Best Organic Fertilizers: Ratings, Reviews, and Top Picks

We all know plants need food. But the nuances of organic fertilizers can get pretty confusing. While most garden soils contain essential nutrients for plant growth, organic fertilizers help boost the availability of certain nutrients to your plants. Sort of like a multivitamin or probiotic, organic fertilizers fuel microbial cycles in the soil to make it easier for your plants to “digest” their food.   

There are so many fertilizer products on the market, and most of them are definitely not organic. If it comes in a neon blue or yellow ball, you can just assume it is synthetic fertilizer that you don’t want in your garden. Synthetic fertilizers are produced in chemical plants from byproducts of petroleum. They are harmful to waterways, soil microbes, and potentially human health.

Ingredients From Nature

The greatest organic fertilizers are simple ingredients that come from nature. They are sourced directly from the Earth and don’t require industrial factories to manufacture them with chemicals.

Safe to Use

This also means that great organic fertilizers are safe for you and your family. Nobody wants toxic chemicals in their garden and a lot of people don’t realize how harmful synthetic fertilizers can be.

The nitric acid and ammonia found in synthetic chemical fertilizers can harm your lungs when inhaled, cause twitching and nausea, burn your skin, hurt children, and increase the risk of certain diseases like cancer or reproductive harm. Just look at the fine print label for any of these products and you will see an alarming amount of warnings, protective gear, and scary toxins.

Boost Plant Growth by Feeding the Soil

Organic fertilizer should boost nutrient absorption for your plants. They do this by “feeding” the microorganisms in the soil, which act like a plant’s digestive system.

As the microbes (bacteria, fungi, protozoa, nematodes, and others) eat and process organic fertilizers, they transform them into compounds that vegetables and other garden plants can uptake.

This is why you will often hear biological and organic growers saying “feed the soil, not the plant.” Organic fertilizers must be broken down and decomposed by our microbial allies in the soil.

Without getting into any complicated chemistry, it will suffice to say that starting with healthy soil is crucial to your garden regardless of what organic fertilizer you choose.

Organic vs. Synthetic Fertilizers

The differences between organic and synthetic fertilizers are pretty drastic. Organic fertilizers come from nature, whereas synthetics come from factories. There is evidence that synthetic fertilizer kills soil microbes and may even harm the probiotic microbes in our guts that help us with our digestion (remember, everything is connected!).

Synthetic Fertilizers

The confusing part is that synthetic fertilizers are technically “instant.” Because of the manufacturing process, they are available for plants to uptake right away rather than going through the microbial processes.

Synthetic fertilizers are sort of like an “IV drip” of nutrients directly into the bloodstream. They bypass the plant’s digestive system (the soil microbes) and go instantly up in the roots. While this may sound like a good thing, it is not the way nature intended for plants to be fed.

Synthetic fertilizers may lead to a quick short-term increase in growth but can have many long-term determinants on the health of your soil, your garden ecosystem, local waterways, and human health.

Organic Fertilizers

While it may not be completely “instant,” organic fertilizer provides just as much “fuel” for plant growth without the risks. Depending on the microbial activity in your soil, some organic fertilizers take a few days or weeks to fully mineralize and become available to plants. This is why they are called “slow-release” because they feed plants over their whole life cycle rather than a quick jolt.

In a society obsessed with convenience and instant gratification, we could all learn from the patience of Earth’s natural ways of processing organic fertilizer. That being said, there are a few “faster acting” organic fertilizers we’ll discuss below.

What to Look For: OMRI Listed Seal

To decipher between synthetic and organic fertilizers without digging into every ingredient, you can use the OMRI seal. When you see the OMRI seal, you know that a given fertilizer is safe for organic gardening.

The Organic Materials Review Institute (OMRI) certifies agricultural inputs as suitable for use on certified organic farms. Although we gardeners aren’t certifying our garden through the USDA National Organic Program, we can still use the label to reliably sort through thousands of fertilizer options. You can also search the OMRI database for specific brands and products.

Best Types of Organic Fertilizer

There are several different types of organic fertilizer you can choose from, as we’ve already discussed. Some of our favorites include an “all purpose” option for ease of use, worm castings, liquid fish or seaweed fertilizers, rock dust, kelp meal, and chicken manure. Let’s take a deeper look at each of these organic fertilizer types, so you can decide which is best for your gardening application.

All-Purpose Organic Fertilizer

If you’re looking for the simplest plant food for your garden, Down to Earth Organic All-Purpose Fertilizer Mix is the way to go. This gentle non-burning fertilizer is great for vegetables, herbs, flowers, and containers. It gives a slow-release steady supply of nutrients to plants while also feeding microbes and enhancing soil fertility over time.

It includes the highest quality OMRI-approved ingredients at a ratio of 4-6-2. I especially love how easy it is to use Down to Earth all-purpose fertilizer at the time of planting. Simply mix the specified amount of powder (on the back of the box) in the soil holes you are transplanting into. You’ll end up with the happiest garden around!

Worm Castings

Worm castings are sometimes called vericompost or vermicast. It’s basically earthworm poop, but it isn’t your average poop. The worms used for this type of composting are actually not earthworms; they’re called “Red Wigglers.”

These worms have specialized digestive systems that microbially enrich the food they eat (plant matter, manure, and combinations of other organic feedstocks). What’s coming out the back end of those worms is pure gold for your garden.

Studies have shown that high-quality vermicompost causes seeds to germinate faster, increases their growth, increases root biomass, improves a plant’s stress tolerance, and increases yields! Plants grown with worm castings have more leaves, more flowers, and more fruit. Plus, worm castings are especially great at improving nitrate and calcium levels for your garden. Casings improve soil structure and biological activity over time.

The N-P-K of worm castings is typically 1-0-0, but don’t be misled by this ratio! Worm castings increase nutrient availability in your soil and lead to more plant nutrient mineralization over time. You can buy worm castings online or from a local worm farm. Add castings at the time of planting or throughout the season to enrich your garden soil.

Liquid Fish

Fish fertilizer is a byproduct of the fishing industry that basically churns up all the leftover fish parts that humans don’t want to eat (eyeballs, bones, guts, gurry, skin, etc.) This slurry makes for an incredible fertilizer because it has naturally balanced N-P-K ratios and loads of minerals from the ocean.

Soil microbes go crazy for liquid fish and make it available to your plants quite quickly. That’s why fish fertilizer is one of those more quickly available organic fertilizers we discussed above. It often is also mixed with seaweed fertilizers that boost micronutrient availability to your plants. I like to use liquid fish right after planting to “water in” my transplants (assuming I haven’t already applied any other organic fertilizers).

My favorite liquid fish fertilizer is Neptune’s Harvest Fish Fertilizer. This multi-generational Italian-American company is based on the coast of Massachusetts where they are directly connected to sustainable fisheries. All of their fertilizers are OMRI listed for organic production.

Liquid Seaweed

Neptune’s Harvest also offers an amazing liquid seaweed fertilizer that loads your garden with trace minerals straight from the north Atlantic ocean. Liquid seaweed is one of the world’s richest sources of plant micronutrients, essential amino acids, alginic acid, minerals, and organic compounds that feed microbes and plants for optimal growth and resilience.

These products are quite concentrated, so it is important to dilute and follow the instructions for application rates so you avoid “burning” your plants with too much fertilizer. Liquid seaweed boosts root growth and plant vigor and can be applied at any stage of growth.

Rock Dust

It may be surprising, but most minerals in our bodies and in plants actually come from rocks. Microorganisms break down rocks very slowly over time into absorbable minerals. But minerals can become depleted in your garden depending on the bedrock materials. This is especially the case in raised beds where bedrock minerals may be unavailable.

Rock dust is a trace mineral organic fertilizer that replenishes these micronutrients and feeds beneficial microbes. The three main types are glacial rock dust, basalt rock dust, and Azomite. Each of these products has different benefits described on their packages, however, they all do the same job of adding micronutrients. Rock dust is typically sprinkled or “top-dressed” around your garden before planting. It can also be added again mid-season.  

Kelp Meal

Kelp meal is another great trace mineral fertilizer that nurtures your soil with potassium, amino acids, and natural plant hormones. Down to Earth offers a great kelp meal fertilizer mix with an N-P-K ratio of 1-0.1-2. It is made from a seaweed called Ascophyllum nodosum in the north Atlantic ocean.

This kelp comes in powder form so that you don’t have to worry about any stinky liquids. It helps with transplant shock and stimulates root development, so it is great to apply at the time of planting or you can add it into your seed starting mix.

Composted Chicken Manure

Humans have been fertilizing their fields and gardens with manure for thousands of years. However, it is safest (and less smelly) to avoid fresh manure and only use composted manure instead.

Composting heats, aerates, and decomposes manure into a rich soil amendment that won’t “burn” your plants nor will it stink up your yard. It also improves the organic matter of your garden soil and helps grow the best tomatoes.

Composted chicken manure is something you will have to find locally or purchase in pelleted form from an organic garden supply store. Sup-R Green Organic Chicken Manure is a great option available online. 

Depending on the materials used in the compost, composted chicken manure typically has around a 1-1-0.5 ratio of N-P-K and pelleted chicken manure will be more like 4-2-1. Pellets are easier to deal with but might be more expensive. Be sure to check the bag for the OMRI seal.

How to Read a Fertilizer Label

When choosing your fertilizer, you need to know what all those numbers and labels mean. To understand the ratios on the front of fertilizer products, you can make a comparison to human nutrition. People need mostly macronutrients (protein, carbs, fats) to survive, but also varying levels of micronutrients (vitamins and minerals). Plants are the same way.

On most fertilizers, you’ll find three numbers that represent a ratio of plant nutrients in the bag. They are often displayed as N-P-K or 28-14-1. NPK stands for Nitrogen, Phosphorus, and Potassium.

These are the major plant nutrients that crops need to grow, however, there are 14 other essential nutrients also required for growth. Even though they are needed in smaller quantities, a deficiency in manganese or zinc can harm a plant just as badly as a deficiency in nitrogen.

If this sounds super complicated, it is! Thankfully, microbes do all the heavy lifting. We don’t have to worry about calculating specific nutrient amounts for our gardens because microbes deliver nutrients to our plants in proper quantities. We just have to put in the right feedstocks (like the organic amendments described above) to support their growth.

Choosing an Organic Fertilizer

To choose the right fertilizers for your garden, consider the different vegetables you are growing. Most of them will want a balanced ratio of N-P-K as well as trace minerals to fuel their growth and fruiting. Some plants, however, are much “heavier feeders” than others. This means they tend to need a bit more fertilizer.

You will also want to assess the type of soil in your garden and potentially use a home soil test or send your soil to a lab for testing to get an idea of the nutrients in your soil at a snapshot in time.

In general, plants need more nitrogen at the time of vegetative (leafy) growth, and more potassium and phosphorus at the time of reproductive (flowering and fruiting growth). For example, when I plant tomatoes I will often water in my transplants with a diluted liquid fish or liquid seaweed fertilizer. Then, when they start to flower, I will side dress with composted chicken manure.

Applying Your Organic Fertilizer

When applying your organic fertilizer, it’s important to get the timing correct. If you do not time it correctly, you can damage your harvest, and make it difficult to reverse course. Let’s take a look at the most opportune times to apply your organic fertilizer, as well as some of the most popular methods you’ll use.

Before Seeding

If you are preparing a first-time garden, this is a great time to add fertilizers like rock dust or kelp meal. You can follow the instructions on your fertilizer package to measure the proper amount. Then, evenly scatter the powder around your new garden beds and use a rake to slightly incorporate it.

At Time of Transplanting

My favorite time to feed my plants is at the time of transplanting. This is especially effective for “heavy feeder” crops like tomatoes, broccoli, cauliflower, squash, and melons.

If using an all-purpose granular fertilizer, I sprinkle the correct amount into the transplanting hole and then plant my seedlings where they can directly access the microbially-transformed nutrients.

If I am using a liquid fertilizer, I will dilute the proper amount in a watering can and use it to “water-in” the transplants. Watering newly transplanted tomatoes with liquid fish is the best way to jumpstart their growth. Liquid seaweed or kelp meal are great options for reducing transplant shock of crops that don’t tend to like root disturbance (melons, pumpkins, cucumbers, and squash).

Always be very careful to use the proper amount of fertilizer, especially when using liquid concentrates. While organic fertilizers are far gentler than synthetic fertilizers, you can still “burn” plants with too many nutrients at once (especially nitrogen). Measure the fertilizer and when in doubt, use less than you think.

Side Dressing

Side dressing is how you apply granulated or powdered fertilizers once plants are already growing. It simply involves using a Tupperware or hand trowel to scatter powder around the base of the plants. You can rub it into the soil to be sure it doesn’t blow away, and that’s it! Super simple.

Soil Drench

Soil drenches are typically used for micronutrient fertilizers like liquid kelp or compost teas. You can make a “tea” by soaking compost or vermicast in an aerated water solution. This can be applied more liberally than other fertilizers because it is diluted and mostly full of microorganisms. The process is exactly what it sounds like: fill a watering can with the solution and apply it directly to the garden soil for a healthy inoculant of microbes and fertility.