Do not start your indoor seedlings in soil.
Does that surprise you? Yet, even good garden loam is not the best choice for growing transplants.
For one thing, soil is more than just dirt. It is full of micro-organisms such as nematodes, bacteria, and fungi. Out in the garden, they keep one another in check. Indoors, it’s another story. One of the most common causes of seedling failure, “damping off” is a disease causes by a fungus. Once infected, it is fatal to the baby plants. The only hope is prevention.
You do have the option of sterilizing your garden soil. You can bake it at about 250º F for several hours. That will kill all those nasty diseases. It will also create a stench in your home. If you want to stay on good terms with your housemates, this is not the best way to go.
Another problem with using soil is that it tends to compact easily in containers. Roots require equal amounts of air and water. If the soil becomes dense enough, there is little room between the soil particles for either one. A clay soil is the worst, but even sandy soils cause problems in containers.
It is much easier, and much better, to simply purchase some sterile potting mix. Containing no soil at all, and steam-sterilized before being bagged, these mixes are made from peat moss (a renewable resource) or coir (a peat-moss replacement made from coconut husks) mixed with some vermiculite or perlite, and perhaps some sand and/or compost. Potting mixes stay fluffy, providing space for water and air.
It’s best to use a product specifically designed for seed starting. The smaller particle sizes of such a mix won’t obstruct a developing shoot or root. I prefer mixes that include a wetting agent, as dry peat moss repels water. Most mixes contain fertilizer, which may or may not be organic.
I fill my seed containers with potting mix straight from the bag. Firming the mix gently helps it keep its shape when you go to remove the transplant from the container. I do this while the mix is still dry, to avoid over-compaction. Then I scrape off any excess mix until it is even with the top of the container.
Some seeds are only scattered on the surface, while others require burying. Your seed packet should have planting directions on it. I use a pencil eraser to make planting holes. Then, if the seed is fresh, I drop one seed into each hole. (I may sow two or three seeds if the packet is older, since fewer of those seeds will germinate.) Smooth the potting mix over the seed, and place the container in a water-proof tray.
Don’t forget to add a label! I recycle white plastic containers (the kind containing yogurt, cottage cheese, etc.) by cutting them into strips. These make great plant labels that last all season. Write on them with a permanent market.
When you have finished planting your seeds, it’s time to add water. I approach this from two directions. Using a household mister set on a fine mist, I gently spray the surface of the containers. This settles the seeds, and wets the surface of the mix. Then I fill the tray with about a half-inch of tepid water. I don’t want to disturb the shallowly planted seeds, so I water from the bottom. The mix will wick the water up into each container through the drainage hole in the bottom. Check back in several hours to see if more water is needed, or if excess needs to be drained off. The goal is damp, but not soggy, potting mix.
Most seedling trays come with a plastic dome. It’s fine to use this at first. The cover mostly eliminates the need to add more water. However, once the first seeds have sprouted, remove the dome. You don’t want to coddle those babies too much, or they’ll be unfit for life in the great outdoors.
Place your seeded trays somewhere warm. About 75º F is optimal for most crops. You can even buy heating mats made especially for this purpose.
Now comes the hardest part—waiting for your seeds to sprout!