The Living Soil - Understanding Vineyard Soils - page 192

Box 5.2 (continued)
• Becauseofthelargeenergydemand,N 2 fixation is less likely to occur when
mineral N, especially ammonium (NH 4 + ), is readily available to the plant
• Inadequatecarbohydratesupplyandwaterstressdecreaseanodulated
root's fixation capacity
• Nodulatedlegumesneedmorephosphorus(P),molybdenum(Mo),and
copper (Cu) than non-nodulated legumes and have a unique requirement
for cobalt (Co)
• NodulationandN 2 fixation can be stimulated by simultaneous infection of
the roots by arbuscular mycorrhizas, mainly because of enhanced P uptake
via the mycorrhiza.
When a legume requires a specific Rhizobium strain, it is cultured in peat and
coated on the seeds with the aid of a sticky sugar solution. The seeds can also be
pelleted with a coating of lime to improve Rhizobium survival in acid soils and the
subsequent infection of legume roots.
Because of these constraints and differences in host-bacterial strain
performance, the quantity of N fixed is highly variable. However, provided an
effective Rhizobium strain is present, N fixation will be directly related to the
proportion of legume in a cover crop. In vineyards where the cover crop occupies
virtually all the land area, the amount of N fixed should be 50 to 100 kg N/ha/year.
This fixed N is made available as nodules slough off and legume residues decompose,
or through the excreta of stock animals if they graze in the vineyard. Note that this
extra N may contribute to an excess vigor problem on more fertile soils.
Table 5.5 summarizes information on different types of cover crops and plant
species that can be used in vineyards. Experience in California suggests that cover
crops can add from 1 to 12 t dry matter/ha/year, with the larger figure applying to
fully organic vineyards (McGourty, 2008).
Manures and Composts
When organic residues are consistently removed and not returned to soil, the soil
becomes more acidic. Thus adding manures and compost from elsewhere can counter-
act soil acidification in vineyards. The nutrient value of composted manures and other
composts is discussed in “Cultural Practices for Organic Viticulture” in chapter 3.
Composting involves accelerating the decomposition of plant residues or ani-
mal manures in well-aerated, moist heaps. Often plant residues and manures are
co-composted to improve the nutrient content of the product, as in the example
given in figure 5.9. Nonindustrial sewage sludge (biosolids) can also be com-
posted, provided bulking-up material such as wood chips or sawdust is added.
Composting is done in heaps or windrows in which the temperature should
exceed 55ºC for at least three days to kill weed seeds and pathogens. The heap
or windrow should be turned regularly to ensure it remains aerobic and main-
tains an adequate temperature. Depending on the nature of the raw materials