Gardening : Now’s the Time for Once-a-Year Fertilizing

Not everything in the garden needs fertilizer, but in my garden, everything gets it once a year anyway. That’s because I’m not the most diligent of gardeners and I tend to forget to fertilize except during March. In March, it’s easy to remember because I’m in a gardening frame of mind practically every day.

Somewhat surprisingly, this March fertilizing works very well, because everything is growing and the fertilizer is put to use right away. You can actually see it take effect. Lawns, for instance, turn greener overnight and grow a couple of inches. Spring flowers seem to double in size and immediately burst into bloom. Shrubs get greener, grow taller and generally look happier. Fruit trees get an enormous amount of fruit or in the case of citrus, lose that yellowish cast they had all winter, and so on.

At other times, it’s more difficult to see the fertilizer take effect, but there are plants that I fertilize then. The flower garden for instance is fertilized once a month, more or less, because it uses up a lot of fertilizer making flowers. Gardenias are fertilized fairly often, because they keep turning yellow, and camellias and azaleas get several feedings during the summer to plump up the buds that are forming.

Sometime in early summer, fruit trees and citrus get fed at least one more time. And, of course, lawns are fertilized fairly often, because grass is the greediest of all plants, though I tend to let the lawn fast between feedings so I don’t have to mow as often.

Container plants are fertilized every couple of weeks if I want them to grow or flower, although some are left a little hungry because they have already filled their pots.

Fertilizing is a great mystery to many. Except for container plants, the plants are already being fed by the soil so what you are really doing is supplementing their diet. In the average soil, they will grow fine without any fertilizer, but fertilizing (which is really the proper term, not feeding ) makes them bigger and better.

Buying a fertilizer is confusing, because you will find so many kinds. An April, 1984, article in Sunset magazine reported there were 169 kinds of fertilizers available on the West Coast.

But this same article proved what some gardeners have long suspected--fertilizer is fertilizer and a lot of what you see at nurseries is packaging. Working with the University of California, they tested many kinds of fertilizer and found that the only ingredient of significant importance was nitrogen.

If you look on the back or side of a package of fertilizer, you will see that it shows the percentage of nitrogen, phosphoric acid (phosphorous), potash (potassium) and perhaps some micronutrients such as iron. A number such as 20-10-10, for instance, indicates 20% nitrogen, 10% phosphoric acid and 10% potash.

The major source of confusion is an old gardeners’ rule that you must use a fertilizer high in nitrogen to encourage growth (such as a 20-5-5), then switch to one low in nitrogen but high in phosphorous and potassium (say, 0-10-10) to encourage flower production.

In fact, what Sunset and the University of California discovered was that this simply wasn’t true. Only nitrogen made a difference in the size or quantity of flowers or fruit. It turns out that you can’t add potassium or phosphorous to the alkaline soils of the West, at least from above ground. They simply can’t move down to where the roots are. Only the nitrogen moves easily through the soil--so easily, in fact, that it is quickly washed out by rain or watering. That’s why you must fertilize at least in March and preferably several other times during the growing season if you want better than average growth, flowers or fruit.

Note that the exact numbers are not important--a 10-6-6 is not much different from a 12-4-2. What is important is that first number--how much nitrogen is in the fertilizer--because that determines whether it’s a good buy. A fertilizer very high in nitrogen (such as ammonium sulfate at 21%) can be tricky to use, because it can chemically burn plants if too much is applied. But it’s the best buy.

I stick to fertilizers such as 8-8-4 or 14-14-14, which I can use with some indiscretion. And I buy the cheapest. At planting time, I add them to the soil, which is the only way to get the phosphorous and potassium into the soil. At other times, I scatter them around the plants and then thoroughly water to get the nitrogen down to the roots. This is not the most sophisticated approach, but it works. I have discovered that gardens that look better than mine are usually fertilized more often.

So how about special fertilizers such as citrus or rose food, or those for acid-loving plants? In a nutshell, only the fertilizer for acid-loving plants offers any advantages over any other fertilizer, though the fertilizer for citrus probably contains more micronutrients than do other fertilizers. But these are not usually required on a regular basis.

Sunset said the acid-type fertilizer was useful in that it helped the potassium and phosphorous work their way down into the soil because it made the soil less alkaline. But drilling holes in the ground for these two nutrients or adding them at planting time would do the same. In short, I found nothing to justify changing my simple routine.

Containers are a different matter. Because there is often no natural soil in the mix, the plants are completely dependant upon fertilizer, so I fertilize these every couple of weeks with a different kind of fertilizer.

In the garden, I prefer a granular fertilizer, which I can scatter or dig into the soil. In containers, I use a powdered fertilizer that is mixed with water. A number of these are available and almost all are very high in nitrogen with numbers like 23-19-17, 23-15-18 or 20-20-20. Miracle-Gro and Peters Professional Soluble Plant Food are two. Some also contain many micronutrients, essential for long-term container growing.

I also use these fertilizers in early winter and at this time of the year. These soluble, high-nitrogen fertilizers tend to be high in nitrate nitrogen, which is the form that goes to work the fastest. Other forms such as ammoniacal or urea work more slowly but last longer. These fertilizers are also expensive and applying them is not as easy as simply scattering granules about.

I also use one of the “slow release” fertilizers. These may be purchased as tabs, stakes or little pellets. They are useful in containers and under certain plants that I want to make sure never go hungry. For instance, I bury a little in the bottom of every planting hole I dig for delphiniums.

If you’re getting confused again, Sunset advises: “We can say that since phosphorous, potassium and micronutrients don’t need to be added to the soil frequently, the only element you should provide with regular fertilizing is nitrogen.”

Your soil may be exceptional, but in general, that’s it. And this weekend is the best time to fertilize everything if you only get around to this job once a year.