Testing Soil Temperature

My multimeter says 11 degrees, still a bit chilly for seeds

Testing soil temperature for seed sowing wasn’t always scientific. It wasn’t too long ago that farmers would test the soil temperature by pulling down their pants and putting their bare bums on the soil (not all farmers I might add) If it felt cold on their cheeks then it was too cold for seed planting.

There were easier, less fun ways of testing the soil without baring all, you could simply place the back of your hand on the ground and again if it felt warm, the seeds could go in. People still check temperatures with their limbs but it’s usually to test the baby’s bathwater which so far as I know has always been the method of temperature gauging and no buttocks have ever been used.

Soil Temperature

Soil temperature is the factor that drives germination, transplanting, blooming and composting. It’s said that the ideal soil temperatures for planting most seeds and plants are 65 to 75 F. (18 to 24 C.). Most seeds will germinate over quite a wide range of soil temperatures but the speed of germination will vary. Too cold and they’ll be very slow to sprout and too hot will also reduce the speed of germination. Far too hot or cold also increases the risk on none of the seeds germinating which is what used to happen to me when I was in a rush to get seeds sown. 

If you have heated propagators and sow seeds under protection most of the guesswork is done. If you are sowing directly into the ground outside you might need to take more notice of the conditions.

I would think the best time to check the soil in the ground would be when the sun isn’t shining on it, the same for a compost bin – these like temperatures of over 60 F (16C) to work well. Soil readings for seeds are done in 1 to 2 inches of soil. Sample at least 4 to 6 inches deep for transplants. You can use your hands or invest in a soil thermometer (the same as on you use in the kitchen for probing meat) with a long metal rod. Insert the thermometer to the hilt, or maximum depth, and hold it for a minute. Do this for three consecutive days. 

The perfect temperature for planting varies dependent upon the variety of vegetable or fruit. Planting before it is time can reduce fruit set, stunt plant growth and prevent or reduce seed germination.

Simple methods

Apart from buttocks and back of the hands, there are other easy ways to check the soil.

If weeds are germinating and growing, it's time for us to get growing.

If spuds in the compost are growing, then you can put spuds in.

One suggestion (non-scientific I might add) is to wait until you can go out and garden without a cardigan on.

A few points to keep in mind:

The temperatures quoted are soil or compost temperature, not air temperature. A sunny day in April may be 20 C but the soil temperature is most likely stuck around 8 C.

 A soil thermometer can be bought very cheaply, typically less than £10.00, which will give you an accurate figure to work from. I use my multimeter which has a very accurate temperature guage.

Beware the sunny day when propagating in the greenhouse. Temperatures can soar, basically cooking your seeds or seedlings.

Higher Temperatures

The ideal temperature for germination is often far higher than we might expect and in some cases it is higher than is ideal for growing. Take carrots for example, their ideal germination temperature is 27 C, which is nearly an oven temperature. The optimum growing range for carrots is between 7 C and 29 C with far faster germination rates once the soil hits 10 C.

Tomato seeds are the same enjoying really high temperatures to kick start themselves into germinating. Like a lot of gardening pursuits it can become an obsession to get just the right conditions but seeds are tougher than we think and only need a few things to thrive. I for one err on the side of caution and leave my seed sowing until late spring which usually meets the seeds needs without us needing to fret or raid the kitchen drawer looking for the meat thermometer. 

You can get an early- weed free start by warming up the soil outside by a few degrees if you cover it with horticultural fleece or better still use a cloche which can make a surprising difference to germination rates and times.