Agriculture Reference
In-Depth Information
The following sections explore the advantages and disadvantages of methods
for building up SOM.
Cover Crops and Mulches
The effect of a mid-row cover crop on the availability of soil water is discussed in
“Cover Crops and Soil Water” in chapter 4. In addition to this effect, cover crops
have important functions such as
• Protectingsoilfromtheimpactofraindrops,whichinitiatesstructural
breakdown and consequent erosion
• Improvingsoilstructure,iniltration,soilstrength,anddrainage
• BuildingupSOMandenhancingbiologicalactivity
• Providingahabitatforbeneicialinsectsandpotentialpredatorsof
grapevine pests
• Suppressingweeds
The relative importance of these functions depends on the soil type and spe-
cies makeup of the cover crop. A cover crop can be a single sown species, such as
the winter cereal shown in figure 4.19 in chapter 4, or a mixture of species involv-
ing grasses and legumes, as seen in figure 5.7. In regions with substantial winter
rainfall but dry summers, winter-growing annuals are preferred because they can
be incorporated into the soil in early summer and so do not compete with the
vines for water. Alternatively, perennials may be preferred in regions where water
availability is not a problem but there is a need to control vine vigor or prevent soil
structural decline and erosion. The deep-rooted perennial chicory can be sown in
situations where improvement in soil structure is required. In organic vineyards
where biodiversity is highly valued, cover crops may consist not only of a mixture
of grasses and legumes but also of herbaceous broadleaf plants such as mustard
(
Brassica
species), Compositae species, sages, buckwheat, and members of the car-
rot family.
Whatever the cover crop, mowing in early spring (at bud burst) may be neces-
sary to reduce the risk of frost damage to the vines. An annual cover crop, espe-
cially one containing a legume, can be cultivated into the soil in spring as a “green
manure” (see “Organic Viticulture,” chapter 3). A legume cover crop that is incor-
porated when young and succulent will decompose rapidly, thus adding little to
the SOM. However, its contribution to available soil N through mineralization
can be appreciable, depending on how much N
2
the crop has fixed. Box 5.2 dis-
cusses the factors influencing N
2
fixation by legumes.
Cover crops potentially compete with vines for water and nutrients. In some
vineyards of the Bordeaux and Dordogne regions in France, for example, a cover
crop is sown in every second mid-row and alternated year by year to reduce the
