Crossville Chronicle, Crossville, TN

April 14, 2014

PLATEAU GARDENING: Cool-season lawn grass fertilization and soil tests

By C. Rae Hozer
Chronicle contributor

CROSSVILLE — Recently, I got an inquiry about the right timing for homeowners who want to fertilize a cool-season lawn which has bare spots that need over-seeding. An email from a new resident in the Crossville area asked how to take a soil sample and where to have it tested. Since problems with the pH or fertility of the soil beneath can result in chronically thin grass with persistent bare places up top, testing the soil then correcting pH and fertility to match plant needs can be an important first step in maintaining your lawn.

Fertilizing and Over-seeding Cool-Season Turf

Those with lawns containing fescues, Kentucky bluegrass or ryegrass (cool-season grasses) should fertilize on/around March 15 and April 15 then hold off until summer’s end (Labor Day). The best growth time for these types of turf is in autumn not spring. Fertilizing later than April 15 stresses cool-season lawns if late spring and summer are hot and dry. Slow-release fertilizers won’t ‘burn’ your grass as fast-release fertilizers can. Don’t use crabgrass prevention or a weed and feed product in areas you wish to over-seed. These products can prevent seed germination and/or weaken young grass plants. Get the University of Tennessee Extension booklet PB1038 Fertilization & Management of Home Lawns for more details.    

Soil Tests

pH — The acidity or alkalinity of soil is measured on a numeric pH-scale (zero to fourteen). The mid-point, a pH reading of 7.0, is neutral. Readings below that indicate soil that is more acidic as the pH-readings go lower. That is to say, soil with a pH of 6.0 is more acid than soil with a pH of 7.0. Readings over 7.0 indicate more alkalinity as the pH-numbers go higher.

Though the values seem close, pH is measured using a logarithmic scale not a linear one. What is a separation of 1.0 on a linear scale is 10 times as much (1.0 X 10) on a logarithmic scale. Envision a regular extension ladder with rungs one-foot apart as "linear" and a "logarithmic" ladder with the rungs between the first and 11th rungs missing leaving a 10-foot gap between the rungs. There would be a big difference going just one step down or up on those two ladders.

The type of bedrock (parent material) in the area as well as precipitation influences natural soil pH values. Because lime is subject to weathering, places that get more rain like Tennessee and other Southeastern states tend to have soils which test lower in pH than soils in the arid states of the southwest.

As mentioned last week, different plants have different soil pH levels at which they grow best. Rhododendrons are acid-loving plants that grow well in soil with a pH of 4.0 to 6.0 according to the Old Farmer’s Almanac webpage www.Almanac.com/content/ph-preferences.Most landscape plants like soil that is not too acidic (within a pH range of 6.0 to 7.0). If your test shows the soil pH is too low for plants you wish to grow, ground up limestone can be applied to raise the pH/sweeten the soil. Ground sulfur can be used to adjust pH downward when soil is too alkaline. Applications of fertilizer also lower pH over time.

Fertility — The three primary plant nutrients are listed in order on fertilizer containers: Nitrogen (N), Phosphorus (P), Potassium (K) each with a number indicating the percentage of that nutrient in the mix. Nitrogen washes out (leaches) and is otherwise lost from soil fairly quickly. Included in soil test results from the University of Tennessee Soil, Plant and Pest Center lab are directions on fertilization which like pH adjustment instructions are based upon the types of plants to be grown in the area as listed on the information sheet submitted with the sample. Soil samples can be taken to your local University of Tennessee Extension office. Telephone UT Extension to talk about how to collect the soil and the test fee. Or go online to the Soil, Plant and Pest Center webpage ag.tennessee.edu/spp/Pages/lawnandgardensampling.aspx.

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Plateau Gardening written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region. Contact UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) answers to horticulture questions, free publications and to learn about the Master Gardener program. Send email comments or yard and garden inquiries to Master Gardener Rae (MGardenerRae@frontiernet.net).