It can be confusing to sift through the many products available to add to your plants. From fertilizer to plant food, plant supplements, and even plant hormones, it all seems very similar yet each product is different. Plants need many types of nutrients. Macronutrients are the nutrients plants need in the largest amounts (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur), which a general fertilizer or plant food will provide. Micronutrients are nutrients needed in much smaller amounts, but are still essential to plant growth, and include iron, chlorine, zinc, molybdenum, boron, manganese, copper, sodium, and cobalt.
Plant supplements can be one or several nutrients that your plant is missing. While the nutrients cross over in fertilizer and plant food categories, it is easiest to think of supplements as a more focused application of a nutrient to get a specific result, heal a deficiency or disease, or for aesthetic purposes, while fertilizer can be seen as a more general nutrient application. For example, adding calcium to tomatoes or peppers will cure or prevent blossom end rot. Adding iron chelate to the soil will help to cure yellowing leaves on citrus, and aluminum sulfate will help acid-loving plants to thrive and hydrangeas to bloom blue.