### Part 18 of a series by Eugene Howell

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Considering how little soil a bonsai tree lives in, fertilizing bonsai trees is especially important. But which fertilizer should you buy for your bonsai? When you look at a bag of fertilizer you’ll see a set of three large numbers written across the front that look something like “8-6-4” or any other combination of three numbers. Every bonsai enthusiast must know how to relate these numbers to the needs of each type of bonsai tree they’re growing.

In a previous article we learned that these numbers represent the content of Nitrogen (N), Phosphorus (P), and Potassium (K) contained in the fertilizer, in that order. The numbers written on fertilizer labels are actually percentages. So, for example, the number 5-3-15 means 5% of the weight of that bag of fertilizer is nitrogen, 3% is phosphorus, and 15% is potassium.

One benefit of knowing this information is that you sometimes can use it to stretch your dollar. Everything else being equal, if the price of a 40 pound bag of 5-10-5 is $5 (as an example) and the price of a 40 pound bag of 10-20-10 is $8 then you get double the amount of active elements for only $3 more by buying the second bag. In other words, the more costly bag will go twice as far as will the inexpensive one. Very frequently you will find this type of price bargain. Just remember to compare numbers on the bags as well as the price of each bag.

Each of these three major elements in fertilizers serves a different purpose for the plant. By knowing what you want your plant to do, you can know which combination of numbers to select while shopping for fertilizer.

Nitrogen, the first number on the bag, has the important function of stimulating vegetative growth. It is essential for the development of leaves and stems. So, if it’s spring time and you have a deciduous bonsai that’s trying to put out lots of healthy new leaves, you’ll want to use a fertilizer that’s high in nitrogen. A number in the range of 20 to 30 would be appropriate for this goal. So you might purchase one that has a numbers such as; 25-6-6.

Phosphorus, the second number on the bag, does the job of developing bright, healthy flowers, fruit and seeds. A fertilizer with a high middle number would be appropriate for use on bonsai such as azalea, bougainvillea, camellia, and gardenia, among others. Again, a middle number in the range of 20 to 30 would be appropriate for these trees when the flower buds are just barely beginning to form. In this example the number combination might be something like; 8-30-4.

Potassium, the last number on the bag, does the job of creating healthy roots. It also increases the plant’s resistance to disease. Obviously, if you have a plant on which you have just done some root pruning, or which needs a more robust root system, then you use a fertilizer with a potassium number in the range of 18 to 25. In this example you might buy something such as 6-4-18.

After having discussed all this, however, don’t be confused about general fertilizing. If you’re doing periodic fertilizing for the normal health and growth of your bonsai, then a fertilizer with all three numbers in the mid-range is perfectly adequate. This might be one with numbers such as 6-5-6, or something similar.

If the numbers are lower than 4, you’re only giving the tree a minimum of nutrition. But don’t assume that more is better. If all the numbers are high and you fertilize too often, you’ll likely burn the roots of the tree. Never overdo it when fertilizing bonsai.

When creating a fertilizing schedule for your bonsai trees, keeping this information in mind will allow you to have bonsai that are robust and healthy.

Photo credit: flickr Creative Commons, Brazilian Rain Tree (Chloroleucon torum) by Cliff1066