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《Acta Pedologica Sinica》 1963-04
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THE CONTENT OF IRON PHOSPHATES IN THE PADDY SOILS OF SOUTHERN CHINA AND THEIR SIGNIFICANCE TO THE PHOSPHORUS NUTRITION OF RICE PLANT

P.F.CHIANG R.K.LU I.C.KU AND A.Y.LI(Institute of Soil Science,Academia Sinica)  
Eighteen samples of paddy soils developed on red earth,alluvial and lacustrine de-posits and Hsiasu loam(a heavily leached loess deposit)were collected from Kwantung,Kwansi,Kiangsi and Kiangsu provinces.They possess pH value from 5.4 to 8.0 andare of different fertility.Fractionations of soil mineral phosphate were conducted accord-ing to Chang and Jackson's scheme.Analytical results revealed that iron phosphate,mostof which are coated with ferric oxide,predominate in paddy soils of acid and neutralreaction.Seven acid and neutral paddy soils were subjected to pot culture experiment.The“A” values for available phosphorus according to Fried were determined at differentgrowing stage of rice.Significant correlations between the content of iron phosphates tothe “A” values were found at all stages of growth of rice after the beginning of tillering,showing γ values 0.95 with 1% level at maturity.Correlations between the contents ofaluminium phosphate and calcium phosphate to the “A” values are rather insignificant,but it does not imply the inavailability of soil phosphorus in this form.A series of supplementary pot experiment were conducted with synthetic strengite(FePO_4·2H_2O),prepared from labelled phosphate.The effective uptake of labelledphosphate by rice plant verified the availability of phosphate ions originated fromstrengite.Solubility curve of strengite,after being transformed under anaerobic condi-tions,shows close coincidence with that of from amorphorus ferric phosphate(FePO_4·xH_2O).It is believed that strengite is firstly changed to more soluble ferrousphosphate under water-logged condition,and the phosphate ions is then reprecipitatedby active sesquioxide(predominantly iron oxide)to form amorphorus ferric phosphate,which has been proved a proper source of available phosphorus to rice plant.Littlechanges in solubility curve even occur when strengite was incubated with soils in anaerobic condition.This gives further evidence of the present conclusion.
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