Fertilisers

Generally speaking, most orchids don't need a lot of fertiliser, many of them won't suffer without it. This is probably because of how adaptable they are. Many epiphytes living on trees would have to rely on the bare nutrients in the bark or collected from droppings of birds and insects. Many terrestrial orchids may naturally live in nutrient poor soils, or live on the top of decomposing leaf litter that may rob them of nitrogen and other nutrients.

A little fertiliser can be beneficial as it speeds up growth and can help make larger blooms, as well as contribute to the plant's hardiness by building tougher leaves and plumper pseudobulbs or stems. Some varieties, such as cattleya, gongora, and some dendrobiums benefit greatly from the application of fertiliser, whereas more sensitive varieties such as masdevallia, disa, and pleurothallis will only benefit from very light applications.

It is important not to apply too much as it can reduce root growth, to trhe point where roots are destroyed by the excessive salts present in the fertiliser. The mantra "weakly, weekly" is widely used by many growers for this reason.

Fertilisers often come with an "NPK" score, this comprises the 3 main elements related to plant growth:

N - Nitrogen - Helps build chlorophyll, which allows plants to gather more energy for growth (and also gives greener leaves)
P - Phosphorous - Builds the core components in plant cells, promoting the growth of new shoots and root tips
K - Potassium - Helps the transport of water within the plant, improving the hardiness of the plant and the fullness of flowers.

Generally, a high nitrogen fertiliser is helpful during growth stages, and a high potassium fertiliser is helpful during flowering stages.

Types of fertiliser

Slow release

Slow release fertiliser, usually in pelleted form, is best for perennial plants, or for deciduous plants only applied at the point where new growth starts to appear after dormancy. Most slow release fertiliser should only be good for about 3-6 months - this will allow the fertiliser to back off as the growth season comes to an end.

The one main problem with slow release fertiliser is that a compromise must be made, as the fertiliser may be present across both growing and flowering times. However for a time poor gardener it's a good option.

Liquid

Liquid fertiliser is often dissolved into a spray can and applied to leaves. From there it is either absorbed into the leaves or trickles down into the root system. This approach allows the best control over the types of fertiliser being applied, and also over when, where (and whether) it should be applied. Many believe it's best only to fertilise on sunny days, or directly after watering.

The other benefit to applying liquid fertiliser is that it can be blended in with other chemicals at the same time, e.g. fungicides and pesticides which must also be applied to leaves.

Some also put a very dilute solution into the irrigation system so plants benefit from fertiliser on every watering, however this can also lead to salt build up over time and does not allow fertiliser to be applied at different amounts to different plants under the same system.

Manure / Natural Solids

Most orchids will not benefit from this type of fertiliser, as it will give a large hit of nutrients then break down rather quickly. It should only be applied to large, healthy plants with extensive root systems that receive regular waterings otherwise it will not be absorbed readily enough, or may destroy a poor root system.

Teas / tonics

There are various products out there - seaweed tonics, tea extracts, plant extracts - most of these are pretty similar in that they contain tannins, which stimulate root growth, as well as a ton of other nutrients derived from the base ingredients, which supplement plant health. These tonics may not boost plant growth nor flower size, but may improve its hardiness and ability to ward off pests and poor conditions. They won't do harm, at least at the recommended concentrations.

Many growers use black tea to promote growth of new roots in plants that have lost their root system, this is a cost effective way to do the same thing as using a tonic.

Applying fertiliser

To leaves

This is the best approach for many orchids when using a liquid fertiliser. It should be done early in the day, and adequate airflow should exist to prevent water collecting on leaves, which may cause them to rot.

To roots

This approach is good for orchids with delicate leaves and also best with liquid fertiliser. A solution of the liquid fertiliser is mixed into a bucket and the plants are dunked in to the top of the pot. The only caution is that pathogens may spread from plant to plant, and the fertiliser strength needs to be weak otherwise it may damage the root system.

Alternatively, slow release fertiliser can be placed in the bottom third of the pot when repotting, to which roots will grow. If the fertiliser burns the roots, the top third will continue to grow. As time goes by the fertiliser will lose its strength and the roots will grow to the base, absorbing whatever is left.

To pots

Spreading solid or slow release fertiliser on top of the media makes it easy to see which plants have been fertilised, and the state of the fertiliser. However it may also lead to the media being oversaturated in fertiliser, leading to root damage. It should be done sparingly, and fertiliser is best place on the edge of the pot away from the plant, in a few places as possible.

Alternatively fertiliser can be mixed in with the media. Do this very carefully and do not spread too much in, following the guidelines given by the manufacturer. If the fertiliser was intended for house plants, divide by 3, for vegetables, divide by 5.

Managing fertiliser

Signs of Imbalance or Deficiency

Most orchids don't need fertiliser, but some will visibly tell you if something is missing:

Yellowing blotchy leaves may indicate a lack of minerals (iron, calcium)
Pale, whitish leaves may indicate a lack of nitrogen
Weak new growths, smaller pseudobulbs and leaves (provided other conditions have not changed) may indicate a lack of phosphorous
Flimsy stems and small flowers may indicate a lack of potassium

Most of the above can be remedied by applying a general use fertiliser, even though the individual causes may have been called out, they may be an indication that the product being used may be inadequate and should be checked. Also regarding the above points, they may indicate an imbalance - for example a high nitrogen fertiliser may be triggering plant growth, however the required components to support this growth (minerals/phosphate/potassium etc) may be missing, leading to a weak plant.

Signs of Excess Fertiliser

When too much fertiliser has been applied, it can be too late before symptoms are obvious. The things to look out for:

Browning of leaf tips and new growths (too much phosphorous)
Spindly, long, weak growths (too much nitrogen)
Burned roots (too much of everything)

When there is excessive damage, best thing to do is repot. If damage is moderate, a good 10 minute soaking in lukewarm water, flushed and repeated 2-3 times could reduce salts in the media.