The Living Soil - Understanding Vineyard Soils - page 184

Box 5.1 Biofertilizers, Biostimulants, and Bioinoculants
In “Organic Viticulture” in chapter 3, we noted some of the organic products,
commonly derived from seaweed, recommended for organic viticulture. These are
usually promoted for their content of growth regulators (gibberellins, auxins, and
cytokinins), sugars, and amino acids. There are also the special preparations used in
biodynamic viticulture, obtained by incubating in soil a mixture of cow manure,
quartz, and flower petals in cow's horn. Winegrowers who use these products claim
benefits for soil and vine health, grape quality, and the distinctiveness of the wines,
especially their own enjoyment of them. The products are best described as possible
biostimulants for soil microorganisms, because the amounts added (as little as 90
g/ha) are far too small to have a direct effect on vine nutrition. Products derived
from fishmeal are also used as biofertilizers.
Humic acid in powder form (called humate) is recommended for improving
soil CEC and soil structure, but to have any such effect it should be added in much
larger amounts than is normally recommended. Although fulvic acid is promoted
as a biostimulant that can penetrate leaves, its stimulatory effect is unproven.
Humic and fulvic acids do not exist as discrete organic compounds because they
are a complex mixture of compounds, artificially derived from the chemical
fractionation of SOM using strong alkalis and acids. The real benefit of these
products in vineyards is uncertain.
Various bacteria and fungi with specific functions have been isolated from soil
(e.g., Rhizobium bacteria; for N 2 fixation in legumes) and arbuscular mycorrhizal
fungi (enhanced P uptake by roots). Other microorganisms can mineralize organic
phosphate or dissolve insoluble inorganic phosphates. An example of the former
is Bacillus megatherium , present in the commercial inoculum phosphobacterin.
Although some isolates have been effective in pure culture and pot trials in glasshouses,
beneficial effects are not consistently produced in the field. When introduced by seed
inoculation into soil, such organisms are overwhelmed by competition from the much
larger number of resident microbes. The introduced organisms do not survive, or they
lose their P-solubilizing activity when they reproduce.
Of the many attempts to manipulate the rhizosphere population by the
introduction of specific organisms, few have produced consistent results except
for the inoculation of legume seeds with Rhizobium (see box 5.2). However,
research in New Zealand suggests that if mulch is inoculated with the fungus
Trichoderma , the pathogen Botrytis can be suppressed. In Australia, a bioinoculant
called Trich-A-Soil is available that contains selected strains of this fungus, but its
effectiveness is unknown.
Studies in the Barossa Valley region, South Australia, showed that the amount
of SOM and the soil's chemical and physical condition influenced the species of
earthworm present and their abundance. Consistent with studies on grasslands,
most species are found to prefer neutral or calcareous soils, and few are found
at pH less than 4.5. Under hot, dry conditions, earthworms burrow as deeply as
possible into the soil and aestivate. As shown in table 5.4, extra organic matter
supplied as straw mulch or from a mown cover crop, together with raising the soil