P. van Quang et al. / Agricultural Science 2 (2011) 375-381
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
380
of mineral N early in the decomposition process. For this
study, soil NH4
+-N levels were above 10 mg·kg–1 at all of
the observed sites, and NO3
–-N levels were excessive
and high for 0 - 20 cm and 20 - 50 cm depth respectively
as evaluated following Marx et al. [27] (Tables 6 and 7).
This exhibited that the soil contained too much fertilizer
residue.
3.6. Available Phosphorous
Phosphorous concentration was 23.0 - 37.6 mg·kg–1 in
young age group and 19.8 - 43.2 mg·kg–1 in old age
group for topsoil (Table 6). For subsoil, the range is 21.7
- 30.0 mg·kg–1 in young age group and 14.6 - 37.2
mg· k g –1 in old age group (Tab le 7). There was no sig-
nificant difference between the two age groups, but
phosphorous was significantly higher in the top soil
compared to the subsoil for the old age group. The re-
sults (Ta bl es 6 and 7) showed that phosphorous (on av-
erage values) was ranked as very high based on Abbott
[28].
4. CONCLUSIONS
The insufficient soil fertility in the raised beds has had
an adverse effect on plant growth. The results indicated
that there existed an imbalance in soil nutrients under
low pH conditions; therefore, this may lead to plant
production problems. Loss of soil quality was also ex-
hibited in reduced organic matter with the aging of
raised beds. Excess nitrogen and phosphorous in soil
may leach out to streams, rivers and groundwater.
In managing these fruit plantations on the raised beds
in the Mekong Delta, fruit growers should pay attention
on the following issues.
Raise the soil pH;
Balance fertilizer application;
Manage the soil moisture;
Improve the soil’s physical qualities.
5. ACKNOWLEDGEMENTS
We gratefully show appreciation to Mr. Ngo Xuan Hien, Agronomy
Bureau, Chau Thanh District, Hau Giang Province, Vietnam for help
and encouragement throughout the field works. We thank all raised
beds’ owners for their kindly support of study locations. We also thank
Huynh Ngoc Duc and Pham Xuan Phu for the assistance during data
collection; Ly Ngoc Thanh Xuan and analysis laboratory staff, Faculty
of Agriculture and Natural Resources, An Giang University, Vietnam
for their help with soil chemical testing. We are grateful to Per-Erik
Jansson and Carin Sjöstedt for providing very helpful comments to
improve this manuscript.
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