How to measure soil temperature

What you need

A probe-style soil thermometer (dial or digital) with a stem of at least 5 inches. A kitchen or meat thermometer works if its stem is long enough and you are not going to use it for food again. Avoid air thermometers laid on the ground: they read the air.

Step by step

1. Pick the spot where you will plant or seed: open lawn in sun, not next to a wall, a driveway or under a tree.
2. Make a pilot hole with a screwdriver or a large nail so the stem does not bend.
3. Push the probe in to the right depth: 2 inches for grass seed and small seeds, 4 inches for lawn pre-emergent, most vegetables and corn. Why the depth matters.
4. Wait 1 to 2 minutes for a dial, until the number settles for a digital one.
5. Read at the same time each day. Mid-morning (about 8 to 10 am) is close to the day's average at 4 inches; the afternoon reading is the warmest of the day and flatters spring soil.
6. Repeat for several days and use the average. Planting rules such as "50 to 55°F for pre-emergent" mean the soil holds that temperature, not one warm afternoon.

Why your reading differs from the map

The map, city and ZIP numbers are a NOAA model's 0–4 inch layer for a grid cell about 25 km across. Shade, slope, mulch, watering, bare soil versus turf and soil type all move a real yard by several degrees. Against NOAA's buried probes the model's daily average is within 5°F on 68% of days (accuracy). Use the model to see when the season is turning, and your thermometer to decide for one lawn.

Apps and online maps

A soil temperature app or map shows a modelled or station value for your area, like this site; none of them can read your soil. Look for one that says where its number comes from, how deep it is and when it was updated: this site states all three on every page.