Saturday, 15 July 2006

Soil Phosphorus Fractionation Dynamics and Phosphorus Sorption in a Continuous Maize-Wheat Cropping System.

K. N. Sharma Sr., Punjab Agricultural Univ, Dept of Soils, Ludhiana, India

Agronomic field studies of Phosphorus (P) management have focused on the plant available inorganic P which are used to estimate fertilizer requirement. However, only crops and most of the applied P remains in the soil in forms take up a small portion of fertilizer P sparingly available to plants. The availability of the applied P is controlled by adsorption and desorption characteristics of the soil. If large amounts of P are accumulated in soil, the capacity of the soil to sorb additional P can be saturated which may lead to an increased movement of P in the sub surface layers. A common way to estimate sorption capacity is to measure the amount of P sorbed by the soil at different P additions and to construct sorption isotherms. We studied the P sorption characteristics and inorganic P fractions (chloride-P, fluoride-P, NaOH-P and H2SO4-P) in an alkaline sandy loam soil (Typic Ustochrept) after 22 years of continuous cultivation of maize (Zea mays) -wheat (Triticum aestivum ) in an annual crop rotation. For adsorption studies, the surface (0-0.15 m) soils were equilibrated with 0.01M CaCl2 solution containing 0, 5, 10, 25, 50, 100 m gm P ml–1 for 24 hours at 25 ±10C and subsequently, the residual soil from adsorption studies, after completely decanting out the equilibrium solution, was suspended in 0.01M CaCl2 for P desorption. The P fractionation scheme of Peterson and Corey was employed to determine various inorganic pools of phosphorus The “Bulid up phase” of Olsen P was reported in the treatments when 17.5 or 35 kg P ha–1 was applied but the “Draw down phase” observed in the treatments where no P fertilizer was applied. Application of P fertilizer resulted in an increase in the readily plant available pools. When the P applied in the fertilizer was less than the P removed by the crops, P in these soil pools declined with time. The amount of increase or decrease also varied with amount of N and K applied. The concentration of chloride-P, fluoride-P, NaOH-P and H2SO4-P in the control plot significantly decreased over their initial contents after 22 years of cropping. A comparison of these inorganic P fractions after 11 and 22 years of cropping indicated that application of P fertilizer over the years significantly influenced the P fractions. A significant decrease in fluoride-P (24.4 %), NaOH-P (44.3 %) and H2SO4-P (24.9 %) while an increase in chloride-P (41.1 %) was observed in the plots receiving balanced fertilization (N120P17.5K33.2) after 22 years of cropping as compared with their content in soils 11 years after NPK application. The chloride P content in N120P17.5K33.2 treatment plot exceeded the control by 2.80 and 2.28 mg kg-1 after 11 and 22 years of fertilizer application, respectively. Among all these inorganic P fractions, saloid P served as a good index of P availability to both maize and wheat in sequence. Phosphate adsorption increased with increasing level of added P in all soils but the extent of adsorption was narrowed down in the plots receiving P fertilizer. Among all the seven pairs of treatments, the maximum adsorption occurred in N180P0K0 plots. The adsorption of P occurred at lower levels of added P in the soil samples of N180P35 K0 and N180P35K33.2 treatment plots. Further, increment of P resulted in an increase in soil solution (equilibrium) P concentration. Phosphorus adsorption data was found to fit best both to classical Langmuir (R2 values ranged from 0.979* to 0.997*) and Freundlich (R2 values ranged from 0.963* to 0.993*) isotherms and the data obtained on P desorption was plotted in accordance with a Langmuir type equation. The adsorption maxima, ‘Xm' and bonding energy ‘b' (calculated from classical Langmuir equation) varied from 345-588 mg g–1 and 0.055-0.087 ml g–1 in the seven differentially fertilized plots. The Freundlich constant ‘a' (extent of P adsorption rate) and ‘n' (rate of P adsorption rate) as calculated from the regression lines indicated that the maximum ‘a' value was recorded in N180P0K0 treated plots whereas, the minimum value was found in N180P17.5K0 treated plots. The ‘n' value varied from 1.10 to 1.65 ml g–1. Application of fertilizer ‘N' alone gave slight higher values than the control treatment. The supply parameter also exhibited similar trend in different treatment plots as observed in Langmuir adsorption constants. The desorption maxima (Dm) and rate constant computed from the desorption study gave highest Dm and lowest Kd values in N180P0K0 treatment plot. The Dm value decreased while Kd value increased with increasing rates of P fertilization. All indices obtained from sorption isotherm were significantly correlated with each other (r= 0.945* to 0.975*). Correlation analysis also showed that adsorption maxima (Xm) were significantly negative correlated with chloride-P (r = -0.831*), fluoride-P (r = -0.718*), NaOH-P (r = -0.896*) and P uptake by wheat (r = -0.747*). Similarly, Freundlich constant ‘a' was negatively correlated with chloride-P (r = -0.939*), NaOH-P (r = -0.841*) and P uptake by wheat (r = -0.884*). The results indicated that inclusion of inorganic P pools in the studies on P management could be helpful in better understanding of P dynamics in soils. The P sorption capacity is a key factor in P cycling and a knowledge about this parameter seems to be a prerequisite for sustainable cultivation.

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