Soil Temperature

[Portland Paul]

Hi, my understanding, is that the growing season begins and ends with a general soil temperature of 6 c (43F).

I measured a soil temp of 11.5 c this morning - rising to 13.5 c by this afternoon. Quite high really, I thought, for this time of year! Our soil was originally a fairly heavy clay seven years ago (good at retaining warmth in autumn), but with the regular addition of compost etc, it is now a nice rather heavy loam.

After moving plants a couple of weeks ago, and cultivating (and digging compost into the) soil in the process, self-seeders have started to pop up. Examples include the germination of Malvas, Hollyhocks and opium poppies in abundance!

Given the well above average air temps of recent weeks, how fast will it take for the soil to get down to 6 c, if a more average autumn climate prevailed, after this current warm spell?

Certainly the plants are enjoying the warmth. My Virginia creeper is still green, and the Hostas are not yet turning yellow or collapsing!

[Stratos Ferric]

Hi, my understanding, is that the growing season begins and ends with a general soil temperature of 6 c (43F).

I measured a soil temp of 11.5 c this morning - rising to 13.5 c by this afternoon. Quite high really, I thought, for this time of year! Our soil was originally a fairly heavy clay seven years ago (good at retaining warmth in autumn), but with the regular addition of compost etc, it is now a nice rather heavy loam.

After moving plants a couple of weeks ago, and cultivating (and digging compost into the) soil in the process, self-seeders have started to pop up. Examples include the germination of Malvas, Hollyhocks and opium poppies in abundance!

Given the well above average air temps of recent weeks, how fast will it take for the soil to get down to 6 c, if a more average autumn climate prevailed, after this current warm spell?

Certainly the plants are enjoying the warmth. My Virginia creeper is still green, and the Hostas are not yet turning yellow or collapsing!

<{POST_SNAPBACK}>

As evidenced by how cold it gets on a clear night, the soil moves pretty quickly at the surface with the air above. The problem this year, and in recent warm autumns, seems to have been, at least in part, increased nocturnal cloudiness holding air temps, and so ground temps, well up.

[Terminal Moraine]

Much depends on the depth at which you're measuring the soil temp' B.B.

At 5-10cm depth, as Stratos states above, the temperature of the soil, dependant on its moisture content, is quite closely synchronised with the air temperature, . A high temperature at 5cm will rapidly fall after one cold night.

By 20cm depth the response to air temperature, though still there, is much more muted and it will take several cool nights and cooler days to give a significant fall. At 30cm the temperature responses are generally not more than 0.5c at a time and by 50cm a diurnal variation of about 0.3c can be expected.

At 100cm depth there is normally a lag of about a week between any major change in air temperature and the corresponding increase at that depth; by 200cm the lag is a fortnight at least, if any response is noted at all. At 100cm and below there is rarely more than 0.1c variation in temperature between one day and the next.

Of course for germinating seedlings it will be the top few cm of soil which are relevant so sustained warmth by day and night will be essential.

You're right about 6c being the threshold for growth although this varies from species to species. 6c was found to be the growth threshold for grass and most cereal crops but it is the mean air temperature which is the most crucial parameter rather than the soil temperature. With mean air temperatures below 6c and soil temperatures above it, vegetative growth will stop but root growth will continue slowly.

Many spring sown vegetables such as Beans, Peas, Lettuce will begin to germinate and grow once the mean temperature reaches 6c but others such as Beetroot, Parsnip, Runner Bean require a slightly higher mean for sustained growth.

T.M

[Paul]

The GFS includes data for soil temps at various depths, would this be a useful thing to have available?

[Portland Paul]

Much depends on the depth at which you're measuring the soil temp' B.B.

At 5-10cm depth, as Stratos states above, the temperature of the soil, dependant on its moisture content,  is quite closely synchronised with the air temperature, . A high temperature at 5cm will rapidly fall after one cold night.

By 20cm depth the response to air temperature, though still there, is much more muted and it will take several cool nights and cooler days to give a significant fall. At 30cm the temperature responses are generally not more than 0.5c at a time and by 50cm a diurnal variation of about 0.3c can be expected.

At 100cm depth there is normally a lag of about a week between any major change in air temperature and the corresponding increase at that depth; by 200cm the lag is a fortnight at least, if any response is noted at all. At 100cm and below there is rarely more than 0.1c variation in temperature between one day and the next.

Of course for germinating seedlings it will be the top few cm of soil which are relevant so sustained warmth by day and night will be essential.

You're right about 6c being the threshold for growth although this varies from species to species. 6c was found to be the growth threshold for grass and most cereal crops but it is the mean air temperature which is the most crucial parameter rather than the soil temperature. With mean air temperatures below 6c and soil temperatures above it, vegetative growth will stop but root growth will continue slowly.

Many spring sown vegetables such as Beans, Peas, Lettuce will begin to germinate and grow once the mean temperature reaches 6c but others such as Beetroot, Parsnip, Runner Bean require a slightly higher mean for sustained growth.

T.M

<{POST_SNAPBACK}>

The GFS includes data for soil temps at various depths, would this be a useful thing to have available?

<{POST_SNAPBACK}>

Many thanks both, that sounds extremely useful.

My soil thermometer has a 3 cm probe, so I guess those seedlings should be slowing down pretty soon. A few days back, it registered a 2 c difference in 9 hours between 8 am and 5pm on a day of hazy sunshine with damp soil, so, critically, it should respond in sinc - as you say, with the air temps.