SciELO - Scientific Electronic Library Online

vol.8 número4Calidad de frutos de dos variedades de fresa mexicana y una introducida frigoconservados en alto CO 2Uso de datos satelitales MODIS y balance de energía para estimar la evapotranspiración índice de autoresíndice de assuntospesquisa de artigos
Home Pagelista alfabética de periódicos  

Serviços Personalizados




Links relacionados

  • Não possue artigos similaresSimilares em SciELO


Revista mexicana de ciencias agrícolas

versão impressa ISSN 2007-0934

Rev. Mex. Cienc. Agríc vol.8 no.4 Texcoco Jun./Jul. 2017 


Effect of conservation agriculture practices on some chemical properties of Vertisols

Aurelio Báez Pérez1  §  

Agustín Limón Ortega2 

Lucila González Molina2 

César Eduardo Ramírez Barrientos3 

Angélica Bautista-Cruz4 

1Campo Experimental Bajío-INIFAP. Carretera Celaya-San Miguel de Allende, km 6.5. Celaya, Guanajuato, México. CP. 38110.

2Campo Experimental Valle de México-INIFAP. Carretera los Reyes-Texcoco, km 13.5. Coatlinchan, Texcoco, Estado de México. CP. 56250.

3Instituto Tecnológico del Valle de Morelia. Carretera Morelia-Salamanca, km 6.5. Morelia, Michoacán. CP. 58100.

4Instituto Politécnico Nacional, CIIDIR-Oaxaca. Hornos 1003, Xoxocotlan, Oaxaca, México. CP. 71230.


El Bajío is an important agricultural area of México and among its soils, the Vérticos have greater potential for agriculture, but the physical-chemical deterioration caused by intensive farming practices has had an impact on its fertility and profitability for grains production. Conservation agriculture practices (CAPs) are a viable option to reverse this problem. The objective of this paper was to evaluate pH, organic matter (MOS), organic carbon accumulation rate (COS), and P-extractable Olsen in five Vertisols: four from Michoacán and one from Guanajuato, with four to eight cycles of continuous cultivation with CAP. Five agronomic practices were evaluated: (1) grass-grass rotation, all crop residues and sewage use; (2) rotating grass-legume-grass, all crop residues and sewage use; (3) same as 2, but with dam water; (4) grass-grass rotation, 30% crop residues and use of dam water; and (5) grass-grass rotation, all crop residues and well water use. The pH was alkaline before the CAP implementation, and decreased to slightly alkaline as a function of the culture time, mainly in the stratum of 0-5 cm. The MOS increased between 1.5 and 2%, after eight continuous CAP cycles. The COS accumulation rate varied between 1.5 and 7 t ha-1 year-1. The accumulation of P-Olsen showed a low correlation with the MOS content (r= 0.44). The highest concentration of P-Olsen was found in the 0-5 cm stratum (up to 47 ppm). CAPs improved the chemical characteristics of the Vertisols evaluated in this study.

Keywords: conservation agriculture; soil organic carbon


El Bajío es una importante zona agrícola de México y entre sus suelos, los Vérticos tienen mayor potencial para la agricultura, pero el deterioro físico-químico ocasionado por las prácticas de agricultura intensiva ha repercutido en su fertilidad y la rentabilidad para la producción de granos. Las prácticas de agricultura de conservación (PAC) son una opción viable para revertir esta problemática. El objetivo de este estudio fue evaluar el pH, materia orgánica (MOS), tasa de acumulación de carbono orgánico (COS) y P-extratable Olsen en cinco Vertisoles: cuatro de Michoacán y uno de Guanajuato, con cuatro a ocho ciclos de cultivo continuos con PAC. Se evaluaron cinco prácticas agronómicas: (1) rotación gramínea-gramínea, todos los residuos de cosecha y uso de aguas negras; (2) rotación gramínea-leguminosas-gramínea, todos los residuos de cosecha y uso de aguas negras; (3) igual que 2, pero con agua de presa; (4) rotación gramínea-gramínea, 30% de residuos de cosecha y uso de agua de presa; y (5) rotación gramínea-gramínea, todos los residuos de cosecha y uso de agua de pozo. El pH fue alcalino antes de la implementación de las PAC, y disminuyó a ligeramente alcalino en función del tiempo de cultivo, principalmente en el estrato de 0-5 cm. La MOS aumentó entre 1.5 y 2%, después de ocho ciclos continuos con PAC. La tasa de acumulación de COS varió entre 1.5 y 7 t ha-1 año-1. La acumulación de P-Olsen mostró una baja correlación respecto al contenido de MOS (r= 0.44). La mayor concentración de P-Olsen se encontró en el estrato de 0-5 cm (hasta 47 ppm). Las PAC mejoraron las características químicas de los Vertisoles evaluados en este estudio.

Palabras clave: agricultura de conservación; carbono orgánico del suelo


Soil fertility and accumulation of organic reserves are determined by the complex interaction of climatic, edaphic, biological and agronomic factors (Lal, 2004). The soil is a natural body with a wide vertical heterogeneity, lateral and over time (Post et al., 2001), so there are difficulties to delimit its quality standards; its study requires evaluating variables that allow to measure their status and evolution. The C organic content in the soil (COS) is an indicator of quality and influences the physical, chemical and biological properties. This parameter is feasible to monitor on soil ecosystems in order to assess their condition and define its productive potential on a given time scale (Karlen et al., 1997; Bautista-Cruz, 2004).

In the Bajío agricultural soils, the continuous tillage practices (fallow, crawl and furrow) implies a constant removal of the soil and an alteration of its physical, chemical and biological properties (Ongley, 1997), together with the removal or burning of crop waste and the lack of incorporation of organic fertilizers, have caused a severe deterioration of soil fertility (Grageda-Cabrera et al., 2004). As a consequence, farmers use excessive doses of chemical fertilizers to try to increase agricultural production, which has a direct impact on production costs and environmental pollution.

Vertisols constitute 8.6% of the total soils in México (Cruz et al., 2007) and in the Bajío they cover approximately 500 000 hectares, with a high potential for grains and vegetables production when there is water available for irrigation (Grageda, 1999); however, intensive agriculture and inadequate agricultural practices have led to a severe soils deterioration, impacting on ecology and profitability for agricultural production. Conservation agriculture practices (CAPs) are based on minimal soil removal, continuous addition of crop residues on its surface, and crop diversification, which is a viable option to reverse the above-mentioned problem. This favors in the medium term the accumulation of COS and biological activity, which directly affects soil quality (Gregorich and Carter, 1997). The aim of this paper was to evaluate the evolution of chemical properties in Bajío’s Vertisols subjected to several continuous cycles with CAP and to study the tendency of organic reserves accumulation.

Materials and methods

Study sites

The study was conducted in the Cuitzeo Basin, located between 19° 59ʼ and 19° 30ʼ north latitude and 101° 00ʼand 101° 30ʼ west longitude. It has an approximate area of 1 050 km2, and occupies an important area of the Morelia-Queréndaro Irrigation District (Figure 1). The altitude is over 2 000 meters above sea level. Four Vertisols were evaluated in this site. Another soil was evaluated at INIFAP, Bajío Experimental Field in Celaya, Guanajuato, located at 20º 3ʼ north latitude and 100º 0ʼwest longitude, at an altitude of 1 754 m. The climate (García, 1984) is BS1hw(w)(e)g with an average annual temperature of 20.6 °C and annual rainfall of 597 mm.

Figure 1 Location of the study area. Morelia-Queréndaro Valley, Michoacán.  

The soils

The textural classification of soils, except one, corresponded to clayey (Table 1), which is characteristic of Vertisols (Table 2). These soils were characterized by being dark and deep (>1 m) and possess expandable clays of smectic type (USDA, 1999).

Table 1 Textural classification of soils evaluated under LC. 

Table 2 Agronomic management under conservation tillage in four localities of the Queréndaro-Morelia Valley and Bajío Experimental Field. 

O-I= otoño-invierno; P-V= primavera-verano.

Agronomic management

Soils were cultivated for four to eight continuous cropping cycles (two to four years) with conservation agriculture practices (CAP) adding 30 to 100% of crop residues on the soil surface.The agronomic history is presented in Table 2. In the Morelia-Queréndaro valley, farmers regularly plant wheat in beds of 1.6 m wide during autumn-winter seasons, re-marking the furrow to facilitate the water irrigation; and in spring-summer they plant maize in a double row, on the same beds. At the end of each crop cycle, after harvesting the grains, the crop residues are crushed and then spread in a mulch-like form as homogeneously as possible on the soil surface.

The soils located in Indaparapeo and Álvaro Obregón, Michoacán, are continuously irrigated with sewage from Morelia city, while the soils of Queréndaro are irrigated with water from the dam. In all soils, the total amount of crop residues on the soil surface was added, except in the soil called Queréndaro II, where approximately three quarters are packed for sale. The amount of crop residues that were added in this soil, after each crop cycle, was variable (Table 2).

In the Experimental Field Bajío wheat is sown in autumn-winter on grooves of 0.76 cm wide, in double row, while maize is sown in spring-summer in the same grooves in single row. Well water was used for irrigation and all crop residues were left on the soil surface in a mulch-like form.

Sampling and processing of soil samples

Four samples were taken in each soil evaluated, since the conservation agriculture practices (CAP) were implemented. In the Morelia-Queréndaro valley these were carried out at 0, 960, 1 200 and 1 440 days of the implementation of PACs; and in the Bajío Experimental Field at 0, 180, 360 and 540 days. The samples consisted of 22 sub samples each, and were collected randomly within each plot. A stainless steel bit was used and three depths were considered: 0-5, 5-15 and 15-30 cm. Samples then were dried in the shade and at room temperature, milled with a wooden mallet, sieved in a 2 mm diameter mesh and perfectly homogenized. For the analytical determination of the organic matter (MOS) a sub-sample of 100 g was prepared, ground and sieved in a number 30 mesh.

The determination was made by the Walkley and Black method, described by Jackson (1976). For the analytical determination of C total, a sub-soil sample of 10 g was used, which was ground and sieved in a number 100 mesh. The C total was measured by a Shimadzu automatic carbon determinator, model TOC-5050a. It was used to measured inorganic C. The COS was calculated by subtracting the inorganic C to the total C. The pH of the soil was measured in wáter at 1:2 ratio. The P-extractable was evaluated by the Olsen method, described by Jackson (1976). Results were related to cultivation time, in order to evaluate the behavior and accumulation tendency of organic reserves by the effect of conservation agriculture practices.

Results and discussion

The pH in the evaluated Vertisols was from alkaline to strongly alkaline prior to the implementation of conservation agriculture (CAP) practices (Figure 2). The pH decreased as a function of the culture time, to about one unit in the 0-5 cm depth stratum. In the strata of 5-15 and 15-30 cm the pH decrease was more tenuous. In the first soil stratum the biological activity was more intense, due to the direct contact with the organic layer of the crop residues. According to Galeana-Cruz et al. (1998), the addition of organic matter (MOS) to the soil surface, as part of the CAP, is the component with the greatest influence on pH depletion. The mineralization of MOS necessarily implies a more intense activity of soil microorganisms, and during the humification process organic acids are produced that react with the mineral fraction of the soil.

Figure 2 Evolution of pH in five Vertisols submitted to conservation agriculture practices.  

The problem of chemical degradation of soils in the Bajío is caused in part by the accumulation of salts from the excessive use of chemical fertilizers and irrigation water with high concentrations of sodium. Castellanos et al. (2000) mentioned that sodium content in Guanajuato soils has increased in recent years due to the high content of sodium carbonates contained in irrigation waters. The sewage used in Michoacán is likely to contain even higher levels of sodium. Therefore, the addition and continuous incorporation of MOS is a viable alternative to reduce alkalinity in soils.

Organic material

The accumulation of MOS in Vertisols, after four to eight cycles of continuous cultivation with the addition of crop residues on the soil surface and its minimal removal, was significant (p≤ 0.05). The increase was linear according to the cultivation time and the amount of agricultural wastes. However, when producers break the conservation tillage system due to the need to level their cropland, MOS reserves falls drastically in a single crop cycle.

This was the case of the Queréndaro II soil where the farmer, after four continuous cycles of CAPs, performed all the tillage practices to level his plot. It was observed that after two years with CAPs the MOS increased up to 5% in the 0-5 cm depth stratum, but when the culture system was broken the MOS decreased drastically to a little more than 3% (Figure 3).

Figure 3 Evolution in the accumulation of MOS and COS in five Vertisols submitted to conservation tillage in the Bajío.  

In this soil, about 38 t ha-1 were added to the surface for 4 continuous crop cycles. The harvest residues in the soil surface until that moment were incorporated to the soil with the fallow, later it was resumed with the CAP; however, due to soil disturbance the MOS accumulation rate was altered and decreased.

Incorporation of crop residues into the soil by fallowing involves flipping the arable layer and exposing the lower layer to the surface, which has a lower MOS content than the upper layer. The latter being buried and in direct contact with soil microorganisms, increases the rate of mineralization of the MOS and the organic reserves decrease. The spraying of soil aggregates by the work of tracking, leveling and furrowing also favors the oxidation of COS (Elliot, 1986; Oades, 1988).

In the soils irrigated with sewage, in Álvaro Obregón and Indaparapeo, after eight cycles of cultivation with CAPs, MOS increased by about 2%, in the stratum 0-5 cm deep, with respect to the content of MOS before implementing The CAPs (Figure 2). This level of high accumulation is explained by a continuous intake of organic waste with the irrigation water. To achieve this increase in MOS, after nearly four years of cultivation, about 50 t ha-1 of crop residues were introduced into the soil. At the depth of 5-15 cm the MOS accumulation was 0.5% smaller, compared to the previous depth.

In the stratum of 15-30 cm the MOS accumulation did not show changes with respect to the amount of organic matter found before starting with the CAP. It was evident that the accumulation of MOS was lower as the depth increased, because the contact with the crop residues with the deeper strata is more distant. In order for a substantial increase of MOS at a greater depth to occur, longer cultivation time is required for the leaching, intemperisation, mineralization and humification processes, among others, to affect the deeper horizons.

In the Queréndaro I soil the percentage of MOS slightly increased in the first 5 cm of depth, in relation to the content it had before implementing the PACs. In this soil about 30% of the crop residues were incorporated after each crop cycle, which was equivalent to approximately 28 t ha-1 (Table 2). This can be explained because the mineralization rate of the MOS is slightly lower than the accumulation rate. In other words, a higher input of crop residues is required to significantly increase the MOS content in this soil (Reycosky et al., 1995).

In the soil evaluated in Celaya, with only four cycles of cultivation with PACs, there was an approximately linear trend in the MOS accumulation, reaching a content greater than 3%; i.e 1.2% more than year zero. For the depth of 5-15 cm this increase was 0.7%. It was estimated that the amount of crop residues added to the soil was about 46 t ha-1 (Table 2). In the stratum 15-30 cm deep the MOS content did not increase (Figure 3).

Organic carbon

The evolution in the accumulation of COS reserves in the evaluated Vertisols showed a significant increase, especially in the 0-5 cm depth stratum, for the soils of Indaparapeo, Álvaro Obregón and Celaya. This increase varied from 0.5 to 2%, which depended on the cultivation time with the PAC and the amount of crop residues added to the soil (Figure 3). The behavior in the accumulation dynamics of the COS was similar to that reported for MOS, which is explained by the close relation (R2< 0.7) between both parameters. Soil MOS contains 58% of C (Jackson, 1976) and its rate of mineralization depends on the amount of lignin, comminution degree, soil distribution, agronomic and edaphoclimatic conditions (Curtin et al., 2000).

In the second stratum, 5 to 15 cm deep, the COS accumulation was more discrete and in some cases was not even evident, whereas in the depth of 15-30 cm in most cases, it was not evident. This is because there is no direct contact with the crop residues, as explained above.

The COS accumulation rate in the Vertisol of Indaparapeo, Michoacán, was the highest of the soils that were evaluated in the valley Morelia-Queréndaro. It entered on average 7 t ha-1 year-1 in the first 30 cm of depth (Figure 4). Meanwhile, in the Vertisols of Álvaro Obregón and Queréndaro II, 1.5 and 2.5 t ha-1 year-1 of this element were accumulated, respectively. But on the Queréndaro I soil there was practically no carbon accumulation by adding only 30% of the crop residues. In the soil of Celaya, where there was an accumulation of greater quantities of agricultural wastes, the accumulation rate of COS was the highest, with 9.3 t ha-1 year-1.

Figure 4 COS accumulation rate in five Vertisols submitted to conservation tillage in the Bajío.  

The accumulation of COS in the soil occurs when the income (organic waste) is greater than the losses (erosion, mineralization, leaching) (Lal, 2004). Báez-Pérez et al. (2009) concluded that the CO2 emission rate (TEC) of the soil is a function of the retained moisture and the COS content. Therefore, soils subject to conservation tillage have a higher content of MOS that is potentially mineralizable, compared to soils where traditional tillage is practiced. This implies a higher CO2 emission as the MOS content increases, as has been reported in another study in these same Vertisols soils (Báez-Pérez et al., 2011).

In traditional tillage systems, in the Vertisols soils of Morelia-Queréndaro Valley, the amount of crop residues incorporated in the soil varies from 1 to 2 t ha-1 and its volumetric moisture in the first 30 cm of depth during spring-summer can vary from 12% in dry season, to 42% in wet season, with a CO2 emission of 0.2 to 1.2 g m-2 h-1, respectively. In CAP systems the volumetric humidity fluctuates from 20% to 52% and CO2 emissions, in the same regard, from 0.4 to 2.6 g m-2 h-1. The above shows that even when there is a constant income of MOS in the culture systems with CAP, the TEC is also larger, so the accumulation of COS can also be limited. In order to achieve a COS accumulation rate greater than the COS loss, additional strategies are required in addition to the incorporation of organic waste. The use of crops with wide vegetation coverage and crop rotation could be additional alternatives to reduce COS losses due to mineralization and erosion.

Extractable phosphorus

The P-extractable content (P) in the evaluated soils showed a low correlation with the soil MOS content (R2= 0.2). The continued contribution of crop residues to the soil surface necessarily contributes to significant reserves of P; however, the constant application of phosphate fertilizers, which farmers apply cycle by cycle to crops, has provided a large reservoir of this element available in the short term to meet the requirement of cereal crops.

No trend was observed in the accumulation of this element that was related to the culture time by effect of the CAP. Because P is an element with low soil mobility and the forms assimilable by plants (H2PO4- y HPO4=) are stable in a medium with slightly acidic pH (Castellanos, 2000), it could be inferred that there is a high availability of this element in the ground. However, the values of alkaline pH that present the majority of the soils in this evaluation could influence the solubility of this element, and therefore the availability for the plants nutrition.

The highest concentration of P-extractable in the evaluated Vertisols was found in the 0-5 cm depth stratum, with the exception of the Indaparapeo soil (Table 3), where strata of 5-15 and 15-30 cm depth were more rich in this element, 42 and 65 ppm, respectively, which is extremely high.

Table 3 Amount of P-extractable Olsen (ppm) in the evaluated soils of the Bajío. 

This is explained by the continuous use of sewage from Morelia city, for irrigation purposes, as explained above. The concentration of P-extractable in the soils of Michoacán can be classified from high to very high, suggesting that it would not be necessary to apply phosphate fertilizers in several crop cycles. According to Castellanos (2000), the requirement of P-extractable for a yield potential in 10 t ha-1 maize for the Bajío region is 90 to 100 kg ha-1.

The available reserve of this element in the soils of Michoacán was estimated to be 169 to 680 kg ha-1, which can satisfy the nutritional needs of two to six consecutive cycles of grasses (Table 3). This can constitute a significant saving in the cost of chemical fertilizer. However, the alkaline pH present in some soils could influence its availability.

The soil of Celaya, Guanajuato, with only two years with CAP, showed the lowest P-extractable values, from 17 to 26 ppm, with respect to the other soils; however, its content is classified as medium to moderately high. The reserves of P-extractable in this soil were estimated at more than 200 kg ha-1.


The conservation agriculture practices in the evaluated soils increased the organic reserves and decreased the pH, mainly in the stratum of 0-5 cm of depth, which contributed to the fertility improvement. There was a significant increase in the accumulation of organic matter and organic carbon, mainly in the stratum 0-5 cm deep, 1 to 2% and 0.5 to 1.5% respectively. As soil depth increased, MOS accumulation decreased. The P-extractable content in soils was high for all three strata, between 20 and 47 ppm; however, it was not related to MOS content. The continuous application of this element by means of chemical fertilizers has provided an important reservoir in the soil.

Literatura citada

Báez, P. A.; Etchevers, J. D.; Haulon, M.; Werner, G.; Flores, G. e Hidalgo, C. 2009. Pérdida de carbono por erosión hídrica y emisiones de CO2 en tepetates habilitados para la agricultua. In: Gallardo, J. F.; Campo, J. and Conti, M. E. (Eds). Emisiones de gases con efecto invernadero en ecosistemas iberoamericanos. Salamanca, España. 25-48 pp. [ Links ]

Báez, P. A.; Huerta, M. E.; Velázquez, G. J. y Bautista, C. M. A. 2011. Acumulación y flujo de carbono en Vertisoles cultivados en labranza de conservación. In: estado actual del conocimiento del ciclo del carbono y sus interacciones en México. Paz, F. y Cuevas R. M. (Eds.). Síntesis a 2011 del programa mexicano del carbono. Instituto Nacional de Ecología. México. D. F. 204-2011 pp. [ Links ]

Bautista, C. A.; Etchevers, B. J. D.; del Castillo, R. F. y Gutiérrez, C. C. 2004. La calidad del suelo y sus indicadores. Ecosistemas. 13:90-97. [ Links ]

Castellanos, J. Z. 2000. Manual de interpretación de análisis de suelos y aguas. INTAGRI. Segunda edición. Celaya, Guanajuato. 187 p. [ Links ]

Castellanos, J. Z.; Hurtado, G. B.; Villalobos, R. S.; Badillo, T. V.; Vargas, T. P. y Enríquez, R. S. A. 2000. La calidad de agua subterránea para Guanajuato. Reporte técnico del proyecto 47/99 de la Fundación Produce Guanajuato A.C. Celaya, Guanajuato. 6 p. [ Links ]

Curtin, D.; Wang, H.; Selles, F. B.; McConkey, G. and Campbell, C. A. 2000. Tillage effects on carbon in continuous wheat and fallow - weat rotations. Soil Sci. Soc. Am. J. 64:2080-2086. [ Links ]

Elliot, E. T. 1986. Aggregates structure and carbon, nitrogen, and phosphorus in native and cultivated. Soil Sci. Soc. Am. J. 50:627-633. [ Links ]

Galeana, C. M.; Trinidad, S. A.; García, C. N. E. y Flores, R. D. 1998. Labranza de conservación y fertilización en el rendimiento de maíz y su efecto en el suelo. Terra Latinoam. 17:325-335. [ Links ]

Grageda-Cabrera, O. A. 1999. La fertilización nitrogenada en el Bajío guanajuatense como fuente potencial de contaminantes ambientales. Tesis de doctorado en Biotecnología y Bioenergía. CINVESTAV-IPN. México, D. F. 145 p. [ Links ]

Grageda, C. O. A.; Medina, C. T.; Aguilar, A. J. L.; Hernández, M. M.; Solís, M. E.; Aguado, S. G. A. y Peña, C. J. J. 2004. Pérdidas de nitrógeno por emisión de N2 y N2O en diferentes sistemas de manejo y con tres fuentes nitrogenadas. Agrociencia. 38:625-633. [ Links ]

García, E. 1984. Modificaciones al sistema de clasificación climática de Köpen. Universidad Nacional Autónoma de México. México, 16-21 pp. [ Links ]

Gregorich, E. G. and Carter, M. R. 1997. Soil quality for crop production and ecosystem health. Develop. Soil Sci. 25:125-165. [ Links ]

INEGI. 2007. Conjunto de datos vectorial edafológico, escala 1: 250 000 Serie II (continuo nacional). México. [ Links ]

Jackson, M. L. 1976. Análisis químico de suelos. 3ra edición. Traducción al español por Martínez, J. B. Omega. Barcelona, España. 282-283 pp. [ Links ]

Karlen, D. L.; Mausbach, M. J.; Doran, J. W.; Cline, R. G.; Harris, R. F. and Schuman, G. E. 1997. Soil quality: a concept, definition, and framework for evaluation. Soil Sci. Soc. Am. J. 61:4-10. [ Links ]

Lal, R. 2004. Soil carbon sequestration impacts on global climate change and food security. Science. 304:1624-1627. [ Links ]

Oades, J. M. 1988. The retention of organic matter in soils. Biogeochemistry. 5:35-70. [ Links ]

Ongley, E. D. 1997. Lucha contra la contaminación agrícola de los recursos hídricos. Estudio de la FAO riego y drenaje núm. 55. 115 pp. [ Links ]

Post, W. M.; Izaurralde, R. C.; Mann, L. K. and Bliss, N. 2001. Monitoring and verifying of changes of organic carbon in soil. Climatic change. 51:73-99. [ Links ]

Reicosky, D. C.; Kemper, W. D.; Langdele, G. W.; Douglas Jr, C. L. and Rasmussen, P. E. 2000. Soil organic matter changes resulting from tillage and biomass production. J. soil and water conservation. 50:253-261. [ Links ]

USDA (United States Department of Agriculture). 1999. Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys. Second edition. 783-784 pp. [ Links ]

Received: January 2017; Accepted: March 2017

Creative Commons License Este es un artículo publicado en acceso abierto bajo una licencia Creative Commons