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Organic acids in the
rhizosphere and root characteristics of soybean (Glycine
max) and cowpea (Vigna unguiculata) in
relation to phosphorus uptake in poor savanna soils
Onyegbula Chike Nwoke1*, Jan Diels2,
Robert Abaidoo1, Generose Nziguheba3
and Roel Merckx2
1Soil
Research Unit, International Institute of Tropical
Agriculture, Ibadan, Nigeria.
2Division of Soil and Water Management, Department of Land
Management and Economics K.U. Leuven, Kasteelpark Arenberg
20, 3001 Heverlee, Belgium. 3Tropical Agriculture
and Rural Environment, The Earth Institute at Columbia
University, USA.
*Corresponding author. E-mail:
o.chik@yahoo.com. Tel:
+234-8034026707.
Accepted
2 September, 2008 |
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Root characteristics associated with phosphorus (P) uptake
under limiting soil-P conditions were examined in two sets
of greenhouse experiments. Average diameter and length of
soybean, cowpea, maize and sorghum roots were assessed after
7 weeks in three low-P soils amended with P fertilizer at 0,
3, 6, 11 and 23 mg P/kg. Organic acids in the rhizosphere of
soybean, cowpea and pigeon pea were separately evaluated in
one soil amended with or without rock phosphate, iron
phosphate, aluminium phosphate, calcium phosphate, or triple
super phosphate. Unplanted soil served as the control. The
growth of soybean, cowpea, maize, and sorghum was
significantly improved with P application in all the soils
and the amount of P applied played an important role. The
shoot dry matter yield and P accumulation correlated
significantly with the root length of cowpea and the average
diameter of sorghum roots. Citric acid was the only organic
acid detected in measurable quantities in the rhizosphere of
all plants tested; on average, it varied from 4 (pigeon pea)
to 17 (soybean) µmol/g soil. For soybean, the
secretion of citric acid appeared important for P
acquisition in P-limiting environments whereas for cowpea,
the size of the roots may be more important.
Key
words:
Legumes, low phosphorus availability, rhizosphere, sparingly
soluble P. |
