Agriculture Reference
In-Depth Information
The Soil
5.3
The basis of
soil testing
is to measure that fraction of the total nutrient in a soil
that is
available
for plant growth (section 4.2.1). However, the concept of nutri-
ent availability is more complex than at first appears because
• Plants take up nutrient ions from the soil solution in the root zone, in
which the total amount of nutrient present is usually insufficient to meet
the plant's demand.
• The concentration of a nutrient in the soil solution is buffered by the labile
pool of the nutrient (section 4.4.3), the greater part of which comprises the
easily desorbable ions on surfaces (for Ca, Mg. K, P, and S). In the case of
N, S, and P, a readily mineralizable organic fraction may also be included.
• The rate of replenishment of a nutrient in solution at a root surface
depends not only on the soil's buffering capacity for that nutrient, but also
on the rate of movement to the root surface by mass flow and diffusion
(section 4.7.1).
Nutrients Held Mainly in an Inorganic Form
Although these three “availability factors” interact to determine the supply of a
nutrient, typically a soil test method involves using a single chemical extractant
to measure the amount of a particular nutrient available. A number of acid or al-
kali extractants have been advocated for available P, for example, but one of the
more widely used is a solution of 0.5M NaHCO
3
buffered at pH 8.5. For Ca,
Mg, and K, the amount of exchangeable cations, as measured by methods de-
scribed in box 4.5, can be used. For SO
4
-S, a solution of 0.01M Ca(H
2
PO
4
)
2
is
sometimes used, and for the micronutrients Fe, Zn, Cu, and Mn, a solution of
0.05M or 0.1M EDTA or DTPA (both metal complexing agents) are used. Fur-
ther details on soil testing are given in specialized texts, such as Rayment and Hig-
ginson (1992) or Jones (1999).
Soil analyses are carried out on a composite sample of soil made up from in-
dividual soil cores, usually taken to a depth of 15 cm, in a way that covers the
range of variability in a vineyard. In the case of N as NO
3
, samples may be taken
to depth in the profile (e.g., to 50 cm). Subsamples from individual soil types
should be mixed separately to provide composite samples for analysis.
5.3.1
Nutrients Held Mainly in Organic Form
Nitrogen is the main element in this category. Grapevines absorb N in the form
of NH
4
and NO
3
ions, but the latter predominate in most soils because of ni-
trification. The availability of soil N therefore depends on the amount of mineral
N in the soil, and the rate of mineralization of organic N.
Ammonium is held as an exchangeable cation that is easily displaced into the
soil solution where it moves with NO
3
, by mass flow to the roots of a transpir-
ing plant. Both ions also move from regions of higher to lower concentration by
diffusion. Thus, all the
mineral N
(except for any nonexchangeable NH
4
held
in micaceous clay lattices) is available to the plant. Mineral N is measured by ex-
5.3.2
