Instead of turning to synthetic chemical fertilizers to feed your plants, consider relying on natural fertilizers to nourish both your plants and your soil. Fertilizers based on natural ingredients not only provide mineral nutrition for growing plants, they also feed the soil’s living organisms. These organisms (most of which are microscopic fungi and bacteria) process these natural fertilizers, breaking them down into the nutrients that plants use to grow. In this article, I’ll introduce you to several natural fertilizer options and discuss which ones are good choices for the garden.
Many soil microbes live in a mutually beneficial relationship with the roots of your plants, providing the plant with certain mineral nutrients in exchange for small amounts of carbohydrates. The health and productivity of your soil is undoubtedly linked to the biological activity found in it. In this horticulturist’s opinion, it’s time to rely on fertilizers that assist with this amazing process rather than harm it. While they do provide mineral nutrition to your plants, popular salt-based synthetic fertilizers can harm soil organisms, which, over time, can lead to multiple issues.
What is a natural fertilizer?
For many gardeners, fertilizer selection is confusing. How do you know which products are natural and which are synthetic? Thankfully, it can be as simple as reading the label. A natural fertilizer relies on plant, mineral, and animal sources for its nutrients. Ingredients such as bone meal, blood meal, fish meal, manures, greensand, rock phosphate, alfalfa meal, kelp, and compost are common in natural fertilizers (more on some of these in a bit).
Good quality natural fertilizers list all of their ingredients clearly on the label. The list will consist of ingredients you’ll likely recognize. Natural fertilizers are available in liquid form, as well as granular, pelletized, and powdered.
Synthetic fertilizers do not have an ingredient list on their label. Synthetic fertilizers can be pelletized, powdered, liquid, granular, or water-soluble crystals.
The N-P-K ratio
Most fertilizers, whether natural or synthetic, are labeled with the percentages of the three primary plant macronutrients found within them: nitrogen, phosphorous, and potassium. You’ll see this N-P-K ratio on the bag or bottle of nearly every fertilizer. You may note that natural fertilizer brands often have lower N-P-K ratios with smaller numbers. That’s another sign that you’re not looking at a synthetic fertilizer whose numbers are often higher. For more on fertilizer numbers and what they mean, check out this article on the meaning of the N-P-K ratio.
Why natural fertilizer is a good choice
Most synthetic fertilizers release their nitrogen (of which only 40-60% is actually useable) within three to six weeks, while fish-based fertilizer releases its nitrogen (of which 90% is useable) over the course of up to 15 weeks. As you can see, although natural fertilizers may seem lower in nutrients because their N-P-K ratios are typically lower, you’ll actually be getting more nutrients over a longer period of time. Since it takes the microbes in the soil longer to process natural fertilizers and release the nutrients they contain into the soil, the nutrients are available in a slow-release form.
An added benefit to using natural fertilizers is that many of them also contain trace nutrients that are not found in synthetic fertilizers that contain only nitrogen, phosphorous, and potassium.
Which natural fertilizer to use
Regular additions of finished compost, aged manure, shredded leaves, and other sources of organic matter are extremely good at keeping many nutrient levels in the optimum target range. However, sometimes an extra boost of mineral nutrition is necessary, especially if a soil test indicates it. That’s where natural fertilizer products and blends become a great tool for gardeners.
Three types of natural fertilizer
These fertilizers are useful for specific nutrient deficiencies indicated by a soil test, or when a boost of one specific nutrient is needed for growing a particular crop. For example, root crops use a lot of phosphorus. This means additions of bone meal (about 3-15-0) help supply this essential nutrient. Remember though, because natural fertilizers take some time to break down, these products are best added a few months before planting. This gives them plenty of time to assimilate into the soil.
Here are a handful of common single-ingredient natural fertilizers:
1.Bone Meal – A by-product of slaughtering facilities, bone meal is created through the steam processing and pulverization of animal bones (definitely not a vegan fertilizer!). It also contains calcium, another essential plant nutrient. Phosphorous is most available to plants when the soil pH ranges between 6.0 and 7.0. Be sure your pH is adjusted accordingly to get the maximum benefit from bone meal.
2. Alfalfa meal – An all-around good source of all three macro-nutrients (2-1-2 to 3-1-2), alfalfa meal also has high microbial content. Though it takes a few months to break down, the many trace nutrients and plant growth hormones present in alfalfa meal stimulate plant growth as well as soil biological activity.
3. Cottonseed Meal – With an average N-P-K ratio of 6-0.4-1.5, cottonseed meal is an excellent source of nitrogen, as well as an added source of phosphorous and potassium. Look for organic cottonseed meal if you can, because many pesticides are used in the production of conventional cotton.
4. Rock phosphate – An excellent source of phosphorous, rock phosphate is a mineral rock powder containing anywhere from 3 to 8% phosphorous (0-3-0 to 0-8-0). The phosphorous contained in rock phosphate becomes more available the second year after application. The highest rates of availability occur when the pH is between 6.0 and 7.0. It’s also a good source of calcium. It is mined from naturally occurring deposits.
5. Blood Meal – Another by-product of slaughtering facilities, blood meal has an average N-P-K ratio of 12-0-0, making it very high in nitrogen. Because of its high ammonia content, it’s easy to over-do it. Foliage and root burn are symptomatic of too much blood meal.
6. Sulfate of potash – Potassium sulfate (aka sulfate of potash) is a soluble salt that contains both potassium and sulfur. Sulfate of potash can be derived from natural, mined sources or created synthetically in a laboratory, so pay attention to its source if that’s a concern for you. It has a lower salt index than many other potassium fertilizers which helps to avoid possible issues with salt build-up and foliage/root burn. With an N-P-K ratio between 0-0-22 and 0-0-50 depending on the source, sulfate of potash provides the potassium necessary for increasing tolerance to cold, heat and drought as well as increasing overall plant vigor.
7. Feather meal – Though it takes four to six months for microbes to process feather meal, it is an excellent source of nitrogen with an N-P-K ratio between 7-0-0 and 12-0-0. It is a by-product of poultry processing.
8. Greensand – This mined, marine-derived sediment (also called glauconite) is greenish in color and contains moderate amounts of potassium (0-0-3 to 0-0-7, depending on the deposit). The potassium it provides improves vigor and hardiness. The micro- and trace nutrients also present in it encourage overall plant health.
Complete fertilizer blends (granular)
There are dozens of different brands of complete natural fertilizer blends. They all use a combination of ingredients such as those listed above, including animal, plant, and mineral ingredients. These are mixed together to create a complete granular fertilizer. These natural fertilizer choices have an N-P-K ratio of 4-5-4 or 3-3-3 or something similar. Some brands are formulated for use on a specific group of plants; acid-loving plants like azaleas and evergreens (such as Holly Tone), tomatoes, fruit trees, or vegetables, for example. What makes them “complete” is that they contain a combination of ingredients that results in the presence of all three macronutrients (in addition to many trace nutrients). Some of my personal favorites include Garden Tone, Dr. Earth, and Coast of Maine.
Types of liquid fertilizers
These products are absorbed into plants via both their roots and their foliage. Liquid fertilizers are made from naturally occurring plant, animal, and mineral ingredients that provide a plant with nutrients. They supply the three macronutrients (nitrogen, phosphorous, and potassium) in small quantities. Many also contain trace nutrients, vitamins, and amino acids. The nutrients in liquid fertilizers are available for use by plants more quickly than those found in granular fertilizers. However, the nutrients are available for a far shorter period of time. Liquid fertilizer can be made from a combination of ingredients or consist of a single ingredient.
Here are a few liquid fertilizer choices:
1.Liquid kelp – Created by processing sea kelp at cool temperatures, liquid kelp is rich in many trace minerals and is a source of several plant hormones.
2. Fish emulsion – Whole fish are cooked and filtered, and the oils and proteins are removed, leaving the resulting fertilizer bereft of many of the amino acids, vitamins, and hormones beneficial to plant growth. Still, fish emulsion is a good natural alternative to chemical water-soluble fertilizers.
3. Fish hydroslate (or liquid fish) – Another, less odorous, form of fish-based fertilizer, hydroslate is created by digesting fish by-products with enzymes at cooler temperatures. Because it isn’t “cooked”, many of the oils, amino acids, vitamins, hormones, and enzymes remain in the product. The result is a nutrient-rich product that has many trace minerals, is less prone to leaching, and is wonderful microbe food. Plus, it isn’t nearly as smelly as fish emulsion.
Personally, I prefer liquid fertilizers that are a combination of natural ingredients. They may also contain liquid bone meal, worm leachate, and other ingredients as well as those listed above. Some of my favorite liquid natural fertilizer brands include Bloom by Espoma, Fox Farm, and Grow Big.
How much natural fertilizer to use
No matter which natural fertilizer product you choose, the rate of application is important. When it comes to fertilizer, remember that more is never better. Even natural fertilizers can be easily over-applied, leading to several different possible issues. These include potential nutrient deficiencies, soil pH imbalance, and fertilizer burn (yes, even some natural fertilizers can burn tender roots or foliage). Follow the label application instructions carefully. Test your soil every 3 to 4 years to determine whether any fertilizer is even needed at all. At the bare minimum, you need to follow the application rates listed on the label of each product.
Please see the following articles for more about how to feed your plants:
– Fertilizer numbers: What they mean and how to use them
– Houseplant fertilization
– How to fertilize containers
– Soil pH and why it matters
– Organic soil amendments for the garden
– How to build a worm bin