Soil or soilless mix: mineral vs organic soils

What is soil? Mineral soils

Soil is found outdoors, on the ground, and is made up of very tiny pieces of rocks, alive and decaying organic matter, water, air and an impressive array of living organisms too small for the eye to see. The size of those tiny pieces of rock determines what we call these soil particles, with sand being the largest soil piece, clay the smallest, and silt in between. Healthy soil is made up of a mixture of sand, silt and clay particles (called texture) that is "glued" together using decayed organic matter (humus) to build a healthy soil structure that should have about 25% air space and 25% water space. The living matter in soil is critical for soil health and a healthy soil is not sterile.

It is the soil texture (amount of sand, silt and clay) that provides the nutrients for the plant. It is the soil structure (how these pieces are put together) that allows the nutrients to be available in a form the plants can actually access. A large part of soil health is unlocking the nutrients that already exist in the soil. If you do not have healthy soil, your plants cannot access these nutrients so you may need to supplement with other fertilizers.

While most prairie soils have an adequate selection of all necessary nutrients within them, soils that have a history of intensive agriculture, soils that are in northern regions, or soils that are in specialized ecosystems may have specific nutrient defeciencies. If you are in a specialized area, you may need a soil test to determine what nutrient deficiencies you have. If you are in a typical prairie soil, odds are good that basic soil stewardship will allow you to unlock your soil's inherent fertility and health.

Soil is permanent. As such, it belongs in the following places:

• on the ground.
• in raised garden beds that have an open bottom on the ground.
• in very large containers as a permanent planting. Think "bigger than a bathtub".

If you want to buy this, it almost never comes in a bag even if that bag says it contains soil. Why? Bagged soils are typically organic soils (potting mix), not a mineral soil that has sand, silt or clay in it. Since mineral soil doesn't usually come in a bag, you probably will not find it in a big box store.

You can purchase soil in bulk from local greenhouses, landscape suppliers, and oftentimes from private individuals. Usually it is either plain top soil or is found mixed with a variety of other amendments such as compost or peat moss. We recommend purchasing a darkly colored top soil with no more than 10% compost and then, once it's placed, following soil stewardship management to ensure it's long term health. We do not recommend purchasing top soil that is mixed with peat moss as it is impractical since peat moss typically acts as an inexpensive filler, is unnecessary for long term soil health, and decays over time. We recommend no more than 10% compost for three reasons:

1. It is important to keep soil textures as consistent as possible to ensure proper drainage and water movement.
2. Too much compost creates overly robust plants that look nice, but have lower yields, fewer flowers, and are more susceptible to pests and diseases.
3. Any more is unnecessary for long term soil health. Investing in mulch is more important long term.

When you purchase soil, ensure that it does not have actively growing grass pieces or other rhizomatous weeds in it. If you are mulching, it is irrelevant if it has weed seeds in it since adequate mulch will prevent or significantly reduce weed seed germination. If you are concerned about residual pesticides, ask your soil supplier how they made this soil weed-free.

What is a soilless mix? Organic soils

A soilless mix is also called potting soil or potting mix or organic soils. While it is technically a soil because you can grow plants in it, it is mostly organic matter without much (if any) minerals in it. That means it doesn't have much sand, silt or clay in it. Because it is mostly organic matter, it:

• is good at providing air spaces for healthy roots,
• is good at holding water, and
• has essentially zero inherent fertility so it is closer to hydroponics than planting into the ground.

These typically contain a large amount of peat moss or coir. Since dried potting mix is somewhat water repellant, it may have a wetting agent in it to allow the peat/coir to get wet more easily, which will have no significant impact on your plants. They may also contain shredded wood products as a filler to retain moisture. To add aeration, they may also be mixed with vermiculite or perlite (both are essentiallly special rocks that have been popped like popcorn). This product almost always comes in a bag and is difficult to find in bulk. Realistically, our market availability is consistent enough that it's safe to assume that anything that comes in a bag and says soil is probably an organic soil.

Potting mixes are supposed to be sterile to reduce disease pressures on sensitive seedlings, but low quality mixes may not be.

One caution - make sure you read the labels on your bag of potting mix. Why? Becasue there are two other potential ingredients that you need to know about to make good decisions.

1. Fertilizers. Potting mixes occur naturally without any essential plant nutrients. Any plant that is past the two leaf stage will need fertiliity for healthy growth. If your soilless mix doesn't have compost or a slow release fertilizer in it, and it probably doesn't, you will need to add it or you will need to water with a water solutable fertilizer at half-strength weekly.
2. Moisture control or polymer crystal gels. Some potting mixes will have special water-retaining crystals mixed into the soils. These absorb water at a tremendous rate and will slowly release it back into the potting mix as it dries. These are similar to the gel balls you find in decorative vases and in the absorbant lining in diapers. These can be effective at extending soil moisture availability short term if you're a relatively consistent waterer. If you are not watering consistently and these are allowed to dry past a certain moisture level, the crystals will strip the plant's roots of moisture, which will kill your plant significantly faster than simple dried potting mix. These crystals may be non-toxic but may not be safe for food plants. You need to read the label to see if this applies to your specific mix.

How can I tell them apart?

Because soilless mixes are primarily organic matter, they will float. Soils are primarily rock pieces so they will sink. If it came in a bag, it's probably a soilless mix but if you want to be sure, toss a handful in a jar of water. If it mostly sinks, it's soil. If it mostly floats, it's a soilless mix.