into a traditional top watering container, or had it's medium replaced. Example: You grow something and give it to a friend.

That's about it for my questions. As you could probably tell I'm new-ish to gardening.

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tapla (mid-MI z5b-6a)

1) You don't need to plan around symbiotic soil life:plant relationships to be able to maintain healthy plants. While some engage in practices purposely aimed at feeding and thus multiplying soil life populations, I prefer practices that don't contribute significantly to their numbers, the reason being the greater the density of soil life munching on the hydrocarbon chains that make up the soil particles, the faster those particles break down. The key to a healthy soil is the relationship between how much air and water a soil will hold - especially at container capacity (when the soil is saturated). You can work around a soil that holds lower volumes of water by watering a little more frequently, but it is very difficult to work around the negative influences of soils that hold too much water.

Many people claim that they work around this by adding 'some bark and perlite' to a peat-based commercial soil and have no drainage problems. But you only need to look at the pudding:perlite example cited to see that indeed these problems do not disappear. They are still there because compaction and a high PWT are still there. When you have a high PWT, you are killing off some fraction of roots every time the soil is saturated. The lower the PWT, the fewer the number of roots killed and the greater the potential for healthier plants.

Some growers don't want to admit that certain practices are not conducive to maximizing root health and ensure lost potential, but we can point to sound reasoning and physical science to show that PWTs limit potential. It doesn't always have to be that we are not willing to deal with the issue of a PWT by building soils that limit their presence to something nearly nonexistant; sometimes we are actually unable to deal with decreased watering intervals because of our schedules or priorities. In either case though, there is no difference in the limiting influence of heavy soils based on the difference between unwilling and unable. It just is what it is and should be recognized like something as simply as 'if you don't wash your car, it's not going to shine,.

2) I think you need to understand that you don't need a large organic fraction to support enough bacteria for mineralization of fertilizer. As long as there is a food source and air, bacteria will break down the fertilizers into usable form, The problem comes in the form of the idea that populations of these soil organisms are erratic in containers, and can't always be relied upon to be there in sufficient numbers when needed. 'Slow release' can either mean 'marginally soluble in water' or they require nitrification/mineralization to be broken down into elemental forms to be available.

3) You understand correctly.

4) It won't work because you're changing the gravitational flow potential by (in effect) using the wick to increase the depth of the soil column.

5) Lots of reasons - habit, convenience, cost, that idea carried over from the garden that a large organic component is required if a soil is to be considered 'healthy', perlite gets kind of ugly in a pot and a few plants react poorly to its fluoride content when it's the primary fraction of a medium, or IS the medium. I've started many, many cuttings in pure perlite & kept them in the same for a year or even two with no difficulty other than that they might have become root bound.

6) It depends on a number of variables.


    Bookmark   February 25, 2011 at 3:02PM
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