University of Vermont Extension 
Department of Plant and Soil Science

Spring News Article


FERTILIZING LAWNS

 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
   
Fertilizer is essential for good lawn growth, yet with good lawn management and soil testing you often can reduce the amount needed.  Soil test kits are available from state Extension Services, and include instructions for proper sampling.

Testing your soils every three years or so is a good practice. A key component of a soil test is the soil acidity, or pH.  If this is not between 6.0 and 7.0 (the latter being neutral, between acid and alkaline), certain nutrients won’t be available to the lawn, no matter how much you apply. 

If available, have your soil organic matter tested too.  It should be above three percent.  If lower, you should sprinkle compost heavily (up to half inch) over the lawn a few times during the season. Make sure the compost is aged, and free of weed seeds.

Mow more frequently to avoid having an excess of grass clippings so you can leave them on the lawn.   This simple practice recycles nutrients back into the soil, and has been shown in research at the University of Connecticut to reduce fertilizer needs by 50 percent or more.

In Vermont and similar cold climates with shorter growing seasons, fertilize lawns in late April, early summer, and then late summer.  If you leave grass clippings, and have good organic matter in the soil, you may need to only fertilize twice—early spring and late summer. At each application time, you should apply one pound of actual nitrogen for every 1000 square feet of lawn.  The key words here are “actual nitrogen”, not pounds of fertilizer.  Of the three numbers on a fertilizer bag, the first is percent nitrogen.  So a 5-3-4 fertilizer has five percent nitrogen.  To figure how much fertilizer will give you one pound of nitrogen, divide one by the first number (one divided by five is 0.2), then multiply by 100.  So 20 pounds of 5-3-4 will yield one pound of actual nitrogen.  If you have 5000 square feet for instance, you’d need five times that, or 100 pounds of fertilizer, evenly spread over the area.

Apply fertilizer with a fertilizer spreader. Spreading fertilizer by hand will always cause some spots to be over-fertilized and others to have none. When using a spreader, be sure to get complete coverage of the lawn. Keep moving at a constant speed to prevent uneven spreading. Any missed spots will appear quite yellow. Do not fill the spreader when it is sitting on the lawn. Fertilizer spills are inevitable and may cause a large dead spot that persists for weeks.

Use caution when applying fertilizer combined with herbicide, especially with broadcast spreaders. These spreaders can throw the material into flowerbeds where the herbicide can injure desirable ornamental plants, or tree and shrub roots can pick these up from under lawns.

Lawn fertilizers vary in composition and price. An ideal composition for a chemical lawn fertilizer is a 4-1-2 ratio of nitrogen, phosphorus, and potassium, for example 20-5-10. Many others created for lawns, that may not be this exact ratio, will work fine too.  For a natural fertilizer, a 3-1-2 ratio is good, but anything close such as a 5-2-4 fertilizer works fine.  Some areas where phosphorus (P, the middle number) may be carried with rain into watersheds, causing potential pollution and algae blooms, restrict P usage.  Check with your local supplier on such rules.  If your soil has sufficient P already, as many do, none may be needed.  In fact, some lawn fertilizers no longer contain P.

The price of the fertilizer relates somewhat to the analysis and the nutrient carriers used in the fertilizer. Less expensive chemical fertilizers are usually water soluble, thus have a high potential to burn the grass if applied when hot, at high rates, and not watered in after spreading.  Being water soluble they are easily leached out of soil with rain and watering, so may pollute water sources.  Yet water-soluble fertilizers will give a response for four to six weeks, become available in the spring when temperatures are still cool, and result in more and darker growth sooner.  If overfed with such fertilizers, succulent grass growth may be more susceptible to insects and diseases.

More expensive fertilizers are not water-soluble (often seen as WIN for water insoluble nitrogen), have low burn potential, and give a response for up to eight weeks, or longer. These fertilizers rely on microorganisms in the soil to release the nutrients. Since the microorganisms are not active when the soil is cool, the fertilizers will not become available early in the spring. Not being water soluble is the reason they stay around longer, and so have less potential for pollution from runoff.  Many good lawn fertilizers will have a mix of both water soluble, and water insoluble nitrogen.  Where lawns are watered regularly, especially on sandy soil, these water insoluble fertilizers should be used.

Many natural-based fertilizers are water insoluble, so last for a long period.  Such natural-based or organic fertilizers have other benefits, such as aiding soil microorganisms needed for healthy soil.  These various forms of soil life provide many benefits, including converting nitrogen from the air for use by plants, producing carbon dioxide which plants need, dissolving nutrients from minerals in rocks, decomposing thatch, aerating the soil, and helping to reduce pests and diseases. So in spite of lower numbers (percent of nutrients) on the bag than with chemical fertilizers, these natural fertilizers enrich the soil in many more ways than the chemical fertilizers.  

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