By: Rubén Parrilla, Education & Technical Assistance Director

In this edition of Be a Better Grower, we’re talking about compost tea & why you should add it to your growing plan this year!

It’s no secret that here at NOFA/Mass, we’re big fans of all things soil health. Earlier this year, NOFA/Mass Education & Soil Technical Services Director, Rubén Parilla, presented a webinar on the huge benefits that supporting your soil biology with compost tea brings.

This edition of the newsletter summarizes the main takeaways & shares other resources for growers of all sizes.

What is compost tea?

Regardless of the process or ingredients you use in creating your compost, it largely functions as a biological inoculum for your soil (in addition to of course providing organic material and fertility for your plants).

Compost tea is an extraction and expansion of the biological and chemical properties of your compost, turned into a liquid (the “tea”) that can then be sprayed onto your plants to support robust, healthy growth, inhibit pathogens, and increase photosynthesis.

This article goes over the benefits of actively aerated compost tea – that is, compost tea that was “bubbled” throughout the brewing process to supply the microorganisms in the tea with more oxygen. This type of compost tea is substantially more biologically active than non-aerated compost tea (the “old-fashioned” kind, where you brew the tea passively in a bucket without moving or aerated water).

Benefits of Compost Tea

You may be asking, “I already use compost. Why should I use compost tea?”

The microbial life that grows in compost tea can be a huge benefit to your plants, supporting growth and reducing pest competition. Here’s an overview of four key benefits of using compost tea:

1. Grow bigger plants

In his study, “Soil Microbes: Their Powerful Influence in Agroecosystems,” Dr. David C. Johnson shows that minerals and fertilizers alone are not a powerful predictor of plant growth.

Instead, the fungal to bacterial ratio of the soil is a useful proxy for predicting how much plants will grow over time, with the most growth occurring in soils with a relatively high level of fungi.

Brewing compost tea provides a place for those useful fungi to grow and multiply, so that you can add a powerful biological inoculum to your soil, increasing the soil’s fungal:bacterial ratio and thereby helping them grow even larger & stronger.
Bar chart comparing plant growth to fungal bacteria ratio.

2. Protect against disease

Plants that have been sprayed with actively aerated compost tea have been shown to be less affected by some diseases than other plants.

For example, a study by Dr. Elaine Ingram of the Soil Food Web school showed that applying compost tea to at least 70% coverage on a plant effectively protects it against gray mold. The compost tea contains microbes that outcompete and inhibit the disease spores.

This kind of disease prevention can be especially important in the Northeast during seasons with unusually high rainfall!

Grey mold under a microscope

3. Draw down carbon & increase the organic matter in your soil

Compost tea applications help plants achieve photosynthesis at their maximum genetic potential, which in turn pulls carbon down into the soil to feed the microbiome there.

This increases the organic matter in your soil, leading to healthier, happier plants.

4. Increase in nutrition available to plants

In Dr. Johnson’s study of biologically managed systems which includes applying compost tea, he found substantial increases in nutrient levels across the board.

Testing over the course of 20 months, he found notable increases in Manganese, Iron, Nitrogen, Magnesium, Phosphorous, Calcium, Zinc, Copper, and Potassium – all of the “foods” that your plants love.

How do I make compost tea?

In essence, making compost tea is a brewing process where you combine finished compost, additional “food” for the microorganisms in that compost, and water. You then aerate the mixture constantly – using an air pump – to give the microorganisms air to “breathe.” This creates an environment where microbes can thrive, grow, and multiply!

Here are some key considerations for making compost tea:

Water:

The water is the largest component of your tea, and the most important ingredient.

Chlorination

Most tap water is chlorinated, to kill off the microorganisms that can make us sick if we drink them. But we definitely don’t want to be killing any microorganisms in our compost tea. If you have access to non-chlorinated water, use that.

If not, dispense the amount of water you’re planning to use, then either aerate it for 20 minutes or leave it uncovered for about 24 hours to off-gas. This should eliminate most of the chlorine in the water source.

If your water source has chloramine, you should use humic acid (enough to change the color of the water) or citric acid to complex out the chloramine. Off-gassing alone won’t remove the chloramine.

If possible, try to keep the water temperature of your tea consistent & warm throughout the brewing process. Water that is too cold can slow down microbial growth, as can overheated water.

Compost

The compost you use in making the tea is also, of course, very important! Use high quality, microbially active compost when brewing compost tea. More microbial diversity in your compost means more microbial diversity in your compost tea. After all, you can’t multiply what doesn’t exist in the first place!

Sampling your compost with tools like a microscope or a microBIOMETER can help you understand its bacterial and fungal content. See the upcoming events list below for NOFA/Mass events that will help you learn how to use these tools to better understand your soil.

Additional Ingredients

In addition to the core ingredients of water, compost, and time, you can add extra ingredients to your tea to support the growth of specific microbes, and provide important growth factors like vitamins, amino acids, and minerals.

For example, simple sugars provide good “food” for bacteria, whereas fungi prefer more complex carbohydrates. You can also add “spices” that actually inhibit the growth of specific microbes.

Here’s a list of ingredients to experiment with in your compost tea:

  • Simple Sugars

  • Kelp

  • Fish Hydrolosate

  • Fruit Pulp

  • Humic/Fulvic Acid

  • Protein Meals, Oatmeal, Barley meal, Feather meal

  • Spices: Garlic, Orange Oil, Citrus, Cinnamon, Lemon

  • Enzymes

  • Yeast

Experimenting with ingredients from this list, and other ingredients, can help you identify what is most helpful for your soil.

Brewing Equipment

Your compost tea brewing equipment can be as sophisticated or as simple as you’d prefer.

It’s important to aerate the tea thoroughly, so make sure to use a pump that is powerful enough for your brewer. The larger the brewer, the more water you’ll have, and the more water you have, the more power you’ll need to aerate the tea.

There are lots of great resources online with examples and inspiration for DIY brewers:

Whatever design you’re using, make sure you’ll be able to clean the components easily. After all, you’re cultivating living microbes that will leave residue and films behind.

Understanding the Biological Content of Your Compost Tea

Once you’ve set up your brewer, made your compost tea mix, and started aerating the tea, you’ll need to wait for 1-3 days before using the tea.

There’s really only one way to know for sure that your tea is “done”: analyze a sample under a microscope.

Here’s an example of the same compost tea under the microscope, as seen at 40 hours and 72 hours:

Even an untrained eye can see that the second image, from 72 hours, has significantly more microbial content than the first.

In fact, sampling and analyzing your compost, compost tea, and soils can be an incredibly useful practice to incorporate into your growing techniques. Microscopic analysis can help you identify microbial deficiencies, determine the bacterial and fungal content of your composts, teas, and soils, and give you a better understanding of your soil biology overall.

NOFA/Mass has a series of microscopy workshops coming up, read more and register for these events on our website:

Other Resources

To dig deeper into compost tea, we recommend checking out the following resources: