Dự án nông nghiệp " Major Cropping Soils of Gia Lai Province, Vietnam, and Soil Constraints to Productivity of Major Upland Crops Grown by Smallholders " ppt

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 of Gia Lai Province, Vietnam, and
Soil Constraints to Productivity of Major Upland Crops
Grown by Smallholders



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
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 report are to:
 review existing information on the identification and extent of upland
cropping soils in Gia Lai Province, Vietnam.
 identify the major soils used for smallholder cropping, and determine
their constraints by the application of SCAMP.
 identify the major crops/cropping systems used by smallholders in Gia
Lai Province, 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.

2. Major Soils used for Smallholder Cropping
The areal extents of the different soil groups (FAO-UNESCO classification)
that occur in Gia Lai Province are presented in Table 1.

Table 1. Areal extent of Soil Groups in Gia Lai Province.
Source: Le Trung Lap, 2000.

Soil Groups Area (ha) Proportion (%)
Chromic and Haplic Acrisols

Acric,Humic and Vetic Ferralsols
756,433 49
Dystric and Gleyic Arenosols
Luvisols
364,638 23
Leptosols 164,752 11
Alumic, Hyperdystric and Chromic Acrisols
Acric, humic and Vetic Ferralsols
90,481 6
Mollic Fluvisols
Luvic and Fluvic Phaeozems
Cambisols
64,218 4
Dystric Gleysols 16,774 1
Others 92,275 6
TOTAL 1,549,571


The land forms of Gia Lai have been classified (Berding et al., 1999) as:
 Hills and low mountains,
 Basaltic plateaus,
 Gently to rolling uplands on metamorphic or granitic rocks or on recent
alluvium and
 Depositional areas.

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Topography (slope) limits the agricultural use of many areas of Gia Lai, and the major
soil groups of agricultural importance are the Ferralsols and Acrisols occurring on the
basaltic plateaus and gently to rolling uplands. These soil groups comprise about 49%
of the land surface of the province (Table 1, line 1).

3. Soil Constraints and Management Practices for Sustainable Crop Production
In a recent study (Moody et al. 2008), sites identified on the 1:100,000 soils
map of Gia Lai Province as 'Red-brown soils on basalt' (Ferralsols) and 'Grey soils on
acid igneous rocks' (Acrisols) (Le Trung Lap, 2000) were selected from the basaltic
plateaus and gently to rolling uplands areas. Fourteen Ferralsols were sampled under a
range of land uses (maize-upland rice, maize-cassava, rubber, coffee, eucalypt
plantation, fruit trees, cashew) in Dak Doa, Mang Yang, Chu Pah, Ia Grai, Duc Co,
Chu Prong and Chu Se Districts of north west Gia Lai Province. Sixteen Acrisols
were sampled from various positions (upper, mid, lower slope) in the landscape under
several land uses (maize, cassava, sugarcane, vegetables) in the vicinity of Dak Po
Village, Dak Po District in the east of Gia Lai Province.
Mini-pits were dug at each site and SCAMP Level 1 and Level 2 assessments
made (Table 2). A composite 0-15 cm soil sample was taken from each site to
represent the plough layer and analysed for Level 3 attributes (Table 2).

Table 2. Attributes determined for each application level of SCAMP.

SCAMP level Attributes
Level 1 Texture, colour (moist) of soil matrix and mottles, structure and consistence (moist),
drainage class, permeability class, slope, erosion hazard, gravel content, compaction
Level 2 Field pH (water and 1M KCl), field EC, dispersion class, infiltration rate
Level 3 Organic C, clay %, P fixation capacity, exchangeable Ca, Mg, Na and K, extractable
acidity (H +Al), ECEC, pH buffer capacity (calculated)


From these data, SCAMP (Moody and Phan Thi Cong, 2008) was used to
identify the constraints of these two major soil groups to sustainable production
(Table 3).

Table 3. SCAMP descriptors for 14 Ferralsols and 16 Acrisols of Gia Lai
Province, Vietnam. Number of sites with the attribute is in parentheses.

Attribute Ferralsols Acrisols
Texture L (9); LC (5) L (10); C (2); LS (4)
Erosion hazard er(slight) (9), er(moderate) (2),
er(high) (2), er(very high) (1),
er(slight) (3); er(moderate) (2);
er(high) (6); er(very high) (3);
er(extreme) (2)
Water pathway drainage (7);
drainage + runoff (7)
drainage (8);
drainage + runoff (5); runoff (3)
Acidity surface a (14); subsurface a (12) surface a (2); subsurface a (1)
Acidification hazard ar(low) (14) ar(low) (2); ar(moderate) (12);
ar(high) (2)
Low nutrient retention e (12) e (3)
High P fixation i (14)
Low organic C om (2)
Low K reserves surface k (1); subsurface k (1) surface k (1); subsurface k (7)
Variable charge characteristics geric (4)
Hard-setting hs (2)
Compaction comp (4)

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These assessments indicate that the Ferralsols are predominantly loamy in
texture, with a range of erosion hazards depending on site slope. The main pathways
of water movement are by drainage or runoff plus drainage. They typically have a low
soil pH (<5.2 in water) but a low acidification hazard because of their high organic C

and clay content. ECEC is typically very low (<4 cmol
c
/kg) and they have a high P
fixation capacity. The variable charge component of several of the soils is near, or at,
the point of zero net charge (pH
KCl
-pH
water
ranging between -0.10 and 0.10).
The Acrisols are predominantly loamy in texture, although a loamy surface
over a sandy subsurface sometimes occurs. Erosion hazard ranges from slight to
extreme depending on site slope. Drainage is the main pathway of water movement in
half of the sites, with the remainder exhibiting runoff plus drainage or runoff
pathways. Acidity constraints are not common, but most of the soils have a moderate
acidification hazard because of their low clay and organic matter contents. Low
subsurface K reserves commonly occur. Hard-setting surfaces or compaction
occurred at several sites.
The main constraints identified for the Ferralsols were acidity (a), low nutrient
retention (e), high P fixation (i), and variable charge characteristics (geric), with
drainage being the most common pathway of water movement (Table 2). With these
constraints, the SCAMP database would indicate the following management strategies
for sustainable productivity:
a: Acid tolerant crops should be grown as a short term response to this constraint.
For long term sustainability, a liming program should be commenced with regular
monitoring of soil pH.
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 & Merckx,
2005). The practicality of adding high activity clays to increase permanent charge

could be assessed (e.g., Noble et al., 2004).
i: The high P-fixation capacity indicates that high rates of P fertiliser will be
required or special P management practices (i.e. sources and method of P fertiliser
application) will need to be implemented. P fertilization in minimum input cropping
systems should be directed toward the use of minimal P rates applied in bands or
pockets close to the seed, and the use of crops with low P-demand. Band or spot
placement of water-soluble P fertiliser applications will decrease the loss of P
availability by fixation. However, such placement will concentrate roots around the
fertiliser and this may reduce root exploration of the soil profile. In areas that have
short term droughts, this may limit yield because of restricted root access to soil
water. An initial, reduced rate, broadcast fertiliser application accompanying a banded
application should allow a more uniform root distribution. Soil P test levels should be
determined periodically to monitor soil P status.
geric: These soils have little net variable surface charge and therefore they have a
very limited capacity to retain nutrient cations (eg. calcium and potassium) or anions
(eg. nitrate) in the surface soil. Fertiliser will need to be applied in small frequent
applications in accord with crop nutrient demands. Liming the surface soil to pH (in
water) 5.5 will increase the ability of the soil to retain cations by increasing net
negative variable charge, and this is an important management option. Addition of
organic materials such as green manure crops should be considered because this may
also increase net negative variable charge.

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With respect to the Acrisols, the most commonly identified constraints were low-
moderate organic matter ratings, low K reserves particularly in subsurface layers, and
the soil physical problems of hard-setting surfaces and compaction. Some individual
sites suffered from impeded drainage leading to waterlogging and ponding of water.
To address these constraints, the SCAMP database would indicate the following
management strategies:
om (low-moderate): 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.
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.


4. Major Upland Crops grown by Smallholders
Agricultural census data were used to identify the major upland crops grown
by smallholders in Gia Lai. Crop areas are presented in Table 4. Crops occupying
greater than 5% of the arable land are: rice, maize, cassava, sugarcane, rubber and
coffee. Rubber is not grown by smallholders.

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Table 4. Crop areas of Gia lai Province. Smallholder crops are shaded.


Crop
Area
(ha)
Proportion
(%)
Rice 63883 20.4
Maize
40486
12.9
Cassava
24297
7.8
Vegetables
7618
2.4
Peanut
3787
1.2
Soybean
35
0.0
Tobacco
2350
0.8
Sugarcane
15543

5.0
Cotton
5184
1.7
Rubber
57307
18.3
Coffee
77530
24.8
Cashew
12354
3.9
Pepper
2609
0.8
TOTAL 312983


5. Soil Suitability for Major Upland Crops grown by Smallholders
SCAMP assessments of the Ferralsols and Acrisols (section 3 above) have
identified several soil constraints to crop productivity. Several of these constraints
have effects on crop productivity, irrespective of the crop grown; erosion (er), low
CEC (e), high P fixation (i), low organic carbon (om), K deficiency (k), variable
charge characteristics (geric), 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 5). To

facilitate the use of SCAMP for this application, individual soil attributes/constraints
identified for the Ferralsols and Acrisols have been rated according to their effects on
the sustainable production of the major upland crops grown by smallholders in Gia
Lai (Table 6). 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 5. 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 6. Suitability class of soil attributes/constraints for production of specific
crops.



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

L 1 1 1 1 1

C 1 1 2 1 1

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

3 3 3 3 2 3

4 4 1 2 1 2

5 4 1 1 1 1

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

2-5 2 1 1 1 1

5-10 3 2 2 2 2

>10 4 3 3 2 3
Soil pH
a
-
1 2 1 1 2

a

2 3 2 2 3
Salinity
s
-
s
2
3
3
4
3
4
2

4
3
4
Drought tolerance
L L H L L
Main nutrient
/water uptake zone
(cm)
<50 80-100 >100 >100 >100
Nutrient
needs
High N
req'd
High N, K
req'd
Tolerates
low
fertility
High N req'd High N, K
req'd



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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 3 above.

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 and Acrisols are the major arable soil groups in Gia Lai. The
constraints to crop production which commonly occur in the Ferralsols are acidity,
low nutrient cation retention, high P fixation and variable charge characteristics, with
drainage being the most common pathway of water movement. For Acrisols,
commonly occurring constraints are low-moderate organic matter ratings, low K
reserves particularly in subsurface layers, and the soil physical problems of hard-
setting surfaces and compaction. Some Acrisols have impeded drainage leading to
waterlogging and ponding of water.

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The crops most commonly grown by smallholders in the province are: paddy
rice, maize, cassava, sugarcane and coffee. Individual crop tolerances to some of the
identified constraints vary, and so soil management responses to ameliorate or
minimise the effects of these constraints on crop productivity will also vary. However
the use of a liming program and application of green manures to the Ferralsols is
required by all crops, while improving drainage, maintaining a surface mulch, and
applying green manures are essential practices for the Acrisols.

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