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. |
Introduction
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.
Figure 1. Generalised
geology map of Cambodia.
Source: Mekong River Commission
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% |
Of the mapped rice soils
(Oberthur et al. 2000b), Prey Khmer and
Prateah Lang Soil Groups which comprise 39% of the rice-growing soils have very
sandy surface horizons. Prey Khmer is sandy
in both the surface and subsoil and
will be the focus of the present
paper (Table 2). However, the Prey Khmer soils even though having
<18% clay and >65% sand in surface
layers (Table 2) would not necessarily classify as
Arenosols because the rice soil classification in Cambodia only considers properties
to 50 cm, whereas Arenosols need to be sandy
to 100 cm or more (Table 2).
Figure 2. Average monthly rainfall for Takeo (40
years)
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. |
Figure 3. Rainfall (mm) in
April at Battambang, Kampong Cham, and Takeo
over the period 1980-2002. Source:
Department of Meteorology,
Cambodia. Note: rainfall
records are incomplete for many stations in Cambodia over the last 35 years and this accounts for missing
entries
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 Fe
2+ 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 CaCl
2 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 CaCl
2 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
|
Table 4. Soil pH and exchangeable Al in soils of
Tramkak District, Takeo
| 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
|
Symptoms
of Mn toxicity have also been observed on mung bean and peanut on acid Prey Khmer soils in Takeo Province.
Hence even where Al toxicity is not a constraint, Mn toxicity may limit crop production on acid sandy soils.
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.
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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
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. |
Introduction
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).
Figure 1. Soil moisture regimes of the Hainan Island
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.
Figure 2. A
schematic map of the general pattern of the annular distribution of the soils
on Hainan Island
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.
Figure 3. Flow
chart of 1:200,000 HaiSOTER establishment
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.
Figure 4. Topsoil attributes developed from
different parent materials (standardized value)
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 |
Land use:
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.
Figure 5. Land use for sandy soil in 2000 year
Crop suitability (Example of Banana)
Figure 6. Banana suitability under different input and technologic
conditions
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.
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1
State Key Laboratory of Soil and Sustainable Agriculture,
Institute of Soil
Science, Chinese Academy
of Sciences East Beijing Road
71, Nanjing 210008, China
