Wireless Underground Sensor Networks (WUSNs) present a variety of new research challenges. For WUSNs, the goal is to establish an efficient wireless communication in the underground medium. A magnetic induction (MI) based transmission technique was proposed to overcome the very harsh conditions of the soil environment. In this paper, we investigate the potential of the MI-WUSNs if, in contrast to some previous proposals, no relays are used. Our main focus is on the throughput of the bottleneck link of the network, which corresponds to the overall network capacity. In order to reduce the number of relevant interferers and maximize the network throughput, we exploit the polarization of the used magnetic antennas (coils) by optimizing their orientation. Additional optimization of the system parameters improves the channel capacity of the bottleneck link. In addition, we consider a special case of the network deployment in mines and tunnels and propose a frequency switching scheme for better propagation conditions.