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Direct and residual effects of phosphorus fertilization of rainfed sorghum (*Sorghum vulgare* L.) and its effect on P adsorption and availability in semi-arid tropical Alfisols

K.L. Sharma[1], K.V. Padmaja^{2}, M. Suryanarayan Reddy^{3}, P. Chandrasekhar Rao^{3}, J. Kusuma Grace^{4}, K. Srinivas^{5} and V. Ramesh^{6}

Central Research Institute for Dryland Agriculture

Saidabad, Santhoshnagar, Hyderabad-59.

Field experiments were conducted to study the direct and residual effect of P fertilization of rainfed sorghum (CSH-9) in an Alfisol at Hayathnagar Research farm (17°18˘ N latitude, 78°36˘ E longitude and an elevation of 515 m above mean sea level) of Central Research Institute for Dryland Agriculture, Hyderabad, India. The farm represents a semi-arid tropical region with hot summers and mild winters and a mean annual temperature of 25.7^{o}C. The mean maximum temperature during March to May varies from 35.6 ^{o}C to 38.6 ^{o}C. Mean minimum temperature during the winter months (December, January and February) ranges from 13.5 to 16.8 ^{o}C. Mean annual rainfall is 746 mm and accounts for approximately 42% of annual potential evapotranspiration (1754 mm). Nearly 70% of the total precipitation is received during the southwest monsoon season (June to September). Soils of the experimental field belong to Hayathnagar soil series (Typic Haplustalf) with a sandy surface layer and increasing clay content in the sub soil. The experiment was aimed to study the direct and residual effect of P fertilization of sorghum (CSH-9), P adsorption and its availability with 0, 10, 20, 40 and 80 kg P ha^{-1} as main treatments and residual fertility of 3^{rd} and 4^{th} years and residual and direct effects during 5^{th} year as sub treatments, in a split plot design, replicated four times. Up to three years (1990-1992), all the plots received P @ 0, 10, 20, 40 and 80 kg ha^{-1}. After 1992, every plot was divided into three sub plots viz., a, b and c. The plots ‘a' received P upto three years (1990 -1992), plots ‘b' received P upto 4 years (1990-1993) and plots ‘c' received P upto 5 years (1990-1994). Thus, the sorghum crop grown during fifth year in plots a and b had advantage of residual effect of P application for three years and four years respectively. Whereas, the crops grown in plots ‘c' had both the residual effect of four years and direct effect of immediate application.

Based on the data on direct and residual response, the P application every year @ 20 kg ha^{-1} was found to be the most optimum dose. With this level of P application, the sorghum grain yields were 11.35, 11.70 and 13.49 q ha^{-1} for a, b and c plots respectively. On an average, there was 20.7 % increase in the sorghum grain yield in the plots receiving 20 kg P ha^{-1} compared to control plots.

When extracted with Olsen's reagent, the soils receiving P @ 80 kg ha^{-1} continuously for 3 years, 4 years and 5 years maintained available P level as high as 81.96, 92.03, and 80.09 kg ha^{-1} with an average value of 84.69 kg ha^{-1}. However, the control sub plots a, b, and c, which did not receive P at all showed available P as low as 17. 02,17.19 and 5.56 P kg ha^{-1} under continuous cropping after 5 years, which indicated the depletion of P when no P is applied.

As an internal response, the P concentration in 3^{rd} leaf at 40 DAS and 50% flowering increased with increase in applied P levels. While studying the chemical P fractions in direct and residual treatments at 0 and 80 kg P ha^{-1}, Al-P, Ca-P, and Fe-P were found to be the dominant inorganic P fractions in the experimental soils. There was a considerable build up in P as Fe-P, Al-P and Ca-P fractions at 80 kg P ha^{-1} compared to control.

Adsorption –desorption pattern of direct and residual P treatments at 0 and 80 kg P ha^{-1} at equilibrium solution concentration was also studied. Of the three-adsorption isotherms used, langmuir equation described the adsorption reasonably and accurately to all solution P concentrations and was found more accurate than the Freundlich and Tempkins equations. The adsorption data fitted to the langmuir isotherm had the adsorption maxima of 303.02 mg kg^{-1} in case of residual P upto three years and it was 333.33 mg kg^{-1} in case of plots which had direct P application upto five years. However, the bonding energy increased from 2.88 at 0 kg ha^{-1} to 5.45 L mol^{-1} in case of 0 and 80 kg P ha^{-1 }respectively in direct treatment and from 1.76 to 2.13 L mol^{-1} in residual treatment respectively. The full paper deals with the direct and residual effects of P application on sorghum grain yields, accumulation of P in different chemical pools, variation in P adsorption-desorption behaviour and its availability.

*Key words:* Residual P, direct P, P fractions, adsorption-desorption isotherms, Alfisols, bonding energy

[1] National Fellow and Corresponding author, Central Research Institute for Dryland Agriculture, Saidabad, Santhoshnagar, Hyderabad-59. Email: klsharma@crida.ernet.in

2. Research Associate, Presently at ICRISAT, Patancheru, Hyderabad.

3. Professors, Acharya N.G. Ranga Agricultural University, Hyderabad.

4. Research Associate, Central Research Institute for Dryland Agriculture, Santhoshnagar, Hyderabad-59. Email: kusuma@crida.ernet.in

5. Senior Scientist, Central Research Institute for Dryland Agriculture, Santhoshnagar, Hyderabad-59. Email: ks@crida.ernet.in

6. Scientist, Central Research Institute for Dryland Agriculture, Santhoshnagar, Hyderabad-59.

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