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Bonsai Feeding and Fertilizing

August 31, 2022Beginners, Care, General

Small round fertilizer holders can be filled with granulated plant food and released over time to feed this juniper bonsai

This article looks at considerations involved in bonsai feeding, including the elements that trees need for nutrition and how to use the various types of fertilizer.

Most of us know the “right” kinds of foods we should be eating to maintain good health. Just as we need to take in nutrition from our diets, plants, trees – and, yes, bonsai – need to absorb nutrition from their environments in order to enjoy robust health.  

Let’s start with an overview of how trees use the elements available to them in their stationary environments for survival.

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The “Big 3 + 3” for Plant Health

To sustain life, trees need three primary elements absorbed from the air and three primary elements absorbed from the soil. From the air, they take in oxygen, carbon and hydrogen. From the soil, they absorb nitrogen, potassium and phosphorus. Trees and plants can synthesize or combine these elements into more complex substances they use for life, growth and reproduction. Nitrogen is essential for leaf growth and chlorophyll production. Phosphorus assists with root growth and energy transfer throughout the plant. Potassium contributes photosynthesis and general health, as well as flower and fruit production. 

Plants in nature grow and thrive by drawing nutrients from the soil. They also return nutrients to the soil by depositing organic matter in the form of fallen leaves and dead plants. Other natural processes also benefit plants by enriching the soil. These include the actions of earthworms and microrganisms that live in soil, the accumulation of minerals through rock erosion, and the activities of higher animal life.

Organic matter is the primary source of the three basic soil-borne elements. Under ideal conditions, the soil in which a tree lives will contain a rich supply organic matter. A good soil will also boast a variety of trace elements that plants need. These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel. 

If a tree finds itself living in less-than-ideal conditions and needs additional nutrients, it has the ability to send out roots far and deep in an effort to gather the nourishment it requires.

How Bonsai Differ

Bonsai, in contrast, live in conditions that require additional consideration and specialized care. In order to survive and thrive within the confines of small pots, bonsai must live in coarse, rocky soil. This type of soil encourages the growth of a dense and fine root system able to sustain the tree. It also drains quickly when watered, essential for minimizing the occurrence of diseases such as root rot. 

Unfortunately, because bonsai soil must dry out quickly, it needs to have low, or sometimes even no organic matter. Organic components, which contain the majority of plant nutrients, also tend to retain moisture. They can promote disease by preventing the soil from drying between watering. So, bonsai soils typically contain a much smaller proportion of organic matter than the soil in which the tree would naturally grow.

It follows that the soil-based nutrients normally carried by organic matter are lacking in bonsai soil mixes. Therefore, hobbyists must supply those nutrients by feeding their trees regularly.

The Well-fed Bonsai

The specific combination of nutrients needed for a tree to thrive varies a bit by species. However, those new to bonsai can safely and simply begin their bonsai feeding protocol by using a balanced fertilizer on all their trees. As their collection expands and their knowledge of tree care grows, they can start to make adjustments in how they care for individual tree species. 

A balanced fertilizer contains around the same amount of nitrogen (N), potassium (K), and phosphorus (P). For example, fertilizers labeled NKP 6-6-6 or NKP 12-10-10 would be considered balanced. 

Examples of the types of individual feeding adjustments a more experienced hobbyist might make include applying trace minerals, using extra potassium for flowering bonsai, or adding soil amendments to adjust PH balance.

Three Types of Fertilizers for Bonsai Feeding

There are three delivery methods for fertilizing plants, including bonsai – granular, liquid and foliar. Let’s review the differences and how to use and apply each type.

Granular Products

Granular fertilizer comes in pellets or small chunks, used by mixing into or applying to the surface of the soil.  Many folks are familiar with applying granular fertilizer to lawns and gardens. The fertilizer pellets rest on top of the soil and disintegrate gradually upon contact with water. As the granules dissolve, nutrients soak into the soil and become available to plants. This helps create green lawns and bountiful garden harvests.

An advantage of using granular fertilizers is that you apply them less often. The pellets break down gradually, extending the length of time between applications. A disadvantage of granular feeding is that it can be easy to overdose the tree. Just as eating too much, too quickly can have negative consequences for us, too large a dose of fertilizer can damage or kill the plant.  Granular fertilizer can also damage delicate foliage if it comes in contact with leaves for an extended period of time.

What type of granular fertilizer is best for bonsai? Look for a slow-release, coated granular fertilizer to use for bonsai feeding, such as this one. Directions for most granular products say to apply them on the surface or mix them into the soil.  

Small basket holding granular fertilizer for an evergreen bonsai.
This juniper bonsai benefits with each watering from granular fertilizer housed in the special basket.

Many hobbyists place their fertilizer pellets in tiny plastic baskets designed for bonsai feeding. The baskets have stakes that insert into the soil, and they allow a little dose of nutrients to release with each watering. Using baskets holds the fertilizer granules in place so that they can’t wash off the surface of the pot. This also prevents the fertilizer from coming into direct contact with the plant’s roots or foliage.

Some baskets are cylindrical with little tops that snap in place. Others are rounded with flexible teeth on top to help contain the granules. One advantage to using baskets is that the hobbyist can easily see when fertilizer needs replenishing.

Check out our related article on proper watering for additional tips on caring for bonsai.

Liquid Fertilizer 

Liquid fertilizer works the same way as granular feeding, except the nutrients are dissolved in water before applying to the soil.  This enables the nutrients to reach the plant roots more quickly and evenly than with granular feeding.  Miracle Gro, a popular product familiar to many, is one example of a fertilizer designed to be applied in liquid form. Liquid products, including those formulated specifically for bonsai, enjoy popularity among hobbyists for feeding trees. 

Applying a liquid fertilizer to bonsai
Applying liquid fertilizer to bonsai

Most liquid fertilizers are less concentrated than their granular counterparts and are designed to be used more frequently.  Bonsai hobbyists who use liquid fertilizers often opt for a diluted solution applied weekly during growing season.

The Foliar Method for Bonsai Feeding

Foliar feeding is another form of fertilizing which involves liquid nutrients being sprayed or misted directly onto the plant’s foliage. Leaves actually have the ability to absorb nutrients and put them to use faster than nutrients absorbed through the root system. 

For this reason, foliar feeding is sometimes used on ailing plants to help them recover from stress or disease. It can also be effective in quickly correcting a problem caused by a known trace element deficiency in the soil.  

For example, a magnesium deficiency causes yellowing of the leaf edges and in between the veins of older leaves. If this situation occurs, soil amendments can correct the problem. However, also spraying a foliar solution that supplies magnesium can help restore the plant to full health faster. Foliar feeding should not replace regular root-based feeding, but can supplement it. 

To apply a foliar feed, spray it directly onto the plant’s leaves and stems. Fertilizer solutions that are too strong can damage the delicate foliage and possibly kill the plant. Many hobbyists opt for natural sprays such as compost tea or seaweed solutions. 

Beginners wanting to try foliar sprays for bonsai feeding might prefer to start with a commercial product.  Superthrive, is a vitamin supplement popular among bonsai hobbyists. It offers a pre-mixed version of the product sold in a spray bottle and designed for foliar feeding. A couple of other sprays that get good reviews are a kelp-based organic spray, and an organic spray made primarily from sea products. Just confirm that any commercial fertilizer used for foliar feeding includes directions for using the product in that way.

In this article, we reviewed the elements involved in tree nutrition, examined why feeding and fertilizing bonsai is especially crucial to the tree’s health, and looked at how to use the various types of fertilizers.

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