Understanding Amendments
for
Planting Mixes & Potting Soils
Garden Tip
TB1488
Technical Bulletin Series
Much of what can be found on the Internet on the subject of Planting Mixes and Potting Soils is Marketing - Not educational Information! Because of this; directions and information are usually confusing or misleading.

Within these websites, when we look at the company that published the information, we see opportunities to purchase. This confirms the bias within.

This isn't where to look when trying to gain understanding of the science behind the product.

If you desire future efforts to suceed, you need to understand the plant science involved.

If science is not for you, then seek advice from an individual trained in plant science. Within this article, the discussions and explanations are all science based - each recommendation can be backed with the scientific reason.



Where can accurate & unbiased info be found?

Universities study and publish information on composting and the characteristics of good compost, but they do not publish studies regarding any particular composted and then bagged product.

It is been easy to see why customers and gardeners get confused about which type of bagged product to use, and whether various brands and varieties are actually a quality value.



So, in this Ad-Free bulletin on this Ad-Free Website; I'll discuss some basic chemistry as it relates to composted products like Planting Mix and Potting Soil.
To start with; what is the diffence between
a "Mix" and a "Soil"?

Unfortuanately many manufacturers of these products create their own definition of soil and mix, so when looking at any marketing material from a manufacturer the answers are only correct with that single company. A different company will likely create their own definition of what is "Soil" and "Mix".


How awesome if there were a standard, and the technical difference between them was clear. Not knowing, creates plant failures,

A "Mix" is supposed catch the gardener's eye that this product is to be mixed before use. The "Higher Carbon" & decomposing "Mix" should be used with conventional native (mineral type) soil or other natural (low carbon) amendment. Not directly, undiluted - as in a garden container.
(explanation for why - later)

A "Soil" is supposed let the Gardener know this is a product that a plant can be directly installed with - Diluting or Mixing is NOT needed.

It's important to know which is which, because it can seriously damage or even kill your plants if installed into the wrong type.

There has been no agency established to set standards for this. The "Buyer" needs to read the small print. Does the bag say use directly for planting, or does it read "mix with native soil"?

Bagged product (Planting Mix, Potting Soil, Raised Bed, etc) is manufactured through the composting process. The various materials used in the raw compost have different and various ratios of Carbon to Nitrogen [C / N ratio]. To start the process a ratio of 25 to 1 [carbon to nitrogen] is good. Enough carbon (through the action of soil microbes.) to break down and generate heat.

As this process continues, the carbon-nitrogen ratio declines. When the product is finished composting the level of between 8/1 and 15/1 is normal and desired.

Sometimes the product when finished is packaged before the ratio becomes stable. This higher C/N ratio product is good for acidifying the soil and multiplying biology in the soil. The microbes produce carbon dioxide (an acid) when they consume carbon. These products are intended to be "mixed" before use.
When used as a MIX, the desire is to build the microbial activity, and in so doing this produce soil acids. In these cases the c/n ratio when packaged will be in the 15 to 20% range. This is active, and will be accompanied by higher inner-soil temps around 104° F. This of course varies based on the ambient temperature of the local air. Another reason why we don't see a practice of measuring these characteristics.

High microbial activity also consumes available soil nitrogen as it feeds the bacteria. This higher c/n ratio product is still composting when purchased; and could be referred to as "hot." If not mixed with native soil or a stable amendment, it can rob the new plants of needed nitrogen. The result is yellowing leaves and poor growth.

The lower c/n ratio reduces microbial activity thus making it less acidic. This more stable product can be used for direct-undiluted planting.

For use as a 100% soil (pots or most raised beds) this ratio needs to be low - around 8 to 10%. This is considered a steady or stable soil.

When performing an annual "refreshing of soil" in a raised bed or container - it's a good idea to add some [15+%], higher C/N product to boost soil biology.
It's also beneficial to add some sand [about ⅜" throughout the surface, and scratched in] during this refreshing event. This will help keep the soil at the bottom of the raised bed from becoming water retentive and poor for drainage.


So, the answer to the question;
Why is it important
to know whether you need to mix
your bagged product or not.

The Carbon/Nitrogen Ratio in the packaged product can damage the plants if too high and not mixed.

Science can be a bit confusing?
Buy from a reputable Nursery
Let them answer your product questions



Now we know why we have these options!


In the effort to improve soil microbe action (a healthy soil is a living soil), a higher c/n product (Like a planting MIX) is added to the amendment process during landscape installation.

If this product is added full-strength, too much nitrogen is often consumed by the soil microbes and yellowing of the plant leaves is the result. Stunted growth and often plant failure are the result.
So, the instruction is given to mix this product 30 to 50% with the native soil to eliminate the problem.

When installing a plant in a container or raised bed, normal practice is NOT mixing or diluting the "Potting Soil". Potting Soil should be produced with a lower C/N ratio of about 8 to 1. Normally smaller raised beds are also 100% bagged product - so Raised Bed Soil should also have this lower C/N ratio.

Why aren't C/N ratios printed on the bag?

The C/N ratio changes continually after it's packaged. So, publishing this parameter on the packaging would inevitably be misleading.

Cheap Potting Soils will have higher C/N ratios as it takes time during the composting process to bring the C/N ratio down to this 8% range. Quicker is cheaper.

One thing the consumer can do is bring a soil [or meat] thermometer to the Nursery and check the temperature in the center of the bag (not the surface) to see how hot it is. There are many things that will affect this (air temp, rain) but it will give you a clue as to whether the product is stable or active (hot).

Maybe one day there will be an agency to sample - test & monitor these bagged products to see if they are hitting the shelf close to the activity rating they should have.