Plants are very sensitive to the temperature of the soil in which they are growing. Regardless of the air temperature, seeds will germinate only when the temperature of the soil warms to a point that they require. For instance, peas germinate when the soil is 45øF and no sooner. Further, the root systems of most plants grow best when the soil temperature is between 65øF and 85øF. When soil temperatures rise above 85øF most plants simply stop growing; they just sit there until the soil cools. How does this all relate to compost?
Soils containing quantities of compost typically have a darker color, since most compost is almost black. A darker soil will absorb more heat from the sun than a lighter colored soil. Consequently, soil containing lots of compost tends to heat up faster in the spring, stimulating plants to start growing sooner in the season. At the same time, a soil mulched with and inch or two of compost in the middle of summer will stay 6ø to 15ø cooler than soil with no organic mulch. The mulch serves as a blanket, protecting the underlying soil from the rays of the sun. If the soil can be kept below that important 85ø ceiling, the plants will do better in the summer heat.