This is part of a series intended to introduce the principles of basic orchid culture.  The series originates on a number of articles I have written over a number of years.  I hope that it will answer a number of questions often raised on these matters.


One very important element of orchid culture concerns the correct feeding of the plants. One reads numerous recommendations and suggestions, and it can be difficult to identify the programme best suited to the plants growing under your system of culture. An appreciation of what fertiliser are and the basic principles of their use can assist in the selection of the most efficient and economical programme. Before looking at fertiliser programs, some background to their use will be of interest and assistance.


Did you know that the average orchid plant comprises mostly water? Different parts of the plant will contain more or less than this, which will be fairly obvious if one looks at a plant. The leaves, for example, contain around 93-95% water, the stem 85% and the flower spike 80%. Obviously some plants are more lush than others, but it is often surprising that the tissues contain such proportions of water.

If one removes all this liquid, one is left with a small quantity of material – the dry matter. This material can be analysed to ascertain what mineral elements or salts the plant has utilised in its growth. Such analysis can be useful to investigate the nutrition of a plant, and can assist in the correct formulation of fertilisers

A typical analysis of a plant will reveal the following. If one has a specimen weighting say one kilogram, 900 grams or 90% of its weight will be water. The 10% dry matter (100 grams in a 1 kg plant) will contain the following. The figures show not only the elements as a percentage of the dry matter weight, but also, in the example of a 1 kg plant, the weight of each element present in grams. (The information represents generalised figures only)



Oxygen 44.5
Carbon 43.5
Hydrogen 6.0 sub total 94%
Nitrogen 1.5
Phosphate .2
Potash 1.0
Sulphur .15
Calcium .2
Magnesium 1.5 =  sub total 3.2%
Manganese .04
Boron trace
Copper trace
Iron .08
Zinc trace
Molybdenum trace
Chlorine .15
Aluminium .1
Silicon 1.2 = 2.8% sub total
TOTAL 100%

Of the total analysis 94 percent of the dry matter is made up of only three elements – carbon, hydrogen and oxygen.

The balance of the dry matter contains a number of elements, some of which are essential for plant growth to take place, and some which are not required at all. Of the elements in the above list, the last two – aluminium and silicon – serve no purpose, and have, with some other unlisted elements, been picked up accidentally. If these are absent there will be no effect on plant growth and development.The thirteen other elements, which are essential for growth, fall into two main groupings. Seven elements are present in the plant and are required from the environment in minute quantities, and are known collectively as the ‘trace elements’. These are often measured in parts per million, or may only be seen as traces, but are essential for plant growth to take place. They are manganese, boron, copper, iron, zinc, molybdenum, and chlorine. These elements are generally naturally available in the environment except in exceptional circumstances, or as impurities in applied fertilisers, and therefore generally growers do not have to make special efforts to meet the plants requirements for them.

Six elements shown above have not so far been discussed, and, with carbon, hydrogen and oxygen, comprise the group of nine ‘major nutrients’. The six others are nitrogen, phosphorus, potash, sulphur, calcium and magnesium. Together they make up to 3.5% of the dry matter. The names of these elements will be recognised as being the ones most feriliisers aim to supply, and they are most likely to be in short supply in the environment. Other elements can affect plant growth, but do not have the influence and importance of the above in allowing the plant to grow and mature.


All living things need to obtain materials and energy from their environment to enable them to grow and function. Complex chemistry is involved, but it has been shown that the processes require certain basic chemical elements to be present to enable the plant to complete its vegetative and reproductive life cycles. Should any of the elements known to be essential not be available, aspects of the plants growth will be inhibited, and it may even die.

The atmosphere and water supply carbon, hydrogen and oxygen, and normally are readily available in the quantities required by the plant. The other elements are contained in the soil or in solution in the water. Fertilisers are involved with the supply of those elements present in the environment in insufficient quantities for the plants requirements, or ensuring the materials are present in a form that enables to be easily utilised by the plant in the completion of its normal growth cycles.

If any of the sixteen essential elements are in short supply, plant growth will be reduced. Plant growth is regulated by the element present in the minimum amount in relation to the plants requirements for each element. Growth rises or falls depending on whether the supply of that growth limiting element is increased or decreased. If, for example, two nutrients are responsible for limiting plant growth, adding only one of them will have little effect. Supplying both will lead to the plant being able to increase its growth substantially, and achieve its full potential as allowed by its genetic makeup and the environment, provided no other limiting aspects are present.

In addition to an adequate supply of the elements required, for growth to take place, the following factors must also be present –

  • Suitable temperature levels
  • Adequate moisture
  • Adequate light levels
  • Adequate air supplies

Even if the mineral levels are satisfactory, growth can still be reduced or affected if all the above environmental factors are not also available in satisfactory amounts. Thus the importance of maintaining ALL cultural elements at optimum levels cannot be over emphasised if maximum results are to be achieved.

While adequate mineral levels are important, the presence of some in too great a concentration can be dangerous, and can poison the plant. Over fertilisation can also lead to weak growth that can be susceptible to disease.

A further factor related to the availability of nutrients and plant growth concerns the pH of the growing media and water, i.e. the degree to which it is either acid or alkaline. Generally orchids appear to appreciate a slightly acid medium in which to grow, a pH of between 5.2 to 6.5 generally being quoted in literature. Occasionally an unsatisfactory pH level may be encountered, and this must be adjusted. One of the most commonly experienced situations where pH correction will be necessary is where pine park is utilised in potting mixes. This material in its natural state has a high level of acidity, and treatment is necessary to make to suitable for orchid growing.


In their natural habitat, the orchid plant receives the necessary nutrients by way of the air and rainfall. The plants often grow in accumulated humus and the natural breakdown of this also supplies much of the plants needs. Bird droppings washed down from higher up also provide additional material.

Some of the older books on orchid culture indicate that the application of fertiliser is not necessary; some even suggest that those who apply fertilisers to their plants are cheating in their culture! However, most authorities now agree that some supplementary feeding is desirable in order to achieve vigorous healthy growth with maximum flowering, although other environmental factors must be appropriate for the plants potential to be achieved.

Early growers utilised soot and animal manure in weak solutions, thrown on to the floor of the glasshouse, relying on the air to carry nutrients to the plants. Obviously this system gave only a minimal benefit to the plants. More adventurous growers took containers of cow mature, or soot or similar products, and soaked this in a larger container of water. When a solution the colour of weak tea was obtained, this was applied direct to the plants, which gave improved results. With the 4resultant increased growth and improved flowering, growers gained confidence with the use of supplementary feeding, leading to experimentation with a wide range of material available. While many natural animal and plant produces continue to be utilised, growers can now select from a bewildering range of alternative products.

The fertilisers available to the grower fall into two broad categories, organic and inorganic products. Organic fertilisers are obtained from plant or animal sources, and include the farmyard animal manures noted above, and also the liquid fish and seaweed solutions, blood and bond, hoof and horn, and similar products. Generally these are comparatively safe materials to use as they supply the elements in relatively dilute concentrations. Thee materials generally require bacterial action to release the elements they contain. The temperatures influence this activity, and therefore their use during the warmer summer months is appropriate. If temperatures are cold, there will be little benefit from their use.

The inorganic fertilisers comprise rocks and minerals containing the required elements, which may or may not have been subsequently refined and manufactured into solid or liquid preparations now so commonly seen. These products are usually scientifically formulated, often for a specific crop or cultural situation, and can be very concentrated when purchased. Generally, greater care must be exercised in their use. The recommended rates of dilution must be carefully followed as they can easily be made up into working strength solutions which can not only harm the plants, but which could even kill them. Because of the way they are formulated, however, they often allow more precise feeding of particular elements, which is often of particular importance to commercial growers, or those wishing to indulge in cultural experimentation.

It will be obvious that different fertilisers can supply different elements apart from water, which makes up around 90% of the weight of the average orchid plant. Carbon, hydrogen, and oxygen comprise the main mineral elements in the plant, but as there are easily available from the environment, it is unnecessary to supply them in fertilisers. Of the other thirteen elements proved to be essential for growth, those required in the greatest quantities, and most likely to be in short supply, are nitrogen, phosphorus, and potassium, and it is these elements most fertilisers aim to supply. Generally the fertiliser containing these materials contain the other minerals required as well, and apart form unusual situations, so specific allowances for the other elements is usually necessary.

In considering individual fertilisers, therefore, the levels of nitrogen (element symbol N), phosphorus (P) and potassium (K) are important. The concentrations of these elements in a particular formulation is usually expressed as a percentage the “N:P:K” or “elemental analysis”. Most commercially available produces will now list the components on the container, as throughout the world this is becoming a universal legal requirement. Obviously the information enables the most effective product to be selected for the purpose required. A list of many fertilisers appears elsewhere. Commercial products have not been listed as availability varies with the locality, and you need to check you local garden centre etc for what is available. Commercial products can change their formulations at any time, so a quick check of the analysis should always be made when replacement supplies are obtained.

For effective comparison of products to be valid, the form of mineral elements in a fertiliser should be analysed on a standard basis. It is now accepted that the minerals in their elemental form should be specified. Unfortunately, by historical convention and by practice in some countries, other chemical forms of the minerals will sometimes be quoted. In some published or advertised elemental ratings, the analytical basis is not specified, and therefore misleading comparisons are possible, although with greater consumer awareness this is becoming increasingly uncommon. Products now will generally show the elemental form, or the oxides of the mineral concerned. In this later cases, phosphorus will be shown as phosphorus pentoxide (P2O5), and potassium as potassium oxide (K2O). If these are shown, the accepted element percentages can be calculated by applying the factors of .44 for P2O5 to obtain the phosphorus level, and .83 for K2O to obtain the potassium level.

In some N:P:K analysis, further figures are included of the sulphur (S) and magnesium (Mg) levels, and perhaps for calcium (Ca). These, and other elements, will, however, be usually clearly identified in the data.

There is one further matter regarding the elements. While the elemental form of the chemicals are listed, it is to be noted that not all of the material may be available to the plant, and it is the ‘free’ or ‘plant available’ content that is important. The actual percentage of plant available elements will vary with the various products, but for superphosphate for example containing 9-10% total P, only 7-8% of the phosphate is available to the plant; some 80 to 90% of the actual P it contains. This is of little importance to amateur growers, but can be significant for commercial operations. The pH of the material can also affect fertiliser uptake of both major and minor elements, but this is normally not of importance in horticulture, although if extremes occur, problems may arise,


As has already been shown, nitrogen, phosphorus and potassium are most likely be be the elements in short supply. As the plant needs vary during the year, depending of the stage of its growth, fertiliser needs also vary. During periods of rapid growth, the plant can accept and use larger amounts of fertiliser, but less frequent and more dilute applications are appropriate when growth is slower such as during the winter months. If a plant goes through a period of natural dormancy, there is no point of applying fertiliser, as it cannot utilise it.

NITROGEN is essential as a component of proteins and chlorophyll, and is required for strong vegetative growth. When deficiencies of this element occur, plants are stunted and many mature too early. Yellowing of the leaves takes place, and they eventually fall off. Too great a supply can lead to excessive vegetative growth and retard flowering.

PHOSPHORUS is primarily involved in energy transfer and as a regulator of vital activity. A deficiency leads to stunting, but the leaves, instead of tuning yellow, become dark green.

POTASSIUM is required for strong growth, a deficiency leads to dwarfness, with the leaves frequently scorched and dead.

The deficiency symptoms noted are general indicators only, and other cultural practices can in certain circumstances produce similar signs in plants.

With respect to the N:P:K analysis, there are two aspects relevant to the figures which are important. In the selection of a fertiliser, the ratio between the elements in the particular product must be considered, as well as the particular concentration of elements that exists.

In the selection of a fertiliser, the absolute N:P:K figures are unimportant – it is the ratio between the elements that is significant. A 30:10:10 and a 3:1:1 fertiliser is identical in the balance between the elements, one is just stronger than the other. If you are considering a fertiliser for a specific purpose, then always look at the basic ratio between the particular elements. During periods of maximum vegetative growth a 3:1:1 ratio is traditionally recommended, although there is some scientific evidence to suggest that with plants growing in bark and peat mixes, a higher nitrogen level may be beneficial (even up to a 6:1:1 ratio). It should be noted that sometimes the ratios are quoted on the basis of the oxide forms of the elements, which will result in misleading interpretation if the previously conversions are not applied to the figures to bring them to the comparable elemental figures.

The second consideration involves the concentration of the elements in the fertiliser, indicated by a analysis figures which show the percentage of the element contained. It will be obvious that a 30:10:10 fertiliser is 10 times stronger than a 3:1:1 product. To give the same final concentration if the applied solution, only one tenth of the stronger produce would be required in this example. Both these products, however, supply the elements at the same ratio.

Knowing the concentration of the elements in a product also allows cost comparisons to be made between similar products. While this will not be of interest to amateur growers, commercial produces find this of considerable importance. Strictly speaking all analysis should be on the basis of the percentage of nutrients actually available to the plants, and not the total figures, but unfortunately this information is often not readily available.

In any fertilising programme, it is the concentration of the element in the final solution applied to the plant which is of critical importance. The figures shown in the N:P:K analysis allows this to be calculated. Often, the solution concentration desired will be published, usually shown in ‘parts per million’ (ppm). A 5:1.5:3 ratio product used in some NZ research was designed to provide an applied solution of 125-175 ppm N; 30=40 ppm P:70-80 ppm K. The dilution rates to achieve the desired working concentrations of the applied fertiliser can be calculated as follows. If we have one percent of an element in a fertiliser, and use it at the rate of 1 gram (ml) per liter (or expressed as 1 gram per 1000 grams) the dilution rate is 1 divided by 1000 = 0.001% or 10 ppm. A 0.01% solution represents 100 ppm, and a 0.1% solution 1000 ppm. Dividing the percentage of the element in a fertiliser by 1000 gives the percentage of that element in a 1 litre solution. In the above example, suing a 10:3:6 fertiliser, taking the predominant element (n = 10%) the dilution is calculated by dividing 10 by 1000 = 0.01% or 100 ppm. To give a 150 ppm final solution, you therefore have to sue 1.5 grams (mils) per litre (15 grams/mils per 10 litres) of that 10:3:6 fertiliser. If you choose the correct ratio fertiliser to start with and calculate the dilution for the major component, the other elements will also be at the desired dilution. For practical use, remember an average heaped teaspoon holds 8 grams and a level teaspoon 5 grams, so in the above example two heaped teaspoons full per 10 litres of water would be satisfactory. If the N level in the fertiliser was double the above at 20:6:12 you would only need to use half the quantity to achieve the same working solution . i.e. 20 divided by 1000 = 0.02% = 200 ppm = need 7.5 grams/ml per litre to give a 150 ppm final solution = 1 heaped teaspoon (8 grams/mls) per 10 litres of water.

With certain plants, and at different times of the year, varying fertiliser application are appropriate. During the spring when maximum vegetative growth is occurring, a high nitrogen ration product is required (3 to 6:1:2). Later in the season, during the period when flower bud initiation can be expected, and during actual flowering, the level of nitrogen should be reduced, and increased phosphorus, and to a lesser extent, increased potassium given i.e. a 1:2:1 or 1:2:1.5 ratio fertiliser used.

As soon as the potting mix is watered, bacterial and fungal action commences to break down the organic material, which it itself releases nutrients which become available to the plant. The organisms involved, however, require nitrogen to complete this process. Especially where pine bark is utilised, but to a lesser extent with the other organically based mixes, the breakdown organisms can use up virtually all the available nitrogen, leaving very little available for the plant. Most authorities, therefore, suggest that for bark based mixes, higher nitrogen fertilisers should be used, whereas for plants growing in other media a more balanced fertiliser may be appropriate.


The following is an indicative list of some fertilisers and their typical elemental analysis. There can be variation even between the same products, so the particular product should always be checked. The organically derived products in particular can show significant variation between samples. The sulphur and magnesium content is also listed for some products where this information is available. The formulations of commercially manufactured products can also be changed without warning so some caution must always be exercised when making purchasers.

  N P K S Mg
Animal manures          
Cattle . 0.6 0.1 0.5    
Horse 0.7 0.1 0.6    
Sheep 1.4 0.2 1    
Chicken 1.6 0.4 0.4    
Blood and bone 5-8 6-9 0    
Meat and bone 0 5-8 6-9 0    
Hoof and horn 5-8 6-9 0    
Bone dust 5-6 8-12 0    
Dried blood 10-14 0 0    
Fish fertiliser liquid 5 1 1    
Seaweed extract 5 5 6    
Wood ash 0.1 0.3 1    
Ammonium sulphate 21 10 10    
Basic slag 0 8 0 0.1 1.5
Calcium amonium nitrate 26 0 0    
Dolomite         11
Epsom salts (Mg sulphate)         10
Magnesite         26
Potassic superphosphate 15% 0 7 7 9 0.1
Potassic superphosphate 50% 0 4 25 5 0.1
Potassium chloride (syn Muriate of Potash) 0 0 48    
Potassium sulphate 0 0 40    
Serpentine superphosphate 0 7 0.1 8 0.1
Superphosphate 0 9 0.2 11 0.2
Urea 46 0 0    
Ammophos 12 10 10    
Lush 17 8 15 0 1
Magamp 7 17 5    
Osmocote flower formula 18 1.8 8    
Osmocote house plant formula 18 2.1 8    

There are many commercial products currently available. Spend some time at you local garden centre and look at what is currently available. Talk to other growers to see what they are currently using. Some commercial orchid nurseries package up in small quantities the products they are using for amateur use.

Note that some seaweed and fish products contain natural growth hormones which supplement their nutrient supply, in some cases boosting their growth effects.


When fertilisers are applied, the elements they contain become available to the plant. However, certain changes to the material can take place, and they can be converted to a form that makes them unavailable. Over a period of time increased concentrations of the salts can build up, sometimes to a level where the health of the plant can be affected. It is essential that the pots be thoroughly flushed out with clean water every month or so, especially if a regime of heavy fertiliser application has been followed, to ensure this does not occur. Plants grown on slabs, where high evaporation occurs, leaving a buildup of salts, should be regularly leached with plenty of fresh water.

Because of the porous nature of most potting mixes, and the relatively large quantities of water used during the watering process, significant amounts of fertiliser will be lost in the water that drains away. Fairly frequent fertiliser application are therefore generally necessary. Some products are now available from which the elements are released over a period of time, one application providing benefit form some months. Such fertilisers obviously have many practical advantages, although the reliability of the element release mechanisms should always be looked at. Some of these products can release the elements faster then desired, which can damage the plants. Some products release the elements at higher temperatures, and their use in a glasshouse requires caution.

Most of the beneficial elements contained in a fertiliser are absorbed by the plant in solution via the roots. Some, however, can be absorbed by the leaves. For this reason several fertilisers can be easily utilised; a top dressing of tome type, with another used in dilute form sprayed on to the foliage. The use of more than one type of fertiliser is often advantageous. While the aim is to supply nitrogen, phosphorus and potassium the other essential elements are usually contained as impurities of the products containing these three elements, but in varying concentrations. By utilising more than one product we can be more certain that adequate supplies of the other elements are also being made available.

Plants will only absorb the elements they require, in the quantity dictated by their growth. As they cannot store appreciate quantities for future use, a continuous supply of nutrients is necessary. If a large quantity of fertiliser is applied at one time, much of the benefit can be lost by the material being leached or of the area penetrated by the roots before the plants can take advantage of it. For the reason it is better to supply a little fertiliser often. This will generally ensure the most economical use of the material, and can also reduce the risk of damaging sensitive parts of the plants such as the young roots by concentrations being too strong.

All fertilisers can harm the plant if applied too strongly, and care must be taken when solutions are being prepared. It is always safer to err on the weak side but apply more often. With dry materials do not allow the fertiliser to come into actual contact with any part of the plant, but apply around the edge of the pot, allowing the water to carry the nutrients to the roots. With all fertiliser, but especially with some inorganic manufactured products, the level of dilution must be accurately assessed. Usually strengths of 25 to 50% of the manufactures general recommendation regarding rates of dilution would initially be utilised on orchids, until experience has been gained with the product at least over some months.

Some of the suggested regimes suggest heavy applications of fertilisers. These programs are based on the principle of maintaining the level of moisture in the potting mix at the highest degree possible, at the maximum water holding capacity. At this level, with high levels of fertiliser, the concentration of fertiliser in the water will be at acceptable levels. If, however, the mix is allowed to dry out, with the water level appreciably less than the maximum water holding capacity, the concentration of fertiliser salts in the remaining water rises considerably, rapidly reaching concentrations which can damage the roots etc. If you halve the water in the mix, the fertiliser concentration doubles, halve again the concentration increases four times and so on. While heavy application of fertiliser will increase growth, where this is followed, special precautions and care are required to prevent the mix drying out, such approaches as capillary watering mats being utilised for this purpose.

Fertilisers may be applied as a dressing to the top of the pot, mixed into the potting media, or as a liquid. Watering and the application of a liquid fertiliser can often be combined. The use of proportioners on the watering hose can also prove effective for liquid preparations.

Before a new product is tried it is advisable to check if other growers have used it. If no information is available on the use of the material on orchids, try on a few plants only first to ascertain its performance.

When a fertiliser is applied in liquid form, certain precautions should be taken because of the particular nature of many orchid potting mixes. Some of these common mixes, especially those contain ping bark, can be very difficult to wet because of the surface tension. Such materials should be thoroughly wetted with plain water first. After leaving for some minutes and ensuring it is thoroughly dampened, the fertiliser solution can be applied. This helps to reduce the buildup of salts and ensures more economical use of the fertiliser.



Site established 9th May 1998
This page first uploaded 15 August 1998