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The How and Why of Biofertilizers

One common question from traditional gardeners that want to switch to organic methods is how to fertilize the garden. Most gardeners are so used to spraying something like Miracle-Gro or another chemical fertilizer that they don’t know what to do. The simplest answer is to add compost, but sometimes compost alone isn’t enough.

Even a good compost might not have all of the nutrients that a particular plant needs. A plant will grow in size and vigor as long as it has all the nutrients it needs. Carbon, nitrogen, and phosphorus are the three biggest components for plants, but there are many micronutrients as well. If one of those nutrients is lacking, growth will be diminished and the food won’t be as tasty or healthy.

The first step to making sure your plants have all the nutrition they need is to use a wide variety of materials in your compost. If you start noticing your plants are getting sick anyway, there are ways to boost your compost even further. The next step is to make biofertilizers.

What is a biofertilizer?

A plant is part of a whole system of processes. There are obvious ones like the soil, the sun, and water, but microbes play a huge role in helping plants get the nutrients they need. Biofertilizer microbes transform other materials in the soil and air into usable nutrition for a plant.

You might know that planting beans and legumes fixes nitrogen into the soil. The way that this happens is that there is a bacteria called Rhizobia that is attracted to the roots of these plants that turns atmospheric nitrogen into ammonium. The plant can then use the ammonium to grow. There are other organisms that convert phosphorous and create other plant-growth promoting compounds as well.

When a soil has a healthy amount of these bacteria then plants can unlock the nutrients in compost and thrive. Also, over time, the bacteria can die off and leave deposits of plant-available nutrients. Healthy microbe-rich soil is another indicator of soil fertility.

Compared to chemical fertilizers, biofertilizers are much cheaper to make and use. However, we have to create the conditions for a healthy biofertilizer colony. Our soils may not have the right conditions to support the health of a biofertilizer. A colony might need supplementation so it can stay alive long enough to unlock the mineral components of the soil.

Compost tea

If you already have a healthy compost pile with a wide variety of materials, that’s the first step. The quality of your biofertilizer depends entirely on the health of your compost pile. The simplest way to make a biofertilizer is to make a compost tea with the finished product of your pile. It’s quite simple to make.

Let’s assume you have a sizeable garden. Get a trash can and fill it up half way with water. Then take a pillow case and fill it up with finished compost. Compost is finished when you can’t recognize any of the original components. Finished compost will likely be at the bottom of the pile, so this is a good project to do when you’re turning your pile into a new bin.

Tie the end of the pillowcase into a knot and put it into the trash can. Wait for 24 hours and check the color of the water. It should be the color of strong coffee. If not, change around the levels of water and compost until you get there. What you’ll have left is compost tea concentrate. The compost in the pillowcase can be added back to the pile once it is done.

Before using it, add water to the concentrate until it is the color of tea. This is important to avoid overfeeding the plant. That’s all there is to it.

There are several different ways that you can use your newly made compost tea. The simplest way is to slowly pour a small cupful around the roots of a plant. You can also fill a bucket with a small hole in the side and leave it near a plant. Move the bucket every so often to fertilize all the plants.

The second method of applying compost tea is called foliar feeding. Plants can absorb nutrients through the leaves. Filter your compost tea well, then add it to a sprayer. Coat the plant thoroughly in tea, including the undersides of the leaves. This will not only feed the plant, but it can also provide an inoculant against plant diseases. It’s also a very fast way to fix up a plant that is in distress, like when your tomatoes leaves go yellow. It’s best to do this in the early morning or late evening so that the sun won’t kill the microbes too soon.

There are things you can do to boost the power of a standard compost tea. Adding a small amount of organic soap to the mixture will make the tea stick better to leaves. You can also dissolve certain additives into your tea like blood meal or kelp for additional nutrition. Just make sure that your additives don’t clog up your sprayer.

Fermented compost tea

What if we really wanted to maximize the microbial growth of our soil? We can take things a step further through the power of fermentation. A finished compost tea has all the nutrients that biofertilizer microbes need to rapidly grow in numbers. By letting the compost tea ferment and using it before all the organisms die off, you can ensure a high microbe population in your soil.

Take your compost tea concentrate and pour some into a bucket. Cover it with a cloth and put it in a shady spot for 7-10 days. When you come back, you’ll have some nasty scum on top.

Carefully take it off and put it back onto the compost pile. There will also be solids at the bottom of the bucket. Take those out too and put them into the pile. The leftover fermented liquid can be diluted as normal and will be much more potent. It is best to use it in the same day you harvest it.

After you use your tea, it’s important to thoroughly clean your fermentation vessel. Otherwise, it’s possible that an unwanted microbe could move in and create a biofilm that can interfere with future batches. Use a stiff bristle brush, hydrogen peroxide, and water to clean everything before making another batch.

Just like making homebrewed alcohol, there are simple ways that make an adequate product and a lot of tricks to make a very fine product. The primary trick is proper aeration.


Soil contains oxygen. Therefore, the microbes that live in the soil also need oxygen to live. When a compost tea bucket is left out to ferment, the microbes begin to multiply and consume the oxygen. If the bucket is left out too long then the aerobic microbes will begin to die off and anaerobic microbes will take over. It will begin to stink really badly. If this goes on too long then the whole batch will need to go back into the pile.

Adding oxygen to the bucket will prevent this from happening. There are two main ways to do it. The first is to give the bucket a strong stirring every day. The second and more efficient way is to install an aquarium bubbler to your bucket. If you use a pump you can make a usable tea concentrate in just 2-4 hours. However, if you’re going to foliar feed, let it bubble for 24 hours and then use it within four hours. Oxygenating the water like this makes the microbes breed much faster than they would in nature, so it must be used quickly before it goes anaerobic.

If you have a very large garden and want a project, you can make a compost trough. Cut a length of PVC pile down the center to make a trough and drill holes into the bottom. Line the trough with burlap and fill it with finished compost. Suspend this over a bin.

Next, install a sump pump at the bottom of the bin and put the output so that it flows over the trough. Fill the bin with water and turn on the pump. The pump will trickle water onto the trough, which will filter through the compost and back into the bin. This will thoroughly aerate a large batch of compost, and create a finished tea in about seven days. Throw the used compost back into the pile and replace it with fresh finished compost for each run.

Getting anaerobics to help

Most of the time you don’t want anaerobics in your brews. But if you want to maximize plant nutrition and you’re willing to take a few months to make your brew, there is a way to have them help out. The Permaculture Research Institute and have a really interesting way to make an extraordinarily potent biofertilizer that can actually remineralize your soil while feeding your plants.

The basic principle is to create an anaerobic environment full of clean water, microbial food, lots of mineral sources, and a source of microbes. This is best done by using a large rain barrel with an airtight lid with an airlock in the lid. The airlock is important so gases can escape!

Microbial food is quite simply complex sugars. A mixture of molasses, milk, and water is perfect. Mineral sources can include soluble silica, soluble phosphorus, basalt, rock dust, and yeast. The best microbial sources for anaerobic fermentation come from fresh manure or fresh rumen (cow stomachs with the fluid inside).

All of these are placed into the barrel and locked tight. Attach the airlock to the top of the barrel and let it sit for 2-3 months. Open it up and smell it. If it smells putrid like something has died, then the batch went bad and must be thrown out. Otherwise, it’s ready for the next step.

Use a strong air pump to push oxygen into the fermented anaerobic tea. This will kill off the anaerobic bacteria and create conditions suitable for aerobic bacteria. With this mineral-rich anaerobic base, you can mix it with fermented compost tea to give it a kick far greater than fermented tea alone.

Example ingredient list:

  • Microbe Food
  • 10lt. Water
  • 2lt. milk or whey
  • 2lt. Molasses
  • 100lt. of water
  • 40 – 50lt. fresh calf or cow manure
  • 3kg Soluble silica and phosphorus (powdered)
  • 2kg Volcanic rock dust or basalt
  • 1/2kg yeast

For more information about this recipe, find out more about making a 200 liter batch here. Be sure to watch the video for full instructions on how to build an airlock.

Things to note

When you use a highly concentrated fertilizer, you should know what the symptoms of overfeeding a plant are so you don’t accidentally kill your plants. The most obvious sign of overfertilization is when there’s an overabundance of foliage and few fruits and flowers. This is a sign of too much nitrogen. Too much nitrogen could also cause brown spots on leaves, or misshapen leaves. Overabundance of other minerals could cause spindly or weak plants, depending on the species. Consider these symptoms the plant form of obesity.

The types of concentrated fertilizers described in this article could overfeed a plant. It’s less likely than when a chemical fertilizer is used, but it is still possible. If you have been feeding your plants with healthy compost and using teas and they still look unhealthy, get a soil test. That will tell you if there is any actual deficiencies. While you’re waiting for the results, stop fertilizing the plants and see if they get better. By the time the results come back (usually about a week), you’ll know whether you gave them too much food.

Everyone’s soil and compost are different. That’s part of the challenge of gardening, and part of the excitement. If you take care of your soil, it will get easier year after year. Use good compost every year, try compost teas, and consider remineralization if your soil is lacking nutrients. You can fix your soil to growth nutritious food for you and your community. Try a tea this growing season.

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