The Limits of Fertilization

by Andy Walsh

Introduction

This article was the response by Andy Walsh to a discussion on the Internet Bonsai Club on the ability of plants to take up fertilizers. The hypothesis was that a plant would respond to ever increasing amounts of fertilizer by producing more and more roots and top growth.
BW

The Premise: More is Better?

It is correct to say that plants will make use of any resources they find in the soil. And it is tempting to assume from this, that adding more fertilizer will result in more growth. This would seem so, since most people have observed that plants grow more when they are fertilized. However this is not so.
Plants will differ greatly in their responses to soil nutrients which is dictated by their inherent growth rates, the duration of their growth periods, their ages, the types of root systems they have, and their ability to absorb nutrients.
Plants have widely different growth rates. Most of us who grow a wide variety of species know that some trees put out lots of growth and others crawl along. Increasing nutrient levels in the soil (fertilizing) will not change these inherent rates of growth. Compare the growth rates of the Trident maple with that of the Boxwood. If you apply equal amounts of fertilizer to a tree of each, that are of the same size and age, will you get the same amount of growth from both, that is, will they both assimilate the same amount of nutrients and incorporate them into new structures? The answer is no. The Boxwood will lag significantly behind the Trident in its amount of growth.
Even trees within the same genus have significantly different nutrient demands and will respond differently to nutrient levels in the soil. The growth of various pines during a season varies greatly in the amount of candle growth and the duration of the growth. White and Red Pines will reach 10 and 15 cm in candle growth by April and not increase significantly thereafter. Shortleaf, Slash, and Loblolly Pines will all reach 15 cm by April also but will continue growing until October by which time they will have reached 50, 60, and 90 cm in length respectively. (Biochemistry and Physiology of Plant Hormones", Thomas Moore).
It is well recognized by Bonsai growers that Pinus thunbergiana, the Japanese Black Pine, has a significantly different growth rate than Pinus parviflora, the Japanese White Pine. These two will never grow at the same rate or the same way. You cannot get a Japanese White Pine to grow like a Japanese Black Pine. So even within the limitations of a genus you will find differing responses to fertilizer levels.

Growth Effects of Increased Fertilizer

Increasing fertilizer does not elicit increasing growth rates even within a given plant either. The responses of many plants to increased fertilization have been very well studied especially in crop plants. There is a point of diminishing, or more appropriately, decreasing returns as fertilizer applications increase. In "Soils: An Introduction to Soils and Plant Growth" by Donahue et.al. shows a graph of plant growth response to increased fertilizer application. The curve is sigmoidal ("S" shaped) rather than linear. There is a plateau reached in the growth response as fertilizer increases and a drop off as it gets higher indicating that increasing fertilizer becomes more harmful than good. Donahue et.al. write:

"As more and more fertilizer is added, the gain in yields from each successive increment is less and less...The downward curve, as excessive fertilization increases, is due to reduced crop yields because of such things as salt problems and unbalanced growth (which may increase the plant's susceptibility to disease and abnormal growth)."

Optimum Amounts of Fertilizer

There definitely seems to be an optimal amount and regimen of fertilizer for each tree (that varies from tree to tree) and exceeding this can be harmful. Dr. Carl Whitcomb in "Plant Production in Containers" also warns about excessive and unbalanced fertilizer application. He writes "The optimum level of a plant nutrient is probably a limited range rather than a specific level. It should be noted that hidden...toxicities can occur long before...toxicity symptoms appear." He also shows a similar growth response vs. fertilizer curve.
I have read such caveats about excessive fertilizer applications in every reference I have on plant nutrition. Again, besides not directly increasing growth, increasing fertilizer levels can cause problems.

Seasonal Timing

Plant growth also differs in when and how long it takes place. There are some plants that put out new growth continuously throughout spring and summer, such as Juniper and Hinoki Cypress, and some that only put out new growth in the spring, such as Beech and Euonymous. Applying high amounts of fertilizer to all four trees may result in more growth in two of them but not in the other two. Adding high levels of fertilizer throughout the summer is certainly wasteful in the later instance since no new growth will be elicited.

Plant Maturity

Trees also differ in their overall growth depending on whether the plant is young or mature. The physiology of young trees is different than in older trees. Young trees tend to grow more rapidly and for longer periods of time. Some seedlings can double in size in a year. Many mature trees will only put out a small amount of new growth in the spring and stop growing early in mid-summer and actually start becoming dormant. (Older trees definitely have to slow down. Obviously a tree that doubles in size each year would be incredibly enormous if it maintained that rate for many years). Adding fertilizer to the soil will not make a tree that is going dormant reverse direction and start growing again. The amount and timing of nutrient uptake will be quite different in an older tree than in an immature tree and just raising the nutrient levels in the soil will not alter this either.

Absorption Rates According to Species

Plant roots also differ substantially in their ability to absorb different nutrients. For example, the ability to absorb magnesium can be as much as 60 times higher in one plant than in another. ("Russell's Soil Conditions and Plant Growth) This is a tremendous difference. One will suck it up and another will choke it down. Most of our Bonsai plants will not differ anywhere near this much but they will still differ significantly. In addition, the ability to absorb one nutrient can be adversely affected by high levels of another. The response to higher levels of nutrients is not equal, and again, high levels of fertilizer can lead to nutrient imbalances and unbalanced growth which can cause health problems with your trees.

And finally

These are some of the reasons that a simple equation of "more fertilizer = more growth" does not hold true.
For a simple analogy, think of the human condition. Can I force feed individuals with dwarfism and get them to grow more? Can I even get my son to grow more by making him eat all of his dinner (and as much as I eat)? He will only use as much food as he needs to grow and the rest will be stored as fat. He will not grow more. I can assure that both the individual with dwarfism and my son have enough nutrients to grow as much as they can - but not any more. I certainly can keep them from growing by limiting nutrients (the old fashioned way of growing Bonsai for that matter;) Neither my son, my father, nor I need as much food as a teenager. And we shouldn't get it. Obviously overfeeding anyone of these individuals is not only wasteful, but can be unhealthy for them also.
There is an old saying "All things in moderation" and this certainly applies to human life as well to fertilizer levels for Bonsai.