Choosing a good deed starting medium or a good medium to transfer seedlings into can be important to your success when you start seeds indoors. For a seed starter you want a mix that does not contain any real soil, to avoid bringing in disease organisms such as dampening off wilt. It should also be a fine textured mix, without large particles but it should take up water readily. Seed starting mixes don’t need fertilizer; seedlings won’t need fertilizer until several pairs of true leaves have developed. However if you intend to leave the seeds in the containers for a few weeks you may want to use a mix with fertilizer, otherwise you’ll have to add fertilizer as the plants grow. A new trend is to mix in soil microbes and mycorrhizae that are found in nature and that are beneficial to plant growth. .
For transplanting seedlings, starting cuttings and repotting plants you need a light weight, moisture retaining mixture. It can be a bit coarser than seed starting mix, the larger particles in this soil aid drainage and aeration of the soil. It’s best to transplant seedlings after they have two sets of true leaves into separate pots if you started them in flats or sowed multiple seeds in pots. (Or thin pots to one plant by cutting off all but one seedling.) Some transplanting mediums have fertilizer mixed in; some will need to have it added. While plants can manufacture their food they need certain minerals found in soil to do well, which you provide with fertilizer. Some seed and transplanting mixes also have wetting agents, products that allow the soil to take in and hold more water. And many now have soil mycorrhizae added.
There is an abundance of seed starting and transplant/potting mediums on the market and it can be hard to decide which one to purchase. To add to the confusion some companies have a variety of places making their brand name mixes and each place can make a slightly different mix depending on local resources. One bag of the mix may not look or perform as well as another of the same brand.
Most commercial potting mixes/seed starters are composed of sphagnum peat, other forms of peat, perlite, ground bark or coir, vermiculite, limestone and various wetting agents, some have worm castings or mycorrhizae added. So which brand is best?
There’s a rating below of what this author likes and dislikes about some common brands of potting soil. To be completely fair the author received samples of Gardeners Supply and Miracle Grow potting soil for testing. Other brands were purchased for comparison. The author has used all of these soils. There are many other brands of potting mix/ seed starting soil on the market and although they aren’t listed here it doesn’t mean they aren’t good mixes.
Gardener’s Supply Planting mixes/seed starter– I love both of these mixtures, seed starting and transplant mix. They are lightweight, the seed starter is fine textured, and they both hold water well. Plants grew very well in both mixtures. These mixtures contain soil mycorrhizae and are pH balanced. (Soil mycorrhizae help plant roots process nutrients and get plants off to a fast start.) The mixes are very similar to Pro-Mix, an advantage is that you can buy this soil in smaller quantities than Pro-Mix and the cost is quite reasonable. Gardener’s Supply offers discounts for multiple bags too. Cost is about $8 for a 9 qt. seed starting mix and $19 for 30 qt. transplant mix. Gardener’s Supply offers an organic seed starter too, (which this author did not try).
Pro-Mix- if you are going to be starting tons of seeds or transplants this is probably the gold standard. Pro Mix is light weight and contains both mycorrhizae and a natural bio-fungicide. It holds water very well and is a delight to use. It does not contain fertilizer. The disadvantage is that Pro-Mix comes in large, 3.2 compressed bales that weigh about 70 pounds. It can be hard to find locally and shipping is very expensive. You can store potting mixes from season to season but be aware that soil mycorrhizae can die over long storage and you will lose that advantage. Cost ranges from $25 a bale up to $65, depending on where you buy it, and discounts are normally offered for multiple bales. Remember to factor in shipping costs if it is shipped to you.
Espoma planting mixes- have the same ingredients as most potting mediums. However they add worm castings which are said to have some nutritional benefits, along with mycorrhizae. There are organic formulas too. They are good mixes, light and water retentive but this author found no advantage over Pro-Mix or Gardeners Supply mixes. Prices are all over the place, the author found Walmart had the cheapest price, at about $10 for a 16 qt. bag. Other places sell the same bag for about twice that.
Miracle Grow- the Miracle Grow brand name is well known but their potting soil mixes are known for variability, depending on where they are manufactured. The company deems to be phasing out the seed starting mix. Some mixes seem fine, lightweight and with good water retention. Others have large pieces of coarse wood, sticks and other debris in them. Some feel heavy and dense. The author has had bags which were great in texture and others not so good. All Miracle Grow mixes have fertilizer mixed in, which some growers don’t like. It’s usually a bit more expensive than other brands; the author found it cost about $10 per 8 qt. bag, although specials and discounts seem frequent. The soil seems to be fine for filling outside planters and large pots.
Jiffy potting mixes- not terrible but so-so, average in weight, texture and water retention. They do not contain mycorrhizae and seem overpriced compared to other mixes. The price the author found was about $19 for a 16 qt. bag. The organic option was terribly expensive at $40 for 12 qt.
Hoffman potting mixes- typical composition, the author did not try it this year but has used the potting mix in the past with few problems. Its pH balanced, seems to be a fine texture and holds water reasonably well. It runs about $13 for a 10 qt. bag.
The author believes the small grower will be happiest with Gardeners Supply transplant or seed starting soil. If you grow a lot of plants and can find it locally Pro-Mix is probably the best choice.
Here are some additional articles you may want to read.
When to start seeds
How to make paper pots for starting seed
Can you re-use soil from last years containers?
You can read the authors weekly garden blog here.