Fertilizers… No, I don’t like to use them on landscape cacti. But, the containerized cactus will need some extra food now and then. I suggest once a month a light solution in the summer months only. Tomato food is fine to use as specialized cactus fertilizer is seldom available. I use a fertilizer that has a high Phosphorous count(the number in the middle). Example 8-20-10:
The 1st number (8) is Nitrogen which enriches the green coloring of the plant and promotes stem growth.
The 2nd number (20) is Phosphorous which encourages flowering and root growth.
The 3rd number (10) is potassium which strengthens stems and helps plants resist disease.
Good Luck and Happy Growing!