Agriculture Reference
In-Depth Information
tracting the soil in a solution of 2M KCl and by calculating the amount in mg
N/kg soil.
The balance between N mineralization and immobilization during decom-
position of organic matter depends on the C:N ratio of the substrate (section
2.3.1.2). The outcome of this “balancing” is the
net
mineralization rate, given by
the equation
dN
kN
(5.1)
dt
where
N
is the amount of organic N per unit soil volume,
t
is time, and
k
is a
dN
decay coefficient (compare equation 2.1). The expression
is the
instantaneous
dt
rate of net mineralization. The way in which this equation is used to obtain val-
ues of mineralized N (
N
min.
) over a period of time is illustrated in appendix 4.
The mineral N first appears as NH
4
, but this is usually rapidly converted to
NO
3
by nitrifying bacteria.
The decay coefficient
k
is an
averaged
parameter, because there are differences
in net mineralization rate among fresh residues (grass mulch and green manures),
animal excreta (chicken manure), and resident soil organic matter (
SOM
). For ex-
ample, fresh residues of C:N ratio
25 decompose and mineralize N much faster
than
SOM
, which has achieved a high degree of stability after many cycles of syn-
thesis and decomposition. Thus,
N
min
.
values will vary according to the following
factors:
• the amount of residues and organic manures returned to the soil, relative to
the
SOM
,
• the C:N ratio of the residues and manure, and their distribution in the soil
(which affects their accessibility to microorganisms),
• soil management (e.g., soil pH, and whether the soil is cultivated or
drained), and
• environmental factors (especially temperature and effective rainfall).
Cover crops
. Cereals, grasses, and legumes, sometimes in combination, are of-
ten grown in the inter-rows of vineyards to protect the soil from erosion and com-
paction, especially during winter (section 7.3.1). When cultivated into the soil in
early summer, cover crops containing legumes provide young green material of
low C:N ratio that decomposes quickly and is an important source of mineral N
to the vines. This is called
green manuring
. Green manure crops that decompose
quickly have little effect on
SOM
; straw and bark mulches, on the other hand,
have high C:N ratios and are slow to decompose. Such mulches can increase
SOM
,
but during decomposition, they “lock up” soil mineral N and make it unavailable
to the vines. Eventually, as the C:N ratio of the mulch residues slowly declines,
some of this immobilized N is mineralized.
Values of
N
min.
for a range of organic materials and soil conditions are given
in table 5.5.
Calibration of Soil and Plant Tests
Irrespective of which soil extraction or tissue analysis method is used, the test must
be calibrated against a measure of plant performance, usually the yield of grapes
5.3.3
