It’s hard to imagine that the granular fertilizer you spread on your lawn last Saturday can end up in the ground water and local waterways within days, weeks, months or years. Yet what you sprinkle onto your lawn and in your garden eventually end up in the water system if not used by plants for growth and maintenance. Before reaching for a bag of commercial fertilizer, it’s important to understand how smart fertilizer use and proper application can prevent run off that can kill fish, wildlife and plants in your local watershed.

Understanding Fertilizer

The ingredients in a bag of fertilizer are the manufactured equivalents of naturally-occurring elements. All brands of inorganic (manufactured) fertilizer are labeled with three letters: N, P and K, which stand for nitrogen (N), phosphorous (P), and potassium K). While each of these elements are found abundantly in nature, a bag of commercially prepared fertilizer concentrates the amount of each in an established balance that is printed on the bag’s label.

The numbers that correspond to the letters represent the percent of each element found in that particular fertilizer formulation. A bag labeled “10-20-10” contains 10 percent nitrogen, 20 percent phosphorous, and 10 percent potassium. These three elements are called “macro nutrients” because they are the major nutrients used by plants. Plants need many other minerals to grow and develop properly, but these are the three major ones gardeners use to prevent and solve problems.

High first-number fertilizers or high nitrogen fertilizers are commonly sold for lawn care because nitrogen is the ‘leaf building’ nutrient that supports strong leaf growth. Turf grass benefits from higher nitrogen fertilizers. Phosphorous boost flower production, and by extension fruit and seed production, so bags of fertilizers sold as vegetable and flower food usually have a higher middle number. The last number, potassium, builds strong roots. A ‘balanced’ fertilizer where all the numbers are equal supports overall plant health.

Soil pH Affects Nutrient Absorption

Once little-known fact among most gardeners is that the soil pH actually changes how well plants can absorb nutrients. If the soil pH is too far outside the plant’s preferred range, it may not be able to use all of the nutrients available to it in the soil. Adding more fertilizer in such a situation is a recipe for disaster; not only is it a waste of money, but much of the excess fertilizer washes into groundwater or nearby waterways.

A good examples of this principle in action occurs in tomatoes. Blossom end rot, a common problem in home-grown tomatoes, is characterized by a black circle on the blossom end of the tomato. It occurs when soil pH is too acidic or when calcium is lacking from the soil. If the soil is too acidic, the plant cannot use the available calcium. If calcium is lacking, adding a typical N-P-K fertilizer can actually be harmful because it temporarily acidifies the soil. Without knowing more about the particular soil in which the tomato plant is grown, it can be difficult to judge how much or what kind of corrective action is necessary to fix cases of blossom end rot. Many other plant diseases are related to soil pH and nutrient deficiencies.

The Importance of Soil Testing

The first rule of smart fertilizer use, then, is to get your soil tested. Before sprinkling fertilizer onto your lawn and garden, get a soil test conducted by your local garden center or Cooperative Extension office. Soil tests are inexpensive, and you prepare the sample at home. Cooperative Extension offices typically provide instructions and guidance on how to prepare your samples.

A basic soil test will tell you:

  • The composition of the soil in your garden - loam, sand, clay, or some variation in between
  • The soil pH
  • Macro nutrient analysis (N-P-K)
  • Minor nutrient/fertility analysis

After a soil test, you will know exactly which nutrient and how much to add to your soil to bring it to the desired consistency. If your soil is too acidic, recommendations will be given to you to raise the pH; if your soil lacks any major nutrient, recommendations are given to boost that particular nutrient without washing extra fertilizer.

Damage Caused by Fertilizer Run Off

Many people follow the adage that if some is good, more is better. Unfortunately with fertilizer, that’s not necessarily so. If you add too much fertilizer to your lawn and garden, the extra doesn’t wait around for plants to need it. Instead, rainwater and sprinklers wash the extra fertilizer off of your lawn. If you live in an urban or suburban environment, chances are it washes the fertilizer into the street, where rainwater carries it into the sewer system. There, the water flows to a sump or groundwater reclamation site, or perhaps it flowers directly into a stream or river. From there, streams and rivers carry it into lakes and oceans.

One household who dumps too much fertilizer onto a lawn isn’t going to do much harm, but multiply that by the number of square miles of lawn in America, including golf courses and athletic fields, and the number is staggering. According to NASA, there are 49,00 square feet of lawn in the United States, making grass grown on more acres than corn! If just a tenth of those households use conventional fertilizer improperly, the chemical run off can be quite damaging to the environment.

As you may recall, nitrogen boosts leaf development, which is why it’s used extensively in turf fertilizers. When excess nitrogen runs off into streams, lakes and oceans, it fertilizes plankton and algae, causing excessive growth. You’ll see this as an unsightly “scum” over the top of a pond, for instance. Such overgrowth makes it hard for fish and other aquatic wildlife to breathe, since the plants deprive the water of oxygen. It also makes water unsuitable for drinking or swimming.

Nitrogen can also be absorbed through soil into ground water, contaminating wells and making the water unsafe to drink. Too much nitrogen in drinking water can cause babies to develop “blue baby” syndrome; stomach bacteria convert the nitrogen to a derivative of nitrate that prevents oxygen from circulating in the blood.

Smart Fertilizer Use Tips

Many gardeners appreciate the ease and convenience of using commercially prepared fertilizer. If you plan to continue using bagged fertilizer, follow these smart fertilizer use tips to prevent waste and run off that can harm the environment.

  • Get your soil tested each spring and add only the amendments the analysis shows are needed.
  • Follow a fertilizing schedule recommended by your local Cooperative Extension. Don’t fertilize more in the hopes that your plants will grow more; the extra fertilizer may be wasted.
  • Consider converting some high nitrogen use areas, like lawns, into naturalized garden areas. Native plants typically need far less (if any) conventional fertilizer, and are better adapted to the local climate.
  • Consider adopting organic gardening methods, using compost, compost tea, manures and cover crops instead of conventional fertilizers.

Just by paying attention to your garden and avoiding excess fertilizer use, you can do your part to help preserve the local ground water, prevent fertilizer run off and waterway contamination. Smart fertilizer use begins with knowledge and continues with action. Take action today to minimize your garden’s impact on the environment.