Green manure probably isn’t what you think. Not a manure at all, green manure refers to certain fast growing cover crops.
Green manure crops are grown to be cut down while they are still green or just after flowering. Traditionally, green manures were plowed into the soil, but it has been discovered that this damages networks of root fungi (mycorrhizae) that help plants absorb nutrients. Like animal manures, green manures provide many benefits to the soil.
Some green manure crops are grown to add nitrogen to the soil. These plants include alfalfa, fava beans, cowpeas, sweet clover, Egyptian (or berseem) clover, crimson clover, lana (or woollypod) vetch, and hairy vetch. These plants are all members of the legume family. Legumes have a working relationship with certain soil bacteria (Rhizobia) that allow them to ‘fix’ atmospheric nitrogen and convert it into a form usable by other plants. Growing these plants as a green manure can increase the amount of nitrogen available to the next crop by 40 to 60%. That’s like dumping 40 to 200 pounds of nitrogen on an acre of land!
In the same way as nitrogen banking, other nutrients are returned to the soil as green manure crops are broken down by soil microorganisms. These important nutrients include calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S).
As green manures break down on (or in) the soil, they tend to lower soil pH. This is because acids are formed in the decomposition process. In the Bay Area, where we tend to have heavy, alkaline soil, this acidification can benefit many acid-soil loving crops, such as blueberries, raspberries, potatoes, and parsley.
As soil microbes, worms, and other critters go to work on a cut green manure crop, fungi and slime add their efforts at getting a portion of the banquet. As they all feed (and poop), the amount of organic matter, or biomass, increases and the soil is aerated, increasing the number of macropores and micropores that carry food, water and air. This also improves water infiltration and retention, and makes it easier for tender, young roots to reach the water and nutrients they need to thrive. Medic, berseem clover, and woollypod vetch are good choices for improving soil structure in the Bay Area. Deep rooted green manure crops, such as mustard, drought-tolerant alfalfa (Medicago sativa), alsike clover (Trifolium hybridum), can also help break up compacted soil and pull nutrients closer to the surface for your garden plants to enjoy in the next growing season.
Attract pollinators and predators
If a green manure crop is allowed to flower before being cut, those flowers can attract and feed a wide range of pollinating insects. Phacelia (Phacelia tanacetifolia) is particularly effective in the Bay Area. Green manure crops can also provide habitat and protection for many beneficial predator insects. This can lead to a reduced need for insecticides and bigger harvests.
Since green manure crops tend to be fast growing, they often block common weeds from getting the sunlight, water, and nutrients they need to thrive and reach seed-producing status. Buckwheat (Fagopyrum esculentum), fenugreek (Trigonella foenum-graecum), sorghum, and sweet clover are especially good at blocking weeds.
Cover crops and green manures can be used to reduce erosion. Plant roots stabilize slopes and protect the top layer of soil from sun and wind damage. White clover, barley, rye, and ryegrass are especially good as erosion control. These crops can also help prevent runoff and urban drool.
Sorghum, crimson clover (Trifolium incarnatum), and rye provide habitat and food for many beneficial insects. If nematodes are problem in your landscape, you can grow a green manure of white mustard (Sinapis alba) and radish (Raphanus sativus). Nematodes hatch and are attracted to the roots of these plants. After burrowing into the plants’ roots, the nematodes are unable to reproduce. Populations of beet cyst nematodes and Columbian root knot nematodes can be reduced by as much as 70 to 99% using this method. If you have citrus trees, planting bell beans, woollypod vetch, Austrian winter pea, or New Zealand white clover will attract a predator mite (Euseius tularensis) that attacks citrus thrips.
Problems with green manures
Like everything else in life, there are downsides to green manures. First, these crops must be cut before they start reproducing. Legumes stop adding nitrogen to the soil once they start their own reproductive cycle. If any green manure crops are allowed to go to seed, they may overtake an area. Also, all that green, shady moisture can attract slugs and snails. Another potential problem is the bacteria that cause club root in members of the brassica family (cabbages, broccoli, turnips, and cauliflower) may also be encouraged by the presence of green manures. A slightly trickier aspect of growing green manures is the carbon to nitrogen ratio.
Carbon-nitrogen (C:N) ratio
A healthy compost pile* will have a C:N ratio of 20:1 to 35:1. This means it contains a mass that is 20 to 35 parts of carbon to one part nitrogen. [Ratios greater than 35:1 will slow the composting process.] Farmers use a 24:1 ratio for simplicity sake. Healthy soil has a similar ratio. So do the microorganisms in soil that break down all that organic matter. These microbes maintain that ratio with the food they eat. If there is too much carbon available, say, if you mulch an area with straw, soil microbes will devour all that straw and then eat all the available nitrogen, leaving nothing for your plants! But don’t panic. When the microbes die, they return all that nitrogen to the soil, it just takes some time.
* Generally speaking, carbon is in the hard stuff, while nitrogen is in the green stuff. That isn’t exactly accurate, but it will help you to understand how to maintain a healthy compost pile. Since plants are made up of both, aim to provide your compost pile with equal parts green and brown.
Non-legume plants have higher carbon contents than the legumes. Also, carbon content changes as a plant ages. You can avoid this problem by planting a mix of green manure crops at certain times of the year and ensuring that they are mowed or cut before they start producing seeds of their own. [For you science nerds, the average adult human body has a C:N ratio of 54:1.] Here is a list of C:N ratios for many green manure crops (animal manure tends to be 20:1):
How to grow green manure crops
Cool season green manures are planted in late summer and allowed to grow through winter before cutting. Summer manure crops are best used for weed suppression and erosion control. To maintain a healthy C:N ratio, plant a mix of low carbon crops, such as clovers, fava beans, peas, mustard, canola, turnips, radish, with just a few high carbon crops. High carbon crops include alfalfa (Medicago sativa), sunflowers, winter rye (Secale cereale), and millet.
You can plant green manure crops in rows, the same way you would for many other crops, or you can simply broadcast seeds over an area and rake them in. Just be sure to keep the area moist until the seeds germinate. Then, simply allow them to do what they do best, until they are just about to go to seed. That’s when you break out the lawn mower, weedwacker, or scythe, and chop your green manure crop down to the ground. Leave the plant material where it falls and allow it to return to the soil the way nature intended.
As you can tell by the photos, these beneficial plants also add beauty to your landscape. Trying adding a few to yours today!
You can grow a surprising amount of food in your own yard. Ask me how!