Responses of canola to the application of slow-release boron fertilizers and their residual effect

Abstract

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 highrainfall 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.