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, 2000KAY, B.D.; ANGERS, D.A.: Soil Structure, Ed. CRC Press, Sumner M: Handbook of Soil Science ed. , ISBN-978-0-8493-3136-7, Boca Raton, USA, 2000. ). It is a multifactorial condition, associated with agronomic and environmental processes, part of edaphogenesis and influenced by management (Lobo & Pulido, 2011LOBO, D.; PULIDO, M.: ¨Métodos e índices para evaluar la estabilidad estructural de los suelos¨, Venesuelos, ISSN-1315-0359, 14(1): 22-37, 2011. ). Lal et al. (2007)LAL, R.; FOLLETT, R.F.; STEWART, B.A.; KIMBLE, J.M.: ¨Soil carbon sequestration to mitigate climate change and advance food security¨, Soil science, ISSN-0038-075X, 172(12): 943-956, 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, 1986KEMPER, W.; ROSENAU, R.: Aggregate stability and size distribution, Ed. Soil Science Society of America, Methods of Soil Analysis, Part 1, Physical and Mineralogical Methods ed. , vol. 5, ISBN-978-0-89118-864-3, USA, 1986. ; Six et al., 2004SIX, J.; BOSSUYT, H.; DEGRYZE, S.; DENEF, K.: ¨A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics¨, Soil and Tillage Research, ISSN-0167-1987, 79(1): 7-31, 2004. ; Lehmann & Rillig, 2015LEHMANN, A.; RILLIG, M.: ¨Understanding mechanisms of soil biota involvement in soil aggregation: A way forward with saprobic fungi?¨, Soil Biology and Biochemistry, ISSN-0038-0717, 1879-3428, 88: 298-302, 2015. ). According to Totsche et al. (2017)TOTSCHE, K.; AMELUNG, W.; GERZABEK, M.; GUGGENBERGER, G.; KLUMPP, E.; KNIEF, C.; LEHNDORFF, E.; MIKUTTA, R.; PETH, S.; PRECHTEL, A.; RAY, N.; KO ̈GEL, K.I.: ¨Microaggregates in soils¨, J. Plant Nutr. Soil Sci, ISSN-1522-2624, 1-33, 2017. the formation of microaggregates comes from the reaction among clays, polyvalent cations and soil organic matter (SOM). According to Tisdall & Oades (1982)TISDALL, J.M.; OADES, J.M.: ¨Organic matter and water‐stable aggregates in soils¨, Journal of soil science, ISSN-0022-4588, 33(2): 141-163, 1982. there is a spatial and hierarchical scale of these mechanisms in which Oades (1984)OADES, J.M.: ¨Soil organic matter and structural stability: mechanisms and implications for management¨, Plant and soil, ISSN-1573-5036, 76(1): 319-337, 1984. suggested the formation of microaggregates within macroaggregates, a concept that has been corroborated based on the dynamics of the SOM (Plante et al., 2002PLANTE, A.; ALONSO, Y.; MCGILL, W.: ¨A modeling approach to quantifying soil macroaggregate dynamics¨, Canadian Journal of Soil Science, ISSN-0008-4271, 1918-1841, 82(2): 181-190, 2002. ; Simpson et al., 2004SIMPSON, R.T.; FREY, S.D.; ABU, J.B.; THIET, R.K.: ¨Preferential accumulation of microbial carbon in aggregate structures of no‐tillage soils¨, Soil Science Society of America Journal, ISSN-0361-5995, 68(4): 1249-1255, 2004. ; Kravchenko et al., 2015KRAVCHENKO, A.; NEGASSA, W.; GUBER, A.; RIVERS, M.: ¨Protection of soil carbon within macro-aggregates depends on intra-aggregate pore characteristics, Sci. Rep., 5, 16261¨, Nature.Scientific Reports, ISSN-2045-2322, (5), 2015. ). These microaggregates constitute the largest carbon reservoir in the soil and are essential for its capture (Blanco & Lal, 2004BLANCO, C.H.; LAL, R.: ¨Mechanisms of carbon sequestration in soil aggregates¨, Critical reviews in plant sciences, ISSN-0735-2689, 23(6): 481-504, 2004. ; Fan et al., 2020FAN, R.; DU, J.; LIANG, A.; LOU, J.; LI, J.: ¨Carbon sequestration in aggregates from native and cultivated soils as affected by soil stoichiometry¨, Biology and Fertility of Soils, ISSN-1432-0789, 56(8): 1109-1120, 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)PIERI, C.: ¨Long-term soil management experiments in semiarid Francophone Africa¨, Advances in Soil Science, ISSN-0176-9340, 225-264, 1995. y Astier et al. (2002)ASTIER, C.M.; MAASS, M.M.; ETCHEVERS, B.J.: ¨Derivación de indicadores de calidad de suelos en el contexto de la agricultura sustentable¨, Agrociencia, ISSN-2521-9766, 36(5): 605-620, 2002. stated that soil fertility integrates physical, chemical and biological attributes. According to García et al. (2012)GARCÍA, Y.; RAMÍREZ, W.; SÁNCHEZ, S.: ¨Indicadores de la calidad de los suelos: una nueva manera de evaluar este recurso¨, Pastos y forrajes, ISSN-0864-0394, 35(2): 125-138, 2012. , physical factors explain much of the decrease in crop yields, while Orellana (2009)ORELLANA, J.A. de: ¨La estructura en suelos de tierra fina.¨, Revista FAVE. Sección Ciencias Agrarias, ISSN-1666-7719., 8(2), 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) MORELL, F.; HERNÁNDEZ, A.; FERNÁNDEZ, F.; TOLEDO, Y.: ¨Caracterización agrobiológica de los suelos Ferralíticos Rojos Lixiviados de la región de San José de las Lajas, en relación con el cambio en el manejo agrícola¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 27(4): 13-18, 2006. ; Hernández et al. (2006)HERNÁNDEZ, A.; MORELL, F.; MORALES, M.; BORGES, Y.; ASCANIO, O.: ¨Cambios globales en los suelos Ferralíticos Rojos Lixiviados (Nitisoles ferrálicos, ródicos, éutricos) de Cuba¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, (2): 41-50, 2006. ; Morell et al. (2006)MORELL, F.; HERNÁNDEZ, A.; FERNÁNDEZ, F.; TOLEDO, Y.: ¨Caracterización agrobiológica de los suelos Ferralíticos Rojos Lixiviados de la región de San José de las Lajas, en relación con el cambio en el manejo agrícola¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 27(4): 13-18, 2006. ; Hernandez et al. (2013)HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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.
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)NC: 110: 2010: Calidad del suelo. Determinación de la humedad, Oficina Nacional de Normalización, La Habana, Cuba, 2010. , apparent density ISO 11272: 2017 (2017)ISO 11272: 2017: Soil quality - Determination of dry bulk density, International Standard Organization, La Habana, Cuba, 2017. , total porosity, capillary and aeration NC: 1045: 2014 (2014)NC: 1045: 2014: Calidad del Suelo. Determinación de la porosidad, Oficina Nacional de Normalización, La Habana, Cuba, 2014. and soil samples to perform organic matter analysis NC: 51: 1999 (1999)NC: 51: 1999: Calidad del suelo-Análisis Químico-Determinación del Porciento de Materia Orgánica, Oficina Nacional de Normalización, La Habana, Cuba, 1999. , mechanical composition (Bouyucos) and distribution and stability of aggregates NC 1044: 2014 (2014)NC 1044: 2014: Calidad del Suelo. Determinación de la estabilidad estructural, Oficina Nacional de Normalización, La Habana, Cuba, 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).
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., 2005LEBEDEVA, I.; TONKONOGOV, V.; GERASIMOVA, M.: ¨Anthropogenic pedogenesis and the new classification system of Russian soils¨, Eurasian Soil Sci, ISSN-1064-2293, 38(10): 1026-1031, 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, 2003OVALLES, F.: ¨El Color del Suelo: definiciones e interpretación¨, Revista Digital del Centro Nacional de Investigaciones Agropecuarias de Venezuela, CENIAP Hoy, ISSN-1690-4117, : 12-21, 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)HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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)HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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.
The values obtained for the apparent density (Da) are shown in Figure 1
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)MARTÍN, N.J.; DURAN, A.J.L.: El suelo y su fertilidad, Ed. Félix Varela, ISBN-978-959-07-1418-4, La Habana, Cuba, 2011. although they coincide with those reported by Hernandez et al. (2013)HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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, 2011MARTÍN, N.J.; DURAN, A.J.L.: El suelo y su fertilidad, Ed. Félix Varela, ISBN-978-959-07-1418-4, La Habana, Cuba, 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., 2004SIX, J.; BOSSUYT, H.; DEGRYZE, S.; DENEF, K.: ¨A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics¨, Soil and Tillage Research, ISSN-0167-1987, 79(1): 7-31, 2004. ; Morell & Hernández, 2008MORELL, P.F.; HERNÁNDEZ, J.A.: ¨Degradación de las propiedades agrobiológicas de los suelos Ferralíticos rojos lixiviados por la influencia antrópica y su respuesta agroproductiva al mejoramiento¨, Agronomía Tropical, ISSN-0002-192X, 58(4): 33-343, 2008. ; Obour et al., 2017OBOUR, P.B.; SCHJØNNING, P.; ALONSO, Y.; MUNKHOLM, L.: ¨Subsoil compaction assessed by visual evaluation and laboratory methods¨, Soil and Tillage Research, ISSN-0167-1987, 173: 4-14, 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. (1994SHAFIQ, M.; HASSAN, A.; AHMAD, S.: ¨Soil physical properties as influenced by induced compaction under laboratory and field conditions¨, Soil and Tillage Research, ISSN-0167-1987, 29(1): 13-22, 1994. ; Totsche et al. (2017)TOTSCHE, K.; AMELUNG, W.; GERZABEK, M.; GUGGENBERGER, G.; KLUMPP, E.; KNIEF, C.; LEHNDORFF, E.; MIKUTTA, R.; PETH, S.; PRECHTEL, A.; RAY, N.; KO ̈GEL, K.I.: ¨Microaggregates in soils¨, J. Plant Nutr. Soil Sci, ISSN-1522-2624, 1-33, 2017. ; Al-Shammary et al. (2018)AL-SHAMMARY, A.A.G.; KOUZANI, A.Z.; AKIF K; KHOO, S.Y.; NORTON, M.: ¨Soil bulk density estimation methods: a review¨, Pedosphere, ISSN-1002-0160, 28(4): 581-596, 2018. , with sensitivity for short-term estimates, as it is accompanied by other physical and biological indicators (Doran, 1994DORAN, J.W.: Defining soil quality for sustainable environment, Ed. Wisconsin, US: Soil Science Society of America, ISBN-0-89118-807-X, USA, 1994. ).
The results obtained are shown in Table 2:
| Sampling point | 1.1 | 1.2 | 1.3 | 2.1 | 2.2 |
|---|---|---|---|---|---|
| Genetic Classification (IS, 1999INSTITUTO DE SUELOS: Nueva versión de la Clasificación genética de los Suelos de Cuba, Ed. Ministerio de la Agricultura, pp. 64, ISBN-978-959-246-022-5, La Habana, Cuba, 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 |
|
|
|
|
|
| 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 |
|
|
| 2.5 YR 4/6 (seco) (dry) 2.5 YR 3/6 (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 |
| 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") |
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., 2013HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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, 1990LOBO, L.D.: ¨Perdidas de agua, suelo y nutrimentos en un alfisol de Chaguaramas, estado Guarico, bajo coberturas diferentes.¨, Agronomia Tropical (Venezuela). Serie Edafologica.(Ene-Jun, ISSN-0002-192X, 40(1-3): 79-89, 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., 2006HERNÁNDEZ, A.; MORELL, F.; MORALES, M.; BORGES, Y.; ASCANIO, O.: ¨Cambios globales en los suelos Ferralíticos Rojos Lixiviados (Nitisoles ferrálicos, ródicos, éutricos) de Cuba¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, (2): 41-50, 2006. ; Hernandez et al., 2013HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 2013. ) and tends to be lower in the eroded parts and higher in the deposition area, due to the effect of the slope.
The results obtained for the evaluation of porosity (total, capillary and of aeration) are shown in Figure 2:
The percentage of total porosity is close to 50%, a value considered ideal (Martín & Duran, 2011MARTÍN, N.J.; DURAN, A.J.L.: El suelo y su fertilidad, Ed. Félix Varela, ISBN-978-959-07-1418-4, La Habana, Cuba, 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., 2018BÜNEMANN, E.K.; BONGIORNO, G.; BAI, Z.; CREAMER, R.E.; DE DEYN, G.; DE GOEDE, R.; FLESKENS, L.; GEISSEN, V.; KUYPER, T.W.; MÄDER, P.: ¨Soil quality-A critical review¨, Soil Biology and Biochemistry, ISSN-0038-0717, 120: 105-125, 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, 1994HILLEL, D.: Introduction to Environmental Soil Physics, Ed. Elseiver Academic Press, pp. 385, ISBN-978-0-12-348655-4, Amsterdam, 1994. ; 2003HILLEL, D.: Introduction to environmental soil physics, Ed. Elseiver Academic Press, ISBN-0-08-049577-X, Amsterdam, 2003. ; 2013HILLEL, D.: Introduction to soil physics, Ed. Elseiver Academic Press, ISBN-0-08-091869-7, Amsterdam, 2013. ), quantitatively showing the structural limitations identified qualitatively in observation of the ground.
The determination of particle sizes was carried out using dry and wet sieving, obtaining the results shown in Figure 3:
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., 2001SHEPHERD, T.; SAGGAR, S.; NEWMAN, R.; ROSS, C.; DANDO, J.: ¨Tillage-induced changes to soil structure and organic carbon fractions in New Zealand soils¨, Australian Journal of Soil Research, ISSN-1838-6768, 0004-9573, 39(3): 465-489, 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)MENON, M.; MAWODZA, T.; RABBANI, A.; BLAUD, A.; LAIR, G.J.; BABAEI, M.; KERCHEVA, M.; ROUSSEVA, S.; BANWART, S.: ¨Pore system characteristics of soil aggregates and their relevance to aggregate stability¨, Geoderma, ISSN-0016-7061, 1872-6259, 366: 114-259, 2020. , quantifying the resistance to change (maintenance of the structure) of the aggregates in response to the application of mechanical stress (Diaz et al., 2002DIAZ, Z.M.; GROVE, J.H.; PERFECT, E.: Aggregation, fragmentation and structural stability measurenment, Ed. Marcel Dekker, Encyclopedia of Soil Science ed. , ISBN-0-8247-0634-X, 2002. ). In general, greater stability was found in the upper layers and poor stability in the points with the presence of compacted layers.
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)SHEPHERD, T.; SAGGAR, S.; NEWMAN, R.; ROSS, C.; DANDO, J.: ¨Tillage-induced changes to soil structure and organic carbon fractions in New Zealand soils¨, Australian Journal of Soil Research, ISSN-1838-6768, 0004-9573, 39(3): 465-489, 2001. ; Pulido et al. (2009)PULIDO, M.M.A.; LOBO, L.D.; LOZANO, P.Z.: ¨Asociación entre indicadores de estabilidad estructural y la materia orgánica en suelos agrícolas de Venezuela¨, Agrociencia, ISSN-1405-3195, 43(3): 221-230, 2009. ; Hernandez et al., (2013)HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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)FORTUN, C.; FORTUN, A.: ¨Diversos aspectos sobre el papel de la materia orgánica humificada en la formación y estabilización de los agregados del suelo¨, En: Anales de edafologia y agrobiologia, ISBN-0365-1797, pp. 185-204, 1989. , since they link physics and chemically the primary particles in the aggregates (Lado et al., 2004LADO, M.; BEN-HUR, M.; SHAINBERG, I.: ¨Soil wetting and texture effects on aggregate stability, seal formation, and erosion¨, Soil Science Society of America Journal, ISSN-0361-5995, 68(6): 1992-1999, 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, 2004BLANCO, C.H.; LAL, R.: ¨Mechanisms of carbon sequestration in soil aggregates¨, Critical reviews in plant sciences, ISSN-0735-2689, 23(6): 481-504, 2004. ; Pulido et al., 2009PULIDO, M.M.A.; LOBO, L.D.; LOZANO, P.Z.: ¨Asociación entre indicadores de estabilidad estructural y la materia orgánica en suelos agrícolas de Venezuela¨, Agrociencia, ISSN-1405-3195, 43(3): 221-230, 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., 2004SIX, J.; BOSSUYT, H.; DEGRYZE, S.; DENEF, K.: ¨A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics¨, Soil and Tillage Research, ISSN-0167-1987, 79(1): 7-31, 2004. ).
Bernal & Hernández (2017)BERNAL, F.A.; HERNÁNDEZ, J.A.: ¨Influencia de diferentes sistemas de uso del suelo sobre su estructura¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 38(4): 50-57, 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)SIX, J.; BOSSUYT, H.; DEGRYZE, S.; DENEF, K.: ¨A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics¨, Soil and Tillage Research, ISSN-0167-1987, 79(1): 7-31, 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)BERNAL, F.A.; HERNÁNDEZ, J.A.: ¨Influencia de diferentes sistemas de uso del suelo sobre su estructura¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 38(4): 50-57, 2017. highlight that in Ferralitic soils, iron, clays and OM interactions influence on the formation of microaggregates and their stability.
According to Hernandez et al. (2013)HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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)HERNÁNDEZ, A.; MORELL, F.; MORALES, M.; BORGES, Y.; ASCANIO, O.: ¨Cambios globales en los suelos Ferralíticos Rojos Lixiviados (Nitisoles ferrálicos, ródicos, éutricos) de Cuba¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, (2): 41-50, 2006. ; Morell et al. (2006)MORELL, F.; HERNÁNDEZ, A.; FERNÁNDEZ, F.; TOLEDO, Y.: ¨Caracterización agrobiológica de los suelos Ferralíticos Rojos Lixiviados de la región de San José de las Lajas, en relación con el cambio en el manejo agrícola¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 27(4): 13-18, 2006. and Hernandez et al. (2013)HERNANDEZ, J.A.; MORALES, D.M.; CABRERA, R.A.; ASCANIO, G.M.; BORGES, B.Y.; VARGAS, B.D.; BERNAL, F.A.: ¨Degradación de los suelos ferralíticos rojos lixiviados y sus indicadores de la llanura roja de La Habana¨, Cultivos Tropicales, ISSN-0258-5936, e-ISSN: 1819-4087, 34(3): 45-51, 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., 2005LEBEDEVA, I.; TONKONOGOV, V.; GERASIMOVA, M.: ¨Anthropogenic pedogenesis and the new classification system of Russian soils¨, Eurasian Soil Sci, ISSN-1064-2293, 38(10): 1026-1031, 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)COOPER, M.; VIDAL, T.P.; CHAPLOT, V.: ¨Origin of microaggregates in soils with ferralic horizons¨, Sci Agric, ISSN-1678-992X, 62: 256-263, 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).
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.