Agriculture Reference
In-Depth Information
If YAN is too low, the fermentation rate is sluggish and may become “stuck.”
Unpleasant odors from hydrogen sulfide (H
2
S) can be produced. Although wine-
makers can correct this problem by adding monoammonium or diammonium
phosphatetothemust,itispreferableforthegrapestohaveanadequateNcon-
centration coming into the winery.
Conversely,ifYANistoohigh,fermentationistoorapidandpoor-quality
wine is produced, particularly in the case of red grapes. Residual protein in the
must after fermentation causes haze in the wine. Especially when coupled with
plentiful water, too high an N supply to the vine leads to excess vigor, a topic con-
sideredagainin“ManagingSoilWaterwithIrrigation,”chapter4.
Phosphorus Cycling and Phosphorus Fertilizers
As illustrated in figure 3.14, P cycling in a vineyard is similar to N cycling with the
followingdiferences:
1. Phosphorus inputs from the atmosphere are negligible.
2. The chemistry of phosphate ions is more complex than N because these
ions are adsorbed onto clays and Fe and Al oxides, and P can also form
insoluble precipitates. The term “P fixation” encompasses these processes
ofsorptionandprecipitation.OneconsequenceofPixationisthatthere
is normally little water-soluble P in the soil, except around dissolving
fertilizer granules, and P does not readily leach from the soil in the way that
NO
3
−
does.
3. Because of low water-soluble P, vines benefit from mycorrhizal symbioses,
asdiscussedearlierin“MycorrhizasandNutrientUptake.”
Soil testing for P aims to measure the amount of available or “labile” P,
comprising soluble P and that fraction of solid-phase P that is readily released
into solution. Water-soluble P fertilizers augment the labile P, but over time
this P reverts to insoluble, less available forms. Thus even though P is not lost
by leaching or as a gas, fixation results in an ongoing need for P fertilizers, com-
post, or manure to maintain healthy vines, unless the soil is naturally rich in P
minerals. Because they contain much Fe and Al oxide and hence have a large
ixationcapacity,soilssuchasredloamsrequiremorePthanothersoils.Also,
morePfertilizerisrequiredatvineyardestablishmentthanforthemaintenance
of mature vines. The maintenance application can be calculated from fruit yield
and the assumption of 0.6 kg P removed per tonne of fruit, with an adjust-
mentforPixation.Mostcommerciallaboratoriescanmeasureasoil's“Psorp-
tion” index, which is a surrogate for its P fixation capacity and can be used to
adjust the P fertilizer recommendation up or down for either establishment or
maintenance.
