Most soils are deficient in nutrients needed by plants. Fertilizers can supply some or all of these nutrients in a variety of forms and combinations. Ask yourself the following questions before deciding what fertilizer or soil conditioner you should purchase.

Plants require a wide range of nutrients for growth and production. Major elements are needed regularly in large amounts, minor elements are needed on occasions and trace elements are needed in minute quantities only.

Major ElementsMinor ElementsTrace Elements
Nitrogen (N)Calcium (Ca)Iron (Fe)
Phosphorus (P)Magnesium (Mg)Manganese (Mn)
Potassium or Potash (K)Sulphur (S)Copper (Cu)
 Zinc (Zn)
 Boron (B)
 Molybdenum (Mo)

By law, the nutrient content of a fertilizer must be shown on the bag or on an attached label. Don't purchase fertilizer products not showing this information. Let's look at a typical fertilizer label:

CROP KING 88. The fertilizer name. N-P-K-15-4.3-11.3. These indicate that it contains 15% nitrogen, 4. 3% phosphorus and 11. 3% potassium. The forms in which the nutrients are present are indicated in the following table:

15%Nitrogen (N) Ammonia form
4.1%Phosphorus (P) Water Soluble
0.1%Phosphorus (P) Citrate Soluble
0.1%Phosphorus (P) Citrate Insoluble
4.3%Phosphorus (P) Total
11.3%Potassium as Muriate of Potash (i.e. the chloride form)
13.6%Sulphur (S) as Sulphates



The remaining 51.5% is made up of elements such as hydrogen, oxygen and carbon that are part of the chemical compounds that contain the nitrogen, phosphorus and potassium nutrients.

Standard fertilizer application rates for each crop are available from the Department of Primary Industries. However, these can only be a guide, as they are calculated for average conditions and soil types. The best basis for a precise fertilizer rate is a soil and leaf analysis. 'Do-it-yourself' soil and leaf sampling kits produced by fertilizer companies are available from most farm supply outlets.

WHAT DOES IT DO ? (Does it suit my crop ?)
These questions are best answered by considering the major groups of fertilizers and soil conditioners.

These are sometimes called commercial or artificial fertilizers. This group is produced by various industrial processes.

Inorganic fertilizers can be divided into two main categories, straights (single element fertilizers) and mixtures, blends and compounds.

These provide one of the three major nutrients, nitrogen, phosphorus or potassium, although other elements may be present. Examples are urea (providing nitrogen), superphosphate (providing phosphorus) and muriate of potash (providing potassium). Occasionally two of the major elements are present, as in the case of potassium nitrate (nitrogen and potassium) and MAP (nitrogen and phosphorus). Straight fertilizers have the following characteristics:

• Provide a known level of nutrients.
• Useful when only one major element is needed
• Easily applied but more time consuming to apply than mixtures.
• Generally fast acting.
• Lower cost in a per unit of nutrient basis.
• If water soluble, can be applied through irrigation.

These contain all three major elements and a range of minor and trace elements. They are prepared by dry blending of straights or by chemical reactions.

Examples are Tropic, 77(S), Fertica, CF flowfeed, Q5 and Nitrophoska. Mixtures have the following characteristics:

• Provide a known level of nutrients.
• Supply all major elements.
• Are generally fast acting.
• Easily applied and convenient to use.
• If water soluble, can be applied through irrigation.
• More expensive than straights.
• Are wasteful if only one element is needed.

Some inorganic fertilizers may be more suited to your particular crop than others. For example, Phytophthora root rot of avocado is encouraged by fertilizers containing chlorides and nitrates. Mixtures containing potash as potassium sulphate and nitrogen as urea or ammonium are therefore the best to use for avocados.

This group includes animal manures and animal or plant by-products. Examples are manures, fish emulsion, mill mud and dynamic lifter. Animal manures contain small amounts of nitrogen, phosphorus and potassium which vary with the animal and its diet and the amounts of straw or litter mixed in with it. They are first class materials for improving soil texture and structure, but must be added in large quantities to benefit the soil in this way. Organics have the following characteristics:

• Stimulate useful soil organisms.
• Improve soil structure.
• Supply organic matter.
• Nutrient levels are variable and low.
• Difficult and expensive to apply.
• Slow acting.

TABLE 1 lists the approximate nutrient analysis of some organic fertilizers.


Nutrient analysis - Organic fertilizers
% content of elements
Cow manure1.00.40.5 
Fowl manure1-1.81-1.60.4-1.0 
Dynamic lifter3.63.61.6Ca 7.7
Blood and Bone4-64-7-Ca 18
Mill Mud0.50.20.04Ca 1.2
Deep litter and sawdust1.41.10.8 

Most inorganic fertilizers are soluble and are readily leached. Nutrients from organics are not easily leached but are slow acting.

To overcome this problem, fertilizers have been developed which provide nutrients in a steady stream to be absorbed by the plant as required. Examples are Nutricote, Osmocote, planting tablets and plant pills. Slow release fertilizers have the following characteristics:

• Supply nutrients over extended periods.
• Fewer applications are needed.
• Supply all major elements.
• Easily applied.
• Most suitable for nurseries and home gardens.
• Expensive.

Foliar fertilizers contain soluble nutrients that can be sprayed onto plant foliage. They are designed to give crops a boost during times of stress or to correct deficiencies. Examples are Wuxal, Complesal and Foliar Nitrophoska. Foliar fertilizers have the following characteristics:

• They are supplementary fertilizers, not a replacement for soil applications.
• Useful for applying trace elements.
• Use low rates to avoid leaf burn and toxicities.
• Expensive.

These are naturally occurring minerals used to change some physical or chemical aspects of the soil, making it more suitable for plant growth. Soil factors which may be improved include pH, soil structure, water penetration, nutrient availability, soil micro-organism populations and levels of calcium and magnesium.

Examples are lime, dolomite and gypsum. pH is a measure of the soil's acidity or alkalinity. It is measured on a scale of 1 to 14 where seven is neutral, higher than seven is alkaline and lower is acid. Optimum for most plant growth is 6 to 7.5. Some plants prefer acid soils, for example pineapples and azaleas, while others prefer alkaline soils, for example lucerne. Lime and dolomite can be applied to acid soils to increase pH to optimum levels. Before applying any conditioner, have your soil tested to determine pH. Rates of lime or dolomite needed for soil type and crop are available from the Department of Primary Industries. Dolomite applies both calcium and magnesium. Lime applies only calcium and reduces the availability of magnesium. Lime is available in various grades based on its fineness and neutralising value (that is, ability to reduce soil acidity). Lime with a high percentage of fine particles is faster-acting and more expensive. Characteristics of the major soil conditioners include:

• Supply nutrients. (Lime supplies calcium; Dolomite supplies both calcium and magnesium)
• Increase pH.
• Increase availability of some nutrients in very acid soils.

Hydrated lime or slaked lime raises pH quickly but reacts badly with some fertilizers and soil humus. Its use is not recommended.

• Supplies calcium to soil.
• Improves soil structure.
• Provides cheap source of calcium but does not reduce acidity.

This is relatively easy to do when comparing straight inorganic fertilizers which only provide one major element. For example, to provide potassium (K) you could use:
• Potassium Sulphate - $580/t.
• Potassium Chloride (muriate of potash)- $299/t.

Other factors such as suitability for your crop must also be considered since some plants are affected by chlorides. The best approach, however, is to assess the cost of application as well as the cost of the fertilizers. This allows you to compare mixed element fertilizers with straights. For example, for avocados, the alternatives and their estimated per hectare costs per year are:

• Side Dressed Straights -Fertilizer $234 + labour $62 = Total $296.
• Side Dressed CK 77(s) + super - Fertilizer $279 + labour $40 = Total $319.
• Side Dressed Fowl Manure + Potash - Fertilizer $292 + labour $125 = Total $417.
• CF Flowfeed RX4 through irrigation - Fertilizer $306 + labour $nil = Total $306.

The cost of the farmer's labour is included. Figures will vary with the crop grown. You should do similar comparisons for your own crop, based on soil analysis results or standard recommendations. All costs are based on November 1985 figures for the North Coast Region.

Poor nutrition steals profits. The use of fertilizers and soil conditioners is essential for profitable crop production. When choosing a fertilizer, ask yourself these questions:

• What's in that fertilizer?
• How much should I apply?
• What does it do?
• Does it suit my crop?
• Is it good value?

G. N. Greer
Extension Horticulturist, DPI Nambour
Extract from Old. Fruit & Vegetable News

DATE: January 1987

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