A comprehensive study was carried out to elucidate the microbial and biochemical properties in relation to phosphorus (P) availability in the Potohar soils. The study as a whole comprised of four experiments. In the first experiment, fifteen prominent soils (0-15 cm) of the Potohar were collected from their relevant agricultural fields in September, 2008 and analyzed for their physico-chemical properties. The soils were also analyzed for different P fractions and microbial parameters like soil respiration, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) microbial biomass phosphorus (MBP) and the activities of enzymes like dehydrogenase (DHA) and alkaline phosphatase (APA). Results revealed that the average size of different P pools in the soils in terms of percentage of total P was in order: dil. HCl-P (63.8%) > con. HCl-Pi (14.8%) > residual P (9.4%) > con. HCl-Po (3.8%) > NaHCO3-Pi (2.4%) > NaOH-Pi (2.0%) > NaHCO3- Po (1.4%) > resin P (1.3%) > NaOH-Po (1.2%). Organic P fractions as a whole constituted a small part (6.4%) of the total P in soils and had significant (P < 0.05) positive correlation with the contents of clay and organic carbon of the soils. The inorganic P fractions comprised of 93.6% of the total P pool in soils and had highly significant (P < 0.01) positive relationship with soil pH and CaCO3 contents. Among the inorganic P pools, the NaOH-Pi had negative correlation with soil pH. The labile P pool (resin-P, NaHCO3-Po & NaHCO3-Pi) had strong positive correlation with Olsen P and the MBP in the soils. A strong positive relationship also existed between organic P pools and the microbial parameters (MBC, MBN, MBP, DHA, APA and CO2 evolution) in the soils. Second experiment consisted of an incubation study conducted in the laboratory to evaluate the dynamics of microbial (MBC, MBN, MBP & CO2 evolution) and biochemical parameters (dehydrogenase & alkaline phosphatase activities), and P fractions in response to various organic amendments (farmyard manure, poultry litter & biogenic waste compost) in two P deficient soils. Moreover, the relationship between P fractions and the microbial/ biochemical parameters was also examined in the study. The soils used in the study were Balkassar (Haplargids) and Qutbal (Ustorthents) which varied from each other for their organic C, CaCO3 and clay contents. Each soil (600 g on oven dry basis) was taken in 2 litre capacity incubation jars and incubated at 25 oC for 72 days after moisture adjustment at 50% water holding capacity (WHC). The organic amendments (farmyard manure, poultry litter & biogenic waste compost) were added separately at the rate of 1% of soil on dry weight basis with four replications. The soil samples were taken at 0, 14, 28, 56 and 72 days after incubation in order to determine MBC, MBN, MBP, DHA, APA, and Olsen P. Phosphorus fractions were measured in the soil samples collected at day 0 and 72 after incubation. Soil respiration was measured as CO2 evolution at 1, 2, 4, 7, 10, 14 days after incubation and from then on weekly basis. Results demonstrated a significant increase in all microbial and biochemical parameters in response to organic amendments with greater increases in poultry litter (PL) amended soils followed by farmyard manure (FYM) and biogenic waste compost (BWC) treated soils. In general, the increases were larger in Qutbal soil compared to the Balkassar soil. However, the microbial biomass P contents were higher in Balkassar soil. All these parameters, except DHA and the CO2 evolution, were highest at day 14 of incubation and significantly declined afterwards till the end of study. With organic amendments addition all the P pools increased significantly except con. HCl-Pi in which increase was non-significant. In general, more increase was observed in labile and moderately labile (NaOH-Pi & NaOH-Po) P pools compared to the stable (con. HCl-Pi, con. HCl-Po & residual-P) P pools in soils. Over the 72 days incubation time, the resin P, NaHCO3-Pi and dil. HCl-P pools declined significantly while the other P pools increased. Olsen P and all the labile P pools were strongly positively related to the MBP which emphasized the role of MBP in P availability in the soils. In the 3rd experiment, a greenhouse research was conducted to elucidate the different organic amendments’ effect on soil microbial parameters, phosphorus availability and its uptake by wheat plants in the two (Balkassar and Qutbal) soils. Three organic amendments used in study-2 were applied @ 1% (on oven dry soil weight basis) with four replications. Wheat cultivar GA-2002 was sown after two weeks of organic amendments addition and soil samples were taken at day 0, 14, 28, 42, 56 and 72 after the sowing (DAS). The samples were analysed for MBC, MBN, MBP, DHA, APA and Olsen P. Plants were harvested at 72 DAS and data regarding plant height, number of tillers per plant and shoot and root weights were recorded. Plant samples were also analysed for the macro (nitrogen, phosphorus, potassium, calcium and magnesium) and micro (zinc, manganese, iron and copper) nutrient contents. The organic amendments caused significant increase in all microbial parameters and Olsen P in both the soils, more with PL followed by FYM and BWC additions. The highest contents of all microbial and biochemical parameters other than DHA were observed at 0 days after sowing which declined later up to 28 DAS and gradually increased again till the end of experiment. The DHA showed a consistent decline from sowing to the end of study. The P availability was markedly higher during the early 28 days of experiment which gradually but consistently reduced during the remaining period of study. Olsen P had highly significant positive correlation with MBC, MBN and MBP which evidenced the importance of microbial biomass in soils with respect to P availability. In response to organic amendments wheat plants growth increased significantly over the unamended control with larger increases in PL amended soils followed by FYM and BWC treated soils. Significant increase in macro and micronutrients uptake was observed with organic amendments, in particular with PL treatment. The fourth experiment was conducted at the University of Adelaide, Australia in order to evaluate the effects of various inorganic and organic P sources on P pools and microorganisms in a P deficient soil collected from Monarto, South Australia. The soil was amended with two composts (low P or high P @ 2% w/w) while the two crop residues (low P or high P), and inorganic P (KH2PO4) were added at equivalent quantities of the P added by the composts. The amended soil was incubated for 56 days at 25 oC after adjusting the moisture content at 60% of soil WHC. The CO2 evolved was measured after 1, 2, 4, 7, 10, and 14 days and thereafter weekly till the end of incubation. Soil samples were taken after 0, 14, 28 and 56 days of amendments addition and analysed for pH, MBC, MBP, resin P and phospholipid fatty acids (PLFA). Different P pools were measured in the soil samples taken at day 0 and day 56. Results showed that MBC and the abundance of bacteria and fungi (assessed by PLFA analysis) increased significantly in soils treated with organic amendments, greater in residues amended soils. Microbial biomass P enlarged in response to all amendments but more with the high P residue (413%) addition. All extractable inorganic P pools including NaHCO3-Pi, NaOH-Pi and dil. HCl-P, increased significantly immediately following the application of high P amendments which demonstrated the rapid conversion of the added P into different soil P pools. However, NaHCO3-Pi reduced over time, perhaps due to sorption/ fixation with soil components and/ or immobilization. The NaOH-Pi and dil. HCl-P pools decreased in soils treated with organic amendments, except the high P compost, whereas increased in the soils amended with inorganic P. The NaHCO3- Po pool increased immediately after amendment addition particularly in high P amended soils. On the other hand, NaOH-Po increased only in soils treated with high P compost and high P crop residues. As a result of this research work the major conclusions drawn are: 1) total P in the soils of Potohar varied between 178- 653 μg g-1 soil which was dominated by dil. HCl-P pool with its 63.8% contribution, while the labile and moderately labile P constituted just 5.1% and 3.1% of the total P, respectively; 2) NaOH-Pi was significantly negatively associated with soil pH where as dil. HCl-P had positive relationship with soil pH and CaCO3 contents; 3) different organic fractions collectively constituted about 6.4% of the total P and were usually controlled by TOC and clay contents of the soils; 4) microbial biomass P performed a significant role in P availability to plants because of its strong relationship with Olsen P and the labile P pool in the soils; 5) microbial biomass and activity parameters in the soils under investigation increased in response to organic amendments and the magnitude of increase was greater with poultry litter additions; 6) generally, P pools increased with organic amendments with more changes in labile or moderately labile compared to the stable P pools; 7) over the time, resin-P, NaHCO3-Pi and dil. HCl-P pools decreased while all the other P pools increased; 8) the effects of the P amendments on microbial activity and community composition were predominantly a function of the amount of C added, particularly water-soluble C; 9) the effects of P amendments on P pools in soil were mainly related to the amount of P added through the source, but the amount of C, particularly the water-soluble C added also played a role and 10) inorganic P addition caused enhancement in inorganic P pools while the organic amendments induced strong increases in organic P pools. Transformation of inorganic to organic P pools highlighted the significance of organic P sources with respect to P availability as organic P may be less prone to fixation and become available to plants after mineralization. Thus, on the basis of this study it is concluded that organic P sources, particularly those with high P concentration and readily decomposable organic C can stimulate microbial activity and biomass as well as the build-up of organic P forms in soils which may later become plant available via mineralization." xml:lang="en_US