Agriculture Reference
In-Depth Information
nutrient varies with its genetic potential, and many of the soil processes affecting a
nutrient's supply to roots are dynamic.
Apart from pH measurement, the most common soil tests are for available P, S,
and the exchangeable cations Ca
2+
,Mg
2+
, K
+
, and Na
+
. Soil testing for N is less com-
mon, mainly because the amount of available N (NH
4
+
plus NO
3
−
) is biologically
dynamic (see box 3.2). Because many chemical extractants have been used, especially
for P, the analytical method must be known in interpreting a soil test result. Not
only do analytical methods vary for any one nutrient, but different laboratories can
also produce different results for the same test on samples of the same soil. Table 3.8
gives an example of interlaboratory variation in soil test results for soil from a single
vineyard. Clearly, field sampling error and laboratory analytical error can engender
uncertainty in a set of soil test results, especially for nutrients such as available N and
P. However, by following a consistent sampling protocol, using the same commercial
laboratory, and making measurements over time, as recommended for plant analysis
(see figure 3.11), a winegrower can see how a soil test value is trending and minimize
uncertainties introduced by temporal, spatial, and human factors.
In addition to differences in analytical methods, there are differences among
commercial laboratories in the interpretation of soil tests. Box 3.7 discusses
the concept of the basic cation saturation ratio (BCSR), commonly known as
the Albrecht System, which is an example of an “alternative” interpretation of
exchangeable cation values.
Table 3.8
Soil Test Results from Two Laboratories for Samples (0-10 cm) of the Same Soil from
a Vineyard in the Gippsland Region, Victoria, Australia
Laboratory A
Laboratory B
Soil test and units
Under vine
Mid-row
Under vine
Mid-row
pH (water)
6.5
6.7
6.3
6.5
pH (CaCl
2
)
5.8
6.0
5.8
6.0
EC(1:5)(dS/m)
0.12
0.12
0.15
0.16
Organic C (%)
2.6
4.1
2.3
3.6
MineralN(NH
4
+
and
NO
3
−
) (mg/kg)
10
14
7
7
Bicarbonate-extractable
P (mg/kg)
33
24
78
87
Available K (mg/kg)
273
351
281
320
Exchangeable Ca (cmol
(+)/kg
7.2
7.8
6.9
7.4
ExchangeableMg(cmol
(+)/kg)
1.3
1.3
1.2
1.4
Exchangeable Na (cmol
(+)/kg)
0.2
0.1
0.4
0.4
CEC (cmol (+)/kg)
9.4
10
12
12
