Fertilization of Cool Season Grasses
Fall is the optimal time of the year to fertilize cool season turfgrass (e.g., Kentucky bluegrass, tall and fine fescues) . The advantages of fall fertilization are increased density, increased root growth, decreased spring mowing, improved fall-spring color, decreased weed problems, increased drought tolerance, and decreased summer disease activity. Cooler temperatures and shorter days provide ideal conditions to maximize root growth and food storage prior to winter. Proper fertilization will help provide quality turf when spring arrives. However, Virginians often overuse fertilizer and lime which can negatively impact surface and ground water quality.
If soil already has nutrients, why do we need to add fertilizer? Although the amount of nutrients in most soils is relatively sufficient in comparison to the grasses’ requirements, much of this potential supply is unfortunately in a form plants cannot use; or else the nutrients are not supplied fast enough at the time the plant is growing in the fall. The value of the fertilizer depends on the total amount of nutrients and the source of nitrogen in the fertilizer.
The best way to determine if your lawn requires certain nutrients is to get the soil tested about every 3 years. The results will provide the amount of phosphorus, potassium, calcium and magnesium available in your soil. (See April 2015 Garden Shed newsletter for further information on soil tests). Since growing conditions are ideal at this time of year, grasses respond quickly to soil test recommended applications of fertilizer. Lime takes weeks and months for benefits to be fully realized, making fall and winter the best time for its application.
The soil test will also tell you the acidity (pH) of your soil and how much, if any, lime is needed. Our region is very acid and may require supplemental lime. The pH of the soil governs what nutrients are available to plants. If the soil pH is above or below the recommended range (6.2 – 7.0), nutrients may not be soluble (absorbable by plants) or they may be so soluble that they become toxic. Therefore, a plant can show signs of nutrient deficiencies or toxicity even when the correct amount of fertilizer is applied . Too many times, homeowners put down lime every year not knowing if it is even necessary. The soil report tells specific amounts of lime, phosphorus and potassium your soil needs to provide adequate nutrition for the turfgrass. Nitrogen, however, cannot be reliably evaluated by a soil test, therefore the test results will not contain that recommendation.
What if I don’t fertilize?
Without the proper nutrients your lawn will gradually thin and weeds will invade. Healthy lawns have less disease, fewer insect and weed problems . If you have a healthy stand of grass, there is less chance for nutrient and soil runoff to surface waters .
All fertilizer packages must have three numbers present on the package such as 10-10-10 or 16-4-8. These numbers represent the percentage by weight of nitrogen or N, phosphorus or P and potassium or K. So a 50 pound bag of 10-10-10 contains 5 pounds each of nitrogen, phosphate and potassium (calculated as 50 pounds times 0.1 or 10% equals 5 pounds). These three elements are the primary minerals needed for plant growth. The package also details the percentage of other nutrients included such as iron and sulfur.
NITROGEN (N) Fall fertilizer with appropriate amounts of N lead to better turf next spring, including improved turf density, color, above ground vegetative growth and food storage. Nitrogen is absorbed by plants from the soil in the greatest quantities.
PHOSPHORUS (P) The second number on the bag is phosphorus which is essential in all phases of plant growth, most notably root growth which takes place over fall and winter.
POTASSIUM (K) Also known as potash, potassium is essential for plant growth and plants take large amounts from the soil. The word potash goes back to colonial days when wood and other organic materials were burned in pots for the manufacture of soap. The ashes were rinsed with water, collected and allowed to evaporate. The residue was largely potassium salts. Today potassium is mined from deposits deep in the earth. Plants use as much potassium as they do nitrogen which is three to four times the amount of phosphorus used.
In general lawns respond better to fertilizer ratios high in nitrogen. However, no more than 1 pound of N/1000sq ft should be applied in a single application. Slow release fertilizers may be used in greater quantities than water soluble fertilizers without the threat of burning the grass. They are more expensive, but are more convenient and may be used less frequently. We’ll look at precise calculations later.
Nutritional needs vary from month to month. The source of nitrogen in fertilizers influences the availability of this element to the turf grass. There are two types of nitrogen sources: quickly available and and slowly available. The label on the bag describes the release characteristics of the nitrogen using the initials WIN and WSN which stand for water insoluble nitrogen and water soluble nitrogen respectively. WIN provides nitrogen that is slowly available because it must be broken down to a simpler form for use. Solubility depends on the kind of material used in the composition of the fertilizer. WSN dissolves quickly and is usually in a simpler form, such as ammoniacal nitrogen.
Plants absorb nutrients continuously, so it is beneficial to provide them with a balance throughout their growth. This is probably best achieved by using slow release fertilizer, which releases nutrients at a rate that makes them available over a long period of time.
Slow release fertilizers (WIN)
- fewer applications
- low burn potential
- comparatively slow release rate
- Unit cost is high. but lawn may not require as much
- release rate governed by other factors other than needs of the plant
Conventional fertilizers (WSN)
- Fast acting
- lower cost
- greater burn potential
- solidifies in bag when wet
- nitrogen leaches readily
Measure your lawn
It is critical to know the size of your lawn in order to calculate the amount needed. Divide the lawn in rectangles, squares or triangles. For help in this calculation, go to http://www.supersod.com/diy/how-to-sod-yard-area-calculator.html and plug in the requested numbers: length, width, and diameter.
Not knowing the size of your lawn usually results in:
- Buying too much fertilizer.
- Storing left over products for next use or disposal.
- Applying too much lawn product which can result in more mowing, reduced plant health and undesirable off-site effects (nitrate leaching/runoff, and phosphorus runoff,).
- Buying the wrong amount of fertilizer to treat the lawn area often results in not achieving the desired appearance.
- Using your time and money ineffectively.
When to fertilize?
The best time to fertilize cool-season grasses in Virginia is from August 15 through November at 4 week intervals.
In general lawns respond better to fertilizer ratios high in nitrogen. However, no more than 1 pound of N per 1000 sq ft. should be applied in a single application when using a water soluble nitrogen. Applications should be distributed at a minimum of 4 weeks apart from September through November. However, slow release fertilizers may be used in greater quantities without the threat of burning the grass.
The amount of nutrients required by an established lawn or turfgrass area depends on the type of grass plants and your management practices. In other words, how much care you decide to give the lawn must be balanced with the demands of that particular type of grass. A vigorously growing, watered lawn from which the clippings are removed requires more added nutrients than a lawn that is not watered during the summer and where clippings are left on the lawn. Consequently, in developing your own lawn fertilizer program, it is appropriate to decide whether your lawn is going to be high or low maintenance.
High-maintenance lawns are characterized by vigorously growing plants. These lawns are watered during the summer to maintain green growth. Va Tech does not encourage watering in the summer , but allow grass to brown as summer gets hotter. It will green up in the fall. Clippings left on the lawn gradually decompose and reduce the need for fertilizer.
Low-maintenance lawns do not commonly receive watering (other than rainfall) during the summer months and grass growth is minimal during hot, dry periods. Clippings are usually left on the lawns.
The following programs were developed by the Virginia Cooperative Extension to help homeowners know how many pounds of nitrogen to apply each month in the fall depending on the quality of lawn desired. As with so many things in life, we all want high quality lawns; but we have to balance our wants with the money and time we’re willing to invest.
Program 1 – Use this table if the fertilizer is less than 50% WIN or other slow release nitrogen source.
|Nitrogen Application By Month|
|Quality Desired||Sept.||Oct.||Nov.||May 15-June 15|
|———–lbs N/1000 sq ft ———–|
Program 2 – Use this table if the fertilizer is more than 50% WIN or other slow release nitrogen source
|Nitrogen Application By Month|
|Quality Desired||Aug 15 to Sept 15||Oct 1 to Nov 1||May 15 to June 15|
|———————-lbs N/1000 sq ft ———————-|
|High||1.5 to 2||1.5||0 to 1.5|
Using fertilizer analysis to calculate nitrogen rates
In order to determine how many total pounds to distribute on your lawn, a calculation must be done. For example:
- If you are using a 16-4-8 fertilizer, the first number(%N) on the bag is 16%N
- Calculate how many pounds are to be applied of N per 1000 sq. ft. by dividing the amount of N you want to add(1 pound of N for example) by the % of N(16% in this case which is the same as .16)
1 divided by 0.16=6.2 pounds of fertilizer in order to apply 1 pound of N per 1000 sq. ft. If you have a 5500 sq. ft lawn multiply the area (5500/1000 sq. ft. which equals 5.5)) by 6.2 pounds and get 34.1 pounds. Round to 34 pounds of fertilizer to apply 1 pound N to your lawn
How to apply
It is important to uniformly apply fertilizer containing nitrogen. Lack of uniformity results in streaking or different shades of green turf in the lawn. Drop type or rotary type spreaders are best to use. When using drop type spreaders, be sure to overlap the wheel tracks since the fertilizer is distributed between the wheels. These spreaders can be difficult to maneuver around shrubs and trees. Rotary spreaders usually give a better distribution because they cover a bigger swath. Apply half the material in one direction and the other half in a perpendicular direction. Avoid application to any non turf areas such as sidewalk, patios, driveways or roads where it may enter the water supply. Blow or sweep any fertilizer away. Do not hose it. Avoid applications if weather forecasts call for heavy rainfall. Having a 1/4 inch of rain after application is an ideal way to move fertilizer into the soil.
Sinnes, A. Cort, All about fertilizers and soils, Chevron Company, 1979.
Rosen, C.J., B. P. Horgan, and R. J. Mugaas, “Fertilizing lawns”, Virginia Cooperative Extension Publication,
Goatley,J.M., ” Lawn Fertilization in Virginia”, Virginia Cooperative Extension Publication,
Goatley, Mike, “Fall fertility strategies for Virginia’s home lawns”, Virginia Cooperative Extension Publication,
Chalmers, David, “Home lawn fertilization in Virginia: Frequently asked questions”, Virginia Cooperative Extension Publication, http://pubs.ext.vt.edu/430/430-003/430-003.html
Chalmers, David, “First step to lawn care: Measure your lawn!” SDSU Cooperative Extension Publication,