I would think that the goal is just to moisten the soil with roots. If you apply more than that, it'll just drain out the bottom. As such, I don't think you can over-fertilize, but you certainly can waste the mixture. Of course, you can also drown the roots, but that's just a matter of making the soil soaking wet before it drains instead of just moistening it. That is, soil can only hold so much liquid. You can pour ten gallons onto one small plant, but the soil around the root ball will still hold on to the same amount of liquid as if you just poured a lot less on.

"on plants both in the ground and in containers."

First, it would be different amounts for both those - in ground need somewhat more than containers.

Second, it would depend on the age and size of the plant and thus it's root ball size - need increases with size of plant but only marginally so.

Third, it instructs you to water the plant well first and then apply the fertilizer mix. In other words, the fertilizer mix is usually NOT also the source of the water the plant needs.

Fourth, need increases the more frequently you have to water so container plants will need it more frequently than in ground plants since container plants have to be watered more frequently and the nutrients wash out.

Fifth, it doesn't have to be used full strength and is often diluted to 1/2 or even 1/4 strength.

And last, it depends on how you deliver it to the plant - root drench, via drip fertigation, foliar spray, etc

So there is no simple answer to your question and no answer that applies throughout the season.

A frequent practice with some container growers using root drench is to (1) dilute it to 1/4 strength and use that mixture to fully water the plant every time you water it, or (2) dilute it to 1/2 strength to fully water the plant every third time you water, or (3) make it full strength, fully water the plant with plain water first, and then feed the plant 8 to 16 ounces (depending on plant and container size) every 7-14 days.

Personally I don't use MG and I use drip irrigation so my containers get fed a well diluted mix (approx. 1/4 strength or less) of the fertilizers I use via the drip lines all the time.

My in ground plants get well watered first and then fed any supplements they may need only as they need them - maybe once a month or less - but that is in my well amended soil that is very high in nutrients. But I don't know your soil so try watering well first and then something like a quart of full strength MG once a month and see if that isn't enough.

Dave

Dave's option #2 works for me... I only use MG in ground as a last resort and use a similar method. The tablespoon or whatever it tells you to use will burn your plants IME...

Don't use MG in containers because it is missing calcium. Use a complete fertilizer that includes Ca, Mg and S.

Never seen a tablespoon of MG per gallon of water burn plants. Though it might if you use that mixture for routine irrigation. Probably don't need application of that mixture more than once every few weeks.

It certainly burned my plants. Using it only once with a watering can.

You will always get differing opinions and methods when it comes to fertilization questions. In my world...these facts stand tall: 1. containers need to be fed more frequently. 2. feeding more often with diluted formulas is better than feeding heavy doses infrequently. 3. liquid ferts give much quicker results...conversely, they leach through soils/mixes quicker.

Fertilizing containers with this stuff can be tricky. If you use full strength, and let the soil dry out, the stuff is going to tend to concentrate. So in some sense, you depend on leaching to control the concentration in the soil.

I'm not sure if I got in on the leaves but I sure wouldn't try again.

It depends on the plant, but IMHO it is best to be dilute and read the plant to see what it needs and how it responds. Less is usually more.