I bought annuals and mostly perennials and was wondering if I can use clay soil as the “base soil” (if there is such a term) and then place the seeds on top of the clay soil and then use a topsoil to cover the flower seeds.
Would this work? Will the flowers grow with clay soil. Most of the flowers require some acidity in the soil.
There are a number of measurements that are used to characterize soil. Texture is one and is determined by the size of the particles and the spaces between them. Clay soil has small particles and retains water. Sandy soil has large particles and drains quickly. The pH level of the soil is another characteristic and is a measurement of the acidity or alkalinity of soil. On the pH scale 7.0 is neutral, below 7.0 and acid and above 7.0 is alkaline. Whether particular flowers prefer neutral, alkaline or acidic soil will depend on the particular plant, although most plants prefer a pH of 6 or 7 (slightly acidic or neutral). Similarly whether a plant will thrive, or indeed perhaps grow at all, in clay or sandy soils also depends on the particular plant.
Although you could try growing your flower seeds as you have described, you may be disappointed.
You want to make sure that the environment for the roots of your flowers is the best possible, as good roots generally mean healthy plants. If the plants cannot develop good roots they are unlikely to thrive and may be more susceptible to damages from insects or disease.
Your seeds should have at least 5 to 8 inches of loose soil for the roots to start growing through so you should loosen your base soil at least this much before planting. While you could put a layer of topsoil this deep on top of your base and this would be less labour intensive it will be more expensive. As well, if the top soil is a layer separate from the existing soil it may create soil drainage problems and the plants roots may stop growing at the beginning of your base soil.
As loamy soils (8-25% clay) are preferred by many plants I suggest adding 2 to 3 inches of topsoil to the top of your garden beds and then mixing the top soil into the existing clay soil to a depth of 5-8 inches before planting your seeds.
You should also add organic material to your garden each year which will continue to improve your soil’s texture each year. (I have been adding soil, leaves and compost to my soil for over twenty years and the clay soil I began with is now a lovely friable loam). This can be done in the fall by adding composted kitchen and yard waste which will continue breaking down over the winter. You can also add new top soil each spring. Putting mulch on your garden after the seedlings have started growing (around but not on top of the plants) will help to conserve moisture and suppress weeds and will also add desired organic matter to your clay soil.
Below are links to Toronto Master Gardener Fact Sheets on improving your soil organically, and composting for the organic garden, which you may find helpful.