Wednesday, October 28, 2009

Green Manures, free nitrogen

It's almost too late plant fava beans, a legume, in the fall. The soil needs to be somewhat warm. This may work in gardens in sunny gardens in the west and south-west gardens. Alfalfa works the best with corn, but this is just for planning next years spring garden. Leave room in your plans to grow some green manures. I'm using another fava bean flower as it's the only one I have without scanning a slide, which I'm to tired to do this morning. :=)

Legumes have gotten a lot of attention because of their ability to fix, via symbiotic bacteria that colonize the plant's roots, atmospheric nitrogen gas into available, "free" fertilizer. Because gardeners are preoccupied with the amounts of nitrogen in their soil, they are often fixated on legumes as a green manure. While legumes are important in a green manure mix, they are not a panacea. And a myth, or lack of understanding, about how this nitrogen fixation really works has confused gardeners and reduced their chances to maximize the nitrogen cycle.

The common perception that growing beans and corn together will help fertilize the growing corn plants is incorrect. Nitrogen-fixing plants, in temperate climates, didn't evolve to directly share the accumulated nitrogen with other plants. The ability to fix free nitrogen "out-of-thin-air", in the bacterial nodules on the roots, is a competitive, evolutionary development. This allows leguminous plants to colonize nutrient-poor soils. Legumes are some of the first plants to inhabit disturbed soils such as landslides, cliffs, beaches, sand dunes, and fire-scorched soil. When you think about it, the compacted clay subsoil that's left after most houses are built is really a disturbed soil. The legumes use all the nitrogen they accumulate to grow and make high-nitrogen seed. While the plant is growing, the nitrogen from the nodules on its roots is temporarily "banked", or stored, in the stems and leaves of the plant. This provides plenty of nitrogen for rapid assimilation into the blossom once seed begins to form. Consequently, the green bean plant growing up a corn stalk isn't about to share its nitrogen with the corn, during the current growing season. The mature beans contain 70 to 90 percent of the entire plant's total nitrogen. After the bean drop is harvested, the roots and their nodules have as little as three to six percent of the plant's total nitrogen--not much of a legacy, or fertilizer portfolio for the "heirs". So, to harvest a bean crop and till under the dry tops of the plant means you'd be getting only a very tiny amount of nitrogen back into the soil because the seed has most of the nitrogen, little nitrogen is left in the stems, roots, and leaves, and the tops are too carbonaceous for easy decomposition. (See illustration on pg. 265 of my "Edible Landscape" book.)

During the growth of a legume, very limited amounts of root nodule nitrogen can become available when the stress of grazing, mowing, drought, or death causes some of the nodules to wither, die, and rot in the soil. This is why the old way of grazing animals on pastures of mixed grasses and legumes works so well. When the cow grazes the pasture, some of the nodules on the legume's roots "shed"--they don't really jump anywhere, they just decompose--the grasses are able to quickly absorb the freed-up nitrogen for their growth. This makes for more pasturage, which entices the cows to eat some more. Meanwhile, the nitrogen-rich legume plants and bulky grasses makes for a goodly amount of nitrogen to be recycled via the bovine posterior--with deposits called meadow muffins, pasture patties, or cow chips.

"Free" Nitrogen with Good Timing

The optimal time to till under a green manure crop for maximum nitrogen (for a subsequent crop) is just before the blooms appear. The conscientious green manurer, must "sacrifice" beautiful blossoms for the optimal amount of nitrogen. I was visiting Ros Creasy one year when almost one third of her front yard had Crimson clover in full, glorious bloom. I timidly mentioned that the best effect for nitrogen occurs prior to bloom, or up to 20 percent of bloom. "Yes," she replied, "I won't get as much nitrogen, but look how absolutely gorgeous the bloom is! I don't mind giving up a bit of the nitrogen for the floral display. Besides, Crimson clover has a decent amount of nitrogen to spare and my soil isn't in horrible shape." Yep.

Let me know what you think.

Visit my web site to learn about my new book on drip irrigation and other gardening books.

NOTE: The comments section at the bottom of the post has disappeared. Click on the "___ Comments" button or the title under the "Blog Archives". Thanks, Robert

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