1. Boron deficiency has been found to develop under field conditions even when applications of borax of recommended magnitude have been made. 2. Differential degrees of boron deficiency have been found on susceptible crops, associated with and apparently dependent on the nitrogen, phosphorus, and potash fertility levels employed. 3. The results of a factorial experiment conducted under greenhouse conditions, employing rutabaga as the test plant and involving two levels each of nitrogen, phosphorus, potash, and boron, are presented. 4. Nitrogen, phosphorus, and boron were found to have positive main effects upon growth. Potash had no main effect but showed a positive interaction with nitrogen. Boron also showed a positive interaction with nitrogen. Phosphorus, however, showed a negative interaction with boron. Nitrogen X phosphorus X boron was also a negative interaction in relation to growth. 5. Nitrogen, phosphorus, and boron all promoted root growth to a greater extent than foliage growth. Potash and boron each amplified the nitrogen effect. 6. Nitrogen, phosphorus, and boron all showed significant controlling main effects on the incidence of boron-deficiency symptoms in rutabaga, the order of efficacy being B> N> P. Potash showed a significant deleterious effect in this regard. 7. Nitrogen interacted with phosphorus to enhance the deficiency and with potash and boron to reduce it. Phosphorus interacted with potash and boron to reduce deficiency. Nitrogen, phosphorus, and potash mutually enhanced deficiency symptoms, but phosphorus, potash, and boron mutually reduced them.