A field experiment entitled “Effect of organics on phosphorus fertilizer optimization to soybean-onion cropping sequence in a high P Alfisol” was conducted during kharif (soybean), 2012 and rabi (onion) 2012-13 in a sandy clay loam, at college farm College of Agriculture, Rajendranagar, Hyderabad to study the response to P levels (0, 30 and 60 kg P2O5 ha-1) either alone or in combination with PSB @ 5 kg ha-1, biochar @ 5 t ha-1, humic acid @ 20 kg ha-1 and citric acid @ 10 mM concentration to study the direct, residual and cumulative effects of the treatments imposed on phosphatase activity, inorganic P fractions, yield, drymatter production and nutrient removals by crops. Incubation study was conducted under laboratory condition to understand the fixation and release of native and applied P from soil at different contact periods under the influence of different combinations of inorganic P and organics. During kharif (soybean), the experiment was laid out in split plot design consisting 3 main levels of inorganic P (0, 30 and 60 kg P2O5 ha-1) and 5 sub levels of organics (no organics, PSB, biochar, humic acid and citric acid). In rabi (onion), the experiment was laid out in split-split plot design, with 2 sub – sub levels (no application, application of best combination from kharif to study the residual and cumulative effects (mode of effects respectively). For this all the plots were divided into two equal halves. For one half, neither inorganic P nor organics were applied to know the residual effect on onion grown during rabi after harvest of soybean crop. In another half, the best combination from kharif was applied to study the cumulative effects. For all the treatments N and K were applied uniformly at the rate of 30 kg N ha-1 and 40 kg K2O ha-1 for soybean, 150 kg N ha-1 and 60 kg K2O ha-1 for onion in the form of urea and MOP respectively. Inorganic P was applied in the form of DAP and N was adjusted with urea. The experimental soil was sandy clay loam in texture, slightly alkaline (pH 7.64) in reaction, non saline (0.195 dS m-1) in nature and medium in organic carbon (0.57 %). The soil was low in available nitrogen (177 kg N ha-1), high in available phosphorus (29.9 kg P EFFECT OF ORGANICS ON PHOSPHORUS FERTILIZER OPTIMIZATION TO SOYBEAN-ONION CROPPING SEQUENCE IN A HIGH P ALFISOL PROFESSOR JAYASHANKAR TELANGANA STATE AGRICULTURAL UNIVERSITY ha-1) and potassium (449 kg K ha-1). Mean available P with the organics was higher i.e., 26.5 mg kg-1 when biochar was integrated with the inorganic P over incubation period. Among different organics, the mean available P followed the order biochar > humic acid > PSB > citric acid > No organics across the inorganic P treatment. Among all the combinations of organics and inorganic treatments at different days of incubation higher available P of 31.3 mg kg-1 was obtained with the integration of inorganic P at 60 P2O5 ha-1 with biochar at 45 days of incubation. In both the crops, alkaline phosphatase activity was superior to acid phosphatase activity and the activity showed a decrease with the increased application of inorganic P fertilizer. Among all the organics, biochar recorded significantly higher phosphatase activity followed by humic acid. In rabi onion crop, cumulative effect showed significant increase in the mean alkaline phosphatase activity at flowering to 143 μg p-nitrophenol g-1 soil h-1, which was higher by 9.2 per cent over the residual effect that showed 131 μg pnitrophenol g-1 soil h-1. In soybean, graded levels of P application at 0, 30 and 60 kg P2O5 ha-1 across the organics showed a significant increase in mean available P content in the order of 33.4, 37.3 and 41.1 kg ha-1 respectively with the corresponding build up of 11.8, 24.7 and 37.5 per cent. Biochar recorded significantly higher mean phosphorus content of 42.2 kg ha-1 against control and organics across the inorganic P treatments at flowering. In onion crop at 45 DAT, cumulative effect showed significant increase in the mean available P to 35 kg ha-1, which was higher by 14.7 per cent over the residual effect that showed 30.5 kg ha-1. Among the inorganic P fractions estimated, saloid-P is the smallest pool and Ca-P is the largest pool which accounted for 1 and 75 per cent respectively at flowering stage of soybean. In both the crops, inorganic P fractions higher with biochar followed by humic acid when they are integrated with inorganic P. In rhizosphere soils, the amount of P availability followed the order: Ca-P > Occluded-P > Fe-P > Al-P > saloid-P. In onion crop, cumulative effect showed significantly higher inorganic P fractions than residual effect. In soybean, among all the organics, biochar across the inorganic P put forth significantly higher mean drymatter of 2184 kg ha-1 over the control (1742 kg P2O5 ha-1). Humic acid alone showed significantly higher drymatter yield of 2431 kg ha-1. Inorganic P at 30 and 60 kg P2O5 ha-1 across the organics significantly increased the mean seed yield of soybean to 1899 and 1683 kg ha-1 over 1389 kg ha-1 in the control which accounted for 36.7 and 21.2 per cent higher yield respectively. The mean seed yield of the soybean with biochar was 2077 kg ha-1 which was significantly higher against the control seed yield of 1329 kg ha-1. In onion, among organics, biochar application led to a statistically significant positive effect on both biomass and yield. Biochar resulted in a significant increase in mean onion yield to 22.1 t ha-1 against 15.8 t ha-1 when organics were not supplemented, the yield response being 39.9 per cent across inorganic P and mode of effect. Cumulative effect was found to show significant influence resulting in a mean yield of 21 t ha-1 which was higher by 22.1 per cent as against 17.2 t ha-1 due to the residual effect. In soybean-onion cropping sequence, N, P and K removals were superior due to the cumulative effect when biochar was combined with inorganic P @ 30 kg P2O5 ha-1. Similar response was observed in Fe, Mn, Zn and Cu. System yield of soybean - onion cropping sequence was higher with biochar and humic acid when applied with 30 kg P2O5 ha-1 with corresponding soybean equivalent yields of 7063 and 6740 kg ha-1. For soybean-onion cropping sequence, residual effect of 30 kg P2O5 ha-1 + humic acid was economically better with higher B: C ratio of 2.0 followed by 30 kg P2O5 ha-1 + biochar in both residual and cumulative effects which showed 1.9. By this, one can emphasize that biochar and humic acid applied for the preceding crop of the sequence will benefit the succeeding crop by the way of sustained residual effect. The present investigation suggests that there is a need to apply 30 kg P2O5 ha-1 along with biochar or 30 kg P2O5 ha-1 along with humic acid to both the crops in soybeanonion cropping sequence grown on high P Alfisol not only to obtain higher crop yields and net returns but also maitain soil fertility. On a high P soil, by integrating with organics one can reduce the inorganic P inputs upto 50 % of recommended dosage.