3. Plant Nutrition
|Observation of plant growth in various parts of the
garden is one way of assessing your soil. More exactly, plant nutrient
content of the soil can be determined through chemical analysis, but this
is only one of the factors that bears directly on fertility. Soil acidity,
organic content, and drainage are also items which must be taken into
Hibiscus need a neutral to slightly acid soil and there are kits available for testing this. These are not expensive and are a great advantage to the home gardener, particularly when a complex range of plants are grown. A soil test has practical value for the hibiscus grower; it enables him to garden more scientifically. The soil test recommendations enable him to make appropriate choices in purchasing fertiliser and other needed materials. This results in the best use of the time and physical effort available, and safeguards the investment in fine hibiscus plants. A soil test is the first step toward soil improvement and better hibiscus. Don't sample unusual areas such as wet spots, burn piles, recently fertilised areas or parts of the ground saturated with pesticides, as they will give an incorrect reading.
What does pH mean? It is a scale expressing the acidity or alkalinity of something, in particular referring to soil or a solution of blood or body fluid etc. Acidity is a measure of hydrogen ion concentration, while alkalinity is a measure of hydroxyl ion concentration. The pH of the soil depends on the chemical composition of its ingredients, such as the mineral content of rock and the origin of the decomposed organic material present.
The pH value of the soil is an extremely important property as it controls root activity, fertiliser availability and the production of plant poisoning salts of iron and aluminium. The correct pH depends on the type of plants you intend to grow in your garden. Do they like acid soil? Or do they like the alkaline (limey) soil.
The pH scale is a set of numbers from 0 to 14, with zero (0) being
extreme acid conditions. The middle point of pH 7 is a neutral state which
is neither acid nor alkaline, and pH 14 represents extreme alkalinity. So
it can be seen that below pH 7 acidity increases with decreasing pH
number, while above pH 7 alkalinity increases with the pH number.
Recommended pH for growing hibiscus is about 6 to 7.
If the soil test shows that the pH is 7.5 or more (alkaline), use
sulphur, iron sulphate, manganese sulphate or aluminium sulphate to lower
it. If your soil is too acid (less than 5.5), it may be rendered less acid
or even alkaline by the application of agricultural lime or dolomite once
or twice a year. Specific advice on these matters should be secured from
your local garden centre or Department of Agriculture or Primary
The Essential Plant Nutrients
All plants need at least sixteen elements in order to grow and bloom normally. Three of these essential elements, carbon (C), hydrogen (H), and oxygen (O) are obtained by plants from the air and water. Of the remaining thirteen essential plant nutrients, six are required by plants in relatively large quantities and are called major elements, while the other seven are needed in much smaller quantities and are called minor elements, or trace elements. The major elements are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S). The minor elements are manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo) and chlorine (C1).
The various nutrients play different roles in the growth and
development of plants. Here are what some of the principal plant foods
A deficiency of any one or more of the sixteen essential elements will cause a plant to show symptoms of abnormal growth, such as yellowish leaves (chlorosis), die back of twigs or dead areas in the leaves (necrosis), reduced leaf size and distorted growth. Although there are different and distinct deficiency symptoms for each of the essential elements, the amateur gardener will often find it difficult to distinguish or separate one deficiency from another. Many of the deficiencies produce symptoms which appear similar to the untrained observer, and different plants with the very same deficiency will show slightly different symptoms. Learning to recognise each of the more commonly seen deficiencies takes practice and experience and familiarity with the plants being grown. Most of the deficiencies of major elements show up on the old or lower leaves of a plant, while minor element deficiencies are observed in the newer leaves towards the tips of twigs. If a complete fertiliser is applied regularly deficiencies of nitrogen, phosphorus and potassium will be uncommon.
Here are the most commonly seen deficiencies of hibiscus:
A fertiliser is a substance used for the purpose of supplying one or more of the elements essential for normal plant growth and development. When you buy a fertiliser the analysis on the label tells you the percentage by weight of nitrogen, phosphorus and potash (potassium carbonate). This analysis appears as three numbers, for example 6-6-6. The first number indicates the percentage of nitrogen (N), the second number gives the percentage of phosphorus (P), and the third number indicates the percentage of potassium (K).
A complete fertiliser contains nitrogen, phosphorus, and potash, while an incomplete fertiliser contains one or two of these nutrients but not all three. Note that the term complete as applied to fertiliser does not mean that a product contains all the essential major and minor elements, but only that all three primary plant nutrients are present. A fertiliser may be complete but still be lacking other important nutrients, such as magnesium, iron, or zinc.
A balanced fertiliser, as the term is used here, contains nitrogen, phosphorus and potash, with nitrogen and potash present in about the same percentages, while the percentage of phosphorus may be lower. Thus a 6-6-6 or 7-2-7 fertiliser would be considered a complete balanced fertiliser. (Some people refer to a fertiliser as balanced only when all three primary nutrients are present in equal amounts.)
Fertiliser ratio refers to the relative amounts of nitrogen, phosphorus, and potash contained in the fertiliser. A 4-6-8 fertiliser contains 1½ times as much phosphorus as nitrogen and twice as much potash as nitrogen, and it could therefore be called a 1 - 1½ - 2 ratio fertiliser. Any other 1 - 1½ - 2 ratio fertiliser such as 6-9-12 would provide the same quantities of the three primary nutrients if the rate of application was adjusted properly. For example, 2 cups of 6-9-12 would provide the same amount of N, P, and K as three cups of 4-6-8. Thus the 6-9-12 fertiliser is essentially the same as the 4-6-8, all other factors being equal, but each fertiliser would be used at a different rate.
The fertiliser label contains valuable information. Unfortunately, the home gardener usually pays little attention to it. One should make it a practice to read the label when one shops for fertiliser. You can select the fertiliser which is best for your plants only if you understand the information on the tag.
Sources of nitrogen The fertiliser label lists four forms of nitrogen which together make up the total nitrogen percentage in the fertiliser; nitrate nitrogen, ammoniacal nitrogen, water soluble organic nitrogen, and water insoluble nitrogen.
Plants can absorb nitrogen in the nitrate and ammoniacal forms only. Nitrate nitrogen is easily leached out of the root zone of plants by heavy rains. Ammoniacal nitrogen is more resistant to leaching than nitrate nitrogen, but it is converted to the nitrate form fairly rapidly, in a period of two to four weeks, by bacteria in the soil. Ammoniacal nitrogen is preferable to nitrate nitrogen for hibiscus because of its greater resistance to leaching and because it seems to benefit flowering.
Water soluble organic nitrogen changes to ammoniacal nitrogen within a few days after its application to the soil. For this reason, the water soluble organic nitrogen reported on the fertiliser label should be considered as the equivalent of ammoniacal nitrogen. The principal source of water soluble organic nitrogen is urea. Urea is not a natural organic material but is synthetically made.
Water insoluble nitrogen comes almost entirely from natural organic sources, such as seed meals, sewage sludge, and tankages. It must be converted to ammoniacal nitrogen by soil organisms before it can be used by plants, and the conversion takes place gradually. Because of its slow release properties and resistance to leaching, some water insoluble nitrogen is desirable in the fertiliser, even though this form of nitrogen is more expensive than the others.
Phosphorus Most plants, including hibiscus, do not require nearly as much phosphorus as nitrogen or potassium. Phosphorus applied to the soil will not leach out and remains available for a long period. Too much phosphorus especially on alkaline soils, will tie up or render unavailable most minor elements, especially iron. Once this situation occurs it can take as long as twelve months to correct or lower the phosphorus level in the soil. For this reason, a fertiliser formula relatively low in phosphorus such as 7-2-7 is desirable.
Potassium Potassium is especially important for the production of good hibiscus blooms. Muriate of potash (KCl), also known as potassium chloride, is the cheapest source of potassium and is therefore frequently found in fertiliser, but it is undesirable because of its high chloride level. Sulphate of potash or sulphate of potash magnesium are preferable sources of potassium.
Sulphur There is usually enough sulphur present in the fertiliser and water which plants receive to satisfy their requirements for this major element. The sulphur may not be listed on the fertiliser label but it is supplied by many fertiliser materials, including sulphate of ammonia, sulphate of potash, etc. Sulphur may be listed as a secondary plant food if it is added in order to make an acid forming fertiliser, but even if sulphur is not mentioned you do not have to worry about it.
Calcium If the pH of the soil is kept in the proper range, there should be enough calcium present for normal plant growth. If agricultural lime or dolomite is applied as needed to keep the pH of the soil above 5.5 you will not have to worry about calcium. Dolomite also contains magnesium which is desirable.
Leaf Sign Language
Iron is one of the most abundant materials found in the soil, but only a small amount of this is present in a form available to plants. Chlorophyll, the green colouring matter of plants, requires iron for its formation, so iron deficiency quickly shows up in chlorosis or yellowing of the leaves due to the absence of chlorophyll. Now this does not mean there is no iron in the soil. There is probably ample for the plant's needs, but excess lime has increased the pH until insoluble iron hydroxide forms, which is not available to the plant. Once soil pH is lowered by adding sulphur, the iron becomes soluble and available again.
Like other plants hibiscus need trace elements, but how do we know if
our soil is deficient in them? This can be observed in the growth of
indicator plants. The best indicator plant for boron deficiency is
beetroot. These will have split roots and some phosphorus which die off
even though they have brightly coloured leaves. Citrus trees are also good
guides, magnesium as without boron the fruit are dry and juiceless. To
correct this soil deficiency of boron (assuming that potassium it hasn't
an alkaline reaction) apply borax to the surface of the soil at the rate
of 40 g to 10 m²
(1 oz. to 10 sq. yards). The borax may be mixed with soil manganese for
ease of application, or spray the foliage with a borax solution, 30 g to 7
L (1 ounce to 1½
gal) of water. Care must be taken in the quantity of borax used. Use only
the quantities indicated, otherwise
If a magnesium deficiency occurs, magnesium sulphate (epsom salts)
should be applied. In the case of severe deficiencies, soil applications
of magnesium will often have no immediate effect. Spraying the foliage
with a solution of 60 g per 4.5
Molybdenum deficiency is not uncommon in hibiscus and is commonly called `strap leaf. Affected leaves, though dark green in colour, are stunted and constricted laterally and veins are prominent and distorted. The flowers of affected plants do not open properly, and the petals are inclined to fuse together. Affected plants should be sprayed with a solution of ammonium or sodium molybdate at the strength of 30 g to 27 L (1 oz to 6 gal) of water, thoroughly wetting the leaves.
Copper and zinc are also important trace elements. The specific effect of copper in plant metabolism is not known definitely, but it has been demonstrated as being present in certain proteins having the character of enzymes. These enzymes aid certain chemical and growth processes. Copper does seem to promote the formation of Vitamin A. Copper deficiency causes leaves to become yellow and fall off and twigs to die back from the tips. Here again citrus trees are a good indication; if the above symptoms occur then your soil is generally copper deficient. Bluestone (copper sulphate) may be applied lightly to correct copper deficiencies.
Zinc deficiencies cause leaves to be small and bunched together in the
form of a rosette and plants may be dwarfed. To overcome the zinc
deficiency plants should be sprayed with a solution of zinc sulphate and
hydrated lime, 45 g (1½
oz) zinc sulphate plus 20 g (¾
oz) hydrated lime per 4.5 L (1 gal) of water. This spray is best applied
in the spring or early summer. Hibiscus are not usually deficient in zinc
if the year's culture has been carried out.
The simplest approach to hibiscus fertilising, obviously, is to use a single formulation the whole year round. A complete balanced fertiliser such as 6-6-6 or 8-8-8 will give good results, but on alkaline soils a fertiliser with slightly more potash than nitrogen may be better. If you can find it, a fertiliser low in phosphorus is much more desirable. The fertiliser you select should contain more ammoniacal than nitrate nitrogen, and at least 20 % to 30 % of the total nitrogen should be in water insoluble form. The fertiliser should derive all its potassium from sources other than muriate of potash in order to keep its chlorine content as low as possible.
A balanced mixture of nitrogen, phosphorus and potash will be inadequate unless it contains certain other essential elements such as magnesium, manganese, copper, zinc and iron. Magnesium is especially important and a fertiliser should contain at least 2 - 4% magnesium as MgO and smaller amounts of the minor elements. The use of a fertiliser containing the minor elements in the proper proportions will minimise the need for foliar sprays and minor element additives. GU 49 iron is desirable as one of the sources of iron. On alkaline soils an acid forming fertiliser may be beneficial.
The general opinion of most hibiscus specialists is that Nitrophoska
Red, a fertiliser put out by B.A.S.F. in Germany, is the best for
hibiscus. The N.P.K. of 12-5-14 suits them admirably. However this
fertiliser is not available everywhere and in these cases substitutes must
be found. In Australia Aboska No 27 is the closest to Nitrophoska and
growers here use this. It is always best to use a fertiliser with a high
potassium content, and this is why many of the foliage fertilisers which
contain a very high proportion of nitrogen should be avoided as they may
cause bud drop. Slow release fertilisers although popular are not as good
for hibiscus as they are for other plants and should be used only in
conjunction with other fertilisers, or on small potted plants. Blood and
bone based fertilisers are good but should be applied mainly in spring as
they take time to break down and become available to plants.
Hibiscus should be fertilised frequently, at least once a month throughout the growing and flowering season. The schedule should be kept up regularly. Heavy rains will leach nutrients out of the root zone of plants so it is recommended that fertiliser be applied after this happens. Your plants are your best guide, if they look well but are dropping buds then they need food. If the growth is slow and spasmodic, they need food. Don't forget to feed your plants right up to winter to give them a chance to build up healthy tissue that will tolerate a greater degree of cold. Remember that numerous light applications regularly are much better than heavy applications now and then.
How Much Fertiliser
This depends on a number of factors but mainly on the size of plants and the time of year. The type and analysis of the fertiliser should also be considered. For recommended fertiliser such as Nitrophoska Red or Aboska 27 the following guide is recommended.
The majority of fertilisers have excellent recommendations for use on their labels. If you have any doubts ask your local nurseryman or someone experienced in growing hibiscus. Two things to remember are that young newly planted hibiscus do not require much fertiliser, and that hibiscus need less fertiliser in winter.
How to Apply
The fertiliser can be applied directly on top of the soil or mulch. It should be sprinkled evenly under and slightly beyond the spread of the branches (drip line), but should be kept off the foliage and well away from the trunks of the plants. A good rule is to apply the fertiliser in a circle which extends from halfway in from the edge of the drip line to an equal distance beyond.
Never apply fertiliser to dry soil. Be sure to water well before and
after fertilising to reduce the chance of burning your plants. While it is
a good idea to fertilise just before rain, do not depend on the rain to
wash the fertiliser into the soil.
Too much fertiliser or inadequate watering after application may result
in an accumulation of soluble salts in the soil, causing damage to roots
and marginal or tip burn on the leaves. The symptoms of fertiliser burn
are the same as those of drought, since the plants cannot absorb water if
excessive salts are present in the soil.
Hibiscus, like all other plants, need small amounts of the seven minor elements in order to grow and flower properly. Plants with minor element deficiencies will decline in appearance and health. The correction of a severe deficiency, if it is allowed to develop, may take many months. The symptoms of any minor element deficiency show up on the new growth with the new and younger leaves being affected. When the deficiency is corrected, the new growth will be normal in appearance.
Most garden fertilisers contain small quantities of the minor elements, and on acid soils regular applications of such fertilisers should satisfy the needs of hibiscus plants. On alkaline soils minor element deficiencies are more common and it may become necessary to add to the soil the particular minor element which is lacking in order to correct the deficiency. There are several commercial products that contain the required trace elements and these are available from garden centres and nurseries.
All the minor elements can be applied, if desired, by spraying the foliage, and this is recommended, particularly for alkaline soils. Foliar sprays are quicker acting but do not last as long as soil applications. Minor element sprays are available as liquids and dry soluble powders. Care should be used in applying these sprays, for they may stain paint or concrete. Do not exceed the manufacturer's recommendation as to the amount used or you will risk injury to your plants.
Correcting Specific Deficiencies
An experienced grower will sometimes recognise specific minor element deficiencies and treat them separately. Use of the following materials is recommended.
Iron Soil applications of iron are not
usually effective unless the iron is applied in a form called GU 49 iron
or iron chelates.
Many of the more successful hibiscus growers have concluded that potassium, when present in amounts larger than nitrogen supply (approximately 1 part nitrogen to 3 parts potassium), has a strong beneficial effect on flower quality, colour, quantity and condition. Accordingly these growers often adjust their fertilising practices to provide their plants with more potassium than nitrogen during the hibiscus show season.
The use of potassium sulphate in small quantities is an effective means
of building up potassium levels to the proper proportions for the highest
quality blooms. One method of application is to alternate the regular
fertiliser with potassium sulphate every two weeks during the flowering
season. Use potassium sulphate at the rate of
½ cup or 120 g (4 oz)
Expert growers may differ in their methods of fertilising, but they all
agree that the key to success with hibiscus is to fertilise little and
often, using a single balanced fertiliser or one with a higher proportion
Hibiscus in pots are susceptible to residue salt burning if they are
not watered properly, due to the accumulation of salts from both
fertiliser and water. To avoid burning your plants in containers, water
them heavily, so that some water comes through the drainage holes and
leaches out any excess salts, and be sure to water after each application
of fertiliser. This is why it is very important to have a good
free-draining soil for the potting mix. Too much peat moss and other
composts in the soil will hold in the salts. Container grown plants may
benefit, just the same as garden plants, from periodic spraying with minor
element solutions and occasional applications of GU 49 iron or iron
Well drained sandy soils are generally low in fertility and dry out quickly. The addition of organic matter will increase the soil's fertility by providing a favourable environment for beneficial food manufacturing micro organisms and by reducing the loss of plant nutrients due to leaching. The ability of the soil to hold water will also be greatly increased by the addition of humus. Organic matter decomposes rapidly and ultimately disappears completely in well drained sandy soils. Thus, it may be necessary to add organic materials each month to maintain a good quality soil. The use of an organic mulch also helps with this.
Animal manures such as cow, sheep, horse or chicken dung provide excellent organic materials for the hibiscus grower. Although manures are used primarily for their soil conditioning value, they are useful also as fertiliser materials, especially for newly transplanted plants which require frequent watering. Manures, sewage sludges, and mature composts can be mixed directly with the soil before setting in plants. The nutrients in these materials resist leaching and are released slowly, so there is little risk of burning the plants. The exception is unleached chicken manure, which is quite high in rapidly available nutrients and which requires careful usage to avoid injury to plants.
Composting improves all manures. Interlayering about 15 - 20 cm (6 - 8
in) of manure with 5 cm (2 in) of soil into a pile and maintaining it in a
moist condition for six to eight weeks, makes an excellent product when
the pile is mixed for usage.
To start a compost heap, build a 15 - 20 cm (6 - 8 in) layer of fresh organic materials such as leaves, grass clippings, plant prunings, and kitchen wastes, including coffee grounds and eggshells. Weeds with ripe seeds, resinous plant material, greasy animal fats, and diseased plants should be avoided. Animal manures and vegetable wastes could draw flies and should be covered or buried within the pile.
Fertiliser added from layer to layer as the pile builds will hasten the decomposition process. Sprinkle a small handful of complete fertiliser to about each barrowful of compost and water thoroughly. You can, if you wish, add a 2 - 5 cm (1 - 2 in) layer of topsoil and a very small handful of dolomite in addition to the fertiliser.
Keep repeating this layering until the compost fills the bin. Air and moisture are important to the decomposition process, so keep turning the material occasionally and water frequently. Make sure the pile is moist but not water logged. A coarse screen chimney can be placed vertically in the centre to ensure better aeration, or the heap can be ventilated by pushing a rake handle down through it in several places.
The compost will be ready to be spaded onto the garden within two to six months depending on the waste materials used and the time of year. Succulent green materials will break down faster than dry ripened growth, and the compost will break down faster in summer than in winter. The process is complete when the original waste materials have decomposed to the point where they are no longer recognisable for what they were. This material is called humus.
Prunings run through a mulching machine are an excellent source of material for starting a compost heap; this may save a trip to the tip or reduce air pollution, as many people simply burn this material.
One good way to maintain a steady supply of mature compost is to build a series of three connecting bins, 90 cm, 60 cm and 30 cm (3 ft, 2 ft and 1 ft) in height. Begin the composting process in the tallest bin, in six weeks, blend the compost and transfer it to the 60 cm (2 ft) bin, and start another pile in the tall bin. The compost will reduce in volume as it breaks down. In another six weeks transfer the material from the middle bin to the 30 cm (1 ft) bin, where it can be stored until needed.
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