Pole beans are prolific summer vegetables that produce long, thin, nutrition-rich green beans all summer, or at least as long as ripe bean pods continue to be picked. Many gardeners prefer pole beans to similar bush bean varieties because of their long season and greater productivity, though pole beans do require a sturdy trellis or other support for their vining stems to climb. In most soils, pole beans require little extra nutrition--especially nitrogen, which legumes such as beans can produce for themselves. Correct soil pH, preferably between 5.5 and 6.0, encourages nutrition uptake.
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Pole beans and other legumes require less nitrogen than other vegetables because they can “fix” nitrogen on their own. Nitrogen fixation occurs in the soil due to a symbiotic relationship between root nodules of legumes and associated microorganisms. These microorganisms get food and energy from the plant’s roots while “fixing” atmospheric nitrogen--converting it into a form that the plant can use for its own growth and development. In reasonably healthy soils about half the nitrogen legumes need is provided through fixation.
Most people think fertilizer is a good thing for plants, but this is not always the case. Applying too much fertilizer can actually damage plants or limit their productivity. Too much fertilizer on pole beans--and excess nitrogen especially--can stimulate too much foliage or vine growth and inhibit the development of bean pods. Adding fertilizer when it’s not needed, or using strong fertilizers or using too much, can damage plants and diminish yields.
In most soils a fairly light low-nitrogen fertilizer, such as 5-10-10, should suffice for green beans. Prior to planting, work fertilizer into the ground to a depth of about 6 inches, applying about 1 cup per 50 feet of row. Commercial growers may use 10-20-20 fertilizer, applying less, then adding a small amount of nitrogen as side dressing when vines begin climbing and again at flowering, the point being to apply nitrogen only as needed--and when needed--to avoid nitrate runoff.
Many organic and almost-organic gardeners keep the emphasis on steady soil building over time, adding composted manure, shredded leaves, straw and garden compost on a regular basis and never adding extra fertilizers for crops such as pole beans. In poor soils, well-rotted horse or steer manure worked thoroughly into the soil to a depth of about 12 inches will begin to improve the moisture-retaining qualities of the soil while also providing slow-release plant nutrition.
In sandy soils and with heavy rains that leach nutrients during the growing season, a side dressing of fertilizer, worked shallowly into the soil on one side of the row, may be advisable after flowering and pod set have started. Another special-needs situation involves growing pole beans in containers filled with potting mix. Apply low or moderate levels of liquid fertilizer at flowering and pod set, to make sure basic nutrition is adequate.