For many years now, we have grown 90% of our heirloom vegetable starts. We do this for several reasons. I have much larger choice if I choose to start our heirlooms from seed. There just aren't that many heirloom vegetable starts available locally. I can grow better quality starts that I can buy, for the most part. It costs quite a bit less even if you buy the potting soil. When you buy potting soil, do not skimp. We grow alot of starts so we buy several large bags of potting soil. WE get it from a company that supplies commercial growers. They cannot afford to use poor ingredients. They would not get any reorders. Ask at local nurseries to find what they recommend and use themselves.
I want to offer you an option to buying potting soil.
1 bucket of each of the following: coarse vermiculite, coarse sand, decomposed leaf mold and good garden soil. The last two should be screened through 1/4 inch mesh. Add a cup of lime and a half bucket of screened charcoal from the wood stove or fireplace. Also add 2 cups of fertilizer.
Here are your fertilizer options:
4 parts composted leaf mold, 3 parts wood ashes, 2 parts bone meal, and 1 part blood meal. Mix and measure out 2 cups for your potting soil.
Our soil is always low on nitrogen, so I prefer to use this recipe:
4 parts leaf mold, 3 parts wood ashes, 2 parts bone meal, and 3 parts blood meal.
Either one of these mixes added to your potting soil will give your plants a very good start. You can also use them at half strength for monthly feedings.
Start with good quality heirloom seeds and you will have good quality heirloom vegetables to harvest.