Lake Safe Fertilizing

If you plan to fertilize your lawn, keep in mind that the type of fertilizer you use and how and when you apply it can have a big impact on the environment and the watershed around you. What happens in the watershed stays in the watershed. Fertilizer contains nutrients that plants need, such as nitrogen, phosphorus and potassium. If not applied properly, those nutrients can be carried away by stormwater runoff into lakes and ponds. These nutrients are causing too much plant growth in the surface water which can cause real damage. Many freshwater ecosystems of New Hampshire are grappling with issues of excess nitrogen and excess phosphorus, and the negative impacts that stimulated plant growth in lakes and ponds leads to.

Excess plant growth in lakes and ponds can: 

  • Stimulate algae blooms including cyanobacteria, which has been proven toxic for humans and animals. 
  • Reduce oxygen levels for fish. 
  • Fill in lakes or ponds over time. 

There are alternatives to using fertilizer; for example, you can get a similar effect by leaving lawn clippings and leaf litter on your lawn. The nutrients grass use to grow are trapped in the blades. Leaving grass clippings to break down in your lawn returns those nutrients to the soil to be used again. (Good news! Leaving clippings does not contribute to thatch.) You are feeding your lawn and saving yourself the work of emptying a mower bag. If you do want to use fertilizer, though, you can help to protect our lakes and ponds, whether you live near them or not, by following some general application guidelines. 

How to pick the Right Fertilizer

No two fertilizers, or lawns, are necessarily equal. You will see that fertilizer packaging comes with a number code, such as “33-0-4” and “5-10-15,” corresponding with an N-P-K. These numbers tell how much of a nutrient is in the fertilizer: the first number is N (nitrogen), then P (phosphorus) and lastly K (potassium). 

The amount of certain nutrients you will need in your fertilizer depends on what your soil is lacking. Most established lawns in New Hampshire do not need phosphorus. New Hampshire soils naturally contain the phosphorus most lawns need to grow.

You don’t want to apply more nutrients than the plants can take up – that wastes money, labor, and causes pollution. Look for N-P-K that are 10 or lower – these are generally organic (natural) sources that are naturally slow release. Look for the OMRI label. In addition to slow-release, look for water-insoluble nitrogen. A water insoluble choice will help decrease the number of applications needed. 

Soil tests will always be the best way to see what nutrients your lawn needs. There are soil testing facilities across the state, including the UNH Cooperative Extension and the Natural Resources Conservation Service in New Hampshire.

How and When to Fertilize

One of the biggest causes of excess nutrients in surface waters is over-application of fertilizers. Know your lawn’s square footage to determine how much is needed. 

Important: It is illegal to apply fertilizer within 25 feet of a lake or pond over 10 acres in size and many streams and rivers that are protected under the New Hampshire Shoreland Water Quality Protection Act. It is a great idea to treat all water bodies in the same way. When storing fertilizer for future use, make sure to store it in a dry place and close the bag with a clip, so there isn’t a danger of nutrients leaking from the bag. 

Fertilize your lawn in the spring – between April and mid-June – or fall – between Labor Day and Oct. 1. Mid-May is a great time to apply fertilizer as the grass is starting to grow, but fall is better because it allows the plants to store nutrients for the next year.  

Never fertilize in July and August as grass grows less in the heat and won’t be able to absorb the fertilizer. The unabsorbed fertilizer is easily be picked up by rainwater and washed into lakes and ponds. Check the weather forecast before application too, and be sure to apply it before a small to medium rain event, but not before a big storm that could cause it to all be washed away.

These simple measures will ensure that your lawn gets all the help it needs without fertilizing the lake. You can find more helpful information on being lake minded on our site under Community Resources.

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