Series No. 911
Lawns are a very important part of our landscape. Besides being aesthetically pleasing, they cover erodible soils, produce oxygen, and fit nicely into our forested areas. However, lawns can be very expensive to care for if they are treated incorrectly. Incorrect fertilization can result in diseased lawns, weedy lawns, or lawns that adversely impact water quality through the leaching of applied nitrates into groundwater.
To fertilize your lawn correctly, you need to understand its nutrient needs. Lawns need four macronutrients: nitrogen (N), phosphorus (P), potassium (K), and sulfur (S). Lawns growing in neutral or high pH soils (pH values greater than 6.8) may also need the micronutrient iron (Fe).
There are two kinds of N fertilizer: slow release and quick release. Slow-release fertilizers become available slowly. Use them in sandy soils, in other soils that drain rapidly, or when grass plants are not growing rapidly -- early spring and fall. Slow-release N fertilizers are often referred to as WIN (water insoluble nitrogen) materials. Quick-release fertilizers provide readily available N to plants. Quick-release fertilizers are best to use when the grass is rapidly growing in early summer.
Phosphorus promotes strong root growth and encourages lawns to thicken quickly. Phosphorus levels are often low in northern Idaho soils. Because P does not move through the soil as N does, you should apply it when you establish the lawn or immediately after aerating it. If the soil is highly erodible, P will run off with sediment.
Adequate K is necessary for disease resistance. It also allows the lawn to stand up to heavy traffic and promotes winter hardiness. Potassium is usually adequate in our soils. Too much K can result in an accumulation of salts.
Sulfur is needed in most of northern Idaho. Be sure to include it in your fertilizer mix.
Soil testing strategy
Soil tests for lawn fertilization should be done in early spring, within a month of the time you are planning the first application of fertilizer. To get a representative sample of soil in your lawn, take 12 to 15 subsamples, mix them together, and take a quart sample from the mixed sample.
Each subsample should come from the top 6 to 8 inches of soil. Make sure you exclude any surface debris that can change the soil test results, including grass blades. The more subsamples you take, the more representative your sample will be. Avoid taking samples from gardens, from under shrubs, or from unusual areas in the yard. If the soil in your lawn varies from one location to another, consider treating each unique area as a separate sample. For more information on how to collect and process a soil sample, see University of Idaho Extension Bulletin 704, Soil Sampling. A standard soil test will also give you the pH of the soil and the percentage of organic matter. Soil test information and the following tables will help you determine the correct amount of each nutrient to apply.
Nitrogen -- Determine the N application rate per 1,000 square feet of lawn based on the soil's organic matter content (Table 1).
capacity of your soil
per 1,000 ft2
|7 or higher||Very high||1|
|5 to 7||High||2|
|3 to 5||Moderate||3|
|1 to 3||Low||4|
|under 1||Very low||5|
Phosphorus -- Determine the P application rate per 1,000 square feet of lawn based on a P soil test (Table 2). Soil-testing laboratories use two different methods of determining soil P -- sodium bicarbonate (NaHCO3) and sodium acetate (NaOAc). Make sure you read the column in Table 2 that matches the method your soil testing laboratory used.
|Soil test P||P-supplying capacity||Application rate|
|NaHCO3||NaOAc||of your soil||per 1,000 ft2|
|(ppm)||(ppm)||(lb phosphate [P2O5])|
|12 or higher||5 or higher||Very high||None|
|9 to 12||4 to 5||High||None|
|6 to 9||3 to 4||Moderate||None|
|3 to 6||2 to 3||Low||1|
|0 to 3||0 to 2||Very low||2|
Potassium -- Determine the K application rate per 1,000 square feet of lawn based on a K soil test (Table 3).
|Soil test K||K-supplying
of your soil
per 1,000 ft2
|(ppm)||(lb potash [K2O])|
|250 or higher||Very high||none|
|150 to 250||High||1|
|100 to 150||Moderate||2|
|50 to 100||Low||4|
|less than 50||Very low||4|
Sulfur -- Apply S if your soil tests less than 10 parts per million (ppm) SO4-S. An application rate of 1 pound S per 1,000 square feet should be adequate for an entire year.
Micronutrients -- Northern Idaho lawns generally do not need micronutrients. The only potentially deficient micronutrient is iron (Fe). Iron is usually deficient only when the soil pH is greater than 6.8. Iron deficiency symptoms -- "Chlorosis" or the yellowing of new growth -- can be corrected by applying a 1/2 percent solution of ferrous sulfate as a foliar spray or a chelated iron at the recommended label rate. Applications may need to be repeated if yellowing reoccurs.
Nutrient ratio strategy
The nutrient ratio fertilization strategy does not involve a soil test. It is based on applying 0.5 pounds of N per 1,000 square feet of lawn for each month of active grass growth. (When daily temperatures average above 80 F, most grasses are not actively growing unless you water them. Most lawns in northern Idaho start active growth in early to mid-April and often continue to grow until mid-October). If, for example, your lawn grows actively 8 months each year, you would apply 4 pounds of N per 1,000 square feet over the year.
Phosphorus, K, and S applications are based on a ratio of those nutrients to the amount of N applied: three parts N, to one part P, to two parts K, to one part S. Thus, if your N recommendation is 4 pounds per 1,000 square feet, your P recommendation would be 1.3 pounds, your K recommendation would be 2.6 pounds, and your S recommendation would be 1.3 pounds.
Let's say you have a lawn that is actively growing 6 months each year. You would calculate N, P, K, and S fertilizer needs for the year as follows: 0.5 lb N per 1,000 ft2 per month x 6 months = 3.0 lb N per 1,000 ft2 3N = 3.0 lb N 1P = 1.0 lb P 2K = 2.0 lb K 1S = 1.0 lb S New Grass Seedings Consider soil fertility needs before establishing new pastures. Both P and K are particularly important as these nutrients are immobile in the soil and are more available when worked into the seedbed before seeding.
At establishment, work 60 pounds of P2O5 per acre and appropriate amounts of K (see table 3) into the seedbed. Add S when a soil test indicates a need. Sulfur does not need to be incorporated into the seedbed because it is mobile in soils and will reach plant root zones with normal precipitation or irrigation.
Adding 20 to 30 pounds of N per acre at seeding will help establish a pasture. Add 50 percent of the recommended N rate listed in table 1 during the first season.
|CIS 811||The Relationship of Soil pH and Crop Yields in Northern Idaho (35 cents)|
|CIS 686||Fertilizer Questions ($1.50)|
|EXT 704||Soil Sampling (50 cents)|
Issued in furtherance of cooperative extension work in
agriculture and home economics, Acts of May 8 and June 30, 1914, in
cooperation with the U.S. Department of Agriculture, LeRoy D. Luft,
Director of Cooperative Extension System, University of Idaho, Moscow,
Idaho 83844. The University of Idaho provides equal opportunity in
education and employment on the basis of race, color, religion,
national origin, gender, age, disability, or status as a Vietnam-era
veteran, as required by state and federal laws.
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All contents copyright © 1996. College of Agriculture, University of Idaho. All rights reserved. Revised: October 10, 1996
ture, University of Idaho. All rights reserved. Revised: October 10, 1996