Post by Deleted on Mar 14, 2005 11:07:29 GMT -6
Pre-warming the soil can help crops germinate earlier than usual, increasing the length of the cropping season. Soil temperature for vegetable seed germination varies, but for hardy crops such as carrots and beetroot it occurs when soil temperatures average above 8° C (46° F), and 12° C (54° F) for tender crops - sweetcorn and French beans for example. Covering the soil will warm it during the day and slow down heat loss at night. Temperatures near the surface will tend to fluctuate but, even if soil freezes occasionally, lower soil levels may retain some of the accumulated warmth.
Dry soil warms more readily than wet soil. Sandy soils hold less water than heavy soils, so tend to warm up earlier. Making raised beds can improve drainage, which will aid soil warming.
Adding organic matter to the soil can reduce warming by raising moisture content, but it can also improve warming by darkening the soil so it absorbs the sun's heat and by improving drainage. As organic matter is beneficial in other ways it is worth adding as much as possible.
Cover the bed with plastic sheeting or with cloches. Transparent plastic is best for warming the soil, although it will not prevent weed seeds germinating. Put in place several weeks before you intend to sow and hoe off weed seedlings as they appear. This has the added advantage of reducing weed problems later in the season.
Black plastic needs to be in close contact with the soil for efficient heat transfer. Polythene is easier to lay over mounded beds. A sandwich of black polythene covered by clear polythene works best.
Fleece traps little warmth, but lets rain through. It is useful for covering seeds and plants after sowing and planting, rather than pre-warming the soil.
