I’m writing from southern Manitoba, Canada. Our zone is a warm 3 and we have very sticky clay soil. We’ve been working it for 5 years now, and we’re still having a lot of trouble getting a proper yield from our vegetable garden. We find a lot of our plants get yellow, spotted leaves easily and the plants are stunted and slow to ripen. We’ve been adding peat moss and compost every year, as well as adding a 4 part top soil last summer to cover our whole garden last year. We had a pretty good yield last summer, but this summer, we’re back to stunted, yellowing plants. Even with fertilizer. We’re thinking of adding sand to it, but we’ve heard that it can making it worse. Is that right?
Thank you for contacting the Toronto Master Gardeners.
Working with clay soil requires much patience. The problem with clay is that the particles (aggregates) are small and fit together fairly tightly leaving very little space between for drainage or oxygen. Because of the lack of oxygen and drainage the plants have difficulty absorbing nutrients and attaining the amount of oxygen they need to function properly.
Some of the standard gardening practices can actually make the clay even denser. Digging, especially when the soil is wet, will further break down air spaces and again starve the roots of more oxygen and nutrient transfer. The less digging you can do the better. Adding any supplements that are fine will also have the effect of increasing the compaction of the clay further.
You have not mentioned if you have raised beds. That would be the easiest and quickest solution as you can add new soil on top and plant directly into the new soil. Over time the clay beneath will start absorbing the nutrients and slowly loosen up.
The best supplements for clay increase organic content. Compost, Leaf mulch (mowed leaves in the fall) and store bought mulch laid on top of the soil will slowly work its way down and start loosening up the clay. As you increase organic matter the worms will follow and oxygen and nutrients should over time become more available to plant roots.
Unfortunately there is no quick solution to clay just persistence, dedication and time. I am including some links for you to read up on clay further.
I hope this helps.