MB2007/V04 - MBfP - Rhizobia and Clover Performance

Summary

Past research indicates that boron deficiency of sub clover results in reduced seed set, plant root development and herbage production. This PIRD's findings confirm that boron deficiency does impact on seed set, growth and nitrogen fixation of sub clover. Other studies have indicated that when boron deficiency of sub clover occurs, it impacts on root volume and this then reduces the plants ability to gather nutrients and moisture from the soil.

Outcomes here gave similar results as the sub clover in the non boron treatments wilted from moisture stress and failed to respond to potassium when applied at the low rate. It is highly likely, that boron deficiency plays a role in survival of effective rhizobia in the soil. Where boron deficiency has existed for some time, results indicate that boron combined with sub clover seed and inoculant are all needed together to ensure sub clover growth returns to normal and significant nitrogen fixation occurs.

Boron was found to move across the landscape via stock in urine along with other major nutrients. Such a loss in time can lead to severe boron deficiency developing in parts of paddocks and significantly reduce the pasture productivity of such areas. Because of this, the amount of boron needed to overcome a deficiency will vary depending on the distance from the stock camp. Water insoluble boron was found to be as effective as the water soluble types in overcoming a deficiency. Its use will significantly lower cost and can also be expected to have a better residual life. In deficient situations, broadcasting boron fertilizer (2.5kg/ha water soluble) at the same time as sowing inoculated sub clover was not found to impact on seedling germination and survival.

Excess boron is known to be toxic to plant growth and its impact at germination time was unknown. Plant tissue testing and soil testing can both be used to diagnose a deficiency, however with soil tests, the Ca:B ratio is possibly a better guide that the singular boron test. Once diagnosed, the targeted topdressing of boron to selective areas based on early spring sub clover growth and color was used as a cost effective method to return the pasture to full productivity, but this only applied when effective sub clover rhizobia were present. Overall, the results indicate that boron deficiency of pasture is hidden when the nitrogen fixing ability of legumes is not functional. When new inoculated sub clover seed plus boron were applied together, pasture production more than doubled.