Hanna Instruments HI 99121 - Manual (Page 3)

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pH is the measure of the hydrogen ion concentration [H + ]. Soil
can be acid, neutral or alkaline, according to its pH value.
Fig. 1 shows the relationship between the scale of pH and
types of soil. Most plants prefer a pH range from 5.5 to 7.5;
but some species prefer more acid or alkaline soils.
Nevertheless, every plant requires a particular range of pH,
for optimum growth.
pH strongly influences the availability of nutrients and the presence
of microorganisms and plants in the soil.
For example, fungi prefer acidic conditions whereas most
bacteria, especially those supplying nutrients to the plants,
have a preference for moderately acidic or slightly alkaline
soils. In fact, in strongly acidic conditions, nitrogen fixing and
the mineralization of vegetable residual is reduced.
Plants absorb the nutrients dissolved in the soil water and the
nutrient solubility depends largely on the pH value. Hence, the
availability of elements is different at different pH levels (Fig.2).
S O I L p H
Each plant needs elements in different quantities and this is the
reason why each plant requires a particular range of pH to
optimize its growth.
For example, iron, copper and manganese are not soluble in
an alkaline environment. This means that plants needing these
elements should theoretically be in an acidic type of soil.
Nitrogen, phosphorus, potassium and sulfur, on the other
hand, are readily available in a pH range close to neutrality.
Furthermore, abnormal pH values, increase the concentration
of toxic elements for plants. For example, in acid conditions,
there can be an excess of aluminum ions in such quantities
that the plant can not tolerate.
Negative effects on chemical and physical structure are also
present when pH values are too far from neutral conditions
(break up of aggregates, a less permeable and more compact
soil).
Management of the soil in relation to the pH value
Once the pH value is known, it is advisable to choose crops
that are suitable for this range (e.g. in an acid soil, cultivate
rice, potato, strawberry).
Add fertilizers that do not increase acidity (for example urea,
calcium nitrate, ammonium nitrate and superphosphate) or
lower alkalinity (e.g. ammonium sulfate).
It is recommended that a cost evaluation is made prior to
commencement of the soil pH modification. Corrective
substances can be added to modify the soil pH, however, the
effects are generally slow and not persistent. For example, by
adding lime, the effects in clay soil can last for as long as 10
years, but only 2-3 years in a sandy soil.
For an acid soil, we can use substances such as lime, dolomitic,
limestone and marl, according to the nature of the soil (Tab.1).
Soil Ameliorants Clay soil Silty soil Sandy soil
CaO 30-50 20-30 10-20
Ca(OH)
2
39-66 26-39 13-26
CaMg(CO
3
)
2
49-82 33-49 16-33
Ca CO
3
54-90 36-54 18-36
Tab.1. Quantity (q/ha) of pure compound necessary to in-
crease 1 unit of pH
Fig. 1. Types of soil according to the pH value
Fig. 2. Solubility of the elements according to varying pH