How much fertilizer do lawns need?

Lawn fertilization can be an effective way to consistently maintain a healthy lawn or improve an unhealthy one.  When applied correctly, the added nitrogen, phosphorus, and potassium of many common fertilizers can stimulate root growth, increase photosynthesis, and benefit flowering, but overfertilization has a number of negative effects.

 

In Connecticut in 2015, close to 8% of our land was turf or lawn, increasing from 6% in 1985, almost the same amount as coniferous forests and over twice as much as water coverage (1). This means that 390 million square miles of lawns have the potential to be overfertilized each year, with many being in close proximity to bodies of water.

 

What can overfertilization do?

  The main worry with overfertilization is that it will lead to eutrophication, over richness of nutrients in bodies of water caused by runoff, in nearby and downstream bodies of water. This eutrophication provides a surplus of nutrients, namely nitrogen and phosphorous to photosynthetic algae and microalgae (cyanobacteria), which in turn bloom to create massive carpets that block out sunlight and create hypoxic conditions (lack of oxygen) that lead to large dead zones. One study, for example demonstrates how this eutrophication leads to hypoxia and dead zones in freshwater systems and their estuaries across New England, reducing ecosystem functions, biomass, and species richness by up to half. By avoiding overfertilization, a recovery of these ecosystems is possible.

 

What are the limits?

Many states, including CT, have limits on when you can fertilize your lawn and what you can fertilize with. In CT, this law applies largely to phosphorus fertilizers, which are the primary cause of eutrophication in freshwater systems. In CT this means that you cannot apply phosphorus fertilizers containing over 0.67% phosphorus to your lawn between December 1 and March 15, and you cannot use them outside of that range unless a soil sample proves your lawn is phosphorus deficient. Other states, like New York, go beyond this ban to limit the amount of nitrogen (1lb per 1000sq miles) you can apply or how close to a body of water you can apply fertilizers. Whatever the case, always look at your town and state regulations before fertilizing!

 

Here are some tips to correctly and safely fertilize your lawn:

• Don't fertilize unless your grass is currently growing (over 55 degree weather)

• Use a controlled release or organic fertilizer to increase effectiveness and decrease nutrient losses (2)

• Leave grass clippings on your yard to maximize organic matter

• Twice as much fertilizer does not mean your grass will grow twice as strong

• The best times for fertilization are late spring and late summer, as grass is consistently growing and heat is manageable (plants will slow growth in harsh mid summer heat and fertilizers can wick moisture from roots)

• Follow your state specific regulations, typically banning phosphorous explicitly unless a test proves otherwise (NY, CT)