You should be making your own compost every year if you have space. That way you control what goes in it. Yard waste and some kitchen waste. Make a circle or square with chicken wire and just start adding layers to it. Feed it a nitrogen source or use a compost starter like Jobes as directed. You can make a compost bin out of tons of stuff....Google it, there are tons of instructions and ideas for compost bins.....its simple.
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How much compost depends on your growing mix and rainfall/temps/ect ect ect. Even type/size of pots. Plus the quality of the compost....how much freaking dirt ends up in it.
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In my climate with my mix i can go with 40% pine fines, 40% good compost and 20% rice hulls. That is a local growers mix they sell to nurseries and it drains like mad. Needs almost daily watering with fabric pots in the summer. If i cut that mix with 50% ProMix i dont need to water nearly as often and it still drains good.
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In my area ive had good luck with Just Natural brand mushroom compost. Some people in other areas said theirs had too much dirt/sand. What ive gotten has been REALLY good stuff for bagged compost. So good ive grown stuff in it with nothing else added.
https://www.lowes.com/pd/Just-Natural-0-75-cu-ft-Organic-Mushroom-Compost/3426612