Horticultural Soils

What is a horticultural soil?

On a very broad level it is a substrate that is used for horticulture.  This can include both soil based and soilless media.  There are many words and phrases used to describe soils and here is just a few that I use interchangeably:
-Media
-Mix
-Substrate
-Soil
-Dirt
They all communicate the same thing: a place for the root system of a plant.  They have various descriptors too.  You can have a growing mix or a soilless substrate or an engineered media.  I guess it depends on the context.  In scientific publications it tends to be substrate, where a salesman sells you mix, and a landscaper installs dirt.

I have a very broad view of horticultural soils as I work with a broad array of materials, oops there's another one.  The characteristics of a media depends on the application.  The considerations you have to have for a plug mix or a greenhouse mix used for annual flat production is different from a mix used for shrub production.

Here is an overview of some of the applications of Horticultural soils.

Container Substrates:

Plug mixes need to be highly consistent and finely graded to provide a seedling proper air exchange to the roots.  There is not a lot of room for error.

Greenhouse mixes need proper air exchange as well but also as important is water holding capacity.  You don't want them to dry out, and you don't want them to hold water too long.  While not a plug you are still dealing with small containers and typically short term crops.

Nursery mixes have a lot of the same things as above. One difference is the need for the components in the media to have long term structural integrity, meaning they need to not break down very fast because the crop may be in the same container for a year or two.

Potting mixes is a term for mixes for the home owner, at least for me.  Plants potted either outside or inside are subjected to different conditions than at a greenhouse or nursery.  Does it need long lasting fertility, what is the price point you are trying to meet. It is important to make it as fool proof as possible.

The above materials can be situation and crop specific.  There might be a special rose mix that is different than what you might use for other shrubs.  If the plants are grown inside under boom irrigation vs outside that can matter.  Needless to say it can get very complex.  On the other hand, a good grower can grow in just about anything as long as they have a grasp on how to adjust cultural practices.

Landscape Soils

Garden mixes or landscape mixes that as you might guess are used in gardens and landscapes.  These may have soil, sand, and/or compost.  These are used to replace existing soils or build planting beds.  They need to have long term viability, and need to drain freely.

Soil amendments on the other hand are there to improve the existing soil.  They will typically have high organic matter content and may be designed to be incorporated or top dressed.

Engineered Soils

Rain gardens and bioswales which I will go into more depth later.  In essence they slow water down to have it recharge the groundwater instead of the sewers.

Greens mix- have very specific needs of water infiltration, and particle size to give golf courses what they need.

Structural soils are used for under concrete to allow roots to grow better in confined spaces and their ability to compact is crucial.

Green roof media-  This is media that goes over structure, media on roofs of buildings of course, but even some parks that are over parking garages are green roofs.  Green roofs primary function is to manage storm water, but have a lot of other benefits that I will go into some time.  Here many times the weight, drainage, long term viability become important.

These are all general groupings based on uses, but by no means should one be limited by some artificial designation.  If you go to a store to buy a "garden mix" doesn't mean it won't work in pots.  Of course it may not be suited for that and hold too much water, but a name is just marketing.

Always at your service,
Your Horticulturist

Got a question?  Let me know and I will try to address it.