Paper Number 7

Soil organic matter and soil fertility in the
long-term experiment of Lao People’s Democratic Republic*

* This country report has not been formally edited and the designations and terminology used are those of the author.

Oloth Sengtaheuanghoung
Soil Survey and Land Classification Center,
National Agriculture and Forest Research Institute (NAFRI),
Vientiane, Laos
and
Khamla Phanthaboun
Northern Agriculture and Forestry Research Centre (NAFRec),
Luangprabang Province

1. Introduction

The Lao PDR has a land area of 23.7 million ha, about one million ha of which are under agriculture. There are 18.7 million ha of hills and mountains. Hills account for 50 percent and mountains 30 percent, with slopes over 8 percent. Most of the hills and mountains are in the northern and central eastern part of the country where shifting cultivation is the major farming system. Luangprabang is the province with the highest area of shifting cultivation in the country.

The topography greatly influences the climate. Areas in the mountains usually receive high amount of rainfall than flat land area. The annual rainfall varies in amount and intensity from one area to another which invariably caused various forms of soil degradation depending on the nature and steepness of slopes and current farming practices.

In 1973 only 12.7 million ha or 54 percent of total area remained under forest cover and was further reduced in 1989 to 11.2 million ha, or 47 percent of total area of the country. Barren land/mountains, and grassland now cover 10.2 million ha where about 5 million ha are under bush fallow due to continuous slash-and-burn practices.

Current farming practice is unsustainable due to reduced fallow periods resulting from population increases. The government instituted a national policy to stop slash-and-burn cultivation as a way to protect the country’s forest resources. However, this policy, which failed to consider management of population growth, unnecessarily resulted in the controlled reduction of access and availability of lands to growing upland communities and individual upland farming families. As a result, fallow periods have rapidly decreased, thereby causing a serious decline in soil fertility, nutrient depletion, soil compaction, weed problems, and consequent soil degradation.

This paper presents the result of the two long-term soil fertility on-farm research sites in Luangprabang (northern part) and Vientiane (central part) from 1994 to 2001.

2. Objectives of the long-term research

1. To monitor the improvement and/or degradation of soil resources under different technologies applied
2. To assess the economical returns of different soil management practices

3. Materials and methods

3.1 Location and site characteristics

Luangprabang’s site

The site is located at longitude 102.10ºE and latitude 19.44ºN, within elevation of about 400 m above sea level in Houaykhot village, Xiengngeun district, Luangprabang province. The topography is classified as low hilly land with slope ranging from 25-35 percent.

The climate is classified as wet and dry tropical climate which is characterized by two distinctive seasons: the rainy season covers the months of April to October and the dry season, November to March. The annual rainfall of the area ranges from 1 100 mm to 1 400 mm. Monthly average rainfall distribution at the site from 1994 to 2001 is shown in Table 1.

The soil is classified as Typic Haplustaft, clay loam, isohyperthermic developed on shales. Soil profile description of the experimental site is show in Table 2.

Vientiane site

The site is located at longitude: 102.20ºE and latitude: 18.17ºN at Hineherb village, Hineherb district, Vientiane province. The topography is a low hill with slope ranging from 20-35 percent, the elevation is about 250 m above sea level, the climate is classified as wet and dry monsoon tropical, with annual rainfall ofabout 1 400-2 000 mm. The rainy season starts from May to September and the dry season start from October to April. Monthly average rainfall distribution at the site from 1995 to 2000 is shown in Table 3.

The soil is classified as typic haplustult clayloam isohyperthermic, developed on sandstone and siltstone. Soil profile description of the experimental site is shown in Table 4.

Table 1. Monthly average rainfall, from 1994 to 2000 at Luangprabang site

Table 2. Soil profile descriptions of Luangprabang site

Table 3. Monthly average rainfall, from 1995 to 2000 at Vientian site

Table 4. Soil profile description of Vientiane site

3.2 Experimental design

The experiment was carried out using a randomized complete block design with 3 replications. Individual plot is 10 x 24 m a Luangprabang site and 10 x 20 m at Vientiane site. The validated treatments were as follows:

Luangprabang site

• T1 Agroforestry (teak, and rice)
• T2 Strip cropping (rice and soybean)
• T3 Alley cropping using vetiver and mango trees as hedgerow
• T4 Hill side ditches with mango trees on the bund
• T5 Farmer’s practice

Vientiane site

• T1 Farmer’s practice (random planting)
• T2 Alley cropping using flemingia congesta as hedgerow
• T3 Alley cropping using flemingia congesta + banana as hedgerow
• T4 Agroforestry (rice and teak)

At Vientiane site, in 1998, 1999 and 2000, 100 kg of Diammonium Phosphate (18N, 46P) with 1 500 kg of powered lime were applied for peanut in T2 and T3. There is no fertilizer use in Luangprabang site.

4. Results and discussion

4.1 Soil loss

An average total annual soil loss from different treatments at Luangprabang site from 1994 to 2001 and Vientiane site from 1995 to 2001 are shown in Tables 5 and 6.

The highest soil loss occurred in farmer practice (T5) with amount of soil loss of 6.07 t/ha. Alley cropping (T3) is most effective in reducing soil loss (0.86 t/ha), followed by the hillside ditches (1.89 t/ha).

The Agroforesty and strip cropping treatment can reduce soil loss equivalent, at 2.88 and 3.05 t/ha, respectively.

Table 5. Total soil loss (t/ha) from 1994-2001 at Luangprabang site

Table 6. Total soil loss (t/ha) from 1995-2001 at Vientiane site

At Vientiane site, among validated technologies, alley cropping using flemingia congesta hedgerow (T2) and alley cropping using flemingia congesta with banana hedgerow were the most effective in reducing soil loss, the amount of soil loss of these treatments were 6.88 and 7.32 t/ha respectively.

The farmer’s practice treatment had the largest soil loss (24.90 t/ha), followed by the Agroforestry treatment which produced the amount of soil loss of 16.00 t/ha.

4.2 Run off

An average total annual run off from different treatments at Luangprabang site and Vientiane site are shown in Tables 7 and 8.

Table 7. Total run off (m³/ha) from 1994-2001 at Luangprabang site

Table 8. Total run off (m³/ha) from 1995-2001 at Vientiane site

At Luangprabang site, the lowest run off was found in the hillside ditches treatment (683 m³/ha), the alley cropping treatment can also effectively reduced run off (829 m³/ha). The strip cropping and Agroforestry treatment could reduce run off similarly, the amount of run off from these treatments were 1 138 m³/ha and 1 436 m³/ha respectively. The farmers practice produced the highest run off (2 386 m³/ha).

At Vientiane site, alley cropping using flemingia congesta hedgerow and alley cropping using flemingia congesta with banana as hedgerow gave the lowest run off 1 470 and 1 516 m³/ha respectively. The farmers’ practice produced the highest run off (3 168 m³/ha), fallowed by the Agroforestry treatment which produced run off (2 554 m³/ha).

4.3 Soil fertility change over time

Soil pH

Soil reaction pH from different treatments at Luangprabang and Vientiane site is illustrated in Tables 9 and 10.

Table 9. Soil pH from 1994-2000 at Luangprabang site

Table 10. Soil pH from 1995-2000 at Vientiane site

At Luangprabang site, soil pH from the farmer’s practice decreased from 6.2 in 1995 to 5.5 in 2001. While, the hillside ditch treatment from 6.0 to 5.6 in 2001 and the strip cropping from 6.2 in 1995 to 5.8 in 2001 . However there is slightly change in soil pH from the alley cropping and Agroforestry.

At Vientiane site, soil pH from alley cropping using flemingia congesta as hedgerow and alley cropping using flemingia congesta with banana as hedgerow increased from 4.9 in 1995 to 5.7 in 2001 and from 4.9 in 1995 to 5.6 in 2001, respectively. This was due to the application of powered lime at the rate of 1 500 kg/ha. While soil pH from the farmer’s practice and Agroforestry decreased from 5.0 in 1995 to 4.5 in 2001 and from 4.9 in 1995 to 4.6 in 2001, respectively.

4.4 Soil organic matter (OM)

Soil organic matter (OM) from different treatments at Luangprabang and Vientiane site is shown in Tables 11 and 12.

At Luangprabang site, soil organic matter content from all treatments tends to decrease from year to year. When comparing all treatments, the farmer’s practice decreased the most amount of organic matter content followed by Agroforestry and hillside ditch treatments. In the farmers’ practice treatment, soil organic matter decreased from 3.71 percent in 1994 to 1.78 percent in 2001. This was due to the removing crop residues from the plot coupled with top soil loss through water erosion. On the other hand, alley cropping and strip cropping showed the least decline in soil organic matter content. This was due to more crop residues that were returned back to the soil.

Similar results were found at Vientiane site with farmers’ practice showing highest decrease in soil organic matter. Alley cropping using flemingia congesta as hedgerow yielded a least decline in soil organic matter content, due to the application of fertilizer and lime enhancing more biomass production by crop.

Table 11. Soil organic matter (percent OM) from 1994-2001 at Luangprabang site

Table 12. Soil organic matter (percent OM) from 1995-2001 at Vientiane site

4.5 Soil phosphorus available

Soil phosphorus available (P₂0₅) from different treatments at Luangprabang and Vientiane site is shown in Tables 13 and 14.

Table 13. Soil phosphorus available (P₂0₅ ppm) from 1994-2001 at Luangprabang site

Table 14. Soil phosphorus available (P ppm) from 1995-2001 at Vientiane site

At Luangprabang site, phosphorus available from all treatments decreased over time this was may due to uptake by annual crops every year and coupled with no input of phosphorus fertilizer.

At Vientiane site, there was obvious evident that phosphorus available under alley cropping using flemingia congesta as hedgerow and alley cropping using flemingia congesta with banana as hedgerow, phosphorus available increased from 3.0 ppm in 1995 to 12.35 ppm in 2001 and from 3.7 ppm to 14.9 ppm in 2001. This was due to the application of diammonium phosphate (18N, 46P) at the rate of 100 kg/ha/year.

Phosphorus available under farmers practice decreased over time from 3.0 ppm in 1995 to 2.7 ppm in 2001, followed by the Agroforestry that decreased from 3.0 ppm in 1995 to 1.7 ppm in 2000.

4.6 Soil potassium available

Soil potassium available (K) from different treatments at Luangprabang and Vientiane site is show in Tables 15 and 16.

Table 15. Soil potassium available (K₂O mg/100 g) from 1994-2000 at Luangprabang site

Table 16. Soil potassium available (K₂O mg/100 g) from 1995-2000 at Vientiane site

At Luangprabang site potassium available from all treatments declined over time. Potassium available under farmer practice decreased from 29.0 mg/100 g in 1994 to 13.4 mg/100 g in 2001, Under Agroforestry treatment, potassium declined from 19.3 mg/100 g in 1994 to 11.2 mg/100 g in 2001, hillside ditches from 31.2 mg/100 g in 1994 to 19.3 mg/100 g in 2001; alley cropping from 18.8 mg/100 g in 1994 to 13.2 mg/100 g in 2001 and strip cropping from 25.6 mg/100 g in 1994 to 21.3 mg/100 g in 2001. his was due to uptake by annual crops every year and coupled with no input of potassium fertilizer. Similar feature was found at Vientiane site.

4.7 Crop yield

4.7.1 Rice yield

Average upland rice yield from different treatments at Luangprabang and Vientiane site is shown in Tables 17 and 18.

Table 17. Rice yield (kg/ha) from 1994-1997 at Luangprabang site

Table 18. Rice yield (kg/ha) from 1995-1997 at Vientiane site

At Luangprabang site, the Agroforestry treatment gave highest yield 862 kg/ha. This due to crop densities is well regularly arranged, when compare with the farmer practice, followed by the farmer practice which produced rice yield of 634 kg/ha.

The alley cropping and hillside ditches gave the lowest yield (634 kg/ha and 533 kg/ha respectively). The lower yields of these treatments were because part of the area on these plots was used for vegetative hedgerow and ditches.

Similar feature, was found at the Vientiane site, the highest rice yield was found under Agroforesty with the yield of 1 036 kg/ha, followed by the farmer practice, alley cropping using flemingia as hedgerow, which produced yield of 924 kg/ha and 890 kg/ha respectively.

Alley cropping using flemingia congesta with banana produced less rice yield than the other treatments with yield of 816 kg/ha, this was due to the effect of shading and nutrient competition from banana.

At both sites, upland rice yield seemed to decline from year to year, this was due to the mono cropping of upland rice, and without legume crop rotation.

4.7.2 Peanut yield

Average peanut yield from different treatments at Luangprabang and Vientiane site is shown in Tables 19 and 20.

Table 19. Peanut yield (kg/ha) from 1998-2000 at Luangprabang site

Table 20. Peanut yield (kg/ha) from 1998-2000 at Vientiane site

At Luangprabang site, the strip cropping produced highest yield (799 kg/ha), this was due to having regular crop density, followed by the farmer practice which produced yield of 681 kg/ha. The lowest yield were under alley cropping and hillside ditches treatments with yield of 472 kg/ha and 444 kg/ha respectively. This was due to the effect of shading canopy of mango threes.

At Vientiane site, the alley cropping using flemingia congesta as hedgerow gave the highest yield (868 kg/ha), this was due to the application of Diammonium Phosphate at the rate of 100 kg/ha and powered lime at the rate of 1 500 kg/ha. It was observed that under alley cropping using flemingia congesta with banana as hedgerow even fertilizer and lime used as the same rate of alley cropping using flemingia as hedgerow, the yield was still low. This was due to the effect of shading canopy and the competition of nutrient element from banana.

4.7.3 Banana yield

Average banana yield from alley cropping treatment using flemingia congesta with banana as hedgerow at Vientiane site is shown in Table 21.

Table 21. Banana yield (kg/ha) from 1996-2000 at Vientiane site

At Vientiane site, banana yield under alley cropping using flemingia congesta with banana as hedgerow was higher in the first two years with yields of 3 800 kg/ha in 1995 and 5 600 kg/ha in 1996, then declined from year to year. It was noticed that banana can not maintain yield for long period of time.

4.7.4 Mango yield

Average mango yield from alley cropping and hillside ditches treatments at Luangprabang site is shown in Table 22.

At Luangprabang site, after three years old of mango trees, since 1998 the yield of mango under hillside ditches increased from year to year with yield of 215 kg/ha in 1998, 636 kg/ha in 1999 and 920 kg/ha in 2000. Similar feature was found under alley cropping treatment.

Table 22. Mango yield (kg/ha) from 1994-2000 at Luangprabang site

4.7.5 Biomass yield

Total biomass (rice straw and peanut residues) of different treatments at Luangprabang and Vientiane site are shown in Tables 23 and 24.

Table 23. Total crop residues (t/ha) from 1994-2000 at Luangprabang site

Table 24. Total crop residues (t/ha) from 1995-2000 at Vientiane site

At Luangprabang site the highest biomass was recorded in the strip cropping which produced biomass of 26.68 t/ha, including, soy been residues, and rice straw, followed by the alley cropping and hillside ditches treatment which produced biomass of 20.65 t/ha and 18.22/ha respectively. While, for the farmer practice treatment crop residues were removed out from the plot.

At Vientiane site, the highest biomass was found under alley cropping using flemingia as hedgerow which gave the biomass of 24.93 t/ha. This was due to the application of fertilizer with lime enhancing biomass production of crops. Followed by the alley cropping treatment using flemingia congesta with banana as hedgerow gave biomass of 19.16 kg/ha. The least biomass was found under Agroforestry treatment, because, after three years old of teak, any crop could not be planted under teak canopy. For the farmer practice crop residues were removed out from the plot.

4.7.6 Net return

Net return (US$/ha) from different treatments at Luangprabang and Vientiane site is shown in Tables 25 and 26.

Table 25. Net return (US$/ha) from 1994-2000 at Luangprabang site

Table 26. Net return (US$/ha) from 1995-2000 at Vientiane site

At Luangprabang site, highest negative net return was found under hillside ditches and alley cropping treatments in the first year of operation. This was due to the high cost of plating material (mango seedlings) coupled with the cost of establishing measure of soil erosion control (ditches and vetiver hedgerow). However, high net return could be received in the succeeding later year with additional income derived from mango trees. Strip cropping gave positive net return for all years due to income from soybean and peanut rather than upland rice. Farmer practice gave negative income in the first four years due to lower price of upland rice. A positive net income could be received when legume crop (peanut) replaced to rice. Agroforestry gave negative net return in the first year of operation (1995) due to the high cost of planting materials (teak seedlings) and in the later year due to crop could not be inter cropped under teak canopy.

At Vientiane site, negative net return occurred for all treatment in the first year of operation. Positive net return could be received in the second, third, fourth and fifth year, however the highest net income occurred in the thirst year (156.86 US$/ha), for the alley cropping using flemingia hedgerow with banana, when high income derived from banana. After that net return decreased from year to year, due to the declining of banana yield. Followed by the alley cropping using flemingia congesta as hedgerow, which could receive positive net return from 1998 to 2000 when upland rice replaced by peanut. Farmer practice gave negative net return for the most of the year due to low crop yield.

5. Conclusions

After seven years of long-term research (1994-2000) on sloping land for sustainable agriculture in Lao PDR, a number of conclusions can be made.

1. Among soil and crop management technologies being validated in Lao PDR, alley cropping using vetiver and flemingia congesta as hedgerow and hillside ditches were very effective in reducing run off and soil loss. The farmer’s practice of planting crop without soil conservation measures caused high run off and soil loss.
2. Soil fertility changed after seven years of cropping, the decreasing soil organic matter under farmer’s practice was evident after several cropping, the alley cropping and strip cropping shown the least decrease in soil organic matter content. The application of fertilizer and lime for alley cropping treatment resulted in increasing level of phosphorus content and soil pH.
3. Crop yield (annual crop) could be maintained in the first two years, after that declined from year to year.
4. Alley copping using vetier with mango trees as hedgerow and hillside ditch planting mango trees on the bunds gave high net income in later year (sixth and seven year) after trees produced fruits.
5. Alley cropping using flemingia congesta with banana as hedgerow gave high net income in second, third and fourth year and declined in later year.

References

National statistic center, 1994, basically statistic on socio-economic development in Lao PDR.

Soil Survey and Land Classification center, 1994, report on soil physical and chemical characteristics in Lao PDR.

Soil Survey and Land Classification center, 1995 report on soil Survey of Luangprabang province.

Forest planning and Inventory Agency, 1995, report on forest cover and land use in Lao PDR.

Annual report, Management of sloping land for Sustainable in Lao PDR, 1994.

Annual report, Management of sloping land for Sustainable in Lao PDR, 1995.

Annual report, Management of sloping land for Sustainable in Lao PDR, 1996.

Annual report, Management of sloping land for Sustainable in Lao PDR, 1997.

Annual report, Management of sloping land for Sustainable in Lao PDR, 1998.

Annual report, Management of sloping land for Sustainable in Lao PDR, 1999.

Annual report, Management of sloping land for Sustainable in Lao PDR, 2000.