Is green manure any good?

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Is green manure any good?

Here at Rolypig.com we focus on making compost out of any organic material that we can get hold of. Turning kitchen and garden waste into a useful fertilizer is an art that anyone can master. One of the issues that most people have is that, when they realise the value of compost, they can’t produce enough.

What are green manure crops?

There is a solution to this. It involves growing a blanket of vegetation on the patch of ground where you would want to apply compost if you had it. This is known as green manuring and it’s a procedure that’s quite easy to manage. 

So, here we look at growing vegetation that can be used as manure and ask: is green manure any good?

Green manure is a good way of improving the soil structure. It will also present a raft of nutrients where the roots of cropping plants can reach them. The vegetation that becomes green manure will be cultivated in the soil that you want to improve. The green manure procedure involves doing some digging. You have to bury the surface vegetation. 

This is the point where it becomes manure. Composting happens but it’s underground and out of sight. It will rot where it sits and release nutrients. It’s important to note that green manure will not add any extra nutrients to the soil, except for nitrates which may be absorbed from the air.

A green manure system will take potash and phosphates from the soil where the green manure crop is growing and then return it back to the soil together with any accumulated nitrates. This procedure may look as though you’re running-on-the-spot but the effort will provide a net gain.

The advantages of a green manure system go beyond feeding plants. Growing the cover-crop, that’s destined to become the green manure, you will suppress weeds. This will only apply during the time that the cover-crop is growing.

Looking at the value of green manure

The vegetation that we see as green manure will be low dry matter. It will contain a lot of water. It may look like a lot when placed in the bottom of the trench before burying it but it will become compressed into a thin layer. However, it will serve a useful purpose. It will be better that it’s there than not.

There is another factor that comes into play when we grow a crop that is to become green manure. Very few people are aware that the roots of the green manure crop are arguably more use than the vegetation that builds up on the surface.

The roots, by their nature, will be evenly distributed throughout the soil beneath the growing green manure crop. When the soil is dug over, the roots will stay in the soil. This will provide a valuable amount of humus forming material right where we want it. It will help to keep the soil open, allowing for better drainage and movement of air.

What is the best green manure crop?

When selecting a crop variety, insist on a legume. These will always make the best green crop manures for one simple reason. As growing plants, legumes have the ability to take nitrogen from the atmosphere and fix it as a soluble nitrate in the soil through their roots. 

This is one of nature’s neat tricks and we should make more use of it. The atmosphere is made up of approximately 80% Nitrogen, so there’s plenty to go round. Of the three main elements that make up the ideal compound fertilizer, nitrogen is the most mobile. Getting it to be where you want it to be and then keeping it there, can be a challenge.

It’s important to note that green manure legumes must be dug into the ground before the plant sets flower. 

When the green manure plants have been buried under ground, the vegetative mass will rot. This process will release nitrogen back into the atmosphere. So, using the green manure method is a 3 steps forward and 1 step back conundrum when trying to enhance soil fertility.

There are a number of legumes that can be used as a green manure crop. Here are some to think about:

  • Field beans
  • Lupins
  • Fenugreek
  • Clover (of any type)
  • Peas
  • Vetches (these can be difficult to dig in)

There are other plants that some have had success with as a cover crop.

These won’t pull in much nitrogen but they will help to suppress weeds and improve soil structure when dug into soil.

Look out for:

  • Poached egg plant
  • Phacelia
  • Winter-hardy salad crops
  • Mustards

How do you use fava beans as green manure?

The fava bean is also known as the broad bean. It’s an example of an ideal legume that can be planted in the Autumn period and left to become a cover crop. It will grow, slowly, through the winter months, provided that any frosts aren’t too severe.

The bean seeds need to be planted approximately 6 inches apart. This is not an exact science. One way of sowing beans is to broadcast them as evenly as possible on the surface and just dig them in so that there out of sight in the soil. You can soak them before planting but this may not be practical.

When the Spring-time comes and your beans have survived the Winter, you should have an established layer of foliage. This is to become your green manure. The bean plants are quite strong and may be tall. To make it easy to dig them in, you need to chop them down into small pieces. There’s a number of ways that you can do this. My favourite, is to use a strimmer.

How long does green manure take to grow?

You need to allow the crop to become established enough to provide a meaningful amount of vegetative material. So, this will probably mean that it will take most of the Summer. If you can establish a start-up crop at the end of a season it may be possible to grow it through the winter. 

The length of time that it takes to grow the crop should neither be a concern or consideration. When you’ve made the decision to grow a green manure crop, just prepare the seedbed for it, sow the seed,and allow the time that it takes for the crop to grow.

To help with the process, you need to try and control any weed population. This can be done by waiting for weeds to germinate before sowing the green manure crop seeds. 

When the ground is covered with weeds, scratch them over with a rake. Then wait for the next batch of weeds to come up, scratch them over, then sow your manure crop seeds.

How do you dig in a green manure crop?

This is the hard work part. The aim is to bury the green manure crop. Ideally it needs to be at least 3 inches below ground. This will allow for free soil that has none of the green material mixed in it.

The best way to do this is to dig a trench about 5 inches deep, along the edge of the patch that you want to cultivate. You then clear sweep the chopped green manure, long the edge of the trench, scraping it into the open trench. 

You then dig the cleared strip of ground, moving it over on top of the green manure in the trench. This will bury the green manure and leave you with another empty trench, going down about 5 inches, ready to load in the next line of green manure. When you do this, you will, essentially, be ploughing. 

Some will tell you to leave the ground for a month before planting any vegetable seeds where you have buried green manure. This is to allow time for the manure to rot down enough to provide nutrients for the vegetables that we want to grow. 

When should I dig in green manure?

To make this work, you need to plan the digging time. Start to dig in green manure as early as you can in the new growing season. This will leave bare earth which will allow weeds to germinate. These can be scratched over early in the season, possibly twice, leaving the ground clear for when you want to sow the vegetables that you want.

What are the disadvantages of green manure?

The only disadvantage of growing a green manure crop is that you may have to allow a full growing season where you won’t be able to take a vegetable crop. This will depend on whether you’ve been able to establish a cover crop at the end of the previous growing season. 

If you can do this, the crop may survive the winter and provide a useful amount of foliage and root activity. This can then be dug into the ground and put to full use in the new growing season.

In regions where this can’t be done, because Winter conditions won’t allow it, you need to manage a garden patch in rotation. Establish a cover crop in the full growing season. When growing legumes as a cover crop, don’t be in too much of a hurry to sow the seeds. Leave it until well into the Summer. 

That way they won’t flower. You don’t want the plants to flower because when legume plants flower they tend to use the nitrates to set seed. We need the green manure to hold onto as much nitrate as possible so that we can make use of it when we grow a vegetable crop.

The green manure crop can then be dug in during the Autumn. This will then rot down and provide the beginnings of an ideal seedbed, ready for the next growing season.

What is green mulch?

If you take freshly cut lawn clippings or any green material that hasn’t started converting to compost and place this around other growing plants, then, this is a green mulch. A green mulch will rot down where you put it. A green mulch is an effective way of feeding the soil around plants without doing any digging. The worms will pull the rotting mass into the soil. It will also help to retain moisture in a dry period.  

Image sources:

Clover green manure

What else would you like to know?

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