Sorry if this has been covered before but one concern with SIPs (sub-irrigated planters) is the possibility of salt buildup, especially if one has hard water.  Because of this I use rain water and haven't had a problem.  For fertilizer I typically use the Espoma products (about 1 cup tree-tone and 0.5 cup iron-tone per 5 gallons of mix).  I had read about some growers adding some water soluble fertilizer to the water every so often as well.  I have some synthetic miracle gro fertilizer (the blue stuff) and added it a couple times to the water reservior of my SIPs (about 1.5 tablespoons per 2 gallons). All seems well; however, I have read recently that there is some salt buildup with synthetic fertilizers like miracle gro.  So I am thinking of switching to an organic liquid fertilizer (foliage pro) instead.  Does anyone have experience using miracle gro over long periods of time in SIPs?

We think alike Steve.  Think it might have something to do with the first name?

I used generic 10-10-10 in my potting mix for my SIP.  This year is the 2nd year so I started adding some mixed liquid fertilizer to the water reservoir since I assumed the 10-10-10 I added last year has been well depleted.  I also added about a tablespoon of pelletized lime to the top surface of the potting mix this year along with a tablespoon of Osmocote.  Once I started addling the liquid fertilizer to the reservoir the tree looked much healthier.  I don't follow the norm and occassionally water from the top since the potting mix looks dry.  This probably leaches some of the salts from the potting mix.  Fall 2015 I will most likely have to do some root pruning and repot so I'm not too worried about salt build up.  Driveway Farmer might be a good member to contact about your question.

Thanks for your comments, Steve.  Unless you have compost in your mix one thing you might want to consider adding is some source of micronutrients.  I started adding the iron-tone for that reason and the plants in SIPs look pretty good.  I think drivewayfarmer's mix has quite a bit of compost in it and I doubt he uses synthetic fertilizers.  But you are right that he would probably know whether adding them could eventually cause problems.

I have Bill's sips and he recommends long release Osmocote, in spring. I supplement with Azomite and Espoma Iron tone. I was thinking of putting some foliage pro into the feeder tube but I have not figured out how to get it from my watering can into a thin pvc tube. But I don't really see the need, my three trees in sips look really good as is. Preto does not grow as vigorously as I hoped but it does have about 40-45 figs on it despite severe pruning in order to provide cuttings to a member this past winter.