When should you fertilize a bonsai?

What is a Fertilizer?

Fertilizers can be characterized by the selection of nutrients they contain, by the compounds or forms, and by the origin of the material (organic or inorganic, natural or manufactured). 

Nutrient Selection.

The main nutrients derived from soil are nitrogen, phosphate, and potassium. These parts of fertilizer mixture are indicated on the container by numbers in that order. The number represents the number of pounds per 100 pounds. Many combinations are available such as 5-10-5, 7-7-7, 6-12-12. When all these primary elements are included, such formulas are called "complete fertilizers".

Some complete fertilizers are sold specifically for certain broad categories of plants, such as flowers, vegetables or more specifically, for a particular plant - roses, tomatoes as an example.

There are fertilizers that provide just one or two nutrients. Blood meal (12-0-0), bone meal (0-10-0) for examples.

Major Nutrients.

Nitrogen Of all the nutrients absorbed from the soil, nitrogen is the most important needed for growth. It promotes strong vegetative growth and dark green foliage. It is used quickly and is easily leached out of the soil. A lack of nitrogen reduces plant height, leaf size, and the production of leaves and branches. A nitrogen-deficient tree will be light green or yellow, with its older leaves affected before the younger ones.

Phosphorus Although it is found in small amounts in comparison to nitrogen, phosphorus plays a key role in the respiration, photosynthesis and the transfer of energy within cells and tissues. It stimulates root branching and the production of root hairs, and helps the tree mature more rapidly. It moves slowly in the soil and is sometimes related to improving disease resistance. Leaves of plants suffering from phosphorus deficiency turn a dark green on top and red to bronze underneath.

Potassium Potassium has several important functions in cell metabolism and plays a key role in regulating the opening and closing of leaf stomata. It acts as a regulator for the other two main elements and helps in producing strong cells which form thick stems and good substance in flowers. Symptoms of potassium deficiency include weak stems, a tendency to wilt, a lower resistance to disease, and a reduction in yield and quality of flowers and fruit. Potassium moves from the lower leaves to the upper leaves when a deficiency occurs so symptoms show up in the lower leaves first.
 

Organic vs. Inorganic.

Fertilizers are formulated from organic materials and from natural or manufactured inorganic compounds but plants absorb all of their nutrients in the form of inorganic ions. Nutrients formulated from natural sources such as cow manure or fish meal and those manufactured in a chemical factory are absorbed by the tree in exactly the same form.

Organic fertilizers have several advantages over inorganic compounds. Because they must first be processed by microorganisms, they are more slowly available to plants. They are safer because they do not burn plants as readily, and they are less likely to leach from the soil. 

They have disadvantages too. They cost more, many are not balanced nutrients, and the release of nutrients is unpredictable.

Inorganic fertilizers provide maximum control over nutrient levels, the nutrients are in soluble form and dissolve into ions that are immediately available to plants. Inorganic fertilizers are also available in specialized forms such as starter solutions which are high in phosphorus and readily available nitrates. They help the tree recover more rapidly from shock or root damage.

Form Selection.

Fertilizer is sold in three basic forms: dry granules; solid pellets, tablets, or spikes; and liquid or soluble powders.

There are both slow and quick release fertilizers. The particles of a slow release fertilizer have been coated with a polymer that reduces the speed with which the nutrient becomes available. Slow release fertilizers provide a steady supply of nutrients over an extended time and minimize the problems associated with leaching of nutrients or an overdose of certain elements particularly nitrogen.

Liquid fertilizers are applied directly on the soil or sprayed on the foliage. They are an excellent source of readily available nutrients and are especially useful for quickly correcting deficiency symptoms.

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