Introduction. The Longan fruit tree of the Vietnam Mekong Delta is grown in raised
beds to improve water drainage during the rainy season and can live as long as 100
years.
Objective. This research explores the extent to which the soil microorganisms as well
as soil physical and chemical properties of these raised beds degrade over a period of
60 years under traditional management practices.
Materials and Methods. Raised bed topsoil samples at depths of 0–20 cm were obtained
from four different Longan orchards raised bed age groups: group 1) 15–25 years (L1–
L5); group 2) 26–37 years (L6–L10); group 3) 38–45 years (L11–L15); and group 4) 46–
60 years. Soil biological properties were tested for nitrogen-fixing bacteria, phosphorus
solubilizing bacteria, potassium solubilizing bacteria, calcium solubilizing bacteria and
silicate solubilizing bacteria, β-glucosidase, urease, phosphomonoesterase, and phytase.
Soil samples were also tested for moisture content, soil texture, soil porosity, and bulk
density as well as soil chemical properties including pH, electrical conductivity (EC),
soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), total potassium
(TK), available nitrogen (NH4C, NO3-), available phosphorus (AP), exchangeable
potassium (K+), exchangeable calcium (Ca2C), available silicate (SiO2), available
copper (Cu), zinc (Zn), boron (B) and manganese (Mn). Key findings: The results
showed that soil moisture, soil porosity, sand content, SOM, TP, TK, available P,
exchangeable Ca2C, available Si, nitrogen fixing bacteria number, β-glucosidase, urease,
phosphomonoesterase, and phytase gradually and significantly decreased in the raised
bedsoil as the Longan orchard increased in age. Pearson correlation analysis between the
ages of Longan orchards and soil properties revealed that raised bed ages were positively
correlated with soil bulk density, but negatively correlated with soil moisture content,
soil porosity, SOM, TN, β-glucosidase, urease, phosphomonoesterase, and phytase.
Principal component analysis (PCA) showed Longan yields had a positive correlation
with available NO3- but negative correlation with NFB, exchangeable Ca2C, pH, and
available B. These findings reveal that traditional long-term management of Longan
trees in raised beds significantly reduce soil organic matter, moisture content, porosity,
and soil fertility with impacts on soil microbial numbers and activity within raised bed
soils.
Future Directions. This suggests that more sustainable management practices, such as
mulch and cover crops that decrease soil compaction and increase soil organic matter,
improve soil porosity, total N, and feed soil microorganisms that are critical to nutrient
cycling are needed to improve raised bed soil quality.