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The Ins and Outs of Making Container Soil

“Then God said ‘Let the earth bring forth grass, the herb that yields seed, and the fruit tree that yields fruit according to its kind, whose seed is in itself, on the earth’ and it was so.”

In the beginning there was dirt, and it was perfect. Perfect for growing in the ground. At Mountain Valley Growers we have spent the last 35 years in search of perfect 100 percent organic container soil. It has not been easy and has often been confusing, conflicting, complicated and toxic. Just using organic materials is not enough, they must be combined correctly. THIS INFORMATION IS EXTREMELY HARD TO FIND.


The bulk of potting soil is composed of just a few inert ingredients. The proportions are different for different kinds of plants. These ingredients perform two basic functions. The first group allows the plant to access nutrients and water. The second group improves drainage.

Water and Nutrient Holders

Organic Peat Moss is our preferred nutrient holder and should make up between 50 and 80 percent to establish a desirable acidic pH. Peat Moss provides a place to store nutrients this is called cation exchange.

Drainage Improvers

These can be used interchangeably and should constitute 20-50% of the soil’s bulk materials. Adjust the percentage to your specific needs. The higher the temperatures and the greater the sun exposure the lower the percentage should be.

  • Large Perlite Preferred
  • Pumice
  • Lava Rock

Don’t use Sand: It offers no nutrient holding ability and does not improve drainage. Tree nurseries like sand because it’s like adding cement to the pots so the trees won’t blow over. If this is a feature you need, use sand for up to 50% of your drainage improving material.

Ground Bark is an inexpensive material commonly added to ready to use mixes, it can be unstable and can release toxic quantities of naturally occurring tannic acid.

Soil Structure

The structure of the soil is defined by the percentages of the different bulk materials that can be used. The smaller the particle size the stronger the capillary action, the result being more water standing at the bottom of the pot. If a fine mix is used in very small pots, plants can drown easily. Using a higher percentage of drainage improvers will offset this. Placing rocks at the bottom of a pot will NOT decrease the water logged area because the capillary effect starts where the soil begins. If rocks are added, the waterlogged region will begin on top of the rocks and the plant will have less available soil. The only practical use for rock is to cover an abnormally large hole in a container.


Most nutrients are available at the slightly acidic pH of 6-6.5. Outside this range, nutrient tie up occurs, becoming more severe the farther from optimum you get. When starting with a mix that has a substantial amount of Peat Moss in it the pH will be 6 or less. It is easy to adjust the pH toward Alakaline with Lime. Make sure to use dolomite lime and not hydrated lime. To test your soil an inexpensive Soil pH Meter can be used.



Nitrogen supports foliage growth. Phosphorus supports PH lowering (flowering) and root development. Potassium supports root development. Calcium must be present for new cells to develop. Magnesium activates enzymes and fights chlorosis. Sulfur is essential to chlorophyll production.


Iron is essential in respiration and chlorophyll production. Manganese is an enzyme activator and assists iron in chlorophyll production. Zinc forms part of the enzyme system that regulates plant growth. Boron regulates water and nitrogen in the plant. Copper is an enzyme activator that stimulates root, fruit and chlorophyll production.

The bulk materials do not contain these essential nutrients. They should be added by using an Organic All Purpose Fertilizer. Organic fertilizers have a distinct advantage over most chemical fertilizers; they break down over time, slowly releasing their nutrients at a consistent rate that fosters healthy plant growth. There is a secret factor that is needed before these nutrients can be used.


It’s not news that compost contains nutrients, organic matter and beneficial organisms. However, the nutrient levels are normally low and the organic matter in compost is not long-lasting. So why is it that it is so important for soil health?

It’s the Fungus! Bacteria! Algae!

Fungi These beneficial organisms take up the space and food supply used by pathogenic (destructive) fungi that attack plants. Some strains can break down organic materials, like cellulose, turning a piece of wood into available nutrients. Others assist roots in nutrient uptake. The mycelium filaments of some fungi trap and feed on soil nematodes. Bacteria There are many different strains of bacteria that perform different functions. Some fix nitrogen out of the air creating unlimited, automatic fertilization. Others break down organic nutrients into plant usable form. Actinomycetes Convert nutrients to plant useable form and release antibiotic compounds that STOP THE GROWTH OF UNDESIRABLE BACTERIA. Dominant species are Thermo actinomycetes, micromonospora and streptomyces. These organisms require fresh carbon as a food source thereby breaking down organic matter in the soil. Pseudomonads Can control plant pathogens and convert nutrients to usable form. Algae Contribute a small amount of atmospheric nitrogen to the soil. The introduction of 2 tablespoons per gallon of compost into your potting mix will add millions of beneficial organisms. They will feed, medicate and protect your plants.

Potting soil needs these beneficial microbes to convert organic fertilizer into available nutrients and safeguard the plant from diseases.


Plant and or animal compost that is fully composted should have a uniformly fine texture with no visible pieces, a rich dark color and no odor. Composted manure is the best choice because there is almost no chance for plant pathogens to be present. Bagged compost should be kept out of the sun so it won’t get too hot and kill the microbes. Composted chicken or steer manure can be found at any nursery center. If you need a large quantity look in the Yellow Pages under Compost. Check to see if it is organic. Remember you are not adding compost for nutrients or organic matter, but for the beneficial living organisms it establishes to release the nutrients in the fertilizer. Any nutrients and organic matter compost contains are a bonus.


Partially composted plant materials: Plant compost has the potential to contain plant diseases if not composted correctly. Avoid any compost that has visible pieces of raw materials. Compost that has sewer sludge: If it has sludge in it, it will smell like it, bad, and the smell won’t go away. Humus and Grow Mixes: These are wood products treated with chemical nitrogen. The nitrogen bonds with carbon, heating the pile and creating an imitation composted product. Any beneficial organisms present in the raw materials are eliminated.


If compost just isn’t your thing, several companies are now offering products that contain laboratory grown beneficial bacteria. It looks and smells like malt. The labs aren’t satisfied with just producing high tech Bacteria, they are also producing sophisticated fungi. Several different companies now have beneficial fungi that protect against plant pathogens.

As simple as 1,2,3,4,5

1-Chose your bulk materials. 2-Adjust the soil structure. 3-Check the pH. 4-Add Organic fertilizer. 5-Add bacteria, fungus and algae.

Voila! Potting soil!