Boron (B) deficiency is directly affected by boron mobility or immobility in plants. In those species in which boron is immobile in the phloem, foliar-applied boron will not be translocated from the site of application. This means there is not enough boron to adequately supply the plant cells that are required for tissue formation. In those species where boron is immobile via phloem, boron must be applied via soil.