ASSESSMENT OF SOIL CHEMICAL PROPERTIES OF GLYPHOSATE

APPLIED SOIL

CLAIRE JANE REMERATA LAGURA

UNDERGRADUATE THESIS PROPOSAL SUBMITTED TO THE FACULTY

OF THE DEPARTMENT OF SOIL SCIENCE, COLLEGE OF
AGRICULTURE, CENTRAL MINDANAO UNIVERSITY, IN
PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE DEGREE

BACHELOR OF SCIENCE IN AGRICULTURE

Soil Science

MAY 2024
 Republic of the Philippines
CENTRAL MINDANAO UNIVERSITY
University Town, Musuan, Maramag, Bukidnon

College of Agriculture
Department of Soil Science

APPROVAL SHEET

The undergraduate thesis proposal attached hereto entitled, “ASSESSMENT

OF SOIL CHEMICAL PROPERTIES OF GLYPHOSATE APPLIED SOIL ”, prepared
and submitted by CLAIRE JANE REMERATA LAGURA, in partial fulfilment of the
requirements for the degree of Bachelor of Science in Agriculture (Soil Science), is
hereby endorsed.

JUNESA U. CRISTOBAL, PhD ______________

Chair, Thesis Advisory Committee Date

ALLAN G. OCTAT, PhD ______________

Member, Thesis Advisory Committee Date

RAUL M. EBUÑA, MS ______________

Member, Thesis Advisory Committee Date

Recommending Approval:

MYRNA G. PABIONA, PhD ______________

Department Chair Date

RONLEY C. CANATOY, PhD ______________

Research Coordinator Date

Approved:

JUDITH D. INTONG, PhD ______________

College Dean Date

Noted:

JUPITER V. CASAS, PhD ______________

Director for Research Date
 BIOGRAPHICAL SKETCH

Claire Jane R. Lagura was born on January 14, 2002, in Bagontaas,

Valencia City, Bukidnon. She was the second from the eldest among the four
siblings of Mr. John Paul O. Lagura and Mrs. Christine R. Lagura. She cur-
rently lives in Purok 9A Dabongdabong, Mailag, Valencia City, Bukidnon. The
author finished her primary education at Valencia City Central School in the
SY 2013-2014, and her junior and senior high school at San Agustin Institute
of Technology, where she graduated with honors in the SY 2018-2019. During
her senior high school years, she was elected as the SSG Treasurer and pur-
sued the Academic track: Humanities and Social Sciences (HUMSS) to com-
plete her high school education. In her tertiary education, she took up a Bach-
elor of Science in Agriculture majoring in Soil Science at Central Mindanao
University (CMU). One of the extracurricular activities undertaken by the au-
thor was serving as the general secretary of the Central Mindanao University
Soil Science Society (CMUSSS) organization.

iii
 ACKNOWLEDGMENT

The author would like to express her gratitude first to the Almighty God
for His greatness and for being her constant strength, source of knowledge,
wisdom, and inspiration throughout her journey. Without Him, everything is
impossible to attain. By His blessings and mercy, He sent people to help the
author make her thesis a success.
The author would likely thank her family, who consistently support,
motivate, and encourage her to finish and overcome the challenges faced
along the way.
To her thesis adviser, Dr. Junesa U. Cristobal, the author genuinely
thanks her for the guidance, support, and patience throughout the thesis
process. The author also extends sincere gratitude to her thesis advisory
committee members, Prof. Raul M. Ebuna and Dr. Allan G. Octat, for their
invaluable feedback, insights, and encouragement in shaping the direction
and scope of this study. Also, to all the faculty of the Soil Science Department
and the staff of the SPAL especially Kuya Troy, Kuya Daniel, Ate Jacqueline,
and Sir Raineer, the author genuinely thanks them for the guidance and
assistance on her laboratory analysis.
The author would also like to thank her friends Shella Grace Coyme,
Aiza Lanayan, and Jilla Han-awon, who put so much effort into helping the
author. To the author's thesis buddy, Andrea Sarong, the author thanks her
for the help during soil sampling. The author also expresses her gratitude to
her partner in crime, Mark, who constantly supports, and encourages the
author to accomplish all the things needed to be done and gives time and
effort to accompany her during sampling days, the author expresses her
gratitude.
To the people who were not mentioned but still contributed to the au-
thor's study success, she would like to express her genuinely thanks for the
effort and time. The author thanks you all.

IV
CLAIRE JANE REMERATA LAGURA

ABSTRACT

ASSESSMENT OF SOIL CHEMICAL PROPERTIES OF

GLYPHOSATE APPLIED SOIL by CLAIRE JANE REMERATA LAGURA,
Bachelor of Science in Agriculture (Soil Science), Central Mindanao
University, University Town, Musuan, Maramag, Bukidnon, May 2024

Adviser: Junesa U. Cristobal, PhD

The study on the assessment of the chemical properties of glyphosate

applied to soil on selected corn farms in Cabanglasan Bukidnon aimed to (1)
determine the chemical properties of soil applied with glyphosate herbicide;
(2) correlate the chemical properties to the years of glyphosate herbicide
application; and (3) recommend rate of fertilizer to sampled corn farms.
A total of 30 sampled corn farms were categorized into three groups
based on the years of glyphosate herbicide application: Short-term (1-5
years), Medium-term (6-10 years), and Long-term (11-15 years). The study
found that long-term application of glyphosate led to an increase in soil pH
and extractable phosphorus levels. Pearson's correlation analysis indicated a
decline in soil properties such as organic matter content and exchangeable
potassium over the years. However, it was noted that the decline in these
properties could not be solely attributed to the long-term application of
glyphosate.
Based on the analysis result, the recommended rate ranges from 80-
100, 0-90, and 0-40 (N-P2O5-K2O kg ha-1). These recommendations translated
to the need for varying amounts in bags of urea, ammonium phosphate, and
muriate of potash.

V
Keywords: Extractable phosphorus, Glyphosate, Soil pH

TABLE OF CONTENTS

PAGE
INTRODUCTION

Objectives of the Study 2

Scope and Limitation of the Study 2

REVIEW OF LITERATURE

Corn 3
Soil Requirement of Corn 3
Glyphosate 4
Chemical Properties of Soil 4
Soil pH 4
Organic Matter Content 5
Extractable Phosphorus 5
Exchangeable Potassium 6
Effects of Glyphosate to Soil 6
Effects of Glyphosate on Soil Chemical Properties 7
Soil pH 7
Organic Matter Content 8
Extractable Phosphorus 8
Exchangeable Potassium 9
Standard Soil Test Value 10
Recommendation Rate of Corn

MATERIALS AND METHODS

Place and Duration of the Study 12

IERC Permit and Entry Protocols 12
Farm Selection Criteria 12
Collection and Preparation of Samples 13
Materials and Equipment 13
Description of Study Area 15
Soil Analysis 17
Methods of Soil Chemical Analysis 17
Soil pH Determination 17
Organic Matter Determination 18
Extractable Phosphorus Determination 18
Exchangeable Potassium Determination 19
Nutrient Fertility Evaluation 20

VI
 Recommended Rate Determination 20
Data to be Gathered 21
Statistical Analysis 21

PAGE
RESULTS AND DISCUSSIONS 22

Soil Chemical Properties 22

Soil pH 22
Organic Matter Content 25
Extractable Phosphorus 28
Exchangeable Potassium 32
Fertilizer Recommendation 35

SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 37

LITERATURE CITED 39

APPENDICES 44

VII
 LIST OF TABLES

PAGE
1 Soil test values of the chemical properties of the soil for
corn 10

2 Nutrient recommendation rate of corn 11

3 Category of sample farms in the study 13

4 Methods used for soil analysis 18

5 Soil pH of the selected corn farms in Cabanglasan,

Bukidnon 23

6 Organic matter content of sampled soils of selected

corn farms in Cabanglasan, Bukidnon 26

7 Extractable phosphorus of sampled soils of selected

corn farms in Cabanglasan, Bukidnon 29

8 Exchangeable potassium of sampled soils of selected

corn farms in Cabanglasan, Bukidnon 33

9 Recommended rate of fertilizer of the selected corn

farms in Cabanglasan, Bukidnon 36

VIII
 LIST OF FIGURES

PAGE
1 Sampled area map of Cabanglasan, Bukidnon 14

2 Location map of Cabanglasan Bukidnon 16

3 Scatterplot of the correlation of soil pH and years of

glyphosate herbicide application of selected corn farms
in Cabanglasan, Bukidnon 24

4 Scatterplot of the correlation of organic matter content

and years of glyphosate herbicide application of
selected corn farms in Cabanglasan, Bukidnon 27

5 Scatterplot of the correlation of extractable phosphorus

and years of glyphosate herbicide application of
selected corn farms in Cabanglasan, Bukidnon 30

6 Scatterplot of the correlation of exchangeable

potassium and years of glyphosate herbicide
application of selected corn farms in Cabanglasan,
Bukidnon 34

IX
 LIST OF APPENDICES

PAGE
1 Informed consent form 53

2 Farmer’s survey questionnaire 54

X
 LIST OF APPENDIX TABLES

PAGE
1 Soil pH of the selected corn farms in Cabanglasan,
Bukidnon 45

2 Soil pH standard values for corn 45

3 Organic matter content (%) of selected corn farms in

Cabanglasan, Bukidnon 46

4 Organic matter content (%) standard values for corn 46

5 Extractable phosphorus (mg kg-1) of selected corn farms

in Cabanglasan, Bukidnon 47

6 Extractable phosphorus (mg kg-1) standard values for

corn 47

7 Exchangeable potassium (cmol kg-1) of selected corn

farms in Cabanglasan, Bukidnon 48

8 Exchangeable potassium (cmol kg-1) standard values

for corn 48

9 Pearson’s correlation of soil chemical properties and

years of glyphosate application 49

10 Summary of survey information for short-term category 50

11 Summary of survey information for medium-term

category 51

12 Summary of survey information for long-term category 52

XI
 LIST OF APPENDIX FIGURES

PAGE
1 Preliminary survey with corn farmers 55

2 Collection of soil samples 55

3 Preparation of soil samples 56

XII
 1

INTRODUCTION

Corn (Zea mays L.) ranks second to rice, not only in terms of are
devoted to its production but being a staple food of about 20% of total
population of the Philippines. Corn has also been used as an important
ingredient in animal feeds. Corn is the most sensitive to drought at time of silk
emergence when the flowers are ready for pollination. Maintenance of high
yield crops under intensive cultivation is possible only by the used fertilizers
(Ayoola & Makinde, 2009).
Glyphosate, or N-(phosphonomethyl)glycine, is a broad-spectrum,
nonselective and post-emergence herbicide, used as an active ingredient in
several weed killing products since 1970. Due to its effectiveness against
wide variety of plants, glyphosate has been nominated as the once-in-a-
century herbicide, and currently, it is one of the most commonly used
herbicides in agricultural and non-agricultural cultivation systems in developed
countries (Duke et al., 2008). When ending up in the soil, glyphosate is
quickly adsorbed to soil particles and has a low probability of leaching along
with surface waters or downwards into the soil profile (Helander et al., 2012).
Glyphosate is also vulnerable to microbial degradation and its main
degradation product, aminomethylphosphonic acid (AMPA), is strongly
adsorbed to soil solids.
This study will be beneficial to farmers on the effects of glyphosate
herbicide to soil fertility for corn crops regardless of the varieties. Through this
study, farmers will gain additional information regarding on the possible
impacts of glyphosate herbicide on the soil quality. Moreover, this will help
future researchers by providing them insights on this topic and serve as a
reference for further studies.
 2

Objectives of the Study

Generally, the study aims to assess the soil chemical properties of

glyphosate applied soil.
Specifically, it aimed to:
1. determine the chemical properties of soil applied with glyphosate
herbicide;
2. correlate the chemical properties to the years of glyphosate
herbicide application; and
3. recommend rate of fertilizer to sampled corn farms.

Scope and Limitation of the Study

The study was focused only on the assessment of chemical properties

(such as soil pH, organic matter content, extractable P, and exchangeable K)
of glyphosate applied soil on selected corn farms in Cabanglasan, Bukidnon.
 3

REVIEW OF LITERATURE

Corn

Corn is one of the first cereal cultivated in the world, before rice and
wheat thanks to its very high productivity. Corn crops can be classified in two
main groups according to their latitude. Corn fields located between equator
and 30° of latitude are considered as tropical corn crop and those located at
higher latitudes are designated as tempered corn (OGTR, 2008). In the
Philippines, corn is considered as the second most important crop. About 14
million Filipinos prefer white corn as their main staple and yellow corn
accounts for about 50% of livestock mixed feeds. Corn is a crop that has a
short life cycle and requires warm weather, suitable apprehension, and
management. It is valuable animal feed, human food, and raw material for a
number of industries (Hiruy & Getu, 2020). Globally, corn is one of the major
vegetables especially because of its high demand and cash value (Borres,
2018). Production of corn was found to be a very impressive source of living
for farmers. Mainly because corn is easier to grow, saves labor, and has
higher market demand for food than growing for grain (Tampus, 2019).

Soil Requirement of Corn

Corn needs loose soils that offer proper aeration and drainage and at
the same time maintain sufficient amounts of water close to roots. The ideal
soil for growing corn is well-drained, preferably a sandy loam. In general, the
plant prefers pH levels higher than 5.5. More specifically, the optimum pH for
corn is between 5.8 and 6.8. Having pH levels close to 5 may reduce
production up to 35%. Corn is slightly sensitive to increased salinity levels
(Torres, 2012). Organic matter such as compost, leaves and grass clippings
can be added to soil to improve its overall quality and drainage, particularly for
heavy clay soil (Delp, 2018).
 4

Glyphosate

Glyphosate is the most extensively used herbicide in the world. It easily

reaches water bodies through surface runoff waters, and this affects
photosynthetic microorganism communities that are often the foundation of
the functioning of aquatic ecosystems. Hence, monitoring the reactions of
microorganisms to glyphosate is an important element of environmental
management (Gonzales et al., 2019). Glyphosate is a non-selective herbicide;
it will kill the majority of plants. It will prevent the certain proteins that are
essential for plant growth that being produced by the plants. The shikimic
acid enzyme route is the one that glyphosate blocks. For plants and some
microbes, the shikimic acid pathway is essential (Cros & Jenkins, 2010). It is
one of the most often used herbicides, has applications in agriculture, forestry,
industrial weed management, grass, gardens, and aquatic environments. The
crops with the most glyphosate use include soybeans, field corn, pasture, and
hay (Covaci, 2014). Using glyphosate-based herbicides in agriculture is
advantageous for farmers, consumers, and the environment. A secure food
supply, efficient weed management, and environmental sustainability all
depend on it. Around the world, weeds, pests, and disease cause farmers to
lose 30–40% of their crops. Without using crop protection methods, it makes
sense that farmers have come to rely on reliable instruments year after year.
A tried-and-true method for preventing damaging weeds from consuming their
crops is to apply glyphosate to fields, especially with glyphosate-tolerant crops
(Baer, 2015).

Chemical Properties of the Soil

Soil pH

Soil pH is a measure of the acidity or alkalinity of a soil. It is a master

variable in soils because it controls many chemical and biochemical
 5

processes operating within the soil. The study of soil pH is very important in
agriculture since soil pH regulates plant nutrient availability by controlling the
chemical forms of the different nutrients and influences their chemical
reactions. As a result, soil and crop productivities are linked to soil pH value.
Though soil pH generally ranges from 1 to 14, the optimum range for most
agricultural crops is between 5.5 and 7.5. However, some crops have adapted
to thrive at soil pH values outside this optimum range (Oshunsanya, 2018).

Organic Matter Content

Soil organic matter is closely linked to soil fertility. It improves soil

structure, porosity, and water-holding capacity, promoting root growth, nutrient
absorption, and overall plant health. The presence of organic matter
enhances the cation exchange capacity of soils, which influences nutrient
availability. Higher organic matter content often correlates with improved soil
fertility and increased crop productivity (Sainju et al., 2018). A review article
by Lal (2015) extensively discusses the importance of soil organic matter for
soil health and sustainable agriculture. The review highlights the positive
effects of organic matter on soil physical, chemical, and biological properties,
emphasizing its role in enhancing soil fertility, nutrient cycling, and carbon
sequestration. It emphasizes the significance of maintaining and increasing
organic matter content in agricultural soils.

Extractable Phosphorus

Phosphorus (P) is an essential nutrient for plant growth and plays a

critical role in soil fertility and crop production. Phosphorus is involved in
various physiological and biochemical processes in plants, including energy
transfer, photosynthesis, respiration, and nutrient transportation. It is a
component of nucleic acids, ATP (adenosine triphosphate), and other
important molecules required for plant growth and development. A study by
 6

Sharma et al. (2013) demonstrated the crucial role of phosphorus in plant

growth and yield. They found that phosphorus deficiency significantly reduced
plant height, biomass production, and grain yield in wheat (Triticum aestivum
L.). The study concluded that adequate phosphorus availability is essential for
optimal plant growth and crop productivity.

Exchangeable Potassium

Potassium (K) is one of the essential nutrients in plants and in recent

years is one of three (including nitrogen and phosphorus) that are commonly
in sufficiently short supply in the soil to limit crop yields on many soil types in
Western Australia. Potassium is commonly found in plants at levels above all
other macro nutrients except carbon, oxygen, hydrogen and occasionally
nitrogen. Potassium has many functions including the regulation of the
opening and closing of stomata which are the breathing holes found on plant
leaves and therefore regulate moisture loss from the plant. For this reason,
potassium is known as poor-mans irrigation because it helps crops finish
better. Potassium is held in the soil by the cation exchange capacity. Soils
with finer particles, such as clay, and organic matter, are able to hold more
positively charged ions than soils with larger particles, such as sand (Reicks,
2021).

Effects of Glyphosate to Soil

Glyphosate (N-(phosphono methyl) glycine; C3H8NO5P), a highly

efficient broad-spectrum and non-selective herbicide, has been widely used in
agriculture, horticulture, parks, and domestic gardens (Wojtaszek et al.,
2009). Its use has increased rapidly with the commercial introduction of
genetically modified corn, soybeans, and cotton; glyphosate-based herbicides
have become the most widely applied herbicide worldwide, especially on
genetically modified crops (Brookes & Barfoot, 2015). After application,
 7

herbicides may evaporate (volatilize) and washed away through surface run-
off or leached into deep soil strata and ground water, they may be inactivated
by plants or adsorbed in soil and become subjected to chemical degradation
(Kortekamp, 2011). Effects of glyphosate residues in soil when it is applied as
a spray in ecological restoration, a situation where the common spray
application technology has a risk of high herbicide delivery rate, regardless of
whether the concentration used conforms to the label recommendation or not.
High delivery volumes will result in run-off from leaves to soil operator error
delivering excessive dose rates appears to present the real problem (Cornish
& Burgin, 2010). Glyphosate use in agricultural land has effect on
environmental and ecological to loosen the soil and for favorable seed bed,
severe erosion and other additional land degradation, in addition the rate of
glyphosate application might be not enough for the weed control on the
farmer’s field of those farmers also might have less awareness, less technical
skill and not convinced about the effectiveness of the herbicide in northern
Ethiopia (Teamti & Tesfay, 2016).

Effects of Glyphosate on Soil Chemical Properties

Soil pH

Soil pH plays an important role in soil fertility as it influences the

availability of essential nutrients for plant uptake (Kabir et al., 2017). Weed
management through glyphosate herbicide application can potentially affect
the soil pH level. Field assessment conducted by Nigussie et al. (2019) has
shown that soil pH declined from the control to under dose, recommended
dose, and overdoses, implying that applying glyphosate to cropland will cause
the soil’s acidity to rise. Oladele and Ayodele (2017) presented that there was
a decrease of soil pH level in experimental plot with glyphosate treatment.
The effects of glyphosate on soil pH in relation to corn cultivation can vary
depending on several factors, such as the initial pH of the soil, glyphosate
application rate, and soil type. While some studies have reported slight
 8

decreases in soil pH following glyphosate application, the extent of pH

alteration may not always be significant or consistent across all situations
(Annett et al., 2014).

Organic Matter Content

Organic matter serves as a reservoir of nutrients and water in the soil,

aids in reducing compaction and surface crusting, and increases water
infiltration into the soil. Through application of glyphosate, it can have
potential effects on the organic matter content of soil in corn cultivation. The
impact of glyphosate on soil organic matter can vary depending on factors
such as application rate, timing, soil type, and management practices
(Shushkova et al., 2010). Glyphosate has been found to affect the
decomposition of organic matter in the soil. Research suggests that
glyphosate can potentially inhibit the activity of soil microorganisms
responsible for organic matter decomposition, leading to a slower breakdown
of organic residues (Druille et al., 2013). This can result in the accumulation of
organic matter in the soil over time.

Extractable Phosphorus

The effects of glyphosate on phosphorus (P) in soil in relation to corn

cultivation have been studied, and it is generally observed that glyphosate
does not directly influence soil phosphorus availability (Carpenter et al.,
2018). Glyphosate primarily affects weeds by inhibiting the shikimate
pathway, which is not involved in phosphorus metabolism. However,
glyphosate can indirectly impact soil phosphorus dynamics through its effects
on weed control and root exudation patterns. By reducing weed competition,
glyphosate can enhance nutrient uptake efficiency, including phosphorus, by
corn plants (Duke et al., 2008). This can potentially result in increased
phosphorus availability to corn. In an evaluation of phosphorus dynamics in
 9

nopal plot with a 5-year history of glyphosate application, it was revealed that
the herbicide usage has significantly increased the concentration of total
phosphorus and hydrogen phosphate (HPO42-) (Chávez-Ortiz et al., 2022).

Exchangeable Potassium

Potassium (K) is an essential nutrient for plant growth. It’s classified as

a macronutrient because plants take up large quantities of K during their life
cycle. Glyphosate is a widely used herbicide, and its effects on soil properties,
including potassium (K) availability. Glyphosate application may enhance the
fixation of potassium in soils, leading to reduced potassium availability for
plants. Glyphosate can form complexes with soil minerals, such as clays and
iron oxides, which can bind potassium ions and reduce their mobility and
availability in the soil solution (Van Ginkel et al., 2020). The amount of
exchangeable potassium (K) in four different soil location that is treated with
glyphosate with different rates of dosage (under dose, recommended rate,
and over dose) have shown increasing result as dosage of herbicide is
increasing (Nigussie et al., 2019). As stated by Lane (2011) exchangeable K
in studied soils significantly increased with glyphosate treatment, it was also
stated that the phenomenon is due to K salt content in the formulation of
glyphosate.

Standard Soil Test Value

Soil samples are analyzed in laboratories to measure the quantitative

value for its properties. Quantitative data obtained from soil analysis are
interpreted following the standard soil test value for a certain land utilization or
crop production. Results of soil analysis which will be interpreted using
standard soil test value will be a baseline for fertilizer recommendations as
well as the evaluation of soil quality status (chemical and physical properties).
Analytical data from soil analysis does not indicate the exact amount of
 10

nutrients present in soil for crop utilization instead it is only through

interpretation that will provide the description of soil fertility status (Nafiu et al.,
2012). Standard soil test value varies depending on a crop since different
crops have variations in soil and nutrient requirements according to its
production. The standard soil test values for chemical properties of corn are
presented in Table 1.

PROPERTY VALUE CATEGORY REFERENCE

<4.5 Extremely Acid
4.5-5.0 Very Strongly Acidic
Soil Survey
Soil pH 5.1-5.5 Strongly Acidic
Staff,1993
5.6-6.0 Medium Acidic
6.1-6.5 Slightly Acidic
<2.0 Deficient
Organic Matter,
2.1-4.5 Marginal PCARRD, 1998
%
>4.5 Adequate
<4.1 Very Deficient
Extractable P, 4.2-8.1 Deficient Crizaldo, 1981
-1
mg kg 8.2-16.3 Slightly Deficient as cited by
16.4-20.5 Adequate Duque, 1990
>20.5 Highly Adequate
<0.25 Low
Exchangeable K,
0.25-0.50 Medium Landon, 1984
cmol kg-1
>0.50 High
Table 1. Soil test values of the chemical properties of the soil for corn

Recommendation Rate of Corn

Soil analysis results are used to determine the specific fertilizer

requirements for a particular field or crop. The analysis results of nitrogen (N),
phosphorus (P), and potassium (K) are utilized to determine the appropriate
rate of fertilizers. In cases where nitrogen is not analyzed, the organic matter
(OM) result can be employed, with a multiplication factor of 0.05 applied to
derive the recommended nitrogen fertilizer rate. By interpreting soil analysis
results, farmers can determine the nutrient deficiencies or excesses in the soil
and adjust fertilizer applications accordingly to optimize nutrient availability for
plant uptake (Smith, 2022). The recommended rate of fertilizers depends on
 11

various factors such as soil nutrient levels and the type of crop. The rates of
the recommended rate of fertilizers of corn are shown in Table 2.

Table 2. Nutrient recommendation rate of corn

STV N (kgs/ha)
Analysis
(%) Hybrid Native
<1.5 100-120 50-60
1.5-2.5 80-100 40-50
Organic Matter,
2.5-4.0 60-80 30-40
%
4.0-5.0 40-60 20-30
>5.0 20-40 10-20
STV P2O5 (kgs/ha)
Analysis
(ppm) Hybrid Native
0-5 90-120 50-60
5-14 60-90 40-50
Extractable P,
14-20 30-60 30-40
mg kg-1
20-30 0-30 20-30
>30 0 0
STV K2O (kgs/ha)
Analysis
(ppm) Hybrid Native
0-20 100-120 60
20-40 80-100 45
40-60 60-80 30
Exchangeable K,
60-80 40-60 20
cmol kg-1
80-119 20-40 15
119-150 0-20 0
>150 0 0
Source:
 12

MATERIALS AND METHODS

Place and Duration of the Study

A field survey was conducted to assess the chemical properties of soil

on the selected corn farms in Cabanglasan, Bukidnon. The soil collected
during the survey was analyzed at the Soil and Plant Analysis Laboratory
(SPAL), at Central Mindanao University, University Town, Musuan, Maramag
Bukidnon. The study commences last November 2023 and was completed on
February 2024.

IERC Permit and Entry Protocols

Prior to the conduct of preliminary survey, a permit from the

University’s Institutional Ethics Review Committee (IERC) was acquired.
Appropriate authorities were formally informed through communication letters
prior to the start of the study for permission. Farm owners of the selected corn
fields in Cabanglasan, Bukidnon was also communicated for the conduct of
the said study.

Farm Selection Criteria

The criteria for the selection of sample farms used in the study were
based on the number of years of glyphosate herbicide application in corn
production without applying lime, the farm size was not less than 1 hectare,
and the topography was flat. The number of years of glyphosate application
was categorized into three namely: Short-term – this includes sample farms
that were applied with glyphosate herbicide for 1-5 years; Medium term – this
 13

includes sample farms that were applied with glyphosate for 6-10 years; and
Long term – were sample farms that were applied with glyphosate herbicide
for 11-15 years. Table 3 presented the different categories of sample farms.
Soil samples were collected from the selected sample farms and chosen
according to the established criteria. Thirty (30) sample farms were selected
based on the criteria and were limited to ten (10) farms in each category.
These farms are all situated within Cabanglasan, Bukidnon.

NUMBER OF
CATEGORY DESCRIPTION
SAMPLES
Short-term 10 1-5 Years of Glyphosate Application
Medium-term 10 6-10 Years of Glyphosate Application
Long -term 10 11-15 Years of Glyphosate Application
Table 3. Category of sample farms in the study

Collection and Preparation of Samples

The collection of soil samples happened after conducting the

preliminary survey. The sampling site was categorized into three (3) terms
according to the number of years of glyphosate application namely: short-term
with 1-5 years of glyphosate application; medium-term with 6-10 years of
glyphosate application; and long-term application with 11-15 years of
glyphosate application. The soil samples were collected in 30 different corn
farm areas that fit the criteria of the study. The collection of soil samples
followed a zigzag pattern with 15-20 subsamples at a depth of 15-20 cm, and
then air dried for seven (7) days. The air-dried samples are then pulverized
using a wooden mallet and passed through a 2 mm sieve. The sieved soil was
stored in a ziplock bag for the determination of the chemical properties of the
soil.

Materials and Equipment

 14

Soil collection and preparation involved the use of various materials

including a shovel, bolo, ziplock bags, record book, ballpen, marker, labeling
tape, wooden mallet, and sacks. For soil analysis, laboratory equipment, a
calculator, and complete personal protective equipment (PPE) such as lab
gown, gloves, and closed shoes were utilized.

Figure 1. Sampled
 15

Description of the Study Area

The study was conducted in the corn farms of Cabanglasan, Bukidnon

to assess the chemical properties of soil. Cabanglasan is bordered by
Malaybalay City to the north and west, by the town of San Fernando to the
south, and by Agusan del Sur province to the east. The municipality covers a
land area of 243.30 square kilometers or 93.94 square miles, accounting for
2.32% of Bukidnon's total area. According to the 2020 Census, its population
was 36,286, representing 2.35% of Bukidnon's total population and 0.72% of
the overall population of the Northern Mindanao region. Nestled in the
highlands of Bukidnon, Cabanglasan is a municipality that boasts stunning
vistas and a tranquil atmosphere. The town is known for its agricultural
activities, with the cultivation of crops such as vegetables, fruits, and corn.
The location map of the municipality of Cabanglasan, Bukidnon is illustrated in
figure 2.
16
 17

Figure 2. Location map of Cabanglasan Bukidnon

 18

Soil Analysis

The laboratory analysis for soil chemical properties such as soil pH,
organic matter content (OM), extractable phosphorus (P), and exchangeable
potassium (K) was analyzed at the Soil and Plant Analysis Laboratory (SPAL),
at Central Mindanao University, University Town, Musuan, Maramag
Bukidnon. The analysis for soil chemical properties was determined using the
methods presented in Table 4.

Table 4. Methods used for soil analysis

PARAMETERS METHODS OF ANALYSIS REFERENCE
Chemical Properties
Potentiometric Method Biddle, 1997
pH
(1:5 soil: water)
Walkley and Black FAO, 2021
Organic Matter, %
Method
Extractable P Bray P2 Method FAO, 2022
Flame Photometer FAO, 2022
Exchangeable K
Method

Methods of Soil Chemical Analysis

Soil pH Determination

The soil pH was determined using the Potentiometric Method

described by Biddle (1997). Briefly, a 5.00 g soil sample was placed in a
centrifuge tube, 25 mL distilled water was then added and shaken for 1 hour.
Measurements were then taken after the display was stable. The calibration
procedure was then repeated, and the results were recorded into the record
book. The pH was read using a calibrated pH meter.

Organic Matter Content Determination

 19

The organic matter content was determined using Walkley and Black
Method described by FAO (2021). A 0.25 g soil sample was placed into a
125-mL Erlenmeyer flask, was slowly added with 2.50 mL of 1.0 N potassium
dichromate (K2Cr2O7) while mixing by rotating the flask. A 5.00 mL
concentrated 18.0 M sulfuric acid (H2SO4) was then added, mixed and allowed
to react for 30 minutes. The reaction mixture was diluted with 50.0 mL distilled
water. The solution was added with 3-4 drops of phenanthroline (Ferroin)
indicator and was titrated with 0.5 N ferrous ammonium sulfate [Fe(NH 4)2SO4]
until the initial color of the solution turned dark red. The percent organic
matter content was calculated using the formula:

V 100
%O.M= ( S-T ) × 0.0069 ×
S wt. of soil

Where:
V - volume of 1 N potassium dichromate (K2Cr2O7) used
S - volume (mL) of 0.5 N ferrous ammonium sulfate
[Fe(NH4)2SO4] required for blank
T- volume (mL) of 0.5 N ferrous ammonium sulfate
[Fe(NH4)2SO4] required for sample

Extractable Phosphorus Determination

The extractable phosphorus was determined using Bray P2 Method

described by FAO (2022). A 2.85 g weight of soil sample placed in an
Erlenmeyer flask was added with 20 mL of extracting solution. The prepared
solution was shaken and filtered using Whatman no. 40. A 5.00 mL aliquot of
the filtrate was transferred to 25 mL volumetric flask, will be added with 4.00
mL of reagent B, and then be diluted up to mark with distilled water. The
extracted phosphorus was measured colorimetrically based on the reaction of
 20

ammonium molybdate [NH4)6Mo7O24] and development of the molybdenum

blue color. UV-Visible Spectrophotometer was used in reading the extracted
phosphorus. The extractable phosphorus was calculated using the formula:

ppm P=reading × slope × d.f

extractant (mL) final volume
d.f. = ×
wt. of soil aliquot (mL)

Where:
Reading - determined from spectrophotometer
d.f - total dilution factor

Exchangeable Potassium Determination

The exchangeable potassium was determined using Flame Photometer

Method described by FAO (2022). A 2.00 g of soil sample was placed into a
50-mL centrifuge tube and added with 10.0 mL ammonium acetate (C 2H7NO2).
The suspension was shaken for 5 minutes. Afterwards, it was filtered using
Whatman no. 40. A 2.00 mL aliquot of the filtrate was transferred into a 10-mL
flask and was diluted to mark with water. The concentration of the extract was
determined using the flame photometer and expressed in K cmol/kg using the
following formula:

reading × d.f.
K cmol /kg=
390
extractant (mL) final volume
d.f. = ×
wt. of soil aliquot (mL)

Where:
Reading - determined from flame photometer
d.f - total dilution factor

Nutrient Fertility Evaluation

 21

The soil chemical properties were evaluated using Soil Test Value
criteria for corn (Table 1) to interpret the soil fertility status of the sampled
farms.

Recommended Rate Determination

The soil analysis results of soil chemical properties were assessed

using the Nutrient Recommendation Rate criteria for corn (Table 2) to
determine the recommended fertilizer rate for the sampled farms.

Data to be Gathered
 22

A. Soil Chemical Properties

1. pH
2. Organic Matter Content (%)
3. Extractable Phosphorus (mg kg-1)
4. Exchangeable Potassium (cmol kg-1)

B. Recommended Rate of Fertilizer

Statistical Analysis

The data obtained from the study was statistically analyzed using
correlation and regression analysis. Analysis was done using Statistical Tool
for Agricultural Research (STAR) Software.
 RESULTS AND DISCUSSIONS

Soil Chemical Properties

Soil pH

Table 5 showed the result of the soil pH of the selected corn farms in
Cabanglasan, Bukidnon ranging from 4.16 to 5.30. Within the short-term
category (1-5 years of glyphosate application), soil pH falls between 4.16 to
5.3, indicating extremely acidic to strongly acidic conditions. The soil pH value
in the medium-term category (6-10 years of glyphosate application) ranges
from 4.24 to 5.63, categorized as extremely acidic to strongly acidic. For the
long-term category (11-15 years of glyphosate application), it had a pH value
of 4.38 to 5.16, identified to have extremely acid to strongly acidic pH levels.
The soil pH values were correlated to the number of years of
glyphosate herbicide application and are illustrated in Figure 3. The trend of
the scatterplot showed an increasing values of soil pH with r=0.5381 (R 2 (%)=
12.8). Pearson’s correlation analysis between soil pH is positively correlated
with years of glyphosate herbicide application, with a value of p= 0.05198, its
correlation is significant. A positive and significant correlation means that
continuous application of glyphosate herbicide can increase the pH levels of
the soil.
Glyphosate, when applied to the soil, can undergo various chemical
reactions that may impact soil pH levels. One of the primary mechanisms
through which glyphosate can influence soil pH is by chelating essential
nutrients such as manganese, iron, and zinc. This chelation process can
inhibit the availability of these nutrients to plants, leading to nutrient
deficiencies and ultimately affecting soil pH levels (Duke & Powles, 2015).
Moreover, glyphosate can also affect soil microbial communities, disrupting
the balance of beneficial microorganisms responsible for maintaining soil pH
levels. This disruption can result in a shift towards more acidic conditions in
 24

the soil, further exacerbating acidity issues in corn production (Helander et al.,
2018).

Table 5. Soil pH of the selected corn farms in Cabanglasan, Bukidnon

YEARS OF
GLYPHOSATE FARM CODE SOIL pH CATEGORY
APPLICATION
A1 4.36 Extremely Acidic
A2 4.29 Extremely Acidic
A9 4.31 Extremely Acidic
A10 4.29 Extremely Acidic
A11 4.22 Extremely Acidic
1-5 years
A12 4.16 Extremely Acidic
A18 4.37 Extremely Acidic
A21 4.22 Extremely Acidic
A24 4.36 Extremely Acidic
A27 5.30 Strongly Acidic
A3 4.24 Extremely Acidic
A8 4.34 Extremely Acidic
A13 4.48 Extremely Acidic
A19 4.51 Very Strongly Acidic
A20 4.26 Extremely Acidic
6-10 years
A22 4.33 Extremely Acidic
A23 4.26 Extremely Acidic
A25 4.50 Very Strongly Acidic
A28 5.23 Strongly Acidic
A30 4.42 Extremely Acidic
A4 5.16 Strongly Acidic
A5 4.40 Extremely Acidic
A6 4.90 Very Strongly Acidic
A7 4.55 Very Strongly Acidic
A14 4.42 Extremely Acidic
11-15 years
A15 4.53 Very Strongly Acidic
A16 4.38 Extremely Acidic
A17 4.46 Extremely Acidic
A26 5.10 Strongly Acidic
A29 4.71 Very Strongly Acidic

Short-term use of glyphosate application in corn fields can potentially

impact soil pH and lead to increased acidity levels. This can occur due to
various mechanisms associated with glyphosate. Glyphosate can affect soil
pH by altering the microbial community in the soil, which can impact nutrient
cycling processes and the breakdown of organic matter (Bockelmann & Ciha,
 25

2016). This disruption can result in the release of acidic byproducts,

contributing to an increase in soil pH.

Figure 3. Scatterplot of the correlation of soil pH and years of glyphosate

herbicide application of selected corn farms in Cabanglasan,
Bukidnon

One study conducted by Zobiole et al. (2016) investigated the effects of

glyphosate application on soil pH in corn fields. The researchers found that
continuous use of glyphosate resulted in a gradual increase in soil pH levels
over several years. This change in pH can impact the soil's microbial
community, nutrient cycling, and overall soil health. The mechanism behind
glyphosate's impact on soil pH lies in its ability to chelate with cations in the
soil, such as calcium and magnesium. This chelation process can release
hydrogen ions, leading to a rise in soil pH levels. As a result, prolonged use of
glyphosate can contribute to soil alkalinity, which may affect nutrient
availability and crop growth.
 26

Research on the direct impact of glyphosate application on soil pH in

corn production systems may yield mixed findings. While glyphosate itself is
slightly acidic, studies have shown that its application can lead to both
increases and decreases in soil pH, depending on various factors such as soil
type, environmental conditions, and glyphosate dosage.

Organic Matter Content, %

Table 6 presented the organic matter content of sampled soils. The

organic matter content (%) for the short-term category has a value of 2.44 to
3.89, for the medium-term category, it has a value of 2.63 to 3.89, and for the
long-term category with 1.87 to 3.62. All sampled soils of selected corn farms
from the short- to medium-term category were categorized as marginal
organic matter content, whereas in the long-term category, there was one
farm has a value of 1.87 which is considered deficient in its organic matter
content.
The correlation of organic matter content (%) and years of glyphosate
herbicide application is illustrated in figure 4. The trend of the scatterplot
showed an increasing values of organic matter content (%) with r=-0.0477 (R 2
(%)= 0.228). Pearson’s correlation analysis between organic matter content
(%) is negatively correlated with years of glyphosate herbicide application, its
correlation is not significant with a value of p= 0.8023. A negative correlation
indicates that as the duration of applying glyphosate herbicide increases,
there is a decrease in organic matter content. Moreover, its negative
correlation indicates that the decline in soil organic matter concentration is not
primarily caused by glyphosate herbicide use.
Glyphosate disrupts the activity of enzymes involved in the breakdown
of plant material, which can lead to a reduction in the decomposition of crop
residues and organic matter in the soil. This disruption can result in slower
decomposition rates and accumulation of plant residues, contributing to lower
organic matter levels. Additionally, glyphosate can also affect soil microbial
 27

communities (Helander et al., 2018). Studies have shown that glyphosate

application can alter the composition and function of soil microbial
populations, which play a crucial role in decomposing organic matter and
cycling nutrients.

Table 6. Organic matter content of sampled soils of selected corn farms in

Cabanglasan, Bukidnon
YEARS OF
ORGANIC
GLYPHOSATE FARM CODE CATEGORY
MATTER (%)
APPLICATION
A1 3.12 Marginal
A2 2.60 Marginal
A9 2.76 Marginal
A10 3.55 Marginal
A11 2.57 Marginal
1-5 years
A12 2.44 Marginal
A18 3.46 Marginal
A21 3.89 Marginal
A24 2.71 Marginal
A27 2.87 Marginal
A3 3.75 Marginal
A8 2.63 Marginal
A13 3.05 Marginal
A19 3.00 Marginal
A20 3.89 Marginal
6-10 years
A22 3.12 Marginal
A23 2.9 Marginal
A25 3.37 Marginal
A28 2.69 Marginal
A30 3.68 Marginal
A4 2.63 Marginal
A5 2.96 Marginal
A6 1.87 Deficient
A7 2.12 Marginal
A14 4.00 Marginal
11-15 years
A15 3.00 Marginal
A16 3.22 Marginal
A17 3.00 Marginal
A26 2.95 Marginal
A29 3.62 Marginal

Changes in microbial activity due to glyphosate use can further impact organic
matter decomposition rates and overall soil health. Furthermore, the repeated
 28

application of glyphosate over time can lead to a decline in soil biodiversity

and beneficial soil organisms, which are essential for maintaining organic
matter levels. This reduction in diversity can hinder the natural processes that
contribute to organic matter formation and stabilization in the soil (Sehata et
al., 2013).

Figure 4. Scatterplot of the correlation of organic matter content and years of

glyphosate herbicide application of selected corn farms in
Cabanglasan, Bukidnon

A study by Zaller et al. (2014) found that glyphosate application

resulted in changes to soil microbial communities and reduced microbial
biomass, ultimately affecting organic matter decomposition rates. This
research provides empirical evidence linking glyphosate use to alterations in
soil health parameters, including organic matter content. Also, in his study, it
found that glyphosate application led to a decrease in earthworm populations,
which are important for organic matter decomposition and nutrient cycling in
 29

soil. This reduction in earthworm activity could potentially impact the organic
matter content of the soil.
Several factors besides glyphosate application can contribute to a
decline in soil organic matter. One significant factor is intensive agricultural
practices such as excessive tillage. Tillage can disrupt the soil structure,
accelerate decomposition of organic matter, and increase erosion, all of which
can reduce OM levels in the soil (Blanco-Canqui, 2017). Furthermore, the
use of synthetic fertilizers can also play a role in diminishing soil organic
matter. High levels of nitrogen fertilization can stimulate microbial activity,
leading to faster decomposition of organic matter and reducing its
accumulation in the soil (Sainju et al., 2016). Another factor to consider is the
lack of crop rotation or monoculture farming practices. Planting the same crop
continuously without rotation can deplete specific nutrients in the soil and
reduce organic matter levels over time (Lal, 2014). Moreover, factors like
urbanization, deforestation, and climate change can also contribute to the
decline in soil organic matter through increased erosion, loss of vegetation
cover, and changes in temperature and precipitation patterns (Batjes, 2017).
Research has shown that glyphosate can have both direct and indirect
effects on soil organic matter. Directly, glyphosate can inhibit the activity of
certain soil microorganisms responsible for decomposing organic matter,
leading to a decrease in organic matter content over time. Indirectly,
glyphosate can also affect plant residues, which are a major source of organic
matter in soil, by altering their decomposition rates.
The decline in soil organic matter (OM) content, as observed in this
study, can have detrimental effects on soil fertility. However, the study
findings indicate that the reduction in soil OM cannot be solely attributed to
glyphosate herbicide application. Instead, there are likely other underlying
factors contributing to the decline in soil OM levels. Therefore, it is essential to
consider various factors beyond glyphosate use when addressing reductions
in soil OM.

Extractable Phosphorus (mg kg-1)

 30

Table 7 shows the levels of extractable phosphorus (mg kg -1) of

sampled soils of selected corn farms. For the short-term category, it has an
extractable p value of 10.61 to 41.59, indicating that the p content is slightly
deficient to highly adequate. For the medium-term category, the extractable p
value ranges
Table 7. Extractable phosphorus of sampled soils of selected corn farms in
Cabanglasan, Bukidnon
YEARS OF EXTRACTABLE
GLYPHOSATE FARM CODE PHOSPHORUS CATEGORY
APPLICATION (mg kg-1)
A1 29.32 Highly Adequate
A2 27.48 Highly Adequate
A9 20.89 Highly Adequate
A10 20.61 Highly Adequate
A11 41.59 Highly Adequate
1-5 years
A12 13.37 Slightly Deficient
A18 32.22 Highly Adequate
A21 10.61 Slightly Deficient
A24 24.4 Highly Adequate
A27 31.23 Highly Adequate
A3 28.38 Highly Adequate
A8 31.64 Highly Adequate
A13 43.34 Highly Adequate
A19 21.3 Highly Adequate
A20 17.2 Adequate
6-10 years
A22 34.17 Highly Adequate
A23 44.09 Highly Adequate
A25 30.47 Highly Adequate
A28 22.19 Highly Adequate
A30 42.14 Highly Adequate
A4 42.81 Highly Adequate
A5 38.09 Highly Adequate
A6 24.82 Highly Adequate
A7 22.22 Highly Adequate
A14 33.46 Highly Adequate
11-15 years
A15 19.47 Adequate
A16 16.36 Slightly Deficient
A17 22.44 Highly Adequate
A26 31.43 Highly Adequate
A29 37.51 Highly Adequate
 31

from 17.2 to 44.09, categorized as adequate to highly adequate p content. For

the long-term category, the extractable p values range from 16.36 to 42.81,
described as slightly deficient to highly adequate p content.
Figure 5 presents the scatterplot diagram of the correlation of
extractable phosphorus and years of glyphosate herbicide application in
selected corn farms. The trend of the scatterplot showed a decreasing value
of extractable phosphorus with r= 0.1642 (R2 (%)= 2.7). Pearson's correlation
analysis

Figure 5. Scatterplot of the correlation of extractable phosphorus and years of

glyphosate herbicide application of selected corn farms in
Cabanglasan, Bukidnon

between soil extractable phosphorus and years of glyphosate herbicide

application shows a positive correlation, indicating that as the duration of
glyphosate herbicide application increases extractable phosphorus levels tend
to rise. However, this correlation was not significant (p= 0.386), suggesting
 32

that the increase in extractable phosphorus is not solely attributable to

glyphosate herbicide application.
One study by Zobiole et al. (2010) conducted in Brazil found that
continuous glyphosate application over several years resulted in higher levels
of extractable phosphorus compared to non-glyphosate-treated soils. This
increase in phosphorus availability can be attributed to the herbicidal activity
of glyphosate, which can promote the release of bound phosphorus in the soil,
making it more accessible for plant uptake.
Furthermore, meta-analysis by Helander et al. (2018) also supported
the findings that glyphosate application can influence soil phosphorus
dynamics over time. The study highlighted that the effects of glyphosate on
phosphorus availability may vary depending on factors such as soil type,
climate conditions, and application rates.
A study conducted by Braman & Hendrix (2015) investigated the
effects of various pesticides, including glyphosate, on phosphorus availability
in soil. While glyphosate did not directly affect phosphorus availability, it
influenced soil
microbial populations, which in turn impacted phosphorus mineralization and
availability.
A study by Sharpley et al. (2013) shows the several factors that can
influence the extractable phosphorus content of soil apart from glyphosate.
One significant factor is the application of phosphorus-containing fertilizers.
When farmers use fertilizers rich in phosphorus, it can lead to an increase in
the extractable phosphorus levels in the soil. Over time, repeated application
of such fertilizers can result in elevated levels of extractable phosphorus.
Another factor to consider is soil pH. Soil acidity or alkalinity can affect the
availability of phosphorus to plants. Acidic soils tend to bind phosphorus,
making it less accessible to plants, while alkaline soils may have higher
extractable phosphorus levels. Therefore, soil pH plays a crucial role in
determining the extractable phosphorus content. Furthermore, soil
management practices such as tillage and erosion can impact soil
phosphorus dynamics. Intensive tillage can disrupt soil structure and increase
 33

phosphorus runoff, leading to higher extractable phosphorus levels in some

cases. On the other hand, erosion can transport phosphorus-rich sediments to
different areas, influencing soil phosphorus availability.
Hence, the result of this current study shows that the observed
increase in phosphorus levels cannot be solely attributed to glyphosate
herbicide application. Other factors may also contribute to this increase.

Exchangeable Potassium (cmol kg-1)

The levels of exchangeable potassium (cmol kg-1) of sampled soils of

selected corn farms were presented in Table 8, which shows that the
exchangeable k values for the short-term category are 0.36 to 0.55, 0.26 to
0.45 for the medium-term category, and 0.32 to 0.49 for the long-term
category. The potassium content in all categories are classified as medium to
high in exchangeable p content.
Figure 6 presents the scatterplot diagram of the correlation of
exchangeable potassium and years of glyphosate herbicide application in
selected corn farms. The trend of the scatterplot showed a decreasing value
of exchangeable potassium with r= -0.1888 (R2 (%)= 3.57). Pearson's
correlation analysis between soil exchangeable potassium and years of
glyphosate herbicide application shows a negative correlation, indicating that
the longer glyphosate herbicide is applied, the lower the level of the
exchangeable k. However, its correlation is not significant (p= 0.3176), which
means that the decrease of exchangeable k is not due to glyphosate herbicide
application.
Research studies have shown that glyphosate can indeed decrease
the exchangeable potassium levels in soil. One study by Zobiole et al. (2010)
found that glyphosate application led to a reduction in exchangeable
potassium levels in soil, potentially affecting plant growth and nutrient uptake.
This decrease in exchangeable potassium can have significant implications
 34

for crop productivity and soil fertility. Potassium is an essential nutrient for
plant growth, playing a crucial role in various physiological processes such as
enzyme activation, photosynthesis, and water regulation. A decrease in
exchangeable potassium levels can result in nutrient deficiencies, impacting
crop yield and quality.
Furthermore, another study by Franz et al. (2018) demonstrated that
glyphosate can alter the microbial community in soil, which in turn can affect
nutrient cycling processes, including potassium availability. This disruption in
the soil microbial community can lead to changes in nutrient dynamics,
potentially impacting exchangeable potassium levels.

Table 8. Exchangeable potassium of sampled soils of selected corn farms in

Cabanglasan, Bukidnon
YEARS OF EXCHANGEABLE
GLYPHOSATE FARM CODE POTASSIUM CATEGORY
APPLICATION (cmol kg-1)
A1 0.55 High
A2 0.43 Medium
A9 0.38 Medium
A10 0.36 Medium
A11 0.48 Medium
1-5 years
A12 0.45 Medium
A18 0.43 Medium
A21 0.38 Medium
A24 0.54 High
A27 0.47 Medium
A3 0.45 Medium
A8 0.32 Medium
A13 0.51 High
A19 0.37 Medium
A20 0.26 Medium
6-10 years
A22 0.28 Medium
A23 0.32 Medium
A25 0.27 Medium
A28 0.31 Medium
A30 0.27 Medium
11-15 years A4 0.39 Medium
A5 0.47 Medium
A6 0.48 Medium
A7 0.49 Medium
A14 0.39 Medium
 35

A15 0.42 Medium

A16 0.32 Medium
A17 0.37 Medium
A26 0.39 Medium
A29 0.38 Medium

Other studies also claim that glyphosate can increase exchangeable

potassium, as one study conducted by Li et al. (2019) investigated the impact
of glyphosate on soil potassium dynamics and found that glyphosate
application led to a significant increase in exchangeable potassium levels in
the soil. The researchers attributed this effect to the ability of glyphosate to
chelate with metal cations, thereby promoting the release of potassium from
soil minerals.

Figure 6. Scatterplot of the correlation of exchangeable potassium and years

of glyphosate herbicide application of selected corn farms in
Cabanglasan, Bukidnon
 36

In the study of Haney et al., (2016), it has been shown that fungi have
the ability to rapidly take up K. Therefore, it is possible that if glyphosate
stimulates fungal biomass, in turn it could immobilize biologically available K
and cause K deficiency in plants. In addition to storing K in microbial biomass
K, fungal hyphae could also be transferring exchangeable K to
nonexchangeable K sites. Alternatively, there could be other shifts in 9
subpopulations that potentially impact crops.
A study Wang et al., (2019) claimed that soil pH plays a significant role
in determining the availability of exchangeable potassium. Acidic soils with a
low pH can lead to decreased potassium availability, as they become less
soluble and more prone to leaching. The texture of soil, such as sandy or
clayey soils, can affect the exchangeable potassium levels. Sandy soils have
lower cation exchange capacity (CEC) and may not retain potassium
effectively, leading to potential losses through leaching. Soils with high
organic matter content tend to have higher exchangeable potassium levels
due to the presence of potassium-rich organic compounds. However,
excessive decomposition or removal of organic matter can reduce the
availability of exchangeable potassium.
With the finding of this study, it is observed that the decline in
potassium (K) levels cannot be solely attributed to glyphosate application.
Other naturally occurring factors and agricultural practices may also contribute
to this phenomenon.

Fertilizer Recommendation

The recommended rate of fertilizers depends on various factors such

as soil nutrient levels and the type of crop. Table 9 presents the
recommended fertilizer rates for selected corn farms in Cabanglasan,
Bukidnon. There are eleven (11) different sets of recommendation rates
distributed across 30 corn farms. The fertilizers used are urea (46-0-0),
ammonium phosphate (16-20-0), and muriate of potash (0-0-60), and were
 37

based on the farmer’s choice of fertilizers. The recommended nitrogen rate

ranges from 80 to 100 kg ha-1, requiring 0 to 3.88 bags of urea (46-0-0) per
hectare. For phosphorus, the recommended rate ranges from 0 to 90 kg ha-1,
which requires 0 to 9 bags of ammonium phosphate (16-20-0) per hectare.
For potassium, the recommended rate ranges from 0 to 40 kg ha-1, requiring 0
to 1.33 bags of muriate of potash (0-0-60) per hectare.

Table 9. Recommended rate of fertilizer of the selected corn farms in

Cabanglasan, Bukidnon
RECOMMENDED FARM INORGANIC FERTILIZER (bag ha-1)
RATE CODE 46-0-0 16-20-0 0-0-60
(N-P2O5-K2O kg ha-1)
80-30-0 A1, A2, A3, 2.43 3.00 0
A24
80-0-0 A4, A5, A11, 3.48 0.00 0
A13, A14,
A18, A26,
A27,
100-30-0 A6, A7 3.30 3.00 0
80-0-20 A8, A23, 3.48 0.00 0.67
A29
80-30-20 A9, A10, 2.43 3.00 0
A17, A19,
A28
100-90-0 A12 0.35 9.00 0
80-60-0 A15 1.39 6.00 0
80-60-20 A16 1.39 6.00 0.67
80-60-40 A20, 1.39 6.00 1.33
80-90-20 A21, 0.35 9.00 0.67
80-0-40 A22, A25, 3.45 0.00 1.33
A30
 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

Field assessment on the soil chemical properties of glyphosate-applied

soil at the Municipality of Cabanglasan, Bukidnon, was conducted from
November 2023 to February 2024 with the following objectives: (1) determine
the chemical properties of soil-applied with glyphosate herbicide; (2) correlate
the chemical properties to the years of glyphosate herbicide application; and
(3) assess the recommended rate of fertilizer. The soil analysis was done at
the Soil and Plant Analysis Laboratory (SPAL), College of Agriculture,
Department of Soil Science, Central Mindanao University.

Criteria were established for farm selection, including: (1) the years of
glyphosate herbicide application in corn production without lime application;
(2) farm size of at least 1 hectare; and (3) flat topography. The years of
glyphosate application were categorized into three groups: Short-term (1-5
years glyphosate herbicide application); Medium-term (6-10 years glyphosate
herbicide application); and Long-term (11-15 years glyphosate herbicide
application).

The soil chemical properties, including soil pH, organic matter content,
extractable phosphorus, and exchangeable potassium, of corn farms were
analyzed. The soil pH in the three categories was identified, showing varying
levels of acidity ranging from extremely acidic to strongly acidic in their pH
levels. Additionally, there was a positive correlation between soil pH and the
years of glyphosate herbicide application, and this correlation was found to be
significant. Most corn farms in the three categories were found to have a
marginal level of organic matter content, with one corn farm identified as
deficient in organic matter content. Despite this, a negative correlation was
observed, although it was not statistically significant. The extractable
phosphorus levels were identified and categorized as ranging from slightly
deficient to highly adequate in phosphorus content. There was a positive
correlation observed with the number of years of glyphosate herbicide
application, but it was not found to be statistically significant. In all three
 39

categories, it was identified to have a result of medium to high levels of

exchangeable potassium. However, despite the negative correlation observed
with the years of glyphosate herbicide application, this correlation was not
found to be statistically significant.

There were 11 different sets of recommendation rates derived from the

analysis, where recommendation ranges from 80-100, 0-90, and 0-40 (N-
P2O5-K2O kg ha-1), which needed at least bags of 0.35-3.48 of urea, 0-9
ammonium phosphate, and 0-1.33 muriate of potash, respectively. The
fertilizers used for recommendations were based on the farmers previously
used.

Continuous application of glyphosate herbicide in corn production

should be further investigated into its effects on various soil properties. Future
research should focus on assessing its impact on soil enzyme activities,
microbial community analysis, and corn yield in response to glyphosate
exposure. Additionally, field studies should be conducted to evaluate
glyphosate's effects in combination with different macro and micronutrient
treatments to understand its influence on nutrient availability. These
recommendations aim to provide a comprehensive understanding of
glyphosate's impact on soil and crop health.
 40

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 45

APPENDICES
 46

Appendix Table 1. Soil pH of the selected corn farms in Cabanglasan,

Bukidnon
Sampled Farms Soil pH
A1 4.36
A2 4.29
A3 4.24
A4 5.16
A5 4.40
A6 4.90
A7 4.55
A8 4.34
A9 4.31
A10 4.29
A11 4.22
A12 4.16
A13 4.48
A14 4.42
A15 4.53
A16 4.38
A17 4.46
A18 4.37
A19 4.51
A20 4.26
A21 4.22
A22 4.33
A23 4.26
A24 4.36
A25 4.50
A26 5.10
A27 5.30
A28 5.23
A29 4.71
A30 4.42
Mean 4.50

Appendix Table 2. Soil pH standard values for corn

CATEGORY STV
Extremely Acid <4.5
Very Strongly Acidic 4.5-5.0
Strongly Acidic 5.1-5.5
Medium Acidic 5.6-6.0
Slightly Acidic 6.1-6.5
 47

Appendix Table 3. Organic matter content (%) of selected corn farms in

Cabanglasan, Bukidnon
Sampled Farms Organic Matter Content (%)
A1 3.12
A2 2.60
A3 3.75
A4 2.63
A5 2.96
A6 1.87
A7 2.12
A8 2.63
A9 2.76
A10 3.55
A11 2.57
A12 2.44
A13 3.05
A14 4.00
A15 3.00
A16 3.22
A17 3.00
A18 3.46
A19 3.00
A20 3.89
A21 3.89
A22 3.12
A23 2.90
A24 2.71
A25 3.37
A26 2.95
A27 2.87
A28 2.69
A29 3.62
A30 3.68
Mean 3.05

Appendix Table 4. Organic matter content (%) standard values for corn
CATEGORY STV
Deficient <2.0
Marginal 2.1-4.5
Adequate >4.5
 48

Appendix Table 5. Extractable phosphorus (mg kg-1) of selected corn farms in

Cabanglasan, Bukidnon
Sampled Farms Extractable Phosphorus (mg kg-1)
A1 29.32
A2 27.48
A3 28.38
A4 42.81
A5 38.09
A6 24.82
A7 22.22
A8 31.64
A9 20.89
A10 20.61
A11 41.59
A12 13.37
A13 43.34
A14 33.46
A15 19.47
A16 16.36
A17 22.44
A18 32.22
A19 21.3
A20 17.2
A21 10.61
A22 34.17
A23 44.09
A24 24.4
A25 30.47
A26 31.43
A27 31.23
A28 22.19
A29 37.51
A30 42.14
Mean 28.51

Appendix Table 6. Extractable phosphorus (mg kg-1) standard values for corn
CATEGORY STV
Very Deficient <4.1
Deficient 4.2-8.1
Slightly Deficient 8.2-16.3
Adequate 16.4-20.5
Highly Adequate >20.5
 49

Appendix Table 7. Exchangeable potassium (cmol kg-1) of selected corn farms

in Cabanglasan, Bukidnon
Sampled Farms Exchangeable potassium (cmol kg-1)
A1 0.55
A2 0.43
A3 0.45
A4 0.39
A5 0.47
A6 0.48
A7 0.49
A8 0.32
A9 0.38
A10 0.36
A11 0.48
A12 0.45
A13 0.51
A14 0.39
A15 0.42
A16 0.32
A17 0.37
A18 0.43
A19 0.37
A20 0.26
A21 0.38
A22 0.28
A23 0.32
A24 0.54
A25 0.27
A26 0.39
A27 0.47
A28 0.31
A29 0.38
A30 0.27
Mean 0.40

Appendix Table 8. Exchangeable potassium (cmol kg-1) standard values for

corn
CATEGORY STV
Low <0.25
Medium 0.25-0.50
High >0.50
 50

Appendix Table 9. Pearson’s correlation of soil chemical properties and years of glyphosate application
pH OM P K Years
pH Coef 1.000
p-value
OM Coef -0.2998 1.000
p-value 0.1075
P Coef 0.1730 -0.0021 1.000
p-value 0.3607 0.9911
K Coef 0.0319 -0.4051 0.0502 1.000
p-value 0.8670 0.0264 0.7924
Years Coef 0.3581* -0.0477ns 0.1642ns -0.1888ns 1.000
p-value 0.0520 0.8023 0.3860 0.3176
n=30
ns= Correlation coefficient is not significant
*= Correlation coefficient is significant at the 0.05 level

49
 51

Appendix Table 10. Summary of survey information for short-term category

Sample Farms
Item
A1 A2 A9 A10 A11 A12 A18 A21 A24 A27
Farm profile
Farm size (ha) 1 1 2 3 1 3 2 1.2 1.2 2
Topography Flat Flat Flat Flat Flat Flat Flat Flat Flat Flat
Farmer’s Practices
Frequency of
glyphosate
herbicide 2 2 2 2 2 2 2 2 2 2
application per
cropping
Number of years
of glyphosate
3 3 4 4 3 3 5 5 4 3
herbicide
application
Time of herbicide 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th
application WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP;
5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5th WAP
th th th th th th th th th

Fertility
IF IF IF IF IF IF IF IF IF IF
management
Fertilizer applied 14-14-14 14-14-14 14-14-14 14-14-14 14-14-14 14-14-14 46-0-0 14-14-14 14-14-14 14-14-14
46-0-0 46-0-0 0-0-60 46-0-0 46-0-0 21-0-0 0-0-60 46-0-0 46-0-0 46-0-0
0-0-60
Methods of Side- Side- Side- Side- Side- Side- Side- Side- Side- Side-
application dress dress dress dress dress dress dress dress dress dress
WAP= Week after planting
IF= Inorganic Fertilizer 50
Appendix Table 11. Summary of survey information for medium-term category
 52

Sample Farms
Item
A3 A8 A13 A19 A20 A22 A23 A25 A28 A30
Farm profile
Farm size (ha) 1 1 1 1.2 2 2 3 1 2 1
Topography Flat Flat Flat Flat Flat Flat Flat Flat Flat Flat
Farmer’s Practices
Frequency of
glyphosate
herbicide 2 2 2 2 2 2 2 2 2 2
application per
cropping
Number of years
of glyphosate
7 8 8 8 7 10 8 8 6 8
herbicide
application
Time of herbicide 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th
application WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP;
5th WAP 5th WAP 5th WAP 5th WAP 5th WAP 5th WAP 5th WAP 5th WAP 5th WAP 5th WAP
Fertility
IF IF IF IF IF IF IF IF IF IF
management
Fertilizer applied 14-14-14 14-14-14 14-14-14 46-0-0 14-14-14 14-14-14 46-0-0 14-14-14 14-14-14 14-14-14
46-0-0 46-0-0 0-0-60 16-20-0 46-0-0 46-0-0 0-0-60 46-0-0 46-0-0 46-0-0
0-0-60 0-0-60
Methods of Side- Side- Side- Side- Side- Side- Side- Side- Side- Side-
application dress dress dress dress dress dress dress dress dress dress
WAP= Week after planting
IF= Inorganic Fertilizer 51
Appendix Table 12. Summary of survey information for long-term category
Item Sample Farms
 53

A4 A5 A6 A7 A14 A15 A16 A17 A26 A29

Farm profile
Farm size (ha) 1 2.5 1.5 1.3 1 2 2 1 1 4
Topography Flat Flat Flat Flat Flat Flat Flat Flat Flat Flat
Farmer’s Practices
Frequency of
glyphosate
herbicide 2 2 2 2 2 2 2 2 2 2
application per
cropping
Number of years
of glyphosate
11 12 13 13 15 12 11 12 11 13
herbicide
application
Time of herbicide 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th 3rd-4th
application WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP; WAP;
5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5 WAP 5th WAP
th th th th th th th th th

Fertility
IF IF IF IF IF IF IF IF IF IF
management
Fertilizer applied 14-14-14 16-20-0 46-0-0 46-0-0 14-14-14 14-14-14 46-0-0 14-14-14 46-0-0 14-14-14
0-0-60 46-0-0 0-0-60 16-20-0 0-0-60 46-0-0 46-0-0 46-0-0 0-0-60 46-0-0
Methods of Side- Side- Side- Side- Side- Side- Side- Side- Side- Side-
application dress dress dress dress dress dress dress dress dress dress
WAP= Week after planting
IF= Inorganic Fertilizer 52
 54

Appendix 1. Informed consent form

Consent Form

I, _______________________________, freely agree to participate in thesis

study entitled “Assessment of Soil Chemical Properties of Glyphosate Applied
Soil on Selected Corn Farms in Cabanglasan, Bukidnon”.

I have understood that my participation in this survey is voluntary and I hereby

sign this consent form.

_______________________
Name and Signature of Participant
Date: ____________________

Pagtugot

Ako, ______________________________________________, lumolupyo sa

______________________________________________________, na-
gakanayon nga akong gabubut-on ang pag-partisipar niining pagduki-duki
kabahin sa “Assessment of Soil Chemical Properties of Glyphosate Applied
Soil on Selected Corn Farms in Cabanglasan, Bukidnon”. Ako mubulig niini
nga pagtuon tungod kay ako nakasabot sa katuyu-an niini.

Isip sa pagpamatuod, nga akong kusang miuyon sa akong partisipasyon,

akong nilagda/mipirma niining pagtugot (Consent Form).

__________________________
Ngalan ug pirma
Petsa: ____________________
 55

Appendix 2. Farmer’s survey questionnaire

Survey Questionnaire

Dear Ma’am/Sir, this survey aims to determine the farm description and
practices that will be the basis in the analysis of the soil samples. May I
humbly request for your full cooperation. The information from this survey will
be used solely for the purpose of study.
CLAIRE JANE R. LAGURA
Researcher
Survey Questionnaire No. : ___________________
Barangay/Purok : ___________________

Farmer’s Profile:
Name: __________________________
Age: ____________________________
Area of farm (ha) : ___________________

Farmer’s Practices:
1. Corn Production
1.1 Years of corn production ( ) 1-5 Yrs. ( ) 6-10 Yrs. ( )11-15 Yrs.

2. Herbicide Application
2.1 Number of years of Glyphosate Herbicide Application
( ) No application ( ) 6-10 years
( ) 1-5 years ( ) 11-15 years
2.2 Frequency of Glyphosate Herbicide Application per cropping
()1 ()2 ()3
2.3 Time of herbicide Application __________________________

3. Fertility Management
( ) Organic fertilizer ( ) Inorganic fertilizer
( ) Liming ( ) Others (specify) _________

4. Fertilizer Applied
( ) Organic Fertilizer Rate: ______/ha
( ) Complete (14-14-14) Rate: ______/ha
( ) Urea (46-0-0) Rate: ______/ha
( ) Ammonium Phosphate (16-20-0) Rate: ______/ha
( ) Muriate of Potash (0-0-60) Rate:______/ha
( ) Others (specify) __________ Rate:______/ha

5. Methods of Application
( ) Basal Application ( ) Side Dress Application
 56

Appendix Figure 1. Preliminary survey with corn farmers

Appendix Figure 2. Collection of soil samples

 57

Appendix Figure 3. Preparation of soil samples