Fertilize cactus in late spring or early summer when the plant begins to grow taller, sprout new sections or develop flowers. The exact timing will vary based on region and species.
Fill a clean 1-gallon jug with lukewarm water. Add in 1/2 tbsp. of a liquid cactus fertilizer to the jug and screw the cap on it. Shake it up to mix the solution.
Pour the amount of fertilizer into the soil as specified by the product label, typically 1 to 2 cups for potted cacti and more for ground-planted ones.
Reapply the fertilizer every two weeks through the beginning of November and again in March.