Temperature of the soil (assuming you're talking outdoors) depends significantly on depth. The top 2" of soil will swing dramatically over 24 hr periods, especially on sunny days compared to cloudy days. Moist soils will have more dampened swings compared to dry soils. As depth increases, diurnal swings quickly diminish, but since you're referencing seed germination, you're likely only interested in the top 1" anyway. Effectively you can just use the air temperatures--the top 1" of soil will not get quite as cold as the air, but it will be within a couple degrees. Moist soils will also warm more slowly than dry soils--a lot of energy required to warm water compared to soil and air pockets in the soil.
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