Studies On Phosphorus Fractionsand Phosphate Sortion Desortion Characteristics Of Paddy Soils

Phosphorus fractionation of 20 rice soils from Jinhua,Zhejiang Province was conducted by the method of Hedley et al.The Indigenous P supplying capacity(IPS)of the soils was determined through P omission plot method in rice fields.Soil phosphorus was fractionated to Resin-P, NaHCO₃-Po,NaOH-Pi,NaOH-Po,HCl-P and residue-P.The characteristics of P fractions of different soils were analyzed.The relationships between P fractions with soil properties and Indigenous P supplying capacity(IPS)were studied.The following are the main results of this investigation.1.The percentages of soil available-P(Resin-P+NaHCO₃-Pi+NaHCO₃-Po),NaOH-Pi, NaOH-Po,HCL-P to the total extractable-P were 27.0,27.0,14.0,32.0,respectively,for the alluvial-origin rice soils;were 23.0,55.0,15.0,6.0,respectively,for the red earth-origin rice soils. The red earth-origin rice soils had much higher NaOH-Pi content than the alluvial-origin rice soils. The NaOH-Pi might be the buffering pool of the available-P in the rice soils studied.2.Soil Indigenous P supplying capacity(IPS)correlated well with the soil available-P(Resin-P +NaHCO₃-Pi+NaHCO₃-Po)fractions.It seems that for rice soils with the same origin(parent materials)in the same region,NaHCO₃-Pi is a good index for P supply considering P uptake by rice.3.Phosphate adsorption isotherm of the 20 paddy soils fitted in with Langmuir equation very well.The correlation coefficients were from 0.970 to 0.999.Parameters of Xm,K and buffer capacity of the Langmuir equation might correlate with soil properties.The highest values of Xm,K, MBC,0.2BC,and X_0.2were found in red earth derived rice soil 701 and 702,which might relate to their high content of clay and Fe,Al oxides which were responsible for phosphate sorption both in quantity and intensity.In the alluvial soils,Xm,MBC,and 0.2MBC might relate to their exchangeable-Ca,Olsen-P and clay content.4.It is realized that equilibrium P concentration of 0.2 mgL^-1may satisfy P demand for most crop growth in upland.Comparing with upland crops,irrigated rice might need a less equilibrium P concentration in solution for its growth,because phosphate moves more easily in submerged soil, the solution P concentration level could be easier compensated through P release from soil particles in rice soils. 5.The resin capsule extractable P was proportional to soil adsorbed P.The linear relationship was valid for all the soils studied,that is RAQ P=A+BX,where A and B are constants(r=0.941 to 0.999).The larger the A value,the higher the Resin-P,and the higher the P supply capacity of the soil;The less the B value,the higher the Resin-P,and the less sensitive of the RAQ P to P application.6.With the extending of incubation time resin could adsorb more P from the submerged rice soils.It could generally reach apparent equilibrium in about one month of incubation.It seemed that the P status and mineral composition of the soil controlled the rate and quantities of P release from soil to the resin sinks.7.A two constant rat model(RAQ P=at^b)could be used to describe the risen adsorption quantities(RAQ)of P as a function of time(t).Where a is the cumulative amount of P adsorbed by the resin at the first 1d,it represents the more easily released P of the soil;and b is a parameter describing the speed of decline in the adsorption rate.