If you’re growing plants in a sub-irrigated planter (or SIP), you’ve likely already been pointed out towards a wicking soilless mix as opposed to using potting soil. The downside to using a mix like this is that there’s little to no nutritional value inherent for plants to use.
If you decide to go with a (mostly sterile) soilless mix that’s 80% sphagnum peat moss, it is essential to keep a couple things in mind.
Not only do plants need the big N-P-K (nitrogen, phosphorus, and potassium), a wide variety of micronutrients must also be included. When I layered the mix in, I also added some slow-release fertilizer that had some micronutrients in it.
If you are making the mix by hand, it’s crucial to add dolomite (agricultural) lime to your mix. There are a couple reasons for this… first off, sphagnum peat is extremely low in Ph, around 4.5. It’s commonly used as a soil acidifier, so the addition of lime helps raise the Ph. But the lime also adds calcium and magnesium. The “general” rule of thumb is approximately 2 tablespoons per gallon, or roughly 1 cup per cubic foot, but testing your Ph can save you grief.
I’m also using (at this stage) a 4-1-1 fertilizer (mostly nitrogen) in the form of liquid fish emulsion fertilizer. It smells like hell (like a really disgusting fishtank, actually), and it looks like a sewer backup. But it is organic and I don’t have to worry (as much) about burning the plants.
Now the rule of thumb is for ordinary containers etc, a fertilizing of once a week. But this is a SIP, which means the fertilizer doesn’t get washed out, only down. But my soil has 0 nutritional value.
I don’t want to overdo it, but I haven’t found very consistent documentation on a top-down fertilizing rule of thumb for a SIP. So this has the potential to be a bit of a crapshoot, and I’m glad I got the 4-1-1, because hopefully that means I’m less likely to kill anything by overdoing. So, for people who wish to be in the know, I’m using the fish emulsion once a week.
Now, this said… I did make one booboo, and I will share that with you. You might find that there’s a root-fertilizer and a foliar spray dilution mix set of instructions. Just because fish emulsion is 4-1-1 doesn’t mean it can be applied by watering can all willy-nilly over the leaves of your greenry at root-fertilizer dilution. I did so, (oops) and I’ve got pinprick dots of nitrogen burn on many of my leaves of all my plants. The plants do seem all OK otherwise, so I think I got lucky. I don’t know if it’ll heal, but if it doesn’t get any worse, then it’s not bad as is.
I think I’ll have to switch to something higher in P & K come blooming season, but for now, the nitrogen seems to be cutting it on the leafies. But I’m definitely going to cut back to the foliar spray dilution level… or I’m going to have to find a watering pitcher so that I can apply directly to the soil.
(Check out the June update to Fertilizing a sub-irrigated planter here.)