Major Land uses on acid Sulfate soils of Hau Giang province, Vietnam

— In recent years, rice crop intensification, which has put a lot of pressure and led to a great change in soil resources and its distribution. In Haugiang province of Vietnam, under different land uses, the previous soil map (2008) has changed and out of date. The research aims to update the soil map of the province under different land uses. The collection of 175 soil profile description data, and 51 soil analysis profiles. Soil classification was followed WRB 2006. The results showed that two major soil groups were found, in which four diagnostic horizons (Mollic, Umbric, Plinthic, and Sulfuric); one diagnostic property (Gleyic); and one diagnostic material (Sulfidic) were identified, and 15 soil types were classified. The Gleysols soil group have 14 soil types (hamoGL, hamoGL(hu), monplGL), (moGL(ptip), moGL(ntip), moGL(dtip), (umGL(ptio), huGL(ptio), umpplGL(ntio), umGL(ntio), huGL(ntio), umGL(dtio), (mowsGL(ntip), umwsGL(ntio)) total area of 9,551.32 ha, accounting for 59.34%; while Anthrosols soil group have one soil type (RGah) area of 66,252.91 ha, accounting for 40.66%. soil group, where acid sulfate soils, low in soil pH and base saturation, are dominated. While rice cultivation is dominated on most of Gleysols soil groups, including alluvial and acid sulfate soils. The acid sulfate soils of the study area have low pH, high acidity, high Al content, and low base saturation, in which crops need high tolerance of low pH such as pineapple, sugarcane, fruit crops, but most of the crops should grow on a raised bed for easy to leach soil acidity and toxicity.


I. INTRODUCTION
Intensive farming in the Mekong Delta has been growing rapidly in recent years. Potential land exploitation is taking place very strongly. The intensive cropping process has greatly altered soil properties, especially accelerated soil degradation processes that depleted the nutrient supply of crops [5]. Formerly established soil map of the region has been changed but have not been updated and no longer respond to the practical situation [6]. Especially in Hau Giang province, in particular, many land-use models have been developed that bring high profit for people [7]. However, land-use changes and intensive cultivation does not pay attention to the conservation of soil resources, along with the changes in natural conditions, since soil properties have been changed [6]. Then, the soil properties and types in the region need to be updated according to the changing of land uses for further land use planning and recommendation.  [3]. Soil sampling: Sampling at 51 sites on the surface (Ap) and surface horizons of the actual depth of the soil horizon, to identify the main diagnostic horizons, properties, and material as described in [1] and [2]. Soil Classification Method: Use FAO's World Reference Based System (WRB) to classify soil based on soil diagnostic horizons, properties and materials and the rules of the system [1], [2]. Soil analysis: Soil samples was analysis for soil chemical a soil physics for major parameters in the lab of Department of Soil science, Cantho University for soil diagnostics, properties and materials identification and classification Soil mapping: Map of soil was updated from previous soil map based on the soil types classified, and contoured from previous soil map (2008), combined with land use status and field observation results. GIS: Mapinfo software was used to create the soil map.

III. RESULTS
Based on the results of soil survey (175 sites for soil profile description) in 7 districts (Chau Thanh A, Phung Hiep, Long My, Vi Thuy, Vi Thanh, Nga Bay and Long My Town) and 51 soil profiles for soil profile description and soil sampling for soil analysis. The soil map of Haugiang was updated. Soil chemical properties of some profiles and major diagnostic horizons, properties and material for soil classification are shown in below tables:

Diagnosis horizon, diagnostic properties and diagnostic materials 3.1.1. Diagnosis horizons
Based on the results of the survey, the description and soil analysis of the area showed that there are 4 major diagnostic horizons, according to WRB (2006) definition: -Mollic horizon: The thickness of the soil horizon at survey sites ranged from 20 to 60 cm, dark colour (Chroma ≤ 3), with the base saturation of 55.29 -81.73% and the organic matter (3.48 -8.31%). (Fig 1) -Umbric horizon: The thickness of the soil horizon at survey sites ranged from 20 to 60 cm, dark colour (Chroma ≤ 3), base saturation ranged from 28.41 to 48.06% organic matter (3.53 -8.24%). (Fig 2) -Plinthic horizon: The survey results show that the Plinthic horizon in Hau Giang province has a depth of 35-80 cm and ends at 70-150 cm depth (Fig 3) -Thionic horizon: the results of the soil survey showing that the actual acid sulfate soil (the Munsell colour of mottle is from 2.5Y 8/6-8/8 occurred at a depth of 40-150 cm and end at 100-200cm. Besides, sulfidic soil material is also identified. (Fig 4)

Diagnostic materials
According to field testing and soil analysis, only sulfidic material identified and often occurs at a depth from 30 to 150cm. (Fig 6)

Major soil groups and soil types in the province
As the above-identified diagnostic horizon, properties, and material, Hau Giang Province has two major soil groups including Gleysols and Anthrosols, which are shown in Table 3 and Figure 7. There are only two major soil groups as Gleysols and Anthrosol, in which Gleysols soil group occupied the largest area (96,709.98 ha, or 50.34%), while Anthrosols soil group is occupied 66,252.91ha (40.00%( The Anthrosols soil group occupied 66,252.91 hectares, accounting for 40.66% of the province and has only one soil type (Gleyic-Anthrosols).

Major land uses on soils of the study areas
According to Table 4, Fig 7 and 8, most of the soils in the province are alluvial soils, then crops can grow well on these soils, but at some soil types with the occurrence of sulfidic material can release toxicity if oxidized, causing high in toxicity and acidity, low pH, high Al 3+ and Fe 2+ , then damage to root crops. These soils have Orthi-Thionic properties, low pH, high toxicity, the result of oxidation of sulfidic material. On these soil types, rice can grow well if under the reduced condition and received freshwater, which can leach toxicity to the canals, and soils get high pH. Otherwise, If freshwater supplied, upland crops can grow well on these soils. On Anthrosol soil group, Orchard and Upland crops occupied the largest area, (35,240 ha), there is no rice on these soil groups, due to most of the soil is acidic, the occurrent of sulfidic material, low pH, high toxicity. While Pineapple and Sugarcane can tolerate low pH and high toxicity, and soils need to make a raised bed for leaching of toxicity. (Fig 9). The rest of the areas are urban, non-Agricultural land, or aquacultural. (Fig 11) On Gleysol soil groups, soils have Gleyic property, it means most of the soils under reduced conditions, higher soil pH and high toxicity, at sometimes of the year, soil can be oxidized to form the soil mottle. Especially, sulfidic soil material is oxidized to release toxicity then the soil has low pH, but in the wet season, because of high rainfall, soil toxicity leached out and rice can grow well on these soils (83,750 ha). However, upland crops such as sugarcane, corn, can growth on these soils if small raised bed created to kept soi dried and toxicity can be drained during the wet season (Fig 9, 10).

IV. CONCLUSION
The soil map of the province was updated based on the previous soil map. Two major soil groups identified from WRB system (Gleysols and Anthrosols). Four diagnostic horizons (Mollic, Umbric, Plinthic and Sulfuric), one diagnostic material (Sulfidic) and one diagnostic property (Gleyic) were identified. Within two major soil groups, 15 soil types have been identified: the Gleysols group of 14 soil types (hamoGL, hamoGL (hu), monplGL), moGL (ptip), moGL (ntip), moGL (dtip), umGL huGL, umGL (ntio), huGL (ntio), umGL (dtio), mowsGL (ntip), umwsGL (ntio)) with a total area of 96,709.98 hectares accounting for 59, 34% and Anthrosols with one soil type (ATgl), occupied 66,252.91 ha, accounting for 40.66% or 66%. Rice cultivation is dominated on Gleysols while Orchard and Upland crops with the raised bed are mainly on Anthrosols soils where acid sulfate soils are dominated. The acid sulfate soils of the study area have low pH, high acidity, Al toxicity and low base saturation, in which crops need high tolerance of low pH such as pineapple, sugarcane, fruit crops, but most of the crops should grow on a raised bed for easy to leach soil acidity and toxicity.