How you approach making soil for your greenhouse will depend on what you want to grow. If you want to use one soil for a greenhouse with a variety of plants, like flowers, herbs and vegetables, you will need to make a balanced blend. If you are only growing tomatoes in your greenhouse, you will want a moist, rich soil. Research what type of soil will best suit your plants before beginning to mix a soil for your greenhouse.
Clean everything first. Before mixing a soil, clean the tools and containers you will be using with soap and water so that you can be sure you don't introduce weed seeds, bugs and diseases into your soil. Wear gloves during the soil-mixing process.
Sterilize garden soil. If you are using soil from your yard, field or garden in your greenhouse, or if you are using homemade compost as a soil amendment, they must be sterilized first. Put layers of the soil a few inches deep in shallow metal disposable baking pans in the oven, and cover with aluminum foil. Bake between 180 degrees and 200 degrees Fahrenheit for at least 30 minutes. See the Resources section for alternate sterilization methods using a microwave or pressure cooker.
Plan your mix. If you are growing all vegetables, you will want to use plenty of rich compost in your mix, perhaps in a ratio of two parts compost to two parts soil, to one part perlite or sand. (If your base soil is already sandy, use less perlite or sand to make up for it.) If you are starting delicate seeds, use two parts perlite or sand to one part compost and one part dry mulch, such as chopped bark or leaves, to give them lots of air to sprout.
Stir the soil mixture well in buckets using a garden fork or hand tools. You can also store it in buckets, covered, until needed. Transfer it to your potting containers or seed flats as needed.
At the transplant stage, reassess the plants' soil needs. Once they are sprouted, you might want to mix soils with more compost or garden soil to get them ready for the outdoors, or just to provide them more nutrients in the greenhouse.
To maintain the viability of greenhouse soils, refresh them more often than you would in an outdoors garden. The soil is shallower and dries quickly in the higher heat of a greenhouse. Add new amendments of loamy soils and compost in the spring and fall, when plants need nutrients the most.