Fertilizer is frequently referred to as plant food. This is a poor concept. The real food for plants is carbon dioxide and water. These two bulk materials with the addition of sunlight are converted to sugars and carbohydrates that do the actual feeding of the plant. Fertilizer is much more analogous to vitamins. The nitrogen, phosphorus, potassium, and minor elements contained in fertilizer are necessary for cell division and enzyme processes that allow photosynthesis and growth to proceed. The amount of these elements that a plant uses in manufacturing growth is really quite small, only a few percent of dry tissue weight.
Nitrogen, N, is responsible (partly) for the green color color of the leaves and the elongation and amount of the new growth. It affects leaf size and internode length. Nitrogen is needed for cell division and the manufacture of proteins.
Phosphorus, P, is also necessary for cell division and is a necessary element in DNA , RNA and fatty substances. It makes plant leaves dark green. It is associated with good root growth, and flowering.
Potassium, K, activates some cell enzymes and is associated with healthy cell activity. A deficiency can lead to a chlorosis of the leaf margin.
The primary method of fertilizer absorption by plants is through the roots. Water in soil creates a soil solution containing N,P, K and the other necessary elements by dissolving them from the soil. The plant roots absorb this solution by osmosis, moving the less concentrated soil solution across their membrane to the more concentrated solution in the cells. By this process water and fertilizer can be pumped to great distances through the plant. In the case of Coast Redwoods, the water is pumped to 300 feet high.
Plants can also take in fertilizer through their foliage. However this is a much over rated method of fertilizing plants. The primary absorption process is through the roots. Foliar feeding can beneficial for a limited number of nutrient deficiencies such as calcium.
When we fertilize plants we supply them with an artificial soil solution. We manufacture this solution by either dissolving fertilizer salts in water or spreading solid fertilizer on the soil and watering it in. In any case the nutrients end up in the soil solution and are absorbed by the roots.
Why do we have to fertilize at all if nature doesn't? The answer to this lies in the soils we use for container gardening, which is a totally unnatural practice. Plants in the earth can tolerate soils that are very much finer grained (heavier) than those in containers. This is because the mechanics of drainage and aeration are different for containers than in the earth. Heavy soils are much better at retaining fertilizer, thus much smaller amounts are needed. Plants grown in containers require soils that are very much more porous than those found in the earth to ensure proper drainage and aeration. These 'light' soils are very lean and poor at retaining nutrients. There is also the limiting factor of the soil volume in a container.
In this totally artificial environment of container growing it is necessary to provide a regular addition of fertilizer to the soil solution in order to maintain the health and growth of the plant.
Fertilizers are carefully crafted to provide an optimum level of salt in the soil solution when used as directed. Let the directions be your guide. Do not use more or less than the recommended amount, or more or less often than listed on the label. You will find that many fertilizers, especially soluble forms such Miracle Gro, Miracid, Peters, etc. will list a concentration for a particular feeding period. The more frequently you wish to fertilize the less concentrated the solution. From my experience I do not see any benefit to feeding more frequently than once every two weeks. This is a standard nursery practice for soluble fertilizers.
Fertilizing more often than this will require more dilute solution and will waste more material as the excess spills to the ground each time. It is also more work to prepare the solutions more frequently. Although I don't have any evidence to prove it yet, it is my suspicion that fertilizers may be more effective when applied at the higher rates at the less frequent intervals. Every other week seems to be a very good period. I can see the green color begin to fade slightly at three weeks if I delay fertilizing. The dark green vibrant color of the foliage is the key to determining the correct amount of nutrition.
All fertilizers manufactured for use in the US will contain a label analysis. Three large numbers will usually be prominent on the front of the package. This is the NPK analysis. Numbers such as 15-15-15, 20-20-20, or 18-6-12 are common. These numbers are the percentage by weight of the N-P-K , nitrogen, phosphorus, and potassium contained in the fertilizer. It is actually a little more complicated than that, but this will suffice for our purposes.
So, is a 20-20-20 fertilizer better than 10-10-10? No, the directions for use take into account the differences in the percentages of dry weight. In other words you will use half as much 20-20-20 per gallon of solution as 10-10-10. You can use this fact to do a cost comparison to get the most fertilizer for your dollar. When all the numbers are the same it is called a balanced fertilizer and for most purposes, including bonsai, this is what you want for general overall health and growth. Fertilizers with uneven numbers such 10-52-10 (a high concentration of phosphorus) are special use fertilizers. This particular one is used for new transplants and for flower production.
On the back of the package there will be a complete analysis of the all the major, minor and trace elements. It will also reveal the source and form of the elements. Nitrogen will be listed by 'nitrate' and 'ammoniacal' forms. In warm weather it makes little difference which form is prevalent. Nitrate forms are utilized a little more quickly, but the difference is only a matter of days. In cold weather, nitrate forms are preferred since they will be absorbed and utilized more readily.
This is the subject of some controversy. The conventional wisdom is to fertilize when the plants are in active growth. This generally is after bud break in the spring until late summer. The contention is that feeding well into the fall and early winter with nitrogen will force new growth that will be killed by frost. It now appears that this is a myth.
Plants begin setting buds in mid to late summer. Nitrogen does fuel new growth, but plays no role in initiating bud break. This is related to day length only. Cold hardiness acquisition is related to lowered temperatures and can actually be enhanced by regular fertilization with balanced levels of nitrogen. Based on these findings there is no reason not to continue balanced fertilizer feeding well into the fall and early winter. As long as the soil temperature is above about 55F plants will continue to absorb and store nitrogen as well as phosphorus and potassium.This storage of fertilizer by the plant plays a very important role in fueling new growth in the early spring when soil temperatures are cold and nitrogen is hard to acquire.
The conventional wisdom also says fertilizing with high levels of phosphorus in the fall will prepare the plant for winter by increasing its cold hardiness acquisition. There is no credible evidence to support this contention.
I now begin balanced fertilizing in the spring after the new leaves begin to harden off and continue well into the fall and early winter.
Bonsai soils are usually pretty much inert and often 'soil less', that is, they have no native soil. Under these conditions, plants sometimes cannot get enough of the minor elements they need. These are calcium, magnesium, sulfur, iron, zinc, copper, manganese, molybdenum, cobalt,and nickel. For this reason, it is best to use a fertilizer that contains these minor and trace elements. Any good soluble chemical fertilizer will add these, it will say so in the analysis on the label, if they are in there. If it doesn't say, then you can be sure that they are not in it. Adding some forms of organic fertilizer may also help supply these minerals, especially animal or marine byproducts. Using a small amount of clean native soil (without a lot of organic material) can also usually satisfy this demand. I say without organic material because it is the rock flour portion of soil that will contain these elements. Many of these elements are also present in ordinary tap water. For example, copper pipes will usually add enough copper.
It is not known precisely how much of these trace elements are needed for plant growth and the amount varies with the species. In general the amounts are quite small, measured in parts per million. The amounts are so small, that the lack of a specific mineral is rare, but it does sometimes occur in soilless mixes. Taking any of the precautions above will prevent it. Some of these elements are needed as catalysts for plant reactions and continually reused.
The analysis on the label will also state the source of the trace elements. While it is good to get a fertilizer with trace elements from any source, the very best source is chelated (KEY-LATED). This is an organic (as in organic chemistry) form which is immediately available to the plant. Smaller doses go a lot farther when chelated. For example, Iron might be available as 'derived from Ferrous sulfate' (not chelated), or 'Iron Chelate', or Iron EDTA (a chelated form).
Fertilizers are designed to operate in a broad spectrum of container sizes and soil composition at full recommended strength. There is no danger of 'burning the roots' when used as directed for anything from a seedling with its first true leaves, to a rooted cutting, to four hundred year old trees. Fertilizer 'burn' is a chemical phenomenon which can be described as backward osmosis. Fertilizers are salts, and in solution they have a salt 'value' or concentration. The liquid inside the cells of roots and all other living tissue also have salts and solutes. Osmosis is the phenomenon of water moving across an otherwise impermeable cell membrane into a solution of higher salt concentration. As long as the concentration of the soil solution is lower than the salt concentration of the cell, the soil solution with its dissolved salts (fertilizer and minerals) will be carried into the cell and utilized.
If you do not follow directions and accidently make a solution that has a higher concentration of salt than the concentration in the cell, the water will move out of the cell and into the soil solution, thus dehydrating the cell. This is a backward osmosis for the plant, and the effects are salt or fertilizer 'burn'. In extreme cases it will cause fatal dehydration and the symptoms will be the exactly the same as if you didn't water the plant, wilt, dull blue green color to the leaves, then yellowing or browning and death of the plant. In milder cases of salt poisoning only the margins of leaves will be affected. Usually they will turn yellow then brown.
To achieve a concentration capable of doing this requires mixing a batch many times stronger than recommended full strength soluble fertilizer. Usually you will see it only if too much solid fertilizer is directly applied to the soil surface or in the case of very hard water.
There are effects of over fertilizing, other than 'burn'. These are subtle effects. The levels of fertilizer that will cause these effects are not clear, and there are some that would argue that they do not exist. The effects include overly large leaves from excess nitrogen, soft turgid leaves and tissues, weak stems, long internodes, low resistance to some fungal pathogens, lowered resistance to insect damage, perhaps even an attractiveness to some insects such as borers. In general, in would take nearly twice the recommended dosage of soluble fertilizers to begin to see these effects.
As far as the general health of the plant is concerned, we know that heavy fertilization will weaken the plant in some ways and strengthen it in others. We also know that under fertilizing will do the same thing. Adequately fertilized plants will grow at moderate rates, have good green color, and usually exhibit excellent health. Pushing the plants beyond these rates can adversely affect the health of the plant. Undoubtedly, there are times, to push plants hard, such as in training to develop trunk caliper. There are also times to keep the nitrogen limited, but in balance, such as when reducing leaves and developing ramification.
Utilization of soluble fertilizers depends greatly on soil composition. Soils high in organic components and clays tend to bind nutrients and hold on to the salts, thus requiring lower levels, or less frequent applications. Bonsai soils on the other hand tend to be even leaner than most other container soils, and much leaner than native soil. Since fertilizer recommendations are based on the average container soil, this would argue for higher effective concentrations, or more frequent full strength concentrations to meet the minimal needs of healthy plant growth.
The half strength fertilizer myth is in a lot of bonsai books, but then too is the myth of 'leaf burn' from watering in bright sunlight. Unfortunately, many bonsai authors are long on style and short on horticulture.
Another common bonsai myth is that sick or recovering plants, or newly transplanted bonsai should not be fertilized. The analogy is that it will over feed the patient or is the equivalent of over dosing with vitamins. I think the proper analogy should be that feeding at half strength or not all is analogous to not taking your medicine, vitamins and nutrition when you are sick.
The recommended strength is designed to produce a soil solution of fertilizer salts at a specific range of electrical conductivity. In this range it is very easy for plants to pick up the various N,P, and K ions. It doesn't matter if the plant is a seedling, newly rooted cutting, newly root pruned bonsai, or recovering plant. They all will pick up nutrients more easily if fed in this range.
Roots are roots. Probably the most tender roots you will ever come across are the newly formed adventitious roots on a cutting. These are very fleshy and extremely fragile. I have fertilized my newly rooted cuttings (and seedlings) for years with full strength soluble fertilizers and they have all thrived. In fact, they quickly show chlorosis if I don't feed them at this level.
There is no evidence of which I am aware that fertilizer will interfere with the process of regeneration of roots. The belief that fertilizer 'burns' young and tender roots cannot be substantiated. Fertilizer is necessary for the health and growth of roots as well as foliage. If new roots cannot find fertilizer from the soil solution, the plant will rob nutrients from existing tissues to fuel new growth. This doesn't sound like a formula for recovery to me.
Research has shown that new roots are often found encasing beads or chunks of pure solid fertilizer. If there were a problem with localized 'burning', there would be a 'dead zone' around such chunks, not fresh new roots. I am not arguing for more fertilizer here, just the continuation of a reasonable fertilizer schedule. Over fertilizing causes a whole new set of problems.
It is my experience that it is important to get a plant back into good growing conditions as soon as possible following transplantation, root pruning, or a root problem. Allowing a plant to languish in the dark without food may relieve light and temperature stress, but does nothing to promote new growth. I firmly believe it is important to maximize all the elements of good growth and nutrition as soon as possible in these situations without inducing stress. This includes allowing the plant to dry slightly before watering, fertilizing on a regular schedule, keeping the soil warm, introducing the plant to increasing light levels as soon as it can tolerate it, and allowing for good air circulation.
There is a careful balance to optimizing these conditions without inducing new stresses, but the reward is to push the plant out of danger by returning it to healthy growth as soon as possible.
This half strength fertilizer myth is pervasive in bonsai books and
culture. Even many nurseryman espouse it. I think what is at work here is
that people read bonsai books and repeat what they have read and heard
without really stopping to think about it, or doing some intelligent
research. Put me in this group as well. Even though I have fed my
seedlings and cuttings at full strength for years, until recently, I too
fertilized at half strength on the advice of bonsai books. Then I started
taking a hard look at the results, which were disappointing. I now treat
my bonsai as what they are: container plants.
The major elements needed for plant nutrition are N, P, and K. The source doesn't matter to the plant. The salts will ultimately be employed by the plant in exactly the same fashion. The major benefit of organic fertilizer is that it releases nitrogen slowly. Nitrogen is quite soluble and quickly leached out of the soil by water unless bound up in some fashion as is the case in organic fertilizers and timed release fertilizers. Since we fertilize bonsai regularly anyhow, I don't consider this a big problem. Timed release fertilizers release these salts in a similar fashion. Some even have a urea (organic compound) based coating which acts to slowly release the salts inside the bead. There is of course, controversy about additional benefits of using organic fertilizers.
Personally, I don't care if someone uses an organic or inorganic (chemical) based fertilizer. I maintain that they ultimately do the same thing, provide the plant with the essential elements. I am not trying to get anyone to give up using organic fertilizers. I do try to dispel the belief that organic fertilizers are far superior in the totally artificial system of bonsai. If we were talking about organic gardening, I would have to agree with the organic school of thought, since organic fertilizers in that system also do wonders for the structure of the soil and the ecology of the microorganisms. In bonsai, this is artificially manipulated, and in fact we try hard not to have the organic portions of the soil degrade (compost) , which causes soil collapse and poor drainage. Additionally, we often drench the soil with fungicides which will kill many of the beneficial microorganisms as well as the pathogens.
All of the foregoing is necessary to understanding how and why fertilizers work. But the bottom line is still the same. In general, all bonsai can be fertilized with a soluble 20-20-20 at full strength every other week from the first new growth in the spring until late fall when the daytime temperatures fail to rise above 50F.