If you’re a regular gardener, you know how important it is to monitor your soil fertility – Higher fertility means better growth, which results in a higher yield. There are many ways to achieve this, ranging from artificial fertilizers to more natural methods such as adding compost.
One way you’ve probably rarely ever come across is ‘green manure’ – a relatively unfamous but effective way of increasing soil fertility. In simple terms, green manure refers to plants that are grown to be dug back in. This is done primarily to improve the quality of the soil.
How does green manure work? Green manure can help to improve soil structure, fertility, and alkalinity. Green manure plants collect nutrients from the air and soil. These nutrients are released once green manure starts decomposing, benefitting all the plants you grow after them.
But that’s not it – there’s really a lot more to green manure. So, without further due, let’s dig right in and have a deeper look at how exactly green manure works.
What’s The Difference Between Cover Crops And Green Manure?
Green manure is often referred to as cover crops, and from the general definition stated above, you’d think they are pretty much the same thing. But that’s really not the case; cover crops and plant manure are two related but different concepts.
Cover crops are basically the plants that are initially grown at the beginning of the fertilization process. For this purpose, they are also called ‘green manure crops.’ Their purpose is not to get harvested but to cover the soil.
Cover crops aim to improve the structure of the soil, offer protection against the forces of wind and water, and discourage the growth of weeds.
On the other hand, green manure is the fertilizer obtained from cover crops, i.e., the plants grown to increase soil fertility. When the cover crops grow, they extract the nutrients and other vitalities from the soil and store them within themselves.
When these plants are later dug back into the soil, they slowly decompose and release everything stored inside. This process transpires gradually, benefitting the future plants grown in the area. This goodness provided is a form of natural fertilizer and is called ‘green manure.’
Green manure adds nutrients to the soil and provides essentials to the plants that are grown with the intention of harvesting.
Now, how exactly will you figure what and how much you need?
Start With Your Goal
The first question you should ask yourself is, “What does my soil lack or need improvement in?”. Once you know, you’ll be done with the most challenging part.
To help you understand when you’ll need green manure, here are some factors that are affected by, or improved, by adding the usage of green manure:
An optimum pH of around 6.5 to 7 is ideal for growing plants and vegetables. If your soil happens to move to the alkaline side, that is, above 7 to 8 pH, you need to neutralize the soil’s alkalinity.
In this regard, green manure comes in handy. It behaves like soil-acidifying matter and brings down the pH of alkaline soils by generating several organic acids.
Henceforth, if your goal is neutralizing alkaline soil, green manure is something you should try.
I’m sure you’ve figured by now that the first and foremost purpose of green manure is to increase soil fertility. It does so by decomposing and releasing all the nutrient goodness it had previously stored within itself. This increases the organic matter in the soil.
Additionally, green manure increases soil humus levels, which also contributes to fertility. Therefore, green manure is your thing if you aim to increase soil fertility.
Poor Soil Structure
If your soil structure has been distorted, or you feel its quality is slowly dropping, green manure can help. In this regard, green manure improves soil structure.
Basically, it increases aeration in the soil, allowing more oxygen and other gases to pass through. It prevents leaching, or in other words, helps retain nutrients, not allowing them to flow out of the soil so easily. Additionally, green manure improves water drainage, preventing soggy and waterlogged soils.
Now, how is this so? It’s pretty simple – if you add nutrient-rich plants, the less fancy term for ‘cover crops’, to the soil and allow them to gradually rot, they will add numerous benefits to the soil, including the few listed above.
There are many different types of green manures…
For different purposes, there are different green manure varieties. Now, you’ve figured what your soil is lacking, but how will you know which manure to select?
Go for Diversity!
The deficiencies in your soil determine which type of green manure should be selected. But it is always a good idea to go for a diverse range of green manures to reap a spectrum of benefits simultaneously.
For example, if you want to use green manure as a substitute for fertilizers, you can use nitrogen-fixing crops. These belong to the family of legumes and help convert or ‘fixing’ atmospheric nitrogen to a form that plants can utilize. Examples of these include beans and peas.
On the other hand, if your soil needs to retain nutrients and minerals more than fertilization or wish to discourage and break the chain of weed growth, you should opt for non-leguminous plants. Examples of these include oats and mustard seeds.
Now, you can either choose ONE of the examples given, for example, beans. But seeding a more diverse cover will help speed up the natural regenerative processes of the soil. Individual monocultures of one crop as a cover crop might provide benefits for that particular species but will lack the advantages other species could provide in the same amount of time.
Therefore, you should choose various cover crops, e.g., one from the legume family, such as peas for nitrogen-fixation and fertilization. Supplementing its benefits with another plant from the non-leguminous family, such as oats, to reduce leaching and weed growth.
Different types of green manure and their effects
Green manures are formed when their respective cover crops are dug back into the soil. To understand the types of green manures and how they work, it is important to look into the related cover crops – after all, they’re two deeply related concepts.
For example, a type of cover crop is peas, and its respective manure provides the benefit of fixing atmospheric nitrogen into the soil.
For a more detailed description of the types of green manures and their effects, let’s look at the varieties of cover crops.
Cover Crops – Effects And Selection
Cover crops are really that one super organic addition (and even substitution for artificial fertilizers) your garden needs. They may require a set amount of space or some additional effort, but really – it’s worth it. For a better look, here are the main type of cover crops:
Nitrogen Sources: Legumes
Legumes are plants whose seeds grow in the form of pods. The special feature that makes them essential to cover cropping is the nodules on their roots. These nodules provide shelter to nitrogen-fixing bacteria.
This bacteria’s purpose is to convert nitrogen from molecular form to a form that can be used by plants, for example, ammonia. This is important because nitrogen forms chlorophyll – the substance used to trap sunlight and make carbohydrates. That’s kind of like a human not having the resources to make their food and eventually starve… no one would wish that on their leafy buddies.
If you wish, you can use inoculant to improve the process of nitrogen fixation further. This inoculating substance is readily available at all gardening stores and boosts yields by an appreciable amount.
Some legumes used as green manures are soybeans, letches, and lupins. If you want plants that are pollinator-friendly in addition to providing nitrogen-fixing benefits, you can go for legumes such as peas, white clovers, and alfalfas.
Weed Management: Buckwheat
Weeds are unwanted plants growing in areas where they are not needed. Weed control involves monitoring and discouraging the appearance of these ‘extra plants’, and stopping the growth of weeds.
In this regard, buckwheat is a very effective plant that can be used as green manure. Its weed-suppressing superpowers make it a handy cover crop for gardeners whose main aim is weed control. But that’s not it; buckwheat provides several other benefits, some of which are listed below.
- Buckwheat grows and regrows rapidly, so it can be used as a quick cover for bare soil
- It survives in less fertile soils
- It loosens up the top layer of soil
- It attracts beneficial insects, boosting pollination
You can grow buckwheat very quickly and easily; it is sown in the rather warmer seasons of spring and summer by distributing its seeds around 5 cm apart and 1 cm deep into the soil. It will take around three to six weeks to fully bloom.
You can grow it while your garden is bare to provide coverage from natural forces, or alongside other plants to provide the benefits listed above.
Reduce Compaction: Mustards
Soil compaction refers to the degradation of soil structure, wherein soil particles come close together and sometimes form big chunks that are hard to break. Such soil retains water by reducing drainage, doesn’t allow roots to spread freely, and results in poor yields.
Mustards are green manure plants that can help improve the structure of your compacted soil. They have large taproots that break through and penetrate hard and compacted soils. This means they can thrive in such soils while also alleviating their compaction.
And every gardener knows how important it is to get rid of compacted soil – it can put years of hard work into vain.
Growing mustards is an easy and quick process. You have to scatter mustard seeds all across your garden evenly.
If you aim to control weed growth, you should spread them out randomly rather than sowing them in rows since weeds show up in unwanted places and not just specific rows. You can then gently rake the soil and water it until green manure is formed.
Erosion Control: Clovers
Erosion refers to the removal of soil, mostly the topmost layer, from your garden (or for that matter, any area that contains soil). This is mainly due to the absence of plants; plants’ roots tend to hold down soil and prevent it from being flown, or ‘eroded’, away.
Clovers are green manure plants that can be used to prevent this erosion of soil by up to a whopping 69%. They do this by covering the bare layer of soil, hence offering protection from the forces of wind, rain, snow and other extreme weather conditions.
The coverage basically helps because the roots of clovers delve right into the soil and hold it together. This ensures the top layer (and layers underneath) is held together and doesn’t wash away too easily.
Clovers add organic matter to the soil, increase nutrients and act as a natural mulch for soil protection. The cherry on the top is that they are pollinator-friendly and attract beneficial insects!
Hence, if your soil is bare for most of the year, or if it has been suffering from soil erosion, mustards should be on the top of your green manure plants.
Mixtures – You Can Really Play Around With Green Manures And Cover Crops!
Many cover crops offer more than one benefit, but if you’re still aiming higher, you can mix and match different cover crops to suit your needs.
For example, if your soil has more than one deficiency that you need to take care of, you can use two (or more) cover crops simultaneously. This will help you solve multiple problems at the same time. A combination of cover crops is ideal as it provides additional benefits that will only enhance your soil’s structure and add to its level of nutrients.
Although you can decipher which cover crops your soil needs by plain research, many widely used combinations might help you decide.
Overview Of Some Commonly Used Mixtures And Their Effects
Mixtures of various cover crops are being tried and tested by gardeners, farmers and research teams. Many positive results have been reported, including higher yields. Some of the most commonly used mixtures are as follows:
- 15 species combined, including legumes, grasses, broadleaves, and mustards. Such a combination is believed to increase organic matter by a considerable amount, in addition to other benefits such as soil erosion control.
- Buckwheat can be paired with soybeans, cowpeas, and Sudangrass. This combination discourages the growth of weeds and fixes atmospheric nitrogen into the soil, hence promoting plant growth.
- Oats can be paired with peas and clovers. The effect of this mixture is providing erosion control, fixing nitrogen, and attracting beneficial insects for pollination all at the same time.
- Forage radish can be combined with mustards and clovers. This mixture enhances soil structure greatly; it loosens compact soil by a great amount while also preventing excessive soil erosion. If soil compaction is your problem, this mixture is what you need.
What benefits do mixtures have as compared to single species?
In addition to the evident advantages of tending to multiple issues simultaneously, mixtures make the most out of the benefits of one single cover crop.
In other words, mustards used alone wouldn’t reduce soil compaction as much as they would in a combination with other cover crops; their function is greatly enhanced.
Furthermore, water resources can be better utilized with mixtures. Plants of varying root length when combined provide optimal usage of the water available, with not too much retained or lost.
Do I Have To Dig In Green Manure?
Very simply – yes. That’s how green manure works. You start by sowing the plants you’ve selected for your green manure. Most of these are easy to grow and involve loosely distributing seeds across the area and watering sufficiently.
During the entire process of growth, green manure plants extract the goodness of the soil and store it within themselves. When they grow into mature plants, they now have all the nutrient-y benefits of the soil inside them. When the plants grow enough and are near reaching maturity, the real game begins.
Now is the time you need to start digging them in. You can do this by using a sharp spade to cut them down into pieces. Side by side, the chopped pieces are placed back into the soil, in a manner similar to when you first sowed them.
If your plants have not yet neared maturity, but you’re anticipating that you’ll be using the ground in another three or four weeks, you should dig the cover crops back in. This is because at least three weeks are required after the digging process before the ground can be used again.
How Long Does Green Manure Take To Decompose?
The still-young, green plants are quick to decompose into the soil after they have been dug in. Within the span of a few weeks, they release all the goodness stored within them. This releases nutrients and organic matter, amongst other things, in a form more readily available to the plants.
The purpose of stating that green manure plants should be dug back in three to four weeks before the ground is reused. This shows the time period required for decomposition – at least three weeks. The preferable waiting period, though, is thirty days.
This is just a general overall estimate and varies from plant to plant. For example, mustard seeds are faster to grow and decompose than other legumes. Just be careful to avoid digging plants as young as seeds or as mature as woody plants.
What Are The Advantages And Disadvantages Of Green Manure Compared To Compost?
Green manure is often neglected when it comes to alternatives to compost. However, Green manure is good enough to stand competition with compost in terms of benefits.
Advantages Of Green Manure Over Compost
Green manure can be prioritized over compost for the following reasons:
- Green manure provides soil coverage and prevents soil erosion as the roots of cover crops hold down soil, while compost is not as efficient in this regard.
- Green manure is better at controlling the growth of weeds.
- Mixtures of cover crops tend to have many different soil needs simultaneously; you can attain a lot more from mixtures than from compost, provided you combine the right crops.
- Compost can merely maintain nitrogen levels in the garden; it cannot convert atmospheric nitrogen into a fertilizer that plants can consume, while green manure can perform this conversion efficiently.
But Why Is Green Manure Not Used So Commonly? – The Disadvantages
There are many reasons as to why gardeners don’t use green manures, including:
- Using green manure is a lot more time-consuming than simply adding compost; you have to wait thirty days before you can even use your ground again when it comes to green manure.
- Cover crops provide competition to other plants in the area. As an example, cover crops, like any other plants, need water to grow. If this resource is limited in the area, the growth of your actual plants can be affected.
- Green manure plants welcome slugs and snails. These may harm your leafy buddies, and in extreme cases, an excessive amount may lead to infestation problems which can potentially destroy your plants.
- Using green manures can be expensive; you must monitor the costs involved and ensure the capital involved in growing cover crops does not exceed that of your actual plants to be harvested.
Green manure is a natural and organic way to enhance soil structure. Despite being less famous, it holds many benefits over compost and artificial fertilizers.
Green manure optimizes alkaline soil pH and increases soil fertility, and when the right mixtures are used, it can improve your soil and yields by whopping amounts.