Inorganic Soil Materials
When people think of soil, most people think of a single category of dirt that encompasses everything from sandy mixes for coconuts to clay rich soils for aquatic plants. This is too simplistic as each mix of soil is composed of various inorganic materials that contribute greatly to the physical properties of soil such as drainage and Cation Exchange Capacity (CEC). In this article, we will cover the most common materials found in most soil mixes, their properties, benefits and how best to use them.
If you’ve seen what looks like little white styrofoam beads in your soil that become a powder when crushed, chances are that’s perlite. Perlite is a common inorganic soil ingredient used in a variety of mixtures. It’s an inorganic volcanic sand that is typically heat treated with steam, causing it to expand. This gives Perlite its fluffy texture. Perlite’s properties include
Lightweight
Moderately water absorbent
Chemically inert
No toxicity or fire risk
Neutral pH
Low CEC
Due to these properties, perlite is utilised everywhere from hydroponic solutions to terrarium mixes. If you need a soil mix with good drainage and want to prevent compaction, perlite is the perfect addition to your mix! Perlite comes in various grades of coarseness depending on which you need so check with your seller on the recommended grade for your use! Coarser grades are more effective at improving aeration and drainage while finer grades hold water better and are used for starting seeds.
Sand is one of the more familiar ingredients that we’ve all encountered before. Technically, sand refers to particles of a certain size (between 0.6mm and 2.0mm across). Sand is a more common ingredient than you think as it is used both to improve the stability of soil as well as to improve soil drainage. Sand is found more commonly in soil mixes rather than specialised gardening application with terrarium and succulent mixes tending to contain more sand than general purpose soil mixes. Sand’s properties include:
Large grain size
Low water absorption
Chemically inert
No toxicity or fire risk
Neutral pH
Low CEC
Largely because of its large grain size, low water absorption, sand is used in soil mixes to improve drainage. Water easily flows out of sandy mixes, bringing with it any soluble nutrients that may be present in excess in the soil which can cause poisoning to plants. Sand is best added to organic materials such as compost or peat before being used as a growing medium. Alternatively, sand can be used as the primary ingredient for soil mixes that need poorer soils such as terrarium or succulent mixes
Leca is an acronym for Lightweight Expanded Clay Aggregate which is formed similarly to Perlite. It comes from natural clay that was heated to the point where it expands giving these iconic red balls that are seen everywhere. Leca’s properties include
Extremely large grain size
Moderate water absorption
Chemically inert
No toxicity or fire risk
Neutral pH
Low CEC
Leca is both used as a growing medium in hydroponic systems and as an aesthetic soil covering. This is primarily due to its moderate water absorption and low CEC properties which provide a moist environment for plants to thrive but does not absorb the nutrients that plants need. Few mixes use leca as an ingredient for soil. However, doing so can greatly assist in improving drainage.
Gravel refers to crushed rock aggregate between 2.0mm and 16.0mm in diameter. While gravel is not used in soil mixes, it is used to line the bottom of flowerpots to improve drainage and as a soil cover to prevent mud splashing. It’s properties include:
Extremely large grain size
No water absorption
Chemically inert
No toxicity or fire risk
Neutral pH
Low CEC
Integrating gravel into your soil mixes is not common but it can assist greatly in improving drainage of the soil. It’s large size and inert nature means it hardly holds on to water. Gravel makes a great addition to succulent or cacti mixes! Not only does it keep the soil drained for these desert plants but it also serves an aesthetic purpose.
Clay, more commonly sold in nurseries under the name ‘burnt soil’ is unique among the inorganic materials we’ve covered in this article. While most of the inorganic materials here have improved drainage, clay will actually reduce drainage in soils. This is not inherently a bad thing as reducing drainage increases soil moisture and it’s nutrient retention capacity. It’s properties include:
Extremely small grain size
High water absorption
Chemically inert
No toxicity or fire risk
Mildly alkaline pH
High CEC
If you’re growing trees or other water hungry plants, you may want to consider adding clay into your soil mix! Clay particles are extremely small and tend to pack together tightly as a result. This forms a firmer soil which helps anchor larger plants and gives them a great deal of stability. Tightly packed clay particles also make it difficult for water to escape, decreasing soil drainage and thereby improving water retention. Thirsty plants such as Lime (Citrus microcarpa) thrive in soils that have some added clay. On top of all of this, clay tends to hold nutrients better due to it’s high CEC, improving soil fertility greatly!
These 5 inorganic soil materials cover most of what you’ll find in nurseries and garden centres!
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