Fertilizer Basics

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Fertilizer Basics

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Grasses require at least 16 different essential elements in their diets, most of which are provided by the grass’s surrounding environment.  However, even with a low-maintenance lawn, you will still need to fertilize with nitrogen (N) to sustain thick, vigorous turf.  In addition to enhancing the grass’s deep green color, nitrogen is also responsible for the sturdy growth and shoot density needed to fight off weeds and to stand up to diseases, insects, and traffic.  Learn when and how to apply fertilizer.

In addition to nitrogen, your lawn may also need phosphorus (P) and potassium (K).  Depending on where you live, your soil may naturally contain adequate levels of these elements.  Phosphorus, which tends to remain in the soil, is necessary in small amounts to aid root growth and improve establishment rates.  Potassium, which is prone to leaching into the soil, enhances grass resistance to cold, disease, drought, and wear.  A soil test will help you determine which nutrients your soil needs.

A fertilizer with a description saying “complete” contains all three of these nutritional elements.  The bag’s fertilizer grade numbers tell you the percentage of nitrogen to phosphorus to potassium.  For example, in a 50-pound bag of 20-10-10 (2:1:1) grade, 20% (10 lbs.) is nitrogen, 10% (5 lbs) is phosphorus, and 10% (5 lbs) is potassium.  The remaining 30 pounds may consist of additional elements such as iron and sulfur, and inert “filler” ingredients.

Ratios are helpful in choosing the appropriate fertilizer to use for specific purposes.  Those with a 1:2:2 ratio (i.e. 6-12-12 fertilizer), are lower in nitrogen but higher in the nutrients needed for planting new grass or renovating old lawns.  Fertilizers with high nitrogen ratios of 2:1:1, 4:1:2, or 3:1:2 are frequently used for maintenance applications.  Maintenance fertilizers are available in grades of 12-6-6, 16-8-8, 20-10-10, 12-4-8, and so on.

Read the back label for the guaranteed analysis of the contents before buying fertilizer.  If your soil test indicates that you do not need to add phosphorus or potassium, choose a bag with a low or zero numeral for that element.  For example, a bag of 20-0-5 would have no phosphorus.

In addition to checking the grade, determine the type of nitrogen used—water-soluble or water-insoluble.  Water-soluble can be immediately used by grass plants.  Ammonium nitrate, ammonium sulfate, and urea are examples of this quick-release form of nitrogen.  They provide rapid greening of the grass but also have drawbacks, as shown in the table below.

Water-insoluble must be broken down by soil microbes into forms grass plants can use.  These slow-release sources include synthetic organics, such as ureaforms, or those derived from natural organic materials, such as composted manures.  Water-insoluble, or other slow-release forms, reduce the amount of time you spend behind your spreader, last longer, and save you time and money.  To induce gradual release of nitrogen over time, fertilizer companies will manipulate the size of particles and sometimes coat them.  Because these forms take longer to dissolve, nitrogen is released at varying rates.  Common examples of water-insoluble are isobutylidene diurea (IBDU) and sulfur-coated ureas.

Determine the type of fertilizer you have by reading the guaranteed analysis on the bag.  Note that many fertilizers have a combination of both fast-release and slow-release types of nitrogen.  You should check carefully to find products that derive a majority of their nitrogen from slow-release sources, such as soybean meal or composted manure.

Slow-Release versus Fast-Release Fertilizers

Nitrogen released gradually Higher initial cost
Less apt to leach Dependent on warm weather for release
Low incidence of burning Takes longer for turfgrass to respond
Fewer applications needed
Lasts longer
Nitrogen available immediately More apt to leach, especially on sandy soils
Generally costs less More apt to burn foliage
More reliable release rate May raise salinity of soil
Releases even in cold weather More frequent applications required
May make soil more acidic and less hospitable to
beneficial microorganisms
May thin cell walls and make plants vulnerable
to disease
Requires more frequent watering