The soil is probably dry in the centre of the pot. This happens with strawberry pots - they're notorious for it. Also the roots will be in the centre, all competing with each other.

Put the entire pot into a large container of water so that the pot is entirely submerged. Leave it there until no more air bubbles come up from the pot. Then let it drain. This will saturate the soil throughout - it will have become water-repellent, you see.

You'll be wise to plant some of the mint into another, much larger, pot on its own just to ensure its survival.

Give the pot a little dose of iron, too. Follow instructions on the packet for quantities. Mint loves a bit of iron. You could poke a rusty large iron nail into the soil for a similar effect.

Rusty nails do absolutely nothing as far as nutrition. For iron to be available to a plant it MUST be in a soluble forum such as Iron Sulfate, or Iron Chelate. Rusty nails is Iron Oxide, worthless.

I hate to disagree with you, herboholic, but plants CAN access the iron from a rusty iron nail. Just as we humans can access iron when we cook things in a cast-iron frypan.

But I'm not going to come to blows over it!

No, that's fine. You can disagree. We're good. :-)

It's just a simple matter of physics. Iron nails produce, when rusty, iron oxide, which is useless to plants to absorb for iron as a nutrient. It has to be in a soluble form, such as stated.

If you've experienced success by putting rusty nails into soil, I assure you the result was purely coincidental. But if it worked for you, then more power to you.

I know people have been doing it for ages, but it's just one of those myths out there that refuses to die.

If you've a plant suffering from chlorosis or some iron deficiency, I would suggest using something like Iron Plus, in low dosages so as not to over-toxify the plant with iron it doesn't need.

The nails on the other hand won't hurt a bit. IMO they won't help one bit, but they won't hurt.

Iron oxide from rusty nails or iron filings is a form of iron that plants cannot take up. Plants are pretty fussy about the elemental bundles that their minerals come in. It's all about soil chemistry and plant genetics.

Chelated iron is, by far, the most efficient method of getting available iron to plants. Iron sulfate can be tied up in the soil, depending upon the pH, as well as the dominance of other elements.

Thank you.

Although there is a huge crowd (mostly people who are on the whole organic gardening train), that believe nutrients need to be in a chelated form to be absorbed into plants. Although that is COMPLETELY correct, their mindset is that rust will stay as rust, but that is completely false. An example people use today is ammonium. Ammonium CANNOT be used by plants in the least bit. It is however converted into nitrates through microorganisms. The same goes for rust. Rust (or Iron III oxide), will either end up chelating to organic acids in the soils (such as humic, fulvic, or acetic). It is there where the oxygen of the rust will combine with the hydrogen in the acids to form water, and the newly freed Iron III ion floats around dissolved in water until is forms an insoluble precipitate via chelation (most likely with the conjugate base that supplied the original hydrogen). It is in this form that it can absorb through the roots. This process is simply accelerated by microorganisms and highly acidic soils. But it is complete BS to say that rusty nails won't help with iron deficient soils, it will just take a lot more time for it to work. And if a plant is already dying from iron deficiency, then it's most likely going to die before it starts absorbing some. That is why it's important to give a dying plant immediate attention with a EDTA or amino chelated form of iron. Or You can dissolve nails (Or better yet, steel wool) in vinegar. You can enjoy this process first hand by placing a fire-stick above the soaking wool and bumping the glass to have the hydrogen bubbles float to the top all at once. It makes cool popping noises.

However, your suggestion of Iron Sulfate is no different from rusty nails soaking in sulfuric acid.