Correcting magnesium deficient soil

Hello, I got my soil test results back and the soil is Mg deficient, in addition to being NPK deficient, which I already knew about. I've had good results doing fertigation with dilute NPK fertilizer at every watering, but the leaves were still lighter green than they should be, which prompted the Mg test. How much epsom salt would you add? I've been watering with 1/2 tsp/gallon of NPK such as Jack's 20-20-20, how much epsom salt would be good to add to this?

Calcium is fine and my pH is fine.

Welcome here. Hope you grow well.

Firstly there is lots of info re Mg deficiency already here. Use the Thread Title Search at the top of the page. What is relevant for organic soil is relevant to your question.

You haven't said how large are your pots.

I haven't seen "Jacks" fertiliser here but would not be using 20/20/20. You just don't need that much P and K at an early stage. Find something closer to 10/10/10.

Good luck!

Teddybrae said:

Welcome here. Hope you grow well.

Firstly there is lots of info re Mg deficiency already here. Use the Thread Title Search at the top of the page. What is relevant for organic soil is relevant to your question.

You haven't said how large are your pots.

I haven't seen "Jacks" fertiliser here but would not be using 20/20/20. You just don't need that much P and K at an early stage. Find something closer to 10/10/10.

Good luck!

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Great advice but he can use 1/2 the recommended amount of 20-20-20

Epsom salt is very good for adding magnesium if you're in a hurry.

Dolomitic limestone also works, albeit at a slower rate.

Epsom at 1tsp/gal mixed and foliar sprayed, NOT in your soil.


If you're already short on NPK, why not just use a complete fertilizer?

MJPassion said:

Epsom at 1tsp/gal mixed and foliar sprayed, NOT in your soil.


If you're already short on NPK, why not just use a complete fertilizer?

Click to expand...

May i ask why not in soil(fertigation)?i know foliar spray works faster but i would fix the def. in the rootzone before flowering.I agree on the complete fertilizer though.

If you've already been feeding salts, I don't think Epsom will hurt the mix. I mean, unless there's some chemistry I'm not aware of. If you're already feeding salt based fertilizers to your dirt, your microbials are probably already dead and it won't make a difference. Hydroponic dirt, essentially, which can grow some good weed, if flushed.

Post your soil test results and we shall see if your soil really is Mg deficient.

MJPassion said:

Epsom at 1tsp/gal mixed and foliar sprayed, NOT in your soil.


If you're already short on NPK, why not just use a complete fertilizer?

Click to expand...

What about top dressing and watering in though? I have a Sour Dubb that is always a needy bitch when it comes to Magnesium but the other plants don't mind as much. I put a teaspoon on the soil last night and watered it in along with some other top dressed fertilizers.

I guess were you speaking more so of not adding it to soil mixes or what? Thanks

half dosage is always safe start with.
You see if it helps with that. Then you can give little more if it works good.
Different strains can handle different amounts
Some like half dose
Some like full dose
Some few like double dose, most if they are huge, and grow them in huge pots.

a cup water finger warm, not cold.
Mix epsom salt in this cup, so it disolves fast.
Then add this cup to the watermix.
Its perfekt if you mix it in cup warm water before you add it to water mix,
warm water, not hot.

i use this magnesium organic

the product to right, thats magnesium powder organic.
This is super quality professional lab tested stuff
Epsom salt is good too anyway
Try them both
They are the same i think. Epson salt is lots cheapier

Why i Use odltimers ? this is 100% good quality, controlled products. dosages is small, its not expensive in the end. you get 100% quality

If you self like try Oldtimers Organic Magnesium cristals/powder you find it in every professional growshop, or ebay, amazon.

Remember mix it in little cup with warm water first before add it to watermix
Also this works perfekt mix halfdosage from the start in soil
And then use it again at 3-5 weeks flowering
Stop use it about 14-21 days before harvest


I know this is needed in start, and once in earlier flowering, this is why i dont spray this stuff, i give it to soil
it works perfekt

The only thing i spray to my leafs in veg is biobizz green algae vitamin mix, never in flower.
Dont ever spray flowering plants. Not even pure water.
iN veg there is lots of good nutes spray , there is aloe, algaes, fish, even epsom , and more

Spray epsom salt and other stuff is only ok in veg.
This only, newer in flower

Most easy is mix it in soil from start halfdosage
then add once again by watering when they flower at 2-3 weeks
Maybe once/third time again if you see signs to that, or if you have longtime flowering strains.
newer use it often than 14days, this must have 2weeks before add again atleast!

And Always stop use this about 2weeks before harvest

Epsom salts won't kill your beneficial organisms. Just mix the epsom with your feeding water before adding anything else.

Big Nasty said:

May i ask why not in soil(fertigation)?i know foliar spray works faster but i would fix the def. in the rootzone before flowering.I agree on the complete fertilizer though.

Click to expand...

EagleWolf said:

If you've already been feeding salts, I don't think Epsom will hurt the mix. I mean, unless there's some chemistry I'm not aware of. If you're already feeding salt based fertilizers to your dirt, your microbials are probably already dead and it won't make a difference. Hydroponic dirt, essentially, which can grow some good weed, if flushed.

Click to expand...

who dat is said:

What about top dressing and watering in though? I have a Sour Dubb that is always a needy bitch when it comes to Magnesium but the other plants don't mind as much. I put a teaspoon on the soil last night and watered it in along with some other top dressed fertilizers.

I guess were you speaking more so of not adding it to soil mixes or what? Thanks

Click to expand...

It has to do with the physical characteristics of Mg...

In water epsom is fine but magnesium is a very small atom and tend to tighten soil mixes creating environments that favor anaerobic conditions.


If you've already got a very very loose soil mix I dont see how a little bit of epsom can hurt.

I have been using apx 12ml of epsom salt per gallon of water for Mg. I think you could even go 15ml if you wanted to. At transplant, and if I think the plants look Mg hungry I give it to them one time and wait until its needed again.


The dose and frequency of dose depends on many variables, whats in your soil already, genetic hunger for Mg, size of plant, size of container, light intensity.. etc.. etc.. There is not a one size fits all answer, but I do agree with starting light, and slowly increasing if you see good results. As well foliar is a safer and quicker test, but sometimes it is just not enough.

Epsom salt, is Mg and Sulfur. The word salt, just refers to it being available, not being sodium.


MjP's concern is real as well. If you put to much Mg in your soil, it can cause some bad problems, that are hard to correct. Soil compaction, and lock outs. So many people fear adding it to the soil. Although you do want some Mg in the soil, I think 10-14% saturation is the general consensus. So if you have a really low Mg saturation, and you are only foliar feeding, your covering up a problem with a band aid, and when you get to deep into flower to foliar your gonna end up Mg deficient.


Have to do some tests, either real life tests adding it in, and watching results dialing in your feed program, and or send the soil for a lab test once in a while to see if your even close to a good saturation. Helps to only change/test one thing at a time, otherwise you will never get any real world data to work from.


Molasses also has a decent chunk of Mg in it. I was talking about, "The Three Little Birds," how they were hitting really high yields organically in soil, decades ago. Iirc TLB's advised adding molasses to every feed, I think it was a tbsp per gallon. Who knows for sure, but I would assume with that much molasses both your microbes will be well fed, and you will be spoon feeding constant Mg and some other stuff.


Dolomitic Lime is a good source of Mg as well, but you are also adding calcium. Calcium is good, you can usually use more, but if you don't need more calcium then Lime is a poor choice to use as a Mg supplement.

Epsom salt, 10% Mg, and 13% sulfur (should double check may vary as well)

Molasses is around 2.4% Mg, has some other stuff, P, K, Fe.

They all will do the trick, just using the right one for the right job is the question. Strong quick dose, go Epsom. Spoon feed a little Molasses. Need calcium as well, go lime. Etc..


Ramble, ramble, starting with a small dose first is always good advice no matter what your adding.

Mr^^

Calimagic!

Calimagic!

Peters/Jacks is the bomb, unfortunately its missing Calcium and Magnesium. This product (Calimagic) will fix your cal/mag deficiency quickly, just start out slow. 1/4 -1/2 teaspoon per gallon added with your normal ferts...

https://www.amazon.com/General-Hydroponics-GH1772-CALiMAGic-Quart/dp/B00GZRKI40

I've been using molasses with at least every other watering but this past one I put in both a tsp of epsom and a tsp of gypsum. I'm hoping that will turn things around but we'll see. I'm heading into flower here in the next couple of days as well.