Though Bonsai are repeatedly pruned throughout their lives to keep their small stature; they are never deprived of nutrients to stop them growing. A tree planted in the ground is able to extend its root system in search of nutrients however a tree planted in a bonsai pot is unable to do so; after a short period of time the soil in a bonsai pot loses its nutritional content as the tree consumes it, much nutrition is also lost from the effects of watering and is literally flushed out of the soil. For a bonsai tree to continue to grow strongly (if at all) the soil it grows in must have its nutritional content repeatedly replenished. 

Plant Nutrients

In order to grow, trees use carbon, hydrogen and oxygen from the air, and water from the soil to manufacture their own starch and sugars. They also need from the soil a number of simple chemicals which they then use to create amino acids, proteins, vitamins and enzymes. 

All organic soils have a stock of these vital chemicals; they come from the mineral part of the soil (sands/clays etc) and from dead organic matter (fallen leaves and dead plant matter). However, in the confines of a bonsai pot these vital elements are soon lost. Inorganic soils such as Japanese clays (Akadama etc) and Seramis or Biosorb that are commonly used in bonsai soil mixes can be completely lacking in one or more of these elements even before use.

The most serious loss concerns three elements; nitrogen, phosphorous and potassium. These are known as the major plant nutrients and are required in relatively large amounts if plants are to grow satisfactorily. There are a further number of “trace elements” that are required to be present in the soil but only in small quantities.

Nitrogen (N) is primarily used by plants for leaf growth, Phosphorous in the form of phosphates (P2O5) is primarily used by plants for root-growth, Potassium in the form of Potash (K20) for flower and fruit production. 


Fertilizers 

To supply your bonsai with a continual supply of essential plant nutrients it is necessary to regularly apply fertiliser. There is NO need to purchase 'bonsai feed' or any other 'special' bonsai fertiliser. Essentially ordinary plants, bonsai are more than able to cope with ordinary soluble garden fertilisers. It is far more important that a fertiliser with the correct nutritional balance is chosen.
By law, the manufacturer of a product described as a fertiliser must declare the nitrogen, phosphates and potash content on the package. The content of most other nutrients must also be declared if they have been added to the product. When purchasing a fertiliser, look for the NPK rating on the packaging. 

The first figure refers to the percentage of Nitrogen (N), the second to the percentage of phosphates (P2O5) and the third to the percentage of potash (K20).

It is important when purchasing fertilisers for bonsai that the three major elements are balanced. As it is possible to control the type of growth a plant makes by applying different fertilisers, many fertilisers available to the enthuisiast are unsuitable; for instance to encourage flower production in garden plants, many fertilisers will contain a higher proportion of Potash to encourage flowering at the expense of leaf production.

ensure that all three elements are balanced, when purchasing fertiliser for bonsai and are listed as being equal (for instance 10:10:10 or 15:15:15). 


Applying Fertilisers

Always use fertilizers that you purchase at half strength.

Soluble fertilisers are mixed with water and are applied to the whole of the soil surface until no more can be absorbed and surplus fertiliser runs out of the drainage holes of the pot. 

Fertilisers that are listed as being 10:10:10 or less can be mixed with water to the specification of the manufacturer as described on the packaging. Fertilisers above this should be diluted with more water to bring the percentage of elements down. For instance; a 20:20:20 should be mixed with twice as much water than as described by the manufacturer to reduce the element concentration down to 10:10:10. 

Fertiliser should ordinarily be applied as indicated on the packaging, or according with the instructions that follow:

Young trees: In the spring feed with a high nitrogen fertilizer and in summer, with a balanced fertilizer; and in early autumn, with a low nitrogen fertilizer. Give food to these bonsai trees every three weeks at the beginning of the season and every four weeks by early summer through to the end of autumn… 

Established bonsai trees: Using fertilizers at full strength is particularly dangerous here as the roots are very tender and they may suffer from being fed. You do not want lush juvenile growth, so feed until early summer with low nitrogen fertiliser. Feed balanced feed in summer food and in autumn with low nitrogen again. Only feed mature bonsai trees approximately every five weeks.


High – and Low - Nitrogen fertilisers

The essential fertiliser for bonsai is one that is balanced; that is to say has equal balance of Nitrogen, Phosphorous and Potassium. There are two other combinations that are useful (though not absolutely essential) to successful bonsai feeding, these should be applied as an alternative to a balanced feed NOT as an additional feed.

A fertiliser high in Nitrogen (for instance 25:15:15) is often applied to many species (particularly when in training) during the Spring, the higher proportion of Nitrogen encourages stronger leaf and shoot growth and can encourage faster growth. It should not however be fed throughout the growing season as this would eventually result in too lush green growth with long internodes. 
A fertiliser low in Nitrogen is very useful when applied to bonsai from late-Summer through to dormancy. The relatively high Phosphorous and Potassium content helps to strengthen the year’s growth and the root system before the cold of winter. It also increases bud production for the following year. Top growth that would otherwise succumb to the first frosts in autumn is slowed. Ultimately a fertiliser that is rated as 0:10:10 and contains no nitrogen is preferred but can be hard to obtain and is relatively expensive. A suitable alternative is to use a low-nitrogen fertiliser (12.5:25:25) that is diluted by half.


Fertilising Lime-Hating/ Ericaceous Plants

Lime-hating plants such as Enkianthus, Azaleas (Rhododendrons) and Stewartia require regular replacement feeds of specially formulated fertilisers which ensure that their soil ph remains acidic, ericaceous fertilisers also contain higher levels of chelated iron which is required by lime-hating species to offset chlorosis.

Many Pine species and Juniper species also benefit from twice yearly 'acid' feeds.

Suitable fertilisers include Miracid or any other fertiliser that is indicated as being specifically for ericaceous or lime-hating plants.


Solid Fertilisers

Japanese bonsai growers traditionally use solid fertilisers such as small cakes of fish-meal or rape-seed that are placed on top of the surface of the soil and these slowly release nutrients every time the tree is watered. The primary advantage of solid fertilisers is that they continually supply the tree with small amounts of nutrients and only require replacing every two to three months so are less time-consuming to apply. However, they can act as a breeding ground for maggots and are in my opinion fairly unsightly.


Overfeeding

It is always better to underfeed trees than to overfeed. If the nitrogen content of the bonsai soil becomes too high it will burn the roots. Always follow the mixing and application rates described on the fertiliser packaging, never mix a higher concentration than is described and do not feed more frequently than is described. Overfeeding results in less growth NOT more.

Trees that are dormant or sick should not be fed as they will not consume the same level of Nitrogen and levels in the soil can build up if care is not taken. 

It is often recommended that newly repotted trees should not be fed for at least six weeks to avoid burning new roots.
However, there is now much unreliable advice and some scientific evidence that promotes the advantages of feeding straight after rootpruning and repotting. Immediately after root pruning, a plant will require additional nutrients to grow and repair roots, particularly phosphorous and potassium. A low nitrogen supply would be very benefical to the plant and improbable to burn the roots.


Do’s and dont’s of fertilising

Do use fertilisers regularly; they are essential to the health of your bonsai and produce strong, vigorous growth.

Do use the accurate NPK balance; by feeding with an incorrectly balanced fertiliser you can support root or flower growth at the expense of leaf and shoot growth.

Do check the needs of individual species; different types can require diverse quantities of fertiliser at different times of the year.

Don't feed trees when they are not in active growth as the fertiliser level builds up unused in the nourishment, resulting in root scorch.

Don't feed trees that are not growing due to poor health for the same reason; feeding a sick tree is unlikely to encourage it to grow and is likely to do more harm than good.


A guide to common garden fertilisers 

Previous to using some of the more generally found garden fertilisers on your bonsai, it is worth taking into account what effect they may have on your trees, here is a listing of the basic NPK ratings of some ordinary garden fertilisers and their suitability for bonsai.

- Tomato feeds (6:5:9) - High in potassium to encourage strong flower and fruit production at the expense of leaf growth.

- Lawn Feeds (40:10:0) - Extremely high in Nitrogen to encourage very lush green top growth at the expense of anything else. Will rapidly burn root system.

- Bonemeal (4:20:0) - High in phosphorous to encourage strong root growth, useful for applying to coniferous bonsai in Autumn.

- Fish Meal (6:6:0) - Lacks potassium. Can be used in conjunction with other fertilisers to create solid slow-release fertiliser.

- Phostrogen (10:10:27) - Suitable for flower and fruit production only.

Toprose (5:6:12) - Encourages flower production at the expense of leaf and shoot growth.