New tomato plants
Espon salt, Magnesium, is what you are most likely thinking of. I can not link to any research based information about using the salt directly but this is what I can share.
If you are purchasing special tomato fertilizers some have magnesium added. So follow directions for application rates. Maybe some of the symptoms below sound like trouble you have had in the past.
Symptoms of magnesium deficiency start at the bottom of the plant on older leaves and then works their way upward. Fruits appear to ripen evenly but maturity is often delayed. Deficiencies may also occur in the middle of the plant during heavy fruit load, but this is usually transient. Where deficiencies are less severe, older leaves look brittle and their interveinal tissue is curled up. Where deficiencies are severe, chlorosis between the veins progresses from the margins to the middle of the leaflets. The small veins also become chlorotic, but the bigger veins remain dark green. With ongoing magnesium starvation, the interveinal chlorosis becomes more intensive, the color turns to bright yelloworange and necrotic spots may appear, which can grow together into brown bands. Finally, the older leaves die and the whole plant turns yellow and fruit yield is severely reduced. Excess magnesium can result in an imbalance in, and expression of, calcium and/or potassium deficiency.
We had some tomatoes a couple years ago which had bottom rot. We have changed locations and had success last year. What I was thinking about was not a product that goes down, but kind of like a tube of some sort around the base of the plant.
Oh, ok. That would be for tomato cut worm prevention. Here is some information for you. There are several things that can act as a barrier to the plant stem.