009/06 VIE Six-monthly Report- October 2008
Attachment 1 009/06 VIE: Improving capability of provincial extensionists for
assessing soil constraints to sustainable production through the use of the
SCAMP decision support system.
Major Cropping Soils and Soil Constraints to Productivity
of Major Upland Crops Grown by Smallholders

2. Southern Central Coast
PW Moody
A
and Phan Thi Cong
B A
Queensland Department of Natural Resources and Water, Indooroopilly, Qld 4068,
Australia
B
Institute of Agricultural Sciences of Southern Vietnam, Ho Chi Minh City,

2. Climate and Landforms
The region has a typical tropical monsoon climate with an annual temperature of
around 24
o
C-27
o
C on average (minimum 20
o
C-21
o
C and maximum 31
o
C-32
o
C). Total
heat accumulation per annum is 8,000
o
C– 9,500
o
C, with annual solar radiation of 140
Kcal/cm
2
. Annual rainfall ranges from 500 mm to 2,500 mm with an average
humidity of 70-80%. Differences in regional rainfall have resulted in the identification
of 3 major agri-ecological zones:
• Nam-Ngai (Quang Nam & Quang Ngai): 2000-2600 mm ;
• Binh–Phu (Binh Dinh & Phu Yen): 1500-1700 mm; and
• Southern Ca Pass (Khanh Hoa, Ninh Thuan): < 600 mm.

The landforms of the Southern Central Coast comprise:

The following general constraints to agricultural productivity have been identified
for the Southern Central Coast:
• the soils are generally low in nutrients and are acidic (low water
and nutrient retention capacity);
• drought is common due to irregular rainfall and high evaporation
during the dry season;
• wind and water erosion is common as a result of the sloping
topography with limited groundcover;
• soil degradation and desertification are common particularly in
Quang Ngai and Binh Dinh Provinces;
• flooding often occurs in the wet season;
• sand encroachment occurs inland due to strong winds and limited
groundcover.

It is evident from Table 1 that Acrisols, Fluvisols and Arenosols occupy the
greatest proportion of the land surface of the South Central Coast. Constraints and
management strategies for Acrisols in the Central Highlands have already been
discussed in Part 1 of this series of reports (Moody and Cong 2008), and these
constraints and strategies would be equally applicable to the Acrisols of the South
Central Coast. Consequently, this report will consider the constraints and management
strategies for Fluvisols and Arenosols.

4
In a limited assessment of the soils of Binh Thuan Province, Moody et al.
(2005) identified several constraints of the sandy soils (Arenosols) and the alluvial
soils (Fluvisols) (Table 2).

Table 2. SCAMP descriptors for 4 Arenosols and 5 Fluvisols of Binh Thuan
Province, Vietnam. The number of sites with the attribute is in parentheses.


nutrient supply, increase CEC, increase water holding capacity and increase pH buffer
capacity. The management of soil organic matter in tropical soils involves mulching
and incorporation of ‘green manure’ crops such as legumes or forage grasses,
retaining all crop residues in the field where the crop has grown, not burning crop
residues, minimum or zero tillage farming systems, strip or alley cropping and
application of organic materials (such as animal manure, composted municipal waste,
sewage sludge, and locally available industrial organic wastes) obtained from off-site.
k: Potassium fertilisers or organic amendments having a significant content of K
will need to be applied. In soils with low K reserves in the subsurface layer, it is likely
that crops will exhibit K deficiency during periods of drought. In these circumstances,
placement of K fertilisers below the seed at sowing or mixing K fertiliser through the
soil in the planting hole is a more efficient management strategy than sidedressing K
fertilisers on the soil surface. Crops should be closely monitored for K deficiency
symptoms.

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hs, comp: Hard-setting surfaces reduce infiltration rate and cause poor crop
establishment, while compaction layers restrict root growth and limit rooting depth
causing drought stress to crops. Retaining crop residues and applying surface mulch
should be used to maintain soil surface moisture thus minimising hard-setting. To
minimise compaction risk, soils should only be cultivated when drier than their plastic
limit, and tillage and machinery traffic should be avoided when soil is wetter than its
plastic limit.

5. Major Upland Crops grown by Smallholders
Agricultural census data (1999) were used to identify the major crops grown
by smallholders in the Southern Central Coast. Where available, crop areas are
presented in Table 3. Smallholder crops with significant areas are rice, maize,
cassava, sugarcane, and peanut. In general, coconut, cashew and rubber are grown in
commercial plantations.

Coconut
?
Rubber
?6. Soil Suitability for Major Upland Crops grown by Smallholders
SCAMP assessments of the Arenosols and Fluvisols (Section 4 above) have
identified several soil constraints to crop productivity. Some of these constraints have
effects on crop productivity, irrespective of the crop grown: low CEC (e); low organic
carbon (om); K deficiency (k); hardsetting characteristics (hs) and compaction layers
(comp). However, crops vary in their tolerance to other constraints such as drainage
and acidity; while a particular soil attribute or constraint might be a major limitation
to the productivity of one crop, it may pose only a minor limitation to another. The
FAO (1976) framework for land evaluation uses five classes to categorise the
suitability of a specific soil/landscape unit for growing a particular crop (Table 4). To
facilitate the use of SCAMP for this application, individual soil attributes/constraints
identified for the Arenosols and Fluvisols have been rated according to their effects on
the sustainable production of the major crops grown by smallholders in the Southern
Coastal Region (Table 5). Ratings are based on collation of information in Williams
(1975), Landon (1984), Page (1984), Schaffer and Andersen (1994), Robinson (1996)
and Dierolf et al. (2001). 7
Table 4. Soil suitability classes [Source: FAO 1976]

Suitability
Class
Criterion Description

L 1 1 1 1 1 1

C 1 1 2 1 1 3

O 4 3 2 3 3 3
Drainage
1 (g) 2 4 4 4 4 4
rating
2 (g
-
) 1 4 3 3 4 3

3 3 3 3 2 3 3

4 4 1 2 1 2 2

5 4 1 1 1 1 1

6 4 1 1 1 1 1
Slope (%)
0-2 1 1 1 1 1 1

2-5 2 1 1 1 1 1

5-10 3 2 2 2 2 2

>10 4 3 3 2 3 3-4
Soil pH
a
-

Nutrient
needs
High N
req'd
High
N, K
req'd
Tolerates
low
fertility
High N
req'd
High N,
K req'd
Ca in
pegging
zone The following comments apply to the management practices that may need to
be undertaken to meet the requirements of individual crops in addition to those
already outlined in Section 4 above.

8

Texture
S: Because of the low inherent plant available water content of sandy soils,
irrigation may be required for crops of low drought tolerance such as maize,
sugarcane and coffee. Using surface mulches of plant residues will reduce evaporation
and conserve soil moisture.

sensitive as deeper rooted crops such as coffee and sugarcane to constraints such as a
compaction layer (comp). Conclusions
Ferralsols, Acrisols, Arenosols and Fluvisols are the major arable soil groups
in the Southern Central Coast. The constraints to crop production which commonly
occur in the Arenosols are low plant available water capacity, low nutrient cation
retention, K deficiency, acidity, low organic matter, and compaction in soils with high
fine sand content. Drainage is the dominant pathway of water movement. For
Fluvisols, commonly occurring constraints are low K reserves, occasional acidity, and
the soil physical problem of hard-setting surfaces. Fluvisols have impeded drainage
leading to runoff on slopes or ponding in low-lying areas.
The crops most commonly grown by smallholders in the region are: paddy
rice, maize, cassava, sugarcane, coffee and peanut. Individual crop tolerances to some
of the identified constraints vary, and so soil management responses to ameliorate or

9
minimise the effects of these constraints on crop productivity will also vary. In
general, however the Arenosols and Fluvisols would benefit from application of K
fertilisers and reduced cultivation (to reduce the risk of compaction and hard-setting).
Increasing the organic matter content of Arenosols is essential. Fluvisols may require
raised beds to improve drainage in the root zone of upland crops.

References
Aitken, R.L., Moody, P.W. and Dickson, T. 1998. Field amelioration of acidic soils in
south-east Queensland. I. Effect of amendments on soil properties. Australian
Journal of Agricultural Research, 49, 627-637.

Dierolf, T., Fairhurst, T. and Mutert, E. 2001. Soil Fertility Kit. Potash and Phosphate

Fruit Crops. CRC Press: Florida.

Williams, C.N. 1975. The Agronomy of the Major Tropical Crops. Oxford University
Press: London.