Geology And Soil
The soil of HCM City was formed upon two sediment classes: Pleixtoxen and Holoxen.
The Pleixtoxen sediment: This ancient alluvial sediment covers most of the northern, northwestern and northeastern parts of the city, encompassing Cu Chi, Hoc Mon, northern Binh Chanh and Thu Duc districts, north and northeastern District 9, and old inner-city areas.
Main characteristics of the sediment class are hilly terrain, with a depth range of three to 25 meters, and oscillation in the southeastern direction. Due to the combined effects of natural factors, including creatures, climate, time and human activities, and erosion and decomposition, the sediment class has developed into grey soil. Grey soil makes up 45,000 hectares or 23.4% of the city’s total soil area.
In the city, there are three types of grey soil: highlands grey soil with some impoverished places, grey soil with red and yellow-speckled layers, and gley grey soil. Soils of the two first types make up most of the total area. In general, the main mechanical constituents of grey soil are sandy soil mixed with heavy soil. The grey soil therefore has a poor water-containing ability. The underground water levels change from place to place and from season to season with a range of one to 15 meters. This is acid soil, as it has a pH of around 4.0 to 5.0.
The grey soil is of low fertility but has thick layers which are suitable for growing agricultural and forestry trees with high yielding and economic efficiency if rotational and intensive cultivation is implemented properly. Also, the foundation of grey soil is appropriate for infrastructure constructions.
The Holoxen sediment: This new alluvial sediment of HCM City had its origin in coastal areas, bays, riverbeds and alluvial plains and consequently formed different types of soil. Specifically, alluvial soil makes up 15,100 hectares or 7.8% of the total area; aluminum soil is 40,800 hectares or 21.2%; and alkaline soil is 45,500 hectares or 23.6%. The remaining area, 400 hectares or 0.2%, is made of sandy soil near the ocean and yellow-brown feralite soil on hills that have eroded.
+ Alluvial soil: Formed in the highlands terrain, which is as deep as 1.5-2 meters, mainly in the southern Binh Chanh District, the eastern part of District 7, the northern Nha Be District and some places in Cu Chi and Hoc Mon districts.
The alluvial soil is classified into three types: alluvial soil with speckled layers, gley alluvial soil, and sweet alluvial soil. The two first types make up a vast area while the third type, which is fertile, covers around 5,200 hectares or only 2.7%. The main mechanical constituent of the alluvial soil is clay with a medium to high amount. Surface layers have a pH of 4.2 to 4.5. The pH degree of deeper layers is up to 5.6 or 6.0 but the acidity is higher. The soil has a medium humus content and fairly high nutrient content. In general, the alluvial soil is fertile and suitable for growing high-yielding paddy.
+ Aluminum soil is classified into two types: with high aluminum content or with a medium content. The high-aluminum type is mainly concentrated in the city’s southwestern area from Tam Tan-Thai My of Cu Chi District to the southwestern part of Binh Chanh District. In the area, the soil is high-aluminum and a pH of 2.3 to 3.0. The area’s soil has the same constitution and characteristics as the aluminum soil of the Mekong Delta region.
The medium-aluminum soil is concentrated along the Saigon River, Tra Canal and in District 9. The soil here has a pH of 4.5 to 5.0 on surface layers but down to 3.0 to 3.5 in deeper layers.
The main mechanical constituent of the aluminum soil is tight-packed clay. The soil, which is one meter deep or less, is softer due to high humus content. The soil has a medium nutrient content. The content of toxin ions is high and consequently the alkaline soil is not suitable for paddy growing.
With irrigation methods taken to reduce the amount of aluminum, the soil can be improved for cultivation and local farmers can raise annual crops from one to two. The aluminum soil is suitable for growing pineapples, sugar canes, cashew trees and industrial trees including cajuput, eucalyptus and some kinds of cuasia.
+ Alkaline soil: This makes up the largest area in HCM City and it is mainly concentrated in Nha Be and Can Gio districts. There are two types of alkaline soil: seasonal alkaline soil and salt marsh.
Seasonal alkaline soil covers 10,500 hectares of Nha Be and northern Can Gio districts. The soil is covered with salt water from December to June or July. The soil is rich and has a high content of humus and a pH of 2.4-2.7 at deep layers.
During the flooding season, the salt water is diluted for four to five continuous months. In the period, the soil is covered with an alluvial layer as deep as 20-30 centimeters so that farmers can grow a crop of paddy with average yield of two tons per hectare.
To gain greater efficiency, the region has carried out crop and husbandry restructuring and given focus to growing fruit and forest trees and breeding shrimp.
Salt marsh soil accounts for 35,000 hectares of the majority of Can Gio District. The ground is heavy and dark grey. The soil has a high content of nutrients and a pH of 5.8-6.5. The soil is suitable for developing mangrove forests in order to serve land protection against sea encroachment and to foster the eco-tourism development along the city’s southern coastal area. The two types of alkaline soil have a weak base and therefore cause difficulties in infrastructure construction. However, the condition is favorable for waterway traffic development as rivers and canals account for up to one-third to the total area of the alkaline soil.