October 2, 2011

When to Lime Your Lawn (& not)

Lime and What it’s For

When I first started going to gatherings of local landscapers at association meetings back in the middle ages, I would listen to those older and more knowledgeable than myself in awe of their experience and knowledge. Pretty much every fall they would expound on how lawns had to be limed religiously  with pelletized lime since powdered lime just was not concentrated enough to be effective.  Much later on, I was to learn there was a little more to applying lime than my peers knew at the time.

Lime is ostensibly applied to adjust soil acidity so that grass can better absorb nutrients. Lime is applied to the soil of home lawns to increase the soil pH.  When the pH is below 7.0, the soil is said to be acidic; when above 7.0, it is alkaline. For turfgrasses, a soil pH between 6.0 and 7.0 (slightly acidic) is ideal. Should the pH be off in one direction or the other, the ability of the lawn to absorb nutrients is inhibited and  thus the need to adjust soil pH. Of course lime only moves pH to be more acid,  if the soil is acidity is too acid already, liming it will make the problem worse.

Several factors cause the formation of acidic soil conditions. One primary cause is the leaching of base nutrients such as calcium, magnesium, and potassium from the soil. This occurs more frequently in areas of heavy rainfall like we have experienced recently. A second cause is the use of acidifying nitrogen fertilizers like hollytone applied to  acid loving shrubs planted next to a lawn and leading to acidification of the lawn.

When the soil pH drops below 6.0, a number of nutrients necessary for proper growth become less available for use by the turfgrass plant. These include the following: nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and molybdenum. As these nutrients become less available, the lawn’s color, vigor, and ability to resist (or recover from) heat, drought, or traffic stress will be reduced. Applications of enough lime to raise the soil pH above 6.0 can increase the availability of these nutrients, thus making it easier to maintain the quality and vigor of the lawn.

Note that an excessively high (alkaline) soil pH (greater than 8.0) is just as undesirable as a low pH. When the pH exceeds 8.0, such nutrients as nitrogen, phosphorus, iron, manganese, boron, copper, and zinc become less available for use by the turfgrass plants in the lawn. The result may be a less vigorous, unhealthy lawn. Sulphur is the recommended countermeasure  under these conditions and applying lime will only make it worse.

Fall is the ideal time to adjust pH as it is sufficiently moist enough for the soil to taking in the amendment and begin to process it. Additionally, lime and sulphur amendments can take up to a year to be effective. Applying them in the fall gives the soil more time for key chemical changes to begin to take root.

Soil testing is  the only sure fire way to determine what needs to be applied and how much. Before you allow your service provider to lime your lawn, make sure they have done their homework. Over-application of liming products may cause the development of alkaline soil conditions that are actually detrimental to your lawn and soil.

Posted in: Lawns