The growing mix for my any Sub Irrigated Planters (SIPS) must have a strong wicking property like a sponge. Soil or compost will not work. A soilless mix with the correct properties works much better. You can buy commercial grow mix usually in large 3.8 Cubic ft. bales made from Sphagnum peat, lime, perlite, and vermiculite or you can mix your own. You can usually find commercial grow mix at most large home and garden centers. The brand is not important, just check the label for ingredients. One large bale should fill 6-8 Alaska Grow Buckets.
Sphagnum peat is a stable organic material that will absorb 15 to 30 times its weight in water and decomposes very slowly. It contains about 1% N but little is released because it breaks down so slowly. It has an acidic pH of about 4 so lime must be added to the mix with sphagnum peat, at the rate of 8.5 lb. per cubic yard of peat to neutralize the acidity.
Coco Coir comes from coconut husks and is a waste product of the coconut food product industry. It has physical properties much like peat but a more neutral pH of about 6. It will absorb up to nine times its weight in water. It can have a high salt content. It comes in compressed blocks that must be soaked in water for several hours or overnight before mixing with other ingredients.
Lime is either calcitic (high calcium) or dolomitic (high magnesium; both are used to increase the pH of a mix but Dolomitic lime is preferable for supplying both Ca and Mg.
Vermiculite helps hold water and fertilizer in the potting mix, and it also contains some calcium and magnesium. It has a pH near neutral. Vermiculite comes in different grades; medium grade is usually used for starting seeds, a coarse grade may be used for larger plants.
Perlite is a volcanic rock that has been heated and expanded. It is lightweight, sterile and has a neutral pH. It can be used to reduce the weight of a potting mix and increase its aeration and drainage.
Mycorrhizae Fungi colonize the root system of a host plant, providing increased water and nutrient absorption capabilities while the plant provides the fungus with carbohydrates formed from photosynthesis.
Most commercial grow mixes have the proper ratio of ingredients plus wetting agents and may contain Mycorrhizae Fungi and fertilizer - check package label for ingredients.
DIY Grow Mix Recipe: Bucket = 5 Gallon Bucket to measure.
3 Buckets Sphagnum peat + 1 cup dolomitic Lime
OR
3 Buckets of damp Coco coir pre-soaked in water
(follow pkg instruction)
Mixed with:
1.5 Buckets Perlite
1.5 Buckets vermiculite
This should make enough to fill 6-8 Alaska Grow Buckets. You can also add your own Mycorrhizae Fungi and a slow release organic fertilizer at planting time.