SOIL DEGRADATION OF RAISED BEDS ON ORCHARDS IN THE MEKONG DELTA FIELD AND LABORATORY METHODS Pham Van Quang September 2013 TRITA-LWR PhD Thesis 1073 ISSN 1650-8602 ISRN KTH/LWR/ PhD 1073-SE ISBN 978-91-7501-857-7 Pham Van Quang TRITA LWR PhD Thesis 1073 © 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 1073 ii Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods D EDIC ATION To my family with respectful gratitude, My wife Nguyen Thi Thanh Xuan, My daughter Pham Xuan Huong, and My son Pham Quang Duy iii Pham Van Quang TRITA LWR PhD Thesis 1073 iv Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods S UMMA RY I N V IETNAMESE (T ÓM LƯC ) Suy thối đất tiến trình phức tạp xuất nơi, lúc làm tác động trực tiếp đến q trình lý, hóa sinh học phẫu diện đất Nó kết hoạt động tự nhiên người sử dụng sai thực hành quản lý đất đai bất hợp lý Cho dù nguyên nhân nữa, suy thoái đất gây ảnh hưởng bất lợi nặng nề lên trồng sức sản suất đất Suy thối đất thúc đẩy hàng loạt q trình xói mịn, nén dẽ, vật liệu hữu sinh vật đất, đóng váng bề mặt nhiễm Luận văn trình bày đánh giá đặc tính đất để mở mang hiểu biết suy thoái đất vườn ăn trái đồng sông Cửu Long Thí nghiệm thực 10 vườn cam quít với khoảng thời gian thành lập vườn từ 1970 đến 1998 tỉnh Hậu Giang Mẫu đất lấy vào mùa khô năm 2010 hai độ sâu cho vườn để phân tích tiêu lý hóa đất Sức kháng xuyên đất đo định kỳ tuần kết hợp với lấy mẫu để xác định ẩm độ đất suốt khoảng thời gian tháng Kết phân tích cho thấy pH đất có khuynh hướng giảm, thiếu cân dinh dưỡng đất ngày lộ rõ, cấu trúc đất xấu theo độ tuổi vườn Các biện pháp phòng ngừa phục hồi cần quan tâm việc phục hồi trì chất lượng đất nước ngầm Các biện pháp nên bao gồm (1) trung hòa độ chua đất, (2) cân dinh dưỡng, (3) trì vật liệu hữu đất, (4) áp dụng chế độ tưới thích hợp v Pham Van Quang TRITA LWR PhD Thesis 1073 vi Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods A CKNOWLEDGEMENTS 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 vii Pham Van Quang TRITA LWR PhD Thesis 1073 viii Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods T ABLE OF CONTENT 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 ix Pham Van Quang TRITA LWR PhD Thesis 1073 x Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods which is increasingly seen as a form of soil degradation (Chan et al., 2003) and is often related to land use and soil/crop management practices Soil structure influences soil water movement and retention, erosion, crusting, nutrient recycling, root penetration and plant yield Soil tillage at inappropriate water moisture as well as the loss of organic matter due to oxidation may cause structural degradation; continued cultivation without organic additions can result in loss of microaggregation leaving a soil very vulnerable to compaction and erosion (Gardner et al., 1999) The evidence from the summary statistics for the observed raised-beds in the correlation analysis (Paper III) showed that soil structure gradually has declined with time It was illustrated by the aging of raised-bed was significantly correlated with bulk density (positive), saturated hydraulic conductivity, wet range suctions (pF 0.4 to 3.0) and organic matter (Tables and 10 in Paper III) It was also demonstrated through a decreased tendency with age of the fast and slow drainage pores (Figure in Paper III), indicating a decreased-level of soil macroporosity; and the increasing in micro-pore proportion with age was identified by soil water retention curves (Table and Figure in Paper III) According to Assouline et al (1998), an increased proportion of microporosity can be as an indicator of soil compaction Next discussion accentuates in soil penetration resistance (soil strength) and its relation to soil moisture and water potential (pF) to understand how soil degradation will develop during cultivations in the raised-beds (Paper IV) These analyses clarify the results of physical properties discussed in previous section The penetration resistance (PR) of soils is an important parameter that influences to nutrient uptake, water infiltration and redistribution, aeration, seedling emergence and root growth, resulting in decrease of plant yields, increased erosion and difficulty in soil cultivation (Bengough et al., 2011.; Taylor and Brar, 1991) The PR is attributed to forces of cohesion and adhesion and varies with soil moisture (Lal and Shukla, 2004) Since the plant root system is mostly growing in porous media, it must therefore overcome mechanical soil resistance The PR has limited to effective rooting depth; if the soil status is too weak, anchor capacity of plant roots will not be adequate to withstand the forces of wind and water; on the other hand, if it is too strong, the plant roots will not have required strength to penetrate to the soil matrix Plant roots will be dramatically affected if the soil strength exceeds the capacity of root penetrability (Jarmillo-C et al., 1992) As the results showed in Fig indicated that, the PR has reached to MPa on over the top 30 cm depth for starting from 30-year-old raised-beds at the field capacity condition and PR is still able to be higher when the soil becomes drier This value was higher than the value reported by Lutz et al (1986) - the study on relationship between citrus root development and soil physical conditions, of which documented that 1.5 MPa is the maximum soil strength restricting root growth Studies conducted by Kees (2005) and Raper et al (2005) showed that roots of most plants are inhibited at PR of 1.5 MPa and roots of many plants stop to grow at PR of 2.5 MPa A penetrometer measurement of MPa generally concerned as sufficient to impede the growth and development of plants (Taylor and Gardner, 1963) At PR larger than 2.5 MPa, root elongation is significantly restricted (Whalley et al., 2007) However, some studies showed significantly higher PR values due to the influence of the soil moisture 15 Pham Van Quang TRITA LWR PhD Thesis 1073 content at the time of penetrometer readings with visually healthy plants (Smith et al., 1997; Sojka et al., 2001; Whalley et al., 2007) The relationship between the PR and soil moisture (and pF values as well) was shown through the regressive equation (1) and (2) The results demonstrated that the less change in PR associated with a change in θ (or pF value) when the soil of raised-beds has been more exposed to wet and dry cycle and cultivating since construction This indicated that the soil in the young raised-beds has been more macropores than that in the old ones, and it has been easier to disturb While the soil in the old ones are in a mode of more inert system with smaller sensitivity In other words, the soil structure has become smaller and dense aggregates with time, resulting in higher PR values It can see that both of predictors (θ and pF values) showed the same phenomena but in the different ways Because the comparisons of PR, predicted by θ and pF values, were not significantly different, so the pF value and either θ can be used to predict the PR However, it will be easier to obtain the θ than pF values In addition, according to the slope coefficients plotted against age of the raised-beds on the different soil layers, drawn from regression equations (1) and (2); the results showed that the explanations based on θ were better than pF values (Fig 7) This suggested that the equation (1) could predict the PR more credible than using (2) In summary, based on above discussions, the soil in the observed raisedbeds has been exhibited a tendency of degradation in term of soil structure decline Management of soil physical conditions to improve the constraints for plant growth will not only conserve the soil functions for the future but also contribute to the minimization of soil degradation Favourable soil structure and high aggregate stability are important role to enhancing porosity and resistance to erosion susceptibility, improving soil fertility, and hence increasing agronomical productivity Soil structural and aggregate stability are affected by many interactional factors including the environment, soil management, plant influences and soil properties The soil properties consist of such as mineral composition, texture, soil organic carbon (SOC) concentration, pedogenic processes, microbial activities, exchangeable ions, nutrient reserves, and moisture availability (Lal and Shukla, 2004) The rate and stability of aggregation generally increases with SOC, clay surface area and CEC The loss of organic matter and consequently soil fertility is often caused by unsustainable cultivation practices e.g unsuitably commercial fertilizer supply, working at an inappropriate soil moisture and/or continuous removal of vegetation without returning biomass residues into the soil Soil amendments Essential Plant Nutrients - their functions, and role of fertilizers Plants require 16 essential chemical elements for normal functioning, growth and completion of their life cycle (Maheshwari, 2012; Roy, 2006) Out of these elements carbon, hydrogen, and oxygen, are usually not considered as nutrient elements because they are supplied by air and water, and the other 13 elements are primarily derived from the soil and are generally managed by the farmers The thirteen classified as macronutrients and micronutrients based on their plant requirements including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn) Plants obtain nutrient elements through root uptake from the soil solution; nutrient 16 Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods uptake depends on nutrient absorbability of the plant, its growth stage and the nutrient level at the root zone (Maheshwari, 2012; Roy, 2006) The sources of these soluble nutrients in a soil can be natural, synthetic or recycled wastes Although most soils can physically sustain more nutrients than a plant needs in a growing season, yet little of these nutrients are available in the soil solution Moreover, plants remove a large amount of nutrients from the soil during the growing season (especially N, P, K), leading to deficiencies of one or more essential nutrients; therefore, application of fertilizers is necessary to compensate Yet, an integrated plant nutrient management needs to be carefully considered to optimize the condition of the soil with regard to its physical, chemical, biological and hydrological properties, for enhancing crop productivity, while minimizing soil degradation Balanced application of appropriate fertilizers is a major component of integrated nutrient management (Maheshwari, 2012) Fertilizers need to be applied at the level required for optimal plant growth based on plant requirements and agro climatic considerations (Maheshwari, 2012) According to Roy (2006), an optimal nutrient supply requires: (1) sufficient available nutrients in the root zone of the soil; (2) rapid transport of nutrients in the soil solution towards the root surface; (3) satisfactory root growth to access available nutrients; (4) unimpeded nutrient uptake, especially with sufficient oxygen present; (5) satisfactory mobility and activity of nutrients within the plant While deficiency of any essential nutrient will become create a limiting factor for plant production, and could have also affected access of other elements, conversely too much of any nutrient could be toxic to plants and reduce the disease resistance of the crop General soil amendments to raised-bed orchards in the Mekong delta Soils in the MD mostly contain kaolinite, illite and montmorillonite in the clay fraction which have been inherited from parent materials (Le Van Khoa, 2002; Nguyen Huu Chiem, 1993) Growers often use a single rather than compound fertilizers, which stresses the use of nitrogen over phosphorus, potassium, lime and micronutrients which are less appropriately considered (Siem, 1997) As a result the soils become acidic, their fertility is lowered, and their phosphorous fixation is high The reasons mentioned above could partly explain the nutrient imbalance as described in Paper II Therefore, it is necessary that soil acidity be corrected before fertilizers are applied because without neutralizing excess acidity, balanced nutrient application could not bring into play effectively (Maheshwari, 2012; Roy, 2006) Target pH and Critical pH Target pH is defined as the soil pH range at which plant nutrients are optimally available for plant uptake and growth Critical pH refers to the maximum pH value at which liming increases plant yield (Adams, 1984) Because the characteristics of growth vary with plant type, target pH also varies with the type of plant to be grown Critical pH is dependent on plant type and reflects the management practices as well as an economic consideration The first step in soil pH management is to identify the suitable target pH For citrus trees, the best range is between 5.5 and 6.5 (Calabrese, 2002) Liming practices are then used to ensure that soil pH maintains close to the target pH and does not fall above or below the critical pH values For this study, critical pH values are about pH 5.5 - 6.5 Hence, lime should be applied whenever the soil pH approaches 6.5 because lower pH values will adversely affect plants 17 Pham Van Quang TRITA LWR PhD Thesis 1073 Lime Requirement A lime recommendation is based on the suitable target pH, the exchangeable acidity level in the soil, and the efficiency of the liming reaction The lime requirement of a soil is the amount of calcium carbonate (in tonnes) calculated to raise the pH of a hectare of soil 20 cm deep, under field conditions, to, and maintain at, 6.5 (Faithfull, 2002) The lime requirement cannot be calculated directly from the pH value because of the need to also neutralize reserve acidity, which is not reflected in the pH value However, pH and soil texture can be used to estimate the amount of lime approximately There are several methods for determining the lime requirement, including Adams-Evans buffer (Adams and Evans, 1962), Mehlich buffer (Mehlich, 1976), SMS buffer (Sims, 1996), and Lime Buffer Capacity (Kissell and Sonon, 2008) All these methods for estimating lime requirement need to be based on the soil samples being representative of the area sampled Fertilizer Recommendations Unfortunately, there is not much specialised literature that would offer detailed approaches on rates of application of fertilizer for citrus trees in the MD One of such was written by Phong et al (1996), the fertilizer recommendations are presented in Table for citrus plantations This promotion was mainly based on the experiences of farmers in the MD and reference materials related to the recommended fertilizer for citrus trees in the world In fact, soil fertility fluctuates throughout the growing season each year because of fertilizer addition, plant growth and development, plant harvest, and nutrient losses Therefore, regular soil testing and leaf analysis are crucially necessary to assess the current status of nutrients for both soils and plants These tests provide the necessary information needed to maintain suitable fertility year after year, and help to adjust and make appropriate fertilizer program decision In addition to yield response functions, fertilizer recommendations should be considered in relation to sustainability, thus, nutrient management should be based on the Fertilizer Best Management Practices (FBMPs), which address on the right source/product, right rate, right time, and right place (4Rs) More detailed discussions on the 4Rs are available in Roberts (2007), Force (2009), and Mikkelsen (2011) Fertilizer recommendation is a component within the FBMPs because understanding the details of fertilizer recommendation philosophy may contribute to sustainable nutrient and soil management and hence toward sustainability for agriculture In general, there are some common methods used to make fertilizer recommendations These methods consist of the following (1) sufficient level of available nutrients, (2) build and maintenance, and (3) base cation saturation ratios All are based on either the plant or the soil Table Fertilizer recommendation for citrus trees (adapted from Le Thanh Phong et al., 1996) Age of tree 1-3 4-6 7-9 > 10 N P2O5 (g/tree) K2O 50-150 200-250 300-400 400- 800 50-100 150-200 250-300 350-400 60 120 180 240 18 Soil Degradatio on of Raised-b beds on Orchaards in the Mekkong Delta - Field F and Labo oratory Methodds Figuree Soil test interpretatio ion categoriees redrawn from f Havlin n et al (19999) Sufficciency level of avvailable nutriennts (SLAN): the t focus of this t approach h is fertilizin ng the plantt To providde the best response, r thiis provides the t greatestt economic benefits to the plant producer p T The amount of fertilizeer recommen nded dependss on the soill test level; fertilizer f is on nly applied when the soil s test leveel drops belo ow some criitical ranges to achievee optimum reesponse of th he plants, as shown s in Figg The SLA AN methodd requires reegular and accurate a soill testing to determine the t nutrient requiremen nt for the cuurrent status of the plan nts, and preccise knowledge of optim mum application rates for particular p plan nts Buildd and maintenaance fertility: Fiigure 10 reprresents a moddel for fertilizzer recomm mendation suuggested by Ohio O State University U In n this approach, build-up p range is co onsidered as an indication n of nutrientt deficiency for f plant growth g Ferrtilizer additiion provides for increasing availab ble nutrients to the criitical test levvel Plant ressponse to ferrtilizer additiion graduallly decreasess as soil test t level reaches r criticcal level For F mainten nance range, nutrients aree added based d on the estim mation of plaant removaal or losses for f maintainin ng the soil nutrient n levell In this ran nge, plant yield y does not n respond to nutrientt application n Soils in the t mainten nance range are usually not n expected to improve yield y but in the t purposee of maintain ning soil test levels over time t In draw wdown rangee is at the soil s test levells above the maintenancee limit Fertillizer addition n is made to o slow the drraw of nutrieent levels dow wn and quickkly become as soil tesst levels incrrease This method m is on nly suitable for f less mob bile nutrients such as P and a K Base cation saturatio ion ratio (BCSR R): fertilizer recommenda r ations are bassed on the concept that maximum plant responsee is only attaiined by creatiing the ideaal ratios of Ca, C Mg and K K If the ratio o is out of baalance, then the t applicattion of fertiliizer is recom mmended Thee ideal ratios of Ca, Mg and a K weree proposed by b Bear et al a (1945), Bear B and Totth (1948) T This approacch often reesults in thee greatest amount a of fertilizer beiing recomm mended These three t method ds may apply as general guuidelines for better b decisio onmakingg in the application of fertilizer f undder condition ns of soils and a farmingg system in the MD To T bring thiss about, it is i necessary to calibratte soil tests to o adapt to thee specific loccal conditionss of the MD In addition n, different methods m are based on th heir own assuumptions abo out what different d plan nts need, andd hence diffeerence of ferrtilizer quanttity recomm mended Thee choice of an a appropriatte method selection shouuld also nsider econom mic, social an nd environmeental targets 19 Pham Van Quang TRITA LWR PhD Thesis 1073 Figure 10 Fertilizer recommendation scheme published in Vitosh et al (1995) Table Different methods of irrigation scheduling Method Soil water managements - Observe and feel - Soil moisture sample - Neutron probes - Time Domain Reflectrometry (TDR) - Tensiometers - Electrical resistance blocks Soil water balance - Water budget approach - Atmometer Measured parameter Equipment needed Soil moisture content by feeling Soil moisture content by taking samples Soil moisture content Soil moisture content Hand probe Soil moisture content Auger, caps, oven Soil moisture content Neutron moisture meter TDR meter Soil moisture content Soil moisture content Tensiometers including vacuum gauge Electric resistance of soil Resistance blocks AC moisture bridge (meter) Soil moisture tension Soil moisture tension Climatic parameters: temperature, radiation, wind, humidity and expected rainfall, depending on model used to predict ET Reference ET Use of plant water stress criteria - Trunk or branch diameter Measuring of stem, change branch or fruit diameter - Leaf water potential Leaf cell water potential Irrigation criterion Soil moisture tension Weather station or available weather information Estimation of moisture content Atmometer gauge Estimate of moisture content Pepista system Estimate of moisture content Estimate of moisture content Estimate of moisture content crop water stress index Pressure chambers - Sap Flow Reference ET Sap flow sensors - Canopy measurements Surface temperature Infrared radiometers Soil moisture content Software Soil moisture content Growth and yield Software Water status of the plants at different stages Use of models - Models based on soil water balance - Mechanistic models Source: ICID/FAO (1996) and FAO-56 (Allen et al., 1998) 20 Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods Soil organic matter management Although soil organic matter (SOM) constitutes only a small fraction of the total matter of most soils, yet the dynamic SOM has a dominant influence on many soil physical, chemical, and biological properties (Bot and Benites, 2005; Brady and Weil, 2002) In the current status of land use in the MD, agricultural practices and cropping patterns have changed rapidly since the economic reforms in the 1980s (Vo-Tong Xuan and Matsui, 1998) The cultivation area has increased thanks to man-made constructions, such as dams, dykes, canal systems, soil rotation systems, agro-chemicals and the reclamation of so-called ‘problem soils’ These practices have led to soil reclamation and irrigation activities converting forest, bare and uncultivated land into agriculture land Changes in agriculture are also a big threat to soil fertility and increase the speed of soil degradation During the 1980s, intensification of cropping patterns was introduced to the MD in an attempt to increase food security Nowadays, most farmers use modern fertilizers, as an alternative to using animal manures or a combination of inorganic fertilizers and animal manures as before, though farmers mostly realise that there will be a problem with soil fertility, if SOM is completely exhausted Because of this, the intensive cropping has tended to reduce the organic matter content of the soil In addition, climate of the MD is characterized by high temperature, humid, and high rainfall, therefore, which accelerates decomposition rates and a quick release of nutrients into the soil Maintaining SOM content requires a balance between inputs and decomposition rates (Bot and Benites, 2005) The maintenance of SOM levels and the optimization of nutrient cycling are essential to the sustained productivity of agricultural systems (Bot and Benites, 2005) There are several ways to add organic materials to the soil such as mulches, composts or covering by crop patterns Especially, Biochar can be considered as another soil conditioner for improving SOM Many studies have shown that biochar can improve soil fertility and increase crop production For a deeper discussion on biochar is available in Lehmann and Rondon (2006), also Shackley et al (2012), and Yamato et al (2006) A recent study conducted by Southavong and Preston T R (2011) on growth of rice in acid soils amended with biochar, showed that biochar increased the biomass growth of rice and water holding capacity, raised soil pH from 4.5 to 5.13 and 5.40 Water management Characterised by two distinct seasons, farmers in the MD often irrigate their plant orchards during the dry season, except during some periods of drought in the rainy season The source of their water supply mainly comes from channels that are linked to the river system The drains are used to regulate water level in the cannels When the soil becomes dry or plant leaves seem to suffer the symptoms of wilting (usually identified by feeling the leaf), it is the time to irrigate the plants Water quality depends on the water characteristics of the rivers’; but the highest tide is usually considered as the best one The method has been commonly applied for irrigating the citrus orchards is a manually operated irrigation pump The pumping system is put on a boat and is moved along the adjacent cannels to spray water over the top of the plant canopy, because this is the most convenient and easy way in situ This research found the quantity of water is often not uniform on all the raised-beds, even on the same one This type of irrigation is more or less similar to a heavy rain and it may result in easily forming surface runoffs, erosion and soil crust, if the soil surface lacks a litter or vegetation cover Also excessive water 21 Pham Van Quang TRITA LWR PhD Thesis 1073 applications may reduce yield and quality, increase waste of water, increase the risk of nutrient leaching and lead to pollution of ground water, all of which are harmful effects on the environment In brief, the growers in the region are mostly relying on the use of intuition or subjective irrigation scheduling methods This method is based on instinct, knowledge, and experience gained over many years of farming However, this subjective technique is not accurate and heightens the chances of soil erosion It is highly recommended that a scientific irrigation schedule should be developed and implemented Improved irrigation water management will lead to water saving which can reduce irrigation costs, increase plant yields and farmers’ incomes Understanding soil moisture dynamic and plant response is of great importance from an economic and an environmental perspective As a component of best management practices, the overall goal of irrigation water is usually not only to maximise net profit over the long term, but also to meet sustainable goals of the environment, economy and society (Lincoln Environmental, 1997) The farmers have therefore to adopt the new technical and scientific approaches of the appropriate irrigation management practices Application of irrigation water needs to be met the aim of high yields and high water use efficiency (WUE) It also aims at avoiding environmental hazards Irrigation scheduling is an extremely important tool in developing a sustainable plant management strategy The core of irrigation water requirements mainly depend on plants, climate, and soils A good irrigation schedule must accurately indicate when to irrigate and how much water to apply In fact, there are many irrigation scheduling methods and models available to help the farmers with decisions in relation to when the plants require water and how much water needs to be applied A large number of techniques and methodologies are introduced in ICID/FAO (1996) and FAO-56 (Allen et al., 1998) These are shown in Table which can range from very simple (observing, felling, soil sampling) to intermediate and high technical methods requiring the participation of multi-lateral partners such as irrigation engineers, scientist and water resources engineers, consultants and farmers High technology equipment, such as computers and microchips has created devices for monitoring and recording that can be used to make real-time irrigation scheduling This involves monitoring of soil moisture content, plant water status or ET that describes the actual conditions of the plants currently growing Realtime monitoring systems are also adaptable to full automation, and capable of interaction with computer systems as well as the use of remote access through telecommunication systems (Phene et al., 1990) Although these high technological methods can bring many benefits, the appropriate monitoring devices are also needed These devices are not commonly available commercially in Vietnam, and so far are only being used for scientific experiments Due to the benefits to be gained from irrigation scheduling, it is recommended that promoting the use of automatic irrigation systems in the MD is necessary now, especially in orchards and upland cropping areas This action has the potential to contribute to sustainable agriculture development in the MD Nevertheless, implementing this is also a challenge for the following reasons: • The area cultivated per household is often small; • The level of farmers’ knowledge and skills is low and needs to be improved; 22 Soil Degradation of Raised-beds on Orchards in the Mekong Delta - Field and Laboratory Methods • Equipment for monitoring is expensive because at present it is imported from other countries and not yet manufactured in Vietnam C ONCLUSIONS AND RECOMMENDATIONS Orchards on the raised-bed system in the Mekong delta (MD) have considerably been affected by the tendency of soil deterioration processes In the present study, it represented that the soils have been deteriorated on both chemical and physical properties with the aging of raised-beds This research showed that raised-beds for growing citrus fruit trees in the Mekong Delta show clear symptoms of soil deterioration and it established a relationship between the aging of the raised-beds and a decline in their physical and chemical characteristics These declines were indicated through low pH soil, nutrient deficiencies and/or imbalances, increase in dry bulk density and decrease in hydraulic conductivity, decline in organic matter, and increase in soil penetration resistance However, the dynamic studies need to be followed by the ageing processes of the raised-beds to find further explanations of the impact on the clay content As indicated, by using a portable electronic penetrometer (or penetrologger) combined with registration of soil water content, measurements can be easily and speedily possible to assess the soil quality in terms of the constraints for root growth This is one of the commonly used non-destructive methods The penetrologger is able to store and process a great amount of data and the procedure can be repeated at many positions within a short period of time This research also raises some concerns regarding the poor fertilizer and irrigation management practices of farmers Because soil is a nonrenewable resource over the long term scales and is dynamic and prone to rapid degradation due to land misuse or poor management practices, appropriate approaches are needed to help farmers develop better management practices Good management practices must be based on plant, soil and water conservation techniques as well as the imitation of the natural environment In this way, one may prevent the many symptoms of soil and plant degradation developing instead of needed to react to them after they develop On the basis of the findings from this study, the following recommendations can be made: • Fertilizer application needs to be based on the plant requirements and the actual nutrient availability of the soil, provided that the soil pH is within a proper range, i.e the soil fertility status is managed to maintain optimal pH levels which make available sufficient nutrients for plant growth but without excess, which may lead to water pollution No more fertilizer and nutrients should be applied than the plant can use and the soil can store • Nitrogen and phosphorus need to be monitored because both can result in environmental damage when soils are too rich with respect to these nutrients • Soil organic matter (SOM) must be maintained at suitable level (3.4% SOM - threshold value) through appropriate practices • Comprehensive soil and plant tests need to be performed regularly to identify possible nutrient deficiencies (at least every two years) • Observing and recording the variability in plant yield and soil tests for each field should take place regularly in order to follow soil health changes and take corrective actions 23 Pham Van Quang TRITA LWR PhD Thesis 1073 • Develop and promote irrigation management that are founded on the technical and scientific approaches such as monitoring of soil moisture content, plant water status and evapotranspiration • Continue to improve our understanding of soil fertility and its associated problems in order to develop better and more suitable management techniques To achieve sustainable soil management targets, it is essential that several different entities cooperate, particularly farmers, scientists, and extension personnel Agricultural researchers play pivotal roles in developing agricultural technologies and management practices Extension personnel can be regarded as a bridge for researchers and farmers to make sure that the desired technologies and management practices smoothly transfer to the farmers Eventually, farmers directly work on the land and implement and adapt new technologies and practices Farmers have some knowledge and skills of technologies and practices but not enough For example, farmers may know more about what happens above ground than about interactions and processes below ground A good education, information and skills training program can improve knowledge and management abilities of farmers, and they can adapt to current and new technologies and techniques Moreover, knowledge gap between scientists and farmers also need to be identified to promote a better communication among the researchers, extension workers, and farmers This is an essential factor for improving transferability of knowledge, management skills, and technology F UTURE WORK Agricultural management is a term which refers to many activities closely related to the farming management, in which, using optimal nutrient and water management practices are required in order to grow healthy plants, improve and retain soil productivity Enhancing soil organic matter, avoiding excessive tillage, managing pests and nutrients efficiently, keeping the surface of the ground covered, preventing soil compaction and erosion, and diversifying cropping pattern systems are good procedures to maintain soil and plant conservation Soil nutrients and water management 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(2013) ? ?soil degradation of raised- beds on orchards in the Mekong delta - field and laboratory methods? ?? TRITA LWR PhD thesis 1073 ii Soil Degradation of Raised- beds on Orchards in the Mekong Delta. .. 43.21 Soil Degradation of Raised- beds on Orchards in the Mekong Delta - Field and Laboratory Methods Figure Average penetration resistance at the different age of the raised- beds and soil depths on. .. major soils Soil Degradation of Raised- beds on Orchards in the Mekong Delta - Field and Laboratory Methods Figure Cross section of raised- bed construction – reverse order compared to original soil