We present a realistic scenario of tracking of scalar fields with varying equation of state. The astrophysical constraints on the evolution of scalar fields in the physical universe are discussed. The nucleosynthesis and the galaxy formation constraints have been used to put limits on $\Omega_{\phi}$ and estimate 𝜖 during cosmic evolution. Interpolation techniques have been applied to estimate $\epsilon\simeq 0.772$ at the present epoch. The epoch of transition from matter to quintessence dominated era and consequent onset of acceleration in cosmic expansion is calculated and taking the lower limit $\Omega^{0}_{n} = 0.2$ as estimated from $SN_{e}I_{a}$ data, it is shown that the supernova observations beyond redshift $z = 1$ would reveal deceleration in cosmic expansion.