I am always cautious about offering recipes for the garden because I don't want people to think they are absolutes. There are 1000 ways to do most things in a garden and this holds true for containers soil mixes. I am introducing my general method that does vary based on what I have available and my general mood. So... please use this video and article for the principles. It is also the least expensive way to build container soil when you have to buy products. We all just don't have endless supplies of compost or a yard for that matter.
The key to container soil is moisture retention. I recommend making your container soil with at least 50% organic matter. Organic matter will retain water. You can use compost which will give it nutrient value. Or you can use peat moss or coco coir. They are outstanding at holding water but offer little if any nutrition to your plants. You of course could make a mix of these 3 items. Use what you have and what fits your budget. Remember, you want 50% of your container soil to be a water holder.
I dump all my container soil out onto a tarp from the previous year. Don't use container soil that had disease or insect problems. That is precautionary. Use that soil in a bed that has unrelated vegetable plants or in your flower beds.
I fill my containers up up half way first, using a mix ratio of 50% organic matter and 50% old container soil. I use peat moss. When I have compost ready I mix about 25% peat moss and 25% compost for my organic matter for my 50% organic matter. The other 50% again, is the old container soil. Remove any large clumps of unwanted debris. Remember 50-50 and fill up 1/2 of the container first.
Fertilizing doesn't have to be difficult although understanding the application directions can be. I recommend the tablespoon. You are not going to over fertilize your container soil. The primary feedings will come from the use of liquid fertilizers during the season. Container vegetables will suck the water and nutrients out of your containers. Regular water and feeding is a must for success.
This year I am using more organic products. The bottom half of the containers are getting 1 tablespoon of bone meal for phosphorous and 1 tablespoon of blood meal for nitrogen. They are both slow release and will sit in the bottom and get ready for future deep roots. I am also putting in 1 tablespoon of a general organic fertilizers that covers N, P and K and that is a more readily available form for your transplants. If you don't have these items 1 or 2 tablespoons of a 5-5-5, 10-10-10 or 15-15-15 synthetic fertilizer is perfectly fine. Plants don't know the difference.
Peat moss is acidic. So I recommend a handful of garden lime in the bottom. It is also good for adding calcium to your containers to prevent blossom end rot and it helps balance out the acidity of peat moss. You could also add your crushed eggshells into the bottom half. This prepares the bottom half of your containers.
I finish the top half off the same way with 50% peat moss or organic matter and 50% container soil. You can add in 1 or 2 tables spoons of any type of balanced fertilizer. A 5 gallon container is loosely equal to a planting hole as directed on the backs of most fertilizers. Or it is equal to just over one square foot. Remember, less fertilizer is better to start and they key to great container soil is 50% organic matter for water management.
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