ORIGINAL ARTICLE

Structure Degradation in an Agrogenic Soil. Analysis of Incident Factors and Associated Conservation Measures

0000-0002-8614-1247MSc. Sol Desireet Santander-Mendoza^{I  *}  , 0000-0002-5703-6632MSc. María del Carmen Falcón-Acosta^II  , 0000-0003-0058-116XIng. Maylin Suárez-Santana^II  , 0000-0001-6251-8348Ing. José Luis Durán-Álvarez^II 

^IInstituto de Suelos. Boyeros, La Habana, Cuba.

^IIUniversidad Agraria de La Habana, San José de las Lajas, Mayabeque, Cuba.

ABSTRACT

The Ferrallitic soils of Havana-Matanzas Plain have been subjected to degradation processes of natural or anthropogenic origin. Their physical properties are often affected, with effects on plant growth, yield and quality of crops, also increasing vulnerability to erosion. The following work was carried out at “El Mamey” Farm, near San José de las Lajas (Mayabeque Province, Cuba) and dedicated to the cultivation of vegetables and flowers. At each sample point, a macromorphological description of the soil and determinations of organic matter, mechanical composition, apparent density, porosity and percentage of aggregation were made. From the results obtained, the structural stability index was calculated, as well as correlations of some of these variables with the organic matter of the soil, in order to identify the main incident factors in physical degradation, which suggested processes of anthropogenic origin. Based on their analysis, conservation measures are suggested in order to promote the recovery of the soil and improve the sustainability of production.

Key words: Ferralitic soils; Physical Properties; Macromorphological Description

INTRODUCTION

The structure is the arrangement of the primary soil particles in hierarchical units, the configuration of their solid and fluid phases at a given moment (^{Kay y Angers, 2000}). It is a multifactorial condition, associated with agronomic and environmental processes, part of edaphogenesis and influenced by management (^{Lobo & Pulido, 2011}). ^{Lal et al. (2007)} its deterioration causes compaction, accelerates erosion, generates water / air imbalance and consequently, reduces fertility.

The structure develops through physicochemical mechanisms (flocculation, cementation, adhesion, cationic bridges, hydrogen bonds), with the intervention of the edaphic biota (^{Kemper & Rosenau, 1986}; ^{Six et al., 2004}; ^{Lehmann & Rillig, 2015}). According to ^{Totsche et al. (2017)} the formation of microaggregates comes from the reaction among clays, polyvalent cations and soil organic matter (SOM). According to ^{Tisdall & Oades (1982)} there is a spatial and hierarchical scale of these mechanisms in which ^{Oades (1984)} suggested the formation of microaggregates within macroaggregates, a concept that has been corroborated based on the dynamics of the SOM (^{Plante et al., 2002}; ^{Simpson et al., 2004}; ^{Kravchenko et al., 2015}). These microaggregates constitute the largest carbon reservoir in the soil and are essential for its capture (^{Blanco & Lal, 2004}; ^{Fan et al., 2020}).

Aggregation can be described qualitatively by observing morphological characteristics in the field and quantitative, using image analysis techniques or by measuring pore size distribution or connectivity. Other analytical procedures are based on the partial rupture of the structural units, the evaluation of the distribution of the sizes of the resulting fragments and their stability in the face of various types of disturbance.

^{Pieri (1995)} y ^{Astier et al. (2002)} stated that soil fertility integrates physical, chemical and biological attributes. According to ^{García et al. (2012)}, physical factors explain much of the decrease in crop yields, while ^{Orellana (2009)} highlights that an adequate and durable soil structure is essential for the development of sustainable agriculture systems.

In this sense, the degradation process of the Ferrallitic soils of Havana-Matanzas Plain has been studied (^{Morell et al. (2006)} ; ^{Hernández et al. (2006)}; ^{Morell et al. (2006)}; ^{Hernandez et al. (2013)} emphasizing, the relationship between the increase in anthropic action and its effects on its physical, chemical and biological indicators. In the present work, an exploration of the available methodologies for the study of the soil structure was carried out, emphasizing the comparison of field observation and analytical determinations and the calculation of various structural stability indices in order to identify the factors generating degradation; conservation measures were defined based on the latter.

MATERIALS AND METHODS

The work was carried out at “El Mamey” Farm, located in the San José de las Lajas Municipality, Mayabeque Province, Cuba and dedicated to various crops of vegetables, meats and flowers. These soils correspond to Havana-Matanzas Plain, of karstic origin, with a humid tropical climate. Five sampling points were chosen based on the topography of the terrain, which has two slight slopes. In the first of them (5% slope) three points were located (1.1, 1.2 and 1.3 at the beginning, middle and end of the slope, respectively) and in the second (8%), of shorter length, the two were located (2.1 and 2.2) remaining at the beginning and end of it.

In each of them, a 50 cm excavation was carried out to evaluate the color, structure and texture, using the Munsell® Table, observation and organoleptic method, respectively. In addition, cylinders were taken to determine humidity ^{NC: 110:2010 (2010)}, apparent density ^{ISO 11272: 2017 (2017)}, total porosity, capillary and aeration ^{NC: 1045: 2014 (2014)} and soil samples to perform organic matter analysis ^{NC: 51: 1999 (1999)}, mechanical composition (Bouyucos) and distribution and stability of aggregates ^{NC 1044: 2014 (2014)}. From these data, the structural stability index was calculated using the equation: Ie = Σ (% ag> 0.25 mm (Ts) / Σ (% ag> 0.25 mm (Th).

RESULTS AND DISCUSSION

Observation of the Macromorphological Characteristics of the Soil

At each sampling point, a 50 cm excavation was made, where two layers were distinguished. The summary of the results obtained is shown in Table 1:

Pedogenesis and agrogenic evolution were evident in the morphological characteristics of the soil (^{Lebedeva et al., 2005}). The reddish tones, typical of ferralitization, are associated with alteration processes of the parent materials under conditions of high temperature, rapid degradation of OM and high release of iron. They are also indicative of high weathering, low fertility and preeminence of oxidation processes (^{Ovalles, 2003}).

The granular structure observed is characteristic of A horizons of soils with little OM, and a consequence of the decrease in the porosity of the aggregates due to the predominance of clay over organic matter in the flocculation process ^{Hernandez et al. (2013)}. This characteristic favors the compaction observed in the second layers of points 1.1, 2.1 and 2.2 (“plow layers”), as well as the presence of orange and gray speckles, which show poor drainage.

^{Hernandez et al. (2013)} associate these conditions with the degradation of red Ferralitic soils as a consequence of inadequate agricultural management. According to these authors, the formation of plow layers is the result of the destruction of the soil structure and the increase in the content of dispersed clays in the upper horizon.

Apparent Density (Da):

The values obtained for the apparent density (Da) are shown in Figure 1

FIGURE 1 Apparent Density (kg / m3). 

Values ​​greater than the 0.9 - 1.16 kg / m3 range were obtained, referential for this type of soil up to 1 m deep (^{Martín & Duran (2011)} although they coincide with those reported by ^{Hernandez et al. (2013)}, who attribute such differences to the intensive anthropogenesis suffered by these soils, without recent subsolation work. The highest values correspond to the second layers of sample points 1.1, 2.1 and 2.2, coinciding with the finding of the compacted layers in the excavation carried out.

At all points there are restrictions for radical growth, except for 2.1.1, since the apparent density exceeds 1.25 kg / m3 (^{Martín & Duran, 2011}). Compaction negatively affects root penetration, gas exchange, infiltration, and water retention; and as a consequence, microbial activity, nutrient absorption and mineralization processes (^{Six et al., 2004}; ^{Morell & Hernández, 2008}; ^{Obour et al., 2017}).

This compaction is conditioned by the history of land use and management, which makes apparent density a very dynamic indicator of physical deterioration ^{Shafiq et al. (1994}; ^{Totsche et al. (2017)}; ^{Al-Shammary et al. (2018)}, with sensitivity for short-term estimates, as it is accompanied by other physical and biological indicators (^{Doran, 1994}).

Mechanical Composition and Organic Matter

The results obtained are shown in Table 2:

TABLE 1 Macromorphological description of the sample points 

Sampling point	1.1	1.2	1.3	2.1	2.2
Genetic Classification (IS, 1999).	Ferralitic Yellowish Leached Soil	Ferralitic Yellowish Leached Soil	Leaching Red Ferralitic Soil	Ferralitic Yellowish Leached Soil	Leaching Red Ferralitic Soil.
Layer 1
Depth	0 - 21 cm	0 - 19 cm	0 - 20 cm	0 - 20 cm	0 - 20 cm
Colour	5YR 4/6 (dry) 7.5 YR 3/4 (damp)	5 YR 5/8 (dry) 5 YR 4/6 (damp)	2.5 YR 4/4 (dry) 2.5YR 2.5/3 (damp)	2.5 YR 4/6 (dry) 2.5 YR 3/6 (damp)	10 R 3/4 (dry) 10 R 3/6 (damp)
Texture	Clay loam	Clay loam	Clay loam	Clayey	Clayey
Structure	Granular, coarse.	Granular, medium.	Granular, coarse.	Granular, medium.	Granular, coarse.
Other features	Accentuated hydromorphy at 17 cm.	Does not present	Shot. Rock fragments (≈ 1% - 2%).	Does not present	Does not present
Layer 2
Depth	22 - 50 cm	20 - 50 cm	21 - 50 cm	21 - 33 cm	20 - 40 cm
Colour	7.5 YR 5/8 (dry) 7.5 YR 4/6 (damp)	5 YR 4/6 (dry) 7.5 YR 4/4 (damp)	10 YR 4/6 (dry) 10 YR 3/3 (damp)	2.5 YR 4/6 (seco) (dry) 2.5 YR 3/6 (damp)	10 R 3/(dry) 10 R – (damp)
Texture	Clayey	Clay loam	Clay loam	Clayey	Clayey
Structure	Granular, coarse.	Granular, coarse.	Granular, coarse.	Granular, coarse.	Granular, coarse.
Other features	Compacted layer (plow layer) at 24 cm. Accentuated hydromorphy (yellow, orange and gray speckles, ≈ 10%). Presence of manganese and iron.	Does not present	Diffuse limit . Mottled (≈ 10%) grayish brown. Rock fragments (≈ 1% - 2%) of the total mass.	Mottled with small lighter spots (≈ 10%). Compacted layer (plow layer) developed. Small clods, evidence of redox processes.	Mottled. Compacted layer (plow layer) at 20 cm. Black small clods, ≈5% of the mass. Sliding faces. ("Slickensides")

TABLE 2 Particle size distribution and percentage of organic matter 

Sampling point	Depth. (cm)	% MOS	< 2mm (%Clays)	0,002 a 0,01 mm (%Fine slime)	0,01 a 0,02 mm (%Thick slime)	0,02 a 0,2 mm (%Fine sand)	0,2 a 2 mm (%Gross sand)
1.1.1	0-21	1,73	62,87	7,54	4,17	13,29	1,73
1.1.2	22-50	1,09	74,82	7,16	8,16	6,45	1,09
1.2.1	0-19	2,12	64,93	9,43	6,51	9,26	2,12
1.2.2	20-50	1,27	73,25	8,53	8,65	7,32	1,27
1.3.1	0-20	2,36	63,77	12,43	10,52	7,45	2,36
1.3.2	21-50	1,32	74,82	6,43	9,71	8,12	1,32
2.1.1	0-20	2,05	64,56	13,03	10,99	6,84	2,05
2.1.2	21-33	1,58	79,82	5,98	4,10	8,43	1,58
2.2.1	0-20	1,75	65,93	10,85	6,96	7,65	1,75
2.2.2	20-40	1,26	78,06	8,67	4,02	8,11	1,26

At all points there was agreement of the analytical results with those from the field observation (Table 1) in relation to the texture and there was a higher content of the percentage of clay particles (less than 0.002 mm) in layers 2; phenomenon attributable to the movement of soil particles through the profile as a consequence of the destruction of the structure in the upper layers (^{Hernandez et al., 2013}).

There is also evidence of a marked predominance of small particles (between 0.002 to 0.01 mm) in the composition of the soil, a condition that provides low structural stability and a high susceptibility to separation from the impact of raindrops due to the less energy than these particle sizes require to separate from the aggregates (^{Lobo, 1990}).

On the other hand, the organic matter of the soil presents low values, coinciding with other authors in the same type of soil (^{Hernández et al., 2006}; ^{Hernandez et al., 2013}) and tends to be lower in the eroded parts and higher in the deposition area, due to the effect of the slope.

Porosity

The results obtained for the evaluation of porosity (total, capillary and of aeration) are shown in Figure 2:

FIGURE 2 Distribution of Porosity Percentages. 

The percentage of total porosity is close to 50%, a value considered ideal (^{Martín & Duran, 2011}), with an equitable distribution of capillary and aeration pores in almost all points of the upper layers, except points 1.1.2, 2.1 .2 and 2.2.2, where the total porosity was found around 10% below this value. As it can be seen, these points correspond to the compacted layers, with "plow layers".

In most of the points there is also a predominance of the percentage of capillary pores over the aeration pores, a characteristic condition of clay textures. This effect is more accentuated in layer 2 samples and increases the moisture retention capacity of the soil; however, it can also affect the water / air balance and the movement of O₂ and CO₂, increasing the areas with anaerobic conditions. This generates a reduction in denitrification, loss of nutrients in the roots and changes in the metabolism of plants, causing adverse effects on the crop (^{Bünemann et al., 2018}). In the compacted layers (1.1.2, 2.1.1, 2.2.2), in addition, percentages of aeration pores less than 10% were found, a value considered minimum according to (^{Hillel, 1994}; ²⁰⁰³; ²⁰¹³), quantitatively showing the structural limitations identified qualitatively in observation of the ground.

Aggregation

Distribution of Aggregate Sizes and their Stability

The determination of particle sizes was carried out using dry and wet sieving, obtaining the results shown in Figure 3:

FIGURA 3 Aggregate size distribution and stability index. 

In general, the percentage of aggregation is close to 40%, resulting lower in the sampling points of transect 2, and in the lower layers of each sampling point. The formation and stability of the aggregates depend both on pedological factors, such as the predominance of clay particles, which limit the spaces in the soil structure and the existence of microaggregates over macroaggregates. It also depends on anthropic factors such as tillage, which repeatedly exposes the organic fractions of the soil related to macroaggregates to aeration and oxidation, causing their decrease (^{Shepherd et al., 2001}).

For its part, the structural stability index (Ie), based on the ratio of stable aggregates in water / total aggregates, allows us to summarize the interaction of the aforementioned factors ^{Menon et al. (2020)}, quantifying the resistance to change (maintenance of the structure) of the aggregates in response to the application of mechanical stress (^{Diaz et al., 2002}). In general, greater stability was found in the upper layers and poor stability in the points with the presence of compacted layers.

Relationship of Aggregation with Soil Organic Matter

In order to explore the possible relationships between% SOM and some evaluated physical indicators, the correlations shown in the graphs of Figures 4, 5 and 6 were calculated.

Figures 4, 5 and 6 show the correlations found between the OM percentage and apparent density, the total aggregation percentage and structural stability, finding R² values ​​greater than 0.6 in the three cases, which coincides with ^{Shepherd et al. (2001)}; ^{Pulido et al. (2009)}; ^{Hernandez et al., (2013)}. Such results highlight the need to quantify the amount and type of OM when evaluating the structural state of the soil. In general, SOM promotes the stability of aggregates by reducing the swelling and permeability of the aggregate, reducing the destructive forces of the burst phenomenon, and increasing their intrinsic strength ^{Fortun & Fortun (1989)}, since they link physics and chemically the primary particles in the aggregates (^{Lado et al., 2004}).

There is a bidirectional relationship between SOM and aggregation: the effectiveness of SOM in forming stable aggregates is related to its decomposition rate, which in turn, depends on its physical and chemical protection and microbial action (^{Blanco y Lal, 2004}; ^{Pulido et al., 2009}) and in turn, the stability of the macroaggregates constitutes a crucial factor in the stabilization of OM in the long term (^{Six et al., 2004}).

^{Bernal & Hernández (2017)} point out that the mineralization and loss of organic matter (OM) is a preponderant factor in the decrease of the aggregation of the Ferralitic soils studied. According to ^{Six et al. (2004)} this effect is evidenced with greater emphasis on the loss of macroaggregates, as has been observed, since their formation is due to the presence of organic matter (OM) recently contributed, with less stability than that of microaggregates. ; whose cementing agent corresponds to OM that is more humified, and therefore more resistant to degradation. ^{Bernal & Hernández (2017)} highlight that in Ferralitic soils, iron, clays and OM interactions influence on the formation of microaggregates and their stability.

FIGURE 4 Correlation between apparent density and organic matter. 

FIGURE 5 Correlation between percentage of organic matter and percentage of aggregation. 

FIGURE 6 Correlation between percentage of organic matter and the structural stability index. 

Soil Conservation Plan for the Soils of "El Mamey"Farm

According to ^{Hernandez et al. (2013)} since the beginning of the 19th century, the need to restore the fertility of red Ferralitic soils has been raised due to the intensity of exploitation since colonial times. ^{Hernández et al. (2006)}; ^{Morell et al. (2006)} and ^{Hernandez et al. (2013)} have found results that show changes in the properties of these soils as a consequence of their agricultural use, a phenomenon that has been described as agrogenic evolution (^{Lebedeva et al., 2005}).

Conventional intensive agricultural exploitation is characterized by numerous tillage tasks, which result in the oxidation of organic matter in the soil, with the consequent breakdown of aggregates and loss of structure ^{Cooper et al. (2005)}, evident both in the observation of the microcalicatas as in the analytical results. Both phenomena lead to compaction, evident in the increase in the value of the apparent density and the decrease in the porous spaces obtained.

From these, five signs of physical degradation and the affected soil "functions" were identified, from which seven conservation measures are proposed, divided into two stages, in order to recover the physical condition of the soil, improve its fertility and increase the sustainability of agricultural activity (Figure 7).

FIGURE 7 Conservation Plan for the “El Mamey” Farm, designed to restore the affected soil functions, according to the analysis of the values obtained in the field observation and the physical indicators. 

CONCLUSIONS

From the work carried out, the usefulness of field methods (direct observation) in conjunction with analytical procedures in the study of soil structure was evidenced. The preeminence of compaction was observed as the main degrading factor of the soil, emphasizing the processes of agrogenic origin, as well as the relationship of the percentage of soil organic matter (OM) with structural stability. Relationships between the evidence of soil deterioration, the affected functions and the associated conservation measure (s) were also presented.

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Received: February 10, 2021; Accepted: September 20, 2021

^*Author for correspondence: Sol Desireet Santander-Mendoza, e-mail: [e-mail]

Sol Santander-Mendoza, Investigadora, Lic. en Biología. MSc. En Agroecología. Esp. Recuperación de Suelos, Instituto de Suelos. Boyeros, La Habana, Cuba, e-mail: [e-mail]

Maria del Carmen Falcón-Acosta, Profesora, Ing. Agrónoma. MSc. Ciencias del Suelo, Universidad Agraria de La Habana, San José de las Lajas, Mayabeque, Cuba, e-mail: [e-mail]

Maylin Suárez-Santana, profesora, Ing. Agrónoma, Universidad Agraria de La Habana, San José de las Lajas, Mayabeque, Cuba, e-mail: [e-mail]

José Luis Durán-Alvarez, Profesor Consultante, Universidad Agraria de La Habana, San José de las Lajas, Mayabeque, Cuba, e-mail: [e-mail]

The authors of this work declare no conflict of interests.