It’s no great mystery that animal manure is something magical for the garden. In reality, this kind of garden manure is more or less relegated to the excrement of domesticated vegetarian animals, as carnivores produce poop far too acidic for plants. In essence, what these grazing animals are doing is speeding up the process of decomposition for plants. In other words, their manure is composed of plant material.
Green manure is when we take the animals out of these processes, and for those of us looking to avoid using animal products, this is the next best option. Instead of relying on domesticated animals to give us manure, we have the potential of growing our own “green manure.” And, green manure has a similarly magical effect in the garden, boosting soil fertility to help with our crops. The process is just a bit longer.
Whether or not using animal manure fits into your ethos, green manure is certainly a valuable tool to try out in the garden.
What Is Green Manure?
While most greenery in the garden does carry with it a dose of nitrogen (good for fertilizing) and minerals (good for healthy plants), green manures are specially selected plants that are particularly good providers of these things. They can additionally be chosen for an ability to break up compact soils, fix nitrogen into the soil, prevent erosion, protect the soil from saturation or dehydration, improve microbiota and/or stop nutrients from leaching away.
Green manures are often quick-growing plants that produce a lot of organic matter to be fed right back to the soil. In general, they are used in beds that need a rest from producing crops (maybe every third or fourth planting) in order to recharge the soil nutrient content. They are also often planted in the fall, after the growing season is over, in order to protect and feed the soil over winter. Some gardeners simply cut the green manure down and let it rot in place while others prefer to compost it, using last season’s composted green manure to fertilize this season’s crop.
These cover crops are referred to as green manure because they, like animal manures, are used for amending the soil and giving nutrients back to the garden.
What Are Green Manures?
Though green manures are certainly part of the new garden movements like permaculture and veganic gardening, they are nothing innovative. Cover crops have long been an important part in organic and sustainable farming methods, and the technique simply suffered in popularity with the rise of chemical agriculture. The bad news, of course, is that the chemicals turned out to be a sham, but the good news is that we already know of a host of great green manure plants to use.
What we look for in a green manure crop is one that is beneficial for the soil. That means we want a lot of organic matter. Nitrogen-fixing crops, like legumes and clovers, add a lot of fertility as well. Dynamic accumulators, with deep-tapping roots, help to pull minerals up from far below the reach of other plants’ roots. We also want cover crops well suited for the terrain and climate, as well as something that won’t require a lot of attention and irrigating.
- Cereal rye is a hugely popular choice because it produces a lot of organic matter, both above and below the ground. It can deal with both drought and cold weather. Some food can be harvested as well.
- Mustard is beloved because it grows very quickly, providing an abundance of green manure for fast turnaround. Mustard works wonders revitalizing soil and tolerating whatever temperature. It’s also pretty good for the occasional forage
- Alfalfa, as a perennial, is noted for its ability to work in long-term soil rehabilitation. It deals with drought well and grows in any season. The downside for some is that it establishes roots that will need to be removed for other plants.
- Fava beans are cold-tolerant, nitrogen-fixing plants that also can provide some protein-rich food for the table. They also produce an abundance of organic matter to feedback to the soil. They’ll grow over winter in places that only mildly freeze.
More and More Green Manure
In reality, the list of green manures is lengthy and diverse. Clover and vetch are great nitrogen-fixing plants that tolerate poor conditions. Different types of radishes, especially the big ones like daikon, are revered for breaking up compacted soils. Barley and oats are recognized for their straw and crop production. We’ve only scratched the surface, somewhat literally.
Green manures are a fantastic option for taking care of the garden without relying on domesticated animals for the fertility. And, they exemplify the crux of sustainable food-production: We can’t simply always take from the land; sometimes we have to give back. Growing green manures does just that.
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