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Soil test will determine fertilizer use

Q. Last week you indicated the optimum times for fertilizing a lawn, but I want to know what's the best fertilizer to use. I've been using 16-16-16 or 15-15-15 for years. What fertilizer do you recommend?

A. It's best to have your soil tested to find out the levels of the these three major nutrients, nitrogen (N), phosphorus (P), and potassium (K). That's the only way to find out what nutrients actually are needed for good grass growth in your lawn. There are two local soil testing labs where you can take your soil to be tested. Your lawn may only need nitrogen or it may need a complete fertilizer with nitrogen, phosphorus and potassium (N:P:K).

Without a soil test to tell you exactly what's needed, look for a fertilizer that applies nutrients in the ratio that the lawn uses them. Fertilizers with an approximate ratio of 3:1:2 (N:P:K) are considered "best" for lawns. Because of our dry climate and hot summer temperatures, we tend to water our lawns frequently. This leaches the soluble nitrogen in fertilizers out of the grass root zone, especially on the sandy soils found in some areas. The best turf fertilizers have at least 30 percent to 50 percent of the nitrogen in a slow-release form. Part of the nitrogen is rapidly available to the grass and the other portion is released over time. This provides for more even grass growth and keeps you from washing the nitrogen away before the grass is able to utilize it.

Q. Do I need to aerate my lawn to control thatch? Aeration with a hollow-tined aerifier that pulls out plugs of grass and soil from the lawn will help reduce soil compaction. Aeration does not adequately control a serious thatch problem. However, aeration can be used as a stop-gap measure to alleviate the problems caused by thatch. Excessive thatch can prevent the air, water, and fertilizers from getting to the grass roots. Aeration aids in penetration of these elements, but does not remove a significant amount of thatch. Thick thatch should be removed with a dethatching or power-rake machine in early spring as soon as the frost is out of the ground or in late August.

Q. My neighbor dethatched his lawn and it was a big job. What causes thatch and how can I avoid it?

A: Thatch is a partially decomposed layer of mostly grass stems and roots that develops between the top of the grass plant and the soil.

It's not caused by grass clippings. Believe it or not, one of the biggest factors leading to a buildup of thatch is the height at which you mow your lawn. Grass should be mowed at the recommended height for the type of grass. Both Kentucky bluegrass and turf-type perennial rye grass lawns should be mowed at a height of 1 1/2 to 2 1/2 inches.

Overwatering also contributes to thatch. Saturated soils lead to surface rooting because that's the only place grass roots can get air. The remedy is to water deeply, less frequently. You also can encourage thatch breakdown by fertilizing your lawn at the recommended rates and timing. Nitrogen is needed by the bacterial organisms that are part of the decomposition process.

GARDEN REMINDER: Join the WSU Master Gardeners at the Demonstration Garden today from 9 a.m. to 1 p.m. for gardening classes and a plant sale.

* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension Office.

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