Agriculture Reference
In-Depth Information
Estimates of
N
min
.
During One Year from Net Mineralization
of Soil Organic Matter and Green Manure
Table 5.5
N
min
.
(kg/ha)
a
Soil Organic Matter (50%
10)
k
Value (1/yr)
b
C, C:N Ratio
Organic N Content,
N
o
SOM
(t/ha)
(kg N/ha)
0.002
0.005
0.01
30
1500
3
7.5
15
40
2000
4
10
.0
20
50
2500
5
12.5
25
N
min
.
(kg/ha)
c
Green Manure (with Legume)
20)
k
Value (1/yr)
b
(40% C, C:N Ratio
Organic N Input,
N
o
DM (t/ha)
(kg N/ha)
0.5
0.75
1.0
1
20
0
8
10
13
1.5
30
12
16
19
2
40
16
21
25
a
Calculated from equation A4.8 in appendix 4
b
The decay coefficients vary with the type of soil and its management, as well as the type of green
manure crop.
c
Calculated from equation A4.4 in appendix 4
per vine or per ha. Calibration requires a number of trials at different sites in
which the response of vines to increasing amounts of a specific nutrient is mea-
sured. It is important that other nutrients are not limiting and the vines do not
suffer serious water stress. These tests are commonly called
fertilizer rate trials
. Ide-
ally, the results should be specific to a particular variety, but they can cover a range
of soil types. The test nutrient can be applied as a commercial fertilizer, such as
urea to supply N, or muriate of potash (KCl) to supply K.
The yield response to nutrient addition usually follows the law of diminish-
ing returns, which states that the yield increase for successive equal increments of
a nutrient becomes steadily less. This relationship is described by the equation
y
A
B
exp(
Cx
) (5.2)
where
y
is the yield per ha and
x
is the amount of nutrient applied in kg/ha. Also,
A
is the maximum yield, (
A
B
) the yield from the soil without added nutrient
(the control or “check” yield), and
C
the rate of change of
y
with
x
. Figure 5.4a
shows an example of a yield response curve to N fertilizer. Because
A
is approached
asymptotically, the practical maximum is often set at 90% of the true maximum
(Goldspink 1996). If the plant tissue is analyzed, the
critical value
for the tissue
test would be determined at 90% of maximum yield. Similarly, if the soil is an-
alyzed, the critical value for the soil test is determined in the same way.
Figure 5.4b has the same response curve plotted to show the increasing value
of the product relative to the cost of fertilizer applied. When the value of the ex-
tra yield from fertilizer (curve AB) is compared with the fertilizer cost line (AC),
the vertical XY indicates the fertilizer rate that gives the biggest profit from fer-
tilizer. If the fertilizer cost changes, XY moves to the right or left to a new point
of maximum profit. Similarly, if the crop value changes due to quality factors, for
example, the curve AB will change and the position of XY moves.
