Resource: What type of soil should I use?

There are so many different types of soil out there, especially those sold in bags in nurseries and hardware stores. What’s the difference between all of them, and which ones should you use for which things? Here’s a resource to help you navigate all the different options and make the best choices for your garden.

The Basics

A quick crash course on soil! It is made up of four ingredients:

• Air - makes up 25% and lives in between all the particles that make up soil

• Water - makes up another 25%, whether you can feel it or not!

• Mineral - the majority of soil - it makes up 45% and includes particles like clay, sand, and silt

• Organic Matter - only a tiny 5% of soil! It’s the nutrient-rich part that comes from decomposed organic material

Different ratios of these elements are beneficial for different purposes while gardening.

Potting Mix

Potting mix is not actually soil! Notice that the name usually omits the word ‘soil.’ It is a lightweight planting medium, designed to be full of air and well-draining. It’s usually a mixture of several substances, things like:

• Perlite - those tiny white, popcorn-like pieces. Thought it may not appear that way, they are naturally occurring minerals made from volcanic rock. Perlite allows the soil to stay light and airy while maintaining some moisture.

• Vermiculite - a natural material (hydrated laminar magnesium-aluminum-iron silicate, to be specific) that resembles mica. Also improves soil aeration and water retention properties without rotting.

• Peat moss - made from a type of decomposed moss. It has a unique ability to hold onto moisture (but not excessively) and allowing plants to hold on to nutrients that might otherwise be leached out of the soil. Unfortunately it is an expensive, non-renewable resource and when harvested from bogs, releases lots of CO2 into the atmosphere.

• Coconut coir - comes from the outside of the shell of a coconut! A more environmentally-friendly component than peat and helps retain moisture.

• Compost - decomposed organic material. Though it is highly nutritious, it does not retain moisture as well as the other elements.

• Worm castings - also known as worm poop! Like compost, it is highly nutritious.

Use potting mix for starting seeds to make your own transplants, and potting up plants in containers up to a foot in depth. It is not meant for long-term use (only a a couple weeks, or as long as it takes to create your transplants) because it does not maintain its fertility. It also requires much more frequent watering than other types of soil.

Planting Mix

Similar to potting mix but with more minerals added - usually sand or sandy loam. Though still not considered true soil, it can be used in containers deeper than a foot while retaining more fertility and allowing for air flow and drainage. A good choice for planting out shallow raised beds and other deep containers.

Planting mix should not be used as a soil amendment, since it does not contain the high amount of organic material that your soil likely needs.

Topsoil

Here’s where a lot of the confusion comes from. Topsoil generally refers to what it sounds like - the top layer of naturally occurring soil, which contains the most amount of organic material due to the decomposition of plants that have fallen to the ground. The soil composition diagram above generally refers to topsoil, but the mineral part can look very different - it could have a high amount of sand, clay, or loam.

Bagged topsoil can be regionally formulated. Use it for backfilling large areas that you plan to plant only if you don’t have any other naturally occurring soil. Otherwise, use a soil amendment (see next section) to improve the soil you already have. Topsoil can also be used to fill tall raised beds (2-3 ft high).

Compost

A soil amendment, not truly soil itself because it does not contain any mineral particles. Compost is the most nutrient-dense part of soil since it contains purely organic material. Commercial compost is usually a mix of materials, such as fallen leaves and byproducts from forestry practices.

Do not plant anything in compost only! This will give the plant lots of short term growth but will hinder its long-term growth. Compost is meant to be used as a soil amendment, that is improving soil fertility, water retention, and texture.

At GFE, we amend our soil with compost every time we turn over a new bed for planting. We usually add a 2-3 inch layer and dig it in to the top six inches of soil. This replaces at least some of the nutrients used by the previous crops and maintains a balanced soil that absorbs water, contributing to our water saving mission at GFE!