Q Is it worth leaving the roots of legumes, such as beans, in the soil to give nitrogen to the following crop?
A Gardening folklore suggests that legumes, such as peas, beans and some green manures, add nitrogen to the soil that will benefit the crops you plant in the same space the following year. Nodules grow inside the legumes’ roots with the help of bacteria that naturally occur in soil. The plant’s root hairs take these up and form a pea-sized swelling. This is pinky-red at first, before turning white.
Once established in the nodules, the bacteria works with the plant, taking some of the carbohydrates produced through photosynthesis and in return releasing nitrogen in a form that the plant can easily use to fuel its own growth.
In theory, the root nodules should retain some nitrogen and release it into the soil as they rot down.
Caption: Is it worth leaving the roots of beans in the ground to fix nitrogen?
Which? Gardening magazine tested this theory by planting eight legumes: runner beans; climbing French beans; dwarf French beans; broad beans; mange tout; garden peas; sugarsnap peas; and winter tares (a green manure in the legume family). We allotted plots measuring 1 x 2m to each legume, enough room to grow two rows of peas and beans. The winter tares were broadcast-sown across their plot. We also cleared two more plots of the same size and left them fallow for a year.
The peas and beans were harvested, and in autumn, we cut down the tops of the plants but left the roots in the soil.
The following spring, we rotovated all the plots and applied Growmore as a base dressing to one of the plots that we had left fallow the previous year.
In total, we had eight plot that potentially had nitrogen produced by the peas, beans and winter tares, one fertilised plot and one control plot, which was relying on just the nitrogen that was already in the soil.
We left the soil for three weeks to allow the nodules to break down and release their nitrogen. We then took soil samples to test for the amount of ‘available’ nitrogen. Plants take up nitrogen only as nitrate and ammonium, so this test only measures these two compounds.
Finally, we sowed a crop of spinach, which needs large amounts of nitrogen otherwise the leaves turn yellow and you experience low yields. In each plot, we sowed two rows of spinach in exactly the same space as where the peas and beans had previously grown. Once the spinach was ready, it was harvested and weighed. We also measured the size of the leaves and assessed their quality.
We found that there wasn’t any advantage in growing peas, beans or winter tares prior to a crop of leafy greens.
The Growmore plot produced the highest yield, at just under 400g of spinach, around twice as much of any of the other plots. This plot also saw crops with the largest leaves.
There was very little difference in yield between the bean plots and the control plot, where no fertiliser was applied and no legumes were grown. All these plots grew roughly half as much spinach as the Growmore.
The plots where the peas and the green manure had been only produced around 50g per plot – far less than the control plot.