Long term filed experiments offer good opportunities to study the influence of long-term management practices on soil nutrient status and crop productivity. The long term fertilizer experiments with finger millet – hybrid maize system at Gandhi Krishi Vignana Kendra, Bangalore, India were started during 1986 – 87 with 11 treatments, which included graded levels of NPK, with and without liming and FYM application. The pH, OC, and available NPK status of initial soil samples were 6.17, 0.46 per cent, 256.7 kg/ha, 34.3 kg/ha and 123.1 kg/ha, respectively. Surface (0 – 15 cm) soil samples were collected from 8 out of 11 treatments (i.e., 50% NPK, 100% NPK, 150% NPK, 100% NPK + lime, 100% NP, 100% N, 100% NPK + FYM and Control) after harvest of maize during 2003 – 04, and were analysed for different forms of phosphorus in order to know the effect of treatments on their contents. In addition, soil P fixation capacity, Bray's 1 available P, total P, pH and OC contents of soil were also estimated by following standard procedures.

The results indicated that the pH of soil was decreased considerably due to the continuous addition of fertilizers. The total as well as available P contents of soil were increased with increase in the levels of added P as well as due to the application of FYM along with fertilizers. The highest (475.5 mg kg^-1) and the lowest (300.0 mg kg^-1) amounts of total P were reported by 100% NP and control, and the highest (84.62 mg kg^-1) and lowest (3.54 mg kg^-1) amounts of available P were recorded by NPK + FYM and the 100% N treatments, respectively. Among the four fractions of P, that of Fe-P was found to be the highest in soil irrespective of the treatments. All the four fractions of P  (saloid –P, Fe-P, Al-P and Ca-P) were very high in the soil under only 100% NP, 150% NPK and 100% NPK + FYM treatments. On the other hand the contents of all the four fractions of P were very low in the control and only 100% N treatments. The values for different P fractions ranged from 2.75 (Control) to 13.44 mg kg^-1 (100% NPK + FYM@ 15t/ha) for saloid P, 83.2 mg kg^-1 (100% N) to 134.4 mg kg^-1 (100% NPK + FYM) for Al-P, 184.5 mg kg^-1 (100% N) to 300.2 mg kg^-1 (100% NP) for Fe-P and 25.02 mg kg^-1 (100% N) to 48.74 mg kg^-1 (100% NPK + FYM) for Ca-P.

Phosphorus fixation capacity of soil at 500 ppm added P was found to be the highest under 100% N (189.6 mg kg^-1) and the lowest under 100% NPK + FYM (146.1 mg kg^-1), respectively. The values for P adsorption parameter (K) varied from 15.9 mg kg^-1 (100% NPK + FYM) to 81.2 mg kg^-1 (Control).

The results suggested that there was considerable build up in available P as well as different forms of P due to continuous addition of P fertilizers. The P fixation capacity of soil had a direct relationship with amount of added P and available P status of soil.

(Key words: Forms of P, Long term experiment, P – fixation)