Welcome to inaugural installment of Know Your Cover Crop! Cover crops are essentially non-cash crops planted to nourish and insulate the soil. During their growth cover crops protect the soil from being swept away by wind or rain or baked to a crisp by UV rays. Their roots retain moisture and aerate the soil, allowing habitat for all kinds of critters, subterranean and otherwise. When cover is mowed down and worked into the ground the plant material provides a boost of nutrition for future crops and forms soil structure. So in a nutshell, cover crops are incredible soil builders and might just be the answer to all your problems, including hair loss and missing kittens. Let’s take a closer look.
First up, the Bell Bean, Vicia faba. Bell beans are a species of vetch, though their sturdy stalks and upright growth are very different from the wispy, creeping tendrils of other vetches. Bell beans are also a little more finicky than other vetches: less frost hardy, less drought tolerant, less content with being waterlogged. However, bell beans do not produce hard seed as rapidly as other vetches, allowing them to be incorporated into the soil without fear of the seeds sprouting and taking over.
Bell beans are a leguminous cover crop, producing nitrogen-fixing nodules during their growth. These nodules take nitrogen from the atmosphere and feed that nitrogen back into the soil in plant available form. So the bell bean is a living fertilizer factory, boosting soil fertility even before the cover crop is turned into the soil as organic matter. Bell bean roots form these nodules by associating with rhiobium bacteria found in the soil. You can actually buy arhizobium bacteria inoculant to apply to the bell bean seeds before planting to ensure the best nodulation. In the picture above you can see the little nodules on the bell bean’s roots. If you were to cut open one of those nodule’s you’d see a pink or redish material, an indicator that nitrogen is being fixed. The color is caused by leghemoglobin (similar to hemoglobin in blood) that controls oxygen flow to the bacteria. Science!
Bell beans are prone to pea-bean aphid attack. This doesn’t particularly effect its usefulness as a covercrop, but can hinder the plant’s ability to produce savable seed. The aphids provide food for other critters, such as this happy ladybug. Actually bell beans are excellent habitat for all kinds of insects. Extrafloral nectaries (sources of sweet, sticky nectar found on the non-flower plant parts) on the bell beans lure in ants and other insects. On the bell bean nectaries are found on the lower part of the stipule, which is a little gland at the base of the leafstalk.
Bell beans are used on the farm as a winter cover, typically in combination with oats and other vetches. My understanding is that bell beans are the same thing as fava beans, except that the variety used for cover crop develops smaller seeds than the culinary fava. Seeds planted in the late Fall in Santa Cruz and then incorporated in the soil in April and May. When we first arrived there were bell beans are far as the eye could see, but now they’ve all been turned into the soil, feeding the beautiful spring crops just starting to mature in the gardens.