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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: 

  • Growth Stage – when plants grow they require more nitrogen and phosphorous. Nitrogen encourages leafy growth and helps plants grow their stems and branches. Phosphorous is needed for seed germination and root development.
  • Fruiting Stage – As a plant enters the fruiting stage it needs more potassium to produce fruit and flowers.
  • Dormant Stage – During winter leaves and branches stop growing so not much nitrogen is needed. But root growth continues so low-nitrogen fertilizer is used during this stage.

Q: Which Fertilizer Should I Buy?

A bit of history first. 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, 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.

  • N-Nitrogen is responsible for green color and new growth. It is needed for cell division and the manufacture of protein.
  • P-Phosphorus is associated with good root growth and flowering. It also aids resistance to disease and pests.
  • K-Potassium is associated with healthy cell activity.

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, Home Depot, 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.

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 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). Dissolve 1 tablespoon per 5-gallons of water and then use a watering can to water your trees. You can splash a bit on the foliage where it will work as foliar feed. Don’t use more fertilizer per gallon because it can cause leaf burn on maples and other deciduous trees.

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 still contain fertilizer. 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 the bead looks nearly clear or you can crush it between your fingers it’s time to replace with new pellets. Timed release pellets can release a lot of fertilizer when the weather is warmer than 90° which can burn your trees. So when the temperature gets hot remove half of the pellets.

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

Pot SizeHow Much Osmocote to use
Small (e.g. 4″ diameter)2-3 teaspoons
Medium (e.g. 12″diameter)
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.

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:

  • For flowering, fruit, and berry producing trees: Use low nitrogen fertilizer (look for bulb and bloom fertilizer) or Seaweed extract when you water.
  • For deciduous trees: Apply 0-10-10, such as E.B. Stone Ultrabloom

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 YellowingDiagnosis
On all leavesLack of nutrients – fertilize!
On younger leavesLack of iron or manganese. Use Ironite
On older leavesLack of nitrogen or potassium
On leaf edgesLack of magnesium and potassium. Use SuperPhosphate
Between veinsLack of iron and manganese. Use Ironite
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