MANAGEMENT OF PHOSPHORUS IN SOILS WITH HIGH PHOSPHORUS STATUS
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Date
2009-07-18
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University of Agricultural Sciences GKVK, Bangalore
Abstract
Phosphorus fertilizers are expensive due to shortage of phosphatic ores.
There is need to explore alternative P sources. Towards this FYM, green manure
and biofertilizers were experimented with to solubilize and mobilise fixed P in
soil. The research activities were conducted at four levels during 2008-09 viz 1)
collection of soils from three agroclimatic zones of Karnataka to assess the total P
status of groundnut growing soils (141 soil samples) 2) Laboratory incubation
study 3) Pot culture experiment and 4) Field experiment to assess the
solubilization and transformations of different P fractions were conducted.
Most of the soils showed P buildup especially Chintamani soil, which
recorded higher available P (33.7-128.7 kg ha-1) and total P (605.3-975.5 mg kg-1).
Laboratory incubation study and pot culture experiment were conducted at
GKVK, Bangalore (Alfisol , pH = 4.91, Av. P2O5 = 141.29 kg ha-1and total P =
1456.2 mg kg-1 ) and the major fraction of phosphorus were in the order of Fe-P >
RS-P > Al-P > Occl-P > Ca-P. The Sal-P decreased from 60.1 to 28.4 mg kg-1.
The field experiment was carried out at Agricultural College Farm, Hassan
(Av. P2O5 = 56.69 kg ha-1and total P = 846 mg kg-1). In Hassan soil, the P fractions
were in the order of RS-P >Fe-P > Al-P > Ca-P > Sal-P > Occl-P. Irrespective of
the treatment combinations, all the fractions of P increased with addition of P
fertilizer and a major portion of it was fixed within seven days. Sal-P decreased
drastically because of fixation especially in treatment receiving N, P and K (from
76.3 to 28.2 mg kg-1 in T3). Interestingly the integration of organics and inorganics
along with biofertilizers (T11 to T15) retained higher level of Sal-P till the later
stages mainly because of less fixation and solubilisation of the already fixed
forms. Al & Ca-P mainly contributed to crop growth and between this Ca-P was
found to be easily soluble. Groundnut growth, yield and P content (T12=0.55%)
were found to be maximum in treatments which received half dose of P fertilizer,
organic manure and multiple inoculation of biofertilizers.
The Phosphatase enzyme activity was also studied. It increased with crop
growth upto 30 DAS both in pot culture and field experiment and the activity was
significantly higher in presence of VAM and organic matter. Thiobacillus
thiooxidans was found to be active in acid soils and B. megaterium in neutral soils.
In conclusion the findings of this study has a practical relevance in that the
recommended quantity P fertilizer could be reduced to one half if organic manures
are applied @ 10 t ha-1 along with multiple inoculation of biofertilizers (PSB and
VAM) for groundnut in soils with high P status.
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