Should fertiliser be required, it must be placed deep enough in the seedbed to allow its utilisation by the crop. Surface applications are ineffective. For winter beans the seed and P and K should be applied together.
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.
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 have a requirement for manganese and boron, while magnesium deficiency occurs very occasionally. Most growers use a prophylactic programme containing a mixture of the micronutrients – this additional input is relatively low cost and can be applied when going through the crop with fungicides.
Manganese – beans are far less sensitive to manganese deficiency than peas, but it can be responsible for marsh spot disorder, which reduces bean quality. Most likely to occur on highly organic or alkaline soils, or when plants are under stress, it can be corrected by a foliar application as soon as the foliar symptoms of chlorosis are observed.
Boron – boron deficiency is becoming more common in both winter and spring beans. Symptoms usually include a cupping of the upper leaves and a stunting of the plants, but in severe cases the growing point can be killed. Corrective foliar treatments only work where the problem has been diagnosed in time.
Magnesium – beans rarely develop magnesium deficiency, but it can occur on acid soils. Foliar sprays are worthwhile where the problem is suspected.
Beans have moderately high demands for phosphate and potash. Any responses to phosphate and potash depend on residual levels of these nutrients in the soil, although beans are more responsive to potash than they are to phosphate. Where soil indices are 2 or above, there is little or no response to applications (see table below).
The following application rates are recommended: