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The Percentage of Organic Materials in Different Types of Soil Mixes

westes Zone 9b California SF Bay
9 days ago
last modified: 8 days ago

I started out my journey making soil mixes by making the gritty mix and 5-1-1 mix popularized by @tapla (mid-Michigan, USDA z5b-6a) . I understand that Tapla uses particle sizes to control the level of the perched water table in container soil mixes. But there is a different aspect of soil mixes that I rarely see discussed anywhere, which is: what percentage of a soil mix should be organic material if we want to create a soil that is optimal for different classes of plants? Now that I have spreadsheets, material suppliers, and most importantly experiences growing many types of plants in dozens of different soil mixes I fabricate, I am finally in a position where I can start to document this issue.

I understand that a purist will never tolerate small particles in a soil mix that goes into a container. But my experience has been that not all plants need to occupy the bottom of all containers, and you can relax this restriction A LOT for many container plantings. That's beyond the scope of this post.

I came up with a general scale from low to high organic content for various plants. These are approximate values and actual percentages will vary based on specific plant genera/species/varieties.

  1. Cacti and Succulents (20-30% Organic Material): Cacti and succulents require a well-draining mix to prevent root rot. A mix may contain about 20-30% organic material, with the remainder being inorganic materials such as sand, perlite, pumice, or small gravel.
  2. Euphorbia (30-40% Organic Material): While also a succulent, Euphorbias can tolerate slightly more organic matter, around 30-40%. The rest would be inorganic materials to ensure good drainage. I will say that I have found some Euphorbias to root rot incredibly quickly if you leave them in a container outside that experiences weeks of rain. I am still experimenting with using a 100% inorganic medium, or soil mixes with 10% organic material, and I lack the experience to have a final say on this.
  3. Orchids (40-50% Organic Material): Orchid mixes typically consist of bark chips, charcoal, and perlite, with around 40-50% organic material. It allows air to reach the roots while still retaining some moisture.
  4. General Houseplants (50-60% Organic Material): Many common houseplants prefer a balance of organic and inorganic materials. A general houseplant mix might contain around 50-60% organic material, such as peat moss or compost.
  5. Vegetables and Fruits (60-70% Organic Material): Edible plants like vegetables and fruits often prefer richer soil with more nutrients. A mix might contain 60-70% organic material, including compost or well-decomposed manure, with the rest being topsoil and a smaller percentage of sand or perlite.
  6. Acid-Loving Plants (70-80% Organic Material): Acid-loving plants like rhododendrons, azaleas, and blueberries often thrive in a mix with high organic content, around 70-80%. This is usually peat moss or bark, which is acidic, combined with a smaller amount of sand or perlite for drainage. I have had tremendous luck growing acid-loving plants in pure composted bark.
  7. Carnivorous Plants (80-100% Organic Material): Carnivorous plants, like Venus flytraps and pitcher plants, typically require a very high organic content, often as much as 80-100%. The organic material is typically sphagnum peat moss, with perlite or sand for drainage. These plants get most of their nutrients from insects, so the soil is nutrient-poor and acidic.

How would you modify the above guidance? Add new categories or provide changes to the guidance. Let's please not spend time in this post arguing that every container needs the minimum particle size to be 2.5 mm to avoid a perched water table. That's a technical issue and a separate discussion. And more importantly, if you are obsessed with perched water tables you can still follow my guidelines above and use larger particles in both your organic and inorganic fractions, so why spend time in this thread talking about this?

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