The role of organic matter in the acidification

of agricultural soils in Ecuadorian Amazonia

El papel de la materia orgánica en la acidificación de suelos

agrícolas de la amazonia ecuatoriana

Luis David Jiménez Jumbo

Jhandry Patricio Sarango Ordoñez

Joana Alexandra Moreno López

Luis Alejandro Fiallos Ulloa

Abstract: The research focuses on understanding how agricultural

practices and the decomposition of organic matter contribute to

changes in soil acidity, a phenomenon critical to soil health and

productivity in this area. The study uses scientific methods to analyze

soil samples, evaluating the composition of organic matter and its

relationship to soil acidity. The influence of organic matter on soil

acidity was evaluated, highlighting that Amazonian soils tend to be

more acidic and clay soils predominate. The study uses a coefficient

of determination of 0.662 to evaluate the relationship between organic

matter and soil acidity, indicating that approximately 66.2% of the

variability in soil acidity can be explained by organic matter. The

researchers conclude that the presence of organic matter plays a

significant role in soil acidification, especially in regions such as the

Amazon, where higher acidity is observed. They also highlight the

predominance of clay soils in these areas, which may influence

nutrient retention and the ability to neutralize soil acidity. These

findings underscore the importance of considering organic matter and

specific soil characteristics when developing agricultural

management and conservation strategies in regions with acid soils

such as the Amazon.

Keywords: hydrogen potential, organic carbon, organic matter, soil,

soil use potential, acidification

Resumen: La investigación se centra en comprender cómo las

prácticas agrícolas y la descomposición de la materia orgánica

contribuyen a cambios en la acidez del suelo, un fenómeno crítico

para la salud y productividad del suelo en esta área. El estudio utiliza

métodos científicos para analizar muestras de suelo, evaluando la

composición de la materia orgánica y su relación con la acidez del

suelo. Se evaluó la influencia de la materia orgánica en la acidez del

suelo, destacando que los suelos amazónicos tienden a ser más ácidos

y predominan los suelos arcillosos. El estudio utiliza un coeficiente

de determinación de 0.662 para evaluar la relación entre la materia

orgánica y la acidez del suelo, lo que indica que aproximadamente el

66.2% de la variabilidad en la acidez del suelo puede ser explicada

por la materia orgánica. Los investigadores concluyen que la

Biochemical Engineer, Universidad Estatal

Amazónica, ld.jimenezj@uea.edu.ec

https://orcid.org/0009-0000-0862-2048

Eng. Environmental Biotechnology,

Universidad Estatal Amazónica,

ja.morenol@uea.edu.ec

https://orcid.org/0009-0008-4427-670X

Biologist, Universidad Estatal Amazónica

Jp.sarangoo@uea.edu.ec

https://orcid.org/0009-0001-4305-6579

Master's Degree in Forestry, Universidad Estatal

Amazónica, la.fiallosu@uea.edu.ec

https://orcid.org/0009-0006-8477-9980

Published

Edwards Deming Higher Technological

Institute. Quito - Ecuador

Periodicity

January-March

Vol. 1, Num. 21, 2024

pp. 12-24

http://centrosuragraria.com/index.php/revista

Dates of receipt

Received: December 19, 2023

Approved: January 30, 2024

Correspondence author

ld.jimenezj@uea.edu.ec

Creative Commons License

Creative Commons License, Attribution-

NonCommercial-ShareAlike 4.0

International.https://creativecommons.org/lice

nses/by-nc-sa/4.0/deed.es

April - June vol. 1. Num. 21 - 2024

presencia de materia orgánica desempeña un papel significativo en la

acidificación de los suelos, especialmente en regiones como la

Amazonía, donde se observa una mayor acidez. Además, resaltan la

predominancia de suelos arcillosos en estas áreas, lo que puede influir

en la retención de nutrientes y en la capacidad de neutralizar la acidez

del suelo. Estos hallazgos subrayan la importancia de considerar la

materia orgánica y las características específicas del suelo al

desarrollar estrategias de manejo agrícola y conservación en regiones

con suelos ácidos como la Amazonía.

Palabras clave: Potencial de hidrogeno, carbono orgánico, materia

orgánica, suelo, uso potencial, acidificación

Introduction

The Ecuadorian Amazon is home to an exceptional wealth of

biodiversity and plays a crucial role in global climate regulation.

However, the growth of agricultural activity in this region has raised

concerns about environmental impacts, particularly on soil health. Soil

acidification, a phenomenon that negatively affects the availability of

essential plant nutrients, has emerged as a significant challenge; this

decrease in soil pH alters the ability of nutrients to dissolve and be

available to plants, leading to nutritional deficiencies and negatively

affecting crop growth and yields (Queiroz et al., 2021)..

This study delves into the intricate relationship between organic matter

and the acidification of agricultural soils in the Ecuadorian Amazon.

Organic matter, consisting of decomposing plant and animal residues,

represents an essential component of soil structure and plays a

fundamental role in soil fertility. Organic matter, composed of

decomposing plant and animal residues, is fundamental to soil structure

and fertility; soil organic matter influences moisture retention,

improves soil structure and promotes beneficial biological activity. In

addition, it serves as a source of plant nutrients, harbors beneficial

microorganisms and helps regulate soil pH. Therefore, the presence and

maintenance of high levels of soil organic matter are crucial to mitigate

the negative effects of soil acidification and ensure the availability of

essential plant nutrients. However, processes involving organic matter

can also contribute to soil acidification, generating a delicate balance

that deserves detailed attention (Lal, 2015).

The objective of this study is to examine in depth the specific role of

organic matter in the process of soil acidification in Amazonian

agricultural contexts; specifically in the crop area of the Amazonian

Research and Production Experimental Center of the Amazon State

The role of organic matter in the acidification of agricultural soils in Ecuadorian Amazonia

University. Through detailed analyses of soil samples and evaluations

of local agricultural practices, we seek to provide a clearer

understanding of the mechanisms driving this phenomenon. These

insights are crucial to inform sustainable management strategies that

reconcile agricultural production with the long-term preservation of soil

health in this unique region.

Materials and methods

Specific tools and techniques were used to collect a representative soil

sample. A stainless steel shovel, airtight plastic bags, identification tags

and disposable gloves were used as key materials. Points within the

study area were randomly selected, ensuring an equal distribution. The

top layer of vegetation was removed at each point and excavated to a

predetermined depth using the shovel. To obtain a three-dimensional

representation of the soil, multiple samples were taken at each point,

which were combined and homogeneously mixed to form a composite

sample. Each sample was accurately labeled, stored in airtight bags, and

transported to the laboratory for subsequent analysis, ensuring

representative and consistent data collection ( (Bismarck et al., 2014)..

Samples were taken within the Experimental Center for Amazonian

Research and Production (CEIPA); where agricultural practices are

implemented with different crops in order to implement a teaching

system to students of the Amazon State University.

Determination of pH

For the determination of pH in a soil sample, the following materials

and method were implemented according to the FAO proposal. A

calibrated glass electrode, buffer solutions of known pH (pH 4.01, 7.00

and 10.01), magnetic stirrer and soil samples were used. Initially, the

glass electrode was calibrated using the prepared buffer solutions.

Subsequently, a soil suspension was prepared by mixing a measured

amount of soil with distilled water. The glass electrode was immersed

in the suspension to record the pH of the sample. This procedure was

repeated with several samples to obtain representative and accurate

results in the evaluation of the soil pH. (Food and Agriculture

Organization of the United Nations, 2020).

April - June vol. 1. Num. 21 - 2024

Determination of organic matter

In the analysis of organic matter in a soil sample, tools and procedures

were used according to the validation of the calcination method. The

materials included a muffle furnace, porcelain capsules, a spatula and

soil samples. In the method, a known amount of soil was weighed into

dry capsules and the initial weight was recorded. Subsequently, the

capsules with the soil were placed in the muffle furnace and heated to a

constant temperature for a given time. After cooling in a desiccator,

they were weighed again to calculate the weight loss as an indicator of

the organic matter present in the soil. This procedure was replicated on

several samples to obtain a representative assessment (Aguilar and

Yuleysi, 2019)..

Organic Carbon Determination

It is possible to estimate soil organic carbon content using the organic

matter content as a reference. Soil organic matter consists mainly of

carbon, but also includes other elements such as hydrogen, oxygen,

nitrogen and small amounts of sulfur and other elements. Although not

all components of organic matter are pure carbon, it is common to use

an empirical ratio to estimate organic carbon content from organic

matter content. The most commonly used ratio is the Van Bemmelen

ratio, which states that approximately 58% of the organic matter content

in soil is organic carbon. However, this ratio can vary depending on

several factors, such as soil type, soil use history, surrounding

vegetation, and soil management practices.

Despite these variations, estimating organic carbon content from

organic matter content is a common and useful practice in soil quality

management and monitoring, especially when specific organic carbon

data are not available (Martinez et al., 2017).

Determination of texture

To determine soil texture by bulk density, soil samples are taken from

different depths using a cylinder of known volume or a suitable

sampling tool. These samples are dried in the laboratory until a dry

weight constant is obtained. The bulk density is then calculated by

dividing the mass of dry soil by the total volume of the sample.

The relationship between bulk density and soil texture is established

through soil theory, where different soil textures, such as clay, silt and

The role of organic matter in the acidification of agricultural soils in Ecuadorian Amazonia

sand, have different bulk densities due to their physical and structural

properties. For example, clay soils tend to have a higher bulk density

due to compaction caused by small particles and high water holding

capacity, while sandy soils tend to have a lower bulk density due to their

looser structure and the presence of large particles.

Although the determination of soil texture through bulk density

provides an indirect and less accurate estimate than other methods of

textural analysis, it can be useful in situations where other resources or

more advanced techniques are not available. (Ibañez, 2007).

Table 1. Texture of soils with respect to their bulk density.

Texture

Bulk density (mg/m )

Sandy

1,55 - 1,80

Franco

1,35 - 1,50

Clay loam

1,30 - 1,40

Clayey

1,20 - 1,30

Source: (Ibañez, 2007)

Relationship between pH and organic matter

GeoGebra, with its interactive and visual capabilities, presents itself as

a highly effective tool in soil laboratories. It facilitates soil texture

analysis by allowing the visualization and evaluation of the relationship

between different particle sizes through scatter plots and curve fitting.

It is also useful for exploring correlations between various soil

parameters, such as the relationship between organic matter content and

water holding capacity, taking advantage of dynamic plots to identify

trends. GeoGebra is also used to visually model soil permeability and

analyze drainage capacity using infiltration rate plots. In spatial

variation studies, the tool creates interactive maps that visualize and

analyze the distribution of soil properties. In addition, GeoGebra acts

as a valuable educational tool, allowing students to interact with models

and graphs to improve their understanding of soil phenomena and

processes. In addition, it enables the simulation of soil laboratory

experiments, providing the opportunity to perform virtual practice

before conducting real experiments, which not only streamlines data

analysis, but also provides a more intuitive and visual understanding of

April - June vol. 1. Num. 21 - 2024

key concepts, thus facilitating informed decision-making in soil

management and study.

3. Result

Table 2. Results of CEIPA agricultural soil parameters

Soil

Organic

Matter (%)

Organic

Carbon (%)

Texture

Coffee

6,01

18,50%

17,3884

Clayey

Pastures

5,52

36,52%

15,0684

Clayey

Medicinal

Plants PB

6,89

15,96%

24,2846

Clayey

Cocoa

6,18

23,56%

10,962

Clay loam

Bio

Fertilizers

6,54

17,56%

11,6

Franco

Citrus

5,59

26,45%

0,2204

Clayey

Forestry

program

(balsa and

guadua)

5,98

23,95%

18,6296

Clayey

Forestry

program

6,5

19,36%

13,8562

Clayey

Banana

orchard

5,86

32,45%

16,762

Clay loam

Banana

study

6,89

15,48%

22,562

Clayey

The role of organic matter in the acidification of agricultural soils in Ecuadorian Amazonia

Old Coffee

5,86

28,50%

8,236

Franco

Grass study

5,48

28,23%

7,4762

Clayey

Medicinal

plants PA

6,795

24,69%

23,950

Clayey

Coffee

research

6,190

22,95%

8,700

Franco

Sajinos

5,871

32,85%

16,026

Clay loam

Pastures 2

5,441

41,20%

14,852

Clayey

Source: Author

Illustration 1. pH vs. organic matter in CEIPA's agricultural soils.

Source: Author

April - June vol. 1. Num. 21 - 2024

The results reflect a clear inverse relationship between pH and organic

matter in agricultural soils that is mainly due to the buffering capacity

of organic matter. Organic matter, composed mainly of decomposing

plant and animal residues, contains organic acids that can neutralize the

bases in the soil, thus lowering its pH. When there is a greater amount

of organic matter present in the soil, microbial activity that decomposes

these residues increases, releasing organic acids and other compounds

that acidify the soil.

On the other hand, organic matter can also help improve soil structure

and nutrient holding capacity, which in turn can influence pH.

However, as organic matter decomposes and nutrients are absorbed by

plants, pH tends to decrease due to the release of organic acids (Toledo,

2016).

Table 3. Regression data between pH and organic matter.

Regression model

Power

Formula

y = a * x

Parameters

a = 5296,844

b = -2,978

Coefficient of determination

= 0.662

Source: Author

The power regression model is used when the relationship between

variables is not linear, but follows an exponential trend. The behavior

of this model is such that when X increases, Y also increases, but not in

a constant manner. It is important to keep in mind that this type of model

can be sensitive to outliers, as small changes in x can result in large

changes in y due to the exponential nature of the relationship (Frias et

al., 2010).

In a potential model, an inverse relationship, as shown in Figure 1,

implies that as an independent variable increases, the dependent

variable decreases exponentially. That is, as X increases, Y decreases.

A coefficient of determination of 0.662 can be considered acceptable in

many contexts of statistical analysis. This value indicates that

approximately 66.2% of the variability in the dependent variable can be

explained by the regression model used. Although there is no single

criterion for determining the acceptability of a DC, in general, a value

The role of organic matter in the acidification of agricultural soils in Ecuadorian Amazonia

of 0.662 suggests a moderately strong relationship between the

variables. However, the evaluation of the acceptability of the coefficient

of determination should also consider other factors, such as the purpose

of the study, the expectations of the investigator, and the nature of the

data. In some cases, a CD of 0.662 may be sufficient to support

meaningful and useful conclusions, while, in other cases, a higher level

of model fit may be required. Ultimately, the decision on the

acceptability of a coefficient of determination should be based on a

comprehensive assessment of the context and the usefulness of the

model for the specific purposes of the research (Novales, 2010).

Table 4. Inverse relationship of pH and organic matter in clay soils.

Source: Author

0 5 10 15 20 25 30 35 40 45

Café

Pastos

Plantas Medicinales PB

Citricos

Programa forestal (balsa y guadua)

Programa forestal

Platano estudio

Estudio de pasto

Plantas medicinales PA

Pastos 2

Organic Matter (%) PH

April - June vol. 1. Num. 21 - 2024

Table 5. Inverse relationship between pH and organic matter in loam

soils.

Table 6. Inverse relationship between pH and organic matter in sandy

loam soils.

Source: Author

The great part of soils according to their topography turned out to be

clayey; according to Astorga (2018): in Ecuadorian Amazonian soils, a

0 5 10 15 20 25

Bio Abonos

Café Viejo

Investigacion café

Organic Matter (%) PH

0 5 10 15 20 25 30 35

Cacao

Huerta de platanos

Sajinos

Organic Matter (%) PH

The role of organic matter in the acidification of agricultural soils in Ecuadorian Amazonia

clay texture predominates. This texture is characterized by having a

high proportion of clay particles compared to sand and silt particles.

Clay is a mineral fraction of the soil with extremely small particles and

a high water and nutrient retention capacity. In Amazonian soils, the

presence of clay contributes to the high fertility of the soil and its ability

to retain moisture, which is fundamental to the diversity and lush

vegetation of the region (Astorga et al., 2018)..

Naturally, organic matter (OM) is integrated into the soil through the

decomposition of organic plant residues, concentrating predominantly

in the surface layers and decreasing in depth. Scientific literature

indicates that the presence of high levels of OM is associated with a

more acidic soil pH, although this behavior may vary depending on the

original soil material. MO and pH are crucial indicators of soil fertility

and health, underscoring the importance of ensuring that these attributes

are at adequate levels and at the optimum depth to support plant growth.

This study focused on determining the MO content and pH in a specific

agricultural soil. The results revealed a high concentration of OM

throughout the soil and a neutral pH, which is beneficial for a wide

variety of crops. An inverse relationship was observed between these

properties, where higher levels of OM were associated with lower pH,

especially in Amazonian soils, due to the anion richness of the original

soil material (Mora et al., 2016).

4. Conclusions

This study has provided significant evidence of the influence of organic

matter on the pH of Amazonian soils. With a coefficient of

determination of 0.662, we have shown that approximately 66.2% of

the variability in soil pH can be explained by organic matter.

Furthermore, we have found that the relationship between these

parameters is inversely proportional, suggesting that as organic matter

increases, soil pH tends to decrease, following a potential regression.

This relationship is especially relevant in the context of Amazonian

soils, where a clay texture predominates. The presence of clay in these

soils not only influences water and nutrient retention, but can also

modulate the influence of organic matter on pH. These findings

underscore the importance of considering the interaction between

organic matter, soil pH and soil texture in the management and

conservation of Amazonian soils, highlighting the need for

management strategies that promote an adequate balance of these

April - June vol. 1. Num. 21 - 2024

factors to maintain ecosystem health and agricultural productivity in the

region.

References

Aguilar, S., & Yuleysi, S. (2019). Validation of the calcination

method in the determination of soil organic matter content.

Universidad Nacional Agraria La Molina, Lima.

https://repositorio.lamolina.edu.pe/bitstream/handle/20.500.12

996/4154/aguilar-silva-sumiry-

yuleysi.pdf?sequence=1&isAllowed=y.

https://repositorio.lamolina.edu.pe/bitstream/handle/20.500.12

996/4154/aguilar-silva-sumiry-

yuleysi.pdf?sequence=1&isAllowed=y

Bismarck, R., Corrales, M., & Espinoza, A. (2014). Guía Muestreo de

Suelos. Water and Soil for Agriculture, 1, 19-33.

https://doi.org/efaidnbmnnnibpcajpcglclefindmkaj/https://repo

sitorio.una.edu.ni/3613/1/P33M539.pdf.

Food and Agriculture Organization of the United Nations (2020). Soil

testing methods. Rome: FAO.

efaidnbmnnnibpcajpcglclefindmkaj/https://www.fao.org/3/ca2

796en/CA2796EN.pdf

Frias, M., Fernandez, J., & Sordo, C. (2010). Estadistica. University

of Cantabria:

https://ocw.unican.es/pluginfile.php/2010/course/section/1746/

tema_02.pdf

Ibañez, J. (2007). How much does a square meter of arable soil layer

weigh: What is Apparent Density (by Régulo León Arteta). An

invisible universe under our feet.

https://doi.org/https://www.madrimasd.org/blogs/universo/200

7/05/16/65688

Lal, R. (2015). Restoring Soil Quality to Mitigate Soil Degradation.

Sustainability, 7(5), 5875-5895.

https://doi.org/https://doi.org/10.3390/su7055875

Martínez, J., Duval, M., López, F., Iglesias, J., & Galantini, J. (2017).

Adjustments in organic carbon estimation by the Calcination

method in molisols of southwestern Buenos Aires. Asociacion

The role of organic matter in the acidification of agricultural soils in Ecuadorian Amazonia

Argentina de las Ciencias del Soil.

https://doi.org/https://www.suelos.org.ar/publicaciones/v35n1-

html/vol35-n1-html/v35n1a16.htm.

Novales, A. (2010). Regression Analysis. Department of Quantitative

Economics. Universidad Complutense:

https://www.ucm.es/data/cont/docs/518-2013-11-13-

Analisis%20de%20Regresion.pdf

Queiroz , A., Wada , M., Perillo, F., Pelissari, C., & Trierveiler, M.

(2021). Effects of serum-free culture media on human apical

papilla cells properties. Archives of oral biology,(104962),

121. https://doi.org/10.1016/j.archoralbio.2020.104962.

Toledo, M. (2016). Manejo de suelos ácidos de las zonas altas de

honduras conceptos y métodos (Vol. 1). Honduras: Dirección

de Ciencia y Tecnología Agropecuaria (DICTA).

https://doi.org/https://repositorio.iica.int/bitstream/11324/3108

/1/BVE17069071e.pdf