by Connie Oswald Stofko
A fertilizer or pesticide may be called “organic,” but that doesn’t mean it’s the best choice for you, contends Dr. Fran Evans, a master gardener and retired chemist.
In a wide-ranging talk about garden conservation this past weekend at the Buffalo and Erie County Botanical Gardens, Dr. Evans touched on “organic” versus “chemical” fertilizers and pesticides.
With fertilizers, you could choose seaweed, which is considered an organic fertilizer, or you could choose something that is considered a chemical fertilizer, such as a 5-10-5. (Commercial fertilizers are labeled with the percentages of the major three nutrients plants need: nitrogen, phosphorus and potassium, in that order. A 5-10-5 fertilizer contains 5 percent nitrogen, 10 percent phosphorus and 5 percent potassium.)
Both the seaweed fertilizer and the 5-10-5 fertilizer are processed from living material, he noted, but the chemical fertilizer has been processed more. The seaweed fertilizer is more expensive. He estimated that for a large garden, it might cost $5 to use 5-10-5 or $20 to use seaweed.
Even if you use compost in your garden, you might need to use some kind of fertilizer, Dr. Evans said. If you use too much compost, the bacteria in the compost could consume the nitrogen that your plants need. You can replace the nitrogen by using fertilizer. There’s no guarantee that you have the proper amounts of other nutrients in your compost either, and fertilizer can help that.
However, you should first get your soil tested to see what your garden needs before adding fertilizer, he said. (The Botanical Gardens will have soil testing for a nominal fee on various Saturdays starting April 9. We’ll post details on our Upcoming Events Page.)
So if we use fertilizer, do we have to worry about the fertilizer getting into the ground water?
“Yes, if you use too much,” Dr. Evans said. “You have to realize what you’re doing. If you use it according to directions, it should be no problem.”
If you want to prevent weed seeds from germinating, you could go with corn gluten, an organic choice, or Preen, a chemical choice.
Corn gluten is fairly new, Dr. Evans said. It’s a waste product that comes from corn. It’s not a foolproof treatment against weeds, but it does inhibit seed growth and acts as a fertilizer as well. Preen is less expensive but doesn’t act as a fertilizer.
When it comes to pesticides, many of us feel reassured when we choose something that is called organic, but being called organic doesn’t mean a product is necessarily safe for humans, pets or wild creatures we wish to protect.
Neem oil is an example of an organic pesticide. The label cautions you to keep it out of reach of children, and gives first aid instructions in case you inhale it, get it in your eyes or get it on your skin. You must take precautions not to contaminate water with neem oil.
While neem oil kills pests, it can also kill bees, Dr. Evans pointed out. Therefore, you have to be careful not to get it on any flowers that bees might land on. Neem oil is also relatively persistent, he said; it remains in the environment for a relatively long time.
Another organic pesticide is Bt, which affects only certain caterpillars and a few other insects. It works fast and lasts only about a day.
Still another example of an organic pesticide is pyrethrins, which are very strong and must be used with considerable caution.
“I won’t use pyrethrins because they’re too strong and too persistent,” Dr. Evans said.
Chemical pesticides have their advantages and disadvantages as well.
Malathion, a chemical pesticide, is less persistent than neem oil and has a half-life of three to four days, he said. (The half-life is the time required for half of the compound to break down in the environment.)
If malathion gets into groundwater, it will last longer than if it is above ground because it won’t be exposed to air and sun, which break it down, Dr. Evans conceded.
“In suggested amounts, it’s not a danger to humans,” he said. “If you use it in the proper way, it’s generally no problem. You have to think about what you’re doing.”
Sevin is a broad-spectrum insecticide used for Japanese beetles and other pests. It is more persistent, with a half-life of about two weeks, Dr. Evans said. It can kill bees and other beneficial insects.
Whichever product you choose, Dr. Evans offers this advice:
“Know what you’re using, know how to use it, and know the consequences.”