The best soil for upside-down tomatoes is a light soilless potting mix that holds water without turning into a brick. You should never use dirt from your yard or garden. That type of soil packs down hard inside a hanging bucket. It chokes your plant roots and drains poorly.
I tested three different mixes during my second year of upside-down growing. Plain store-bought potting soil got too heavy after a few weeks of watering. It started packing down tight. Straight peat moss dried out too fast and turned crusty on top. My best results came from a 2:2:1 blend of potting mix, coconut coir, and perlite. Plants in this custom mix grew bigger and made more fruit.
Penn State research shows why your mix choice matters so much. High-peat mixes compress as they break down over the summer. This squeezes air pockets out of your soil. Roots need oxygen just as much as they need water. Packed soil starves them of air and slows growth. A good potting mix hanging tomatoes setup stays fluffy from spring planting right through fall harvest.
The ideal recipe calls for 2 parts quality potting mix, 2 parts coconut coir or peat moss, and 1 part perlite. The potting mix gives you base nutrients and structure. Coconut coir holds moisture while staying light and airy. Perlite adds drainage and keeps everything from packing down. This ratio makes lightweight tomato soil that works great all season long.
A coconut coir tomato mix beats straight peat moss in several ways. Coir comes from coconut husks. It's a renewable resource that breaks down slower than peat. Coir also rewets much easier if you let it dry out. Peat moss turns crusty and water-resistant when it dries. Water just runs off instead of soaking in. Coir absorbs water even after going bone dry. This saves your plants when you miss a watering day.
You should pre-moisten your mix before you fill any containers. Dump your dry ingredients into a wheelbarrow or large tub. Add water while stirring with your hands or a trowel. Keep mixing until the blend feels like a sponge you squeezed out. You want it damp enough to clump when you grab a handful. But it shouldn't drip water when you squeeze.
Dry mix causes problems if you skip this step. It creates pockets inside your bucket that resist water for weeks. You think you're watering well but dry spots stay bone dry. Your roots can't grow into those areas. Your plant suffers even though you water every single day.
Your 5-gallon bucket needs about 15-18 quarts of prepared mix after everything settles. Fill your container in stages. Press down gently to remove large air gaps. Don't pack the soil too hard or you'll squeeze out all the drainage. Leave about two inches below the rim for easy watering. Your mix should stay loose and drain well while holding enough moisture to keep roots happy between your daily watering sessions.
Read the full article: How to Grow Tomatoes Upside Down Successfully
