Soil degradation of raised beds on orchards in the mekong delta field and laboratory methods

Soil degradation can accelerate a series of processes such as erosion, compaction, loss of organic matter, loss of whole soil biota, surface sealing and contamination.. This thesis prese

TRITA-LWR PhD Thesis 1073

ISSN 1650-8602

ISRN KTH/LWR/ PhD 1073-SE

ISBN 978-91-7501-857-7

Pham Van Quang

September 2013

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© Pham Van Quang 2013

PhD Thesis in

Land and Water Resources Engineering

Department of Sustainable development, Environmental science and Engineering

Royal Institute of Technology (KTH)

SE-100 44 STOCKHOLM, Sweden

Reference should be written as: Van Quang, P (2013) “soil degradation of raised-beds on orchards in the Mekong delta - field and laboratory methods” TRITA LWR PhD thesis

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To my family with respectful gratitude,

My wife Nguyen Thi Thanh Xuan,

My daughter Pham Xuan Huong, and

My son Pham Quang Duy

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Suy thoái đất là một tiến trình phức tạp xuất hiện ở mọi nơi, mọi lúc làm tác động trực tiếp đến các quá trình lý, hóa và sinh học trong phẫu diện đất Nó có thể là kết quả của các hoạt động do tự nhiên hoặc do con người như sử dụng sai hoặc thực hành quản lý đất đai bất hợp

lý Cho dù nguyên nhân thế nào chăng nữa, suy thoái đất cũng gây ra các ảnh hưởng bất lợi nặng nề lên cây trồng và sức sản suất của đất Suy thoái đất có thể thúc đẩy hàng loạt các quá trình như là xói mòn, nén dẽ, mất vật liệu hữu cơ và sinh vật đất, đóng váng bề mặt và

ô nhiễm Luận văn này trình bày sự đánh giá về các đặc tính của đất để mở mang sự hiểu biết về suy thoái đất trên các vườn cây ăn trái ở đồng bằng sông Cửu Long Thí nghiệm thực hiện trên 10 vườn cam quít với khoảng thời gian thành lập vườn từ 1970 đến 1998 tại tỉnh Hậu Giang Mẫu đất được lấy vào mùa khô năm 2010 ở hai độ sâu cho mỗi vườn để phân tích các chỉ tiêu lý hóa đất Sức kháng xuyên của đất được đo định kỳ mỗi tuần kết hợp với lấy mẫu để xác định ẩm độ đất trong suốt khoảng thời gian 5 tháng Kết quả phân tích cho thấy pH đất có khuynh hướng giảm, sự thiếu và mất cân bằng dinh dưỡng đất ngày càng lộ rõ, và cấu trúc đất đang xấu đi theo độ tuổi của vườn Các biện pháp phòng ngừa và phục hồi cần được quan tâm đối với việc phục hồi và duy trì chất lượng của đất và nước ngầm Các biện pháp nên bao gồm (1) trung hòa độ chua đất, (2) cân bằng dinh dưỡng, (3) duy trì vật liệu hữu cơ trong đất, và (4) áp dụng chế độ tưới thích hợp

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I would like to express my sincere gratitude to the Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden and the many people who have helped and supported me in various ways, during my field survey in Vietnam as well as data analysis in Sweden This statement is to thank them for their tremendous enthusiastic help

I greatly appreciate Mr Ngo Xuan Hien, section of agriculture and rural development, Chau Thanh district, Hau Giang province in Vietnam for help and encouragement during the fieldwork activities I thank all the local farmers for their kind support at the study locations College of Agriculture and Applied Biology at Can Tho University

in Vietnam are gratefully acknowledged with the facility support

I would like to express my deepest gratitude to my main supervisor Per-Erik Jansson for his great guidance, understanding, encouragements, advice and support for my research works I would also like to thank my co-supervisor Dr Le Van Khoa for his kind advice and comments during the study

I would like to thank Britt Chow and Aira Saarelainen for their efficient and kind help with all administrative matters Joanne Fernlund was able to share her precious time to help me to meet all the format regulations of my thesis by KTH

Many thanks go to Dr Ewa Wredle, department of Animal Nutrition and Management, the Swedish University of Agricultural Sciences for arranging the comfortable accommodation during the time I stayed in Uppsala, Sweden

I also thank Mr Huynh Ngoc Duc and Mr Pham Xuan Phu for their assistance during data collection Miss Ly Ngoc Thanh Xuan, Mr Nguyen Van Chuong and analysis laboratory staff, Faculty of Agriculture and Natural Resources, An Giang University, Vietnam for their help with soil chemical testing

I thank my close friends from Vietnam for sharing many funny stories and parties during my stay in Uppsala, Sweden I also thank my colleagues in Faculty of Agriculture and Natural Resources, An Giang University: Pham Huynh Thanh Van, Thai Huynh Phuong Lan, Ly Ngoc Thanh Xuan, Huynh Ngoc Duc, Pham Xuan Phu, Pham Duy Tien, Nguyen Van Kien, Tran Van Hieu Many thanks go to Mr Nguyen Thanh Trieu and Nguyen Thanh Tinh for many kinds of help

Many thanks go to Dr Charles Howie for checking the English language

I would also like to express my deepest gratitude to my parents-in-law for their continuous supports, taking care of my children and encouragements during my study

I wish to express my heartfelt gratitude to my parents for their holy motivation, which led to successful completion of this thesis

Last, but not least, I want to thank to my wife Nguyen Thi Thanh Xuan and my children Pham Xuan Huong and Pham Quang Duy for their understanding, supports and encouragements, which inspired me to accomplish this work

Financial support from the Vietnamese Government, partly by KTH and the utilization of enormous facilities at the Royal Institute of Technology (KTH), Stockholm, Sweden, is gratefully acknowledged

Pham Van Quang

Stockholm, September 2013

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Dedication iii 

Summary in Vietnamese (Tóm lược) v 

Acknowledgements vii 

Table of content ix 

List of tables xi 

List of figures xi 

Abbreviations and symbols xiii 

List of papers xv 

Abstract 1 

Introduction 1 

Study objectives 3 

Scope of the study 4 

Soil compaction formation concept 4 

Methods 5 

Field experiment performance 5 

Sites descriptions 5 

Sampling and analysis 6 

Soil penetration resistance measurement 7 

Results 8 

Chemical properties on raised-bed soil (Paper II) 8 

Physical properties on raised-bed soil (Paper III) 8 

Soil penetration resistance (Paper IV) 8 

Discussion 12 

Soil properties assessment 12 

Chemical properties 12 

Physical properties 13 

Soil amendments 16 

Essential Plant Nutrients - their functions, and role of fertilizers 16 

General soil amendments to raised-bed orchards in the Mekong delta 17 

Conclusions and recommendations 23 

Future work 24 

References 25 

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Table 1 Major soil groups in the Mekong Delta 2 Table 2 10 study locations selected in Hau Giang province, Mekong delta, Vietnam 6 Table 3 Ratings of chemical properties on observed raised-beds 8 Table 4 Fertilizer recommendation for citrus trees (adapted from Le Thanh Phong et al., 1996) 18 Table 5 Different methods of irrigation scheduling 20 

Figure 1 Cross section of raised-bed construction – reverse order compared to original soil 3 Figure 2 Cross section of a raised-bed construction – the same order as in the original soil 3 Figure 3 Conceptual diagram of soil formation processes in raised-bed soils 5 Figure 4 (a) Provincial Administrative Boundary Map of the Mekong Delta, (b) Map of Hau Giang Province, (c) Studied Sites Map – Extracted from Google Earth 6 Figure 5 Average penetration resistance at the different age of the raised-beds and soil depths on all the observed period of 22-01-2010 to 02-06-2010 Red dashed line: critical PR of 2.5 MPa 9 Figure 6 Soil penetration resistance recalculated by PR = a1θ + a0 (θ estimated from water retention curves at pF value of 2, 2.5, 3 and 4.2) for soil depths 10 Figure 7 Slope coefficient plotted versus age of raised-beds on different soil layers 11 Figure 8 Slope coefficient plotted versus age of raised-beds on different soil layers 12 Figure 9 Soil test interpretation categories redrawn from Havlin et al (1999) 19 Figure 10 Fertilizer recommendation scheme published in Vitosh et al (1995) 20 

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FBMP Fertilizer Best Management Practices

4Rs Right source/Product, Right rates, Right time, and Right place

mEq Milliequivalents

MPa Megapascal

SPSS Statistical Package for the Social Sciences

SWRC Soil Water Retention Curves

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I Van Quang P., Jansson P-E., 2008 Development and description of soil compaction on orchard soils in the Mekong Delta (Vietnam) Scientific Research and Essays 3 p:500-504

II Van Quang P., Guong V.T., 2011 Chemical properties during different development stages of fruit orchards in the Mekong delta (Vietnam) Agricultural Sciences Vol.2, No.3, p:375-381

III Van Quang, P., Jansson, P-E., and Guong, V T., 2012 Soil physical properties during different development stage of fruit orchards Journal of Soil Science and Environmental Management Vol 3(12), p: 308-319

IV Van Quang, P., Jansson, P-E., and Le Van Khoa, 2012 Measurements of soil penetration resistance to investigate tendencies and explanations of compaction for orchard soil International Journal of Engineering Research and Development, Volume 4, Issue 8 (November 2012), p: 87-96

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Soil degradation is a complex process which may occur anywhere and at any time It directly affects the physical, chemical and biological processes within the soil profile Soil degradation can either be as a result of natural hazards or due to manmade actions, such as mismanagement on cropping patterns, soil preparation and cultivation practices Regardless of how it is caused, soil degradation has strong negative effects

on plant and soil productivity Soil degradation can accelerate a series of processes such as erosion, compaction, loss of organic matter, loss of whole soil biota, surface sealing and contamination This thesis presents the assessment of soil properties to improve our understanding of soil degradation on raised-bed orchards in the Vietnamese Mekong delta (MD) Measurements were made on 10 citrus plantations which had been established during a range of years from 1970 to 1998 at Hau Giang province Soil sampling was made in the dry season of 2010 at two soil depths for each raised-bed to determine soil chemical and physical properties The soil penetration resistance (PR) was periodically measured once a week together with soil sampling for moisture measurements during a period of 5 months Analysis indicated the pH value of the soil was tending to decrease, nutrient imbalance and deficiency was developing, and the soil structure was deteriorating during the age since the raised-beds were originally constructed Preventive and restorative measures need to

be considered for restoring and retaining the quality of the soil and the ground water These measures should consist of (1) neutralizing of excess acidity, (2) balancing of nutrients, (3) maintaining of soil organic matter, and (4) application of appropriate irrigation schedules

Key words: citrus orchards; nutrient; soil fertility; soil strength; soil degradation; alluvial soil; Mekong delta

The Vietnamese Mekong Delta (MD) covers an area of 3.9 million ha located in the southern territory of Vietnam It extends from latitudes 104°30’E to 107°E and longitudes 8°30’N to 11°N, bordered by the Gulf of Thailand to the west, the Eastern Sea to the east and to the south, Ho Chi Minh City to the north, and Cambodia to the north-west There are 13 provinces located in the MD with a population estimated to

be 17.3 million people (20 % of the total of the Vietnamese population

in Vietnam) The average population density is 426 persons per square kilometer over the whole delta (General Office of Statistics, 2010) The MD has a monsoon tropical climate, characterised by two distinct seasons - the dry season and the wet season The dry season runs from December to April and the wet season from May to November The average rainfall ranges from less than 1500 mm to more than 2500 mm,

90 % of which occurs during the rainy season The mean temperature ranges from 23 to 25 °C during the coldest months and from 32 to 33

°C during the warmest months The humidity is high in the rainy season, the highest in September (91%), and the lowest in the dry season (79 – 82%)

The MD is a young land mass formed and developed during the Holocene period by transgression and regression of the sea (Nguyen Huu Chiem, 1993) The soils are mainly formed by the deposition of sediment from the rivers (Mekong and Bassac rivers) and the sea The sediment is carried by flood water deposited along the banks of the

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Table 1 Major soil groups 1 in the Mekong Delta.

There are seven major soil types in the MD, as summarised in Table 1 (Le Van Khoa, 2002; Nguyen Bao Ve and Vo-Tong Anh, 1990)

Agriculture in the MD is based on land that has been reclaimed and used since the 17th century (Huynh Lua, 1987) Many management methods have been used to grow paddy rice, such as burning, ploughing, making dikes, etc Traditionally, a single rice crop was cultivated but due to food security issue and the rapid population growth, a double rice crop system was initiated in the 1930s, and a triple rice crop system was introduced around 1980 In addition to rice cultivation, large areas are used for upland crop and fruit trees

The topography of the MD is generally flat and low The elevations range from 0 to 4 m above sea level, with the exception of some hills and mountains (Mount Cam in An Giang Province 716 m, Mount Co To in Kien Giang Province 258 m) The MD receives annual flooding It usually happens in August - November, with an average flooding depth from 0.8 to 1.5 metre Consequently, farmers have had to build so-called

‘raised-beds’ to avoid submergence of upland crops, most of which are grown in lowland areas with alluvial soils, to keep the trees above the flood level

Excavating and heaping of soil materials have pushed the raised-beds up from the adjacent lateral ditches The raised-beds are the long soil strips that are higher than the original ground surface as illustrated in Figs 1-2, which show cross-sections of stages in raised-bed constructions Soil layers in the raised-beds can be arranged in reverse order (Fig 1) or in the same order as the original soil (Fig 2)

Efforts have been made to improve soil quality/primary production, plant varieties and management methods Research on plant varieties and farming systems has resulted in positive effects on crop yield, disease tolerance, better practical techniques and a more suitable cropping calendar (e.g Buu and Lang, 2004; Dang Quang Tinh and Pham Thanh Hang, 2003; Hien and Thi, 2001; Lang et al., 2007; Lang et al., 2001; Thi Lang and Chi Buu, 2003; Thi Lang et al., 2001; Tu et al., 2003; Vo-Tong Xuan, 1991)

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Soil chemical processes, soil nutrients and soil biology have also been taken into account, e.g in studies conducted by Vo Thi Guong et al (1995), Tran Kim Tinh (1999), and Tran Kim Tinh et al (2001), Berglöf

et al (2002), Nguyen My Hoa (2003), Tan et al (2003), Chau Minh Khoi

et al (2006) and Chau Minh Khoi et al (2008)

Process-orientated models which describe the behaviour of the entire soil-plant-atmosphere system are important tools in understanding water management and the extent to which soil physical conditions impact on plant growth (Jansson, 1996) Previously, Van Quang (1998; 2009) and

Vo Khac Tri (1998) applied a soil physical model to the conditions in the MD Experimental studies on soil physics in the MD have also been reported by Le Quang Minh (1996), Uppenberg et al (1997) and Le Van Khoa (2002)

There have been warnings of soil degradation in the MD during recent years, not only in rice farming areas but also in fruit-growing areas especially in intensive cropping Some studies of soil physics indicated that soil compaction has occurred between 20-40 cm soil depth and its thickness varies from 35 to 50 cm in the intensive rice cultivation areas; this compaction was apparent at higher soil density, lower soil porosity (less than 50 vol-%) and lower saturated hydraulic conductivity (Le Van Khoa, 2002) Chemical soil degradation has also occurred, as evidenced

by nutrient depletion, acidification and sodification (Le Van Khoa, 2002) Vo Thi Guong et al (2005) reported soil physical and chemical degradation in citrus plantations in the MD and identified trends of soil compaction during the ageing of the raised-beds The cross-sectional area and length of soil pores decreases during the ageing of the raised-beds, resulting in a decrease in bypass flow rates (Le Quang Minh, 1996)

Study objectives

Research aims to improve the knowledge on soil degradation of the raised-beds on fruit orchards in the Mekong delta Specific objectives were:

• To review the available literature from the MD and the impacts of agricultural activity on soil degradation on fruit orchard soils

• To assess soil fertility based on soil chemical properties on beds of different age

raised-Figure 1 Cross section of raised-bed construction – reverse order compared to original soil

Figure 2 Cross section of a raised-bed construction – the same order as in the original soil

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• To assess the soil compaction on soil physical properties during

different development stages of fruit orchards

• To examine soil penetration resistance and its dependence on soil moisture and age of the raised-beds in the MD

amendments/conditioners should be used to increase the fertility and adjust the pH as well as improve the water management practices

Scope of the study

Paper I reviews the previous available studies with respect to soil

degradation from the MD on both rice farming systems and upland

crops The results in Papers II-IV are further steps performed at the field scale on 10 selected citrus plantations in the Hau Giang province in the

MD This information is based on Paper I and a previous study on soil formation and soil moisture dynamics in agricultural fields in the MD (see Van Quang, 2009) In Papers II and III, an examination of soil chemical and physical properties was made in order to evaluate soil fertility and soil compaction during different development stages of fruit orchards by analyzing soil samples in the laboratory Paper IV dealt with soil penetration resistance measured by using a portable electronic penetrometer The relationship between PR and soil moisture was

analyzed by using linear regression methods based on the penetration

resistance data series and soil moisture data

Soil compaction formation concept

Due to raised-beds’ construction, the soil rearranges itself to get a stable state under natural conditions and human activities by time The reconstruction processes of soil physical characteristics in the raised-beds can briefly be described as follows:

• The alternate drying and wetting cycles of the raised-bed soils in the

MD are in combination with the fine soil texture

• Cracks on the raised-beds are normally formed during the dry season

or during drought periods in the rainy season

• The soils in the raised-beds have been subject to overtopping and internal erosion processes

• When the bank is built for the first time, the space distribution in situ

is very high

• Larger soil blocks break down into smaller blocks through weathering processes and the mechanical process of preparing the soil surface for planting The soil particles are gradually rearranged until they become stable

• Fine-grained soil particles move with the flow of rain or irrigation water and gradually fill up the soil gaps If the clay content of the soil

is high, this can cause an increase in soil resistance

• Soil aeration exposes soil organic matter, and speeds up the breakdown of organic matter by oxidation processes This is harmful

to soil structure and leads to a rapid increase in soil density

• The rise in water level due to the rainy season or irrigation and the fall in water level due to the dry season or drainage will lead to changes in soil volume

• Cultivation activities, such as irrigation, drainage and weed removal from the raised-beds can accelerate the soil compaction rate

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Figure 3 Conceptual diagram of soil formation processes in raised-bed soils

Therefore, the soil in the raised-beds is affected by a complex process comprising of a core component of physical, chemical and biological factors, together with factors relating to climate and agricultural practices The physical process can be summarized as a diagram as shown in Fig 3

Breakdown of soil blocks Wet and dry circle Forming cracks

All soil particle size

Smaller soil particle sizes

Finer soil particle sizes

Soil loss/bulk density

Bulk density Aggregate/structure Conductivity Compaction

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Figure 4 (a) Provincial Administrative Boundary Map of the Mekong Delta, (b) Map

of Hau Giang Province, (c) Studied Sites Map – Extracted from Google Earth

Table 2 10 study locations selected in Hau Giang province, Mekong delta, Vietnam

Locations Latitude Longitude Age (years)

Sampling and analysis

Data collection started at the beginning of the dry season (January 2010) after the flooding level had receded and the ground water was below the surface of the raised-beds Soil samples were randomly taken on each of the selected raised-beds at 0-20 cm (topsoil) and 20-50 cm (subsoil) depths with four replicates including undisturbed core samples (5 cm long and 5 cm in diameter - approximately 100 cm3 in volume) and disturbed soil samples of about 4 kg for each layer All undisturbed core samples were taken by augering to a desired depth using a bulk density auger Disturbed samples were collected by digging using a shovel Disturbed soil samples were air-dried, homogenized and sieved through

a 2 mm mesh screen to determine soil chemical properties including pH, organic matter, CEC, total nitrogen, NH4+, NO3– and exchangeable

Ca2+, Mg2+, K+ as well as available phosphorous Statistical analyses were performed by using the SPSS statistical package (version 16)

Soil physical analysis included bulk density, saturated conductivity, water retention, and soil texture Data were analyzed by using the SPSS statistical package (version 16) A two-way ANOVA with the two factors (age of raised-beds and soil depths) was performed to evaluate the soil physical properties In case no interaction effect between age and soil depths on soil properties was detected, a one-way ANOVA with an