Agriculture Reference
In-Depth Information
Mulches affect soil temperature. Because mulches have a higher albedo than
bare soil, the energy absorbed during the day, and hence soil warming, is less under
this type of cover. The fact that soil stays moister under mulch also influences its
temperature,becauseawetsoilrequiresmoreheatenergythandrysoilforits
temperaturetoriseonedegree.Soilwetnessisthereforeimportantindetermining
when root growth starts in spring. Vine roots start to grow around 6ºC and have a
temperature optimum close to 30ºC.
Surface albedo is also afected by color, with black and dark-red surfaces
relectinglessradiationandthereforewarmingmorequicklythanlight-colored
surfaces. However, at night, dark surfaces radiate energy faster than light-colored
surfaces, adding to the loss of heat by convection. Bare soil surfaces with no veg-
etative cover lose heat rapidly at night, especially if there is no low cloud cover.
In the Châteauneuf-du-Pape appellation of the southern Rhone Valley, France,
largequartzitestones(called galets )depositedbytheriverovermillenniaactas
“heatsinks”duringthedayandreradiatethisheatatnight(igure4.21).hevine
canopy traps some of this heat with the result that the surface soil temperature and
near-surface air temperature remain higher than otherwise expected. This is an
important factor in decreasing frost damage to green tissues.
Organic mulches, such as straw or bark pieces, have a low thermal conductiv-
ityandslowthetransferofheatintoandoutofthesoil.Soilundermulchthere-
fore remains cooler during the day and warmer at night than bare soil, resulting in
Figure 4.21 A vineyard with heat-absorbing galets to the east of Châteauneuf-du-Pape
appellation in the Rhone Valley region, France.
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