Coastal sandy soils and constraints for crops in Binh Thuan Province,
Southern Central Vietnam

Nguyen Cong Vinh¹

Keywords: sandy soils, Central Vietnam, nutrient limitations

Introduction

Vietnam is a humid tropical country, covered by large areas of soils of light texture classified as Acrisols and Arenosols (marine sands). These soils are distributed in Thanh Hoa, Nghe An, Quang Binh, Thua Thien Hue, Ninh Thuan, Binh Thuan, Dong Nai, Vinh Phuc, and Phu Tho Provinces. The total area of light textured soils is estimated to be 20,504,076 ha, of which Arenosols comprise 533,434 ha and Acrisols, 19,970,642 ha. In Binh Thuan Province in the central coastal area of Vietnam there are 12,935 ha of Arenosols and 154,210 ha of Acrisols. This coastal region is characterized by very hot and dry weather, and low soil fertility, making agricultural production problematic.

Facilitating management practices that enable farmers to improve soil fertility is essential for economic development of the coastal zones. A participatory study with farmers was carried out in Bac Binh District, Binh Thuan Province to find out the limiting nutrient factors and to develop appropriate strategies for sustainable crop production. This study aims to: evaluate the soil fertility status of the dry coastal land and to identify the main limiting factors for crop growth; increase farmer understanding of soil characteristics and fertility of their fields for better crops; and facilitate farmer adoption of soil amelioration practices and soil environmental protection.

Materials and methods

Field survey

A field survey was carried out of the agricultural land of 4 communes (Luong Son, Song Luy, Song Binh and Binh Tan). Seventy six soil samples from 14 profiles were taken for analysis. The following analyses were undertaken: pH (1:2.5 water), Walkley Black organic C, Kjeldahl total N, total P₂O₅ and total K₂O using a concentrated H₂SO₄+ HClO₃ digest, available P₂O₅ using the Bray 2 extractant, and exchangeable Ca²⁺ , Mg²⁺ and CEC using NH4COOH, 1 M, pH 7.

Omission Trials

Glasshouse trials were undertaken following the guidelines for rate trails and omission trails recommended in “Diagnosis of nutritional limitation to plant growth by nutrient omission pot trials” (Asher and Grundon, 1998). The methodology comprises two main steps. The first defines suitable nutrient rates for the crop. This is done with rate trials of nutrients applied at rates ranging from 0 (control) to 4 times. Results indicate the most suitable rate of applied nutrients for crop growth in each soil. The second step in the methodology, based on the results of the first step, comprises an omission trail for 14 nutrient elements (Table 1). This step aims to find out the most limiting nutrients to the growth of the plant. If any element omitted, while other elements are applied at the most suitable rates, and plants grow weakly, then the tested element is a limiting factor for crop growth. Conversely, if any element is omitted, but plants are healthy, then that element is not a limiting factor for crop production. Pot trails were carried out in the greenhouse at the National Institute for Soils and Fertilizers. In this report, only the results of the omission trials are presented.

Table 1. Treatments applied in the omission trial in the glasshouse

Nutrients were applied in solution form to the soil in pots. Soil moisture was maintained at field capacity with distilled water during 7 days after germination. Maize (Zea mays) seedlings were thinned to 3 plants/pot. Distilled water was added as often as necessary to keep the soil moisture around 80-85% of field capacity. This moisture content is considered to be the most effective for plant growth.

Field trials on fertilization for peanut

Field trials were laid out in a completely randomized block design with 4 replications. Treatments are indicated in Table 2.

Table 2. Treatments in field trial carried out for peanut on an Arenosol

Results and discussion

Site characterization

There are 4 soil types in Bac Binh District: Arenosols (sandy coastal soil) (53,670 ha = 29.1%), river alluvial soils (16,000 ha = 8.7%), Acrisols (21,900 ha = 11.9%) and Ferralsols (87,290 ha = 47.4%). Agricultural land comprises 38,000 ha. In general, soils in Bac Binh are light textured in the surface, have loose structure and non sticky plasticity. Soil is compact and has a low water holding capacity. Water and wind erosion occurs throughout the year. Climate data for the district are as follows: mean air temperature: 26-27ºC; difference between mean air temperatures day and night: 5-7ºC; mean relative humidity: 78.9%; rainy days in year: 50-60; annual precipitation: 750-1,200 mm; highest monthly rainfall (month): 157 mm (July); lowest monthly rainfall (month): 3.4 mm (March). The dry season lasts from October to next April with an average rainfall of 5 mm/month, although monthly rainfall can be less than 1 mm/month for 3 consecutive months. In addition, during the dry season there is very strong wind, causing sand movement.

Current farmer practice

Farmer awareness of environmental degradation and desertification is limited. Findings from a survey of the local indigenous knowledge in Binh Thuan found that farmers have a low understanding of soil fertility and fertilization, especially organic fertilizers. Farmyard manure (FYM) is not applied to the field but sent to others off-site. Moreover, farmers have no understanding of the role of phosphate fertilizer and lime, so neither of these is applied to the crop. As a consequence, cultivated soils are very poor in organic matter and phosphate, and are strongly acidic.

Table 3. Soil acidity and total nutrient contents of soils of four communes

Soil chemical characteristics

A soil survey was undertaken and 76 samples were collected for analysis. Data in Table 3 show that soil acidity ranges from moderately acid to strongly acid with pHKCl values of 3.6-5.8, average 4.79, with coefficients of variation (CV%) of 7.04-12.32%. Soil organic carbon (OC%) in the surface layer varied from 0.12-1.56%. Nitrogen varied from 0.016-0.078% N. Percentage of total P₂O₅ ranged from 0.006-0.112% while the percentage of total K₂O ranged from 0.012-0.85%. Values of total OC, N, P and K had high CV%.

Table 4 presents the extractable P, exchangeable cations and CEC of the soils. Both Bray II extractable P and exchangeable K have large ranges (0.21-18.65 mg P₂O₅/100 g soil and 1.2-26.13 mg K₂O/100 g soil). Data indicate that in general the soils are very poor in available P and K. Cation exchange capacity is generally low with an average of 7.35 meq/100 g soil. Exchangeable cations averaged 2.2 meq/100 g soil for Ca²⁺ and 1.43 meq/100 g soil for Mg²⁺.

Distribution of nutrients in the soil profile

White dry sandy soil: This soil type is common in Bac Binh Distinct, being distributed on undulating hills and sandy dunes. Almost all of the district’s cashew is grown on this soil type. Cashew trees grow very well, but have very low yields, in some years producing no yield at all. This may be caused by nutrient imbalance or deficiency, or extremely low plant available water.

Profile LS1 is under cassava-cashew production (Table 5). Cashew is 10 years old with healthy trees. Cassava shows poor growth. Chemical properties are presented in Table 5. These data indicate that this type of soil is strongly acidic, and poor in organic matter and total N. Phosphorus and K are low in both total and available forms.

Red dry sandy soil: Red dry sandy soils are distributed in the Hoa Thang, Hong Phong, Binh Tan communes. These areas are being seriously degraded. Soils are very dry, and susceptible to wind and water erosion. Profile LS12 indicates that the soil is moderately acid and poor in plant nutrients, and has low cation exchange capacity throughout the profile.

Grey sandy soil derived from sandstone: This type of soil occurs in Song Binh and Binh Tan communes. Cropping systems are cassava, cashew, water melon, peanut and corn. Crops generally show very poor growth and yields. The soil is strongly acid, with low organic carbon and poor in plant nutrients (Table 7). Cation exchange capacity is very low.

Table 4. Available contents of P₂O₅ and K₂O and exchangeable cations in the soils of four communes

Table 5. Soil chemical properties of a white dry sandy soil profile (Profile LS1)

Table 6. Soil chemical properties of a dry red sandy soil profile in Hong Phong (Profile LS12)

Table 7. Soil chemical properties in the profile of a grey sandy soil in Binh Tan (Profile BT5)

Nutrients limiting growth in the soils in omission trials

Red dry sandy soils: Plant growth was markedly hindered by omission of macroelements. Compared to the control treatment, omission of P reduced plant height by 53%, omission of N caused a 45% reduction, and omission of K reduced height by 36%. Plant dry weight was reduced by 18-48% when macroelements were not added (Table 8). The lowest weight was recorded in Treatment 2, in which N was omitted. The second greatest macroelement deficiency factor was P, followed by K.

Table 8. Nutrient effects on corn growth on a red sandy soil, Hoa Thang

Omission of Ca and Mg impaired corn growth more than the omission of S. Compared to the control treatment, omission of Ca reduced plant height by 36% and omission of Mg reduced height by 42%. Plant dry weight was reduced 6-12% when mesoelements were not added. The lowest weight was recorded in Treatment 7, in which Ca was omitted. The next most deficient element was Mg, then S. In the case of the micronutrients, plant height and dry weights were lowest when B and Mo were omitted, followed by Zn. However, when Mn was omitted, plant height and dry weight were similar to the control treatment. This indicates that Mn may be in surplus supply in the soil.

White dry sandy soils: The effects of nutrient omission on the white sandy soil are presented in Table 9. Plant heights were 42.5 cm (77%) when N was omitted, 44.3 cm (81%) without P, and 45.3 cm (82%) without K (Table 9). Plant weight was 50-55% less than the control treatment when N, P, and K were omitted (Table 9). These results show that N, P, and K are deficient in white sandy soil.

Table 9. Nutrient effects on corn growth on white sandy soil, Hoa Thang

Omission of S only reduced plant height by 14%, but omission of Ca reduced height by 36% and omission of Mg caused a 41% reduction (Table 9). Dry weight was reduced 34-58% when mesoelements were not added. The lowest weight was recorded when Mg was omitted, followed by Ca and S. For the micronutrients, plant height and weight were greatly reduced when Zn and B were omitted (Table 9). Plant dry weight was reduced by over 50% when either of these nutrients was omitted. Plant height was the same as the control treatment when Cu and Mn were omitted. The plant dry weights were reduced by 24% in the -Cu treatment and by 29% in the -Mn treatment compared to the control treatment.

Appropriate fertilization to improve soil fertility and crop yields

A field trial on fertilization for peanut was carried out on a white sandy soil inter-cropped under a cashew nut plantation more than 10 years old. This field experimentation aims to help the farmer get a better understanding of soil fertility/fertilization and economic returns from intercropped annual crop with perennial crops on white sandy soils.

Field trial results are presented in Table 10. Farmer practice applied 20 kg NPK (20:20:25) and 5 kg urea per sao (100 m²). Lime and organic manure were not applied. This type of soil is acid and low in organic matter. Peanut in farmer practice can give pod yields of 2,505.6 kg/ha. At the same rate of inorganic fertilizers as farmer practice, but with liming and application of organic fertilizer (T2) pod yield increased to 2,791 kg/ha, 11.4% higher than farmer practice.

Table 10. The effect of balanced fertilization on peanut yields (Mr. Hue’s field)

Table 11. Economic return (thousands of VND) in peanut experiment

With balanced NPK at the rates of 45 N, 90 P₂O₅, and 90 K₂O/ha, liming and FYM (T3), pod yield of peanut increased to 3,812.8 kg/ha, 52.2% higher than farmer practice. White dry sandy soil is very poor in organic manure, and NPK. It has low cation exchange capacity and consequently increasing high inorganic fertilizer may not increase pod yield because of the limited capacity of the soil to hold nutrient cations. In Treatment 4, increasing the rates of NPK up to 60 N, 120 P₂O₅, 120 K₂O/ha gave pod yield of 2,753 kg/ha, only 9.9% higher than farmer practice.

Economic effect of fertilizer for peanut

Investment in farmer fertilizer practice for peanut production is estimated to be 3,410 thousand VND/ha (Table 11). Improved treatments increased the outlays to 8,660-11,756 thousand VND/ha. Estimated economic returns were calculated as 15,033.6 thousand VND/ha for farmer practice. However at the higher fertilizer application rates, incomes can reach 16,746 thousand VND/ha in T2, 22,876 thousand VND/ha in T3 and 16,521 thousand VND/ha in T4. Improved fertilization increased returns from 1,712.4 to 7,843.2 thousand VND/ha/crop more than farmer practice (Table 11). This represents an increase in economic returns from 11.4-52.2%, compared to farmer practice.

White sandy soil is acid and poor in organic matter and available plant nutrients. Farmer practice is cultivation without organic manure, no liming and imbalanced low fertilization. In order to transfer the technologies on soil fertility and fertilization, field demonstrations were implemented with 4 treatments in 4 farmers fields (Table 12).

Table 11. Economic return (thousands of VND) in peanut experiment

Table 12. Treatments for field demonstrations of fertilization for peanut in Hoa Thang commune

Fertilizer and liming for autumn peanut on white sandy soils increased plant growth and yields remarkably (Table 13). Balanced application and liming for peanut increased branches by 16%, plant height by 29%, number of pod/plant by 67% and filled pods/plant by 111% compared to farmer practice. FYM addition in combination with fertilizer and liming, increased branches/plant, plant height, pods/plant and filled pods/plant over farmer practice by 32%, 49%, 147% and 228% respectively.

Table 13. Yield components and yields of peanut in field trails (mean of 4 field demonstration trials)

Table 14. Peanut yields in participatory study in Hoa Thang. Values are means for 10 farms

In the second peanut crop (next spring), further demonstration trials were carried out on 10 farms. Results are shown in Table 14. Balanced fertilization (N, P, K, lime) treatments increased peanut yields by 55% for stover and 66% for pod, compared to farmer practice. Combination of NPK, lime and FYM increased stover yield by 122%, and pod yield by 149%, compared with farmer practice.

Conclusion

In general, coastal sandy soils in Bac Binh District, Binh Thuan Province are strongly acid, low in organic matter and poor in plant nutrients. Sandy land is suffering from serious wind and water soil erosion, and these are constraints for agricultural production. Pot trials indicated that the corn grown on two different soil types of Hoa Thang and Phan Hoa have different responses to the nutrient rates and elements. Omission trials indicated that deficiencies of nutrient elements for crop growth were arranged as follows:

• Red sandy soil: N > P > K and B > Mo >Zn.
• White sandy soil: P > N > K and B > Mo > Zn.

In both soils, omission of Mn increased yields. Balanced fertilizer, liming, and soil organic man­agement could increase peanut yields by 66-149%, compared with farmer practice.

Figure 1. Bac Binh Province and project sites in the Bac Binh District

References

Asher, C.J. and. Grundon, N.J. 1998. Diagnosis of nutritional limitation to plant growth by nutrient omissions pot trials.

Vinh, N.C. and Phien, T. 1997. Limiting factors on plant nutrition and fertilizer effect on plants grown on acid upland soil.

Vinh, N.C. and Trinh, M.V. 2001. Result of surveying on land use and local knowledge at Bac Binh District. Project report.

¹ National Institute for Soils and Fertilizers, Tu Liem, Hanoi, Vietnam: [email protected], ncvinh_nisf2003 @yahoo.com