Agriculture Reference
In-Depth Information
Box 3.6 Testing for Soil pH and Lime Requirement
Soil pH indicates the likely effect of H
+
ions on plant growth and the potential for
toxicities and deficiencies to occur. The pH stands for the negative logarithm of the H
+
ionconcentration,measuredinmolesperliter(M)ofsolution.hescaleisfrom0to14
pHunits,withpH7beingtheneutralpoint(equalconcentrationsofH
+
and OH
−
ions).
Soil pH is measured in the field with a universal indicator and color chart to
an accuracy of ±0.5 pH units; alternatively, in a laboratory it is measured with a pH
meter to an accuracy of ±0.05 pH units. Because soil is mixed with water (usually
one part by weight of soil to five parts by volume of distilled water), the laboratory
pH differs from the field pH. To minimize this difference, the soil can be shaken
withasolutionof0.01MCaC1
2
(1:5ratio),whichgivesapHvalueabout0.5to0.8
units lower than in water and closer to the field pH. For this reason, it is important to
knowwhichmethodofmeasurementhasbeenusedwhenpHvaluesarequoted.
The recommended pH(CaCl
2
)forgrapevinesisbetween5.5and7.5.Vines
will grow outside this range but are more likely to suffer a toxicity or deficiency.
Forexample,theavailabilityofFe,Cu,Zn,andMndecreasesasthesoilpH
increases; conversely, at a pH(CaCl
2
) of less than 5.5, the hydrated aluminum cation
Al(H
2
O)
6
3+
begins to accumulate and can impair root growth and P uptake. In fact,
clay in very acidic soils (pH(CaCl
2
)<4.5)isdominatedbytheseAl
3+
ions. Hydrolysis
of the hydrated Al
3+
ions produces a continuing supply of H
+
ions, which means that
considerableamountsoflimearerequiredtoraisethepHtoanacceptablevalueof
5.5 or above. A special laboratory soil test involving titration of a soil with an alkali is
usedtomeasurethelimerequirement,whichcanrangefrom1to5tCaCO
3
/ha.
Laboratoryanalysisisalsonecessarytomeasuretheneutralizingvalue(NV)
ofalimingmaterial.TableB3.6.1givestheNVofseveralsuchmaterials.Particle
size is also important, because the smaller the particles, the faster they dissolve in
thesoil.Ahigh-qualitylimingmaterialshouldhaveanNVgreaterthan85%and
more than 60% of the particles less than 0.15 mm in diameter. Because lime is
slow to dissolve, it is most effective when cultivated into the soil so that the contact
between soil and lime particles is maximized.
Table B3.6.1
Liming Materials for Vineyards
Materialanditschemical
composition
Neutralizing value of
commercial grade (%)
a
Comments
Burnt lime, CaO
>150
Reacts vigorously with water
Hydrated lime,
Ca(OH)
2
120-135
Occurs as a very fine powder;
difficult to handle
Dolomite,(Ca,Mg)CO
3
95-110
MoresolublethanCaCO
3
;
containsabout11%Mg
Limestone, CaCO
3
plus
impurities
50-85
Neutralizing value depends
on the concentration of
impurities such as clay, silica,
and Fe and Al oxides
Cement kiln dust,
CaCO
3
, CaO, K
90-110
Fine powder, by-product of
cement manufacture; often
pelleted; also supplies K
a
Calculated relative to pure CaCO
3
as 100%.
Compiled from Goldspink and Howes (2001) and the Fertilizer Federation Industry of Australia
(2006).
