Work out each group's share directly as a fraction of its chart before converting to weight. Group A+B+C on the Fertilizer-P chart covers $15+22+21=58$ percent, and since total Fertilizer-P is $2520/0.21=12000$ Kg, this group sells $0.58 \times 12000 = 6960$ Kg. Group C+D+E on the Fertilizer-Q chart covers $30+10+17=57$ percent, and since total Fertilizer-Q is $3150/0.21=15000$ Kg, this group sells $0.57 \times 15000 = 8550$ Kg. The gap between the two groups is $8550-6960=1590$ Kg. \[\boxed{1590}\]