Fertilizing lawns

Fertilizers are important in lawn care because they supplement the soil with needed nutrients.

• Nutrients are needed by grass plants because of the many stresses people apply to them.
• Grass uses nutrients to repair damage done by normal wear and mowing.
• Other plants in the landscape, such as trees, flowers and weeds, also consume the nutrients.
• Nutrients are lost when clippings are removed while mowing.

The three main nutrients needed by plants are nitrogen (N), phosphorus (P) and potassium (K).

• For a home lawn, the most important nutrient to add on a regular basis is nitrogen. 
• Phosphorus is present in high levels in many Midwestern soils.
  • Adding phosphorus in the form of fertilizer is usually not needed.
  • Established lawns show little response to added phosphorus.
  • Because phosphorus does not easily move in the soil, additional amounts are often beneficial during establishment of new turf.
  • Only apply phosphorous if a soil test indicates a need.
• Potassium may need to be supplemented.
  • Coarse-textured (sandy) soils often need added potassium.
  • Use a soil test to determine when to add potassium.

Plants also need many other nutrients, but in very small amounts. These are called micronutrients.

Concentration of nutrients

The label on a bag of fertilizer includes an analysis of the concentration of nutrients. Each number in the analysis represents a percentage of the total weight of the product that is actual nutrient.

• The first number indicates the amount of nitrogen, or N.
• The second number is the amount of phosphorus in the form of phosphate, P₂O₅.
• The third number is the amount of potassium in the form of potash, K₂O.

For instance, a 10-10-10 is a balanced food with equal parts nitrogen, phosphorus and potassium.

High numbers of each nutrient in a particular fertilizer usually means the type of fertilizer can be applied less often. This is similar to differences between a concentrated laundry detergent and a more dilute one.

Natural organic fertilizers usually have a low concentration of nutrients compared to the manufactured types. As a result, more organic fertilizer is needed to supply the same amount of nutrients.

It's important to consider nutrient amounts when figuring cost, since more bags of the low analysis fertilizer will be needed to supply the same amount of nutrients.

A soil test from the University of Minnesota Soil Testing Lab can help you determine if your lawn is missing or has too much of these essential nutrients.

Many factors affect the amount of nutrients needed for an established home lawn, including:  

• Turfgrass species. 
• Access to water. 
• Percentage of soil organic matter. 
• Amount of shade. 
• Expectations of the home owner. 

Turfgrass usually needs more nitrogen than other nutrients and lawn fertilization programs are based around this nutrient.  Turfgrass gets some of the nitrogen it needs from the soil.  However, homeowners may want to add supplemental nitrogen to maintain turfgrass quality that they find acceptable. 

Below is a table that outlines when and how much nitrogen is recommended for established lawns based on the amount of maintenance required for the lawn.

Soil organic matter levels: Low = less than 3.1%; Medium = 3.1 – 4.5%; High = 4.6 – 19%; Organic soils = more than 19%. If soil organic matter level is not known, guidelines for medium soil organic matter can be used. Assuming each application does not exceed 1 lb N/1,000 sq ft (for annual applications of 1.0 lb N or greater).

There are many kinds of fertilizers available for the homeowner. 

• Natural fertilizers include: Milorganite, Ringer, Sustane, manure, grass clippings, phosphate rock, potash (source of potassium)
• Manufactured fertilizers include: ammonium nitrate, urea, IBDU, sulfur-coated urea, urea formaldehyde, super-phosphate, triple super-phosphate

Compost is not included because it has little nutrient value. Composts can be very beneficial as a soil amendment, adding organic matter and improving many soil properties.

Natural organic fertilizers are commonly made from waste products of various sources ranging from chicken feathers and manures to treated sewage sludge from city waste systems. Rock phosphate and potash are mined from the earth.

The manufactured fertilizers normally are made from petroleum and natural gas. The exceptions are superphosphate and triple superphosphate, which are rock phosphate that has been concentrated using acid reactions.

Whether natural or manufactured, the nutrients in fertilizer are generally not in a form the plant can use directly.

• Only two forms of nitrogen are readily available to the plant: ammonium (NH₄) and nitrate (NO₃).
• All nitrogen fertilizers must be broken down to these forms regardless of their source.
• Some chemical changes and decomposition must occur before the plant takes up the nutrients.

Speed of nutrient release

The speed at which nutrients in the fertilizer become available to the plant varies depending on the product.

• Natural organic fertilizers are typically slow release.
  • They deliver nutrients over a period of time.
  • Decomposition of the organic matter occurs slowly.
• Some of the manufactured fertilizers are also slow release.
  • This group includes sulfur coated ureas, urea formaldehyde and IBDU.
  • Microbial activity in the soil gradually breaks down the fertilizer.
• Fast release fertilizers are rapidly converted to available nutrients.
  • Examples include ammonium nitrate and urea.
  • They are applied more frequently, at low rates.

• "Burning" occurs when fertilizer is so concentrated around the roots that it essentially pulls water out of the plant.
• Burning most commonly occurs when a lawn is fertilized in hot weather or without being watered.
• This is particularly a problem with fast-release fertilizers.
• Slow-release products, both natural or manufactured, also have the potential of burning, but the nutrients usually do not build up such high concentrations in the soil.
• To prevent burning of the lawn, always water after applying any fertilizer, unless the label recommends otherwise.

All fertilizers must be broken down to the same nutrient form. All types of fertilizer can contribute to pollution, if used incorrectly.

• The leaching of nitrates may be of concern in areas of sandy soils when over-watering and over-fertilization occur, particularly with the use of fast-release forms.
• Excess phosphorus has also been involved in lake and stream pollution.
  • If the lawn is dense and vigorous, however, it contributes very little, if any, to runoff. The grass roots bind the soil tightly, promoting infiltration of water and nutrients.
• Do not leave fertilizer and organic matter (lawn clippings, leaves) on sidewalks, driveways and gutters. These contribute substantially to nutrient runoff into lakes and streams.

See Water wisely.

Some nitrogen fertilizers tend to reduce the pH of the soil and thatch slightly.

• pH is a measure of acidity, with 7 being neutral, below 7 is acidic, and above 7 is alkaline.
• Most commonly grown turfgrasses do best at a pH in the 6.0 to 6.5 range.
• In areas of already low pH (less than 5), acidifying fertilizers may be of concern.
  • Low pH reduces the population of earthworms and certain microbes.
• In Minnesota, and many other parts of the Midwest, pH tends to be rather high (above 7) along with high buffering capacity.
  • Buffering capacity refers to the soil's ability to resist pH change.
  • In a soil with high buffering capacity, it is very difficult to reduce the pH by means of fertilizers and other soil amendments.
  • In those soils, the use of acidifying fertilizers may actually be beneficial. 

• You should apply fertilizer as part of your regular lawn maintenance.
• Only use herbicides when specific weed problems occur.
• Fertilizers that include weed killers (i.e. herbicides) may not be the best choice for your lawn.
• The correct timing and post-application procedures are important for weed-and-feed products to be effective.
• Spot treating weeds may be more effective than using weed-and-feed over the entire lawn.
• Environmentally, applying fertilizers separately from weed killers is the best approach.

Timing

• For effective weed control, use herbicides at specific times during the year depending on which weed species is being targeted.
• The time of herbicide application to kill a specific weed may not be the ideal time for fertilizer application.
• Never use fertilizer-herbicide products when the herbicide would be ineffective or unnecessary.

Post-application

Water-in fertilizers following application for maximum effectiveness and to reduce the chance of damaging the lawn (i.e. fertilizer burn). Many herbicides, however, need to remain on the plant leaves for effective weed control.

Weed-and-feed products often compromise the effectiveness of the fertilizer, herbicide or both during application. Always check the specific product label for any post application requirements (e.g., whether it should be watered-in or not).

Spot treatment

Fertilizer-herbicide combination products often apply much more herbicide than is needed to kill the target weeds. Where there are only scattered weeds throughout the yard, it is just as easy to spot treat those individual weeds or small areas of weeds rather than applying a weed-and-feed product over the entire lawn.

Broadleaf weed control

• Combination products specifically formulated for broadleaf weed control, such as for dandelion, require contact with the leaves of the weed.
• Weeds must be visible and growing at the time of application to be most effective.
• This type of product will not prevent broadleaf weeds from germinating and establishing.

Fertilizer-crabgrass preventer (pre-emergent) combinations contain a weed preventer specifically targeting the tiny seedlings of warm season annual grasses (crabgrass and yellow foxtail). The weed preventer works as the grasses just emerge from the seed, but before they are visible above ground.

A spring application of fertilizer that contains a crabgrass preventer could be an appropriate choice if you wait until early May. However, crabgrass preventers should be put down much earlier in the spring than fertilizers.

If fertilizers are applied to a lawn too early, the environmental loss of these nutrients can be a concern. Applying fertilizers and crabgrass preventers separately is the best approach.    

As with most weeds, crabgrass will not likely be uniform over an entire yard. A crabgrass preventer may only need to be applied along those areas where the lawn grasses are not as competitive. These include areas along curbs, sidewalks, narrow boulevards and driveways.