Temperature Monitoring
Part of Soil Science
Soil temperature is one of the most practical and most overlooked variables in agricultural decision-making. Seeds will not germinate below their minimum threshold regardless of calendar date, moisture, or soil fertility. Transplants planted into cold soil sit dormant for weeks and are overtaken by later plantings made at the correct soil temperature. Compost thermometers tell you when turning is needed; soil thermometers tell you when to plant. This article covers how to measure soil temperature without instruments, how to interpret readings for planting decisions, and how to manipulate soil temperature to extend the growing season.
Why Soil Temperature Matters More Than Air Temperature
Air temperature and soil temperature diverge significantly:
- On a sunny spring day when air temperature is 18°C, soil at 5 cm depth may still be only 8°C — too cold for warm-season crops
- In autumn, soil retains summer heat for weeks after air temperatures drop — warm soil allows continued growth while nights are already cool
- Soil temperature changes slowly (buffered by thermal mass of the earth); air temperature fluctuates daily by 10–15°C, but soil temperature fluctuates by only 2–5°C
This buffering is why planting decisions should use soil temperature, not air temperature.
Measuring Soil Temperature
Method 1: Calibrated Soil Thermometer
A commercial dial or digital soil thermometer is inserted to a standard depth (usually 5 or 10 cm). Take readings at the same time each day — either early morning (represents minimum temperature) or mid-afternoon (represents maximum). Early morning readings are most relevant for germination decisions.
Taking a reading:
- Push the probe to the target depth (5 cm for shallow-rooted crops, 10 cm for general purposes)
- Hold in place for 60 seconds to allow equilibration
- Read and record with date and location
Method 2: Glass Candy or Lab Thermometer in a Probe Tube
A glass laboratory thermometer in a thin-walled metal or PVC tube inserted into a pre-drilled hole works identically to a commercial soil thermometer.
- Drill or push a hole 10 cm deep in the soil
- Insert the thermometer, packing moist soil around the tube to ensure contact
- Read after 2 minutes, removing quickly before air temperature affects the reading
Method 3: Hand Testing (No Instrument)
The hand test is less precise but immediately available:
- Push your bare hand or fingers into the soil to 5 cm depth
- Hold for 15–20 seconds
| Hand Sensation | Estimated Soil Temperature | Planting Implication |
|---|---|---|
| Painfully cold, numbing | Below 5°C | Plant nothing (except garlic, onion sets, early broadbeans) |
| Cold, uncomfortable | 5–8°C | Cool-season crops only; brassicas, spinach |
| Cool but bearable | 8–12°C | Most cool-season crops; peas, lettuce, chard |
| Neutral — neither warm nor cool | 12–16°C | Brassicas produce well; borderline for tomato |
| Noticeably warm | 16–20°C | Good for tomato, capsicum, bean, zucchini |
| Clearly warm | 20–25°C | All warm-season crops; excellent germination |
| Hot | Above 25°C | Most crops struggle; mulch to cool |
Calibrate Your Hand Test
Practice the hand test alongside a thermometer for a few weeks so you build a reliable sense of the temperature ranges. Once calibrated, the hand test is accurate to within 3–4°C — sufficient for most planting decisions.
Method 4: Water Temperature Proxy
A simple approximation: in a shallow bowl of water placed on the soil surface in a shaded location, the water temperature equilibrates roughly with the soil surface temperature by early morning. This gives a conservative (slightly lower) estimate of soil temperature.
Germination Temperature Thresholds
Minimum temperatures for seed germination represent the absolute lower bound — germination is very slow at the minimum. Optimal temperatures produce fast, even germination. Maximum temperatures inhibit or kill germinating seeds.
| Crop | Minimum | Optimal | Maximum |
|---|---|---|---|
| Spinach | 2°C | 15–18°C | 25°C |
| Peas | 4°C | 16–20°C | 28°C |
| Onion | 5°C | 20–25°C | 35°C |
| Lettuce | 4°C | 16–22°C | 29°C |
| Broccoli/Cabbage | 5°C | 16–22°C | 35°C |
| Carrot | 5°C | 16–22°C | 35°C |
| Beetroot | 5°C | 18–24°C | 35°C |
| Chard | 5°C | 18–24°C | 35°C |
| Bean (common) | 10°C | 24–30°C | 35°C |
| Cucumber | 16°C | 26–35°C | 40°C |
| Tomato | 10°C | 26–32°C | 35°C |
| Capsicum/Pepper | 16°C | 30–35°C | 40°C |
| Melon | 18°C | 30–35°C | 40°C |
| Pumpkin/Squash | 16°C | 26–32°C | 40°C |
| Maize | 10°C | 26–32°C | 40°C |
| Sweet potato | 18°C | 28–35°C | 40°C |
Implications for Planting Calendar
Do not plant maize when morning soil temperature is below 12°C. Even if seeds germinate eventually, they are vulnerable to seed rot, damping off, and early-season insect damage during slow cold germination. A planting 3 weeks later into 18°C soil will often overtake an earlier cold planting within 4 weeks.
For transplants, the root zone temperature governs uptake. Transplanting warm-season crops into cold soil (below 15°C) results in transplant shock and very slow establishment even when air temperatures are warm.
Compost Temperature Monitoring
Compost thermometers serve a different function — monitoring the heat generated inside an active compost pile to determine when turning is needed and when decomposition is complete.
| Compost Temperature | Interpretation | Action |
|---|---|---|
| Under 30°C | Pile is inactive — too dry, too compact, or C:N imbalance | Add water and nitrogen material; loosen pile |
| 30–45°C | Warm composting — active but not hot | No action needed; check moisture |
| 45–60°C | Hot composting — optimal range | Monitor; turn when temperature drops |
| 60–70°C | Very hot — pathogens and weed seeds killed | Excellent; turn to oxygenate when drops below 55°C |
| Above 70°C | Too hot — beneficial organisms dying | Turn immediately to introduce fresh air |
| Stable below 40°C | Maturation phase — composting nearly complete | Allow to cure 2–4 weeks then use |
The compost thermometer should be pushed to the centre of the pile (the hottest zone). Measuring only the outside surface gives falsely low readings.
Pasteurisation Temperature Targets
For compost to be reliably safe to use on edible crops:
- Must reach 55–70°C at the centre
- Must be turned so all material passes through the hot core
- Must maintain target temperature for at least 15 days (three 5-day periods at heat, with turns between)
This protocol kills most human pathogens, weed seeds, and plant disease organisms.
Soil Temperature and Nutrient Availability
Soil biological activity drives nutrient cycling. As soil temperature drops, microbial activity slows and nutrients become less available:
| Soil Temperature | Microbial Activity Level | Nutrient Release |
|---|---|---|
| Under 5°C | Near zero | Minimal |
| 5–10°C | Low | Slow — 10–20% of peak |
| 10–15°C | Moderate | 30–50% of peak |
| 15–25°C | High | Good — 70–100% of peak |
| 25–35°C | Very high | Peak activity |
| Above 35°C | Declining | Organisms begin dying |
This explains why plants grow slowly in early spring even after soil has warmed to the minimum germination threshold — nutrients are not yet cycling at full rate. Liquid feeds (compost tea, manure water) supply immediately available nutrients to bridge the gap until soil biology activates.
Manipulating Soil Temperature
Warming Cold Soil
| Method | Temperature Gain | Setup Time |
|---|---|---|
| Clear plastic mulch (solar heating) | 5–10°C at 5 cm | Lay 2–3 weeks before planting |
| Black plastic mulch | 3–5°C at 5 cm | Lay 2–3 weeks before planting |
| Clothes / low tunnels | 3–6°C inside | Immediate |
| Cold frame (glass or plastic box) | 5–15°C inside | Build before needed |
| Raised beds with dark facing | 2–4°C | Permanent installation |
Clear plastic is more effective than black for soil warming because light penetrates to heat the soil directly. Black plastic heats the surface but shades the soil below from some radiation.
Cooling Hot Soil
In hot climates, peak summer soil temperatures exceed 35–40°C — lethal to many crops and beneficial organisms.
| Method | Cooling Effect |
|---|---|
| Straw mulch (8–10 cm) | Reduces surface soil temperature by 8–15°C |
| Wood chip mulch (8 cm) | Reduces surface temperature by 10–18°C |
| Shade cloth (30–50% shade) | Reduces surface temperature by 5–10°C |
| Irrigation (cooling effect) | Immediate 5–10°C drop during application |
Temperature Monitoring Summary
Soil temperature drives germination, transplant establishment, nutrient availability, and compost decomposition. Measure it with a soil thermometer at 5–10 cm depth in the early morning. When no instrument is available, the hand test (push bare fingers 5 cm into soil for 15–20 seconds) distinguishes planting zones by feel. Never plant warm-season crops (tomato, bean, cucumber, maize) until morning soil temperature consistently exceeds 15°C at 5 cm — and ideally 18–20°C. In compost management, target 55–70°C at the pile centre and turn when temperature falls below 55°C. Manipulate soil temperature with clear plastic (warming), straw mulch (cooling), and structures (cold frames, tunnels) to extend productive growing windows beyond what air temperatures alone would permit.