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How we use Green Manures

🍀 Nature hides its soils with great care. For this reason, we say that a soil lover does not want to see his/her soil. After all, plants are the driver of all life on earth, by virtue of their invaluable ability to convert sunlight and air into complex and storable food. Aboveground and belowground life (including us) could not survive without them. So it is important to keep our soils planted with living plants.

🌱 For this reason, in agriculture, in the absence of a veg crop, green manures (also called Cover Crops) are commonly planted into a bed or field to cover the soil and protect it from the elements, manage soil erosion, increase soil fertility (think of nitrogen-fixing legumes), compete with and deter weeds, repel pests and attract pollinators and wildlife. Isn’t that a lot of benefits? And it’s not even all!

🔎 Very recently we have discovered that cover crops have got even more beneficial effects. Indeed, they can unlock useful minerals, by releasing acidic exudates that break the bond between locked-up calcium and phosphorus in the soil and make both of these available.

🥬 Brassica cover crops such as Mustards and Radishes release root exudates called isothiocyanates, which are toxic to root knot exudates. Mustards also produce glucosinalates, which press wireworm to complete their life cycle in record time and move away from our veg beds in search of grass. Isothiocyanates can also protect against some of the most destructive fungal diseases, such as Fusarium, Pythium and Sclerotium.

🌾 In the last few years, the USDA has reported that cover crop mixes of five species can create a surprising biochemical synergy. The roots of neighbouring plants message each other and collectively trigger the release of phenolic compounds into the surrounding soil. These are exceptionally health-promoting compounds in humans (think of green tea!) but soil microbes seem to be as passionate about them as we are! Soil life goes bonkers and multiplies, decomposes and unlocks fertility at a record rate. As a consequence, soil structure improves more rapidly and humus is created more efficiently.

🌿 The five species include grasses, cereals (such as Rye and Oats), brassicas, chenopods (the Spinach/Beet family) and legumes (the nitrogen-fixing beans, peas, vetch, etc). A typical cocktail might include a dozen different plants, but the best mixes must contain all of the five species.

👩‍🌾 In the case of small-scale, regenerative systems, cover crops are the "supreme" tool: they build soil, attract life and reduce the need for importing compost and fertility from other ecosystems.

However, they are not as popular as they could be - mainly because they take up space, time and effort to be managed.

❓ How do we use green manures at Living Soil Garden?

During the last two years we have been exploring with species, timings, planting and suppression techniques.

🥕 We currently use green manures only for short-term coverage, which means that we never leave a bed planted with a green manure for an entire growing season. The reason for this is that we only grow on a small area (1/2 acre, 2500 sqm), and therefore our best way to cover the ground is by growing vegetable crops! However, when we sell a vegetable, part of the fertility (the part that went into the tissues of the harvested part) is not returned directly to the soil. In the case of Beans and Peas, for instance, most of the nitrogen is removed with the actual harvest, although lots of organic matter and other minerals are returned into the system with the roots, leaves and stems.

📆 There are two times of the year when green manures are really useful in our garden: in spring and in late autumn to late winter.

🌼 In spring, some beds are empty after we finish harvesting the last winter crops: cabbages, cauliflowers, fennels, leeks, turnips, beetroots, etc. If we time things well and choose the correct cover crop mix, we can keep the soil planted until a late-summer or autumn crop goes in - for instance: chicories, winter brassicas, fennel, autumn salad leaves, late carrots and many others. For this purpose, Phacelia, White Mustard, Field Beans and Oats work quite well. We sow them in March and aim to remove them when the space is needed (late May to July). The beans do not fix much nitrogen until the soil warms up, but oats grow well in the cold and phacelia provides lots of food for pollinators and predatory insects.

🌨 The second time window which lends itself to green manures is late autumn to late winter. Quite a few beds end up empty from October to early December, and while some get planted with Broadbeans, Garlic, some quick Asian Greens, Rocket and Radish crops, most beds are not needed until the following spring. So, a great thing to do is to sow a nice cocktail of cover crops.

In the last two years we have had great success with a mix of Tillage Radish, Winter Rye, Black Oats, Field Beans, Vetch and White Mustards. Clovers and Buckwheat have struggled, probably due to our soil and the time of the year that wasn’t particularly suitable for them. A September sowing lends itself to lots of options, but as we approach the end of October the only suitable species are winter rye and field beans.

🚜 Wait a moment - you might be thinking - but if you are no-dig, how do you incorporate all the cover crop biomass into the soil? This is indeed a great question, and one that has required lots of experimentation. The belowground part is easily dealt with - we just leave the roots rot into the soil. For this reason we avoid planting perennial green manures whose roots are not easily killed.

✂ In order to deal with the aboveground biomass, we can do several things. The best one is to mow as low as possible (ideally we would have a flail mower, but so far we have had to make do with a normal rotary mower). Once the top is mown, we can either compost all the biomass, or leave it as a mulch. In the latter case, it’s often best to either cover with a plastic sheet or with a thin layer of compost, otherwise most of the carbon oxidises and literally flies into the air.

🍃 There would be a lot more to say about green manures in biointensive systems such as ours, but that’s it for this post. If we have forgotten anything, if you want to share your strategy or if you have any questions - please fire away in the comments!

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