Fall Soil Testing: Why Now Beats Spring

On a golf course, nobody would dream of fertilizing without a soil test. At home, most people buy whatever bag is on sale and spread it four times a year. A soil test costs about the same as one bag of fertilizer and tells you exactly what your lawn needs. And fall is the best time to take one.

Why fall

• Labs aren't slammed. In spring, university and private labs get buried in samples, and results take weeks. In fall you get them back quickly.
• You have time to act. Lime, in particular, takes months to change soil pH. Apply it in fall and it works through the winter.
• The soil is in a steady state after the growing season, before freezing.
• You can plan next year's fertilizer program over the winter instead of guessing in April.

How to take a good sample

A bad sample gives you a bad answer. Do it right:

1. Use a clean trowel or soil probe and a clean plastic bucket (no galvanized metal, which can skew zinc results).
2. Take 10 to 15 small samples from around the lawn, zigzagging across the area you're testing.
3. Sample the root zone: about 3 to 4 inches deep for lawns. Discard the grass and thatch on top.
4. Mix the cores together in the bucket, let it air dry, and send about a cup.
5. Test problem areas separately. If the front and back lawn look different, or one area is thin, sample them as separate tests.
6. Don't sample right after fertilizing or liming. Wait several weeks.

Where to send it

Many state land-grant universities and their extension services run soil testing labs, and there are private labs too. A basic test usually reports pH, phosphorus, potassium, and organic matter, with recommendations. Some labs ask what you're growing; tell them it's a lawn and what kind of grass.

Reading the results

Our guide on how to read a professional soil test report walks through it in detail. The short version:

• pH: most cool-season grasses do best around 6.0 to 7.0. Below that, the report may recommend lime. Above it, you generally don't need to do anything for a lawn. See our guide to pH management.
• Phosphorus: many lawns already have plenty. If yours does, use a phosphorus-free fertilizer, which many states and towns require anyway.
• Potassium: helps with stress and winter hardiness; the test tells you whether to add it.
• Organic matter: low numbers mean compost topdressing would help.

How often

Every three to four years is plenty for an established lawn, or before a big renovation. Keep your reports so you can see the trend.

The test turns lawn care from guessing into a plan. Take your samples this month, send them off, and you'll have your answers before the snow flies.

Related: Moss in the Lawn: What It's Telling You