5
Porosity is important for the exchange of gases and liquids. Micro-porosity (porous < 2 -
10 µm) permits water to be retained while macro-porosity (porous > 10 µm) contrib-
utes to a fast circulation of air and water.
Plants therefore are in need of a correct relationship between micro and macro porosity.
Clay soils have a greater micro-porosity than sandy soils and hence hold more water and
remain wet for a longer period.
Because of the greater tenacity and adhesivity of clay soils, they are called heavy while
sandy soils are referred to as light.
Organic matter, caused by animal and vegetable residues, is another important constituent
of the solid part of the soil. Organic matter has a positive effect on the soil fertility by
adding nutrients, stabilizing the pH reaction and permitting a good retainment of water.
Organic matter is also important for the activity of microorganisms and, in general,
contributes towards prevention of soil erosion.
The colloidal portion, composed of micro-particles (1-100 µm), is important for holding
nutrients. Since most of these particles have a negative charge, the colloidal portion has a
particularly large capacity to retain cations (NH
4
+
, K
+
, Na
+
, Ca
++
, Mg
++
, etc.). The CEC
(Cation Exchange Capacity) is higher in soils rich with clay and organic matter than in
sandy soils.
The chemical composition of soil includes pH and chemical elements. Their analysis is
necessary for better management of fertilization, tillage and in order to choose the most
suitable plants for best results.
By using the HANNA Soiltest, it is possible to measure pH and the most important
elements for plant growth, that is, nitrogen (N), phosphorus (P) and potassium (K).
CHEMICAL
COMPOSITION
pH
pH is the measure of the hydrogen ion concentration [H
+
]. A soil can be acid, neutral or
alkaline, according to its pH value.
Fig. 3 shows the relationship between the scale of pH and kind of soil. The pH range from
5.5 to 7.5 include the most of plants; but some species prefer acid or alkaline soils.
Fig. 3. Types of soil
according to the pH value
Nevertheless, every plant need a particular range of pH, in which can better express its
potentiality of growth.
pH strongly influences the availability of nutrients and the presence of microorganisms
and plants in the soil.
12
Asparagus very low 160 120 180
low 120 100 150
medium 100 70 130
medium-high 90 50 110
high 80 40 90
very high 70 20 80
Barley very low 140 130 170
low 110 90 120
medium 90 70 80
medium-high 80 50 60
high 70 40 50
very high 60 30 40
Corn silage very low 340 200 230
low 300 150 150
medium 280 120 120
medium-high 260 90 90
high 240 60 60
very high 220 40 46
Maize very low 300 200 230
low 270 150 150
medium 240 120 120
medium-high 230 90 90
high 210 60 60
very high 200 40 40
Soybean very low 0 150 220
low 0 130 170
medium 0 100 130
medium-high 0 80 100
high 0 60 80
very high 0 40 60
Sugar beet very low 160 150 230
low 120 130 180
medium 100 100 150
medium-high 90 80 120
high 80 60 90
very high 70 40 60
Tomato very low 150 250 250
low 130 180 200
medium 110 150 150
medium-high 90 120 120
high 80 90 90
very high 70 60 60
Wheat very low 180 150 170
low 160 100 120
medium 150 80 80
medium-high 140 60 60
high 130 50 50
very high 120 40 40
Tab. 7.
CROP SOIL CONTENT ADVISED DOSES (kg/ha)
N P
2
O
5
K
2
O
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