Fertilizing Lawns

Contact: Dr. Leonard Perry, Extension Professor University of Vermont

There are no two ways about it. Fertilizer is essential for good lawn growth. Depending on the type of grass, from one to three fertilizer applications should be made during the growing season, usually early May, early to mid-June, and late summer.

Lawn fertilizers vary in composition and price. The ideal composition for a lawn fertilizer is a 4-1-2 ratio of nitrogen, phosphorus, and potassium, for example 20-5-10. However, most available lawn fertilizers will not have this exact ratio but will still give good results. The price of the fertilizer relates somewhat to the analysis and the nutrient carriers used on the fertilizer.

Cheap fertilizers are usually water soluble, thus, have a high potential to burn the grass. Water-soluble fertilizers will give a response for four to six weeks.

Many of these fertilizers have disclaimers on the bag stating they will not burn the grass if the directions are followed. The directions usually state that the fertilizer must be watered-in immediately after spreading. Because these fertilizers are water-soluble they become available in the spring when temperatures are still cool.

More expensive fertilizers are not water-soluble, have low burn potential, and give a response for up to eight weeks. 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.

Where lawns are watered regularly, especially on sandy soil, the more expensive types of fertilizers should be used. Heavy watering will dissolve water-soluble fertilizer and flush it below the root zone of the grass plants.

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. Any missed spots will appear quite yellow. Most lawn fertilizers are packaged so that the right amount of nutrients is applied per 1,000 square feet. Generally, about one pound of actual nitrogen is required at each fertilization (five pounds of 20-4-12 containing 20 percent nitrogen).

Do not fill the spreader when it is sitting on the lawn. Fertilizer spills are inevitable. Spilling water-soluble fertilizer causes a large dead spot that persists for weeks. Begin applying the fertilizer by making "header" strips around the border of the lawn. Then start at one edge and go back and forth across the lawn. Make sure each strip overlaps the previous strip. Turn off the spreader when the header strip is reached. Do not turn the spreader while fertilizer is dropping through onto the grass. Such corners are over-fertilized and the grass could be burned.

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.

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