Lawn Fertilizer FAQs


What is fertilizer?

Fertilizer is a product that combines the nutrients that plants need to grow – nitrogen, phosphorus, and potassium. Think of it as plant food.

What is phosphorus?

Phosphorus is an element found in rocks and minerals. The phosphorus used in fertilizers comes from the fossilized remains of ancient marine life found in rock deposits in the U.S. and other parts of the world, or from plant and animal materials. Phosphorus helps plant health, density, and root growth. Phosphorus is the plant world’s equivalent of carbohydrates – it provides the energy that a plant needs to grow.

What is nitrogen?

Nitrogen is a naturally occurring element and makes up about 78% of the air we breathe, It is a major component of foods and fertilizers. Nitrogen also comes from the earth – soil contains organic matter, which contains nitrogen compounds. Nitrogen is essential to the growth of all living things, both plants and animals. In plants, nitrogen is important for growth and helps keep the plant green. Part of the proteins and DNA found in cells is made of nitrogen.

Which number on the bag of fertilizer is phosphorus and which is nitrogen?

The three numbers on the label are nitrogen (N), phosphorus (P) and potassium (K), N-P-K. Phosphorus-free fertilizer has a zero (0) in the middle. Nitrogen-free fertilizer will have a zero (0) as the first number.

How do I know if the fertilizer is phosphorus or nitrogen free?

Three numbers are displayed prominently on the bag of fertilizer. Look for a zero (0) as the middle number of the three numbers. These numbers represent the percentage by weight of nitrogen (N), phosphorus (P), and potassium (K), respectively. Phosphorus-free fertilizer has a zero (0) in the middle and nitrogen-free fertilizer has a zero (0) as the first number.

With the exception of lawn starter fertilizers, almost all lawn fertilizers found in home improvement stores are now phosphorus-free.

How do nutrients move around and affect our water quality?

Nitrogen (N) and phosphorus (P) are essential to plant and animal life. Problems with these nutrients as a water pollutant result not from their presence, but from the addition of excessive amounts. Nutrients enters lakes and streams in one of three primary ways:

  1. stormwater run-off (sediment, leaves and pollen, dead plants and animals, animal waste, septic seepage, fertilizer),
  2. wastewater treatment facility discharges and
  3. agricultural run-off.

When transported into aquatic systems, nutrients cause weed and algae growth. When nutrient levels are too high, excess plant and algal growth creates water quality problems. Plants begin to die and decompose, depleting the dissolved oxygen supply in the water – a condition called hypoxia, which can lead to fish kills in some cases. Nutrients are also released from the sediment and from decomposing plants back into the water column, continuing the cycle or algal or plant growth. The reaction of the aquatic system to an overloading of nutrients is known as eutrophication. Hypoxia and eutrophication, to some extent, occur within many of our rivers every year, and occur on a larger scale in the Chesapeake Bay. Eutrophic lakes and streams are also more likely to have higher levels of blue-green algae, which can produce blue-green algal toxins.

Is organic fertilizer the best option?

Not necessarily. Organic fertilizer is made of natural materials. Examples of organic fertilizer include livestock manure, feather meal, blood meal, cotton seed meal, compost, etc. Organic fertilizers also typically contain phosphorus. If the organic fertilizer phosphorus number on the bag is low, often is the nitrogen number. Turfgrass managers will usually recommend nitrogen numbers in the twenties (20s); therefore, if organic fertilizers with low nitrogen percentages are used, the recommended application rates (total amount applied) are often high resulting in a larger amount of phosphorus being applied than is ideal. Even low phosphorus products can add up to large loads of phosphorus depending on how much is applied.

Types of Fertilizers

To have a good looking lawn, do I need to fertilize?

Grass (and all plants) need nutrients to grow. A mature lawn, with healthy soil requires less fertilizer than a new lawn. If you leave your grass clippings on the lawn (natural fertilizer) you are recycling the nutrients back into the soil and no additional fertilizer is usually needed. If you choose to fertilize, apply a fertilizer designed for lawns. Having a soil test done before fertilizing will tell you if your soils needs to be fertilized. If you are establishing a new lawn, then it is recommend to use a lawn starter fertilizer to aid in root establishment and new growth.

What are the best practices for lawn care?

Mow the lawn at a high mower setting. Taller grass is stronger grass and is better able to capture and absorb rainfall and prevent runoff.

Keep grass clippings and other plant materials, and fertilizer, off of hard surfaces (sidewalks and streets) where they can be washed into storm drains.

Maintain healthy greenspaces (trees, landscape and lawn) to capture rainfall and prevent run-off and sedimentation. Unless a soil test shows a need for phosphorus, there is no reason to add more.

If you choose to fertilize your lawn, the best times to do so are in late Spring and early Fall. Fertilize little to none during the summer. A fall fertilization program will produce the healthiest turf throughout the year. Do not apply fertilizer if the forecast calls for a heavy rainfall. No fertilizer should be applied between November 15 and March 1. Never apply fertilizer to frozen soil.

What about weeds? Aren't most fertilizers "weed'n'feed" fertilizers?

Most lawn fertilizers are sold as “weed ‘n’ feed” fertilizers. This is the case because when you apply an herbicide to kill some plants in your lawn (‘weeds’), it’s important to simultaneously encourage the growth of the plants you’re trying to maintain. Herbicides, however, come with additional unintended consequences to pollinators, pets, and water pollution. Please consider your priorities and choices carefully in order to balance your needs and the environment.

Does bad taste or smell of my drinking water have anything to do with nutrients in the water?

If you have “city water” that comes from a lake or river rather than a well or aquifer, algae may cause bad tastes or smells in your water. Two common taste and odor compounds produced by algae as a byproduct of their growth are MIB (methyl-isoborneal) and Geosmin that cause the water to taste and / or smell earthy or musty when there is a large algal bloom. The compounds are harmless – but unpleasant. The treated water coming from your faucet is still safe to drink.