How to Amend Clay Soil for Your Garden: Improve Drainage and Nutrient Absorption

Soil is maybe the single most important thing when trying to grow dahlias and other flowers. Proper nutrients and drainage, give your plants what they need for a strong season of growing.

Start with a Soil Test

For growers in Southern States, and elsewhere with heavy clay, this can be a challenge. When I started growing dahlias, I greatly underestimated how bad the soil was on my subdivision lot. Thanks to a soil test I did early in the season, I was made abundantly clear that with optimum amounts of sulfur and sodium, and extreme deficiencies in everything else; I was attempting to grow dahlias on Mount Doom.

When it comes to clay soil, it is notorious for having drainage problems. This can be a major issue when trying to grow dahlias, as their tubers tend to rot in poorly draining soils. There are steps you can take, however, to amend clay soil and improve both its drainage and nutrient absorption.

Adding Organic Matter

The first thing I did to improve drainage was skelping the top 3" of clay out of the new patch. Then I tilled the ground to loosen it up. My next job was to add organic matter to the clay soil. This can include compost, garden soil, peat moss, soil conditioner, or leaf mold. All of these will help loosen up the clay and allow water to drain more easily. It will also give the soil nutrient content over time.

Another option is to till in sand or gravel into the clay soil. This will also help with drainage but keep in mind that it will not add any nutrients to the soil.

Adding Fertilizer and other Amendments

If you have a specific nutrient or pH problem with your clay soil, there are products available to help amend it. For soils that are too acidic, lime can be added; and for soils that are too alkaline, sulfur can be used. In addition, adding your routine fertilizers while amending your soil can hit two birds with one stone, so plan ahead.

Here's what I know, amending clay soil is not an overnight process. It will take time for the organic matter to break down and add nutrients to the soil. Even in my first year, I lost one out of every 10 dahlias from poor drainage and rot, and that was AFTER amending. But by following these tips, you can make your clay soil into a hospitable place for dahlias and other flowers.

If you have any other thoughts or questions, make sure to leave them in the comments below. If you’re interested in a method to bypass soil amending for growing dahlias in heavy clay regions; see Coty’s blog on growing dahlias in Grow Bags.