Supplying nutrients to plants can be done in several different ways. Some nursery producers try to supply all nutrients through the base dressing in the potting mix, others may not incorporate anything into the potting mix but supply all the nutrition through irrigation. In general, a combination of the two provides the best result.
Relying totally on base dressings to supply nutrients can be risky. Sudden hot spells can cause high quantities of fertiliser salts to be released into the mix, quickly killing or burning large numbers of plants.
Using liquid fertilisers alone can result in considerable nutrient waste because irrigation systems are seldom perfect. Nutrient solution is lost between pots and onto pathways in addition to losses from leaching through potting media.
Concerns about contamination of groundwater mean that nutrient leaching is no longer acceptable. For example in Europe, legislation now prohibits the leaching of any nutrient-laden water into the groundwater or general drainage system. This has meant that European nurseries are now often totally enclosed, frequently with concrete, so that all runoff may be captured and recycled.
Such legislation is not yet in place in Australia but unless nurseries and other horticultural enterprises can show they use fertiliser and water efficiently, the possibility for such severe measures to be implemented is real.
The principal advantage of liquid feeding is its flexibility. Fertiliser regimes can be changed from week to week if desired. Once a system is set up, the labour required is low.
Try and aim for at least 50% of the nutrients to be supplied through the base dressing and the balance through liquid feeding.
How often to apply liquid fertiliser?
The frequency of application determines the strength of the fertiliser to be used. For weekly feeding the optimum concentration of nitrogen is about 200-250ppm. For continuous liquid feeding, 50-100ppm is sufficient. It may even be possible to fertilise with a 500ppm solution of nitrogen on a fortnightly basis provided the mix has a reasonable ability to retain nutrients.
Salinity problems may occur when feeding stronger solutions on an irregular basis. In many wood-based mixes, applying fertiliser on a little and often basis is safer and will probably give better results. The form of nitrogen is also important. A mixture of both nitrate and ammonium forms is better than one source alone.
It is usual to supply potassium (K) at about 80% of the rate of nitrogen (N). However some nursery plants such as palms and other foliage crops may benefit from a higher N:K ratio. Excess nitrogen results in soft, lush vegetation which can be susceptible to diseases such as powdery mildew or inviting to insect pests. During winter it may be desirable to reduce the N:K ratio to harden the plant.
In a constant liquid feeding program, phosphorus may be supplied at the rate of 10ppm. On an intermittent basis, when nitrogen is being supplied at about 200ppm, phosphorus at 30ppm is optimal.
Calcium and magnesium
Calcium requirements are generally supplied by the lime in a potting mix. When there is suspected calcium deficiency it is often the result of impaired uptake rather than actual deficiency. Supplementary calcium is often supplied in a foliar spray of calcium nitrate.
Magnesium is usually supplied in sufficient quantities by the dolomite (if used) in a potting mix. It may be supplied in liquid feeds at the rate of 20-30ppm.
Occasionally it may be necessary to include trace elements in a liquid feed. Frequently the easiest way to do this is to use a proprietary mix such as Librel-BMX® , Hi-Trace® or Fetrilon-Combi®. Alternatively, use the following elements at the rates listed in Table 1.
Iron chelate (13% iron)
|Manganese sulphate (25.5% manganese)|| |
|Copper sulphate (25% copper)|| |
Zinc sulphate (36% zinc)
|Borax (11% boron)|| |
|Sodium molybdate|| |
The rates in Table 1 are relatively safe to use on a long-term basis. Higher rates may be used with caution on an intermittent basis. Consult your agronomic adviser. Many companies also produce individual trace elements as solutions which can be easier to use if accurate scales are not available.
Topdressing with dry fertilisers
The availability of slow release fertilisers has overcome the main disadvantage of topdressing which is the labour cost of application. However, care is still required both to apply the fertilisers evenly and in the correct place.
Ideally, fertilisers should be placed below the soil surface to minimise fluctuations in moisture and temperature which can upset the pattern of nutrient release. Special tools are available for this purpose.
If fertiliser is being applied to the surface of the pot, take care with plants which are vase-shaped or have their foliage in whorls, as granules can collect in the centre of the plant (lodging) causing severe burning. It is always wise to irrigate after the initial application of fertiliser.
Topdressing rates may be calculated by taking the suggested fertiliser rate per cubic metre and applying it according to the volume of potting mix held by the pot.
Foliar feeding is no substitute for the practices described above. It should only be used as a means of overcoming transient nutrient deficiency problems due to sudden climatic changes or growth spurts. To use foliar feeding as the only means of supplying nutrients would be time-consuming and expensive. Large amounts of fertiliser are also wasted.
When applying nutrients to the leaves, care is needed to apply the correct form of the element for optimum absorption by the plant. A wetting agent should be used. With iron, zinc or manganese sulphate the addition of 0.5‑1.0% urea aids plant uptake. Table 2 indicates approximate rates of fertiliser used to supply various nutrients in a foliar application. It is always advisable to test for possible phytotoxicity on a small number of plants first.
|Element||Fertiliser||Spray strength |
10g/L is the usual rate. Use the higher rate for plants with waxy leaves
|Potassium||Potassium nitrate||15-45|| |
|Calcium||Calcium nitrate||5-15||Can be phytotoxic at higher rates|
The most cost-effective way to supply copper
Iron chelate (EDTA)
Can stain leaves
Poor application can result in green spots on the leaves
|Zinc||Zinc sulphate||2-10|| |
|Boron||Boric acid||2|| |
|Molybdenum||Sodium molybdate||0.25|| |
Chelates versus sulphates
Chelates are available for almost every element. Chelates are often said to be capable of inducing growth beyond that expected from their elemental content alone. They are certainly useful in certain soils, for example, iron chelates in alkaline situations. However excessive use of chelates, particularly of one element, can induce problems in the same manner as other forms of the nutrient. For example, large quantities of iron chelate can interfere with the uptake of manganese.
With foliar applications, the uptake of the chelate form by leaves is undoubtedly better than the uptake of the sulphate form. In the case of iron there may be a three-fold benefit to using the chelated form.
For copper, the chelate is taken up twice as efficiently as the sulphate form. However, when cost comparisons are made, per unit of element absorbed, the sulphate forms are cheaper, as shown in Table 3.
|Fertiliser||Cost||Cost per unit of element|
Iron sulphate (heptahydrate)
Iron chelate (EDTA)