Envision the lawn you’d be happy with and fertilize accordingly, says OSU expert

Last Updated: 
April 30, 2008

CORVALLIS, Ore. - For many gardeners, summer wouldn't be summer without a smooth expanse of dark green grass and the smell of freshly mowed clippings. If you're in this group, you need to know some basics about fertilizing your lawn before the grass starts growing this spring.

Most lawns in Oregon can survive with little or no fertilizer, explained Tom Cook, associate professor and turf grass specialist at Oregon State University. But adequate fertilizer can make all the difference between a straggly weedy lawn and a lush green one. Added nutrients can help produce a better looking and more robust lawn that resists diseases and competes with weeds.

When people ask Cook how much fertilizer to use, he asks them to envision the kind of lawn they want. If a homeowner wants a consistently dark green carpet of grass, they may need to pay the price of more frequent mowing, more water and more fertilizer. If someone isn't that obsessed with having a perfect ultra-green lawn, then less fertilizer and work will be required.

For his own lawn, Cook is a minimalist.

"I coast along with as little fertilizer as I can and then make a strategic application about three weeks before my in-laws visit," he said. "The lawn looks good when they show up, and in between visits I don't have to mow so much grass."

The basic approach is to apply the least amount of fertilizer to produce healthy grass and the appearance you want, he said.

"Complete" fertilizers contain nitrogen, phosphorus, and potassium in varying proportions. Nitrogen applied in the proper amounts makes the grass green. Potassium may increase hardiness. According to Cook, lawns don't need phosphorus unless the soil is deficient in it. The "vast majority" of lawn soils already contain enough phosphorus, he said. However, since it's included in complete fertilizers, he suggests picking one with low phosphorus content if you have a choice.

All complete fertilizers are labeled to show the proportions of these three components, always in the same order: nitrogen, phosphorus, potassium (N-P-K). A label that says "6-1-4" refers to the ratios of nitrogen to phosphorus to potassium. Cook says you can be confident in ratios ranging from 6-1-4 to 8-1-2 "or anything close to that range."

Other nutrients that are often included in fertilizers are sulfur and iron. Sulfur is an important plant nutrient, especially for bent-grass, common in western Oregon lawns.

Iron supplement isn't usually needed for healthy grass, but it is often included in commercial fertilizers because it provides a fast, though temporary greening response.

If you apply a fertilizer with iron, be careful applying it near concrete sidewalks and driveways. "If it gets on these surfaces and then gets wet, you'll have brown stains that are difficult and expensive to remove," he warned.

A drop spreader for fertilizing most lawns is best, said Cook. "You might want to use a rotary spreader if you have an exceptionally large area. But rotary spreaders are less accurate, so it's difficult to control just where the fertilizer ends up."

A major complaint about lawn fertilizer is that it gets leached or washed away and causes surface and groundwater pollution. Cook says this association is over-exaggerated and cites a study by Pennsylvania State University in 1999 that concluded that only 0.3 percent of nitrogen pollution and 0.2 percent of phosphorus pollution came from golf courses and lawns.

"If you use common sense, you can be confident you are not contributing to pollution," he said.

Cook also busts the commonly held belief that grass clippings contribute significantly to thatch buildup. "In fact, they extend the effect of fertilizers by recycling nutrients during decomposition."

For more information, Cook's OSU Extension Service publication, Fertilizing Lawns, includes detailed guidelines for why, when and how to fertilize a lawn. He also writes about common lawn grasses and the optimal pH range for each. If your soil needs to be adjusted for pH, you can follow the instructions in the booklet. The publication also includes tips to help prevent pollution with fertilizer and discusses the difference between organic and synthetic fertilizers, plus has other topics on lawn health and nutrition.

The OSU Extension Service offers several lawn care publications, available to download for no charge.


Author: Davi Richards
Source: Tom Cook