My neighborhood has sandy silt loam soils with near neutral pH around 6.8. I see symptoms of Mg deficiency on some weeds by the roadside. When starting my garden long ago, I added a lot of peat and dolomite limestone. The resulting pH was 6.5, and the dolomite provided a long-lasting source of Mg. Over time, as the original OM decayed, the pH rose to 6.8 and occasional iron deficiency symptoms appeared. Perhaps the Mg I added has been used up over the years. I don't want to add more dolomite because it would make the pH too high. It seems there is a simple, natural shortage of Mg in the local soil, and probably in my leaf compost as well. Routine application of epsom salt seems like the appropriate treatment. I would not recommend using epsom salt unless there is a known deficiency of magnesium. I had my amended soil tested long ago, and everything was fine at that time. It does have a low CEC (nutrient retention) caused by the low proportion of clay. Lately I have been adding clay (plain kitty litter) when replanting. I wish I had known to do that sooner.

About soil tests......if you have never had one it is not a bad idea to get one done just to see what you are working with. That is the only way you can determine if you have any nutrient deficiencies you need to address. (caveat: home test kits are pretty much worthless - pay the bucks and have a professional lab prepare the test). You don't do multiple tests around the property. You gather soil from a variety of areas (your prime planting areas) and mix it together in a single sample. Most labs will tell you how to prepare the test sample and will ask what you are growing - crops, lawn, ornamentals. That will help them customize your test results. Once you get the results back, they will tell you your current nutrient load and if you need to amend at all. Generally, the only addition recommended will be nitrogen as that is the most mobile and transitory of the primary plant nutrients and the one most often lacking. And adding organic matter - compost - is the easiest and one of the most environmentally friendly ways to add nitrogen to an existing garden. Additionally, compost provides a full range of other nutrients in a low dose, organic process that encourages a healthy soil biology and discourages leaching into groundwater as one encounters with synthetic nutrients (from chemical fertilizers). Compost also improves soil structure, loosening heavy or clay soils, improving drainage and adding a degree of moisture retention to very sandy soils. If used as a mulch it will supress weed development, reduce evaporation of soil moisture and provide insulating capacities. It does however make a rather fertile seed bed for windblown weed seeds so some monitoring is required. I guess you are all going to do what you are going to do but fwiw, there is NO scientific evidence that dosing plants with epsom salts/magnesium sulfate encourages blooming. There is absolutely nothing in the product that affects a plant's biological inclination to bloom or not to bloom. Understanding how necessary plant nutrients work - or don't work - would go a long way in dispelling some of these myths about "miracle" fertilizers (ie. MiracleGro and its cousins) and other miraculous plant potions.

blondboy47, The soil might be a little too wet -- that's the other problem that I was considering. There are some roots growing out of the drainage holes in the pot, and they look healthy though, so I don't think there's a root-rot problem. As for fertilizer, I've just been using general purpose miracle-grow assuming that it had a good balance. I'll try a little epsom salt too, maybe that'll help. A little probably can't hurt anything, even if Mg isn't the problem...

Try using Miracle Gro for Acid Loving Plants and see what happens. This has extra iron included in the formulation and works almost like a tonic to "green up" virtually any broadleaved evergreen plants....very fast acting, you should see color improvement in a week or two. btw, iron chlorosis will be far more likely to be the problem with acid lovers than would be an Mg deficiency. FWIW, I would always recommend using a fertilizer formulated specifically for rhododendrons or other acid loving plants when growing these in containers. Regular ferts simply do not contain the right nutes in the right concentrations otherwise.