Compost Extract vs Actively Aerated Compost Tea (AACT)

You’ve heard great things about AACT, Actively Aerated Compost Tea, and have also heard of compost extracts. But most growers still are unclear on the differences. Which ones are better? How should they be used? What are the drawbacks?

If you are involved in biologic farming in any way, you have likely heard of AACTs and almost certainly want to use your biologically active compost as efficiently as possible. Compost teas and extracts is a method of increasing the biodiversity in your soil that has been proven to be more effective than just topdressing. Think of biologically active, or Robust Compost, as a complex community of soil microbes in a solid transport medium. Topdressing, as the name implies, is putting this compost on top of the soil – usually at 3/8″ to 1″ depending upon how poor the quality of the soil is to start, and how dramatic your need for improvement is.

In Efficiency and Optimization in Agriculture Today the benefits of compost teas and extracts were discussed, as well as and the necessary procedure of aerobic brewing. We wanted to dive a bit deeper, and discuss the difference between Actively Aerated Compost Teas (AACT) and Compost Extracts, and the methods of constructing these soil rehabilitators. 

What makes the Compost Extract unique is that what is delivered to the soil often contains a more diverse spectrum of microbial life than Actively Aerated Compost Tea. By allowing the reproduction of organisms to the brewing process, the higher trophic levels also reproduce and eat the life that is lower in the food chain.

What’s the difference?

Time: The first differentiator between AACT/Teas and Extracts is time. Compost Teas require a brewing time of 24 to 36 hours. In contrast, Compost Extracts take 60 to 90 minutes per batch for the for microbial extraction.

Microbial Food: In Actively Aerated Compost Teas, microbial food is added to the water. These foods may include kelp, molasses, bat guano, humic acid and others. The microbes begin to reproduce, consuming oxygen, food and each other. There is more biology and the balance changes over time. Some growers use warm water to speed the process up further.

In Compost Extracts, no additional food is added. The water is seen as a delivery method and the bag with compost is shaken and bubbles are used to push the biology out into the water. This can be done with active biology, spores or dormant biology. The biology is less active than in an AACT and, thus, does not change rapidly from its original ratios.

Oxygen: In AACT, the microbes are active and reproducing. In addition to food, they consume oxygen at a much faster rate than in the Compost Extracts. Without the bubblers, the solution will go anaerobic in as little as 3 hours. As the grower/brewer, you will often smell that the brew has gone bad, but it may have gone bad well before the stink started. Compost Extracts on the other hand, can last days if the water was well oxygenated by the bubbler.

The drastic difference in brew time and shelf life will result in different microbial life and different amounts of microbial life. Because of the longer brew time and the addition of microbial food, your AACT can result in a more specific group of microbes, and requires on site and real time microscopy.

Price: Prices vary by region and by crop, but nationally, the price per gallon for AACT , made on site and confirmed by microscope, is $24 to $28 per gallon. Compost Extract is more forgiving in the brewing, takes less time and has a longer shelf-life. Compost Extracts usually sell in the range of $5.00 to $8.00 per gallon.

How should they be used?

Both Compost Extracts and AACT can be delivered to the soil via irrigation systems. (Make sure the nozzle size is sufficient to allow the larger fungal hyphae and beneficial nematodes to get through the system without being crushed. ) They can also be used as foliar sprays. At Cline Cellars Vineyard, the first year they carried one cup of AACT to each vine, and poured it into the base of the plant. Even something as simple as that worked to get the biology to the roots.

While both extracts and AACT help bring soil back to life, and the benefits are well documented, Dr. Elaine Ingham believes Compost Teas are more beneficial for foliar applications. Metabolically active bacteria produce a sticky film (which makes brewers hard to clean), but the same film is viscous and causes bacteria and other microbes to stick to plant surfaces. Due to the stickiness and higher microbe to water ratio, AATCs used as foliar sprays are able to coat the plant with beneficial microbes more efficiently, which is helpful for some conditions or pathogen suppression.

What are the drawbacks?

Unlike other products, compost extracts can’t be overdone. The same is true of AACT, they can’t be over done IF THE BIOLOGY IS CORRECT. ONE MAJOR DRAWBACK, AACTs can go bad if the solution sits too long and/or if the microbes lack sufficient oxygen. We strongly recommend users of AACT develop their soil biology microscope skills. If you are considering getting a microscope, please check out Dr. Ingham’s thoughts on What to Look for in a Microscope for Soil Biology.

Which is Better?

Because of the longer brewing time, some people believe that Actively Aerated Compost Teas are the superior amendment for your soil. Others, because of the shorter brewing time, feel equally strongly about the benefits of Compost Extracts. There are studies showing that extracts are just as beneficial as teas in their ability to improve soil health and both result in healthier, more vibrant plants.

If you are more concerned with shorter brew times, more diverse microbes and longer shelf life, Compost Extracts should be your preference. But if you are more concerned with amounts of microbial life, foliar applications and have the extra time to go towards brewing, Actively Aerated Compost Tea will suit your needs more effectively. 

Robust Compost (AKA biologically active compost, Soil Food Web Compost) with broad biodiversity across 7 types of life is difficult to find and expensive. We hope this information provides a useful roadmap for people deciding which way to utilize their compost.

Compost Extract, where the microbes are suspended in water, without the stimulus to reproduce provided by kelp, molasses and other foods, has the same proportions of all microbes as were found in your Robust Compost. (Robust Compost, Soil Food Web Compost, Biologically Active Compost are different words to describe the same complex microbial community, with broad biodiversity across 7 types of life.) By allowing the reproduction of soil organisms to be triggered by plant extrudates, or signals to the soil microbe biome, as to the plants’ needs, you increase the odds of growth in the exact group of life forms which do best in the mineral, weather and plant environment.

On the other hand, you deliver more biology … and faster acting microbial life using Actively Aerated Compost Tea. The microbes are already in the reproduction mode, liquid food is delivered along with the water, so they literally “hit the ground running.”

Thus, which is better varies by your growing needs. This is where your experience, advice from a Soil Maven or a Soil Food Web consultant becomes valuable.

What is Robust Compost?

Robust Compost feeds plants the way nature intended with the complete soil microbe biome.  Sometimes called Living Soil, Soil Food Web compost or biologically-active compost, the full spectrum soil ecosystem includes bacteria, fungi, archaea, beneficial nematodes, amoeba, flagellates and micro-arthropods. Often it has other types of life, including animals (earthworms) and it may contain diatoms and other algae, phages and other types of protozoa.

Healthy soil contains in excess of 10,000 species of life, Robust Compost has in excess of 1,000 species and includes at least 7 types of life. SymSoil® RC is Robust Compost, with over 2,000 species and 7% biochar, which acts as a soil conditioner as well as a tool to revive or reseed the soil microbe biome.

About SymSoil® Inc.

SymSoil is an evidence-based, soil health company with products and services to improve the profitability of all growers, based on a deep understanding of soil biology.  The company has soil amendments for agriculture that restore the full-spectrum of microbes that provide the right food to the plant roots, improving plant health, and making food more nutrient dense and flavorful, the way nature intended. SymSoil has products and services for growers using regenerative agriculture methodologies which improve profitability. SymSoil was named one of 2019’s AgTech Companies to Watch. Accredited Investors can learn more about SymSoil as an impact investment here.

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