Nitrogen (N), phosphorus (P) and potassium (K) are called “macronutrients” because they are needed by the plant in relatively large quantities. Nitrogen (N) is the nutrient that primarily controls turf growth and density and is required in the largest amount.
When adequate nitrogen is provided the turf will have a vigorous root system, high shoot density, maximum recuperative potential and tolerance to environmental stress. Also, varieties that have a color response to nitrogen will darken up when fertilized. Inadequate or low levels of nitrogen will reduce shoot density, stress tolerances and ability to recover as fast from traffic damage, and favor weed encroachment and certain diseases like rust, red thread and dollar spot. Excessive amounts of nitrogen can be detrimental to the turf by reducing rooting, stress tolerance and wear tolerance. Excessive nitrogen can also increase thatch and favor diseases that thrive in high nutrient situations like snow mold, leaf spot and brown patch.
There is no reliable soil test for nitrogen so other factors are used in determining the amount of nitrogen that is needed. However, soils with more than 6% organic matter require less nitrogen. You can select nitrogen fertilizer sources that are water soluble and have quick release properties, fertilizers that have slow release properties or a combination of both. See tables for advantages and disadvantages of different nitrogen sources.
Advantages and disadvantages of nitrogen fertilizer sources
| Nitrogen form | Advantages | Disadvantages |
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Quick release:
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Slow release:
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Slow release:
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Phosphorus
Phosphorus (P) promotes rooting and is especially important in new seedings because it enhances establishment, especially with tall fescue. Phosphorus should be incorporated into the seedbed since it is not very mobile in the soil. Applications of phosphorus to established turf are rarely needed but applications of P may be necessary when overseeding. Soil test to determine needs.
The availability of phosphorus in the soil is influenced by pH. Phosphorus is most available at pHs above 6.5 so be sure to check your soil pH.
Some common sources of phosphorus include superphosphate (16-21% P2O5), triple superphosphate (40-47%), Monammonium phosphate (48%) and Diammonium phosphate (46-53%). A natural organic source is steamed bone meal (23-30%) and animal based composts.
Potassium
Potassium (K) promotes rooting, rhizome and stolon development and is said to improve heat, cold, wear and drought tolerance (although there is not enough research to back up this claim). It can help reduce some diseases such as leafspot, brown patch, red thread and dollar spot.
Potassium is very mobile in the soil and is easily leached in sandy soils. So when soil tests determine potassium is needed it should be applied in smaller amounts several times during the year or applied in slow release forms especially on sandy fields. If soil test results are not available a minimum ratio of 2:1 (nitrogen: potassium) is desired. Common potassium sources include muriate of potash (KCL) with 60-62% K2O and sulfate of potash containing 50-53%.