IJERD – International Journal of Environmental and Rural Development (2010) 1–1

Research article
erd

Rice Soil Fertility Classification and Constraints in the
Mekong Delta
VO QUANG MINH
Department of Land Resources, College of Agricultural and Applied Biology,
Cantho University, Vietnam
E-mail: vqminh@ctu.edu.vn

LE QUANG TRI
Department of Land Resources, College of Agricultural and Applied Biology,
Cantho University, Vietnam
Received 29 December 2009

Accepted 5 March 2010

Abstract In the Mekong Delta, Vietnam, areas of intensive rice production have been
rapidly enlarged. Soil fertility degradation in this system can be one of the most importantfactors contributing to the yield decline. Information on soil fertility and recommendations
on improving soil constraints will provide basic data for proper soil management, landevaluation and land use planning. A Fertility Capability Classification (FCC) system
incorporates characteristics of soil morphology, soil physics, and soil chemistry. Data from300 soil profiles in rice fields and 28 field experiments on fertilizers efficiency werecollected, showing that there were 25 rice soil fertility types, in which types CC (clay in
top and subsoil) and CCs (clay in top and subsoil and saline effected) occupied in large
extent. The major soil constraints for rice cultivation can be listed as follows: low organiccarbon content (o); high P fixation and high Fe toxicity potential (i); potential salinity (s-);
low available P (p); high acidity and Al toxicity (a); the separation of actual acid sulfate
soils (c, c-) and potential acid sulfate soils (f, f-).
Keywords FCC, soil fertility, constraints, modifier

INTRODUCTION
In the Mekong Delta, rice cultivation is usually irrigated and highly productive, featuring multiple
rice crops. They also face serious problems, including the unsustainable exploitation of water andsoils, inefficient use of chemical inputs, and emerging or worsening disease and pest problems. The
intense and increased pressure on land leads to its degradation and pollution, which may result in apartial or complete loss of its productive capacity.
According to Sanchez (2001), differences in soil taxonomy are important to establish thebroad picture, but what does it mean in agronomic and ecological terms? The problem with soil
taxonomy is that it quantifies only permanent soil parameters, most of which are located in thesubsoil. To overcome this limitation, a Fertility Capability soil Classification system (FCC) was
developed more than 25 years ago to interpret soil taxonomy and soil tests in a quantitative mannerthat is relevant to growing plants (Buol et al., 1975; Sanchez et al., 1982). It is now widely used
and is included in the worldwide FAO soils database (FAO, 1995). Most of class limits areborrowed from Soil taxonomy (Soil Survey Staff, 1994) or the FAO/Unessco soil classification
system (FAO, 1974). Emphasis is placed on features that are easily detectable in the field, such as
texture, color, depth of horizons, presence or absence of mottles, etc. Soil analytical laboratory data
are only used to support the classification if available. The strength of this system is its ease of use,which allows the soil to be classified at several locations simply and quickly. To facilitate the easytransfer of information about soil properties and constraints, the system consists of a series ofindividual letters to describe the soil. These properties signify fertility limitations with different
interpretations, and represented by small letters.
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Based on the integrated system for soil fertility evaluation, it was used for soil fertility
classification and recommendation for rice soil of the Mekong Delta, where rice cultivation
dominantly applies, which can assist the land use planner or agricultural extension officers in
identify the constraints, and recommend for proper use.
METHODOLOGYThe Fertility Capability Classification (FCC) from Sanchez et al (2003), with some modifications
from Vo Quang Minh (2007) was used as reference system. Approximately about 300 top andsubsoil samples in rice fields were collected within the Mekong Delta from 1996 to 2007 (Fig. 1)
for soil chemical and physical analysis (soil texture, pH, ECe, Al3+, Fe3+, K+, Na+; Ca2+, Mg2+, CEC,
organic matter, P available) and soil profile description based on the guideline of FAO (1974),
which was recommended by the system, and data of 28 field experiments on fertilizers efficiency
for rice cultivation. Fourteen (14) modifiers that indicate the soil fertility status and effect to rice
growth in the Mekong Delta were determined: a, a- (Al toxicity, low pH), c, c- (actual acid sulfate
soils), e (high leaching potential), k (low Nutrient Capital Reserves), f, f- (potential acid sulfatesoil), g+ (constant saturation), i (high phosphorus fixation), n- (potential Sodic), s, s- (saline), o (loworganic matter status), and p (low inherent P content), which efftect to rice root. In which,modifiers p, o, c, c-, f, f- were added by Vo Quang Minh (2007), and superscripts + or - indicate agreater or lesser expression of the modifier. The soil fertility was named as soil texture of each soil
layer (C, L, S) plus modifiers.

Fig. 1 The Mekong delta in Vietnam

RESULTS AND DISCUSSION
Soil fertility classification for rice cultivation in the Mekong Delta
The term “FCC” is used to indicate adaptation of the Fertility Capability Classification, as
developed by Sanchez and Buol (1982), and Sanchez et al (2003), and modified by Vo Quang
Minh (2007) to the soil and rice growing conditions present in the Mekong delta, which deal withsoil morphology, physic, and chemistry characteristics. We set out below the meaning of each soil
fertility type, including 25 soil fertility types, which was converted from soil map at 1/250.000
scale, identified by its corresponding letter and the corresponding limits (Table 1).

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Table 1 Summary of soil fertility capability classification (FCC) for rice cultivation in the
Mekong Delta (Based on a system from Sanchez et al, (2003), Vo Quang Minh(2007) and the conversion of rice soil map - WRB 1998 at scale 1/250,000)
FCC
CC
CCs
LC
CCcCCi+
CCai+
CCv
LLacp
LL
LLai
CCacps-CCfLLsLLcCCc-sLLfsLLfLLc-sLLi
CCacp
LLf-sLLi+
CCf-sLLf
LLacps-Soil fertility capability interpretation
texture is clay (C) within 50 cm from the soil surface
texture is clay (C) within 50 cm from the soil surface, severe salinity insubsoil (s)texture is loamy (L) between 0 and 20 cm, and a clay (C) between 20 and 50cm
texture is clay (C) within 50 cm from the soil surface, Moderately actualacid sulfate soils (c-)
texture is clay (C) within 50 cm from the soil surface, iron toxicity if
prolonged, soil submerged (i+)
texture is clay (C) within 50 cm from the soil surface, soil is very acid (a),iron toxicity, if prolonged soil submerged (i+)
texture is clay (C) within 50cm from the soil surface, cracking clays(v)texture is loamy (L) within 50 cm from the soil surface, soil is very acid (a),strongly actual acid sulfate soil (c), low inherent P content(p)texture is loamy (L) within 50 cm from the soil surface
texture is loamy (L) within 50 cm from the soil surface, soil is very acid (a),high phosphorus fixation (i)texture is clay (C) within 50 cm from the soil surface, soil is very acid (a),strongly actual acid sulfate soil, low inherent P content (p), slightly salinity
in subsoil (s-)
texture is clay (C) within 50 cm from the soil surface, moderately potential
acid sulfate soils (f-)
texture is loamy (L) within 50 cm from the soil surface, potential salinity (s-)
texture is loamy (L) within 50 cm from the soil surface, Moderately actualacid sulfate soils (c-)
texture is clay (C) within 50 cm from the soil surface, Moderately actualacid sulfate soils (c-), potential salinity (s-)
texture is loamy (L) within 50 cm from the soil surface, shallow potential
acid sulfate soils (f), slightly salinity in subsoil (s-)
texture is loamy (L) within 50 cm from the soil surface, moderately potential
acid sulfate soils (f-)
texture is loamy (L) within 50 cm from the soil surface, Moderately actualacid sulfate soils (c-), slightly salinity in subsoil (s-)
texture is loamy (L) within 50 cm from the soil surface, high phosphorus
fixation (i)texture is clay (C) within 50 cm from the soil surface, soil is very acid (a),strongly actual acid sulfate soil (c), low inherent P content (p)texture is loamy (L) within 50 cm from the soil surface, moderately potential
acid sulfate soils (f-), slightly salinity in subsoil (s-)
texture is loamy (L) within 50 cm from the soil surface, iron toxicity if
prolonged soil submerged (i+)
texture is clay (C) within 50 cm from the soil surface, moderately potential
acid sulfate soils (f-), slightly salinity in subsoil (s-)
texture is loamy (L) within 50 cm from the soil surface, shallow potential
acid sulfate soils (f-)
texture is loamy (L) within 50 cm from the soil surface, soil is very acid (a),strongly actual acid sulfate soil (c), low inherent P content (p), slightly
salinity in subsoil (s-)© ISERD93ha
328,382
327,099

%
18.6
18.4

160,0139.0

128,952

7.2

117,015

6.6

109,560

6.1

102,027
100,863

5.7
5.6

61,99054,0033.4
3.0

44,7392.5

35,7732.0

30,17528,4941.7
1.6

25,287

1.4

25,2101.4

21,7491.2

16,1800.9

15,9410.9

14,001

0.7

11,472

0.6

5,051

0.2

3,760

0.2

3,506

0.2

15

0.001

1,771,267

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IJERD – International Journal of Environmental and Rural Development (2010) 1–1

The constraints for crops cultivation
The attribute used in the system is the lower-case letters of the constraints that have been identified
for that soil indicated in the soil fertility type. Based on the field observations, soil analysis ofchemical, physical, rice field behavious, and from several field experiments results, the major soil
constraints for rice cultivation in the Mekong Delta can be identified and grouped as below.
Constraints related to soil mineralogy
High leaching potential (e): Soils with a low cation exchange capacity (CEC) have topsoils with alow organic matter content, a low clay content, clay minerals with low CEC, or all these properties.
These soils have a low inherent fertility and also a low capacity to retain nutrients added as
fertilizer. More exacting N management needed; identifies degraded paddy soils and low organic
matter; if potential H2S toxicity can occur if (NH4)2SO4 is used as N source; in coarse texture soils
often Mn deficient; application of large organic material keeps pH low even after flooding.
High phosphorus fixation (i): This constraint is caused primarily by a high content of free ferricoxides (Fe2O3) in the clay fraction, which fix phosphate ions in unavailable forms. It is a feature
also found in strongly acidic soils, and hence commonly associated with the a, or a- constraint,
aluminium toxicity. High P fixation by Fe; P deficiency likely; Fe toxicity potential; soils difficult
to puddle and will regenerate original structure rapidly.
Low Nutrient Capital Reserves (k): Low inherent fertility because of low inherent reserves of
wethearable minerals; potential K deficiency depending on base contents of irrigation water.
Low organic matter status (o): N deficient; response to N fertilization very likely; low ECEC on
sandy soils; N fertilizer should be applied in frequent, small doses.
Low inherent P content (p): Plant available P deficient; response to small additions of P
fertilization very likely.
Constraints related to soil reaction
Al toxicity, low pH (a, a-): These are soils in which the exchange complex is dominated byalumina. The problem is commonly described as one of strongly acidic soils, can be caused bystrong leaching from high rainfall, and mainly from oxidation of sulfidic material, which often
associated with c, c- modifiers. Aluminium toxicity will occur in aerobic layers.
Actual acid sulfate soils (c, c-): Al and Fe toxicity, low pH, and P deficiency, which originatedfrom oxidation of sulfidic material.
Potential acid sulfate soil (f, f-): Potential acid-sulfate soils, causing Fe and S toxicity whenanaerobic and Al toxicity when aerobic; depth at which f modifier occurs determines feasibility ofrice production; Zn deficiency common; prevent seepage from this areas.
Constant saturation (g+): Prolonged submergence causes Zn deficiency. N loss increased if soil isintermittently flooded and drained.
Saline (s, s-): Defines saline soils; drainage needed, consider conductivity of irrigation water.
Potential Sodic (n-): This soil has a slightly high content of sodium but is low in calcium andmagnesium salts causing soil dispersion, puddling, poor infiltration and poor aeration, and if
sodium is high in the plow layer, increased probability of surface crust formation. Defines sodic
soils; reclaiming with drainage and gypsum applications may be needed; Zn deficiency common.
Strategies for better utilization of soil and soil fertility conservation
The management requirements are given per interpreted soil property or group of properties. A
complete listing of all possible combinations is not given because only a limited number of
combinations of soil properties will be found in any area under consideration. At large scale,
however, interpretation of the soil properties in relation to farming systems, local expertise or rice
varieties could be a valuable extension tool.
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Table 2 Extent of modifiers and soil fertility constraints in rice soils of the Mekong Delta(Based on the conversion of rice soil map-WRB at 1/250.000 scale)
Modifiers
p
s
a
i+
cc
sv
fi
f

Soil fertility constraints
low available P
Strongly salinity of subsoils
Acid soil, iron, aluminium toxicity
High phosphorus fixation and high Fe toxic potential
Depth actual acid sufate in subsoil
shallow actual acid sufate soils
slightly salinity in subsoils
Cracking clays soils when working the soil, we meet obstacle because soil
usually flood out, rice root can be broken when soil is dry
Potential acid sulfate in subsoil
High phosphorus fixation
Shallow potential acid sulfate soils

Area (ha)327,099.4
327,099.4
323,183.1
231,628.0
198,914.3
159,619.2
156,840.9
102,027.1
72,757.0
69,944.7
28,716.8

Each soil unit can have more than 1 indicator

The management requirements are based on Smith (1989), Sanchez et al (1982, 2003), Vo
Quang Minh (2007), field observations, and several experiments in the Mekong Delta on soil
management, reclamation, fertility adaptation, rice varieties, etc. A description of soil fertility or
management constraint identified is given below.
Al toxicity, low pH (a, a-): Soluble and exchangeable acidity should be removed as much as
possible by leaching before applying amendments. Leaching with fresh water is efficient inremoving free H2S04, and efflorescences of soluble Fe and Al salts from the soil. Due to directly
effected by low pH, and high Al contents (Vo Thi Guong, 1997), on very low soil pH and very highAl, the amount of lime should be changing from 6 to 10 ton/ha (Vo Tong Xuan et al, 1982).
Actual acid sulfate soils (c, c-): Iron and aluminium or manganese toxicities and phosphorus
deficiency are common. Physical properties are very poor. Jarosite mottles occur at 10 to 50cm
depth (c) or more than 50cm (c-). This soil should not be drained. Draining results in a dramatic
decrease in pH. High liming rates (greater than 10t/ha every 3 to 4 years) or long term leaching
would then be required for crop production (Van Breeman and Pons, 1978). The most profitable
practice is shallow drainage to grow one crop of a medium-term rice (Vo-Tong Xuan et al., 1982).
High leaching potential (e): These soils suffer from a combination of low organic matter content
and a coase texture, resulting in an extremely low cation exchange capacity. Soil management
interventions to remedy these constraints are bound to be expensive and often unprofitable as they
imply a change in the clay-humus complex through considerable organic matter and/or highactivity clay inputs.
Potential acid sulfate soil (f, f-): When f soils are exposed to air and are low in calcium carbonate,
FeS2 is oxidized to ferric sulfate and free sulfuric acid, producing pH values on the order of 2 or 3.
Drained acid sulfate soils are extremely infertile. Flooded rice is often grown, since underconstantly reduced conditions, the pH is sufficiently high to eliminate aluminium toxicity.
High phosphorus fixation (i): High P-fixing soils can be identified as those with clayey topsoils
having red or yellowish colours indicative of high contents of iron oxides, usually accompanied bya strong granular structure. These soils require high levels of phosphate fertilizers or special P
management practices.
Low Nutrient Capital Reserves (k): Potassium fertilizers must be added. Usually the K fertilizer
rates are low on newly cleared land but they increase with time. Generally these soils have also
limited capacity to retain nutrients and the potassium, calcium and magnesium added can be easily
lost (Vo Thi Guong, 1997; Nguyen My Hoa, 2003).
Prolong submergence (g+): Prolonged submergence causes Zn deficiency, especially on all year
round cultivation soil remittently flooded and drained. H2S toxicity symptom can occurred if soil
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Potential Sodic (n-): Reclamation requires the replacement of Na+ on the exchange complex by
Ca2+ and leaching of Na+ out of the root zone. Soil permeability and internal drainage must also be
improved so the displaced sodium ions can be leached out of the root zone. Common mineralamendments used are: gypsum, phosphogypsum, calcite and other acid-forming salts like iron andaluminium sulfates, limesulphur and pyrites.
Low organic matter status (o): N deficient; response to N fertilization very likely; low ECEC on
sandy soils; N fertilizer should be applied in frequent, small doses. Increasing the levels of organic
matter in these soils would improve nutrient supply, increase CEC, increase water holding capacity(P. W. Moody et al, 2008).
Low inherent P content (p): P management should be considered as a long-term investment insoil fertility, and it is more effective to prevent P deficiency than to treat P deficiency symptoms. P
requires a long-term management strategy because P is not easily lost or added to the root zone bybiological and chemical processes that affect N supply.
Saline (s, s-): Na, Ca, Mg, Cl, and SO4 are the major ions involved. Presence of soluble salts
requires drainage and special management for salt-sensitive rice varieties. Total reclamation ofsaline soils is often impractical because of the lack of high quality water for irrigation and leaching.
Wetland rice production may be an economical alternative.
CONCLUSION
A Fertility Capability Classification (FCC) system, based on the work of Sanchez et al (2003), Vo
Quang Minh (2007) was used for rice soil fertility in the Mekong Delta relies mostly on the topsoil
properties that indicate the soil fertility capability and affect rice production and some properties at
subsoil which related to topsoil properties. There were 25 rice soil fertility types, in which clay soil
texture in both layers and without modifiers (CC) and the same as soil with saline (s) modifier(CCs) occupied largest extent. The major soil constraints for intensive rice cultivation in the
Mekong Delta including High P fixation and potential Fe toxicity (i+), Potential salinity (s-), Low
available P (p), acid and Al toxicity (a), respectively. However, constraints of actual acid sulfate (c,c-), potential acid sulfate (f, f-), and low organic carbon status (o), are major constraints forintensive rice cultivation in the Mekong Delta. The major strategies for better utilization of soil
could be the reclamation of acid sulfate and saline soils by leaching acid and soil toxicity, andimproving soil nutrient status such as N, P, K fertilizers and organic matter application.
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