Fertilizer Basics

There's a lot of opinions about fertilizer. Our goal is to give basic information and guidance that will help your bonsai thrive. This article won't debate whether "organic" is better than "inorganic" or the pros and cons of different brands.

Fertilizer Isn't Plant Food

Although it is common for many fertilizers to be called plant food, that's not accurate. Plants produce their own food, using water, carbon dioxide and solar energy. This food, which consists of sugars and other carbohydrates, is combined with plant nutrients (minerals) to produce enzymes, proteins, vitamins and other things necessary to plant growth. Bonsai soils don't contain a lot of minerals that the plant can absorb so we must supply the minerals via fertilizer. Fertilizers are sold as a pellet, solid, powder or liquid that contains minerals as salts. Side note, although plants typically pull minerals from the soil they can also absorb them through their leaves. (More on this later).

Q: When should I Fertilize?

The simple answer is "all the time" because bonsai soil doesn't contain a lot of minerals that trees can absorb. But, trees need different minerals at different times of the growth cycle. So, let's explain those first. There are three stages:

Q: Which Fertilizer Should I Buy?

First a bit of history. Before commercial fertilizers were available people used fish meal (liquefied fish, very smelly!), or blood meal, bone meal, cottonseed meal, etc. Manure might be used to supply nitrogen. Because these materials were slow to decompose the nutrients usually wouldn't harm the plants. Many of the early issues of our club's newsletters recommended using these "natural" fertilizers. They're still fine, but some have drawbacks - like being smelly and attracting vermin.

In the early days of commercial fertilizer you might buy 25-50 pound bags labeled 8-8-8 or 16-16-16. You'd use these on your lawn, garden or orchard. The pellets dissolved quickly and made your plants look great, but if you added too much you'd kill your plants. The problem is, when the fertilizer / mineral salts concentration is too high it can interfere with water and nutrient transport within the plant or actually be toxic. Too much of a good thing can hurt your trees!

Now most fertilizers are timed-release to minimize the danger of damaging lawns and gardens. Osmocote brand timed-release fertilizers are easy to find at nurseries, Orchard Supply, Home Depot, Lowes, etc. Usually you'll find 1 to 2 pound containers. But there are other brands, such as Ajax, that work just as well. Most members of our club use a timed-release fertilizer during the spring / summer growth stage.

Q: What do the numbers on fertilizer packages mean?

When you buy a package of commercial fertilizer you will see a label with three numbers, like 12-8-6. These numbers represent the percentage of available nitrogen, phosphorus and potassium (N-P-K) found in the bag. That means a 12-8-10 fertilizer has 12-percent nitrogen, 8-percent phosphorous and 10-percent potassium. These are referred to as Macro-nutrients and come from mineral salts, such as ammonium nitrate or urea (for nitrogen), phosphoric acid (for phosphorus) and potash a mined mineral containing potassium in water-soluble form.

Micro-nutrients are elements required by plants and bacteria, in proportionately smaller amounts, for survival and growth. Boron, cobalt, copper, iron, manganese, molybdenum and zinc are micro-nutrients. When the micro-nutrients are missing the growth rate for the plant will be reduced - sort of like vitamin deficiency in people. Many fertilizers now advertise that they contain micro-nutrients. We recommend choosing one of these over a "regular" version.

Q: What About Organic Fertilizer?

As mentioned earlier, some people prefer to use an "organic fertilizer" instead of the timed released "chemical" variety. You can find bags of components like bone meal, bone meal, etc. at nurseries, Orchard Supply, etc. You can also find bags of mixtures of these components so you don't have to mix them yourself. But, many of these "mixtures" have a high percentage of poultry manure and you're warned to wash your hands after applying. And, in at least some cases after applying the powdered mix to the top of bonsai soil the result has been a slimy mess.

Q: How Is Fertilizer Absorbed?

The tiny feeder roots, work together with MYCORRHIZA (literally meaning "Fungus-root") to absorb mineral nutrients from the soil and water. These live inside the root ball so fertilizer applied to the top of the soil has to dissolve and be taken into the soil where the mineral salts can be absorbed.

Foliar feeding - nutrients can be also absorbed by the leaves of tree. Foliar feeding is especially helpful for trees that have recently been repotted (fine roots destroyed) or need an extra boost. Here's how it works. Underneath the leaves are nearly invisible pores, known as "stoma", where the plant exchanges gases. Carbon Dioxide is absorbed and Oxygen is expelled. Water vapor is also expelled from the stoma through a process known as Transpiration. These stoma can also absorb nutrients and water from mist and droplets. But, when the temperature becomes too warm the stoma close to prevent the tree from drying out. The rest of the leaf surface, the epidermis, can also absorb nutrients, but not as quickly - so we prefer to foliar feed when the stoma are open. That means late morning or early evening.

Image of stoma
Carbon dioxide enters, while water and oxygen exit thru a leaf's stoma. Fertilizer can also enter.
(Photo Credit evolution.berkeley.edu)

Use a dilute solution of Miracle-Gro or VF11 in a spray bottle and mist the leaves every two weeks. Spray all surfaces of the leaves and stems. If you’re using Miracle-Gro make sure you use 1/2 teaspoon (not tablespoon) per gallon of water. If using a typical spray bottle that would be less than 1/8 teaspoon. Too much fertilizer (aka salt) sprayed on leaves can cause them to “burn” or turn brown.

Q: How Much Fertilizer Should I Use?

Repotting - We recommend adding Super Phosphate granules and bone meal into the soil mix before tying your tree into the pot. Both are available from Orchard Supply and nurseries. Super Phosphate dissolves quickly, giving a shot of phosphorus to help strong root growth. Bone meal is slower dissolving and will be there after the Super Phosphate has washed away. Because there won't be many fine roots we also recommend light Folar feeding.

Growth Stage - Beginning late-February or early-March, this is when major growth of leaves and branches occur. More nitrogen is needed at this stage.

Most people in our club prefer the convenience of a timed-release fertilizer like Osmocote. The advantage is that every time you water a bit of fertilizer will dissolve and feed your tree. But these fertilizers typically don't release fertilizer when the weather is cooler than 70° so at the beginning of the growing season you might want to use a liquid fertilizer like Miracle-Gro (or Miracid for pines, junipers and azaleas). And they can release a lot of fertilizer when the weather is warmer than 90° which can burn your trees. So when the weather turns hot you need to remove some of the pellets.

With daily watering Osmocote pellets won't last more than a couple of months, so you need to occasionally look at the pellets and see if they are still full. The pellets you see are actually a plastic bead that contains the fertilizer. As the fertilizer leaches out you'll be left with an empty plastic bead. If you can crush the bead between your fingers it's time to remove and replace them.

After the candles have been removed from pines you shoul dadd additional fertilizer.

Pot Size How Much Osmocote to Use

Small (e.g. 4" diameter)

2-3 teaspoons

Medium (e.g. 12"diameter)

2-4 tablespoons

Large (> 12" diameter)

4-8 tablespoons

Younger plants, or those that you want to grow larger, will require more fertilizer (esp. nitrogen) than older trees that you are maintaining. So, you might double the amount shown in the table above. But, too much nitrogen can give you excessive, leggy growth that you have to cut off. It can also cause leaf "burn" (See "Too Much Fertilizer" at end of this article.)

For a quick boost of fertilizer some people use a diluted version of Miracle-Gro. Use Miracid on acid loving plants such as pines, junipers and azaleas. You can foliar feed or simply apply a dilute solution to the soil.

Available at nurseries everywhere
Available at nurseries everywhere
Also known as MirAcid
Available at most nurseries

Dormant Stage - And as the weather cools and trees enter the dormant stage remove the high nitrogen fertilizer and replace it with blooming fertilizer. Use twice the amount shown in the table above. That's because during our rainy season a lot of fertilizer will be washed away. We recommend:

  1. For flowering, fruit, and berry producing trees: Use low nitrogen fertilizer (look for bulb and bloom fertilizer) or Seaweed extract when you water.
  2. For deciduous trees: Apply 0-10-10, such as E.B. Stone Ultrabloom
Bloom Fertilizer
From Orchard Supply Hardware
No Nitrogen Fertilizer
From Summer Winds Nursery

Symptoms of Fertilizer Deficiencies

Or, Why are my leaves yellow?

The proper name for this condition is chlorosis or chlorophyll deficiency. There are variety of reasons for this condition.

Location of yellowing Diagnosis
On all leaves Lack of nutrients - fertilize!
On younger leaves Lack of iron or manganese
On older leaves Lack of nitrogen or potassium
On leaf edges Lack of magnesium and potassium
Between veins Lack of iron and manganese

Nitrogen deficiency will show up as up as chlorosis of the entire plant – leaves and needles turning yellow. Often this will show up on older leaves / needles first. A quick fix is to use a liquid fertilizer.

Phosphorus deficiency will cause a variety of symptoms that are difficult to identify. If the stem or underside seems to be purple, or you see a gray or brown-netted veining then that’s probably a phosphorus deficiency.

Magnesium deficiency may show up as chlorosis of the tip of needles. When the yellowing is seen primarily on new growth then it’s probably an iron deficiency. For magnesium deficiency sprinkle a teaspoon of Epsom Salt to the top of your soil. Because Epsom Salt dissolves so quickly you’ll probably need to do this a few times over a month. For iron deficiency sprinkle a tablespoon of Ironite over the top of your soil. This dissolves more slowly and one application may be enough.

For more information about how Nitrogen is used in plants, see the article - Nitrogen Cycle

Too Much Fertilizer

Just as too little fertilizer can cause problems, too much fertilizer can also cause leaf "burn". The classic symptom is dry brown edges of the leaves or needles. Of course leaves can also turn brown because the heat was too much and the tree wasn't watered enough. Once you've ruled those out, evaluate the amount of fertilizer on top of your soil. If it looks like a lot more than the recommendations above remove most of the fertilize for a week or two to give time for the excess to wash out of the soil and for the tree to begin to recover. Later you'll want to add some new fertilizer but use less this time - half the amount you removed.

Once leaves have burned they won't recover, so avoid too much heat and fertilizer and water appropriately to avoid leaf burn in the first place.