Phosphorus: friend and foe

You can find three numbers on every bag of fertilizer. They tell you, the consumer, just how much nitrogen (N), phosphorus (P) ,and potassium (K) are in that container.

The obvious question is what numbers do you need? To answer this question, Don Kretchmer of Wolfeboro, limnologist and Certified Lake Manager, tells people to have their soil tested before applying any chemicals to a lawn, garden, shrubbery or trees. (Limnology, by the way, is the study of inland waters, and Kretchmer’s certification comes from the North American Lake Management Society.)

Why don’t I just go for the bigger numbers? Well, too much of a ‘good thing’ can in fact be bad. For example, too much nitrogen can cause a plant not to flower or bear fruit. Too much potassium can inhibit the absorption of other nutrients. This can then lead to problems caused by nutrient deficiencies. Plus, excess of any of these chemicals is free to go elsewhere, causing other problems.

The second number on the bag represents the amount of phosphorus. “Phosphorus is a necessary ingredient for plant growth. About 0.2% of a plant’s dry weight is composed of the element” according to Theodorou and Plaxton. Phosphorus provides essential components for processing nutrients as well as enzymes and other factors that contribute to the plant’s health.

But Kretchmer explains that added phosphorus in streams and lakes can, and often does, contribute to excessive growth of vegetation and algae. A common example of this is phosphorus’s role in algae blooms.

“One of the ways we judge the quality of water is, literally, its color”, notes Kretchmer. He cites two experiments: one by his daughter, repeated over several years, using Lake Wentworth water, and another by the Canadian government on a lake in Canada.

In both cases, the water samples without added phosphorus remained bluer; the portions with added phosphorus turned green due to increased algae and other vegetation. Green, “cloudy” water is not attractive to lake users, and some of the species of algae can be toxic to humans and pets.

Another unwelcom impact of phosphorus is increased growth of aquatic plants, including some invasive species. A prime example of this is milfoil, an invasive plant in this area. Because we have no effective natural treatment options for milfoil at this time, once it is established, it may need to be pulled or require herbicide treatment to keep it from spreading further.

Either of these efforts can be thwarted by too much phosphorus. (Of course, prevention of a milfoil problem rather than trying to fix it is the best solution. To do this, it is important to keep milfoil from being transferred into an unaffected body of water.)

So, what can you do to prevent excess phosphorus from entering surface waters?

First, you have to determine the ways excess phosphorus is getting into the water. There are myriad sources. Among them are runoff from lawns and gardens; soil erosion from gravel and dirt roads; water that flows over hard surfaces such as paved roads, parking lots, and roofs; non-functioning septic systems; and waste from waterfowl and other animals. These sources can found anywhere in the watershed.

For some of these sources you can control or redirect the excess phosphorus and runoff; for others there are natural solutions. Kretchmer notes “traditionally developed sites send 5 to 10 times more phosphorus into nearby surface water than similar sized forested sites.” So thoughtful land development and management are key.

A natural solution is to have a buffer of native bushes or trees between the lawn and the waterline. This slows the runoff and allows the water to infiltrate the soil, which helps to remove the phosphorus. Kretchmer offers one very encouraging note: “Phosphorus loves to adhere to soil.” If we can ensure that runoff doesn’t enter the rivers and lakes directly, but infiltrates the soil, we will keep the waters cleaner.

An example of controlling excess phosphorus is when a soil test shows you don’t need phosphorus, apply a zero-phosphate fertilizer, sold in many locations, including Bradley’s in downtown Wolfeboro. Better yet, since most of our soils contain sufficient naturally occurring phosphorus, released when water is added, watering your lawn is usually all that is needed. Even better, the most environmentally friendly solution is to have natural landscaping, with no lawn or plantings that require fertilizer or watering.

A winding path to the shore rather than a “straight shot” into the water is helpful. The bends in the path prevent unfiltered runoff from reaching surface waters by encouraging the water to flow into soil at each curve in the path.

Not feeding waterfowl near the shore makes a difference. And, of course, fixing a leaky or non-functioning septic makes a huge contribution in nearby water quality.

On a larger scale, paving with permeable asphalt, building retention ponds and ensuring road runoff from paved and gravel roads is buffered are all good practices. Like many parts of life, best management practices have benefits. And many of these practices are free or reasonable in cost.

So keep phosphorus-laden sediment from entering nearby rivers and lakes by making changes on your own property: reduce erosion, force stormwater to infiltrate into the ground, leave a buffer of vegetation along the shoreline, use phosphorus-free products, maintain your septic system, do not feed wildfowl, and properly maintain your gravel road and driveway. If you find you need to use lawn fertilizer, buy the brand with the second number (phosphorus (P)) equal to “0”.

As an added benefit, if residents in the Lake Wentworth and Crescent Lake watershed do these things, they will help us reach the goal of a 15% reduction in phosphorus in these two lakes over the next ten years.

For other ways you can control stormwater runoff on your property, check the Best Management Practices section of the Watershed Management Plan Report Appendices on this site for ideas.

Inexpensive soil testing is available from UNH Cooperative Extension. For more information go to: