The composition and health of our soil are essential for plants in gardens and pots. In most of Beaumont-Wilshire, our natural soil is clay and river rock. Those who garden are experienced at digging out rocks that range from huge to pebble-size that made their way to our area during the Missoula Floods.
Know the soil needs of your plants. We grow plants that have varying requirements for air at the roots, drainage, and nutrients. Plant tags give useful information about sun exposure and hardiness, but less often about moisture and soil composition requirements. Go online. Read books. Talk to a neighbor who is successful with the plant you want to grow. Learn about the environment where your plant or its antecedents grow naturally. Meeting the plant’s drainage, moisture, and nutritional requirements is key to success.
I like rock garden plants, so I make sure that the soil I use has plenty of gravel and coarse river sand along with organic material to match their native soil environments. I also grow vegetables and annual flowers that need more nutrients and moisture to do their job in one short season. And I have many plants in pots, including vegetables, which are at risk for overheating, drying out, or sitting in too much moisture. These three environments have different soil needs. The moisture and nutrient needs of the various plants and their environment dictate how I amend the soil.
The clay in our native soil has important nutrients. The problem is that the very fine clay particles hold too much moisture and prevent the air at the roots necessary for many plants to thrive. Amend clay soil with lots of organic matter every season to improve drainage. For plants in pots, I also add ¼”–10 crushed rock with no fines (rock dust), and coarse sand. Do not use beach sand. These important amendments are available at landscape supply businesses, such as Oregon Decorative Rock. The rock increases drainage and air and the finer particles of sand hold some moisture. I also have a worm bin in our basement from which I gather the rich worm castings to add to our soil.
For all of my pots I use good quality organic material such as Gardner and Bloome raised bed potting soil (available locally at Garden Fever or Portland Nursery) together with added coarse sand and crushed gravel, a method I learned from friends who are commercial growers of alpine and native plants. I also use a high quality slow release fertilizer that does not release in hot weather.
On the other hand, I also garden with plants that need lots of moisture, such as some primroses, iris, and carnivorous plants. Those plants are in areas where the clay soil is not amended or are in pots set in large saucers of water.
Treat your soil gently. Do not till the soil. Tilling kills earthworms and helpful microbes, and crushes the important aerated soil structure you have been developing over the years. Using a shovel is good exercise.