I recently answered this question from a customer, and I spent so much time on it, I thought it might interest the group:



It's not simple. You need to understand the role of fertilizer, it is not food. Food for plants is carbon dioxide, water, and light- to produce carbohydrates. Fertilizer, NPK, role is more of a catalyst, than anything else, although P and K are needed for DNA. N is needed in larger quantities because it plays a larger role in forming DNA and also because it is not stored for long periods in the plants and is a mobile nutrient, going where it is needed. Also it is soluble and not stored for long in the soil either. So, NPK would be the ticket if your plant is sick because it is too low on NPK, that is, it hasn't been fed in a long time. I'm talking months here. Normally this doesn't happen in bonsai because we are fertilizer nuts and feed too much most of the time, not too little.



Plants can be sick for many reasons, low fertility usually isn't one of them. Probably the biggest cause of plant decline (if you can rule out pests and diseases) is lack of oxygen to the roots, or root rot. Root rot is really a misnomer because the roots die from lack of oxygen and THEN rot because they are dead. This happens due to harmless composting bacteria. There is also pathogenic fungi and bacteria that cause disease by attacking living tissue. Actually these are quite rare in container plants if you are careful (clean). If you get one of these you are in serious trouble as they are often fatal. They usually show symptoms but not always, like part of the plant is wilted, discolored stems or foliage, etc. Most of the time there isn't anything you can do about this since the cures are as bad as the disease, chemicals kill the beneficial organisms too. The only chemicals I will use are fixed copper oil sprays in winter, and systemic insecticides which will only kill sucking insects (granular, not sprayed).



Most of the time a sick plant will simply have the wrong environmental conditions, and the biggest factor is poor aeration of soil. So, the best cure is to change the soil to something with excellent drainage. You can't do this for some plants out of season, mostly conifers. Deciduous plants can take it, and if you are going to lose it any way if you don't do something, why not? Plants decline from too little light, but this is usually obvious and easy to correct. The symptoms are larger than normal leaves and internodes. Plants can also suffer from excess salts (mostly carbonates) in the soil and water (hard water), but this is pretty rare if you are using any kind of city tap water. It will show up as dead leaf margins and obvious salt build up on pots, etc. Again, this is an environmental defect that is fairly easily corrected.



There is much misunderstanding about fertilizing sick plants. Some pathogenic diseases are favored by high nutrition, others are favored by low nutrition. Unless you know exactly what you are dealing with, you don't know which way to go. Since plants need NPK to GROW, is it better to just lightly fertilize and hope for the best? Since most sick plants have a compromised root system, it is only the roots that need to grow. Shoot growth will not happen until the root system is healthy and begins to send the hormone cytokinin to the buds. What do the roots need to grow? A small amount of nitrogen to support DNA growth. Good soil aeration. Auxin from the leaves and stems, if possible. Recent studies have shown that, contrary to popular belief, the presence of high amounts of P can in fact RETARD root growth in newly transplanted plants. Only a small amount of K is necessary for plant enzyme function and DNA, an amount that is probably already held by the plant.



Therefore, the most sensible route it seems to me is to fertilize ONCE with a dilute solution of N only, such as a teaspoon to a gallon of water of ammonium sulfate, 20-0-0, and withhold any other fertilizer until new growth appears. This is of course assuming that the problem is a compromised root system which is the most common problem. Use of a weak fertilizer solution using full NPK ratios probably won't make any difference if that makes you feel better. Since the biggest factor is almost always a change to more conducive environmental conditions, especially better soil, this should be accomplished as soon as possible.



As an addendum, withholding fertilizer from newly transplanted plants only slows down the root colonization process, especially with regard to nitrogen. Burning of roots from a properly applied fertilizer is a myth that I hopefully no longer have to address. For this reason I always incorporate a low to medium dose of timed release fertilizer to my soil mix to make sure all the nutrients are there and available when the plants needs them, rather than guessing when it is time to feed.



Brent

EvergreenGardenworks.com