Field beans like all legumes are capable of fixing their own nitrogen from the atmosphere. They achieve this by playing host to rhizobium bacteria, which reside in root nodules attached to the roots of the bean crop. There is generally little demand for nitrogen fertiliser as applications simply suppress nodulation and reduce the contribution the rhizobium bacteria make to nitrogen nutrition of the plant. However, in an experiment where 560 kg nitrogen ha-1 was applied to two varieties of spring beans at Aberystwyth University, yield increases of over 1t ha-1 were produced (Figure 1). Similarly, whilst applications of up to 200 kg N ha-1 as a soil treatment suppressed yield whilst 80 kg N ha-1 as foliar applications during seed development increased yield7. This demonstrates that beans may have the ability to respond to more nitrogen than they can supply themselves. This leads us to try to improve nodule health and productivity to endeavour to maximise nitrogen fixation.
Root nodule growth is optimised in warm soils (optimum 30oC), pH 5.5 reduced in compacted, waterlogged or droughted soil and activity may be reduced due to shading in the late season e.g. during pod fill or root pests such as pea and bean weevil or nematodes. However, one of the main mechanisms of enhancing nodule development is to pay attention to their nutrition. Various micronutrients are essential to optimise nodule activity and nitrogen fixation. Molybdenum is an essential constituent of nitrogenase, the enzyme present in rhizobium bacteria which converts nitrogen into ammonia. Cobalt is a constituent of leghaemaglobin, the compound that supplies the oxygen to add to the nitrogen forming nitrate molecules and has been shown to increase yield in some legumes. Beans also have a high demand for boron. The element does not appear to have a direct role in nitrogen fixation although it is important for pollination and production of vascular connections. It may have a role in the transport of nitrates from the root nodules to the plant.
Various experiments (Figure 2) have shown the response of beans to these nutrients, applied alone or in mixture. Research conducted in Lincolnshire has demonstrated the ratio of boron, cobalt and molybdenum required for nodule development and this has been used to determine the content of various micronutrient products for use on legumes e.g. ‘Headland Pulse mix’. ‘Pulse mix’ or a mixture of the three micronutrients should be applied when the first flower bud is visible or at the onset of spring growth. Nodule activity is high during this period but declines once pod set occurs as the pods become the main sink for assimilates.
Beans have moderately high demands for phosphate and potash. They respond to applications of potash up to 200 ppm potassium in the soil. Phosphate demands are less well understood but are thought to play a role in nitrogen fixation. Basal nutrient recommendations as set out in RB209 are shown here.
Further research is being conducted to determine the worthiness of standard fertiliser recommendations. When the results of that research become available the website will be updated.Table 1 Fertliser recommendations for peas and beans (MAFF, RB209)
‘’The amounts of potash and phosphate are appropriate to bean yields of 3.5 t ha-1. Where yields are likely to be greater or smaller, phosphate and potash applications should be adjusted accordingly.
Different potash recommendations are given for the lower half (2-) and upper half (2+) of K index 2. If there is no long term policy of building up soil P and Kat Index 0 or 1, the recommendations may be reduced by 50 kg ha-1 at Index 0 and 25 kg ha-1 at Index 1.