Easy Pedology - Poll 27

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Poll 27 – Knowing more about pedological chemical analysis

The pedological chemical analysis includes routinely the following determinations: calcium; magnesium; potassium; sodium; aluminum; hydrogen; pH (H₂O); pH (KCl); and carbon (C).

The pedological interpretation is referred to the B horizon, but if there is no B horizon, it is referred to the C horizon; or else, if there is no C horizon, it is referred to the A horizon. This interpretation is used to classify the soil in the higher level group, according to the EMBRAPA nomenclature, and to properly accomplish the soil chemical management and the plant allocation in function of their nutrient requirements.

The sum of bases is represented by SB and it expresses the concentration of bases in the soil (Ca+Mg+K+Na). The base saturation level is represented by V and it expresses the level of bases in the cation exchange capacity (CEC) at pH 7; the aluminum saturation is represented by m and it expresses the exchangeable aluminum concentration in the effective CEC; the clay cation retention is represented by CR and expresses the soil weathering level.

The equations are as follows:

SB = Ca + Mg + K + Na = cmol kg^-1 of soil

CTC = SB + Al + H = = cmol kg^-1 of soil

V= (SB / CTC) x 100

m = Al / (SB + Al) x 100

RC= (SB+Al) / % clay x 100

In 1999, the Brazilian Soil Classification System (of EMBRAPA), introduced the terms eutrophic, dystrophic, acric, alic and aluminic in the pedological interpretation, based on the chemical attributes of soil subsuperficial horizons.

In 2004, H. Prado suggested the terms mesotrophic and mesoalic (PRADO, H., FERTIBIO 2004) to separate groups of soils within the large range of base saturation (V%) values observed, specifically for the dystrophic soils of EMBRAPA (1999) classification.

See more details about this new proposed approach in the site www.pedologiafacil.com.br.

Table 1 presents the pedological chemical criteria:

Table 1. Chemical conditions of soil subsuperficial horizons (EMBRAPA, 1999; PRADO, 2004).

Character	V ⁽¹⁾	SB⁽²⁾	m⁽¹⁾	Al³⁺⁽²⁾	RC⁽³⁾
Eutrophic	>= 50	>= 1,5	-	-	-
Mesotrophic	30-50	>= 1,2	-	-	-
Mesotrophic	> 50	< 1,5	-	-	-
Dystrophic	< 30	-	< 50	-	> 1,5
Acric	-	-	-	-	<= 1,5*
Mesoalic	-	-	15 < 50	>= 0,4	-
Alic	-	-	>= 50	0,5 - 4,0	-
Aluminic			>= 50	>= 4,0

(1): percentage (%); (2): cmol kg^-1 of soil; (3): cmol kg^-1 of clay;
(*) a soil is considered acric when CR < 1.5 cmol kg^-1 of clay and pH (KCl) >= 5.0 or ∆pH is positive.

Figure 1 shows the large range variation observed among V%, m%, calcium and aluminum values of hundreds of soils included in the dystrophic class.

Figure 1. Mesotrophic soils (green) within the soil classes traditionally considered as dystrophic soils by the EMBRAPA classification (green + red), according to their chemical attributes.

Dystrophic soils, according to EMBRAPA (1999), show calcium ranging between 0.5 and 1.6 cmol kg^-1, V% ranging between 20 and 50%, SB ranging between 0.8 and 1.8 and aluminum ranging between 0.0 and 0.9 cmol kg^-1.

Before such large range variation, how to allocate plants with different nutrient requirements?

When m is close to 50%, but it is not higher than 50%, it means the soil presents high aluminum levels, and in this case, the soil is classified as mesoalic.

The mesotrophic soils tend to be close to the chemical attributes of eutrophic soils, and the mesoalic are close to the alic soil attributes.

A high V value means that the soil presents high base levels, especially calcium; and a high m value means the soil aluminum levels are high.

Usually, when the soil V value is high, the m value is low; and the opposite is true, that is, when the V value is lowered, the m value is increased. Therefore, there is an inverse relationship between V and m for most soil types, except for the acric soils, because these are predominantly oxidic soils and, in this case, both calcium and aluminum values are reduced.

Eutrophic soils usually present high base levels, especially calcium that enhances root growth, contrarily to the alic soils that show high aluminum values, inhibiting root cell division and root growth.
VASCONCELOS observed large differences between sugarcane rooting in eutrophic and alic soils (Figure 2).

Higher sugarcane root volume was observed in the eutrophic soil, meanwhile in the alic soil, thicker and shorter roots were present in response to the presence of toxic aluminum content (Figure 2).

Figure 2. Sugarcane root growth in chemically contrasting soils.

The proposed approach objective (of subdividing the Brazilian dystrophic class) was to make easier the studies on precision agriculture, because the soil variations in nature occur gradually and not abruptly. In the landscape, it is usual to find intermediary or intergraded soils. On the other hand, the pedological literature abruptly separates eutrophic soils (V = 55%) from dystrophic soils (V = 48%).

Studying the impact of the mesotrophic character on sugarcane productivity, LANDELL et al. (2003) observed that sugarcane yields, from the third cut then on, depends on the B horizon chemical conditions (Figure 3).

Figure 3 shows that a mesotrophic Oxisol was ranked in second for sugarcane yield (only surpassed by a eutrophic soil) and that a dystrophic soil, classified according to PRADO (2004), was ranked in third, corroborating that mesotrophic soils present a richer chemical environment compared to the dystrophic soils!

Figura 3. Produtividade de cana-de-açúcar ao longo dos cortes (FONTE: LANDELL et al., 2.003).

Figure 3. First, second and third sugarcane cut yields (source: LANDELL et al., 2003).

In 2006, EMBRAPA presented a new nomenclature for the Brazilian Soil Classification System, maintaining the terms eutrophic, dystrophic, acric and aluminic for specific chemical characteristics; but introduced the term alitic (Table 2) and persisted in maintaining the alic character at the family level instead of the high level group, since 1999. The family level is referred to the detailed soil survey, but Brazil has rarely done such detailed surveys. This fact restricts the possibility of the alic character being at the higher level group, where it should be in order to use its attribute information for a rational soil management and crop allocation. Brazilian sugarcane varieties have been genetically improved and adapted to different environments. Therefore, there are sugarcane varieties adapted to alic soils, and others adapted to dystrophic soils, etc.

Table 2. Chemical criteria of EMBRAPA (2006)

Character	V ⁽¹⁾	SB⁽²⁾	m⁽¹⁾	Al³⁺⁽²⁾	RC⁽³⁾	T
Eutrophic	>= 50	>= 1,5	-	-	-	-
Acric	-	-	-	-	<= 1,5	-
Dystrophic (*)	-	-	-	-	-	-
Alitic	-	-	>= 50	>= 4	-	>= 20
Aluminic	-	-	>= 50	>= 4	-	< 20

(*) V < 50% since it does not accomplish the requirements for acric, alitic and aluminic characters;
the soil is considered acric when CR < 1.5 cmol kg^-1 of clay and pH (KCl) >= 5.0 or delta ∆pH is positive.

You may want to input your own chemical data and get your soil pedological classification. For that, enter the site www.pedologiafacil.com.br and the window “Chemical conditions below the arable layer”; input your data and automatically get your soil classification.

Finally you may follow the Brazilian official recommendations, but without loosing the chance of knowing practical aspects of the Field Pedology.

Since 2004, the chemical attribute criterion has been suggested to be considered within the higher level group of soil classification, but an official evaluation of the respective EMBRAPA Committee is still being expected.

The suggested approach might be compared to the white light that, under an accurate and close examination, is the combination of all the contrasting colors of the visible light spectrum.
Figure 4 illustrates the comparison between visible light spectrum and the continuous soil variation observed in nature and that should be considered in Field Pedology.

Figure 4. Symbolical comparison between the white light decomposition in color nuances and the dystrophic character (traditionally defined by EMBRAPA) decomposition in "chemical nuances", as suggested by Hélio do Prado to the EMBRAPA Pedological Committee.

Poll result

Correct alternative : F

Alternative	Vote %
a) In an alic soil, the V value is > 50%.	18,4%
b) In a eutrophic soil, the m value is > 50%.	2,6%
c) In an acric soil the CR value is > 1.5 cmol kg^-1 of clay.	7,9%
d) Aluminum enhances root growth.	5,3%
e) Calcium inhibits root growth.	5,3%
f) All above are false alternatives.	60,5%
Totoal vote number:	38