Seng,
V.1, R.W. Bell2, P.F.
White3, N. Schoknecht3, S. Hin1 and W. Vance2
Keywords:
Cambodia,
drought, erosion, farming systems, field crops, rainfed lowlands, rice, sandy
soils, soil fertility, shallow groundwater, soil water| Abstract Siliceous sedimentary formations underlie much of Cambodia, consequently there is a propensity for sandy surface soils. Only the soils fringing the Tonle Sap lake, those of the alluvial plains along the major rivers (especially the Mekong), and soils developed on basalt deviate from the characteristic of sandy soils. Substantial areas of sandy, high permeability soils are used for lowland rainfed rice production. Due to their inherent high hydraulic conductivities, standing water in rice fields of the deep sandy soils drains rapidly after rainfall predisposing rice crops to drought and high rates of nutrient leaching. However, loss of soil water saturation may limit rice yield by inhibiting nutrient uptake more often than drought, per se. Prospects for growing field crops in sandy lowland soils are contingent on the amounts and reliability of early wet season rainfall or on amounts of stored water after harvesting rice. Apart from drought, waterlogging and inundation are significant water-related hazards that influence the growing of field crops in lowland soils. In addition, soil fertility constraints in the early wet season and dry season will likely differ from those encountered by rice due in part to the different soil water regime they encounter. In particular soil acidity, low nutrient status, hardsetting and shallow rooting depth have been identified as significant constraints for field crops. Vast areas of sandy upland soils occur in Cambodia but are only poorly described. Low soil fertility is likely to limit upland farming systems on the sandy uplands and erosion is a concern for their sustainable use. There is a need to hasten the pace of research and resource assessment of these uplands so that land suitability assessment and sustainable farming systems are available to guide the expansion of agriculture in these areas. |
Sandy materials cover a large proportion of the landscape of Cambodia, on account of the siliceous sedimentary formations that underlie much of the Kingdom (Workman 1972). Due to their prevalence in the lowlands of Cambodia, sandy, high permeability soils are commonly used for rainfed rice production (White et al. 1997). Increasingly in Cambodia, attention is being turned to the potential for crop diversification and the prospects for other land uses in sandy lowland soils (Bell et al. 2005). A key constraint for the use of sandy soils in Cambodia is the amount and reliability of rainfall during the early wet season (April to July) and main wet season (July to October) or the amounts of stored water after harvesting rice. Apart from drought, waterlogging and inundation are significant water-related hazards that influence the growing of field crops in lowland sandy soils (White et al. 1997; Bell and Seng 2004; Bell et al. 2005). In addition, soil acidity and low nutrient status have been identified as significant constraints for crops on sandy soils in Cambodia.
Vast areas of sandy upland soils occur in Cambodia but are only poorly described, and at present not extensively used for agriculture. Low soil water storage, and low soil fertility, including soil acidity, are likely to limit upland farming systems on the sandy uplands and erosion is a concern for their sustainable use. There is likely to be pressure to develop agriculture on these sandy uplands over the next 20 years. There is a need to hasten the pace of research and resource assessment of these sandy uplands so that land suitability assessment and sustainable farming systems are available to guide the expansion of agriculture in these areas.
In this paper, we review the geological setting of Cambodia which helps to explain the prevalence and distribution of sandy soils. Since Cambodian agriculture is heavily dependent on lowland rainfed rice, we review the nature and properties of the sandy soils in the lowlands. Finally we review the limited knowledge-base of sandy soils in the upland areas that are likely to experience development pressure over the next two decades.
Surface geology and distribution of sandy soils
Mesozoic sandstone dominates most of the basement geology in Cambodia (Workman 1972: Figure 1) and hence will have a dominating influence on the properties of upland soils. Recent and Pleistocene alluvial/colluvial and lacustrine sediments that now form the parent material for most of the lowland agricultural soils of Cambodia are substantially derived from the weathering and erosional products of the Mesozoic sandstone (White et al. 1997). However, low hills from felsic igneous intrusions particularly in South and Southeast Cambodia have also supplied siliceous sediments for the recent and older alluvial/colluvial terraces. In the Northeast of Cambodia, basaltic lava flows from the Pleistocene covered significant areas of older alluvial terraces. The soils formed on weathered basalt and on the alluvial/colluvial sediments derived from basalt have very different properties to those of the siliceous parent materials that dominate most other soils (White et al. 1997). In the West of Cambodia, bordering Thailand substantial areas of siltstone limestone and marl occur (Figure 1), and this area is emerging as significant for upland crop production. Finally the sediments deposited by the Mekong River along its flood plain and in the basin of the Tonle Sap have resulted in a large part of Central Cambodia being dominated by recent alluvial/lacustrine sediments derived in part from the Mekong River basin and in part from the immediate basin of the Tonle Sap (Oberthur et al. 2000b).
No specific mapping of sandy soils has been undertaken in Cambodia. A soil map (1:250,000) of most of Cambodia was recently completed based on the FAO World Soils Map (1988) as part of a soil resources map for the lower Mekong Basin (MRC, 2002). Parts of Cambodia fall outside the lower Mekong Basin and hence were excluded from mapping, including the eastern provinces of Prey Veng and Svay Rieng, and parts of the southern provinces of Kampot, Kampong Som, Koh Kong, and Pursat. The rice growing soils have been mapped (Oberthur et al. 2000b) based in part on an old small scale map (1:900,000) of soils of the whole country. However, soil mapping coverage of the upland regions where soils are predominantly developed on sandstones and related siliceous formations are poorly described (Seng and White 2005).
In Cambodia, the Arenosols (sandy soils featuring very weak or no soil development) are mapped on only 1.6% of the land area (Table 1). Sandy surface textures are more prevalent than the deep sandy soils that fit the definition for Arenosols. Sandy textured profiles are common amongst the most prevalent Soil Groups including Acrisols and Leptosols (MRC, 2002). The Acrisols are the most prevalent Soil Group occupying nearly half of the land area of Cambodia. The main subgroups are: Gleyic Acrisols (20.5%, Haplic Acrisols (13.3%), Plinthic Acrisol (8.7%) and Ferric Acrisol (6.3%).
Table 1. Chemical properties of surface layers of Prey Khmer (White et al. (1997) sandy rice soils in Cambodia and the percentage of the rice area they occupy (Data source: Oberthur et al. 2000a; White et al. 2000 and Seng et al. 2001b)
| Property | Typical surface soil values |
| Sand | 730 g kg-1 |
| Silt | 220 g kg-1 |
| Clay | 50 g kg-1 |
| pH (1:1 H2O) | 5.6 |
| Organic C | 4.7 g kg-1 |
| Total N | 0.5 g kg-1 |
| Exch K | 0.04 cmol kg-1 |
| Exch Na | 0.05 cmol kg-1 |
| Exch Ca | 0.61 cmol kg-1 |
| CEC | 1.45 cmol kg-1 |
| Olsen P | 1.3 mg kg-1 |
| Percentage of rice area | 10-12% |
Rainfall and cropping systems
In Cambodia, mean annual rainfall mostly falls in the range from 1,250-1,750 mm (e.g. see Figure 2) with increases up to 2,500 mm in the south, and east of the country (Nesbitt, 1997). The variations in average annual rainfall produce changes in cropping patterns, and options for pre-rice and post-rice cropping with field crops. The East and South of Cambodia has higher early wet season rainfall and may therefore be a more prospective area for expanding field crops on sandy soils (Figure 2).
Cropping in Cambodia revolves around three season: the early wet season (EWS) from April to July; main wet season from July to October; and dry season from November to March (Nesbitt, 1997). Rice is the dominant crop on lowlands in the main wet season with transplanting occurring as soon as sufficient rainfalls to allow cultivation of soils and the accumulation of standing water in the fields. This may vary from June to later August depending on the season and landscape position of the field. Harvesting coincide with the early part of the dry season. Dry season crops can only be planted where there is sufficient stored soil water, as in some lowland rice fields, or where irrigation water is available. Throughout Cambodia substantial year-to-year variation in total rainfall is experienced as well as rainfall distribution pattern (Figure 3).
Table 2. Soil profile description for a deep sandy soil from Tramkak District, Takeo Province, Cambodia. Classified as similar to Prey Khmer according to White et al. (1997) and Plinthic Alisol (World Reference Base 1998). Described by: N. Schoknecht, 6/6/03 Location: Datum: IND60 Zone: 48 448326 mE 1220774 mN
| Horizon | Depth (cm) | Description |
| A | 0-6 | strong brown (7.5YR 5/6 moist), medium sand; very friable moist consistence; single grain structure; very fine, medium porosity, clear, smooth boundary. |
| A | 6-20 | brown (7.5YR 5/4 moist) medium sand; very friable moist consistence; single grain structure; very fine, medium porosity, gradual, wavy boundary. |
| A | 20-60 | light brown (7.5YR 6/4 moist) medium sand; medium faint reddish yellow (7.5YR 6/8 moist) mottles; very friable moist consistence; single grain structure; very fine, medium porosity, sharp, tongued boundary. |
| Ctv | 60-85 | grey (10YR 6/1 moist) sandy clay; medium prominent reddish yellow (5YR 6/6 moist) mottles; hard dry consistence; weak, medium, angular blocky structure; fine, low porosity, gradual, wavy boundary. |
| Ct | 85-100+ | yellowish brown (10YR 5/4 moist) clay; fine prominent reddish brown (2.5YR 4/4 moist) mottles and fine distinct grey (10YR 6/1 moist) mottles; firm moist consistence; few segregations, fine elongated black soft; fine, channels void. |
Rice Soils
Rice is the dominant crop in Cambodia, with a production area equivalent to 90% of the agricultural land in Cambodia (Nesbitt 1997). The sandy Prey Khmer Soil Group comprises 11% of the rice-growing soils (White et al., 1997). The soil groups have been defined using the Cambodian Agronomic Soil Classification (CASC) which groups soils according to their effects on lowland rice production (White et al. 1997). It is adapted from the Fertility Capability Classification (Buol et al. 1973) rather than a soil classification based on concepts of soil genesis that emphasizes subsoil properties. The Cambodian Agronomic Soil Classification emphasizes surface soil properties since rice roots are relatively shallow. Even Prey Khmer Soil Group when used for lowland rice would generally not fall into Arenosols since only the 0-50 cm layers are considered in CASC (White et al. 1997).
The Prey Khmer soil has very low CEC, organic C, total N, exchangeable K and Olsen P (Table 1). In field trials in Cambodia, strong responses to N are generally reported in sandy rice soils (Seng et al. 2001b). However none of these soils respond to N alone. On the sandy Prey Khmer soils, N alone either has no effect on yield or decreases it (White et al. 1997, Seng et al. 2001b). On sandy soils, responses to P alone may be obtained although strongest responses generally require N and P, and on the lower fertility soils K and S fertilizers are also required for rice. Low levels of Mg and B have also been identified as potential production constraints for crops on the Prey Khmer soils, but have not been verified in rice in the field (Lor et al. 1996). Leaching of N and other nutrients may also limit productivity of these soils even when water is not limiting. The Prey Khmer soil in Cambodia has low potential productivity even with fertilizer application (White et al. 1997).
The dominant rice ecosystem in Cambodia is rainfed lowlands (Wade et al. 1999). The shallow, drought- and submergence-prone sub-ecosystem, is most widespread of the rice sub-ecosystems in Cambodia, in part due to the erratic rainfall, topography and the prevalence of sandy textures in the root zone of the rice crop. While the sub-ecosystem concept is useful in regional classifications of rice growing areas, in practice local surface hydrology can vary to such an extent as to overrides the influence of rainfall. Within a single farm or among adjacent fields, the upper terraces which are commonly sandy may be classified into the drought-prone sub-ecosystem and the lower terraces may belong to the submergence-prone or drought- and submergence-prone sub-ecosystem. Fields in the high or upper terraces of the lowlands lose large amounts of water, particularly after heavy rainfall, through surface runoff and subsurface lateral water movement, while those in the lower terraces may intercept the flows from the upper paddies (Fukai et al. 2000). Location of on-farm drains, road embankments and drains under roads can markedly affect where the runoff is directed. Water balance models are particularly useful for identifying key aspects of the surface hydrology experienced by rainfed rice. Fukai et al. (1995) have developed a water balance model for sandy soils in N.E. Thailand and this model may be useful for the sandy soils of Cambodia. Maintaining water in the root zone for rice is hindered on sandy soils by high percolation rates that are a common problem in the sandy lowland rice soils of Cambodia (White et al. 1997).
In the rainfed lowlands, significant periods of loss of soil-water saturation occur intermittently throughout the growing season (e.g. Seng et al. 1996; Fukai et al. 2000). Based on rainfall, its distribution and variability, it could be assumed that drought was the main soil water-related constraint for rice in the region. However, the more common effect of low soil water may be to limit nutrient availability and uptake rather than to cause drought per se. The implications of the temporary periods of loss of soil-water saturation for nutrient availability are not fully understood (Fukai et al. 1999), although variations in soil water saturation interact with nutrient availability (Bell et al. 2001). Fluctuating soil water regimes will have major effects on the forms and availability of N (Seng 2000), P (Seng et al. 1999) and on Fe and Al toxicities (Seng et al. 2004b).
Options for minimizing the impact of periods of loss of soil-water saturation are either to use cultivars that are efficient in P uptake and use, and presumably would be best able to cope with a temporary decline in P availability (Fukai et al. 1999); or to treat soil with straw (Seng et al. 1999). Straw keeps the redox potential lower during the period of soil-water saturation loss, thus decreasing the extent of Fe2+ oxidation and minimizing losses in P availability due to reaction with Fe oxides. Other forms of organic matter added to the soil at planting, including cow manure, or residues from pre-rice pulse crops or green manures like sesbania, can all help minimize losses of P during periods of soil-water saturation loss.
Iron toxicity has been reported for Prey Khmer soils in Cambodia. However, the impact on yield has not been quantified. Neither is there direct evidence of the consequences of intermittent loss of soil water saturation on the incidence and severity of Fe toxicity.
Application of clay to sandy soils has been suggested as a semi-permanent treatment to enhance water and nutrient retention (Noble et al. 2004). Initial research on the sandy soils of N.E. Thailand suggests very strong responses in growth can be achieved by clay amelioration. The use of claying presumes a ready local supply of clay. N.E. Thailand has numerous deposits of high activity clay in lacustrine sediments (S. Ruaysoongnern, personal communication). The relevance of this technology for the Prey Khmer (Arenosols) of Cambodia, warrants further research.
Upland sandy soils
Important upland crops in Cambodia are maize, rubber, soybean, mung bean, cassava, sesame, peanut and sugarcane (Bell et al. 2005). There is very limited information on the sandy upland soils of Cambodia. Only generalized comments can be made at this stage, based largely on understandings developed for rice soils with similar properties and on recent studies carried out in the west of Takeo Province (Bell et al. 2005) where sandy soils are prevalent.
The Prey Khmer soil is defined for rice production as having a sandy layer <50 cm deep, because deeper sand is unsuitable for rice. However, similar soils to the Prey Khmer are encountered in Tramkak with deeper sandy layers up to 80 cm. These soils are suitable for non-rice field crops and so the deep phases have been distinguished from the Prey Khmer as defined by White et al. (1997). A typical soil profile is shown in Table 3. The surface soil properties are similar to those reported above (Table 1). That is, low levels of organic C, N, Olsen P, exchangeable K are commonly found in surface layers. In addition, KCl 40 extractable S levels, DTPA Cu, and Zn, and hot CaCl2 extractable B levels were low.
From preliminary analysis of a range of upland soils from Takeo, soil acidity appears to be a significant limiting factor for a range of field crops (Table 4). In Prey Khmer soils in uplands of western Takeo Province, Al saturation values of 50-80% were found in the subsoil (Table 4). Aluminum saturation >20% is commonly regarded as a potential Al toxicity in sensitive crops, whereas in very tolerant crops >80% Al saturation is required to impair crop growth (Dierolf et al. 2001). Seng et al. (2004a) showed strong responses by upland rice to lime application on the acid Prateah Lang soils (pH CaCl2 4; Al saturation 80%) when maintained in an aerated state whereas no response was found when these soils were flooded.
Table 3. Soil chemical properties of two profiles from the District of Tramkak, Takeo Province classified as sandy soils. Profiles were classified as Prey Khmer (White et al. 1997). Site 5 has no phase specified; site 52 has a coarse sandy phase specified
| Site | Depth (cm) |
Total N g kg-1 |
Olsen P mg kg-1 |
KCl40 S mg kg-1 |
DTPA Cu mg kg-1 |
DTPA Zn mg kg-1 |
DTPA Mn mg kg-1 |
Hot CaCl2 B
mg kg-1 |
| 5 | 0-6 | <0.2 | 16.0 | <1 | 0.14 | 0.19 | 3.46 |
0.2
|
| 6-20 | <0.2 | 26.0 | <1 | 0.14 | 0.16 | 3.31 |
0.2
|
|
| 20-60 | <0.2 | 3.0 | 2.5 | 0.11 | 0.15 | 1.47 |
0.4
|
|
| 60-85 | <0.2 | 2.0 | <1 | 0.29 | 0.18 | 5.29 |
0.3
|
|
| 85-100 | <0.2 | 3.0 | <1 | 0.36 | 0.14 | 5.36 |
0.3
|
|
| 52 | 0-45 | 0.2 | 2.0 | 1.1 | 0.12 | 0.16 | 25.08 |
0.2
|
| 45-95 | 0.1 | 2.0 | 1 | 0.18 | 0.12 | 11.6 |
0.2
|
|
| 95-120 | 0.01 | 1.0 | 1 | 0.16 | 0.04 | 3.53 |
0.2
|
| Soil Type | Depth (cm) |
Phase
|
pH
CaCl 2 |
Al
(cmol kg-1) |
ECEC
(cmol kg-1) |
Al saturation
(%) |
| Prey Khmer (Site 5) | 0-6 |
4.3
|
0.14
|
0.45
|
31
|
|
| 6-20 |
4.3
|
0.29
|
0.56
|
52
|
||
| 20-60 |
4.5
|
0.32
|
0.65
|
49
|
||
| 60-85 |
4.1
|
3.24
|
5.6
|
58
|
||
| 85-100 |
6.4
|
0
|
10.7
|
0
|
||
| Prey Khmer | 0-12 |
fine sandy phase
|
4.5
|
0.28
|
1.83
|
15
|
| 12-60 |
4.2
|
1.57
|
1.81
|
87
|
||
| 60-100 |
4.1
|
1.4
|
1.6
|
88
|
||
| 100-120 |
4.2
|
1.32
|
1.48
|
89
|
Water supply is a key limiting factor for most areas of Cambodia because of the monsoonal rainfall pattern and the erratic rainfall distribution during the early wet (Figure 2) and main wet seasons. Most of the crops grown in the early and main wet season receive less than optimal rainfall in total (Bell et al. 2005). Hence the water storage capacity of the soil would have a large bearing on the regulation of water availability to crops especially on sandy soils. Deep sands are generally considered unsuitable or of low productivity for paddy rice because water is not retained in the shallow root zone of rice, and because a plough pan does not readily form to retain water (White et al. 1997). Deep sands (75-100 cm) will have a higher potential for production of deep rooted field crops than for rice. Subsoil Al may impede root growth and act as a limit on access to stored subsoil water (Table 4).
Discussion and further research needs
A major hindrance to the management of sandy soils in Cambodia is the dearth of knowledge about the distribution and properties of such soils in the uplands. There is a need for a land resource assessment of uplands of Cambodia. There will also need to be parallel development of sustainable farming systems for the sandy uplands.
The geographical proximity of Cambodia, Laos and Northeast Thailand, and the prevalence of rainfed lowland rice as the major crop in their agro-ecosystems suggest that the cross-flow of research information about sandy soils amongst these regions should be helpful. Coordination and collaboration amongst these countries could minimize duplication of research, and maximize synergies in their collective research. However, exchange needs to be based on a critical examination of the similarities and differences amongst them in agro-ecological classifications, in the prevalence of rainfed rice ecosystems, and in the soils used for rice and field crop production (Bell and Seng 2004).
Acknowledgements
ACIAR for support of some of the research reported in the present paper, and CARDI for the provision of facilities for the conduct of the research.
References
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1 Office of Soil and Water Sciences, Cambodian Agricultural Research and Development Institute, P.O. Box 01, Phnom Penh, Cambodia
2 School of Environmental Science, Murdoch University, Murdoch, Western Australia 6150
3 Department of Agriculture of Western Australia, Baron- Hay Court, S. Perth, WA 6151
Soil characteristics and crop suitability of sandy soils in Hainan, China
Zhao, Y. G.1, G. L. Zhang, 1, Z.
Wen-Jun1, and Z. T. Gong1
Keywords: HaiSOTER, sandy soil, crop suitability
| Abstract Sandy soils in Hainan Island are mainly distributed on marine sediments which cover nearly 10% of the island according to a recently established Hainan Soil and Terrain Digital Database (HaiSOTER). There is also a secondary, very small area of sandy soils associated with granitic parent material. Crop production in sandy soils is mainly limited by nutrient conditions. The main nutrient attributes, such as soil organic matter, cation exchange capacity (CEC), and N content, are significantly lower for sandy soils than for most other soils. Most sandy soils are covered by natural or crop plants, such as Casuarina equisetifolia, coconut, eucalypt, peanut and cassava, in the meteorologically favourable parts of the island in the east and northeast, where rainfall is abundant. However, in the west and southwest part of the island, vegetation is sparse due to low rainfall and the very low water-holding capacity of these coarsely-textured soils. Crop suitability for particular regions of sandy soils is evaluated based on land quality classifications. For most sandy soils, nutrient availability is the most limiting factor. However, in the southwest part of this island, aridity becomes the most important limiting factor, and in the northeast, typhoon is another limiting factor. In such areas, wind-resistant trees and crops are suitable for planting. According to the Automated Land Evaluation System (ALES) land evaluation system, the sandy soils can play a more important role in tropical crop cultivation. It is concluded that when the technological and financial conditions are improved, the sandy soils in Hainan can be used for tropical crops more efficiently. |
Hainan Island is located in the northern fringe of the tropics, and therefore enjoys advantageous hydrothermal conditions and rich plant resources. Agriculture is the dominant economic sector. Hainan Island is an important base for developing tropical crops in China, and therefore exports tropical fruits and winter vegetables throughout China. The sustainable and efficient use of land resources is an important issue for agricultural development of the island. Sandy soils cover a large area of the island. Much of these areas are cultivated, but often at a low production level. Using the HaiSOTER database, which was constructed from 1999 to 2003, soil quality and crop suitability were evaluated as a method of identifying limitations and potential uses of sandy soils on the island.
Climate
Mean annual temperature on Hainan Island ranges between 23ºC and 25ºC, and mean annual precipitation from 900 mm to 2,600 mm. Precipitation is unevenly distributed, with less received in winter and spring and more in summer and fall. During the summer and fall, typhoons are frequent, bringing about one-third of the year’s precipitation. As shown in Figure 1, there are regional differences in precipitation on the island. In the eastern parts, such as near Wanning and Qiongzhong, precipitation may reach 2,000 mm or more, while in the southwestern part, precipitation is less than 1,200 mm. Most soils of Hainan Island have a Hyperthermic soil temperature regime. With respect to soil moisture, soils in the broad central northern part have an aridity <1 and belong to the Udic moisture regime; those in the southwestern part have an aridity >1 and belong to the Ustic moisture regime; and those in the central mountainous region, where precipitation increases with elevation, belong to the Udic and Perudic moisture regimes (Gong et al., 2003).
Landform
The Hainan Island landform is characterized by high elevation in the centre surrounded by a low and flat circumference. With Wuzhishan (1,867 m) and Yinggeling (1,811 m) mountains at the centre, three concentric zones can be defined (Figure 2). The centre zone accounts for 25.5% of the island’s territory and consists of mountains and hills with elevations >400 m. It has steep slopes and mostly young soils seriously affected by erosion. The zone makes up 45.8% of the territory, and contains hills and plateaus with elevations ranging from 20 m to 400 m that are predominantly mature soils. The outer zone contains 28.7% of the island’s area, and has mostly flat coastal plains below 20 m in elevation. It is in this outer zone that human activities are the concentrated and is dominated by mostly anthropogenic soils.
Materials and Methods
SOTER Methodology
SOTER (Soil and Terrain Digital Database) has been widely used as a world soil and terrain digital database (FAO, 1995). Underlying the SOTER methodology is the identification of areas of land with distinctive, often repetitive, patterns of landform, lithology, surface form, slope, parent material, and soils. Tracts of land distinguished in this manner are named SOTER units. Each SOTER unit represents one unique combination of terrain and soil characteristics.
There are two types of data in a SOTER database: geometric data and attribute data. The geometric component indicates the location and topology of SOTER units, while the attribute part describes the non-spatial characteristics. The geometric data is stored and handled by GIS software, while the attribute data is stored separately in a set of files, managed by a relational database system. A unique code is set up in both the geometric and attribute databases to link these two types of information for each SOTER unit.
A SOTER database at a scale of 1:200,000 was compiled for Hainan Island (HaiSOTER). Figure 3 shows the procedure of HaiSOTER (Zhao et al., 2005). The database consists of spatial and attribute data of the soil and terrain conditions, and associated data such as climate and land use. HaiSOTER database collected 153 soil profiles accoss the island. Each SOTER unit has its representative soil profiles.
Crop Suitability Evaluation
An expert model for physical land evaluation developed in the Automated Land Evaluation System (ALES) was used to separate potentially suitable AEU (Agricultural Ecological Unit)’s from unsuitable ones. Soil depth, surface horizon depth, texture, structure, bulk density, cation exchange capacity (CEC), pH, total nitrogen (TN), total phosphorus (TP), exchangeable Ca, Mg, K, growing period, rainfall and typhoon occurrence were considered during the modeling processes.
The evaluation model for crops distinguishes between management types with different levels (low, medium and high) of input and degree of mechanization. Such specific types of land use are called ‘land utilization types’ (LUT). To illustrate how the evaluation model works, we take the case of banana, which is a common crop in Hainan with favourable marketing prospects. Four land utilization types of banana growing were defined for this study (Mantel et al., 2003):
Low input and low technology. This LUT includes a low application of organic fertilizer and simple implements for weeding and soil tillage. No terracing or artificial drainage is practiced.
Medium input and low technology. This LUT includes modest applications of inorganic or organic fertilizer and agrochemicals. It does not include use of mechanized tools for weeding and soil tillage. No terracing is practiced. Artificial drainage is not applied.
Medium input and medium technology. This LUT includes modest applications of inorganic or organic fertilizer and agrochemicals. Mechanized tools are used for weeding and soil tillage. No terracing is practiced. Artificial drainage is applied where required.
High input and medium technology. This LUT includes applications of inorganic or organic fertilizer and agrochemicals and mechanized tools for weeding and soil tillage. No terracing is practiced. Irrigation and artificial drainage is applied where required.
Other levels of input and degree of mechanization were not defined in the assessment model for banana in this study.
Results and Discussion
Soil Chemistry Attributes
There are four major parent materials in Hainan island (GPGSB, 1965): acid igneous rock, which forms Cambosols and Ferrosols; marine sediments, which form Primosols and Cambosols; inner land clastic sediments, which form Cambosols; and basic igneous rocks, which form Ferrolosols. Soils from these four parent materials cover 83.2% of the island. 138 of 153 soil profiles in HaiSOTER database were taken from these four types of parent materials, in which, 65 soil profiles on acid igneous rock, 35 sandy soil profiles on marine sediments, 23 on clastic sediment, 15 on basic igneous. Topsoil chemical attributes of sandy soils formed from marine sediments and soils from the other three parent materials are listed in Table 1, and illustrated in Figure 4 using standardized values to better compare and contrast the soils (Zhao et al., 2005).
Exchangeable bases: Because the silt and clay contents were very low for sandy soils, they contained less exchangeable bases. In terms of average values, exchangeable K, Ca, and Mg of sandy soils were lower than in soils developed from other parent materials. Exchangeable K content of sandy soils was lower than that of soils developed from acid igneous material and clastic sediments, and exchangeable Mg was lower than in soils formed from clastic sediments. There was no significant difference in exchangeable Na content among soils developed from four parent materials.
Sandy soils had the highest pH and lowest exchangeable acidity and Al. Normally in tropical climates with high precipitation such as those of Hainan, strong desilication and allitization are the main soil forming processes, and exchangeable Al dominates soil pH. However, sandy soils contain fewer weatherable minerals, making desilication and allitization weak. Also, there are shell and coral sediments in sandy soils with high Ca content that neutralize acid quickly.
Sandy soils in Hainan also had the lowest CEC, total carbon (TC), and TN values compared to the other soil types, which makes them unfavourable for growing most types of vegetation. Because of their coarse texture, many plant-essential nutrients elements can be easily leached. The TP content for soils developed from basic igneous material is lower than that of soils developed from clastic sediments. P content in clastic sediment maybe higher because of the erosion process brings P sediments from upper reaches.
Table 1. Differentiation of topsoil attributes developed from different parent materials (rocks)
| ID | Parameter | Soils on Acid igneous n = 65 |
Sandy Soils on
Marine sediment n = 35 |
Soils
on
Clastic sediment n = 23 |
Soils
on
Basic igneous n = 15 |
| pH | pH (Water extractable) | 5.0 bc |
6.0
a
|
5.5
b
|
4.8
c
|
| EXAC | Exc. acidity cmol kg-1 | 1.5 ab |
0.9
b
|
2.1
a
|
1.2
b
|
| EXAl | Exc. Al cmol kg-1 | 1.2 ab |
0.7
b
|
1.6
a
|
1.0
ab
|
| EXCa | Exc. Ca cmol kg-1 | 1.4 a |
1.2
a
|
1.9
a
|
1.7
a
|
| EXMg | Exc. Mg cmol kg-1 | 0.8 ab |
0.6
b
|
1.2
a
|
0.7
ab
|
| EXNa | Exc. Na cmol kg-1 | 0.2 a |
0.3
a
|
0.3
a
|
0.2
a
|
| EXCK | Exc. K cmol kg-1 | 0.5 a |
0.2
b
|
0.4
a
|
0.3
ab
|
| CEC | CEC cmol kg-1 | 7.4 a |
3.9
b
|
7.7
a
|
8.6
a
|
| TC | Total C g kg-1 | 15.2 a |
7.3
b
|
14.6
a
|
19.6
a
|
| TN | Total N g kg-1 | 1.14 b |
0.59
c
|
1.20
b
|
1.56
a
|
| TP | Total P (P2O5) g kg-1 | 1.36 ab |
1.56
ab
|
1.97
a
|
0.86
b
|
| Note: letters (i.e., a, b, ab) following table values is the results of multi comparison, different letters in one row mean significant difference existed for the soil attribute among 4 kinds of soils. SPSS10.0, Duncan | |||||
Figure 5 shows the land use distribution for sandy soils in Hainan, as interpreted from the TM satellite in the year 2000. As the data indicate, agriculture is the most important economic resource for Hainan. Nearly 70% of the area of sandy soils is under cultivation. The main crops include vegetables, cassava, coconut and peanut. In the eastern part of the Island, where water supply is abundant, rice is cultivated on sandy soils.
Agricultural exploitation on sandy soils mainly occurs in the middle circle (Figure 2) and not in the newly formed, unsuitable sand areas in the outer circle, where Casuarina equisetifolia, coconut and salt-tolerant grasses are the main vegetation types.
Crop suitability (Example of Banana)
We use the example of banana in this paper to illustrate how crop suitability was evaluated for different soil and terrain types in Hainan. Banana suitability at four different input and technological conditions was demonstrated by Mantel et al. (2003). Poor native soil productivity is reflected by low suitability values under low input and low technological conditions. Because banana has high requirements for water and nutrients, and for low wind climates, almost none of the island’s land was found to be suitable for banana production under conditions of low input and technology (Figure 6). When the input and technological conditions were improved, the area suitable for production increased gradually for the whole island. For sandy soils, however, the increase is steeper (Figure 6). More than 30% of sandy soils can be used for banana planting at high input and moderate technological conditions. The sharper increase in suitability demonstrates that some sandy soils can be improved easily, because poor nutrient availability, their most limiting production factor, can be addressed by fertilizer input.
The sandy soils suitable for banana production are mainly distributed in the west and northern part of the island. Almost none of the sandy soils in the east part can be used for banana planting because of the typhoon risk. The prevailing direction of typhoon is from the south and southeast towards the north and northeast. Nutrient deficiencies are similar for sandy soils in both the western and eastern parts of the island. Water limitation is especially critical for sandy soils of the west part, because annual precipitation is less than 1,000 mm and evaporation is very high. However, with financial support to construct irrigation systems, this constraint can be removed in some areas.
Soils suitable for banana planting are mainly distributed on the old marine sediments towards the interior of the outer concentric zone shown in Figure 2, where soil texture and nutrient conditions of these soils have been improved by longer weathering and a longer history of cultivation.
Conclusions
Sandy soils play an important role for Hainan’s agriculture. Nearly 70% of the area of sandy soils is under cultivation. But sandy soils in Hainan are limited by poor nutrient conditions. The main nutrient attributes such as soil organic matter, CEC, and N content are significantly lower than those in other soils. However, they have the highest pH and lowest exchangeable acidity and Al.
Some sandy soils can be improved easily, because poor nutrient availability, their most limiting production factor, can easily be addressed by fertilizer input. More than 30% of sandy soils can be used for banana production at high input and moderate technological conditions.
Acknowledgement
The construction of HaiSOTER database was cooperatively done by Chinese Academy of Tropical Agricultural Sciences. Methodology was supported by the International Soil Reference Information Center. Mr. VWP. van Engelen, Mr. S. Mantel and Dr. X.L. Zhang completed the main work on assessment of crop suitability.
References
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GPGSB (Guangdong Provincial Geologic Surveying Bureau). Geologic map of Hainan Island (1:200,000). State Printery No. 543, Tianjin, 1965.
Mantel, S., Zhang, X.L. and Zhang, G.L. 2003. Identification of potential for banana in Hainan Island, China. Pedosphere. 13(2): 147-155.
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Zhao Yuguo, Zhang Ganlin, Gong Zitong, Deng Wangang, 2005, Soil type, soil quality and crop suitability of soils developed from different geology environment in Hainan Island, Quaternary Science, 25(3): 389-395.
1 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences East Beijing Road 71, Nanjing 210008, China
