When to dig in mustard green manure?

Whether you have a small veggie patch or an expansive forest garden, adding green manures is a great way to replenish or build soils with minimal input.

Thanks to nature and photosynthesis, sowing a ‘green manure’ mix of seeds will give your garden a crop of specialty plants that convert sunlight into biomass for your soils and add maximum nutrients, too – and all for free.

The ‘manure’ part is a touch confusing though – there’s actually no animal poo involved.

Instead, green manuring is all about sowing fast-growing, specific plants for a few weeks or months, then purposefully cutting down their juicy foliage and digging all that green matter back into the soil, where it decomposes – enhancing your garden’s soil balance, and eventually coming back around as food for us all.

Green manures are grown from the seeds of fast-growing, specific, usually annual plants – often a legume combined with a specific grass (e.g. oats). And there’s different combinations of green manure seed mixes that suit different soils and climates.

The effect of using green manures is a bit of magic for your garden – as the legumes (clover, lucerne, peas or beans) provide nitrogen, while the plants from the grass family provide plenty of organic matter.

But a note of caution here – when I say ‘grass’, I’m referring to cereal grains such as barley, oats or sorghum, not problematic running grasses such as couch or kikuyu, like you find in your lawn. That is not a mistake you want to make!

Planting green manures as part of your yearly rotation in parts of your veggie garden is an outstanding way to improve your veggie harvest and, as a bonus, break diseases cycles in your soil.

Check out the list below of green manure benefits, compiled by Green Harvest, an ace Australian seed company:

If you have access to appropriate seeds, you can plant a green manure crop at any time of year (depending on your climate, obviously).

But, as a starting point, summer or winter is the best time to plant a green manure crop – for digging in the following autumn or spring.

In general, it’s a process that helps ensure you’re moving your soil forwards, not backwards. Regeneration is the name of the game here.

You might decide to plant a green manure crop to:

Our favourite way to plant a green manure crop is by broadcasting a good-quality seed mix over the patch we want to use it on, and then raking it all in so the birds can’t eat all the seeds. Sometimes we put a layer of shade cloth over the area to help with this, until everything’s germinated well. Shadecloth is also super useful to help contain soil moisture while your green manure crop germinates.

You’ll then need to keep this bed moist, to ensure the seeds germinate.

For a winter green manure, you’ll hopefully be blessed with rain – check the weather and aim to broadcast your seeds just before a rainy spell, and rake them in well. For drier weather, you may need to rig up a sprinkler, irrigation or remember to water daily, to keep the soil moist at least until germination happens.

And then watch as a lush green carpet develops…

Back in the days of our Milkwood Farm, sowing winter green manure crops was a crucial part of the organic vegetable growing regime – the green manures convert sunlight to nutrients, then get dug into the soil in early spring, and become food for us all.

We used them extensively all over our Milkwood Farm for the years that we were there.

In the more exposed parts of the farm, we used green manures when building earthworks such as dams and swales. (That led to mixed success, with some big disappointments due to incorrect species selection, drought, rabbits and the like.)

But in the more protected micro-climates, like the Milkwood Farm forest garden or the market garden – where there was certainty of regular watering – green manures quickly became an essential part of the winter cycle of establishing and managing plantings.

Adding winter biomass builds the soil’s water-holding capacity – a crucial thing to help safety steward summer annual food crops through the long, hot summers we experienced on the farm.

Our favourite winter green manure seed mix – the one we used over and over, every year – is Green Harvest’s Cool Season Green Manure Kit. If you can’t access this fine Australian seed company’s seeds because of where you live, the species combo is:

We’ve found it suitable for everything from the dry and hot conditions of Wiradjuri country near Mudgee (New South Wales) to the cooler climate of Djab Warrung country near Daylesford in Victoria, where we grew huge green manures while living at David Holmgren and Su Dennet’s Melliodora property.

Summer green manures are great for shielding soil from the harsh sun, which helps prevent erosion and retain moisture in hot, dry conditions.

These crops can also help reduce soil compaction, fix nitrogen and draw nutrients up to the surface from the subsoil.

Again, our go-to mix has been from Green Harvest – their Warm Season Organic Green Manure Kits are great, and come in a range of sizes suitable for small veggie beds up to market-garden sizes.

Please support your local open-pollinated seed companies whenever you can to source these seeds! If you’re operating on a small farm scale, it may make more sense to buy 20kg of each seed variety from your most ethical bulk seed supplier. The combination is:

Note that both these seed mixes are suited to temperate and cool-temperate climates – if you’re in another climate, ask around (at your local nursery or community garden is a good place to start) and find out what a good green manure seed mix is for where you live… remember that that combination of a nitrogen-fixing legume and a cereal-like grass species is a great place to start.

While planting green manures is pretty easy, a bit of future planning is also important here – you need to ensure you leave enough time for the green manure seed to germinate, grow and (maybe) begin to flower, to make the most of it.

At that point, your green manure crop is ready to be chopped down or dug in, where it will start to decompose. (Tip: cut down the green manure crop before it starts fully flowering, otherwise it will start using nutrients from the soil.)

Green Harvest provides this timeframe as a guide: “Generally allow a minimum of 8 weeks for the green manure to grow and 6 weeks for it to decompose.”

There’s a couple ways to chop and incorporate your green manure, depending on how big your patch is and how quickly you need to plant into it.

On a small scale – say a veggie bed or two, you can simply hack it all down with a rice knife or similar, lay it down on the bed, wait a week for it to start decomposing a bit, and then plant seedlings directly into your lovely pre-mulched bed. Or, you can run a mower over the bed, making sure you empty the mushed green manure biomass back onto that bed, and maybe dig it in. Wait a few weeks, and plant into that.

In the past, we have also tarped beds after chopping-and-dropping our green manure crop (tarping is literally putting a plastic tarp on the bed and holding it down with a brick in each corner) and left it for 4 weeks before prepping for the next rotation.

Tarping holds the moisture in, speeds up the decomposition of the green manure biomass, the worms in your soil have an absolute party, and four weeks later, that bed will be pretty much ready to either plant into (if you’re doing no-dig) or prep for your next crop.

Back in our Milkwood Farm days, in our organic market garden, we would first mow the green manure (without a catcher, so the mushed plants ended up back on the bed), then roto-till it all into the soil, and then plant directly into that.

For intensive vegetable production, turning the green manures into the soil is a great way to cycle nutrients quickly and create high production rates.

Green manures are excellent for pumping your soil full of nutrients and life – so heavy feeding veggies will often produce excellently if planted next in the bed.

For example, if you’ve had a spot where you’ve planted a green manure over winter, your spring tomatoes – which are heavy feeders and LOVE nutrients – will appreciate those extra nutrients in the soil.

Crop rotation does take a bit of planning and forethought, but where we can, we like to do it this way – root veggie beds give way to fruiting plants, fruiting plants give way to leafy greens, leafy greens give way to legumes or green manures.

And around the cycle it all goes.

Yes, you can use aquatic plants for your green manures, too. Plants like azolla, for example, make an excellent aquatic green manure to top-dress or dig into your veggies beds.

In some places, azolla (a type of very small-leaved, floating water fern) is a problematic plant when it establishes on dams, but that’s all the more reason to harvest and use it on your veggie beds. Instant green manure, if you will

Until recently, azolla was used extensively in rice paddies across Vietnam, China and other parts of Asia, where it would cover the surface of the water in the paddy and out-compete weeds while the rice grew, and then fix nitrogen and contribute fertility to the soil, once the paddies were drained, later in the rice growing process.

We have an article on azolla here, if you’re interested in learning more. You might have other water weeds, or fast growing water plants, that you can look into to do this same job, too.

If you’re wanting to set up your own bathtub greywater system (we have a how-to guide on exactly how to do that over here), you’ll need to create a holding pond for water that has cycled through your worm farm and reed bed.

It’s best to deposit that trickle of outgoing water into a large garden bed used for summer main crops like corn and wheat. Over winter, this damper area is an excellent spot to grow green manures.

Movable greenhouses (on rails, skids or wheels) can be a great idea for the serious backyard grower or small-scale veggie farmer.

To help ensure disease-free soil, you move the greenhouse to a new spot every now and then – or rotate between two patches – and use the cleansing powers of sunlight on the earth and photosynthesis in the forms of green manures to assist your garden’s nutrient cycling. Which ensures good food grown in healthy soil – all year round. More info on moveable greenhouses here.

You can also use green manure mixes in fixed greenhouses to regenerate the soil just as well.

So, scatter your clover seeds and some oats and vetch also – and let us know how you go?

Lupin and Mustard sown in winter is right ready early spring to dig into the soil. It is the nitrogen fixing properties from this green manure crop which will promote a boomer crop of veggies ahead in the summer months.

Ideally, you should dig in your green manure three to four weeks before planting new crops, and at least a month before sowing seed. This gives the manure time to rot down and return its nutrients to the soil before you use it again. However, if the crop looks like it’s approaching maturity, you should dig it in sooner. Young green growth decomposes and feeds the soil in a matter of weeks but older, woody material takes longer. What’s more, if flowers and seeds develop you could end up with a “weed” problem.

It’s easy to tell if green manures are approaching maturity. Some manures, such as clovers and vetches, will start producing flowers. Others, such as grazing rye, form flower buds in the centre of the plant, which you can feel with your fingers. Check your green manure regularly and ensure it doesn’t outstay its welcome.

To dig in a green manure, cut the foliage down and let it wilt for a couple of days, then use a sharp spade to unearth the roots and turn them back into the soil, digging everything in in the process. If the manures have been growing only for a few weeks you can hoe them off and then leave the foliage to rot back into the soil. If you’d rather not dig the soil, you can strim the plants down and then cover the whole area with plastic sheeting, for around four weeks.

When left to rot back into the soil, green manures can suppress plant growth, so make sure you leave at least least two weeks, ideally a month, between digging in green manure and planting or sowing.

If growing crops in the brassica family you could unwittingly create the perfect conditions for the fungal disease club root to develop. Take care to use green manures as part of your crop rotation, if using brassicas such as mustard.

Alfalfa (Medicago sativa): leguminous perennial that can be left for up to two years. Sow from April to July. Best for alkaline soils.

Alsike clover (Trifolium hybridum): leguminous perennial that can be left for up to two years. Sow from April to August. Best for wet, acidic soils.

Bitter blue lupin (Lupinus angustifolius): leguminous perennial that should be left for up to three months. Sow from March to June. Best for light, sandy and acidic soils.

Buckwheat (Fagopyrum esculentum): half hardy annual that should be left for up to three months. Sow from April to August, and dig in before autumn. Best for nutrient-poor soils.

Crimson clover (Trifolium incarnatum): leguminous perennial that should be left for up to three months or until it flowers. Sow from March to August. Best for light soils.

Red clover (Trifolium pratense): leguminous perennial that overwinters well and can be left in for up two years. Sow from March to August. Best for loamy soils.

Grazing rye (Secale cereale): annual that works well as an overwintering crop. Sow from August to November and dig in the following spring. Best for improving soil structure.

Mustard (Sinapis alba): annual that should be left for two or three months before digging in. Sow from March to September. Don’t follow with other brassicas as this could lead to the build up of the disease clubroot.

Phacelia (Phacelia tanacetifolia): half hardy annual that overwinters in mild areas. Otherwise sow from April to August and dig in after two to three months.

Trefoil (Medicago lupulina): annual or biennial that can be left for up to two years and overwinters well. Sow from March to August. Best for light, dry alkaline soil.

Winter field bean (Vicia faba): leguminous annual legume that can be left for up to three months. Sow from September to November. Best for heavy soils.

Winter tares (Vicia sativa): leguminous hardy annual that overwinters well. Sow from March to August and leave for two or three months, or July to September for overwintering.

Mustard produces large volumes of green matter and residual fibre which when chopped up and dug in is particularly good for soil that lacks organic matter as it helps to improve soil texture and moisture retention. It is not very hardy but may come through a milder winter. If it does get frosted, the foliage can be left on the soil surface as a mulch. It will gradually be incorporated by worms etc. and the remainder can be dug in during early spring.

Mustard does not fix nitrogen but is a rapidly growing annual nitrogen lifter.

After 4-8 weeks of growth chop down the plants and cultivate into the top few inches of your soil. On sandy ground let mustard reach 40cm in height before digging in. This will produce more fibrous plant matter that can help very free draining soils retain more moisture and nutrients.

It is also thought that mustard may reduce the population of wireworms by stimulating the pest to complete their life cycle much quicker - these insects can devastate a crop of potatoes. (We sow it before planting our seed potato beds).

Mustard Green Manure is part of the brassica family so caution is needed if sown before a brassica in a crop rotation plan as it may increase the likelihood of diseases.

Need Some Help On Green Manure?Then we have 2 invaluable resources on our website that will provide the answers to all of your Green Manure questions:

1. A Beginners Guide that covers everything you ever wanted to know about Green Manure in 5 quick and simple steps - read it

2. How To Choose Which Green Manure To Sow...a summary of all the Green Manures showing which soil types, plant families, and benefits that each Green Manure can bring to your veg patch or allotment - read it

Using green manure is among the environment protection trends that help the agricultural industry to shift towards sustainability through preserving resources and satisfying food demand. Seeking ways of switching to effective and environmentally friendly production, modern farmers face a number of issues. One of those is to maintain soil fertility while using fewer chemicals. The solution to this problem would be to consider green manure crops.

Farmers discovered a favorable impact of this technique on soil back in ancient times. Now, green manure is irreplaceable for those growers who want to minimize the use of harmful chemicals for soil fertilization.

Green manure is a specific crop grown with the purpose of being dug into the soil while still green. Such crops are usually planted on unoccupied land between the main crops. While growing, they act as ground cover, preserving the soil structure with their roots system, preventing erosion and the washout of nutrients, suppressing the growth of weeds, and also enriching the soil with nitrogen.

When incorporated into the soil, the plant’s residues decompose and turn into a mass of green manure that can be either dug into the soil or used as mulch – an organic material that covers the soil surface and improves its organic matter.

In fact, green manure is among the natural fertilizers that the supporters of organic farming advocate for. The average amounts of nitrogen accumulated by such crops can fully substituteBecker, M., et al. “Green Manure Technology: Potential, Usage, and Limitations. A Case Study for Lowland Rice.” Plant and Soil, vol. 174, no. 1/2, 1995, pp. 181–94. JSTOR. Accessed 28 Dec. 2022. mineral nitrogen fertilizer under the condition of average application rates. Still, there are several challenges one should consider before implementing this approach.

Here are the main reasons to use manuring instead of leaving the ground open.

Manuring is often performed to shield the main crops from heavy rainfall and strong winds, and enrich the upper and lower soil layers with useful substances. Decomposed plant residues “feed” the soil with organic matter and nitrogen. And while growing, plants saturate the ground with phosphorus, iron, potassium, and calciumAnnette Wszelaki. Sarah Broughton. Cover Crops and Green Manures. University of Tennessee Institute of Agriculture.

The root system of green manure plants loosens the soil, which ensures a better supply and retention of oxygen and moisture. Besides, the roots perform natural soil tillage, which offers farmers a possibility of growing green manure without tilling the ground. Applying a reduced or no-till farming approach is critical for sustainable agriculture.

When the plant residues decompose, the nutrients they have accumulated are eventually released into the soil, preventing the leaching of these useful substances into the environment.

Green manures effectively hinder weeds development, making it impossible for them to break through the dense biomass to get sunlight, hence performing natural weed management.

Some crops can repel pests and prevent the development of diseases, decreasing the populations of major pests that threaten the cash crop. Some plants are even capable of trapping pests in their roots.

Flowering crops attract pollinating insects, increasing the number of beneficial fauna. For instance, Phacelia’ flowers attract bees and bumblebees.

Besides, manuring benefits soil microbes and other organisms. These organisms and their activities also play a huge part in the creation of soil aggregates, enhancing its porosity and organic matter. The plant roots serve as a source of nutrients for soil microbes. When the plants are dug into the ground, they decompose and facilitate further microbial activity in the soil.

Despite the undeniable benefits, there are some points to consider before using green manure.

Cutting back and rotating cover crops before sowing the main crop is a time-consuming process. It may take up to a month if a crop is allelopathic, meaning it inevitably leaves some toxic substances in the soil, restricting the germination of the successor.

Like any other plant, green manure crops also utilize moisture. Hence, if planted in the area with limited moisture levels, they might use up all of the available water. This entails additional irrigation to support the cash crop growth.

Manuring means adding another crop to the crop rotation plan, which will also have to be adjusted to ensure it doesn’t exhaust the land. Planting green manure crops right after harvesting the main plant might deprive the soil of the necessary time for rest and rejuvenation.

Depending on the time of sowing, green manures are divided into the following types.

Established for two or three years to form a basic part of organic field vegetable and arable rotations. The list of green manure plants for long-term planting includes:

Sown in fall to be incorporated in the following spring and to act as a fertility building crop by utilizing land that would otherwise be fallow. The best green manure for overwintering includes:

Used to provide sufficient nitrogen amid crop rotation. Can be grown either for a whole season from April to September, or just for a period between two main crops. Green manure crops for summer include:

The term implies integrating green manure crop into an established cereal crop in spring to enable better weed control. Such crops include:

Term implies sowing several crops simultaneously to magnify the benefits. Among the best green manure crop combinations for mixed planting are:

The mix depends on the types of cultivating cash crops, soil characteristics, and other factors.

In general, planting green manure at any point during a growing season will have its advantages. Although manuring usually serves the purpose of covering the ground in fall and winter due to its ability to prevent erosion, it also offers similar benefits in the summer months, providing protection against the drying effects of wind and sun.

The best time for digging in the plants is two or three weeks prior to using the soil again, or when they are reaching maturity stage. The young plants’ mass will decompose fast enough to “feed” the soil. That is why it’s critical not to wait until flowering. Besides, consider digging in before the stems become woody and harder to break down in the soil.

To determine the best time for digging in green manure, you can use software tools like EOSDA Crop Monitoring. The platform is of help throughout the whole cultivation span, be it the cash crops or manures. All you have to do is enter the data on crop name, sowing date, and maturity.

Now you can easily track stages of plant growth to decide on when to dig it in for getting the maximum benefit. Find the time before the plants start flowering, and especially before they go to seed or before the stems become woody and hard to break down into the ground.

Green manures return nutrients to the soil, protect the soil from water and wind erosion, and also prevent overheating of the top layer of arable land by solar irradiation.

Does green manure add nutrients to soil? Yes, it provides the land with all the necessary elements. Decomposed plants’ mass enriches the soil with macro- and microelements (including nitrogen, phosphorus, and potassium), which are essential for healthy crop growth.

In case growing a green manure crop is not enough to cover the soil needs for fertilizers, additional application is required. Use EOSDA Crop Monitoring Zoning feature to apply the VRA approach to soil fertilization, avoiding overapplication and pollution of soil and water, while saving time and money.

It’s critical to choose the type of crop depending on the soil. For instance, the best green manure for clay soil would be:

These plants have deep roots, which allows them to pull nutrients from the subsoil to the top, while breaking up the compact clay. Plant them in fall and dig them into the soil in spring before they go to seed.

The best green manure crops for sandy soils would be those capable of enhancing the land’s water retention ability and nutrients content. These include:

Integrating green manures into crop rotation allows for maintaining soil fertility according to the key principles of sustainable agriculture. Offering numerous benefits, green manure enables soil protection, fertilization, and its organic content improvement, which is critical for eliminating the threat of land degradation. And more healthy soils means more sustainable food production due to fewer chemicals’ application needed and less tillage required.

The possibility of using less synthetic fertilizers and heavy machinery throughout crop cultivation implies a significant reduction in air and water pollution rates. Hence, growing green manure crops enables growers to build an organic farm.