Soil Temperature and Plant Growth

It’s useful to be able to take your soil’s temperature. Use a probe-type thermometer made for soil, or any other thermometer you can put into the soil. Measure about 2 inches below the surface for most uses, including starting seeds. Take the soil’s temperature in early afternoon for 3 days in a row to be sure it is stable.

Seed Germination

Soil temperature is useful information for many gardening tasks. Probably the most common of them is knowing when to plant vegetable seeds in the spring. The table below gives minimum temperatures for some common vegetable seeds.

Minimum Soil Temperature Plants
32 degrees Onion, spinach, parsnip
40 degrees Radish, carrot, beet, cabbage, peas
50 degrees Corn, asparagus, tomato
60 degrees Squash, cucumber, bean, pepper, melon

Nutrient Release

Decomposition in the soil depends on soil bacteria. Manure, compost, and organic fertilizers release nutrients as they decompose, so the rate of nutrient release is dependent on soil temperature. Bacterial activity is very low below 50 degrees. Above that temperature, it doubles for every 10 degrees rise in temperature until about 90 degrees, after which it falls off rapidly as the bacteria are killed by the high temperatures.

When using fertilizers that depend on bacterial activity for their nutrient release, remember the effect of soil temperature. Most of the time, plant growth proceeds at about the same rate as bacterial activity, so nutrient release coincides with nutrient needs by plants. But organic fertilizers don’t work so well for some uses. For example, turfgrasses need nitrogen in the fall or spring when soil temperatures are too low to break down organic fertilizers. Either place organic fertilizers in place in time for them to release nutrients before the soil temperature drops too low, or use inorganic fertilizers for these times.

Fertilizers that depend on bacterial action for nutrient release include:

  • All organic matter
  • Ammonium-based fertilizers
  • Urea
  • Sulfur-coated slow-release
  • Isobutylidene diurea (IBDU)
  • Urea formaldehyde (UF)

Water-soluble fertilizers and controlled-release fertilizers other than those named above are only effected slightly by soil temperature.