Excessive Use of Fertilizers & Pesticides

If there are two things that help define what gardeners think about on a regular basis they might well be nutrients and pests -- nutrients because they benefit and promote the crop and pests because they often inflict damage. An important part of a gardener’s mission is to exert some control over those two factors. Fertilizers and pesticides provide some means for that control, but excessive use can be detrimental to the immediate surroundings and to the environment at large.

Because many pesticides can be harmful, not only to the target pest but also to humans, pets and non-target organisms -- including beneficial ones -- excessive pesticide application is something to be avoided. Most pesticides are non-selective, meaning that, not only is the target pest being affected, but that other organisms -- including many that you might not intend to harm -- suffer as well.

Amphibian populations, for example, have been shown to suffer from insecticide and herbicide exposure -- not just locally but worldwide -- and bees are another much-publicized victim of carelessly used insecticides. All insects, including predators that would ordinarily help you keep pest populations down, are killed by insecticides. In addition, pesticide residues in food in reach our tables in many of our favorite fruits and vegetables, as well as meat, poultry and dairy products.

Excessive fertilizer use has negative consequences, both to the pocketbook and to the environment. Nutrient levels beyond what plants can use tend to leach downward, where they can pollute groundwater. A good rainfall can wash it off your lawn, down the gutters and into a waterway, causing a cascade of ill effects in nearby streams, ponds and wetlands. The excess aquatic plant growth, and its inevitable death and decomposition, can lead to reduced oxygen levels, killing fish and other aquatic organisms.

These harmful consequences occur at scales ranging all the way from the local pond or wetland in your neighborhood to massive waterways, deltas and estuaries like the Chesapeake Bay and the the outflow of the Mississippi river, even causing large, persistent dead zones in those marine resources. Collectively, backyard gardeners, small-scale operations, small- to medium-sized commercial growers and large agricultural operations all contribute to the problem.

Knowing how much fertilizer to use is just as important as knowing, for example, how much to water the garden. The decision of whether, how much and with what material to fertilize is further complicated by factors that influence nutrient availability, such as soil composition and pH.

The people who sell fertilizers often advise a specific schedule of use, but this may end up being unnecessary if the right soil amendments are used to begin with. It is often best to test soil for nutrient content and availability before embarking a regular schedule. Do-it-yourself kits from local garden centers offer one way to do this, but sending a sample to a soil testing lab is probably the best option. One of your state colleges or universities will have a Cooperative Extension System office where you can send or drop off soil samples for a modest fee. Your nearest Cooperative Extension Service office can offer soil testing and guidance on how much fertilizer to use.

Integrated Pest Management, or IPM, is an approach developed to look beyond the immediate pest in isolation to get a sense of the broader picture and develop a more targeted, less damaging approach. Its main focus is long-term prevention, managing the ecosystem, monitoring and identifying pests to decide if controls are necessary, and combining methods of management for greater control. Combined approaches include biological controls like introducing predators, cultural controls like changing watering practices, mechanical and physical controls, such as traps and barriers, and chemical controls that are selective to the targeted pest and will do the least damage to the environment.

For example, if an insect is eating the leaves of a crop, a gardener using the IPM approach would do several things before reaching for the pesticide spray:

Accurately identify the pest

Assess the magnitude of the problem and decide if control is necessary. Is it a massive infestation doing great damage, or are there just a few individuals doing minimal damage?

What natural predators or other controlling agents, such as pest-targeted diseases, are present to limit the pest's numbers?

The gardener would then apply the least harmful solution -- perhaps a large release of predator wasps, or a spray of Bacillus thuringiensis, or Bt -- before resorting to a poison spray. The wasps and Bt offer the added benefit that they are safe for the environment and are not toxic to humans or wildlife. IPM is more about management than control -- limiting pest numbers, rather than eliminating them entirely. States and municipalities and the Cooperative Extension System provide gardeners with information and training on IPM.

Since insects have to eat plant parts treated with Bt for it to be effective, spray entire plant surfaces with the product. It can take days to a week for the results to show.

Bt degrades in sunny conditions and depending on the formulation can last up to one day to less than one week. Follow product instructions and reapply as recommended.

For more information on using pesticides and fertilizers, see "The Pros & Cons of Pesticides & Fertilizers."