‘Organic’ not always best choice for fertilizers & pesticides, chemist says

bee in Buffalo
Some pesticides, both “organic” and “chemical,” can harm bees and other pollinators.

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.”

6 Comments on “‘Organic’ not always best choice for fertilizers & pesticides, chemist says

  1. Compost is used to feed the soil so the microbes, insects, worms, and so forth are able to break down the components of the soil making minerals and so forth available to plants. Chemical fertilizers are mineral salts extracted from rocks. They contain inert fillers which do little to nothing. Thus a 5-10-5 fertilizer is 80% inert fillers. The effect of the addition of organic materials to soil is slower than that of a chemical fertilizer but they feed the soil! Another advantage of organic materials is that frequently they are free (Know a farmer?) and thus recycle wastes as well. A disadvantage is that you really don’t know how much to apply,

    Chemical fertilizers are water soluble. This has the advantage of being a quick plant treatment. However they can be leached during a heavy rain or by overwatering. Where do they go? Into the ground water.

    I do not understand how bacteria in well rotted compost could use up the nitrogen. The aerobic bacteria present convert material into ammonium. Other bacteria fix it into available compounds.

    Corn gluten was patented as an herbicide in 1991. it works by preventing the formation of roots by emerging seeds.

    Neem oil is used as a skin treatment in India. I know this from a lady from U.B.

    Bt is from the spores of a species of bacteria, Bacillus thuringiensis. It takes about 2 to 3 days for the caterpillars to die.

    Pyrethrin is one of the safest insecticides as it disintegrates in sunshine.

  2. Carol Ann,
    You’ve added a great deal of information and have raised some great points.

    While I see that neem oil is used in medicines in parts of the world, I would still follow package instructions when using pesticides containing neem oil. It’s possible that neem oil in the pesticide is more concentrated than the neem oil used medicinally, or it may be mixed with other ingredients that are harmful to humans. The product I have directs users not to get it on their skin or clothing, and I follow those precautions.

    You raise many other great points that I simply don’t have the expertise to address. I will see if Dr. Evans will clarify.

  3. Compost is generally used to only to put organic material into the soil-not as a fertlizer since the nutrient value is low. Nitrogen may be significantly depleated because the bacteria that do the composting require nitrogen as part of the digestion cycle.Since this nitrogen is not soluble it cannot be easily used by the plant. With reference to the “fillers”in commerical fertlizer-nitrogen and phosphorus may be present as ammonium phosphates and other salts while the potassium may be present as potassium phosphates or sulfates or other anions that help solublize the salts. All of the things in commercial fertlizers are there for a reason not just as “fillers”.
    Since all of the elements needed by the plant must be soluble ,if you add too much of ANY type of fertlizer it can end up in the ground water.
    So far as pesticides are concerned–all of them are decomposed by sun and air -the rate of decomposition is the important consideration. DDT lasted for years. Used as instructed all are safe. I certainly would wash amything I eat that had Neem oil or Pyrethins several extra times since these organic oils could soak into the organic surface of your fruit of veggie and cling more that some of the symthetic pesticides

  4. Dr. Evans,
    Thanks so much for your reply. The issue of pesticides and fertilizers is definitely complicated. I appreciate you clarifying those points for us.

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