Selecting compost or planting mix can feel like gambling. You know soil is the heart of your garden, that what you grow and how it grows is directly linked to the quality of what you’re growing it in, but which soil or soil mixes are best? Whole seasons can be lost due to poor soil, regardless of any other effort you give your garden or conversations, poems or songs you sing to your plants. In the end the success and flavor of your harvest comes back to soil.
Making your own soil, soil mixes and compost is ideal but understandably time consuming and sometimes labor intensive. Your next best option is to purchase soil at a landscape supply company or your local garden center.
Fortunately (or unfortunately) you’re faced with nearly endless choices. How is potting soil different from planting mix for instance? The label says organic but is it organic and what does that mean? Do you need compost or loam builder? It can be incredibly perplexing but it doesn’t need to be.
Here are the basics for deciphering the soil question:
When starting from scratch, fill your beds with a 60/40 to 50/50 organic top soil to compost blend. When amending raised beds, add compost. Top with a coarse compost or mulch in winter and summer to protect soil structure, maintain soil moisture and temperature. *Substitute planting mix for top soil as needed.
Fill with an organic potting mix. Line the bottom of containers with drain rock, especially for larger containers and containers holding plants that require “well draining” soil. Amend with compost, compost tea and liquid fertilizers. Top dress with compost or other soil protecting materials. Gravel, pebbles, fir bark and coarse compost for example. *Treat larger containers such as troughs as raised beds.
Two solutions for prepping an in-ground planting area include using the no-dig approach and simply top dressing your planting area with compost, or double digging and adding a compost or loam builder. With either, consider planting fava beans, cover crops or other plants that add organic matter and assist in making soil workable through root development while also adding structure and tilth.
Organic vs. organic
Soil blends and composts made from organic materials are usually labeled as “organic”. Organic materials can be anything that is biodegradable. With soils it generally refers to plant based organic matter. However, not all of these materials are chemical free. Chemical free means free of pesticides, insecticides and chemical fertilizers and other pollution. A product that is “certified organic” has been tested to be chemical free or grown and harvested in a chemical free environment, one in which chemicals are not added or applied in the growing or making of the product. Look for an official seal stating “certified organic” such as an OMRI certification. This is one of the highest standards for approving soils and other garden products as chemical free in the US.
Adding organic matter to soil is feeding soil. Think of soil as a living system. Our job is to copy nature as closely as possible but we’re doing so in a semi-controlled environment — our gardens. In a forest, for example, leaves, branches and entire trees fall to the ground, creating habitat and food for all sorts of creatures above and below the soil surface. Worms, beetles and various insects live in the soil along with microscopic organisms such as bacteria. Fungi run like highways throughout the soil. All are working away, doing what they do — eating and pooping — and in the process decomposing matter and, inadvertently building soil. Leaves, trees bark, straw and kitchen scraps are all “organic matter,” improving soil structure, holding moisture and also providing drainage while adding nutrients.
Compost is any mix of organic materials, however not all are created equally. Worm compost is quite different from leaf mold but both are compost and both are fabulous. When choosing a basic, all around compost for your garden look for one made with local materials and is a mostly plant based mix. Worm castings (vermicompost) are always good. Manure is good but sewage sludge is not. Buy certified organic.
Potting Mix vs. Planting Mix
These are often very similar. Both are typically made with fir bark or another, locally sourced tree product, sawdust, mycorrhizal fungi, worm castings, alfalfa and/or kelp meal, feather meal, perlite and sometimes mushroom compost, gypsum and bat guano. Potting soil is designed for containers so ingredients to improve drainage and decrease compaction are added, such as pumice and sand, along with organic material to hold moisture like peat moss or coconut coir. Look for products that don’t use peat whenever possible. Coconut coir is a fabulous substitution and far more sustainable.
Generally a mixture of a coarsely refined tree product such as fir bark and manure, often chicken manure. Loam builders and other, similar products such as “forest” blends are excellent ways of adding organic matter to soil.
Top soil is generally the upper two inches of soil found in the average environment. It’s typically filled with beneficial fungi and bacteria as well as grit such as sand and some gravel. It tends to be high in organic matter compared to soil found at greater depths. The quality of top soil varies by region.
Loam is the technical term for a soil that contains balanced proportions of sand, silt and clay along with organic matter. It’s considered an ideal growing medium.
Seed Starting Mix
These are soil-less mixes, sterilized mixes or sterilized soils. They are designed to be bacteria and fungi free and thus disease free which can be helpful when growing plants from seed. See How to Make Your Own Seed Starting Mix: https://passthepistil.com/make-your-own-seed-starting-mix-diy-gardening/
Mulch is a term used to describe a top dressing. Soils are top dressed to protect soil structure, in particular from rain which can very quickly degrade soil structure. Mulch also helps maintain soil temperatures, keeping them cool on hot days and trapping warmth when temperatures drop. It also helps preserve soil moisture and, if using a mulch made from an organic material such as fir bark, it adds organic matter to soil through decomposition. Leaves, needles, bark, straw, husks, coarse compost and gravel are all different types of mulch.
If all else fails and you’re still unsure, choose a certified organic product that is locally made whenever possible. Living near the coast for instance, I would expect to find a soil mix containing kelp, oyster shell and other products found near the sea. It didn’t need to travel far to get to my garden which means I’m supporting the local economy, using fewer fossil fuels and using materials that are endemic to the local environment.