ISU Extension recommends equal parts of top soil, peat moss and perlite. I prefer to have less sand or perlite because you don’t have to your transplants as often if there is more organic material in the soil. I use the following combination when I make my own. It also works better with the soil block makers.
I don’t use peat moss due to the negative environmental impact. Organic Gardening ran an article that explains that all very well, if you are interested.
2 parts top soil
2 parts coco peat or compost
1 part sand or perlite
I mix a few organic additives in the mix to provide nutrients to the seedlings, although I don’t add nitrogen at this point. We want to focus on healthy roots at this stage, not green leafy growth. Phosphorous encourages root growth. Bone meal and azomite are good sources.
If you make your own soil, it needs to be sterilized.. This is very important. Soil can be sterilized by heating it in a 180 degree oven for 30 minutes. I use a crockpot or my granny roaster depending on how much soil I need.
You can use many sorts of containers to plant the seeds. To begin with I planted them in cardboard egg cartons. Then I moved up to the coco-peat containers. Now I use a soil block maker system. I’ve had my seed block makers for five years now. While it has definite space-saving benefits and conserves soil, my favorite part about using it is that my transplants won’t become root-bound and experience very little transplant shock.
Roots are smart. If they encounter too much resistance, they try to find a way to go around it. They stop growing when they no longer encounter resistance. This is called air pruning. If you leave room for airspace around your cubes, the roots will NOT grow together. I have never had that problem. If the cubes are touching, the roots will continue to grow into the next.
If you use egg cartons, you will need to transplant into a larger container eventually as they are not larger enough to support a seedling. You can save larger containers for transplanting. It important to make sure that you have cleaned and sterilized these containers.
If you make your own newspaper pots, they need to be a minimum of 2 ½ inches high by 2 inches in diameter. Paper pots may disintegrate within five weeks.
To fill any of the containers:
Pack them with potting soil, moisten the soil with warm water and tamp it down a bit. Allow the soil to drain and repeat as necessary to fill your containers.
Before you plant your seeds it is good to know which seeds are cold germinators and which seeds need a warmer start so that they can be placed in different spots. A kale seed will germinate at 55 degrees, while tomatoes germinate at 75 degrees. Watermelon may not successfully germinate until it is 85 degrees. Additionally some seeds germinate overnight while others take weeks. As containers need to be covered until germination, it is good to group them accordingly. Most of your vegetable seeds will germinate within 7 – 14 days. Some herb and flower seeds need a period of cold stratification before they will germinate. The best guide on how to germinate seeds I have found is available on the Internet at http://tomclothier.hort.net The Seed Site is also a great site, but I find his germination charts to be confusing.
1. Plant seeds. It is not necessary to plant seeds deeply. If you think about how seeds self-sow in nature, you realize that they are just lying on the surface of the soil. Thick and thin is the best motto when planting seeds.
Fine seeds simply be sprinkled on the surface or you can use the paint brush trick.
Medium seeds can be covered to a depth that no greater than the thickness of the seed. It is always better to cover the seed with too little dirt than to bury it too deeply.
Large seeds can simply be pressed into the surface and covered lightly.
2. Cover the top of your container with transparent material such as plastic film or a plastic lid to keep moisture in. Remove this cover as SOON as you see germination.
Your seedlings should be receiving at the very least eight solid hours of light per day. A 12 hour light-dark cycle is more desirable. You will likely need to use artificial light to accomplish this. It doesn’t have to be a fancy set up. You can use a shop light with one warm bulb and one cool bulb on a timer. The lights should hang between 6-8 inches above the top of the seedlings, which may mean raising the lights, as the seedlings grow. If they are trying to grow too quickly to “reach” the light, they become long and leggy. Putting your seedlings in a window will work but you will probably find that the plant grows toward the light and gets a little leggy.
It is best to keep the soil moist by lightly misting it with a spray bottle or a pump sprayer, rather than actually pouring water on top of the seedlings. This causes seeds to drift and may result in fungal problems. I like to bottom water my larger soil blocks but I wouldn’t do this with my small blocks.
Once your seedlings are growing it helps to have air circulating above them. This discourages fungal growth and simulates the wind triggering thigmomorphogenesis. This is beneficial to the plant because it results in shorter sturdier stems. (Stern: Introduction to Plant Biology – 198). If you don’t have a fan, you can just ruffle your hand gently across your seedlings. For plants that need to be started at warmer temperatures such as peppers, tomatoes and basil, you can use one of those heater fans to heat the air around the trays. I have found this to be more effective than bottom heat.
The first two leaves that emerge are known as the cotyledons. These are not true leaves and they will yellow and fall off as true leaves appear. If you need to transplant seedlings to a larger container this can be done any time after the first true set of leaves appears but I like to wait until the second set appears.
Seedlings grown inside need to be hardened off by gradually exposing them to daylight and outdoor growing conditions. 7-10 days before you are going to plant them in the ground, set your seedlings in a protected shady place. Every day after this put them in the sunlight for successively longer periods, until they are ready to put out.
I like to try to plant my starts out on a cloudy day which is usually easy to accomplish in the spring. Don’t hurry the plants be sure to wait until temperatures are appropriate and soil crumbles easily in your hands. If you work damp soil too soon in the spring, you are likely to create compaction and other problems.