The number of gallons in a container are tricky to calculate because the container's shape affects its volume. Location is also a factor in determining the amount of nutrition to apply. For example, indoor containers generally require less plant food than do those that sit outside (on a patio, deck, sidewalk or what have you). With those qualifiers stated, I suggest using 335 grams (12 oz.) for the 20 gallon container, 408 grams (14 oz.) for the 25 gallon container and 554 grams (20 oz.) for the 3o gallon container. A key advantage in using Osmocote is its slow-release characteristic. The proprietary coating controls the release which reduces the risk of over-feeding so even if too much is applied, the plant will not be harmed. Bob Stohler, PlantersPlace manager, for Marianne.