Foodgrains, specially rice (uncooked), have the capacity to absorb and transmute negative energies. On the eleventh day of the waxing and waning moon cycle, this absorption is at a maximum. The day is called 'ekadasi' (eleventh day). Hence the rice is kept near the place of worship. Chanting of 'mantras' and 'japas' also create pure environment because of the effect of sound vibrations. But silent 'japa' of mantras is equally effective in purifying the environment as it produces pure thought vibrations on the energy atmosphere ground.
In olden times men gave a lot of importance to Tulsi. That is because its leaves are very effective in destroying the negative energy and purifying the atmosphere. That is why many keep Tulsi plant in terrace/balcony.