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"Chemical Fertilisers"

The inverted comma's are used as all fertilisers are chemicals really, even the organic ones!

Certain compositions of chemical fertilisers are recommended for various seasons, dependant upon their nitrogen, phosphate and potash (NPK) composition. The most important thing to remember is that for trees in small containers, like bonsai dishes, they should usually be used at half or even quarter strength, to prevent over-fertilising. Younger developing trees, in training boxes or larger pots, may be fed more regularly. For these you can use strengths closer to, but never more than, the full manufacturers recommendation. The last thing that you want is to kill off the tender root feeder hairs. Trees rely upon these fine hair roots to take up their water and nutrients, if they are all killed off, losses will almost inevitably occur. A tree that is over-fertilised will rapidly become sickly and the only hope is to flush all the excess from the pot with running water and then treat the tree as a cutting. i.e. Mist the top, keep the root area barely moist and keep in a shaded frame or cold greenhouse. With luck, some trees may be able to recover by generating new roots.

Well balanced, regular feeding will encourage lush growth and more rapid trunk thickening. With mature or "finished" bonsai, under-feeding is desirable as it helps preserve the detail of refinement and prevents rampant new growth with longer internodes and larger leaves or needles.

Generally speaking, the recommended fertiliser for spring and summer application is a balanced one. A breakdown of 10:10:10 or 7:7:7 is ideal. For flowering and fruiting trees Tomorite or Phostrogen are used as they contain potassium and magnesium in the correct proportions to encourage flowering and the setting of fruit 4:4½:8. Acid-loving plants, such as Rhododendrons, azaleas and many conifers, will benefit from feeding with Miracid or Phostrogen Acid Plant Food & Tonic both of which include trace elements.

Toward the end of the growing season a low nitrogen feed is advantageous as this helps to harden off shoots and prepares the tree for winter frosts. The breakdown for zero nitrogen is 0:10:10.

Trees in development can be fed with less effort using controlled release fertiliser pellets. You will often find these little pale yellowish spheres in the soil of container grown plants. These rely on moisture and temperature to allow the foods to pass through an organic resin coating into the soil. Their viability in the soil varies from a few months to six months after which they will have to be supplemented with other fertiliser. Products such as Osmacote have a good balance of the essential NPK plus essential trace elements.

The breakdown below may appear to give a lot of detail but it does not show the whole picture. Many of these also contain the trace elements - refer to the packet before purchasing.

Chemical or Inorganic Fertilisers % N % P % K
  Nitrogen Phosphate Potash
Chempack Formula 2 25 15 15
Chempack Formula 3 20 20 20
Chempack Formula 4 15 15 30
Chempack Formula 8 12½ 25 25
Growmore 7 7 7
J.A.Bowers Sangral 15 10 25
Miracle Grow 15 30 15
Miracid 30 10 10
Osmacote (typical) 10 11 18
Phostrogen 10 10 27
Phostrogen Acid Plant Food 15 10 15
Potassium sulphate 0 0 50
Superphosphate 0 20½ 0
Tomorite 4 8
Urea 47 0 0

(There are several different versions of Osmocote available for different applications.)

Mycorrhiza = Mycelium = Fungi

More and more relationships are being discovered between plants that have a symbiotic relationship with the mycelial growths of beneficial fungi. This could indicate a need to rethink our repotting routines. Introducing certain species of live fungus can help when transferring from ground to training box, from training box to bonsai pot. Mycorrhiza is commonly noted in the soils of many kinds of pines, larches and cedars and is known to coexist with many more.

I completed some research recently growing approximately 50 identical seedling Scots Pines. All were pricked out, showing similar healthy growth at 1 year. There was no deliberate addition of mycelium to the growing mix. I noticed several looked sickly but many were romping away with healthier, deep green needles and more rapid growth. Inspection of the rootballs confirmed that spontaneous mycelium growth was present with the healthiest seedlings roots and there was none at all in the sickly seedlings soil.

What does the mycorrhiza do? In association with the roots, it acts in the same way as root hairs, gathering moisture and food from the soil, and also widens the tolerable pH range for the plant i.e. from pH 4.5 to 6.6 or wider, depending upon species. In exchange the plant supplies carbon to the mycorrhiza.

The physical condition of the soil greatly influences the levels of mycorrhiza present. The soil must be well aerated and above pH 5.5 to support mycorrhiza. High levels of phosphorus (usually from chemical fertilisers) within the plant causes reduced mycorrhiza colonisation of the roots. This also applies to some extent with nitrogen. Studies indicate it is not the fertiliser itself in the soil, but the amount of phosphorus present in the plant.

The healthiest potted trees are ones that have good aeration and contain a good concentration of mycorrhiza.

It still boils down to an important single element and that is the condition of your soil, without aeration nothing will thrive, not even fungi.


Feeding trees when they have been lifted or repotted has been considered inadvisable. I was left until after about one month and they are showing healthy growth. Many experts are now revising their opinion and feeding weakly but regularly straight after these operations.


Two needle pines are best fed from January through to March monthly with Miracid at half strength or 7/7/7 granules. From April to July use quarter strength fortnightly to continue their development. Use 0/10/10 Monthly from August to October to harden growth off for winter.

For five needle pines or the maintenance of older specimens and the reduction of needle size, use quarter strength Miracid or 7/7/7 granules fortnightly from January to March and don’t feed from April to July.

A shorter feeding period is usual for other conifers. Fortnightly from March to July using Miracid at half strength or 7/7/7 granules. 0/10/10 half strength August to October.


From bud burst (0/10/10 if early sprouting is caused by mild weather) feed weekly using 10/10/27 at half strength for about a month and then alternate weekly with 7/7/7 granules until July. Use 0/10/10 monthly in August and September to harden off growth for winter.

Trees may be fed for slightly longer if they are provided with sheltered growing conditions i.e. a polytunnel or greenhouse.


From bud burst 0/10/10 fortnightly up to flowering. Use 10/10/27 if yellowing of leaves occurs. Don’t feed during flowering up until fruit has set. Then feed fortnightly with 10/10/27 until August and monthly with 0/10/10 to October.

Further Reading

A very useful program called Superfeeding was developed by Michael Persiano and presented in 1997 in Bonsai Today, the American Bonsai magazine. Discussion of this can also be found in the IBC (Internet Bonsai Club) archives.

Specific deficiencies have not been discussed owing to space limitations. For detailed information on the causes and remedies of most feeding problems refer to the RHS Encyclopaedia of Gardening p554/555.

In Conclusion

To sum up then, well fed trees are healthy trees. If you stint on this aspect of your bonsai, they may be undernourished and will probably become unhealthy. If you overfeed them, they will very likely die. If anyone can add to the pool of knowledge with suggestions, further information on breakdown's or corrections to any of the figures contained in this article, I would be very grateful. email kevin@actionvideo.freeserve.co.uk

All Text & Photographs © Kevin Bailey 1998 - 2008


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