Green Manure Crops

July 16, 2020

Green Manure Crops

Green manures are a vital part of soil management and integral for successful vegetable gardens. Here's how to use green manure crops to improve your veggie garden.

What is a Green Manure Crop?

Green manure or cover crops are fast growing crops specifically grown to be dug back into the soil to improve soil structure and return vital nutrients to the soil. Sadly their use has dwindled in recent times with the rising popularity of quick fixes such as synthetic fertilisers.

How do they work?

Green manures have the ability to draw nutrients up from deep within the soil, they store these in their plant cells so when they are dug back into the soil they slowly release as the plant breaks down. Legumes also have the added benefit of fixing nitrogen and returning this to the soil. Other benefits of manure crops include acting as weed surpressors between plantings by out competing undesirable weeds whilst also aerating compacted ground and preventing erosion or hydrophobic soil. Some even have the added benefit of fumigating the soil to control root knot nemotodes and fungal pathogens.

What green manure crop should I sow?

You may be surpised to learn that anything can be used as a green manure crop (within reason)... however there are some crops that are better than others. Things you need to consider when selecting a green manure crop include:

  1. Will the variety germinate and grow at the time you are sowing them? Choose something that is suitable for your climate and the time of year that you are sowing. Cool climate crops such as peas, broad beans, oats, radishpoached egg flower, fenugreek, lupins and mustard are perfect for Autumn/Winter. For warmer seasons, consider crops such as millet, marigolds, mung beans, buckwheat, soybeans, beetroot and even sunflowers for Spring/Summer. 
  2. What soil improvements do you need? For nitrogen fixing, choose legumes such as peas, beans, lucerne, oats and lupins. For aeration look for varieties with long/comprehensive root systems. If fumigation is your main focus, plant marigolds, brassicas or mustards
  3. How easy are they to dig back into the soil? Woody stemmed plants may be OK if using machinery to dig in, but for the smaller home garden be sure to use crops with soft stems that are easy to hand turn and will break down in an appropriate timeframe. Ryes and many clovers are notorious for being hard to turn so avoid these if you're not in for a huge workout!
  4. What soil type do you have? Not all crops will grow in all soil types. Select something suitable for you soil type and pH.
  5. What are you planting in the garden next? Some crops may interfer with your rotations so be sure to use a variety that complements your next crop roation. Mustards are part of the brassica family and therefore may negatively affect future brassica crops.

There is no harm in using a mix of seeds for your green manure crop, in fact it is quite beneficial to sow a mix to ensure best coverage and success. 

How to grow your green manure crop

  1. Prepare your soil as you would for general sowing - remove weeds, break up clods and loosen the soil.
  2. Check the seed packet for instructions on how best to sow the varieties you have chosen.
  3. Broadcast your seed onto the ground ensuring good coverage and lightly rake to cover.
  4. Water in well.
  5. Once grown, and before flowering (approximately 4-6 weeks), cut down your crop - by hand or using a clean (weed-free) mower. Green manures should be dug in before they flower which is when they start to consume many of the nutrients stored (especially nitrogen). If green manures are left to go to seed, it will result in new plants growing which may compete with your crops.
  6. Depending on when you want to start planting you can either dig the crop in or lay the crop on the surface as a mulch. The former takes longer to complete but the results are much better.
  7. If you're in no rush to get planting then proceed to dig the crop into the top layers of soil. A few weeks later repeat digging in. After approx 2-4 weeks your garden will be ready for its next sowing.
  8. If you're in a hurry to get planting you can lay the manure crop on the surface more as a mulch. 

We hope this article helps you with your garden, and remember green manure crops are your friend and are a much better alternative than leaving ground bare for any amount of time, use them to your advantage!