Recording soil temperature

G.1
Procedure

Chose a maintained grass site that is level, freely drained, unshaded and with the soil unobstructed by stones or severe induration to a depth of 300 mm (12 in). Purchase from an ironmonger or garden centre a 300 mm (12 in) long, metal-sheathed soil thermometer and make up, or purchase, a 12 mm (1/2 in) wide, 300 mm (12 in) long, strong metal probe fitted with a handle—an old fashioned, long chisel is ideal. The probe is required to create the hole into which the thermometer is to be neatly inserted to 250 mm (10 in) depth without risk of damage, the idea being to enable close contact to be maintained between the metal sheath and the soil.

Measure soil temperature at 250 mm (10 in) depth weekly (or twice monthly), between midday and late afternoon, leaving the thermo

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FIGURE G.1 Graph of 1992 soil temperature records as listed in Table A7.1.

TABLE G.1 1992–93 soil temperature records at 250 mm (10 in) depth beneath garden lawn near Aberystywth. Grass maintained during growing season by weekly mowing at not less than 25 mm (1 in); clippings mostly returned. Soil a freely drained, clay loam with earthworms present, therefore no surface mat

Date Temperature (°C) Observations
January 19 8 Mild.
29 5.5 Cold and frosty but also sunny.
February 27 9 Changeable; grass now needs first cut; some buds bursting; snowdrops and crocuses flowering; daffodils in bud; rhubarb shoots emerging; tortoise stirring.
April 1 10 Colder; some snow; grass needs second cut; crocuses, primroses and daffodils in flower; tortoise dormant again; birds nesting.
12 12 Changeable; grass seeded end of March now germinating; buds of larger trees now bursting; tortoise lethargically active.
26 13.5 Sunny but cool wind; fern shoots unwinding; apple blossom starting; tortoise still somewhat lethargic.
May 28 18.5 Dry and very hot; soft fruit forming up; tortoise lively and eating well.
June 17 20 Continuing hot and dry; strawberries ripe.
27 21 Still continuing hot and now very dry; soil hard and no longer sticking to chisel; no growth, grass beginning to show signs of scorch.
August 4 20 Changeable; grass growing again; last of bilberries now being picked.
S­e­p­t­e­m­b­e­r 5 18 Changeable, wet and windy; grass growing well; starting to pick early apples and sweet corn; killing caterpillars on broccoli by hand.
22 17.5 Wet weather continuing.
October 8 15 Colder and drier; grass growing well; pears ripening; caterpillars still very active on broccoli; earwigs and slugs on brussel sprouts but under control;
tortoise dug in and hibernating since September 30.
17 13 First frosty morning; weather mixed—cold, sunny and occasional showers.
Autumn colours starting on aspen, fig and bilberries.
21 12 Continuing wet/cold/sunny; pear leaves yellowing; grass seed sown October 14 now germinating.
N­o­v­e­m­b­e­r 5 12 Mild; full autumn colours; leaves starting to fall.
15 10 Wet and windy; most trees now bare.
29 10 Mild with some wind and rain; snowdrop and daffodil shoots emerging.
D­e­c­e­m­b­e­r 10 9 Frosty morning but previously mild, wet and windy, grass-seeded patches still only sparsely covered with seedlings.
23 7 Cold and frosty but sunny; main lawn still growing slowly and should be trimmed to tidy.
28 5.5 Continuing frosty and sunny; soil surface hard frozen.
January 1 6 Sunny and mild following a week of hard frost.
23 9.5 Very mild, wet and windy; snowdrops all in flower plus occasional crocus and primrose; daffodils in bud; late-seeded grass still thin.
28 9 Still mild and moist; main lawn requires mowing but too wet; bluebell shoots now emerging; tortoise still hibernating under her sheltered pile of pine needles—will she respond to this false spring as she did last year?
February 1 8.5 Mild, dull and damp.
19 9.5 Continuing mild; fruit buds bursting grass growing well.
March 3 7.5 5 cold days; light snow and frost.
11 9 Mild and sunny; tortoise stirring.
24 10 Daffodils in full bloom and plum blossom fully out; grass due for third cut.
April 15 12 Apple blossom starting. Spring on its way. Tortoise active.

meter in place to equilibrate for a period of about 5 min. Record in a diary, along with notes about weather conditions, vegetation and other related observations, and enter as an additional plot on a continuously recorded, one-year graph. (Refer to Figure G.1 and associated observation records in Table G.1.)

Interest will develop as data accumulates to enable comparisons to be made, one year with another, and ideas about local norms and seasonal trends developed. This local knowledge will help to interpret variations in sward performance from one site to another and one year to the next, and will assist in the timing of management procedures such as fertilizing and seeding where the response is likely to be temperature dependent.

G.2
Aids to interpretation
  1. The site to which the data in Figure G. 1 refers is an unshaded garden lawn at 15 m (50 ft) above sea level, on the north side of a broad, east-west valley, 1 1/2 km (1 mile) inland from the coast of mid-Wales, just north of Aberystwyth.
  2. The soil temperature at 250 mm (10 in) tends to vary between a winter low of about 5° C and a summer high of about 20°C, with values of 9°C and above likely to promote growth, all other conditions being favourable. On this basis, the 1992 growing season began mid-March and continued until late November.
  3. The soil temperature tends to rise at an average rate of 0.75°C per week through spring and into summer, falling again at a similar, or slightly slower rate, through autumn into winter. At these transition periods, however, a spell of low air temperatures will affect the surface of the soil and limit germination from seed and seedling growth.
  4. Think of soil temperature, at or below 250 mm (10 in) depth, as reflecting the heat store available to warm the soil from below. However, once the soil surface becomes frozen the effect will reduce the influence of this heat store at least temporarily, but because it is maintained geothermally from below, the 250 mm (10 in) depth value will readily recover to its more normal winter range of 6–8°C, just enough to maintain a very slow rate of grass growth.
  5. There is no point in applying highly soluble, readily leached fertilzers before soil temperature is adequate to enhance growth. Therefore, delay application of seed and nitrogen-rich fertilizer, until the spring surge of soil temperatures exceeds 9–10°C. In the autumn the fall in soil temperatures will lag behind the fall in air temperatures and this will affect the performance of seeds and seedlings at the surface. It is advisable, therefore, to avoid seeding in the autumn beyond the point at which soil temperature at 250 mm (10 in) falls below 14 or 15°C.
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