Boost Your Soil with Green Manures


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Boost Your Soil with Green Manures

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Boost Your Soil with Green Manures

  1. 1. Boost your soil with green manuresEnhance soil fertility, suppress weeds, improve soil released in to the soil, giving a boost to followingstructure and build organic matter with these miracles. Nutrient recycling: Other nutrients such asWhy grow them? phosphorus that would otherwise be leached from the garden are absorbed by the manure crop as it growsGreen manures have many benefits and are an and made available to following crops in a form theyinvaluable tool for an organic gardener. They are not can use.harvested, but incorporated into the soil when they arestill green to contribute to the care and fertility of the Weed control: Green manures tend to outcompetesoil. weeds for water, light and nutrients. As they grow, manure crops prevent weeds from colonising bareOrganic matter: A healthy soil should contain about ground left after the preceding crop has been removed.5% organic matter. While this might seem like a smallcomponent of the soil, it is a vital one. Worms and Soil stability: Growing a green manure crop protectsmicroorganisms decompose organic matter - such as is the soil surface from wind and water erosion andprovided by green manure crops - into humus and prevents leaching of nutrients.other organic compounds, providing structure and Pest and disease management: Using green manuresbiological activity to soil. The presence of humus also in a crop rotation system can help disrupt pest andincreases the amount of water that can be held in the disease cycles. Some manures release naturalsoil, and hence the drought resistance of the garden. chemicals into the soil, some act against soil-borne diseases and nematodes and some attract beneficial parasitic insects to the garden. When to grow them? Although many green manures can be grown year round - whenever a patch of garden is free of crops for six weeks or more - there are three main ways to grow manures: 1. As a crop during the main growing season. A disadvantage of this approach is that the manure crop takes up valuable space at the most productive time of the year.Soil structure: Water holding capacity can also be 2. As an undercover crop grown with the main crop. Aincreased by using green manures with deep, fibrous green manure planted after the main crop has beenroot systems. Their roots penetrate the subsoil established can be a useful approach in cold areas(particularly of compacted soils), giving subsequent where there is not enough time to plant manure aftervegetable crops channels for their roots to access water the summer harvest. It is also useful in warm seasons,and nutrients from lower levels in the soil structure. In where an early green manure crop can protect newaddition, most vegetable crops are able to put down seedlings from weeds and drying winds.large root systems if the soil is loose enough - which isanother important role of green manures. 3. As an overwinter crop. This is the most common approach in the ACT region as manure crops are idealNitrogen fixing: Leguminous manures are able to to grow during the cold months, when vegetable plotsabsorb nitrogen from the air and transfer it to their might otherwise be left vacant. The time to plantroots via Rhizobium bacteria. When the green manure overwinter crops is now - , as soon as the summercrop is incorporated into the soil, the nitrogen is crops have been harvested, to allow time for establishment before winter. Canberra Organic Growers Society Autumn 2008
  2. 2. Green manures are an important component of croprotations. It is suggested that once soils have beenmade relatively fertile, it should be necessary to usegreen manure crops only once every four years. Anexample of such a rotation is:Year 1: Tomatoes, capsicums, eggplants, leafy greensYear 2: Onions, garlic or peas, beans, followed by brassicasYear 3: Root cropsYear 4: Cucurbits, sweet corn, followed by a winter green manure crop.Be aware that onions do best without a precedingmanure crop and most root vegetables prefer soilwithout a lot of organic matter.How to grow themPrepare the plot by removing weeds, turning the soiland raking until level. Scatter seeds over the surface,using the density recommended by the supplier. Bagsof COGS green manure mixes cover about 30 squaremetres of ground. control, try a clover. If you need more organic matterTap over the surface to ensure the seeds are in firm in your soil, choose a bulky manure with the soil and water in. Germinationgenerally occurs within a few weeks and green Other factors to consider when choosing a greenmanures will do the most good if left for at least 8 manure are your soil type, the availability of waterweeks. (particularly important when using a green manure in hotter or dryer seasons), the length of time from sow-An overwinter crop should be either dug in or cut in ing to flowering (choose quicker growing varieties ifspring, in preparation for summer plantings. If you are you have only a short ‘window’ between vegetabledigging in, wait for about 4 weeks before planting crops) and adaptation to Canberra’s climate.summer crops to allow the green manure to properlydecompose. It is often a good idea to grow a mix of green manures for optimise benefits.To dig or not to dig? Studies in the US suggest that theamount of nitrogen released from many cereal crops The table over the page gives details of green manurehardly differed between crops that had been dug in and crops suited to this region, and focuses on varieties tocrops that had been left untilled. However, decomposi- plant in autumn as an overwinter crop.tion is likely to be faster if the crop is at least partiallychopped and roughly incorporated into the soil. References and further readingDon’t let the green manures go to seed as they can • Organic Farming: Which green manure will Ibecome a nuisance. Try to get to flowering crops just grow (Christiane Jaeger, Victorian Departmentbefore flowering and cereal crops before they produce of Primary Industries Agriculture Notes,a head of grain. October 2003) • Organic Farming: Green manures for vegetableWhich ones to grow cropping (David Madge and Christiane Jaeger, Victorian Department of Primary IndustriesWhich green manures you use will depend on your Agriculture Notes, January 2003)objectives for using them. For example, if your soil is • Grow your own green manures (Organiccompacted, try a green manure with deep roots. If you Federation of Australia’s Organic Living)need to boost your nitrogen levels for your next crop, • BBC Gardening ( a legume. If you need soil stability and weed Canberra Organic Growers Society Autumn 2008
  3. 3. Green manures The following table gives details of green manures Words: COGS members and Monica van Wensveen commonly used in the ACT region. Photos: Monica van Wensveen Best sowing Winter Soil type Root sys- Weed con- Other comments time hardy tem trolBarley Autumn, Yes Fibrous Some Vigorous grower; increases early spring weed uptake of phosphorus in control following cropBroad Autumn Yes Likes heavy Shallow Poor weed Adds nitrogen; producesbeans soil; tolerates control considerable organic matter water loggingBuckwheat Mid-spring No Tolerates Short Smothers Flowers attract hoverflies; poor soil fibrous weeds extracts phosphorus; short taproot growing season (8-10 weeks)Clovers Early autumn Yes Prefer loams Taproot Smother Flowers attract beneficial with weeds insects; effective nitrogen spreading fixers; not large amounts of laterals foliage; loosens compacted soilsCow pea Spring- No Most types Strong Smother Adds nitrogen; builds organic summer taproot weeds matter; prefers warmer with later- weather; somewhat drought als resistantField peas Early autumn Yes Likes heavy Shallow Not Adds nitrogen; produces soil; tolerates vigorous considerable organic matter water logging enoughLupins Early autumn Yes Tolerates Deep, Attracts bees; effective most types strong phosphorus and nitrogen taproot gatherer; not susceptible to fungal diseases that can affect peas and beansOats Autumn Yes Range of Fibrous soils; tolerates acidityRye Autumn Yes Range of Fibrous; Large amount of organic soils; stronger matter; drought resistant tolerates than other acidity cerealsTic peas Autumn Yes Likes heavy Shallow Poor weed Adds nitrogen; cheaper soil control alternative to broad beansVetch Autumn, Yes Taproot Competes Considerable organic matter; spring, well with nitrogen fixer; host for Summer weeds ladybirdsWheat Autumn Yes Fibrous Canberra Organic Growers Society Autumn 2008