Plants that fertilize the soil as they grow

Lupins, one of many stunning plants that enrich the soil naturally, make a grand alternative to springing for a bag of fertilizer.

Lupins, one of many stunning plants that enrich the soil naturally, make a grand alternative to springing for a bag of fertilizer.

Imagine if gardeners could improve their soil by planting a perennial that produces spectacular cutting flowers as well as feeding bees and other pollinators. Or a tree native to our region that blooms profusely in bright pink when other trees are just starting to wake up from winter. Or tasty vegetables that will be among the first food crops to ripen in spring.

All this can be done and the heroes of the first paragraph are: Lupins, the redbud tree and peas. All these plants, with very different roles in the garden, share an extraordinary talent. They capture nitrogen, one of the three main ingredients in fertilizer, from the soil. Then they use it, not only for themselves, but to feed their neighbors and enrich the ground they grow in.

The ability is called nitrogen-fixing and some other plants do it, especially legumes, which to most gardeners will mean peas and beans. However, legumes – defined as a dry fruit with two seams in its outer wall — also include alfalfa, crimson clover, hairy vetch and the black locust tree, to name a few.

Alfalfa, which you may know as a crop which feeds cattle and sheep, produces purple flowers. Crimson clover has beautiful, deep pink blossom in spring and hairy vetch pale purple or white blossom that resembles sweet peas, but smaller. These three are “green manure” crops, which need to be turned over to fertilize the soil. The first two can be planted in spring and turned under in fall, or planted in late summer and turned over the next spring.

Vetch should be planted in late summer or fall and tilled under in spring, when in flower. Although, I would add, not before pollinators have had time to sample its nectar, because pollinators love vetch. The “University of Maryland Master Gardener Handbook”says vetch needs to be “inoculated” with a certain Rhizobium bacteria – more later on the role of bacteria in nitrogen-fixation. And it warns that vetch can become a weed. But what a useful one!

Back to the three nitrogen-fixing plants mentioned at the beginning: Lupins, among the most colorful of border plants, can be sown in spring or fall. They flower from May to July, can grow to three feet tall and have flower spikes almost two feet long. The key to growing lupins is to cut off the faded flowers (or the plant will last only a few years) and to avoid planting in water-logged soil. They like slightly acidic, loose soil and sun or part shade.

The plant’s protein-rich seeds were once eaten as food by the poor and scientists recently discovered how to make a tofu-like substance from lupin that, according to the grower Bakker’s website, has become quite popular in a number of countries, particularly with vegetarians.
Annual as well as perennial lupins exist and the sweet lupin variety is grown as animal feed, green mulch and even for use as a coffee substitute, in addition to improving the soil.

As for the redbud tree, now, before the weather turns hot, or late fall are the perfect times of year to plant one. Peas usually should be planted in March, when it’s still cool, but seeing as we’ve had an unusually cold spring this year, it’s probably not too late to put them in now.
Beans, another delicious nitrogen-fixing vegetable, typically should be planted later, when the temperature hits the 60s F. Incidentally, inoculating beans with nitrogen-fixing bacteria (which should be available wherever one buys seeds) can reportedly increase their yield by up to a third.

Some shrubs native to our area, so guaranteed to thrive, are also nitrogen-fixing. The bayberries – wax myrtle (or southern bayberry) and northern bayberry (or candleberry) – are medium-sized examples. I particularly like the first one, an evergreen, for its prolific, pale blue berries in winter. Both sorts have fragrant leaves and don’t mind occasional flooding with fresh or brackish water.

New Jersey tea, a three-foot shrub which produces white flowers from May to September and attracts warblers and butterflies, is described as “tough” in the U.S. Fish & Wildlife book, “Native Plants for Wildlife Habitat and Conservation Landscaping/Chesapeake Bay Watershed.” It will tolerate moist soil, if well drained, as well as dryness, and in fall produces black fruit and yellow to tan color in its foliage.
Sweetfern, another small bush, is fragrant, has leaves that may persist into fall and appeals to warblers, too.

On the other hand, smooth alder (also called hazel alder) grows 12 to 20 feet tall. It produces purple flowers in early spring, tolerates flooding and has high wildlife value.

There’s even a native herbaceous emergent plant that fixes nitrogen – black needlerush (also called needlegrass rush). It lives in brackish and salt marshes, grows one to four feet tall and appeals to warblers and waterfowl. There’s a lichen, too, but my space, unfortunately, is limited.
As for how the process of nitrogen-fixing works, some plants are able to do it thanks to various sorts of bacteria that live on their roots.

Together, they convert nitrogen into a form that can be used by plants, animals and other sorts of life. Planting lupins between apple trees in an orchard, for example, will reportedly make the trees healthier and more productive. Then, when a nitrogen-fixing plant dies and decays, more nitrogen is released into the soil.

But, and there’s always a “but,” the efficiency of nitrogen-fixation in the soil depends on many factors, including weather, soil conditions and the nitrogen-fixing plant’s health. One study found the nitrogen yield of an acre planted with red clover varied from 50 to 200 pounds.
To boggle the mind even further, nitrogen-fixing bacteria also live in the sea. Some bacteria in coral reefs can fix twice as much nitrogen as bacteria on land. Although, how scientists discovered that fact, I can’t imagine.

This spring, why not give up commercial fertilizer, a pollutant of the Bay, and plant some of these amazing nitrogen-fixers instead? In the meantime, happy gardening!

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