Chemical properties during different development stages of fruit orchards in the mekong delta (Vietnam)

doi:10.4236/as.2011.23049

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Vol.2, No.3, 375-381 (2011)

doi:10.4236/as.2011.23049

Agricultural Sciences

Chemical properties during different development

stages of fruit orchards in the mekong delta (Vietnam)

Pham van Quang1, Vo thi Guong2

1Department of Soil Science and Natural Resources, Faculty of Agriculture and Natural Resources, An Giang University,

Long Xuyên; Vietnam; Corresponding Author: [email protected]

2Department of Soil Science and Natural Resources, Agriculture and Applied Biology College, Can Tho University,

Can Tho, Vietnam, Corresponding Author: [email protected]

Received 1 July2011; revised 28 July 2011; accepted 6 August 2011.

ABSTRACT

This study to examine soil fertility status was

conducted on 10 citrus plantations in Hau Giang

province within the Vietnamese Mekong Delta,

Vietnam. Fruit trees are mostly grown on the

raised beds to avoid annual flood with alluvial

soil type. Soil sampling was done in the dry

season of 2010 at two soil depths, for each

raised bed. Development ages of raised beds

were represented by two groups, young age

group (≤30 years) and old age group (>30 years).

For chemical analysis, pH, organic matter, CEC,

total nitrogen, 4,

–

3 and exchangeable Ca,

Mg and K were determined. The results showed

that the pH (water) was strongly acid. The CEC

was in average 19.2 cmol·kg–1 in topsoil (0 - 20

cm depth) and 18.7 cmol·kg–1 in subsoil (20 - 50

cm depth) for young age group. Similarly, the

CEC was 16.7 cmol·kg–1 in topsoil and 15.8

cmol·kg–1 in subsoil for old age group. Organic

matter on young age group (7.38% and 5.47% on

average for topsoil and subsoil respectively)

was significantly higher than that of old age

group (5.20% and 3.81% on average for topsoil

and subsoil respectively). Total nitrogen was

not significantly different between the sites for

the age groups of raised as well as the soil lay-

ers. Ammonium-N levels were excessive, and

–

3-N levels were high. Potassium and Mg2+

were significantly different between age groups

of raised beds and the same pattern between

soil layers, while Ca2+ did not vary significantly.

Potassium and Ca2+ levels were moderate, Mg2+

was high and P levels were very high. Soil fer-

tility in the raised beds subjected to an adverse

on plant growth and an imbalance in soil nutri-

ents under low pH conditions. Loss of soil qual-

ity was exhibited in reduced organic matter with

the aging of raised beds.

Keywords: Citrus Orchards; Soil Properties;

Alluvial Soil; Nutrient Balance; Soil Fertility; Mekong

Delta; Vietnam

1. INTRODUCTION

There are seven major soil types in the Vietnamese

Mekong Delta (MD); those are alluvial, acid sulphate,

saline, saline acid sulphate, old alluvial, peat and moun-

tainous [1,2].

The MD is the main area of fruit production that cov-

ered approximately 285,300 ha by the year 2009 ac-

counting for about 38% of the fruit tree area of Vietnam

[3]. Most of the land used for fruit trees in the MD is

lowland alluvial plain; therefore, raised beds are com-

monly constructed to avoid annual flooding. The raised

beds are long raised soil trips that are higher than the

original ground surface by piling up soil materials and

excavating from adjacent lateral ditches. The ages of

raised beds are also widely different which lots of them

have lasted more than 30 years old. Soil layers on the

raised beds are commonly arranged in reverse or the

same order as natural soil. Thus, the soil is disturbed by

the effect of human activities. With time, the soil may

have changed under influence of physical and natural

conditions as well as management practices. Soil degra-

dation is considered as a subsequent limitation on plant

growth. Major reasons of soil degradation are compac-

tion, loss of organic matter, salinization, nutrient deple-

tion and pollution [4,5]. Degradation of top soils may be

easily resilient, whereas degradation of sub soils is much

more difficult to restore and the degradation may even

be permanent [6].

A record from this investigation was that the plant

productivity has decreased as from the third or the fourth

P. van Quang et al. / Agricultural Science 2 (2011) 375-381

376

harvests due to young fruit drop and root rot. Many

growers had to plant/sow new seeds into their soil. In

some cases, they also had to introduce new soil below

the stumps but this did not improve the situations either.

A hypothesis can be raised related to soil degradation on

fruit plantations that the old raised beds may expose

more adverse restrictions to plant growth than the young

ones. Additionally, there are many factors, which may

cause the decrease in soil productivity, such as natural

conditions and soil tillage. Although soil is a major

source of nutrients that plants need to grow, the compo-

sition of fertilizer supplement is also necessary in return

for the losses by plant uptake. Studies on soil chemical

fertilizers, plants and their relationship were performed

to recommend various formulations of NPK fertilizers

by Phong et al. [7], Chau [8] and Hau [9], and these

have helped fruit growers achieve balanced fertilization.

However, the formulations of fertilizer application may

be out of date if they are not periodically updated based

on the nature of the soil in the field. Nutrient deficien-

cies and/or excesses have an adverse effect on plant

growth and resulting plant yields. High concentrations of

one nutritious substance in soil may cause imbalances or

deficiencies of other elements [10].

A problem is that most of the land growing fruit plan-

tation in the MD has acid soils with a low level of cal-

cium (Ca2+), magnesium (Mg2+), and high levels of iron

(Fe3+) and free aluminum (Al3+). Due to these character-

istics, phosphate is fixed in less available forms [10].

The fixation of phosphate and potassium may be severe

in some soils with a high clay content [11 ,12]. The acid-

ity and salinity effects of fertilizers are important con-

siderations in the selection and application of fertilizers

[13]. In addition, the fertilization habits of growers in the

MD often use a single rather than compound fertilizer,

which stress the use of nitrogen while phosphorus, po-

tassium, lime and micronutrients are less appropriately

considered.

The objectives of this paper were to 1) assess the soil

fertility by measuring pH, organic matter, CEC, total

nitrogen, 4



, 3



and exchangeable Ca, Mg and K;

2) compare different ages of the raised beds to see if they

differ in soil fertility; 3) identify possible nutrient im-

balances.

2. MATERIALS AND METHODS

2.1. Site Description

This study was conducted on 10 selected citrus plan-

tations in Hau Giang province, Mekong Delta, Vietnam

with different ages (Figure 1). The plantations were di-

vided into two main categories based on age: young age

group: less than 30 years of age (normally 10 - 30 years),

n = 4, and old age group: more than 30 years, n = 6 (Ta-

ble 1). The soil was classified as alluvial soil [14, 15].

Figure 1. (a) Provincial Administrative Boundary Map of the Mekong Delta, (b) Map of Hau Giang Province, (c) Studied Sites

Map―Extracted from Google Earth.

P. van Quang et al. / Agricultural Science 2 (2011) 375-381

377377

Ta b le 1. The ten study locations selected in Hau Giang prov-

ince, Mekong Delta, Vietnam.

Locations Latitude Longitude Construction year†

1 9˚53′44.38″ 105˚43′41.09″ 1995

2 9˚53′28″ 105˚43′57.14″ 1993

3 9˚53′35.74″ 105˚43′47.75″ 1991

4 9˚53′31.7″ 105˚43′42.78″ 1980

5 9˚53′28″ 105˚43′29.46″ 1979

6 9˚53′35.7″ 105˚43′41.27″ 1978

7 9˚53′29.18″ 105˚43′50.7″ 1978

8 9˚53′38.47″ 105˚43′45.34″ 1977

9 9˚54′2.7″ 105˚43′49.4″ Prior 1975

10 9˚53′29.22″ 105˚43′41.16″ Prior 1975

† the year that raised beds were established

This study was conducted on 10 selected citrus plan-

tations in Hau Giang province, Mekong Delta, Vietnam

with different ages (Figure 1). The plantations were di-

vided into two main categories based on age: young age

group: less than 30 years of age (normally 10 - 30 years),

n = 4, and old age group: more than 30 years, n = 6 (Ta-

ble 1). The soil was classified as alluvial soil [14,15].

The climate is characterized by two distinct seasons, a

dry season from January to April and a rainy season

from May to December. The annual rainfall ranges from

less than 1000 mm to over 1300 mm, however most of

the rain (90%) falls during the rainy season. The mean

temperature ranges from 23 - 25˚C during the coldest

months to 32 - 33˚C during the warmest months. The

humidity is highest in September (91%), and the lowest

in the dry season (79% - 82%).

The data was collected in the beginning of the dry

season, January 2010. Soil samples were taken from

each of the raised beds for chemical analyses at 0 - 20

cm and 20 - 50 cm depth with 4 replications at each site

and about 4 kg for each layer.

2.2. Soil Analysis

Soil samples were air-dried, homogenized and sieved

through a 2mm mesh screen to determine soil chemical

properties including pH, organic matter, CEC, total ni-

trogen, 4

H, 3

O and exchangeable Ca2+, Mg2+ and

K+.

pH was measured in water at 1:2.5 soil:water ratio.

Organic carbon was determined by the Walkley and

Black method [16] with the correction factor of 1.33

(recovery of 75%). The percent of soil organic matter

was calculated by multiplying the percent organic car-

bon by a factor of 1.724, following the standard practice

that organic matter is composed of 58% carbon [17].

Cation Exchange Capacity was determined by the bar-

ium chloride method. Total nitrogen was analyzed using

the Macro Kjeldahl method, where the soil sample is

digested by sulphuric-salicylic acid with a catalyst, and

titrated by volumetry with 0.1N H2SO4. The available

phosphorous of the soil was extracted according to Bray-

2 method and the concentration was then determined

colorimetrically [18]. The exchangeable cations Ca2+,

Mg2+, K+ and Na+ were extracted with BaCl2 and ana-

lysed by flame atomic absorption spectrometer. Inorganic

N (3



and 4



) was extracted with 2 M KCl fol-

lowed by determination according to standard colorimet-

ric pro- cedures.

Statistical analyses were performed using SPSS soft-

ware. One-way ANOVA with four treatments were per-

formed using a 5% significance level. The treatments

included:

 Topsoil: 0 - 20 cm depth and young age group of

raised beds: 10 - 30 years old;

 Subsoil: 20 - 50 cm depth and young age group of

raised beds: 10 - 30 years old;

 Topsoil: 0 - 20 cm depth and old age group of raised

beds: >30 years old;

 Subsoil: 20 - 50 cm depth and old age group of raised

beds: >30 years old.

3. RESULTS AND DISCUSSIONS

3.1. Soil pH (Water 1:2.5)

The results of soil pH showed that the pH was higher

in the young raised beds compared to the old raised beds

and the same pattern for soil layers (Tables 2 and 3).

However, there were no significant differences between

the two age groups of raised beds as well as between soil

layers for the young age group, while the pH was sig-

nificantly different between topsoil and subsoil for the

old age group.

The low pH may be caused by some factors such as

plant uptake of base cations, nutrition leaching, accumu-

lation of hydrogen ion (H+) and unbalanced fertilizer

Table 2. Soil pH (water), cation exchange capacity (CEC) and

organic matter of young-group and old-group of raised beds for

0 - 20 cm depth.

Group Site pH-water

(1:2.5)

CEC

(cmol·kg–1)

Organic matter

(%)

1 4.71 22.07 12.19

2 4.44 20.02 5.18

3 4.35 19.23 6.61

Young

4 4.24 15.59 5.54

Means4.43 19.23* 7.38*

5 4.43 17.37 4.45

6 4.36 14.87 4.57

7 4.35 18.73 4.46

8 4.49 16.78 4.36

9 4.21 16.17 4.53

Old

10 3.93 16.35 8.81

Means4.29‡ 16.71* 5.20*

*The mean difference is significant at the 0.05 level between age group;

†The mean difference is significant at the 0.05 level between soil layers for

young age group; ‡The mean difference is significant at the 0.05 between

soil layers for old age group.

P. van Quang et al. / Agricultural Science 2 (2011) 375-381

378

Table 3. Soil pH (water), Cation Exchange Capacity (CEC)

and Organic Matter of Young-group and Old-group of Raised

Beds for 20 – 50 cm Depth.

Group Site pH-water

(1:2.5)

CEC

(cmol·kg–1)

Organic matter

(%)

1 5.11 21.60 9.59

2 4.89 20.40 3.55

3 4.52 17.70 4.34

Young

4 4.13 15.15 4.38

Means 4.67 18.71* 5.47*

5 4.80 16.27 3.41

6 4.55 14.18 3.25

7 4.58 18.07 3.11

8 4.70 15.71 2.99

9 4.27 14.93 3.65

Old

10 4.23 15.74 6.47

Means 4.52‡ 15.82* 3.81*

*The mean difference is significant at the 0.05 level between age group;

†The mean difference is significant at the 0.05 level between soil layers for

young age group; ‡The mean difference is significant at the 0.05 between

soil layers for old age group.

supply. The range of soil pH for citrus growth is between

5.5 and 8.0, however, the best soil pH for citrus planta-

tion is between 5.5 and 6.5 [19]. When the pH falls be-

low 5.0, it can lead to aluminum toxicity and manganese

toxicity in root zone. A low pH condition can also cause

a deficiency of nutrients such as calcium, magnesium,

phosphorus, and molybdenum [20]. Decreasing soil pH

can also cause decline of the mineralization and nitrifi-

cation [21,22]. Soil pH at all sites was rated as strongly

acid [23], therefore, deficiency of major plant nutrients

such as calcium, magnesium, nitrogen and phosphorus

may occur. In the low range of soil pH, the adsorbed H+

and Al3+ may predominate the cation exchange capacity.

3.2. Cation Exchange Capacity (CEC)

Cation exchange capacity (CEC) in the two groups of

raised beds is shown in Tab les 2 and 3 for topsoil and

subsoil. The CEC of the old age group was significantly

lower than that of the young age group, while there were

no significant differences between topsoil and subsoil

for both age groups.

Cation exchange capacity of a soil refers to the amount

of positively charge ions that a soil can retain. When a

soil becomes more acidic the base cations are replaced

by H+, Al3+ and Mn2+ and this also produce higher CEC

values [24]. The higher the CEC, the more amounts of

nutrients the soil is able to supply [24]. The CEC of the

soil depends on the kind of clay, organic matter content,

pH and soil formation. The CEC of all study sites was

ranked as moderate [25], this may indicate the restriction

of soil nutrients to plants. However, the aging of raised

beds is also an important factor. Because decreases in

soil pH and organic matter with the aging of raised beds

may contribute to the decrease in negatively charge

components and this may then reduce nutrient holding

capacity of the soil.

3.3 Organic matter

Organic matter content for the young age group (7.4%

and 5.5% on average for topsoil and subsoil respectively)

was significantly higher than that of the old age group

(5.2% and 3.8% on average for topsoil and subsoil re-

spectively). The topsoil contained more organic matter

than the subsoil in both age groups (Tables 2 and 3). The

results showed that the amount of organic matter was

remarkably high for site 1 and site 10 compared to the

others. For site 1, the raised bed was built up on low

landform and there had existed a humus layer in the soil

profile with many decayed and semi-decayed materials

from 0.5 to 2 m depth. Because of the raised bed con-

struction, part of the humus layer below was brought to

the surface; this was probably a reason for the higher

organic matter content of site 1. For site 10, rather good

management practices of the owner’s raised bed garden

were executed by returning the residual materials of

plant or vegetation into the soil.

Organic matter play a crucial role in soil fertility,

buffering capacity, water holding capacity and stabiliza-

tion of soil structure [10,26].

3.4. Exchangeable Cations

Tab les 4 and 5 show the mean values for exchange-

able cations (K+, Na+, Mg2+ and Ca2+) between different

ages of raised beds and between soil layers.

The results showed that the concentrations of K+ and

Mg2+ were significantly lower for the old age group,

while it was not significantly different for Ca2+ (Tables 4

and 5). The base cations concentrations were higher in

Ta ble 4. Exchangeable cations of young-group and old-group

of raised beds for 0 - 20 cm depth.

K+ Mg2+ Ca2+

GroupSite (cmol·kg–1)

1 0.46 3.45 9.85

2 0.53 5.02 6.99

3 0.81 3.20 7.72

Young

4 0.26 2.52 5.24

Means 0.52*† 3.55* 7.45

% CEC 2.65 18.38 38.33

5 0.24 2.94 8.00

6 0.27 2.75 6.42

7 0.33 2.62 6.90

8 0.40 3.34 6.76

9 0.33 1.45 4.58

Old

10 0.46 2.81 7.46

Means 0.34*‡ 2.65* 6.69

% CEC 2.03 15.91 40.05

*The mean difference is significant at the 0.05 level between age group;

†The mean difference is significant at the 0.05 level between soil layers for

young age group; ‡The mean difference is significant at the 0.05 between

soil layers for old age group.

P. van Quang et al. / Agricultural Science 2 (2011) 375-381

379379

Ta ble 5. Exchangeable cations of young-group and old-group

of raised beds for 20 - 50 cm depth.

Group Site K+ Mg2+ Ca2+

1 0.54 3.25 9.69

2 0.33 6.43 5.71

3 0.58 2.66 7.08

Young

4 0.23 2.08 5.06

Means 0.42*† 3.60* 6.89

% CEC 2.23 18.83 36.56

5 0.25 2.52 6.96

6 0.22 2.29 5.99

7 0.29 2.93 6.09

8 0.26 3.17 6.23

9 0.22 1.69 4.41

Old

10 0.34 3.10 6.63

Means 0.26*‡ 2.62* 6.05

% CEC 1.66 16.51 38.34

*The mean difference is significant at the 0.05 level between age group;

†The mean difference is significant at the 0.05 level between soil layers for

young age group; ‡The mean difference is significant at the 0.05 between

soil layers for old age group.

topsoil compared to subsoil for within the same age

group as well as for between age groups. The amount of

K+ and Ca2+ was ranked as a moderate level, while Mg2+

was ranked as a high level [27]. The percent base satura-

tion of calcium, magnesium, sodium, and potassium in

the soil may indicate that there was a deficiency and/or

excess in the balance of available nutrients to plants. The

following proportions of the various cations suggested

by Abbott [28] for optimum crop performance are usu-

ally within the following ranges: potassium 1% - 5%,

magnesium 10% - 15%, calcium 65% - 80% and sodium

<2.5%. Based on most efficient range suggested above,

for this study, only exchangeable potassium/CEC (%)

was in a good range, while exchangeable magnesium

/CEC (%) was slightly higher and exchangeable cal-

cium/CEC (%) was rather lower (Ta b l e s 4 and 5). This

indicated that there existed an imbalance of cations in

both young and old age group of raised beds.

3.5. Total Nitrogen, Ammonium and

Exchangeable Nitrate

Total nitrogen varied in the range from 0.14% - 0.27%

in young age group and 0.15% - 0.22% in old age group

for topsoil (Table 6 ), from 0.10% - 0.13% in young age

group and 0.10% - 0.16% in old age group for subsoil

(Ta ble 7). Total nitrogen was not significantly different

between the two age groups, while it was significantly

higher in topsoil compared to subsoil for both age

groups (Tables 6 and 7).

Ammonium concentrations were 21.0 - 32.8 mg·kg–1

in young age group and 14.3 - 37.4 mg·kg–1 in old age

group for topsoil (Ta ble 6). For subsoil, the values are

15.6 - 33.6 mg·kg–1 in young age group and 20.8 - 32.8

mg· k g –1 in old age group (Table 7).

Exchangeable nitrate changed between 27.2 and 64.8

Table 6. Total Nitrogen, Exchangeable Ammonium, Ex-

changeable Nitrate and available P of Young-group and Old-

group of Raised Beds for 0 - 20 cm Depth.

GroupSite Ntotal NH4+ NO3– P

(%) (mg·kg–1)

1 0.2732.10 41.85 26.90

2 0.1421.02 27.18 23.01

3 0.1532.79 64.83 37.64

Young

4 0.1624.39 63.40 24.41

Means0.18†27.57 49.31*† 27.99

5 0.2214.31 44.62 31.87

6 0.1525.32 26.38 41.94

7 0.1918.00 24.25 43.21

8 0.1537.44 40.03 16.60

9 0.1928.21 44.81 41.97

Old

10 0.2234.67 33.87 19.78

Means0.19‡26.32 35.66*‡ 32.56‡

*The mean difference is significant at the 0.05 level between age group;

†The mean difference is significant at the 0.05 level between soil layers for

young age group; ‡The mean difference is significant at the 0.05 between

soil layers for old age group.

Table 7. Total Nitrogen, Exchangeable Ammonium, Ex-

changeable Nitrate and available P of Young-group and Old-

group of Raised Beds for 20 - 50 cm Depth.

GroupSite Ntotal NH4+ NO3– P

(%) (mg·kg–1)

1 0.2033.25 14.63 28.61

2 0.1015.65 15.06 25.75

3 0.1031.76 17.99 21.71

Young

4 0.1318.08 17.12 30.01

Means0.13†24.68 16.20† 26.52

5 0.1632.84 21.29 18.25

6 0.1120.78 13.37 31.68

7 0.1325.41 19.39 37.22

8 0.1032.61 19.47 24.23

9 0.1622.55 9.37 26.40

Old

10 0.1628.16 13.25 14.64

Means0.13‡22.18 16.02‡ 25.40‡

*The mean difference is significant at the 0.05 level between age group;

†The mean difference is significant at the 0.05 level between soil layers for

young age group; ‡The mean difference is significant at the 0.05 between

soil layers for old age group.

mg·kg–1 in young age group, between 24.2 - 44.8 mg·kg-1

in old age group for topsoil (Table 6). The values for

subsoil changed between 14.6 - 18.0 mg·kg–1 in young

age group, between 9.4 - 21.3 mg·kg–1 in old age group

(Table 7). The 3



amount in young age group was

significantly higher than that in old age group for topsoil,

however, for subsoil, there was no significant difference

between the age groups. Furthermore, there was a sig-

nificant difference on NO3

– between topsoil and subsoil

for the young age group as well as for the old age group.

Rating according to Bruce and Rayment [23], the soils

of this study were moderate in total nitrogen, high in

organic carbon and in C:N ratio. However, the C:N ratio

was lower in old age group than in young age group.

This indicated that the decomposition process was faster

in old age group; and therefore, there is usually a release

P. van Quang et al. / Agricultural Science 2 (2011) 375-381

380

of mineral N early in the decomposition process. For this

study, soil NH4

+-N levels were above 10 mg·kg–1 at all of

the observed sites, and NO3

–-N levels were excessive

and high for 0 - 20 cm and 20 - 50 cm depth respectively

as evaluated following Marx et al. [27] (Tables 6 and 7).

This exhibited that the soil contained too much fertilizer

residue.

3.6. Available Phosphorous

Phosphorous concentration was 23.0 - 37.6 mg·kg–1 in

young age group and 19.8 - 43.2 mg·kg–1 in old age

group for topsoil (Table 6). For subsoil, the range is 21.7

- 30.0 mg·kg–1 in young age group and 14.6 - 37.2

mg· k g –1 in old age group (Tab le 7). There was no sig-

nificant difference between the two age groups, but

phosphorous was significantly higher in the top soil

compared to the subsoil for the old age group. The re-

sults (Ta bl es 6 and 7) showed that phosphorous (on av-

erage values) was ranked as very high based on Abbott

[28].

4. CONCLUSIONS

The insufficient soil fertility in the raised beds has had

an adverse effect on plant growth. The results indicated

that there existed an imbalance in soil nutrients under

low pH conditions; therefore, this may lead to plant

production problems. Loss of soil quality was also ex-

hibited in reduced organic matter with the aging of

raised beds. Excess nitrogen and phosphorous in soil

may leach out to streams, rivers and groundwater.

In managing these fruit plantations on the raised beds

in the Mekong Delta, fruit growers should pay attention

on the following issues.

 Raise the soil pH;

 Balance fertilizer application;

 Manage the soil moisture;

 Improve the soil’s physical qualities.

5. ACKNOWLEDGEMENTS

We gratefully show appreciation to Mr. Ngo Xuan Hien, Agronomy

Bureau, Chau Thanh District, Hau Giang Province, Vietnam for help

and encouragement throughout the field works. We thank all raised

beds’ owners for their kindly support of study locations. We also thank

Huynh Ngoc Duc and Pham Xuan Phu for the assistance during data

collection; Ly Ngoc Thanh Xuan and analysis laboratory staff, Faculty

of Agriculture and Natural Resources, An Giang University, Vietnam

for their help with soil chemical testing. We are grateful to Per-Erik

Jansson and Carin Sjöstedt for providing very helpful comments to

improve this manuscript.

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