Dự án nông nghiệp " Major Cropping Soils and Soil Constraints to Productivity of Major Upland Crops Grown by Smallholders - Southern Central Coast " pdf

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,

Vietnam

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1. Introduction
The Soil Constraints and Management Package (SCAMP) has been developed
to identify soil constraints to crop production by considering a range of key soil
properties (Moody and Phan Thi Cong 2008; Moody et al. 2008). Once these
constraints have been identified, management practices that ameliorate or modify
these constraints can be formulated. Crops vary in their tolerance to soil constraints
and a soil property limiting the productivity of one crop may not be a limitation to the
productivity of another crop. Therefore, once the constraints of a particular soil type
have been identified, the soil can then be assessed for its ability to potentially support
the production of specific crops.
The objectives of this series of reports are to:
 review existing information on the identification and extent of
upland cropping soils in focus provinces of Vietnam.
 identify the major soils used for smallholder cropping in the focus
provinces, and determine their constraints by the application of
SCAMP.
 identify the major crops/cropping systems used by smallholders in
the focus provinces, and document the specific soil requirements of
these crops/cropping systems.
 link soil constraints to the soil requirements of the major upland
crops and develop management guidelines for the management of
specific soil types for specific crops.
Part 2 focuses on the soils and cropping systems of the Southern Central Coast
comprising Quang Nam, Quang Ngai, Binh Dinh, Phu Yen, Khanh Hoa, Ninh Thuan,
and Binh Thuan Provinces.

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:

(i) mountains and midlands,
(ii) river deltas, and
(iii) coastal sand dunes, floodplains and swamps.



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3. Major Soils used for Smallholder Cropping
Acrisols, Fluvisols and Arenosols comprise the major soil types of the South
Central Coast of Vietnam. The areal extents of the soils are given in Table 1. Of the
total land area of the Southern Central Coast, about 19% comprises land that is
suitable for agriculture (of which about 21% is cultivated), 39% comprises forestry
land, and 35% comprises unused land, rivers and streams.


Table 1. Areas of major soil groups in the South Central Coast of Vietnam

Soil type Area Proportion
(ha) (%)
Ferralitic grey soils (Ferric Acrisols) 299,512 54.9
Acidic alluvial soils (Dystric Fluvisols) 59,466 10.9
Grey soils with yellowish red mottles (Plinthic Acrisols) 53,465 9.8
Humus grey soils (Haplic Acrisols) 39,280 7.2
Sandy soils (Haplic Arenosols) 19,640 3.6
Others 74,196 13.6
Total 545560 100.0
Source: Nguyen thi Ngoc Hue et al. (2008)


4. Soil Constraints and Management Practices for Sustainable Crop Production

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.

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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.

Attribute Arenosols Fluvisols
Texture S (4) L (1); C (4)
Water pathway drainage (4) drainage + runoff (1);
runoff/ponding (4)
Acidity surface a (1) surface a (2)
Acidification hazard ar(high) (2); ar (very high) (2) ar(low) (3); ar(moderate) (2)
Low nutrient retention e (4) e (1)
Low organic C om (3)
Low K reserves surface k (4) surface k (3)
Hard-setting hs (4)
Compaction comp (2)


These assessments indicate that the Arenosols are sandy in texture, with low
organic matter content, low CEC, and high to very high acidification hazard.
Potassium status is low. Drainage is the main pathway of water movement.
Compaction is a constraint in those Arenosols that have a predominantly fine sand
component (20-200 µm).
The Fluvisols are predominantly clayey in texture. Depending on site slope,
runoff or ponding is the main pathway of water movement. Acidity constraints are not
common. Low soil K reserves and hard-setting surfaces are common.
With these constraints, the SCAMP database would indicate the following
management strategies for sustainable productivity:
e: CEC should be increased by increasing soil organic matter content (retaining
crop residues, adding organic residues, growing cover crops or companion green
manure crops) in association with a liming program to increase soil pH and therefore
the variable charge component of CEC (e.g., Aitken et al., 1998; Phan and Merckx,
2005). The practicality of adding high activity clays to increase permanent charge
could be assessed (e.g., Noble et al., 2004).
om (low): Increasing the levels of organic matter in these soils would improve

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.

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Table 3. Crop areas of the Southern Central Coast. All crops except rubber are
grown by smallholders.


Crop
Area
(ha)
Rice 540000
Sugarcane
71067
Maize
41300
Cashew
32426
Cassava
?
Coffee
5196
Peanut
?
Cotton
1787
Pepper
988

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).


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Table 4. Soil suitability classes [Source: FAO 1976]

Suitability
Class
Criterion Description

1 Highly suitable Soil is suitable for sustainable
production of the crop without
ameliorative measures.
2 Moderately suitable Soil is suitable for sustainable
production of the crop if minor
ameliorative measures are applied (e.g.
liming, mounding to improve local
drainage).
3 Marginally suitable Soil is only suitable for sustainable
production of the crop if major
ameliorative measures are undertaken
(e.g. large scale drainage works).
4 Currently not suitable Soil is not suitable for sustainable
production of the crop.



Table 5. Suitability class of soil attributes/constraints for production of specific
crops.



SCAMP
descriptor
Paddy
rice
Maize Cassava Sugarcane Coffee Peanut
Texture
S 4 2 2 2 2 2

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
-

1 2 1 1 2 1

a

2 3 2 2 3 2
Salinity
s
-
s
2
3
3
4
3
4
2
4
3
4
3
4
Drought tolerance
L L H L L M
Main nutrient
/water uptake zone
(cm)
<50 80-
100
>100 >100 >100 50-
100

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.

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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.

For crops with high nutrient demands such as maize, sugarcane and coffee, the
low ECEC of sandy soils requires that nutrient cations such as potassium are applied
in split applications at rates in accord with crop demand. Growing green manure crops
or applying plant material from these crops (eg. Tithonia) will temporarily increase
the nutrient holding capacity (i.e. CEC) of the soil.
C: Root crops such as cassava are not suited to clayey soils because of
harvesting difficulties.
Clayey soils are unsuitable for crops that do not tolerate prolonged soil
wetness such as coffee; the low permeability of clayey soils causes them to remain
wet for a longer period than soils of lighter texture.

Drainage
Soils with imperfect or poor drainage are unsuitable for crops that cannot
tolerate waterlogged conditions such as coffee and maize, and raised beds and large
scale drainage works must be undertaken if such crops are to be grown.

Acidity
a: Soils with this constraint are unsuitable for crops with a low or moderate
tolerance to Al and/or Mn toxicity such as maize and coffee unless a comprehensive
liming program is undertaken.
a
-
: These soils require a liming program if they are being used to grow crops
of low tolerance to Al toxicity such as maize and coffee. Applying Tithonia residues
to acidic soils has been shown to ameliorate soil acidity by increasing soil pH. An
added benefit of using fused magnesium phosphate (FMP) as a P fertiliser is that it
also has a liming effect.

Main nutrient/water uptake zone
Crops with a comparatively shallow active rooting depth will not be as

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

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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.

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