by George Buehler

We first want to investigate some of the factors that affect plant growth and then look at fertilizing bonsai.

Plant growth and development are influenced by physical, chemical and biological components in the plants' environment. Any of the factors in the plants' environment that are less than optimum, whether deficient or in excess, will limit the plants' growth. Some of the external factors affecting plant growth include light, temperature, gases, air pollution, water, and nutrients.

Light

Light is needed to initiate and sustain photosynthesis. A plant's response to light will vary depending on the intensity, duration, and wavelength of the light it receives. During summer, when the light a plant receives is the strongest and brightest, the rate of photosynthesis is the highest and growth is greater. As the days become shorter and cooler, the rate of photosynthesis slows and dormancy is initiated.

Temperature

Optimum temperatures for growth vary with the species, but is generally from 40-97 degrees F. Several growth processes are sensitive to temperature. One of these is respiration, part of the photosynthetic process. Respiration rates increase rapidly as the temperature increases and, conversely, decrease as the temperature decreases. Photosynthetic rates are determined by light (as we have already seen), CO2 levels and temperature1.

Temperature has little effect on the photosynthetic rate from 50-86 degrees F unless light and CO2 are readily available. Root temperatures also affect the rate of plant growth. Increasing root temperatures up to about 76 degrees F may increase top growth and the uptake of inorganic ions. If the root temperature exceeds approximately 85 degrees F, uptake of water and inorganic ions cease and death of the plant can occur.

Gases

Green plants require oxygen for normal growth and development. The energy released in cellular respiration, from the breakdown of carbohydrates and complex organic molecules, consumes oxygen and releases CO2. Most plants respire continuously and therefore require a continuous supply of oxygen. Roots also require oxygen for aerobic respiration. The absorption of salts and root extension are dependent upon the energy supplied from respiration. If the plant is root bound the respiration will slow or even cease and the roots stop growing and are more susceptible to disease1.

Air Pollution

It has been shown that air pollution can cause poor growth, as well as leaf discoloration, spotting, and disfigurement. Unless bonsai are grown near a source of air pollution, there is generally little effect from air pollution. It has been shown2 that "acid rain", a result of air pollution, can cause various plant problems and, in some cases, death.

Water

Most growing plants contain about 90% water3. Water acts as the medium for transfer within the plant, the solvent system of the cell, and is required for the production of new compounds. Moisture stress is generally detrimental to plant growth and depending on the degree of the stress, will determine the severity of reduced plant growth. Unfortunately, water quality can also affect plant growth. Water with high concentrations of calcium and/or magnesium (hard water) can slow the uptake of necessary nutrients by the plant. This can be somewhat overcome by using rain water or by periodically treating the plant with a mild solution of an acid fertilizer, which helps wash out some of the mineral deposits by dissolving them.

Nutrition

Sixteen elements are considered to be essential for growth and development of plants4. These essential elements are divided into two groups: the macronutrients -those required in relatively large quantities (carbon, hydrogen, oxygen, nitrogen, phosphorous, potassium, calcium, magnesium and sulfur), and the micronutrients which are required in lesser amounts (iron, chlorine, manganese, boron, zinc, copper and molybdenum).

Carbon, hydrogen and oxygen are obtained from the environment; while chlorine is obtained from municipal water supplies. The other elements have to be added by way of fertilization.

• Nitrogen - Nitrogen produces rapid growth and gives stems and leaves their deep green color. Nitrogen is a mobile element in the plant, and deficiencies will develop first on lower leaves as the nitrogen is removed for use in new developing leaves. The older leaves turn yellow and eventually die. Too much nitrogen produces lush plant growth.
• Phosphorus - Phosphorus is needed for formation of seeds, flowers and fruit. It also helps plants store energy for winter and to harden off roots and buds. It is used in the plant's energy transfer system, and a deficiency can slow growth considerably5. With phosphorous deficiency, older leaves can develop a purplish color as anthoyanin pigments accumulate. Phosphorus uptake is directly influenced by temperature.
• Potassium - Potassium acts as a coenzyme or activator of many enzyme systems. It improves stem and trunk rigidity, improves the plant disease resistance, and helps the plant overcome poor weather or soil conditions. A deficiency of potassium appears as marginal leaf chlorosis (yellowing), giving the appearance of burned edges. Unfortunately, an exceedingly high concentration of potassium in comparison to calcium and magnesium can cause the plant to suffer stress6.
• Calcium - Calcium is required to maintain membrane integrity. Calcium deficiency generally only effects fruits, but can also effect leaf size in some trees.
• Magnesium - Magnesium is an essential constituent of chlorophyll and is required for activating certain enzymes in the energy transfer process. A deficiency of magnesium affects plant growth and development as photosynthesis is directly affected.
• Sulfur - Sulfur is a constituent of a number of chemicals contained within the plant. Deficiency symptoms appear similar to nitrogen deficiency symptoms as an overall plant yellowing or chlorosis.
• Copper - Copper is a constituent of certain proteins and enzymes. Almost all commercially available fertilizers contain adequate concentrations of copper.
• Iron - Iron is required for the synthesis of chlorophyll. Iron deficiencies appear as a chlorosis (yellowing) of leaves. Chelated iron can be routinely added through out the growing season to promote production of chlorophyll.
• Boron - Boron is required in minute quantities by most plants. Boron deficiencies will slow growth and often stunt the whole plant. Because boron is required is such low concentrations, there is normally more than enough supplied by any fertilizer.
• Chlorine - Like boron, chlorine is required in minute quantities and more than enough is supplied by use of municipal water or normal fertilizers.
• Manganese - Manganese is involved in production of oxygen from water in photosynthesis and may be involved in chlorophyll synthesis5, 6. Deficiencies almost never occur in normal bonsai horticulture.
• Molybdenum - Molybdenum is required for nitrogen fixation and for the reduction of nitrates. Molybdenum deficiency may appear as nitrogen deficiency and results in restricted plant growth and flower development5, 6.
• Zinc - Zinc is required for formation of hormones and enzyme activators. Zinc deficiency results in stunting of plants and leaf growth.

 

Nitrogen	Stunted growth, chlorotic foliage starting in older leaves
Phosphorus	Plants are dwarfed with thin stems and small leaves. Anthocyanin pigments may accumulate in older leaves
Potassium	Older leaves develop marginal browning and possible forward curling of leaves
Calcium	Shoot tips yellow and die back. New leaf may have distorted leaf growth. Root tips die back
Magnesium	Withering of old leaves. Interveinal chlorosis developing first on older leaves
Sulfur	Yellowing in leaves usually beginning in new leaves
Iron	Interveinal chlorosis beginning on younger leaves
Manganese	Interveinal chlorosis on leaves near the tip of the plant
Boron	Die-back of shoot and root tips
Zinc	Shortened internodes
Molybdenum	Interveinal chlorosis beginning on older leaves moving up to younger leaves

Having looked at some of the factors affecting plant nutrition, we now turn to what and how we should fertilize our bonsai. A proper fertilization program is important for all types of plants. However, it is of particular importance when growing bonsai, due to the bonsai being grown in containers and the use of soil mixtures made up primarily to promote drainage. The roots have little space to grow, so they can not obtain nutrients like a landscape tree. Our bonsai soil contains little organic materials that could hold nutrients or decompose and release nutrients. Therefore, we must give the plants the nutrients they need.

If the bonsai is to grow (and more importantly survive), it must be put on a regular feeding schedule during the growing season. Feeding during the dormant season is not required for non tropical species. For tropical varieties a greatly reduced program should be followed during the winter months.

Fertilizers for bonsai are available in two forms: organic and inorganic. There are pros and cons to both types. The main criterion is that whichever type is used, it should contain both the macro- and micro- nutrients reported above.

All fertilizers contain three primary elements which are nitrogen, phosphorus, and potassium. The amount of these ingredients in any fertilizer is given in a series of three numbers printed on the package. One bonsai fertilizer I use (Dyna-Gro) has a listing of 7-9-5 which translates into 7 % Nitrogen content, 9 % Phosphorus, and 5 % Potassium. Whenever you see such a series of numbers on a fertilizer, the first number represents the nitrogen component, the second the phosphorus, and the third potassium. In actuality, these percentages are not really the elements nitrogen, phosphorus and potassium but the percent of the compounds containing the elements. As with any good fertilizer, the label also lists the percentage of the other micronutrients contained in the bottle. This type of fertilizer is called a "balanced" fertilizer.

A fertilizer is chosen based on the type of growth the bonsai artist wants to promote. If the goal is to promote green growth, then the first number should be high - as in 30-10-10. If flowering or fruiting is the goal, then the second number should be the highest - as in 5-20-10. If root growth is desired, then the third number should be highest - as in 5-5-20.

As stated above, when using a fertilizer, too much can be added. In practice, if a 'garden' fertilizer is used on bonsai, unless care is exercised, the tree can be hurt because the concentration of components can be too high.

Most of the liquid fertilizers and granular 'garden' fertilizers are considered inorganic because they are mostly composed of man-made materials. Inorganic fertilizers can be purchased in liquid form (or powdered which is mixed with water to make a liquid solution), granular or pelletized, and in slow release form.

Some bonsai artists use organic fertilizers because they are purported to improve soil structure and release plant nutrients slowly. Organic fertilizers are typically in the form of balls or cakes which are placed on top of the soil. As the tree is watered, the cakes start to dissolve and migrate into the soil. The main problem with organic fertilizers is that they have a tendency to grow unwanted fungus or molds and can serve as a home for insect larvae.

The most common organic fertilizers used for bonsai are bone meal, cottonseed meal, blood meal, and fish emulsion.

Bone meal (a ground up mixture of animal bones) is high in phosphorus and decomposes slowly. Because it is naturally alkaline, it also helps to neutralize the acidity in acidic soils.

Cottonseed meal is slightly acidic and is good for acid-loving plants such as azaleas.

Blood meal (dried, powdered blood from slaughterhouses) is high in nitrogen. It also contains a number of trace elements.

Fish emulsion is a blend of decomposed pulverized fish and is high in nitrogen. It also contains a number of trace elements.

Because both blood meal and fish emulsion are high in nitrogen, they can burn your trees. A problem with organic fertilizers is the smell and potential problems with varmints thinking you have buried a fish or piece of meat in the soil. The second problem is the tendency to grow fungi or molds reported above. Many bonsai artist use a combination of organic and inorganic fertilizers.

Fertilization Frequency

Every bonsai artist has his own schedule for fertilizing his trees. Some do it weekly, some bi-weekly. Others add a much diluted fertilizer solution to their daily watering schedule. Some use a combination organic and inorganic protocol. I couldn't find a consensus of what is the best method. The key is to add fertilizer on a fixed schedule.

One thing that must be mentioned is that plants differ in their response to nutrients. That response is dictated by their inherent growth rates, their age, type of root system they have, and their ability to absorb nutrients.

Various species have different growth rates. If we compare the growth rates of a Trident maple with that of the Boxwood, we find that, no matter how much fertilizer we put on a Boxwood, it will not grow as fast as the Trident. However, if we compare the growth rate of two identical Boxwoods, we see that if one is fertilized and the other not, the one that receives the fertilizer will have a better growth rate than the one not being fertilized.

By no means should one consider the "more is better" practice. Donahue, et. al.7 showed a graph of plant growth response to increased fertilizer applications. The curve was "S"-shaped (the curve reached a maximum then dropped off as more fertilizer was added) rather than linear. A plateau was reached in the growth response as fertilizer was added and then there was a drop off as the concentration of fertilizer continued to be increased. This indicated that increasing fertilizer concentrations actually did more harm than good. What this maximum concentration is varies with tree species - each having a different maximum.

My fertilizer routine is dependent on the season, the age of the tree, the health of the tree, and what I perceive the tree needs. I have chosen to use almost exclusively inorganic fertilizer. I don't like the smell of some of the organics and definitely don't like the 'fuzzy' white stuff that grows on the organic cakes after a few days. I also use the fertilizer full strength or as directed on the manufacturers' label - most of the time.

If the tree was under stress the previous year or if it is currently under stress, I use a ½ strength solution of a balanced fertilizer and use it every other Sunday until the tree's health improves. For my tropicals, I use a ½ strength solution during the winter months.

In early spring I lean more toward a higher nitrogen fertilizer for deciduous trees and change to a high phosphorus fertilizer toward the end of the growing season. The general procedure I use is listed in Table 2. Bear in mind that even though the table calls for high nitrogen, I alternate the schedule with a low nitrogen fertilizer every second or third week.

Season	Tree Type	Fertilizer types
Early Spring Mar-Apr	Deciduous, Young Evergreens	High Nitrogen
	Flowering	High Phosphorus
	Older Evergreens	Balanced
Mid - Late Spring Apr-May	Deciduous, Young Evergreens	High Nitrogen
	Flowering - after flower fall
	Older Evergreens	Balanced
Early Summer June	Deciduous, Young Evergreens	High Nitrogen
	Flowering
	Older Evergreens	Balanced
Summer July	Deciduous, Young Evergreens	Balanced
	Flowering
	Older Evergreens
Summer August	All types	None
Fall September	Deciduous, Young Evergreens	Balanced
	Flowering
	Older Evergreens
Fall October to storage	All types	High Phosphorus Low Nitrogen

During my research on this article, I ran across a NABF Newsletter article on a fertilization program8 that I found interesting. Llorens used 12 trees of the same type, age, size, and planted in the same type soil and containers. One tree was labeled the reference tree and only received water - no fertilizer. The others were individually treated with granular, slow release or liquid fertilizers once a week and were watered daily with equal amounts of water. The trees were placed on the same table so that they all received the same climatic conditions. The fertilizers selected were:

• 4 granular - 3 had levels of 20-20-20; 1 had low nitrogen and high phosphorus
• 4 liquid - 2 high phosphorus, 2 balanced
• 1 slow release - apparently like Osmocote
• 1 vitamin B-1
• 1 hormone - type not specified

The experiment was carried out over a 4 month growing period. The growth was measured by the width of the foliage and the height increase. His reported results were:

• The trees treated with granular fertilizer and those treated with slow release fertilizer showed exceptional growth
• Trees treated with liquid fertilizer showed moderate growth
• Those treated with hormone and vitamin showed the same growth as the tree receiving water only

These results were somewhat unexpected for Llorens, so he checked with a PhD botanist to determine if he could shed light on the lop-sided results favoring the granular and slow release fertilizers. After much discussion and observations, they theorized that since the plants were watered on a daily basis, the trees treated with the liquid fertilizer would have the fertilizer in contact with the roots for a 24 hour period before more plain water was added to the pot, which would wash out or dilute some of the nutrients. For the granular or slow release fertilizer, the daily watering would wash a certain amount of nutrients to the roots with each watering. They concluded that the granular or slow release fertilizers are much more efficient for bonsai.

 

There was no indication of how much of either the granular or slow release fertilizer was added to the top of the soil. Obviously, if a large amount of either was spread around the pot, the tree could be over fertilized; and, as we previously discussed, too much nutrients can be just as bad as not enough.

Dave Bogan, from Evansville, uses a dilute liquid fertilizer solution that is siphoned into his water hose with every watering. With this technique he can continually give his trees a mild dose of nutrients.

Personally, if I can figure out how much granular fertilizer to put onto the top of the soil, I would try this. Using Dave's technique would be a safer solution and less likely to burn the tree.

In summary, our bonsai need to be treated regularly with nutrients both macro as well as micro. With proper adjustment of the type of nutrient added, we can promote growth and/or trunk growth.

Reference

1. Mastalerz, J.W. 1977. The Greenhouse Environment, John Wiley and Sons, NY. pg 629
2. Janick J. 1979. Horticulture Science. W.H. Freeman and Company, San Francisco. pg 608
3. Hartman, H.T., et. al. 1981. Plant Science Growth. Development and Utilization of Cultivated Plants. Prentice-Hall, Inc. pg 676
4. Amon, D.I. and P.R. Stout. 1939. The Essentiality of Certain Elements in Minute Quantity for Plants with Special Reference to Copper. Plant Physic. 14:371-375
5. Bergman, E.L. Nutrient Solution Culture of Plants. The Pennsylvania State University College of Agriculture, Extension Service Hort. Mimeo Series II:160. pg 21
6. Jones, J.B. Jr. 1983. A guide for Hydroponic and Soilless Culture Grower. Timber Press, Beaverton Oregon. Pg 124
7. Donahue, R.L. 1990. Soils: An Introduction to Soils and Plant Growth. Prentice Hall, New York
8. Llorens, E. 2004. Bonsai Fertilizer - Most Efficient. North American Bonsai Federation Newsletter. 7:#4