Hanna-instruments HI 3896 Manual do Utilizador Página 7
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Once the pH value is known, it is advisable to choose crops that are indicated for this range
(e.g. in an acid soil, cultivate rice, potato, strawberry).
Add fertilizers that at the same time 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 modifi-
cation of the soil pH. Corrective substances can be added in order 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. 2).
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
High pH levels can depend on different elements, hence, there are different methods
for its correction.
– Soils rich with limestone:
Add organic matter (this is due to the fact that non-organic ameliorants such as
sulfur and sulfuric acid might not make economic sense due to the large quantities
needed).
– Alkaline-saline soils:
Alkalinity is due to the presence of salts (in particular a high concentration of
sodium can be harmful).
Irrigation washes away salts hence an appropriate use of irrigation can provide positive
results (drop-irrigation being the most recommended).
If alkalinity is caused by sodium, it is recommended to add substances such as gypsum
(calcium sulfate), sulfur or other sulfuric compounds (Tab. 3). Also in this case, a cost
evaluation is necessary.
Soil ameliorants (pure compounds) Quantity (Kg)
Calcium chloride: CaCl
2
· 2H
2
O 85
Sulfuric acid: H
2
SO
4
57
Sulfur: S 19
Iron sulfate: Fe
2
(SO
4
)
3
· 7H
2
O 162
Aluminum sulfate: Al
2
(SO
4
)
3
129
Management of the
Soil in Relation with
the pH Value
Tab.2. Quantity (q/ha) of
pure compound necessary to
increase 1 unit of pH
Tab.3. Quantities provide
the same result as 100 Kg
of gypsum
10
It is important to note that whereas an insufficient dose of nutrients decreases the potential
crop production, an excess can have a negative effect on the physiology of the plants and
the crop quality. In addition, too much fertilization can be unnecessarily costly as well as
being harmful to the environment.
Before sowing or transferring plants, use a slow-acting fertilizer to enrich the soil for long
term. This is particularly important for Nitrogen which unlike Phosphorus and Potassium
tends to become less present over time. Compound fertilizers which contain nitrogen
(preferred in ammonium form), phosphorus and potassium can also be used.
Adding organic substances (such as manure and compost) help to increase the soil fertility
(Tab. 5).
ELEMENT QUANTITY (%)
N 0.4-0.6
P
2
O
5
0.2-0.3
K
2
O 0.6-0.8
CaO 0.5-0.6
MgO 0.15-0.25
SO
3
0.1-0.2
If possible, add the fertilizer more than once. In case of lack of Nitrogen, use fertilizers
containing Nitrate due to their faster absorption by the plants. It is important to add the
necessary elements at particular phases in the plant’s life cycle (for example, before
sprouting or wheat raising).
Do not give nitrate to crops such as lettuce (in which the product is the vegetable part) at
the end of the plant cycle, in order to avoid their accumulation in the leaves (nitrate is
carcinogenic).
Tab. 6 below shows average quantity of element absorbed by the principal crops based on
their yield (note that the relationship between absorption and fertilization is not exact).
CROP YIELD Nitrogen Phosphorus Potassium
(q/ha) N (kg/ha) P
2
O
5
(kg/ha) K
2
O (kg/ha)
Alfalfa 120 280 75 300
Asparagus 50 125 40 110
Barley (whole plant) 60 110 25 95
Bean 100 130 40 100
Cabbage 200 110 60 150
Carrot 300 130 55 200
Colza 30 175 70 140
Tab.5. Composition of
manure
Top dressing
Tab.6. Experimental
average quantity of
elements absorbed based on
crop yield
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