Responses of Canola to the Application of Slow-Release Boron Fertilizers and Their Residual Effect

Boron (B) is essential for plant growth and the use of soluble B fertilizers is common, but leaching can result in poor fertilizer use efficiency in high-rainfall areas. We assessed the response of canola (Brassica napus L.) in two consecutive crops to the application of co-granulated slow-release B fertilizer under simulated leaching conditions. The slow-release B sources were boron phosphate (BPO4) synthesized at 500 or at 800°C for 1 h. For comparison, the more soluble B sources borax, ulexite, and colemanite were also included. All B sources were co-granulated with monoammonium phosphate (MAP) at a nominal concentration of 1.0% (w/w) B. The pure B sources, not co-granulated with MAP, were also included (first crop only). In the first crop, B concentrations in shoots were in the toxic range when B was supplied with the more soluble B sources, and dry matter yield was reduced for the treatments with borax and ulexite. The dry matter yields were lower and B concentrations in the shoot were higher in the treatments with pure B sources than in those with the co-granulated B sources. In the second crop, B concentrations in shoots were in the deficient range for the treatments with borax and ulexite and the plants showed B visual deficiency symptoms, although dry matter yield was not significantly affected. In contrast, the B supply for the treatments with BPO4 was adequate across both crops. The cumulative leaching loss of B for the treatments with co-granulated BPO4 was low compared with that for the more soluble B sources. These results indicate that BPO4 co-granulated products have potential as slow-release B fertilizers in high-rainfall areas.