Agriculture Reference
In-Depth Information
Box 5.2
Determining the Best Fertilizer Option to Meet a Nutrient Requirement
Suppose the optimal N requirement, determined from figure 5.4b, is 40
kg/ha. The cost per kg N content determines the slope of the fertilizer cost line
AC in figure 5.4b. The unit cost is given by
Cost/tonne
Cost/kg N
(B5.2.1)
10
If the fertilizers have similar costs per tonne, clearly the most economical
source of N is the fertilizer with the highest N content (expressed as a percentage).
Usually, the price of urea per tonne is the lowest, so the unit cost is much lower
for urea than other N fertilizers commonly used in vineyards (table 5.6). However,
losses of N by volatilization can be higher, and the acidifying effect of ammonium
oxidation can be a problem with urea (section 5.4.1.2). These factors must be
balanced against the economic advantage.
The quantity of fertilizer Q required to supply 40 kg N/ha is given by
40
N content
100
Q
(B5.2.2)
N content
If the fertilizer chosen is ammonium nitrate (34% N), Q
118 kg/ha.
Because there are 10,000 m
2
per ha, this is equivalent to 11.8 g/m
2
. If the fertilizer
is to be dissolved in irrigation water and applied through drippers—the process of
fertigation
—we must know the quantity of water to be applied per vine (chapter
6). The solubility of the fertilizer then becomes an important factor.
the preferred form of fertilizer and its cost. An example of the calculations is given
in box 5.2.
Precision Viticulture
As with other intensively managed crops, yield and quality can vary markedly
within a vineyard due to subtle changes in soil type, soil depth, drainage, and nu-
trient supply. The spatial pattern of variation may be consistent from year to year,
in which case blocks of vines can be identified where soil management and fer-
tilizer practice are tailored to achieve the desired soil physical and chemical con-
ditions. This is the basis of
precision viticulture
.
The spatial pattern of variation is mapped by manual soil surveys or real-time
sensing with mobile instruments. The position of each measurement is determined
by a Global Positioning System (GPS) and the coordinates entered into a Geo-
graphical Information System (GIS), together with the “soil attribute” data (pH,
profile depth, soil texture, salt content, and so on). Also, yield monitors are avail-
able for mounting on grape harvesters; these monitors allow the spatial pattern of
yield variation to be recorded digitally, entered into the GIS, and mapped. An ex-
ample of such a yield map is shown in figure 5.5 for a block of Ruby Cabernet
grown under irrigation in Sunraysia, Victoria. The generally low yields (
9 t/ha)
in the northwest sector were due to poor drainage (a shallow impermeable sub-
soil), whereas the high yields (
20 t/ha) in the eastern part were obtained on a
deep, permeable sandy loam. The recommended action is that drainage in the
5.3.5
