hiệu quả của phân hữu cơ trong cải thiện chất lượng đất, năng suất và phẩm chất trái trên vườn măng cụt và chôm chôm tại huyện chợ lách, tỉnh bến tre (tt e)

1

The PhD study was completed at College of Agriculture and Applied Biology,
Can Tho University.







Scientific supervisor: Prof. Dr. Vo Thi Guong.

Reviewer 1:………………………………
Reviewer 2: ………………………………
Reviewer 3: ………………………………








The Doctoral thesis is defensed in front of the Examination Council at University level held in
Can Tho University
Time:…………………… Date:……………………

This thesis can be found at:
1. Learning Resource Centre of Can Tho University
2. National Library of Vietnam




2

GENERAL INFORMATION
1. Introduction
Rambutan and mangosteen orchards are the major fruit orchards with about 2.350 ha at Cho
Lach district. Theses fruit are high economic value and high export potential. However the
average yield of rambutan and mangosteen was very low which was only about 2.1 and 0.8
tons per ha, respectively. Currently, surface area of these fruit orchards has been reduced
dramaticaly due to low yield and low fruit quality. The leaf blight of the rambutant trees and
the gamboge disorder of mangosteen fruit is a severely problem which lead to loose
remarkable income of farmers. Consequently, farmers have to destroy these fruit orchards and
replace by other plants. Finding the solution to improve mangosteen and rambutant fruit yield,
better quality, higher benefit return and sustainability production are very essential study to
contribution to the social-economic development of the area.
2. Aims of research
- To survey and to evaluate soil fertility in terms of chemical and physical and
biologicalsoil properties of mangosteen orchard in various age range of raised beds
- To evaluate the effect of different organic manure in improvement of soil fertility, fruit
yield and and fruit quality of rambutant and mangosteen orchards.
- To find the solution for mitigating gamboge disorder of mangosteen fruit and leaf blight

of rambutant through water management, balanced inorganic fertilization in
combination with organic amendment.
3. Meaning in science and practice
3.1 Meaning in science
- Providing scientific data on using compost in combined with balanced inorganic fertilizer in a
relatively longterm to improve soil chemical-physical and biological fertility, significantly
increasing fruit yield and fruit quality of mangosteen and rambutant orchards.
- Organic amendment in combined with covering the soil surface with plastic sheet resulted in
significantly decrease gamboge disorder ratio on mangosteen fruits.
- Providing scientific data on improvement of leaf blight of rambutant. A combination of
balanced N and K fertilizer and compost led to reduce remarkly leaf blight ratio on rambutan
orchards in comparision with farmers’ practice with a low K/N ratio.
3.2 Meaning in practice
Providing scientific information on the soil degradation in terms of physical, chemical and
biological aspects of mangosteen and rambutan orchards with the ages of raised beds over 20
years,. The research results indicated the important role of organic amendment in
improvement of soil constraints, enhancement of productivity, fruit quality and economic
return of mangosteen and rambutan orchards. A combination of organic amendment and
proper soil moisture management led to mitigating of gamboge disorder of mangosteen fruits.
3

Whilst, an application of organic fertilization combined with balanced K and N fertilizers
resulted in significantly reduce rambutants’ leaf blight. This management practices have
contributed to significant increase in fruit yield, economic value and farmers’ income. These
results need to be recommended to farmers for improving the cultivation technique to obtain
sustainable and higher economic efficiency of mangosteen and rambutan cultivation.
4. New findings of the research
The degradation of raised bed soils of mangosteen orchards with the age of over 20 years. The
soil was very acidic with the mean pH value of 3.5. Soil chemical properties such as soil
organic matter, exchangeable cations (Ca, Mg and K), avalible N and P were very poor. Soil

micro-organism activities were very low. Also, soil physical properties such as water capacity
and structural stability were low.
For mangosteen soil, an application of 22.5 kg of organic fertilizer per plant in combination
with balanced inorganic fertilizers resulted in improving of soil degradation and increase fruit
yield significantly. The practice for mitigation of gamboge disorder of mangosteen was highly
effective. It led to decrease about 45% of gamboge disorder of mangosteen through applying
organic and balanced inorganic fertilizers and covering the soil surface with plastic in rainy
season (the rate of gamboge disorder decreased from 64% to 19%) and consequently increase
fruit yield significantly.
With respect to rambutant orchards, application of 18kg of organic fertilizers in the form of
sugarcane filter mud, biogas sludge, Ox-eyes daisy compost resulted in improving of soil
fertility in terms of physical, chemical and biological aspects. Soil chemical properties such as
pH, organic matter, percentage of base saturation increased significantly. Soil physical
properties such as structural stability, water holding capacity and hydraulic conductivity also
increased significantly. Biological activity remarkably increased though increase in enzyme
catalase content in the soil. Application of organic fertilizer provided a significant increase in
rambutan yield compared with farmer’s fertilization or only application of inorganic fertilizers.
Application of inorganic fertilizers with a K/N rate between 0.9 and 1.3 combined with 18 kg
of organic fertilizer per plant helped reduce 60% of leaf blight rate of rambutan.
The benefit return obtained from cultivation of magosteen and rambutan was highest when
applying sugarcane press mud, biogas sludge in combination with balanced inorganic
fertilizers.






4

CHAPTER 3: METHODOLOGY
The research was carried out since March 2009 to August 2012 on four experimental orchards.
The contents of the research are presented generally through a diagram in Fig. 3.1.


Figure 3.1: Schematic prepresentation of the research contents
Object and scrope of the research
Object of the research: The research focused on mangosteen and rambutan orchards at Long Thoi,
Son Dinh, and Phu Phung Commune, Cho Lach District, Ben Tre Province, and the soil properties of
these orchards.
Scope of the research:
- Survey and evaluate farming practices of local farmers and factors that cause gamboge disorder of
magosteen and leaf blight of rambutan.
- Analyse and evaluate soil quality of orchards with different age of raised beds.
- Evaluate the effects of the improvement of magosteen and rambutan soil fertility in terms of
chemical, physical and biological aspects
- Applying balanced inorganic fertilizer in combination with organic fertilizer types in the form of
biogas sludge, sugarcane press mud, Perionyx excavatus’ manure, and Sphagneticola trilobata compost
in order to improve soil fertility, enhance fruit yield and quality. Applying organic fertilizer, together
with coverage of the tree foots with plastic aimed to reduce gamboge disorder of mangosteen and leaf
blight of rambutan.
Trial (1):
Applying
organic
fertilizer to
improve soil
fertility and
fruit yield

Surveying orchard soil

properties and gamboge
disorder in mangosteen
MANGOSTEEN
Improving
soil quality
and
enhancing
fruit yield
and quality
in
mangosteen
and
rambutan
orchards
RAMBUTAN
Trial (2):
Reducing
gamboge
disorder
and
enhancing
fruit yield

Surveying leaf
blight desease of
rambutan

Trial (4):
Applying
organic

fertilizer to
improve
soil
fertility,
fruit yield
and
qualtity

Trial (3):
Applying
organic
fertilizer and
K fertilizer
to prevent
rambutan
leaf blight
and improve
fruit yield

5

A. RESEARCH ON MANGOSTEEN
3.1.1 Methodology
The surveyed households had cultivated land area of 0.3 ha or more. Mangosteen tree was in the age of
giving fruits. Studied orchards were divided into four types based on the age of raised beds: (i) below
20 years; (ii) between 20 and 40 years; (iii) orchards with the cultivated duration between 40 and 60
years; and (iv) above 60 years.
3.2 Estimation of the effects of organic amendent on the improvement of soil fertility and
magosteen fruit yield.
Aim of experiment: Based on the survey and estimation of soil quality of mangosteen orchard,

experiment was carried out to aim at improving soil fertility and fruit yield of mangosteen.
Experimental design: The experiment was laid out in a randomized complete block design (RCB)
with 5 fertilizer treatments and 4 replications per treatment. The mangosteen orchard which was
experimented had been cultivated over 50 years and had the age of raised beds of 65 years. This
orchard had low productivity.
The experimented soil belonged to developing alluvial soils and was categorized as Endo Protho
Thionic Gleysols according to the soil classification system of FAO-UNESCO. The potential acidity of
the soil was medium.
The treatments of the experiment are the followings:
Treatment 1 (control treatment): Farmer’s inorganic fertilizer dose (1.8 kgN + 1.8 kgN + 2.0 kgP
2
O
5

+ 0,02 kg K
2
O.plant
-1
)
Treatment 2: Recommended balanced inorganic fertilizer dose (1.6 kgN + 1.5 kgP
2
O
5
+ 2.2 kg K
2
O.plant
-1
).
Treatment 3: 22.5 kg of Sphagneticola trilobata compost/plant + recommended dose in treatment 2
Treatment 4: 22.5 kg of Perionyx excavatus’ manure/plant + recommended dose.

Treatment 5: 22.5 kg of sugarcane press mud/plant + recommended dose
Treatment 6: 22.5 kg of biogas sludge/plant + recommended dose
The improvement of soil fertility and fruit yield and economic efficiency were analysed and evaluated
in the third crop after 3-year application of organic fertilizers.
3.3 Effect of organic amendment and plastic coverage on soil fertility, yield and gamboge
disorder of mangosteen fruits.
Aim of experiment: The experiment was done to aim at (i) evaluating properties of mangosteen
orchard soil and gamboge disorder status of mangosteen fruits, and (ii) studying methods for
mitigating gamboge disorder of mangosteen fruits and improving fruit yield through supplying
nutrients and adjusting soil moisture.
Experimental design: The experiment was laid out in a randomized complete block design (RCB)
with 5 treatments which one included two plants and 3 replications per treatment.
The treatments of the experiment are the followings:
6

Treatment 1 (control treatment): Farmer’s dose (0.4 kgN + 0.22 kgP
2
O
5
+ 0.02 kgK
2
O/plant);
without covering the soil surface with plastic.
Treatment 2: Inorganic fertilizer dose (1.5 kgN + 1.0 kgP
2
O
5
+ 2.2 kgK
2
O./plant) + 14,4 kg/plant of

biogas sludge; without covering the soil surface with plastic
Treatment 3: 28.8 kg of biogas sludge/plant; without covering the soil surface with plastic.
Treatment 4: Farmer’s dose + covering the soil surface with plastic at the onset of rains
Treatment 5: Inorganic fertilizer dose (1.5 kgN + 1.0 kgP
2
O
5
+ 2.2 kgK
2
O./plant) + 14,4 kg of biogas
sludge/plant; without covering the soil surface with plastic
Observed parameters
- Initial samples: Before fertilization, soil samples were collected to analyse some of soil
physical and chemical properties, such as soil pH
- Mid-cropping samples: After applying organic fertilization for three months, soil samples were
taken to analyze the following properties: pH, organic matter, available nitrogen, phosphorus
and potassium and exchangeable Ca, Mg, bulk density and soil structural stability.
- Fruit yield: Fruits were weighted to determine fruit yield at every harvet.
- Economic efficiency from application of organic fertilizer and balanced inorganic fertilizer was
analyzed.
- Soil water content was determined once a week since the soil surface was covered with plastic.
- For calculation of gamboge disorder percentage, on each tree 20 fruits were uniformly
randomly harvested in four directions of the canopy. Then the number of fruits with gamboge
disorder was counted. The percentage of gamboge disorder of mangosteen is calculated as the
following:

B. RESEARH ON RAMBUTAN
3.4. Experiment on evaluation of effects of organic and K fertilizer on the mitigation of leaf
blight of rambutan and improvement of fruit yield.
3.4.1 Suveying leaf blight status of rambutan

Eighteen orchards from two different land regions (riverine land and mound) were involved in the
survey.
Parameters relevant to cultivation were observed including the rate of inorganic and organic fertilizer,
K/N ratio of the fertilizer which farmers used. The relationship between K/N ratio and the degree of
leaf blight of rambutan was also evaluated.
In order to evaluate the degree of leaf blight, three orchards per each commune were chosen and in
each orchard choosing randomly three plants and on each plant collecting 120 leaves contributed
uniformly in four directions. The number of leaves getting leaf blight was counted relying on visual
7

obersivation. The degree of leaf blight is divided into three levels consisting of strong, moderate and
light level accoding to Le Van Be (2006). Table 3.1 represents different degrees of leaf blight.
Table 3.1 Assessment of leaf blight degree of rambutan
Level
The number of leaves getting leaf
blight in percentage
0
< 10% of total leaves
1
10 - 30% of total leaves
2
> 30 - 50% of total leaves
3
> 50% of total leaves
3.4.3 Evaluation of the practice for the improvement of leaf blight of rambutan
Aim of experiment:
The field experiment of application of organic and K fertilizer with various K/N ratios was carried out
to aim at mitigating leaf blight status of rambutan, improving soil quality and rambutan fruit yield.
Experimental design:
The experiment was established at Son Dinh Commune, Cho Lach District on a rambutan orchard with

the garden’s age of 12 years and the raised beds’s age of 20 years. Rambutan trees did not grow well
which provided low yield and low fruit qualtity. The leaf blight percentage was high. The research soil
lies on high landscape and belonged to developing alluvial soil types with medium potential acidity.
The soil was categorized as Endo Protho Thionic Gleysols according to FAO-UNESCO (2006).
The experiment was laid out in a randomized complete block design (RCB) with 5 treatments which
one included two plants and 3 replications per treatment.
Treatment 1 (Control treatment): Farmer’s dose with the K/N of 0.1 (2.0kgN + 3.0kgP
2
O
5
+ 0,2
kgK
2
O per plant)
Treatment 2: Farmer’s dose with the K/N of 0.1 (2.0kgN + 3.0kgP
2
O
5
+ 0.2kgK
2
O per plant ) + 18 kg
of biogas sludge per plant
Treatment 3: Farmer’s dose with the K/N of 0.9 (1.4kgN + 1.0kgP
2
O
5
+ 1.3kgK
2
O per plant) + 18 kg
of biogas sludge per plant

Treatment 4: Farmer’s dose with the K/N of 1.2 (1.4kgN + 1.0kgP
2
O
5
+ 1.7kgK
2
O per plant) + 18 kg
of biogas sludge per plant
Treatment 5: Farmer’s dose with the K/N of 1.3 (1.4kgN + 1.0kgP
2
O
5
+ 1.8kgK
2
O per plant) + 18 kg
of biogas sludge per plant
Observed parameters
Potassium content in leaves and soil was analysed. The leaf samples were taken according to four
degrees of leaf blight on different orchards which was gotten leaf blight.
On each tree, four branches were taken to determine green leaf area and count the number of leaves
getting leaf blight to evaluate the degree of leaf blight.
8

At the harvest, fruit yield was recorded and economic return was calculated.
3.5. Effect of organic fertilization on the improvement of soil fertility and yield and quality of
rambutan fruits.
Experiment was established to evaluate the improvement of soil fertility and fruit yield and quality.
Plants of the experimented orchard were grown for 17 years and the raised beds were established for
20 years. Under the SANSED project which experiments were carried out for tree crops before, the
experiment was continued from the fouth and fifth crop.

Aim of research
The experiment was done to aim at evaluating the effects of organic amendment on the improvement
of soil quality and yield and quality of rambutan fruits.
Experimental design
The experiment was designed in a randomized complete block design (RCB) with five treatments of
fertilizer and three replications per treatment. Each treatment had an area of 30 m
2
and consisted of
two plants. The studied soil was categorized as Endo Protho Thionic Gleysols according to FAO-
UNESCO (2006). Organic fertilizer amount used for the experiment was 18 kg per plant equivalent to
3.6 tons per ha.
The treatments of the experiment are the followings:
Treatment 1: Farmer’s dose (2.2 kgN +1.5 kgP
2
O
5
+ 0,3 kgK
2
O per plant).
Treatment 2: 18 kg of sugarcane press mud per plant + (1.5 kgN + 1.0 kgP
2
O
5
+ 1.7K
2
O per plant).
Treatment 3: 18 kg of biogas sludge per plant + (1.5 kgN + 1.0 kgP
2
O
5

+ 1.7 kgK
2
O per plant).
Treatment 4: 18 kg of

Perionyx excavatus’ manure per plant+ (1.5 kgN + 1.0 kgP
2
O
5
+ 1.7 kgK
2
O
per plant).
Treatment 5: 18 kg of Sphagneticola trilobata + (1.5 kgN + 1.0 kgP
2
O
5
+ 1.7 kgK
2
O per plant).
Observed parameters
Soil samples were taken at harvest of the fifth crop to analyze physical, chemical and biological
properties.
At harvest, about 4 kg of fruits were collected per each plant. Fruits were equally harvested in four
different directions of the canopy. Then the number of fruits per kg unit was counted.
Assessment of fruit quality: 30 fruits per plant were collected to assess the brix (sugar content) of pulp,
and weight of pulp. Nutrients in pulp such as Pb, Zn and K were also analysed.
Economic return or efficiency was calculated.
3.7. Method of soil analysis
Chemical properties: pH, NH

4
+
and NO
3
-
, easily decomposed organic N, available P, organic matter,
exchangeable cations (Ca, Mg and Na), Zn, CEC and base saturation.
Physcial properties: Bulk density, structural stability and available water content.
9

Biological properties: Microbiological population was determined by counting the number of colonies
grown in the environment of agar. The intergated environment TSA (Trypon Soya Agar) was used to
qualtify the total population of microorganisms in the soil. The Hutchinsion-Clayton environment
supplied with 1% CMC (Carboxyl Methy Cellulose) was used to grow and quantify the population of
microorganisms which are capable of decomposing cellulose (Subba Rao, 1984 và Ulrich và ctv.,
2008). Activity of enzym catalase in the soil was determined by standardized method of Drawgan-
Bularda (2000), while enzyme β-Glucosidase was determined by the color testing method of Eivazi and
Tabalabai (1988) with the substratum p-nitrophenyl- β glucopyranoside (PNG 0,05 M).
3.10. Content of nutrients in organic fertilizer
Nutrient content of various types of organic materials are presented in Table 3.2
Table 3.2 : Content of nutrients in organic materials
Organic material
Nutrient content (%)
N
P
2
O
5

K

2
O
CaO
MgO
C
Biogas sludge
1.45
0.55
0.36
0.06
0.27
37.0
Sugarcane press
mud


1.90
2.50
0.34
0.35
0.27
29.8
Perionyx
excavatus’ manure
0.60
0.21
0.81
0.003
0.34
5.4

Sphagneticola
trilobata (3-day
compost)
0.76

0.36

0.11

0.025

0.22

13.9

3.11. Data analysis
Experimental and surveyed data were arranged by using Microsoft Excel programme and analyzed
using statistical programme SPPSS version 11.5. The ANOVA was used to compare the diference
among treatments. Grapths and tables were made in Excel.










10

CHAPTER 4: RESULTS AND DISCUSSION
A. STUDIED RESULTS ON THE MANGOSTEEN ORCHARD
4.1 The general information on mangosteen orchards
In Cho Lach district, the mangosteen orchards that were between twenty and forty years old accounted
for 40%, whilst the younger orchards (below twenty years old) accounted for 9% of the total orchards
in the region. It was about 60% of fruit orchards that were grown between 20 and 60 years. However,
there were orchards that have been grown between twenty and seventy years. The cultivation
technique that used excessive and unbalanced inorganic fertilizers resulted in low yield and gamboge
disorder of mangosteen which occurred mainly on the younger ones (below 20 years old) and between
June and August of the year.The soil quality was low which was indicated through low pH, poor in
organic matter and nutrients, low in activities of microorganisms as well as water holding capacity and
structural stability.
Based on the surveyed results above, the experiment was carried out to aim to aim at investigating the
effects of organic fertilizer on the improvement of soil properties, mangosteen yield and mitigation of
gamboge disorder of mangosteen
4.2. The efficiency of organic amendment on soil properties and mangosteen yield.
The research results showed that the obvious effect of organic fertilization on the improvement of soil
fertility was indicated in the fourth crop. Also, the research results suggested that application of
organic fertilizer helped in improving soil pH, increase organic matter, promoting the supply of
available nutrients such as available N, exchangeable K and Ca (Figure 4.4; 4.5; 4.6; 4.7; 4.8 and 4.9),
and increasing cation absorption capacity and base saturation (Figure 4.11 and 4.12). The most
effective of organic amendment was found in the treatment applied with sugarcane press mud and
biogas sludge.

Figure 4.4: Effect of organic fertilizer on soil pH
11


Figure 4.5: Effect of organic fertilizer on soil organic matter content

Figure 4.6: Effect of organic fertilizer on soil available nitrogen

Figure 4.7: Effect of organic fertilizer on soil available phosphorus
12


Figure 4.8: Effect of organic fertilizer on soil exchangeable potassium

Figure 4.9: Effect of organic fertilizer on soil exchangeable Ca

Figure 4.10: Effect of organic fertilizer on soil cation exchange capacity (CEC)
13


Figure 4.11: Effect of organic fertilizer on soil base saturation
4.3.10 Efficiency of organic fertilizers on mangosteen yield improvement
Research result indicated that fruit yield was significantly increased since the third crop (Figure 4.13).
The highest yields were found in the treatments applied with biogas sludge, sugarcane press mud, and
balanced inorganic fertilizer and significantly differed from the other treatments (210% increase in
yield). The Peronyx excavatus’s dung also helped significantly increase mangosteen yield comparing
with the control treatment which only inorganic fertilizer was applied. On the other hand, blanced
inorganic fertilization also resulted in increase in yield compared with the control treatment.

Figure 4.13: Efficiency of organic fertilizers on mangosteen yield improvement
Treatment 1 (Control treatment): Farmer’s dose (1.8 kg N + 2.0 kg P
2
O
5
+ 0.02 kg K

2
O/plant);
Treatment 2: Recommended dose (1.6 kg N + 1.5 kg P
2
O
5
+ 2.2 kg K
2
O/plant);
Treatment 3: 22.5kg of Sphagneticola trilobata compost/plant + recommended dose;
Treatment 4: 22.5kg of Perionyx excavatus’ manure /plant + recommended dose;
Treatment 5: 22.5kg of sugarcane press mud /plant+ recommended dose;
Treatment 6: 22.5kg of biogas sludge/plant + recommended dose.
14

4.3.11 Economic efficiency of organic amendment on mangosteen orchard
It can be observed from the Table 4.6 that a combined application of inorganic fertilizer and biogas
sludge could produce highest profit. Next, sugarcane press mud is also considered the effective organic
compost. Besides, application of Perionyx excavatus’ manure and Sphagneticola trilobata could also
brings about high profit compared with inorganic fertilization. In comparison with farmer’s
fertilization (Treatment 1), recommended fertilization (Treatment 2) provided higher profit and
increased by 134%. Whilst, the profit of the biogas sludge amendment (Treatment 6) increased by
162% in comparison with the recommended fertilization (Treatment 2). Especially, biogas slude
amendment (Treatment 6) could result in an increase in profit by 218% compared with the control
(Treatment 1) or without organic fertilization.
Table 4.6: Economic efficiency of organic fertilizers on mangosteen orchard (Unit: 1,000 VND)
Treatment
Contents
1
2

3
4
5
6
Total expenses (ha/year)
16.322
23.583
29.983
36.383
28.703
28.703
Yield (kg/plant)
27,5
37,5
41,25
47,5
56,25
57,5
Number of plants (plant/ha)
160
160
160
160
160
160
Price
18
18
18
18

18
18
Gross values (ha/year)
79.200
108.000
118.800
136.800
162.000
165.600
Profit (ha/year)
62.878
84.417
88.817
100.417
133.297
136.897
Treatment 1 (Control): Farmer dose 1.8 kg N + 2.0 kg P
2
O
5
+ 0.02 kg K
2
O/plant);
Treatment 2: Recommended dose (1.6 kg N + 1.5 kg P
2
O
5
+ 2.2 kg K
2
O/plant);

Treatment 3: 22.5kg of Sphagneticola trilobata compost + Recommended dose
Treatment 4: 22.5kg of Perionyx excavatus’ manure + Recommended dose;
Treatment 5: 22.5kg of sugarcane press mud + Recommended dose;
Treatment 6: 22.5kg of biogas sludge +Recommended dose.
Urea (11.000 VND/kg); Superphosphate (3.000 VND/kg); Potassium Chloride-KCl (12.000 Vnd/kg); Biogas
sludge and sugarcane press mud (800 VND/kg); Sphagneticola trilobata compost (200 VND/kg); Perionyx
excavatus’ manure (1.500 VND/kg); Lime (2.000 VND/kg); Labour (120.000 VND/day)
4.4 Efficiency of organic fertilization and mulching of soil surface by plastic cover on yield and
gamboge disorder of mangosteen fruits
4.4.1 The efficiency of organic fertilization on mangosteen yield improvement.
It can be observed from the Figure 4.14 that organic and balanced inorganic fertilization in
combination with coverage of soil surface with plastic could significantly increase fruit yield
compared with only inorganic fertilization. In the case that the soil surface was not covered with
plastic or exposed to rains, organic fertilization brought about higher effect than only inorganic
fertilization regime or farmer’s dose. A combined application of organic and balanced inorganic
fertilizer increased fruit yield by 134%. Similarly, in the case that the soil surface was not covered with
plastic, orgnic fertilization in combination of balanced inorganic fertilizer increased yield by 385%.
Consequently, application of organic and balanced inorganic fertilizer in combination with coverage of
soil surface with plastic could noticeably improve mangosteen yield.
15


Figure 4.14: Efficiency of organic amendment and mulching of the soil surface on mangosteen yield.
Treatment 1: Farmer's dose (0.4kgN + 0.22kg P2O5 + 0.02kg K
2
O/plant); without mulching of the soil surface
by plastic cover (the soil surface exposed to rains)
Treatment 2: 14.4kg of biogas sludge + (1.5kgN + 1.0kg P
2
O

5
+ 2.2kg K
2
O/plant); with mulching of the soil
surface by plastic cover;
Treatment 3: 28.8kg of biogas sludge; without mulching of the soil surface by plastic cover;
Treatment 4: Farmer's dose; with mulching of the soil surface by plastic cover;
Treatment 5: 14.4kg of biogas sludge + (1.5kgN + 1.0kg P
2
O
5
+ 2.2kg K
2
O/ plant); with mulching of the soil
surface by plastic cover.
4.4.2 The efficiency of organic fertilization on the mitigation of gamboge disorder of mangosteen
fruits.
In the situation that the soil surface was covered by plastic a combined application of organic and
balanced inorganic fertilizer resulted in a decrease in gamboge disorder of mangosteen fruits by 45%,
although fertilization with farmer’s dose mitigated by 45% of gamboge disorder of fruits. Under
condition that the soil surface was exposed to rains (without mulching by plastic cover) and high soil
water content, balanced inorganic fertilization resulted in a decrease in gamboge disorder by 22% (Fig.
4.15). In the case that the soil was only applied with organic fertilizer, the gamboged fruit rate was
reduced by 22%. The results showed that reducing soil moisture in rainy season by muching the soil
surface by plastic cover could be considered an effective method to mitigate the gamboge disorder of
mangosteen fruits. In addition to above method, applying only balanced inorganic or inorganic
fertilizer without mulching the soil surface by plastic cover was also an effective method to reduce
gamboge disorder of mangosteen fruits. A combined application of organic and balanced inorganic
fertilizer, together with mulching the soil surface by plastic cover was the most effective method to
mitigate the rate of gamboged mangosteen fruits.

16


Figure 4.15: Effects of cover of the soil surface with plastic and organic amendment on gamboge disorder
of mangosteen fruits
Treatment 1: Farmer’s dose (0.4kgN + 0.22kg P2O5 + 0.02kg K
2
O/plant), without mulching of the soil surface
by plastic cover (the soil surface exposed to rains)
Treatment 2: 14.4kg of biogas sludge + (1.5kgN + 1.0kg P
2
O
5
+ 2,2kg K
2
O/plant); without mulching of the
soil surface by plastic cover.
Treatment 3: 28.8kg of biogas sludge, without mulching of the soil surface by plastic cover;
Treatment 4: Farmer’s dose, with mulching of the soil surface by plastic cover;
Treatment 5: 14.4kg of biogas sludge + (1.5kgN + 1.0kg P
2
O
5
+ 2,2kg K
2
O/plant), with mulching of the soil
surface by plastic cover.
The question was given that whether soil moisture was a factor that contributed to gamboge disorder of
mangosteen fruits. According to the study of Osman and Milan (2006), much water uptake by plant
after dry season could result in gamboge disorder of mangosteen fruits. Some other studies also

suggested that gamboge disorder of mangosteen fruits was caused by a variation in soil water content
from low to high especially the stage between 45 to 50 days before harvest (Peet et al., 1995; Sdoodee
và Udom, 2002). The analysed result showed that soil moisture had possitve correlation with gamboge
disorder of mangosteen fruits in the period of 32 to 40 days before harvest (R
2
= 0.55). This result
fortified the effects of the method of mulching of the soil surface by plastic cover at the beginning of
rainy season.
On the other hand, application of organic and balanced inorganic fertilizer combined with cover of the
soil surface by plastic improved some soil chemical, physical and biological properties as well as
enhanced soil organic matter and biological activities and therefore soil fertility. The improvement of
these soil properties resulted in a significant increase in fruit yield. Analysed results showed that soil
properties such as soil pH, available N, base saturation percentage, microorganisim density and
activity of enzyme Catalase all increased (Fig. 4.16; Fig. 4.17; Fig. 4.18; Fig. 4.19 and Fig. 4.20)


17


Figure 4.16: Effect of soil surface cover and organic fertizer on soil pH.

Figure 4.17: Available N content in the soil.

Figure 4.18: Effect of soil surface cover and organic fertizer on percentage of base
saturation.
18


Figure 4.19: Effect of soil surface cover and organic fertizer on the soil microorganisim density

Figure 4.20: Effect of soil surface cover and organic fertizer on the variation of enzyme
Catalase content in the soil.
Treatment 1: Farmer's dose (0.4kgN + 0.22kg P2O5 + 0.02kg K
2
O/plant), without mulching of the soil surface
by plastic cover (the soil surface exposed to rains)
Treatment 2: 14.4kg of biogas sludge + (1.5kg N + 1.0kg P
2
O
5
+ 2.2kg K
2
O/plant), without mulching of the soil
surface by plastic cover;
Treatment 3: 28.8kg of biogas sludge, without mulching of the soil surface by plastic cover;
Treatment 4: Farmer's dose; with mulching of the soil surface by plastic cover; Treatment 5: 14.4kg of biogas +
1.5kgN + 1.0kg P
2
O
5
+ 2.2kg K
2
O/ plant) with mulching of the soil surface by plastic cover.
The research results indicated the effect of organic and balanced inorganic fertilization, together with
mulching of the soil surface by plastic cover on the decrease in soil moisture, improvement of soil
environment in terms of fertile and biological aspect and therefore fruit yield. Thus, magosteen orchards
which have been cultivated in a long duration were applied with organic and balanced chemical fertilizer
together with soil surface cover in the rainy season contributed to the improvement of fruit yield and
mitigation of gamboge disorder of magosteen fruits.
4.4.3 Economic efficiency

The economic efficiency for treatments are presented in Table 4.7. Calculated resuts showed that a
combied application of organic fertilzier and balanced inorganic fertilizer, together with covering the
soil surface with plastic resulted in very high benefit for gardeners.
19


Table 4.7: The economic efficiency for each treatment in mangosteen orchard (Unit: 1,000 VND).
Contents
Treat 1
Treat 2
Treat 3
Treat 4
Treat 5
Total expenses (ha/year)
9,100
36,040
33,640
14,100
41,040
Yield (kg/ha)
1.750
5.500
4.500
2.250
6.750
Gamboged fruits percentage (%)
64.17
36.67
42.22
25

19.25
Price
18
18
18
18
18
Gross value (ha/year)
31,500
99,000
81,000
40,500
121,500
Gross value – gamboged fruit
percentage (ha/year)
11,286
62,697
46,802
30,375
98,111
Profit (ha/year)
2,186
26,657
13,162
16,275
57,071
Notes: Number of trees: 250 trees/ha; Fruit yield = Fruit weight per tree x number of trees per ha
Treatment 1: Farmer's dose (0.4kgN + 0.22kg P2O5 + 0.02kg K
2
O/plant, without mulching of the soil surface

by plastic cover;
Treatment 2: 14.4kg of biogas sludge + (1.5kgN + 1.0kg P
2
O
5
+ 2.2kg K
2
O/plant), without mulching of the soil
surface by plastic cover;
Treatment 3: 28.8kg of biogas, without mulching of the soil surface by plastic cover;
Treatment 4: Farmer's dose, with mulching of the soil surface by plastic cover.
Treatment 5: 14.4kg of biogas + (1.5kgN + 1.0kg P
2
O
5
+ 2.2kg K
2
O/ plant), with mulching of the soil surface
by plastic cover.
Urea (11.000 VND/kg); Super phosphate (3.000 VND /kg); Potassium Chloride - KCl(12.000 VND/kg);
Organic fertilizer (2.000 VND/kg); Lime (2.000 VND/kg); Labour (120.000 VND.ngày
-1
); Gross value = (actual
yield – percentage of gamboged fruits) x price; Total expenses (inorganic, organic fertilizer, lime, canvas and
labour); Profit = Gross value - total expenses
B. RESEARCH RESULTS ON THE RAMBUTAN ORCHARD
4.5. Current status of rambutan leaf scorch disease
The surveyed results revealed that all the rambutan orchards were affected by leaf scorch disease
because most of the rambutan orchards have been applied with a very low potassium rate in
comparation with that of nitrogen. The ratio of K/N was calculated based on the total amount of

potassium and nitrogen fertilizer applied that was related to the rate of rambutan leaf scorch disease. In
general, the rambutan orchards which have been applied with organic fertilizers resulted in lower rate
of rambutan leaf scorch disease (less than 10% of the total scorched leaves per tree).
4.6. Approaches for improvement of the rambutan leaf scorch disease and fruit yield
4.6.1 The efficiency of organic amendment on the mitigation of rambutan leaf scorch disease
The data presented in Table 4.9 show that the rambutan leaf scorch disease was significantly decreased
with increasing the K/N ratio from 0.9 to 1.3 compared to farmers' fertilizer practice with the K/N ratio
of 0.1. The rate of the rambutan leaf blight decreased considerably in which the scorched leave area
was decreased to about 19 - 22% of the total leave area (level 1) compared to treatment of low K/N as
farmer’s dose ( level 3). The analysed result indicated that the treatments applied with chemical
20

fertilizer with the K/N ratio between 0.9 and 1.2 incorporated with organic fertilizer had high soil
exchangeable potassium content and the leaf blight rate was significantly decreased. This result was in
accordance with the previous studies that the ratio of K/N of the fertilizer applied to rambutan soils
should be one (Watson et al., 1988).
Table 4.9: The effects of K/N ratio and organic fertilizer on exchangeble potassium
concentration and scorched leaf area of rambutan (levels of disease).
Treatments
Exchangeable K
(cmol/kg)
The scorched leaf area
(%)
Levels of
disease
Treatment 1
1,00
b

79,58

a

Level 3
Treatment 2
0,99
b

46,24
b

Level 2
Treatment 3
1,43
a

22,33
c

Level 1
Treatment 4
1,47
a

22,16
c

Level 1
Treatment 5
1,77
a

18,95
c

Level 1
CV (%)
16,21
11,80

Treatment 1 (Control): Farmer’s dose 1 (K/N = 0.1) (2.0 kg N + 3.0 kg P
2
O
5
+ 0.2 kg K
2
O) per plant;
Treatment 2: Farmer’s dose 1 +18 kg of biogas sludge per plant;
Treatment 3: Farmer’s dose 2 (K/N = 0.9) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.3 kg K
2
O) per plant + 18 kg of biogas sludge per plant;
Treatment 4: Dose 3 (K/N = 1.2) (1.4 kg N + 1.0 kg P
2
O
5
+ 1,7 kg K
2

O) per plant + 18 kg of biogas sludge;
Treatment 5: Dose 4 ( K/N = 1.3) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.8 kg K
2
O) per plant + 18 kg of biogas sludge.
The results showed that organic fertilization together with increased K fertilization reduced
significantly rambutan leaf scorch. Application of inorganic fertilizer with the K/N ratio of about 0.9 to
1.3 supplied an adequate amount of potassium to the soil and contributed to mitigation of leaf scorch.
The analysed result of the total potassium uptake in leaves related to levels of leaf scorch suggested
that the balanced potassium concentration in leaves is a key factor for improvement of rambutan leaf
scorch. The leaf potassium uptake is shown in Figure 4.21. The leaf potassium uptake found was
highest in the rambutan orchards without leaf scorch disease and significantly differ from that in
orchards which the leaf potassium uptake was low (leaf blight: level 3).

Figure 4.21: Total leaf potassium uptake in orchards with different levels of leaf scorch
21

4.6.2 The efficiency of organic amendment on fruit yield improvement
Mitigating leaf blight disease is an important factor which contributes to improvement of fruit yield.
The results presented in Figure 4.22 show that the fruit yield of the treatment applied with organic
fertilizer and low K as famer’s dose increased significantly. However, organic fertilization in
corporation with reduced nitrogen and phosphate fertilizers and increased potassium with the K/N ratio
from 0.9 to 1.3 significantly increased fruit yield by 95% compared with that of the control treatment.
This fruit yield improvement might be due to a balanced nutrients amendment, improvement of soil
fertility and acceleration of soil microbiological activities and therefore fruit yield.

Figure 4.22: Effects of various K/N ratios and organic fertilizer on fruit yield

Treatment 1(Control): Farmer’s dose (K/N = 0.1) (2.0 kg N + 3.0 kg P
2
O
5
+ 0.2 kg K
2
O) per plant.
Treatment 2: Farmer’s dose + 18 kg of biogas sludge per plant
Treatment 3: Dose 2 (K/N = 0.9) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.3 kg K
2
O) per lant + 18 kg of biogas sludge per lant
Treatment 4: Dose 3 (K/N = 1.2) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.7 kg K
2
O) per plant + 18 kg of

biogas ludge per plant;
Treatment 5: Dose 4 (K/N = 1.3) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.8 kg K
2

O) per plant + 18 kg of biogas sludge per plant.
4.6.3 Economic efficiency
The results presented in Table 4.10 show that balanced inorganic and organic fertilization by
increasing the amount of potassium simultaneously reducing nitrogen and phosphorus resulted in an
increase in profit by 134%. The highest profit was found on the treatment with the K/N ratio = 1.2.
Consequently, the proper fertilizer formula which should be disseminated to rambutan gardeners
equals an amount of balanced inorganic fertilizer with the K/N ratio of 0.9 to 1.2 which is equivalent to
1.4 kg of N + 1.0 kg of P
2
O
5
and 1.3 to 1.7 kg of K
2
O per plant in combination with 18 kg of organic
fertilizer.
Table 4.10. Economic efficiency of different fertilizers treatment on rambutan orchards. (Unit:1,000 VND)
Content
Treatment
Treat 1
Treat 2
Treat 3
Treat 4
Treat 5
Total expenses (ha/year)
24,714
34,448
30,263
31,863
32,263
Yield (kg/plant)

40.67
54.5
73.33
78.33
78.33
22

Price
8
8
8
8
8
Plant number/ha
200
200
200
200
200
Goss value (ha/year)
65,072
87,200
117,328
125,328
125,328
Profit (ha/year)
40,358
52,753
87,065
93,465

93,065
Treatment 1 (Control): Famer’s dose (K/N = 0.1) (2.0 kg N + 3.0 kg P
2
O
5
+ 0.2 kg K
2
O) per plant.
Treatment 2: Farmer’s dose +18 kg of biogas sludge
Treatment 3: Dose 2 (K/N = 0.9) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.3 kg K
2
O) per lant + 18 kg of biogas ludge per plant
Treatment 4: Dose 3 (K/N = 1.2) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.7 kg K
2
O) per plant + 18 kg of biogas sludge per plant;
Treatment 5: Dose 4 (K/N = 1.3) (1.4 kg N + 1.0 kg P
2
O
5
+ 1.8 kg K
2
O) per plant + 18 kg of biogas ludge per plant;

Urea (11,000 VND/kg); Super phosphate (3,000 VND/kg); KCl (12,000 VND/kg) Biogas slugde (800 VND/kg;
Vôi (2,000 VND/kg); Labour (120,000 VND/day)
In short, leaf scorch disease of rambutan was considered to be related to the balance of nutrients.
Application of approximately 18 kg of organic fertilizer per plant, together with increased potassium,
decreased nitrogen and phosphorus resulted in improvement of soil physical properties through
increasing water infiltration, decrease in the rate of leaf scorch disease, increase in rambutan fruit yield
and therefore economic efficiency. However, the effectiveness of organic fertilizer on soil chemical
and physical properties and rambutan yield need to be estimated in long-term application.
4.7 Efficiency of organic fertilizers on soil fertility and rambutan fruit yield of improvement.
The issue is given that the research of the effectiveness of organic fertilizer and balanced inorganic
fertilizer on soil fertility in terms of physical, chemical and biological properties and fruit yield and
quality need to be done in the long-term.
4.7.1 Efficiency of organic fertilization on the improvement of rambutan soil fertility.
Application of fertilizer with a high and unbalanced amount resulted in the degradation of soil fertility
in terms of physical, chemical and biological properties of orchard soils. The experient was carried out
on four different organic fertilizers consisting of sugarcane press mud, biogas sludge, Perionyx
excavatus’ manure and Sphagneticola trilobata. The organic fertilizer amount used for the experiment
was 18 kg per plant. The combined application of organic and balanced inorganic fertilizer was aimed
to compare its effectiveness with that of the control treatment or farmer’s fertilization. The analysed
results showed that soil properties of the treatments applied with organic and balanced inorganic
fertilizer such as pH, organic carbon, available N and P, exchangeable Ca and K, percentage of base
saturation, soil aggregate stability and microbiological activities were much improved and statistically
significantly (P < 0.05) differed from those of the control treatment (Fig. 28, Fig. 29, Fig. 30, Figure
31, Fig. 32 and Fig. 33). In order to enhance the rambutan soil fertility, application of the research
results should be recommended.
23


Figure 4.28: Efficiency of organic amendment on soil pH

Figure 4.29: Efficiency of organic amendment on soil organic carbon.

Figure 4.30: Efficiency of organic amendment on soil available nitrogen.
24


Figure 4.31: Efficiency of organic amendment on soil available phosphorus.

Figure 4.32: Efficiency of organic amendment on soil exchangeable potassium.

Figure 4.33: Efficiency of organic amendment on soil CEC
Treatment 1(Control): Farmer’s dose (2.2 kg N +1.5 kg P
2
O
5
+ 0.3 kg K
2
O) per plant;
Treatment 2: 18kg of sugarcane press mud per plant + (1.5 kg N + 1.0 kg P
2
O
5
+ 1.7 K
2
O) per plant;
Treatment 3: 18kg of biogas sludge per plant+ (1.5 kg N + 1.0 kg P
2
O
5
+ 1.7 kg K

2
O) per plant;
Treatment 4: 18kg of Perionyx excavatus’ manure per plant + (1,5 kg N + 1,0 kg P
2
O
5
+ 1,7 kg K
2
O) per plant;
25

Treatment 5: 18kg of Sphagneticola trilobata per plant+ (1,5 kg N + 1,0 kg P
2
O
5
+ 1,7 kg K
2
O) per plant
4.7.2 Efficiency of organic fertilizers on the improvement of soil physical properties
* Soil aggregate stability index
The research results presented in Fig. 4.36 show that the sugarcane press mud treatment had the
highest soil aggregate stability index and was significant different from the control. However, there
was not any significant difference in soil aggregate stability index between the sugarcane press mud
treatment and the other organic fertilization treatments.

Figure 4.36. Efficiency of organic amendment on soil aggregate stability index
* Soil water holding capacity
The influence of organic fertilizer on water holding capacity of the soil is presented in Fig. 4.39. In
general, all organic fertilizers increased the soil water holding capacity which was significant higher
that of the control treatment. Thus, application of organic fertilizers in a relatively long period

increased the soil water holding capacity.

Figure 4.39: Effect of organic fertilizer on soil water holding capacity.
Treatment 1(Control): Farmer’s dose (2.2 kg N +1.5 kg P
2
O
5
+ 0.3 kg K
2
O) per plant;
Treatment 2: 18 kg of sugarcane press mud per plant + (1.5 kg N + 1.0 kg P
2
O
5
+ 1.7kg K
2
O) per plant;
Treatment 3: 18 kg of biogas sludge per plant + (1.5 kg N + 1.0 kg P
2
O
5
+ 1.7 kg K
2
O) per plant;