Blog Microbiology

What Can We Learn About Microbiology From a Bag of Fertilizer

On this post, I will examine a common bag of Starter Fertilizer and explore the many opportunities to learn microbiology by examining its contents.

Last Updated on June 14, 2020

My wife recently purchased some starter fertilizer for a couple of indoor plants that needed re-potting. When she showed me the bag, my mind immediately realized that there was much about microbiology that could be learned by observing and researching its contents. Right off the bat, I spotted a few items that caught my attention:

TrueBiotic® Inside – Beneficial soil microbes plus Mycorrhizae

It’s clear Dr. Earth is using this mixture as a selling point to his fertilizer, but what if you don’t know what soil microbes or mycorrhizae are and why you might need them? Let’s see if we can dig into this a little more

What’s a soil microbe?

A soil microbe is a really small microscopic organism, such as bacteria, algae, fungi, amoebas and slime molds, that live and reproduce in soil. Microorganisms make up most of the total living mass on Earth.

What’s a mycorrhizae?

Mycorrhizae describes fungi that have a symbiotic relationship with a plants root system known as the rhizosphere.

Also on the front of the bag I see:

2-4-2 Feeds 55 square feet or 80 one-gallon transplants.

What do the hyphenated #-#-# numbers mean on fertilizer bags?

When I moved into my new (100 year old) house last year, I had to quickly get up to speed on how to care for all of the mature trees and plants on our property. And when I would go to a lawn center and see various kinds of fertilizer and nutrient supplements, I noticed a lot of the bags had this #-#-# labelled on them. So what is it?

The #-#-# fertilizer label represents something called the NPK or N-P-K label. What does NPK stand for? It stands for the chemical elements Nitrogen, Phosphorus, Potassium.

Different plants require different proportions of these essential elements to grow and live. On this bag, the proportions 2-4-2 indicate that the 1.8 kg bag contains 2% Nitrogen, 4% Phosphorus and 2% Potassium.

Naturally, the next question I had when discovering this was:
What proportions do I need for my yard?

And I won’t get into too much detail on that here, but so far, my approach has been to search for the species of grass, trees, flowers and other plants and looking for published info on what the various species need.

It would be great if I could find an authoritative source that defined the necessary proportions for species, time of year to fertilize and incorporated the climate zone. If you know about this, please share.

Moving on, let’s take a look at what I was most excited to see and that is the back of the bag.

Dr. Earth Root Zone Starter Fertilizer – Back
Dr. Earth Root Zone Starter Fertilizer – Bag (Close)

Hey, look at that. Under Guaranteed Analysis, we see Total Nitrogen (K) 2%, Available Phosphate (P₂0₃) 4% and Soluble Potash (K₂O) 2%. That confirms the 2-4-2 on the front of the bag, but it does prompt some other questions that I’ll have to dig into later:

  • Why state how much of the nitrogen is water insoluble? Presumably that has something to do with the availability of the Nitrogen to the plant.
  • What does the “Available” in Available Phosphate mean?
  • Why call the Potassium proportion “Soluble Potash”?

On the right side of the bag, we see a listing of the “Non-Plant Food Ingredients”. That must mean that the following ingredients are beneficial to new root systems, but that the benefits are made available through a different mechanism than just food that the plans absorb directly. These ingredient quantities are measured in Colony Forming Units per gram.

What is a Colony Forming Unit?

A CFU is a measurement used to estimate the number of viable microbes in a sample. Presumably, the microbiologists at Dr. Earth have cultured these bacteria on a smaller scale (like a petri dish), counted the viable cells (either by hand, aid of a microscope or software) and extrapolated the counts to the total volume of substrate which is a 1 lb pound bag.

Let’s take a cursory look at the bacteria in the fertilizer.

Bacillus amyloliquefaciensFights harmful root pathogens.
Bacillus_licheniformisFacilitates nutrient recycling.
Bacillus megateriumHelps prevent plant disease.
Bacillus pumilusProvides hardiness to soil conditions.
Bacillus subtilusHardy plant nutrient facilitator.
Bacterial Amendments in the Starter Fertilizer

Most soils already contain an abundance of microbes but not all and certainly not in the proportions that can help new plantings get established in a new environment. A fertilizer such as this one can supplement existing soils and provide beneficial bacteria to the plant where it may not otherwise be available or in short supply.

What is the difference between endomycorrhizae and ectomycorrhizae?

We know that mycorrhizae fungi interact with a plants root system (rhizosphere). But how?

When soil fungi grows, it creates these tiny branches resembling the veins in a leaf called hyphae. Some of these hyphae branches will penetrate the cells of the root and some do not. The mycorrhizae fungi that penetrate the individual cell walls are called endomycorrhizae while the ones that do not are called ectomycorrhizae.

As you might imagine, the two different types have different underlying chemical mechanisms for exchanging nutrients with plants.

What does propagules per gram mean?

A fungi propagule is a spore, hyphae and/or root fragment that can reproduce more of the same organism. In a similar way that biologist estimate the quantity of bacteria by counting colonies of bacteria in a sample using a microscope, propagules are also counted from a smaller sample and then extrapolated to the total bag contents.

SpeciesMycorrhiza Type
Glomus aggregatumEndomycorrhizae
Glomus clarumEndomycorrhizae
Glomus deserticolaEndomycorrhizae
Glomus etunicatumEndomycorrhizae
Glomus intraradices (Rhizophagus irregularis)Endomycorrhizae
Glomus mosseaeEndomycorrhizae
Glomus monosporumEndomycorrhizae
Paraglomus brasilianumEndomycorrhizae
Glomus margaritaEndomycorrhizae
Laccaria laccataEctomycorrhizae
Laccaria bicolorEctomycorrhizae
Pisolithus tinctoriusEctomycorrhizae
Rhizopogon villosullusEctomycorrhizae
Rhizopogon luteolusEctomycorrhizae
Rhizopogon amylopogonEctomycorrhizae
Rhizopogon fulvigleba*Ectomycorrhizae
Scleroderma cepaEctomycorrhizae
Scleroderma citrinumEctomycorrhizae
Fungi in the Starter Fertilizer

* No information on this species could be found. I have contacted Dr. Earth for more details and will update this post if I receive any updates.

Lastly, we see that the bag states Contains 12% Humic Acids (derived from Leonardite). What’s Humic Acid? What’s Leonardite? Why 12%?

Humic acid is an organic (carbon-containing) acid. An acid is an ion or molecule that can either give up a proton (Hydrogen ion H+) or form a covalent bond with an electron pair. Humic compounds are apparently one of the earliest studied natural materials in chemistry and there are still many aspects of how humics are formed that we still don’t know.

The role of humic acid as a soil amendment and how this organic material benefits crops is a wide-ranging field with ongoing research. The gist of it is that it’s helpful in the mineralization of poor soils due to the fact that it generates useful byproduct of microbial decomposition such as essential amino acids. I’ve also read that it aids roots in receiving water and nutrients due to it’s negative charge, but how this is done I will save for another time.

Finally, I accidentally placed the open bag of fertiziler in a place where ran could penetrate it and when I went to take these pics, I looked inside and noticed something.

Inside Bag of Starter Fertilizer with Myco Growth
Visible Growth Inside of Bag

See the grey / white fuzzy stuff on the left side? 🙂 Looks like the rain spurred some growth of one or more of the mycorrhizae. Or maybe it’s a bacterial colony? I’m not sure – but it’s cottony, fuzzy and definitely a growth of some sort.

Additional Research Opportunities in How Fertilizer Works

This exploration into fertilizer touches on many aspects of science, in particular microbiology, chemistry, mycology and more. As usual, learning something new usually prompts many more questions.

Here are some of my remaining questions that I hope to come back and fill in as my knowledge of DIYBio advances:

  • What is the best resource for determining fertilizer ratios?
  • How are the proportions of bacteria and fungi determined for this product?
  • What is the bulk substrate the non-plant food ingredients live on?
  • Why use just these bacteria and fungi and no more or less?
  • How is the humic acid proportion determined?
  • How does one determine the amount of humic acid needed?

Future Experiment Ideas

Here are some experiments or labs I’d like to run in the future using soil fertilizers:

  • Learn how to isolate individual strains of bacteria and fungi from a fertilizer sample.
  • Analyze and photograph fertilizer under a microscope.
  • Calculate CFU using various plating and counting methods, including manual (pen+click), software and automated.
  • Calculate Propagules Per Gram for mycorrhizae using a microscope.
  • Identify the species of the pictured growth.
  • Test a plant growth with and without the fertilizer and analyze the results.
  • Extract, amplify and sequence DNA of one or more fungi or bacteria.

Closing Remarks

Who would have guessed that a bag of starter fertilizer could make so many connections to science! I’m fascinated by how much we can learn by simply looking at the world around us with a curious eye and a quench for knowledge.

Nature is incredible and it seems to me that we keep discovering that it has already provided all of the tools and solutions to our problems. Starter Fertilizer embodies this in that its very contents are comprised of what nature already provides to nourish life, only now we can wield it to our benefit.

I’m excited to embark on this journey and will definitely be revisiting the science of fertilizer in the future.

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