Choosing a bag of potting soil from your local garden center can be a bit overwhelming for the first-time gardener. When trying to plant seeds for the very first time, rows of regular potting soil, organic seed starting mix and other potting soil can seem overwhelming. It's obvious that they have different uses, but what are those uses and which ones do you need for your project? Understanding the difference between seed starter mix and potting soil is actually pretty easy, and knowing the difference can make starting seeds indoors or transplanting tender seedlings that much easier.
Growing seedlings is a fun way to try new varieties of vegetables, flowers and herbs, and if you're a plant lover, you're bound to want a specific plant that you can't find at a local garden store. If you are growing plants from seed, you will want to understand the difference between the seed starting mix you use when you first germinate seeds and the potting soil you transplant your baby seedlings into to give them more space and nutrients to grow. Let's go through the differences in more detail.
When you first start your outdoor garden, you will find that plant growth is highly dependent on the type of soil you use. When you first germinate your seeds, your tender seeds will sprout delicate little roots that have a hard time struggling through dense soil. They thrive best in the loose and light soil of a seed starter mix, where they have the best luck growing their delicate roots. At this stage of plant growth, your optimal growing medium is to choose from a variety of seed-starter mixes.
All about seed starting mixtures
On the left is Espoma Seed Starter Mix. At right is Kellogg Patio Plus potting soil. Note the different particle sizes. Source: Lorin Nielsen
seed starter mixes are not always made of earth. They are made from a variety of ingredients such as sphagnum peat moss, vermiculite, coir, perlite, and rice husk. All of these ingredients are selected for their use. An organic material like sphagnum peat moss is chosen for its organic matter and water retention, which helps keep the seeds hydrated. Perlite, which is an expanded rock and is useful for aeration and drainage due to its porosity, is often used as a starting mixture. The addition of coconut adds moisture to your seed starter without making it too mushy. This humectant acts as a wonderful moisture retention agent at a crucial time, as young seedlings can easily dry out and discolor before they reach maturity. Moisture retaining granules such as vermiculite are also sometimes added to help with moisture at a critical stage of seed development.
The root growth experienced at this point is relatively rapid. Another reason why soilless seed-starter mixes are widely used is that nutrient-rich soil is not required during the germination stage. Seeds don't actually need any additional nutrients during this time. The structure of the seed contains both the plant embryo and enough nutrients to sustain the plant until it forms its first two true seeds.
To complicate matters further, there are very few regulations as to what seed starters should be called. They can be called anything from soilless seed starter mixes to soilless mixes to pot mixes.
If you're short on space, sow early in the year, or choose to sow indoors, try a light and fluffy seed starting mix and be prepared to transplant your healthy seedlings into larger pots once they reach a healthy one seedling stage.
One of the appealing aspects of using a seed-starter mix is the fact that the seed-starter mix is technically sterilized when it first enters the pouch after it has been manufactured. It may not stay that way between the factory and when you buy it, but as a result it may contain fewer pathogens than a homemade seed-starter mix or a wet and used seed-starter mix lying around in the garden. Additionally, one of the main reasons for using sterilized seeds is the lack of weed seeds in these mixes, but the bag must not have been contaminated while in your possession. By starting seeds in a weed-free mix, it's easier to know which your own seeds will germinate and which won't. For beginners who need the help of seed packets to know if their seed has germinated or is a weed, this is a great help.
If you're looking for a quick and easy way to tell the difference between a commercial potting mix and a commercial seed starting mix, there's an easy way to tell the difference. If the ingredient label doesn't list soil, compost, or sand, it's a seed starting mix, and if it does contain soil, sand, or compost, it's potting soil.
Watch our video below for a great way to make your own homemade seed starter!
On the left is Espoma Seed Starter Mix. Center is Kellogg Patio Plus potting soil and right is Kellogg Raised Bed Potting soil. Source: Lorin Nielsen
Potting soil and seed starter mix primarily differ in the size of their media. With potting soil, your growing medium tends to be thicker. These soil mixes tend to contain larger pieces of organic material such as bits of bark or sticks. While this medium isn't the best for delicate seeds, it is the perfect medium for container plants once they've reached a large enough size (once again, after they've put out their first set of two true leaves).
potting soil generally consists of compost, composted manure or manure soil, plus sphagnum peat moss (or sometimes coco coir) and vermiculite. It may have fertilizer mixed in, something to be aware of when sowing seed starting soil cannot contain too much fertilizer without accidentally 'burning' the seeds. Potting soil ingredients usually differ between brands, which can be frustrating for the gardener just starting out. However, once you begin to understand which ingredients can help with which aspect of the grow, you might find that this can often work to your advantage. But there are other things to consider as well; large chunky mixes may or may not actually come into contact with the seed, meaning moisture penetration into the seed can be slowed down. Soil-based mixes can contain large amounts of clay, which can absorb far too much moisture.
In general, potting soil can actually contain soil, which means it can also contain microbiological soil matter like fungi or bacteria, and depending on the soil, it can be things that are left behind, such as the fungal causes of problems like damping-off.
Mixed potting soil is usually soilless (and can be sterilized) but often contains larger particles such as forest products (aka pieces of wood) and may or may not contain a fertilizer that is entirely unnecessary for seed starting. Some potting mixes are optimized for specific uses, such as: B. a raised bed mix with lots of added woodland products added for long-term moisture retention and excellent drainage.
Peat moss has a slightly acidic pH and is great for plants that prefer a more acidic variety like blueberries. For other crops, like beans and cabbages, you can opt for a mix that includes not only peat but also other ingredients like compost, as this usually provides a more neutral environment.
Take tomato seeds for example. These seeds are smaller in size and would have trouble starting in most potting soil. The coarser texture of potting soil or garden soil can stunt growth by not providing the right amount of continuous moisture. However, after the tomato plants have grown into healthy young plants, say in a soilless mix consisting of seeds, you can start transplanting your seeds into potting soil of your choice. Tomatoes have an aversion to 'wet legs', i.e. constantly damp roots, and as such you may want potting soil that has better drainage. Familiarizing yourself with your seeds and what's available will give you a better idea of what to look for in a seed starter.
While some gardeners use potting soil to start seeds, you may want to consider what type of seed they are using. When sowing seeds large enough for potting soil, sow seeds about the size of your pinky fingernail. Imagine one of the larger sunflower seeds, pea seeds or even beans. Starting seeds is more of an art form than a science, and it can take time to get to know your own seed options well enough. Organic gardening is different for every gardener, and it takes a lot of trial and error to figure out what works best for you.
Home gardeners can easily make their own potting soil at home for transplanting seedlings, for their potted plants, or for any other all-purpose potting soil. Farmers and gardeners can make their potting soil at home with a simple formula. Try mixing equal parts compost, peat moss, and perlite. Mix and strain through a fine-mesh hardware cloth and (optionally) add a natural fertilizer such as seaweed meal or worm droppings. Powdered azomite is another great addition to a homemade potting mix as it contains trace minerals that are harder to find.
When learning about potting soil and seed starter mix, remember that only OMRI-listed potting soil and mixes are actually guaranteed organic. Potting soil and seed starting mixes may claim to be organic without the label showing, but the OMRI designation means the mix has been certified organic by the Organic Materials Review Institute or OMRI. All components used in an OMRI-rated organic soil mix must be proven to be from organic sources, lest you end up with any weird chemical fertilizers or other questionable components. Companies that are not OMRI certified may use some organic ingredients, but some inorganic or chemically derived additives may also be included.
Hopefully this gives you a good idea of seed starting mix and potting soil and why they have such different uses. Keep in mind that there are many different variations, so it's important to make sure you have the right one for your specific plants.
frequently asked Questions
On the left is Espoma Seed Starter Mix. On the right is Kellogg's raised bed soil mix. There is a drastic difference in particle size. Source: Lorin Nielsen
Q: Can I use potting soil instead of potting soil?
A: The seed starting mix is optimized to help seeds germinate and generally does not contain much, if any, nutritional value. It's said to be ideal for germination, but not great for later growing. Since most people repot or transplant their seedlings into other soil, seed starters can be used to get things started and you can then plant them in a potting soil mix.
Q: What is the best soil for planting seeds?
A: While the best soil is "the one that gets you planting seeds," if you're using older seeds or seeds with low germination rates, a special seed starting mix will give you a better overall chance of success. The fine particles ensure good contact between moisture and seed, allowing the seed to be fully hydrated and fully alive. Larger seeds like sunflower seeds can be planted in potting mixes as previously mentioned as the larger seed size gives them more surface area to absorb moisture from the soil. Ideally, the potting soil should absorb at least enough moisture to feel like a wrung-out sponge, but not so much that it becomes mushy.
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