The Best Organic Fertilizers to Double Your Harvest

Garden fertilizer basics

Fertilizers are any naturally or chemically derived material containing the nutrients essential for plant growth. They are available in the form of manures, compost, and granular or liquid amendments. Most often we apply them to a plant’s root system or as a foliar spray.

To the new gardener, a stroll through the fertilizer aisle can be a very confusing exercise triggering memories of chemistry class. What do these numbers mean to my garden? is the question often circling as we consider what to add and how much.

When you buy fertilizer, you’ll see the ratio of three macronutrients—nitrogen (N), phosphorus (P) and potassium(K)—listed on the container in bold numbers. These numbers tell you the combination in percentages of N, P, and K in the fertilizer blend. Educate yourself and take heed: not all fertilizers are created equal and feeding your plants with fertilizers does not guarantee a successful crop.

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• Up, Down, and All Around: A Simple Way to Understand Fertilizers

Plant nutrition 101

In truth, plant nutrition is determined by a complex system of chemistry, microbes, fungi, and organisms working in the soil and processing the organic materials into accessible elements. It’s a symbiotic relationship where microbe feeds plant and plant feeds microbe. This cycle offers your garden nutrition in perpetuity.

The benefits of soil organisms

All those organisms living in your soil provide benefits to your growing crop. They help:

• Decompose organic material to its essential elements.
• Improve nutrient accessibility to plants.
• Protect from attacks by insects and disease.
• Subdue stresses caused by heat and drought.

What do plants need?

Of the 118 elements and the 3800 plus minerals on earth, plants need only about 20 in total. Exposure to air and water provides carbon, hydrogen, and oxygen. Plants get the remaining 17 elements through their roots, which take these elements from the soil.

These 17 elements are divided into two groups. The elements that plants require in high doses, such as nitrogen, phosphorus and potassium are considered macronutrients. Those needed in smaller amounts are considered micronutrients.

Common Nutrients Needed for Healthy Garden Plants

Under natural conditions, plant nutrition comes from two sources:

1. Decomposing organic material converted to simple elements by microorganisms.
2. Inorganic materials in the form of minerals from rocks, sand, or clay.

Without these essential elements, plants can’t complete a normal life cycle. When soil is overly tilled, compacted, flooded, or exposed to long-term chemical use, the results are poor soil quality low in nutrition. Under these circumstances, feeding gardens and landscapes with complete natural fertilizers can help rebuild soil nutrition and the microbial community.

Other beneficial plant nutrients

In addition to the macro and micronutrients, there are other beneficial nutrients that can enhance growth or offer protection. Silicon, for example, has been found to improve tolerance to heat and drought as well as improving resistance to insect and fungal infections.

Furthermore, silicon can help alleviate toxicity caused by manganese, iron, aluminum, and phosphorus, as well as compensating for zinc deficiency. So while silicon is not considered an essential element, it gives plants an advantage.

How soil pH makes nutrients more available

Your soil’s pH has a decisive influence on how accessible nutrients will be. If your soil is too basic or acidic, elements can become toxic or unavailable to your plants.

For example, if a plant is showing signs of phosphorus deficiency and the soil is too acidic, it wouldn’t matter how much phosphorus fertilizer you apply. Your plants won’t be able to access them. A neutral pH or one that’s slightly acidic for vegetables allows the highest availability to all plants.

To adjust the pH of a soil, you’ll need to find out what the pH is according to a soil test. From there you can add either lime or sulphur depending on whether you want to raise or lower the ph.

Raising your soil’s pH

To raise the pH from acidic to alkaline, adding lime is necessary. There are two types of lime available to the home gardener: agricultural lime made from ground Calcium carbonate and dolomitic lime made from ground limestone, which also contains magnesium. Each should be added at least 6 weeks before planting to take effect on the soil. Though there are other types of commercial lime such as slaked lime or quick lime, they pose a high risk of burning and are not suitable for the home garden.

Lowering your soil’s pH

Sulphur is an essential macronutrient for plants and also can lower the pH in alkaline soils. However, sulphur is much riskier if misapplied. That’s because microbes will convert sulphur to sulphuric acid in soil temperatures above 55 F. For this reason, be sure to apply sulphur only in the spring for effectiveness. Also watch for soil that is oversaturated or anaerobic: in these conditions the sulphur is converted to hydrogen sulphide, which kills developing roots. And be aware that sulphur takes a minimum of one year to take effect on the soil.

Organic vs. chemical fertilizers: what’s the difference?

It’s true that plants don’t differentiate between the nutrients in chemical or organic fertilizers. It’s also true that you can apply higher concentrations of nutrients using synthetic options, but this should not be the only determining factor when choosing a fertilizer. The production of fertilizers and components used to make them have many implications and different results in the soil.

Organic, natural fertilizers

There are many types of organic fertilizers that are beneficial for your garden. Organic or natural fertilizers are created using composted or dried organic matter such as cow manure, concentrated compost, crop residue, earthworm castings, seaweed, seed meal, and animal sources. They are applied by side dressing or by incorporating into the soil of potted plants. These materials, when applied in combination to crops, contribute all essential macronutrients, micronutrients, and trace elements. More importantly, they help build a healthy, supportive environment for beneficial microbes, fungi, and bugs.

Some naturally sourced products such as rock phosphate, guano, lime, and green sand are mined, which isn’t necessarily a sustainable method. But these materials offer particular nutritional components in concentrations that other organically derived materials do not. Granular materials are applied in the same way as organic fertilizers; they break down slowly and are beneficial to a healthy macrobiotic system.

Chemical, synthetic fertilizers

Chemical or synthetic fertilizers are chemical compounds synthesized from a natural origin. They’re often derived from the by-products of the petroleum industry and are commonly acidic. Their primary purpose is to address the macro-nutritional needs of a plant by providing nitrogen, phosphorous, and potassium. Examples of these would be ammonium nitrate, ammonium phosphate, and potassium sulphate.

Some fertilizer blends offer secondary nutrients of calcium, iron, molybdenum, sulphur, and magnesium, but almost none contribute the complete spectrum of micronutrients a plant needs to complete growth. The most significant downside to using chemical fertilizers is that they can kill healthy microbes and create an unsuitable environment for other organisms due to the inherent acidity.

Comparing natural, organic fertilizers with chemical, synthetic fertilizers

General purpose, organic fertilizers: can I use just one thing?

So what is the best organic fertilizer? Or more specifically, what is the best organic vegetable garden fertilizer? If you have to choose just one, there’s a clear winner.

Adding compost or manures is an absolute must as an overall general fertilizer and conditioner for proper garden maintenance. Not only do these materials add essential nutrition for your plants, but they also condition and aerate soil, improve water retention, and moderate temperature.

Compost

Composts are what are left after plant material decomposes. There are two main forms: slow compost, which has degraded with naturally occurring microbes, and worm compost, otherwise known as castings.

Related:

• Worm Composting Basics for Beginners
• The Best Compost Tea Recipe to Help Your Plants Thrive

Slow compost offers a material high in nutrition, mycorrhizae, and beneficial microbes, while worm compost is rich in plant growth promoters like cytokinins and auxins, along with increased levels of micronutrients like calcium, magnesium, and sulphur. You can also buy concentrated compost, which offers a combination of ingredients.

Manure

Manures are decomposed animal waste from farms. They can also come from mushroom production. Like compost, manures offer balanced nutrition and soil conditioning while also supporting beneficial microbes. The number one benefit of manures is accessibility.

Every garden center and many grocery stores sell manure by the bag. If you are lucky and you live close to a farm, it’s likely you have access to an unlimited amount. If this is the case, it’s important to note that locally collected manures should be composted before using so you don’t damage plants or introduce weed seeds.

The most common manure choices include:

• Well-composted manure from steers or horses produce a cooler material, making it ready to use in the garden without worry of burning. However, horse manure can contain many weed seeds since they don’t get processed while digesting.
• Chicken manure is a source of high nitrogen making it suitable for blueberries, leafy greens and plants in vegetative growth. However, it’s also a hot item and if not allowed to compost fully can burn plants with its high nitrogen content.
• Sheep and rabbit manures offer a rich nutritional content and arrive in the easily spreadable pellet form naturally. Both are especially good for flowering plants but are not often found in abundance, so may have to be used sparingly.

All-in-one fertilizers

Sometimes organic raw materials (detailed below) are combined to create all-purpose organic fertilizers. These can be especially effective if applied to the soil two or three weeks before seeding or transplanting your crop.

Raw materials

There are many other amendments you can add to the garden to boost your harvest and improve your blooms. Raw materials created from the grinding of dry, organic materials speed the decomposition process for microbes and help convert the elements more readily. Use these raw materials individually or combined to achieve multiple benefits. You can find most of these products at your local garden center, feed supply store, or online.

Raw organic fertilizers and their uses

Speciality amendments and fertilizers

In addition to the 17 macro and macronutrients required by the garden, there are other amendments you can add to improve the production and quality of plants.

• Glacial rock dust is a readily available source of calcium, iron, magnesium, and potassium. Glacial rock dust also contains many trace elements which improve soil structure, moisture holding properties, bacterial action, and nutrient availability.
• Granite sand contains over 60 elements, including many trace elements that help your plants grow nutrient dense fruits and vegetables.
• Gypsum is calcium sulphate and is touted as very beneficial for breaking up compact clay soils. It is useful in changing the soil structure in excessively heavy soils, which is impacted by heavy traffic, flooding, or over cropping.
• Pumice is a soil amendment made from igneous rock. It’s most commonly used as an alternative to perlite for its water retaining properties. It’s also good for improving soil aeration, drainage, and nutrient content in both vegetable and flower beds.

DIY organic garden fertilizer

Steve Solomon, founder of the Territorial Seed Company, developed a recipe that works as a general all purpose food for plants. The original recipe appears in his book, Gardening When It Counts (©2005) and is reprinted below with permission from New Society Publishers.

Apply at 4-6 quarts per 100 square feet or a half a cup per 10 gallon potted plant.

Complete Organic Fertilizer (COF) Recipe

Mix uniformly, in parts by volume:
4 parts seed meal
1/4 part ordinary agricultural lime (calcium carbonate), best finely ground
1/4 part gypsum (or double the agricultural lime)
1/2 part dolomite lime
Plus, for best results:
1 part bone meal, rock phosphate or high-phosphate guano
1/2 to 1 part kelp meal (or 1 part basalt dust)

Best fertilizer mixes for particular vegetables

If mixing a recipe sounds too daunting, there are many pre-made organic choices to choose from to address individual crops needs. Here is what you need to look for when purchasing fertilizers for specific crops.

Tomatoes 5:6:8
Corn 6:3:3
Fruit Trees 5:3:4
Berry Bushes 5:3:4
Root Crops 2:8:4
Brassicas 6:3:2
Squash 5:10:10

The role of mycorrhizae

The process of feeding your plants does not end with adding fertilizer to the soil. The real mechanics of feeding plants happens with the billions of microorganisms that process that fertilizer into a usable and accessible form for the plants to ingest.

One such group of organisms are the mycorrhizae, which are the root systems of the many fungus species that live in healthy soils. These mycorrhizae form a mutual relationship with the host plant’s root system. The plant and the fungus have a symbiotic relationship, whereas the fungus facilitates water and nutrient uptake in the plant, and the plant provides food and nutrients created by photosynthesis to the fungus.

Mycorrhizae can be viewed as an extension of the plants own root system, allowing it a greater area to access resources. They also act as a line of defence, protecting the plant’s root environment and improving soil characteristics. Soils that have been freshly tilled, compacted or waterlogged, lack any existing fungi putting the plant at risk of malnutrition and disease.

Related:

• Who’s At Work in the Compost Pile?
• 7 Signs Your Compost is Struggling (and What You Can Do About It)

The good news is that you can encourage mycorrhizae in soil by exercising healthy gardening practices. Certain types can also be purchased and added to your soil to boost your community of organisms. These are called inoculants and can be applied to target specific needs such as germination, root development, and nitrogen fixing.

The best time to apply the inoculant is at seeding or transplanting stage. Either coat seeds with inoculant powder before spreading or gently rub the product directly onto the root ball to ensure contact between the fungi and root system. You can also add the material to existing landscapes. Using a pitchfork, create holes around the drip line of trees or into the soil of garden or sod. Apply inoculant powder or liquid over the holes to ensure it reaches the root system. Water thoroughly.

Liquid fertilizer vs. granular

The choice between applying a liquid fertilizer or a dry granular fertilizer is a matter of purpose. What are you trying to achieve? Is it a quick fix? Are you aiming for long-term nutrition or something to work within one growing season? Here are a few things to consider when making your choice.

Fertilizer timing and application

Plants require different foods at different times in their life cycle. In the beginning, as leaves are developing, they require nitrogen and elements that support cell division. At bloom time they require potassium and trace elements. At dormancy for perennials, root crops, or trees they are looking for phosphorus and the material required for a robust root system that will last over the winter.

When fertilizing organically, think of your applications as pre-emptive—to address the future needs of the plant. At seeding, a liquid application of fertilizer provides immediate access to nutrients for developing roots but should be used in combination with granular fertilizers, which ensure consistent availability as the plant develops. With each transplant, bone meal or rock phosphate should be applied to encourage root growth as well as potassium in the form of kelp for the future needs in bloom.

Fertilizing the lawn

Grass by its very nature is perennial and for the most part does not need much additional fertilization. It has an extensive root system fed by the perpetual cycle of death, decomposition, and growth. However, if you are aiming for a lawn that is lush and green, more effort is required.

Grasses eat a lot of nitrogen, hydrogen, and oxygen, so your choice of fertilizer and maintenance must reflect this if a green lawn is the goal. When planting your lawn for the first time or when overseeding to boost your lawn’s growth, use a turf starter fertilizer to get things off to a good start.

When performing routine maintenance, consider the following steps as you fertilize and care for your lawn:

1. Aerate soil mechanically or with pitchfork.
2. Apply high nitrogen fertilizer such as manures, seed meals, or compost 4 times per growing season.
3. Water frequently. Soil should not dry out below 2”.

Related:

• Natural Lawn Care
• Lawn Alternatives

Garden fertilizer FAQ's

Are there plants that do not like fertilizer or amendments?
Yes! Lilacs prefer a more sterile soil and potatoes can develop rust from un-composted manures.

I fertilized my plants but they are not growing.
Fertilizer may not be the problem. Check your soil pH and adjust accordingly. (See above)

Tomatoes are brown on the end of the fruits
The major cause of this issue, known as blossom end rot, is moisture fluctuation in the soil making it difficult for plants take up calcium. Apply a layer of mulch to your beds to help moderate changes in soil moisture. Water regularly to keep your soil evenly moist.

Are organic fertilizers harmful to children or pets?
Although there are fewer risks associated with organic fertilizers, they can still harm children and pets if consumed in concentrated amounts. Dogs are often attracted to the scent of blood or bone meal, while cats may prefer fish emulsion fertilizers. Both may dig in areas where these fertilizers have been applied, disturbing soil and plants. To prevent harmful ingestion, apply fertilizers at root level and avoid concentrating your application by working into the soil and burying. Store fertilizers in a safe place away from curious visitors.

A final word

Organic fertilizers won’t provide a quick fix like synthetic fertilizers, but they do offer a long-term source of nutrition that requires less effort over time. The result is a healthier environment for your family and the earth.