Gardeners who start a number of seeds indoors or who do a great deal of container gardening will quickly find that buying potting soil and seed starting mix can get expensive very quickly. I am all about saving money and doing things myself around the garden. I would like to share with you the mix I use for my potting soil and seed starting.
Potting Soil and Seed Starting Mix All in One
- 4 parts sphagnum peat moss
- 1 part compost
- 1 part vermiculite
- 1 part perlite
- 1 part worm castings
To make it easier on myself, I use this same mix for BOTH seed starting and potting soil needs. Keep in mind that some differences exist between the requirements for potting soil versus seed starting mix. I will share why this mix works for both.
Qualities of a Good Potting Soil and Seed Starting Mix
A good potting soil and/or seed starting mix share a lot of the same requirements.
- Oxygen! The roots of plants need air to breathe. When in a container, the soil has a greater chance of settling and compacting because it doesn’t have anywhere else to go. This is especially important for a young plant trying to establish its roots.
- Good drainage. The soil must allow for water to flow easily. If the soil is soaking wet or pooling with water, the roots of the plants will drown and die!
- Good moisture control. At the same time, the soil needs to retain moisture so it doesn’t dry out. I know, this seems contradictory! Think of how a wrung-out sponge feels. That is about how wet the soil needs to be.
- Space for easy growth. Once a seed sprouts, the roots need a nice loose mixture to be able to easily grow. The roots need to be able to spread easily. Too much compaction would prohibit this root growth whether it is a new seedling or a more mature plant in a pot.
These are pretty basic qualities that we have talked about in other articles about seed starting and growing in containers. A few minor differences exist between a good potting soil and a good seed starting mix, which we will discuss next!
Difference Between Potting Soil and Seed Starting Mix
The one key difference between potting soil and seed starting mix is that potting soil needs nutrients for the plant to grow and seed starting mix actually does not require any nutrients!
Surprisingly, seeds can germinate and grow their first set of true leaves without any nutrients being added to the soil. The seed itself is packed with nutrients that the plant uses to get off to a good start.
In fact, a big mistake people make is buying synthetic fertilizer and using it on little seedlings too early. Seedlings do not need this much fertilizer! If you do feel the need to fertilize, then only use about a quarter of the strength of what the directions say on the package.
This is the beauty of organic fertilizers. Plants cannot be harmed and they will decide when they need to start using the extra fertilizer. This is why the same mix I use for my potting soil can also be used for seed starting. The compost and worm castings will feed the plant at the right time. It is also much easier to just make one batch of mix for all my planting needs instead of making two different mixes!
DIY Potting Soil and Seed Starting Mix Steps
When searching for potting soil or seed starting mixes online, you will undoubtedly find ten or more different recipes. I am sure they all work perfectly great, so do not think that there cannot be some variations to what I provide below. Modify it as you wish and experiment with different ingredients to see what works best for you!
So again, here is the recipe that I use for both seed starting and growing in containers. It has been working very well for me for many years now.
- 4 parts sphagnum peat moss
- 1 part compost
- 1 part vermiculite
- 1 part perlite
- 1 part worm castings
Peat moss is an organic material that holds water very well, similar to a sponge. It also improves soil structure and holds nutrients in the soil instead of letting them leak out.
Vermiculite is a group of minerals that also increases water and nutrient retention. It is very light and helps plants absorb nutrients such as ammonium, potassium, calcium, and magnesium.
Perlite is actually volcanic glass that is heated up and popped like popcorn! It results in a very lightweight material whose main purpose is for great soil aeration.
Compost is a good source of organic matter that will slowly provide nutrients to plants. It is also light, holds water well and does not compact easily. I make my own compost at home from shredded leaves, grass clippings, used coffee grounds from a local coffee shop and plant clippings from the garden and landscaping. If for some reason you cannot make your own compost for free, it can be purchased at a garden store in bags or in bulk.
Worm castings are often referred to as “black gold” by gardeners. They provide a great source of nutrients for plants and they can be made for free! I have a worm bin in our basement and I feed these worms many of our food scraps. The worms turn it into great worm castings for the garden fairly quickly!
If compost or worm castings are not easily accessible, it is possible to purchase and add granular fertilizers to the mix. I list some good options in the article I wrote Does Potting Soil Need Fertilizer? Keep in mind that if this is being used for seed starting, not nearly as much fertilizer is needed, if any. It may be ideal to wait to add the fertilizer at transplant time.
Steps for Mixing My Potting Soil and Seed Starting Mix
Step 1: Gather Materials and Tools
First, I simply gather up my supplies. I have bags of peat moss, vermiculite, and perlite from the store. My own finished compost is usually already stored in five-gallon buckets, as well as my worm castings.
I find that a wheelbarrow works well for mixing all the ingredients together. This could also be done on a tarp depending on how much of the mix is needed.
For measuring the “parts” of the recipe, I simply choose to use a bowl from the kitchen. Anything you have will work perfectly fine!
A nearby water source is also necessary. If I am doing this during the winter in the garage (to prepare for seed starting), I get a large pitcher of water ready. A large bucket would also suffice. In the spring, I use the hose, which is much easier because quite a bit of water is needed to moisten the mix.
Step 2: Sift the Compost and Worm Castings
This is an optional step that is really only necessary for seed starting. When starting seeds, a coarse mix with a lot of big chunks of material is less than ideal. This is why I sift my compost and worm castings. I normally already have this done ahead of time and it is stored in five-gallon buckets.
To sift the material, I made a simple square screen using some scrap wood and a one quarter inch metal screen that I purchased at the hardware store. I made the width of the sifter about the same size as my wheelbarrow so that I can set it down over the top of the wheelbarrow to sift out the material.
The larger material, I put back into the compost or worm bin and the sifted material is perfect to add to your mix!
Step 3: Add the Ingredients
Now, I simply add the ingredients listed above into the wheelbarrow, using a bowl as my measuring device.
I scoop out four bowls of peat moss, one bowl each of vermiculite, perlite, compost, and worm castings. Wearing a mask for this step is a good idea for those who are sensitive to breathing in this material. It is very dry and may create a lot of dust; especially outside in any amount of wind.
Step 4: Water it Thoroughly!
Now it is time to water the mixture very thoroughly. It will take more water than one might think because the mixture is very dry and, as we discussed before, these materials are great at holding moisture!
I use my pitcher of water or the hose and water it down a bit. Then, I start mixing all the material together with my hands.
Next, I add more water again and continue to mix and add water until the entire mixture has the right amount of moisture. It should be as wet as a wrung-out sponge. What I do is I take a handful of the mixture and squeeze it. When I have the correct amount of moisture, one or two drops of water will drip out.
Step 5: Put Your Mixture to Good Use!
Normally, immediately after making this mixture, I want to use it right away. For seed starting, I use it to fill my seed trays and then plant my seeds.
For my seed starts, I use this seed starting mixture and never have to fertilize my plants before going into the ground. My tomato plants have grown over twelve inches tall indoors in this mix with no additional fertilizer! It is so easy and I do not have to worry about hurting my plants from over fertilizing.
I also use the same mixture when transplanting my small seedlings into larger pots when the time is right. It is not necessary for me to have different mixtures or buy anything else from the store for the different stages of my plants.
For container gardening, I simply use this as potting soil and fill up my pots before transplanting my plants. One tip: for decorative larger sized pots used for flowers or other more shallow-rooted plants, save some of the precious homemade potting soil by filling the bottom of the container with other material. Things I have used are shredded leaves, wood chips and smaller plastic pots turned upside down. You could also use milk jugs or something similar to take up the extra space.
Container gardening may require additional fertilizer, but this potting soil should give plants a great start. I have added a very small amount of granular organic fertilizer to some of my containers as well as a good nitrogen source, such as fish emulsion or blood meal. Read more about this topic in an article I wrote called Does Potting Soil Need Fertilizer?
Step 6: Store Your Extra Soil for Later
I normally have extra soil that I will use later for additional seeds or containers. I simply store the soil in five gallon buckets and put them in my garage or shed. I do use a lid for the buckets but I am not sure if it is necessary. If the soil dries out, it will just need to be watered again before use.
This DIY potting soil and seed starting mix can definitely save time and money; especially if a large quantity is needed. Potting soils and seed starting mixes can be quite expensive to purchase by the bag, which really does not go very far; especially if filling up many pots for container gardening.
I find it so easy to make this mix myself and use it for all my seed starting and container needs. I also do not have to worry about purchasing additional fertilizer for my seed starts.
If you are not making your own compost and worm castings yet, I would highly suggest looking into it! I truly feel this is one of the most important pieces to gardening success. Not to mention, it is easy to do, free and saves so much money compared to expensive fertilizer.
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I am the vine; you are the branches. If you remain in me and I in you, you will bear much fruit; apart from me you can do nothing.John 15:5