INTRODUCTION
The Mekong River releases around 475 km3 of water yearly and is the 10th world’s most extensive and 12th longest river. Because of its large size and length, it is not surprising that the Mekong River flows through many countries in the East Asia region. During the Holocene era, the Mekong River’s sediments had a significant role in the formation of the Mekong Delta. Sea level has shifted numerous times as a result of sedimentation. As a result, seawater seeps deep into the rainfed portions of the delta during the dry season, affecting an area larger than one million hectares. The shallow groundwater is still salty even if the rainy season flushes and freshens the soil surface. When the river’s course changed in the past, it generated several river branches in the low terrain, where sulfur-rich and organic deposits have collected (Minh et al., 2020).
Besides, the intense agriculture raised soil pressure,
which in turn caused soil deterioration and pollution.
Soil properties are dynamic, because plants continuously take nutrients, so the nutrients in the soil are reduced. However, soil also gets additional nutrient elements from plant and animal remains that have accumulated in the soil decomposition and neutralization by soil microorganisms. The Mekong Delta is Vietnam’s primary rice-producing region.
Developing various dike systems for flood control also results in less alluvial material buildup, less fertilized soil, and more deteriorated soil (Guong, Hoa, Khoi, Dung, & Vien, 2007).
More than 25 years ago, the Fertility Capability Soil Classification system (FCC) was designed to evaluate soil taxonomy and soil tests quantitatively crucial to growing plants (Boul, Sanchez, Cate Jr., &
Granger, 1975; Sachs et al., 2012; Sanchez, Couto,
& Buol, 1982). Currently, It is widely used and listed in the FAO’s global soil database (FAO, 1976;
ARTICLE INFO Keywords:
Conversion Constraints FCCRecommendation WRB
Article History:
Received: February 20, 2021 Accepted: February 8, 2023
*) Corresponding author:
E-mail: vqminh@ctu.edu.vn
ABSTRACT
The rice crop intensification led to soil degradation and yield decline in the Mekong Delta, Vietnam. Therefore, it is necessary to classify soil fertility and identify the constraints for proper soil use. The soil legend classified by World Reference Based was converted to the Fertility Capability Classification system. The soil constraints and recommendations for appropriate services are suggested. The findings indicate seven primary soil categories, ten diagnostic layers, three diagnostic features, and three diagnostic materials in the rice soils of the Mekong Delta. Eleven soil constraints were found, most of which were associated with acid sulfate and saline soils. These constraints included low acidity, strong acidity, low available P, high P fixation, high potential Fe toxicity, slightly actual acid sulfate and shallow potential acid sulfate, deep potential acid sulfate, slight salinity, and strong salinity, low mineral supply capacity, limited organic carbon content, and low nutrient retention capacity.
Reclamation of acid sulfate and saline soils by leaching soil toxicity and boosting soil nutritional status with organic matter and P, K treatment were recommended for degraded, acid sulfate, and saline soils.
ISSN: 0126-0537
Cite this as: Minh, V. Q., Tri, L. Q., Khoa, L. V., Du, T. T., Vu, P. T., Dung, T. V., & Dong, N. M. (2023). Rice soil fertility classification in the mekong delta, Vietnam. AGRIVITA Journal of Agricultural Science, 45(1), 56-68. http://doi.
org/10.17503/agrivita.v45i1.2943
Rice Soil Fertility Classification in the Mekong Delta, Vietnam
Vo Quang Minh1*), Le Quang Tri1), Le Van Khoa2), Thai Thanh Du1), Pham Thanh Vu1), Tran Van Dung2),andNguyen Minh Dong2)
1) Department of Land Resources, College of Environment and Natural Resources, Cantho University, Vietnam
2) Department of Soil Science, College of Agriculture, Cantho University, Vietnam
2003; 2006; 2015). For instance, Chandrakala et al. (2021) classified soil fertility in the Dry Semi- Arid Land of the South Telangana Plateau, Andhra Pradesh, using this method. Most class boundaries are taken from soil taxonomy or FAO/UNESCO soil categorization system (Soil Survey Staff, 2014) to emphasize field-detectable properties, including horizon depth, texture, color, presence or absence of mottling, etc. The technique uses distinct letters to convey information about the soil’s limitations and physical properties. These properties, denoted by small letters, have various interpretations and recommendations (Sanchez, Couto, & Buol, 1982;
Sanchez, Palm, & Buol, 2003).
The strategic planning of sustainable soil management will be aided by evaluating rice soil fertility and recommendations. However, this resource’s data source is insufficient and difficult to comprehend, thus making it incapable of providing practical advice. Typically, it only contains information that is pertinent to assessments.
Therefore, the study aims to use soil categorization as the standard for evaluating soil fertility, identifying soil constraints, and recommending appropriate soil use. The analysis is based on the correlation between the FAO (2006) WRB categorization
standards and the Sanchez, Palm, & Buol (2003) FCC modifiers.
MATERIALS AND METHODS Data Collection
Data collection of 300 soil samples, and 423 profiles from various location in the Mekong delta, Viet Nam from 2007 to 2017 for soil profile description, classification (Fig. 1). Soil samples analyzied for chemical and physical properties determination, which were used for soil fertility classication, and soil use recommendations.
Soils and Soil Fertility Classification
The soil analysis and classification were carried out in the Land Resources Department, Can Tho University, Vietnam. Identification of soil diagnostics horizons, properties, materials used for WRB soil classification, constraints recommendation, and correlation were conducted based on FCC system criteria..
The Fertility Capability Classification (FCC) method of Sanchez, Palm, & Buol (2003), with revisions from Minh (2007), was used as a reference system in correlation with WRB system criteria to identify soil constraints and recommendations.
Fig. 1. Location of soil data collection in the Mekong Delta, Vietnam
RESULTS AND DISCUSSION The Soil of the Mekong Delta
The definition of soil diagnostics in the WRB system is based on the soil’s morphology and physical and chemical properties. The following descriptions are the classifications of the rice soil in the region: seven (7) major soil categories, ten (10) diagnostic horizons, three (3) diagnostic features, and three (3) diagnostic materials.
Diagnostic horizons:
- Albic E is a light-colored subsurface horizon in which clay and free iron oxides have been removed or separated. The color of the horizon is governed by the color of sand and silt, not by coatings on these particles.
- Argic B: The subsurface argic horizon has a higher clay composition than the horizon above - it.Histic H: The histic horizon is an organic soil layer poorly aerated and occurs at shallow depths on the surface or in the subsurface (Fig.
2).
- Mollic A: The mollic horizon is a dark-colored surface horizon, well-structured, with a moderate to high amount of organic matter and high base saturation.
- Umbric A: is a low base saturation and moderates to a high amount of organic materials. As a result, the Umbric horizon is a well-structured, dark-colored surface horizon.
- Plinthic B: A subsurface horizon known as the plinthic horizon is composed of kaolinitic clay, quartz, and other elements in an iron-rich,
humus-poor mixture. Exposure to repeated wetting and drying with unrestricted oxygen access permanently alters a hardpan or irregular aggregates (Fig. 3).
Fig. 2. Peat soil with Histic horizon and organic soil material
Fig. 3. Plinthic horizon
- Salic: The salic horizon is a shallow subsurface or surface layer that has been secondarily enriched with easily soluble salts.
- Sulfuric B: The sulfide oxidation process produces sulfuric acid in the highly acidic subsurface horizon known as the sulfuric horizon (Fig. 4).
- Umbric A: The umbric horizon is a thick, dark- colored, base-desaturated surface horizon rich in organic content.
- Vertic V: Due to shrinkage and expansion, The vertic horizon is a clayey subsurface horizon with ped surfaces that are polished and grooved
Fig. 4. Sulfuric horizon with Jarosite mottle
The soil properties of horizons are presented in Table 1.
Diagnostic properties:
- Alic: The term “Alic property” refers to highly acidic mineral soil containing much exchangeable aluminum.
- Gleyic: Soil materials develop gleyic properties when declining conditions persist over time (varying from a few days in the tropics to a few weeks in other areas); they become entirely saturated with groundwater unless drained and have a gleyic color pattern.
- Stagnic: If soil is not drained for a sufficient amount of time (in the tropics, it may range from a few days to a few weeks in other regions), it exhibits stagnant properties and color pattern, at least for the temporary period (Fig. 5).
The soil characteristics of diagnostic properties are presented in Table 2.
Diagnostic materials:
- Fluvic: Fluviatile sediments that regularly receive fresh material or have just received it are referred to as having fluvic soil material (Fig. 6).
Fig. 5. Stagnic soil property
- Sulfidic: Sulfidic soil material is a wet deposit that contains sulfur, primarily as sulfides (Fig. 7).
- Organic: Organic soil material is made from organic detritus collected at the soil’s surface whether it is wet or dry, and in which the mineral content has little or no impact on the soil’s properties, as Peat soil with Histic horizon showed above Fig. 2.
Fig. 6. Fluvic soil material
Fig. 7. Sulfidic soil material (identified by H2O2, pH
<2)
Table 1.Soil properties of major soil diagnostic horizons
Diagnostic Horizons pHH2OEC1:5N Organic matter
Avai P
Total Acid Exch exAl Total Base Base sat
Al sat
Na sat
CECClaySiltSand
ms/ cm
---%--- ---mg/100g---meq/10 0g------%---meq/100g---%--- --- Argic5.170.630.212.563.271.130.135.0039.878.101.0312.5455.1125.2519.64 Sulfuric3.221.210.252.531.909.983.564.9732.5423.311.5615.2753.1426.6320.23 Umbric5.540.590.222.373.440.480.508.4139.042.321.0021.5455.498.7334.78 Mollic4.681.030.291.942.602.700.7114.9777.723.681.9019.2653.0821.4825.44 Salic5.433.800.201.132.941.690.105.7235.720.623.3416.0149.9427.4522.61 Plinthic5.350.560.171.101.282,901.124.5824.576.000.6918.6451.1529.1919.66 Vertic5.320.780.200.803.101.130.035.3535.120.201.2115.2357.0123.6819.31 Histic5.001.030.9032.128.901.120.5018.5672.670.001.2125.5455.2233.1611.62 Table 2.Soil properties of major soil diagnostic properties Soil propertiespHH2OEC1:5N Organic matter
Avai PEx AlNa satBase satAl satNa satCECClaySiltSand ms/cm---%---mg/100g---meq/100g------(%)---meq/100g---(%)--- --- Alic3.440.710.221.091.826.540.96100.770.007.3212.17--- Rhodic4.540.620.241.562.271.011.6899.010.00-15.48-17.76- Stagnic5.410.540.202.801.560.15--0.00--54.2724.6327.97 Eutric5.610.290.181.671.090.251.3393.446.145.7617.3753.95-10.04 Dystric6.251.310.181.320.910.241.0917.280.451.6225.1757.1723.6518.94
Major soil groups classification:
- The soil of the Mekong delta can be divided into 9 (nine) primary soil groups according to the FAO (2015) soil classification criteria, cited from a World digital soil map (Sanchez, 2019).
Fig. 8 illustrates the distribution of the major rice soil groups. Only soils with rice cultivation were selected in the study area, and the rocky mountains were not included.
- Albeluvisols (AB): soils at 100 cm depth or less from the soil surface and have an argic horizon (as indicated above) with an uneven upper border brought on by albeluvic tonguing into the argic horizon.
- Alisols (AL): soil with an argic horizon (as described above) with a CEC of > 24 cmol/kg clay, either beginning within 100 cm deep from the soil surface and Alic properties are in the
middle part between 25 and 100 cm from the soil surface.
- Arenosols (AR): Loamy sand-textured soil or soil with a coarser texture that is at least 100 cm below the soil surface
- Fluvisol (FL): soil with fluvic soil material (as described above) that extends at least 50 cm from the soil surface and begins within 25 cm from the soil surface
- Gleysols (GL): which are within 50 cm from the soil surface and have gleyic properties (as described above)
- Luvisols (LV): soils with an argic horizon (as described above) and an overall CEC is at least 24 cmol/kg clay.
- Plinthosols (PL): soil with a plinthic horizon that begins within 50 cm from the soil surface and follows the definition given above.
Fig. 8. Major soil groups of the MD (as classified by WRB system)
Soil properties:
The rice soil properties of the study area were defined in the FCC system related to WRB soil classification criteria and described as follows:
a-: from 10 to 60% Al saturation at a depth of 50 cm topsoil; This characteristic is mainly in soils with a sulfuric horizon at depths of > 50 cm.
a: Soils with > 60% Al saturation to 50 cm depth of topsoil; or < 33% base saturation at pH 7.0; or pHH2O (1:1) < 5.0.
i: Soils with > 4% free Fe; or > 35% clay or Clay (C) type and mottles with hue = 7.5YR or 5YR or 2.5YR. Corresponding to Plinthic horizon or Rhodic Properties.
p: applied to topsoil only, with the phosphorus <
2 mg/100g (with Olsen testing method) or < 1 mg/100g (with Bray-II testing method).
s-: Soils with ECe < 4 mmhos/cm (25°C) within 100 cm of topsoil.
s: Soils with ECe > 4 mmhos/cm (25°C) within 100 cm of topsoil.
c-: Applied to Sulfuric horizon with pHH2O (1:1) < 3.5 after drying, with Jarosite mottle and with hue = 2.5Y or yellower, with chroma 6; at a depth of between 50 cm and 100 cm.
c: Applied to Sulfuric horizon with pHH2O (1:1) < 3.5 after drying, with Jarosite mottle and hue = 2.5Y or yellower, with chroma 6; at a depth of < 50 cm above ground level.
f-: Applied to potential acid sulfate soils, soils with sulfidic material between 50 cm and 100 cm of subsoil; or have pHH2O2 (1:1) < 2.0 in field conditions, no jarosite mottles with hue = 2.5Y at depth from 50 cm to 100 cm topsoil.
f: Applied to potential acid sulfate soils, soils with sulfidic material within 50 cm of topsoil; or have pHH2O2 (1:1) < 2.0 in field conditions, no jarosite mottle with hue = 2.5Y at a depth of 50 cm topsoil.
k: Soils with low cation base reserves, or low K exchangeable; usually < 0.20 cmol/kg of soil; or
< 2% if base ≤ 10 cmol/kg of soil. Hence, the cation exchange capacity on sandy soils is low, and the K content is also low, usually < 0.2 cmol/
e: Applied to low cation exchange capacity (CEC) kg.
soils; < 4 meq/100g of soil by the method of Σ Base + Al can be extracted by KCl (ECEC), or CEC < 7 meq/100g by Σ Cations at pH 7.0, or CEC < 10 meq/100g soil by Σ Cations + Al + H at pH 8.2.
o: Applied to topsoil only, with organic matter < 2%
or organic carbon < 1.16%.
Table 3 and Table 4 illustrate the relationship between modifiers and soil texture from the FCC system for rice production in the Mekong Delta, based on the primary soil group and diagnostic horizons, characteristics, and materials from the WRB system to soil chemical and physical data (Minh, Trí, Vu, & Dung, 2016). Finally, Table 5 shows the extent of soil fertility constraints and modifiers on rice soils and areas within the study area. The main constraints to growing rice in the region are the low available P (p), high subsoil salinity (s), acid soil, iron, aluminum toxicity (a, a-), high phosphorus fixation, and high Fe toxic potential i.
Soil Constraints and Recommendations for Rice Production
In Vietnam’s Mekong Delta, rice farming is expanding quickly. These advancements require better soil knowledge to facilitate effective planning, correct data interpretation, and extend outcomes to the new area. The horizons, materials, and properties of soil are meant to reflect characteristics of soils that are well known to occur in soils. They can also characterize and define soil classifications (Arrouays et al., 2014). The diagnostic horizons described in Revised Legend FAO (2014) have served as the foundation for WRB. The management requirements are based on the recommendations of Fanzo, Remans, & Sanchez (2011), Minh (2007), Sanchez (2020), Sanchez, Couto, & Buol (1982), Sanchez et al. (2009), Sanchez, Palm, & Buol (2003), and Smith (1989). The Mekong delta’s field observations and several experiments on soil management, reclamation, fertility adaptation, and rice variety selection were examined.
Within the rice soils, the strongly actual acid sulfate soils (c), slightly actual acid sulfate soils (c-), shallow potential acid sulfate soils, high P fixation and Fe toxicity capability (i), low accessible P (p), and 13 additional types of agricultural production constraints were identified by the inquiry. Acid sulfate soil has a deep potential (f-), is light (s-), and has a high salinity (s), Low organic carbon content, low nutrient retention, low ability to feed minerals, and low nutrient supply (o).
Constraints related to soil mineral
The potential of high leaching ability (e): soils that are predominantly members of the Arenosol soil group and have low cation exchange capacity (CEC), low levels of organic matter, clay content, clay minerals with low CEC, or all of these properties.
Soils are not very fertile and not very good at holding
on nutrients. Therefore, it is essential to apply more
N fertilizer; it is often accomplished by locating paddy soils that have degraded and contain minimal organic matter.
Table 3. The modifiers of soil properties (based on FCC) related to diagnostic horizons, properties, and materials (WRB)
No. WRB system FCC system
Diagnostic Horizons Diagnostic Properties Diagnostic Materials Modifiers
0-20 20-50 50-100
1 Thionic EpiOrthiThionic - a and p c -
2 Thionic EndoOrthiThionic - a- and p - c-
3 Thionic EpiProtoThionic Sulfidic - f -
4 Thionic EndoProtoThionic Sulfidic - - f-
5 Salic - - s s s
6 - HypoSalic - s- s- s-
7 - EndoSalic - - - s
8 - Sodic - s s s
9 Plinthic - - - i -
10 - Rhodic - - i -
Remarks: a-: Slightly acid, a: Strongly acid, i: High P fixation, p: Possible lack of P, s-: Slightly salinity, s: Strongly salinity, c-: Moderately actual acid sulfate soils, c: Strongly actual acid sulfate soils, f-: Deep potential acid sulfate soil, f: Shallow potential acid sulfate soil, k: Ability low supply mineral, e: Ability low nutrient retention, o: Low organic carbon
Table 4. The modifiers of soil properties (based on FCC) in different rice soil groups
No. Major Soil Groups FCC system
Modifiers Soil texture
0-20 20-50 50-100 0-20 20-50 50-100
1 Albeluvisols - - - C C C
2 Alisols - - - C C C
3 Arenosols k, e and o k - S S S
4 Fluvisols - - - L L C
5 Gleysols - - - C C C
6 Luvisols - - - C C C
7 Plinthosols - i - C C C
Remarks: C: Soil Texture is Clay, L: Soil Texture is Loam, S: Soil Texture is Sandy, k: Ability low supply mineral, e: Ability low nutrient retention, o: Low organic carbon
Table 5. The extent of modifiers and soil fertility constraints of rice soils and the areas in the MD
Modifiers Soil fertility constraints Area (ha)
p Low available P 327,099.4
s Strongly salinity of subsoils 327,099.4
a Acid soil, iron, and aluminum toxicity 323,183.1
i+ High phosphorus fixation and high Fe toxic potential 231,628.0
c- Depth of actual acid sulfate in the subsoil 198,914.3
c Shallow actual acid sulfate soils 159,619.2
s- Slightly salinity in subsoils 156,840.9
f- Potential acid sulfate in the subsoil 72,757.0
i High phosphorus fixation 69,944.7
f Shallow potential ASS 28,716.8
Remarks: Each soil unit can have more than one indicator
If (NH4)2SO4 is utilized as an N source, potential H2S toxicity can arise in coarse-texture soils. Additionally, soils with a coarse texture are susceptible to Mn shortage. Even after flooding, a substantial amount of organic material is applied to keep the low pH since low nutrient capital reserves (k) restrictions always occur. These soils have a very low cation exchange capacity due to their low organic matter concentration and coarse texture.
Interventions in soil management to address these limitations are pricy and frequently unsuccessful.
Additionally, they suggest that significant organic matter and/or high-activity clay inputs have altered the clay-humus complex.
High fixation of phosphorus (i): This limitation is caused mainly by the clay fraction’s high free ferric oxide (Fe2O3) content, which fixes phosphate ions into inaccessible forms. Furthermore, it is frequently linked to the a, or a- constraint, aluminum toxicity since it is found in very acidic soils. Fe toxicity potential, with high P-fixed by Fe in the root zone (Fig. 9), difficult-to-puddle soils, and rapid restoration of the original structure are the constrained factors.
It is primarily on acid sulfate soils with orthiThionic properties or Plinthosol soil groups. Iron oxide is high in high P-fixed soils, characterized by clayey topsoils with red or yellowish tints and a strong
granular structure. Because of this, these soils need high phosphate fertilizers or specific P management techniques.
Low Nutrient Capital Reserves (k): Limited inherent fertility due to low intrinsic stores of weatherable minerals; possible K deficit depends on the elemental makeup of irrigation water.
It primarily belongs to the Arenosol soil group.
Therefore, fertilizers high in potassium must be supplied. However, these soils typically also have a limited ability to hold onto nutrients, and additional potassium, calcium, and magnesium can be lost readily (Hoa, 2003).
The low status of organic matter (o): Low ECEC (Effective Cation Exchange Capacity) on sandy soils, N deficiency (Fig. 10), response to N fertilization extremely likely; N fertilizer should be administered in small quantities. It primarily belongs to the Arenosol soil type. Low ECEC on sandy soils; N fertilizer should be administered frequently in tiny doses; N deficiency; reaction to N fertilization extremely likely. According to Moody, Cong, Legrand, & Chon (2008), these soils’ nitrogen supply, CEC, and water-holding capacity would all be enhanced by adding more organic matter to them.
Fig. 9. Fe toxicity symptoms on rice root and leaf (a: Fe fixed in the root zone; b and c: P deficiency symptom on leaves)
Fig. 10. N deficiency in rice
Low content of inherent P (p): Deficiency of available P (Fig. 11); response to small P fertilizer applications is very likely. The soils are predominantly Arenosols with orthiThionic characteristics. P management is more effective in preventing P insufficiency than treating its symptoms, and it should be viewed as a long-term investment in soil fertility. The application of P fertilizer necessitates a long-term management strategy since it is not immediately removed or supplied to the root zone by biological and chemical processes that affect N supply.
Constraints related to soil reaction
Low pH, high Al toxicity (a, a-): These are soils where alumina’s the main component of the exchange complex. One of the severely acidic soils is how the issue is typically described. It can result from heavy rains that cause severe leaching, primarily from the oxidation of sulfidic material, which is frequently linked to c, c- modifiers. Aerobic layers will experience aluminum toxicity (Fig. 12), mainly on soils with orthiThionic properties.
Before adding amendments, leaching should eliminate as much soluble and exchangeable acidity as possible. Freshwater leaching eliminates
the soil’s free H2S04 and soluble Fe and Al salt efflorescences and as a result, these impact the low pH and high Al levels. In highly acidic soils with high levels of aluminum, Guong (2016) suggested increasing the amount of lime from 6 to 10 tons/ha (Xuan, Quang, & Tri, 1982).
Actual acid sulfate soil (c, c-): Low pH, Al and Fe toxicity, and P shortage were caused by the oxidation of sulfidic substances.
Phosphorus shortage is prevalent, as are iron, aluminum, or manganese toxicities. These soils are poor physical properties. At a depth of 2 to 50 cm (c), or greater than 50 cm, jarosite mottles appear (c-). The soil should be drained. The pH dramatically drops after draining. For crop productivity, high liming rates (more than 10t/ha every three to four years) or long-term leaching would be necessary (van Breemen & Pons, 1978). For one crop of medium-term rice, shallow drainage is the most promising technique (Xuan, Quang, & Tri, 1982).
Potential acid sulfate soils (f, f-): The depth at which the f modifier impacts the viability of rice cultivation. Potential acid sulfate soils can induce Fe and S toxicity when Al toxicity and anaerobic conditions occur. Zn shortage can happen.
Fig. 11. P deficit likely symptom
Fig. 12. Al toxicity symptom (a) and lime application for reclamation (b)
FeS2 is oxidized to ferric sulfate and free sulfuric acid when exposed to air in low-calcium carbonate soils, resulting in a drop of pH values up to 2 or 3. (Fig. 13). acid sulfate soils that have been drained are utterly barren. Flooded rice is frequently cultivated in poorer environments. The pH is high enough to remove aluminum toxicity completely.
Saline (s, s-): High soluble salt levels in saline soils necessitate drainage and soil testing of irrigation water.
The primary ions involved are Na, Ca, Mg, Cl, and SO4, which harm crops (Fig. 15). For salt- sensitive rice types, soluble salts require drainage and special handling. Due to a shortage of high- quality water for irrigation and leaching, complete reclamation of saline soils is frequently difficult. Rice production in wetlands might be a more affordable option.
Fig. 13. Acid salt (Al2(SO4)3OH6) accumulation on the surface of acid sulfate soil
Constant saturation (g+): Continual submersion results in a Zn deficit (Fig. 14).
Additionally, N nutritional loss rises if the soil is repeatedly inundated and dried for a long time.
Long-term submersion causes Zn shortage, especially in soil used for year-round farming that intermittently floods and dries out. Additionally, if the soil has a lot of organic debris, H2S poisoning symptoms may manifest (Ponnamperuma, 1977).
Fig. 14. Zn deficiency symptom on rice
Fig. 15. Rice crop damage by saline CONCLUSION
According to the WRB method, the rice soils of the Mekong Delta , Vietnam have seven (7) major rice soil groups (for rice cultivation only), ten (10) diagnostic horizons, three (3) diagnostic features, and three (3) diagnostic materials. There are 13 soil constraints for rice cultivation, including slight and strong acidity, high P fixation, high Fe toxicity potential, low available P, strongly and slightly actual ASS, shallow and deep potential ASS, slight and strong salinity, low ability of material supply and nutrient retention and low organic carbon. The reclamation of saline sulfate and saline soils involves releasing toxicity and acidity while improving nutritional status.
ACKNOWLEDGEMENT
The Land Resources Department and Soil Science Department at Cantho University have provided some funding for this study in exchange for sharing data, experiment results, and reports, as well as CanTho University’s support of the annual research and VLIR (CTU-Belgium) programs.
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