What is the Biomass Belt?
The biomass belt is a simple closed system, perennial polyculture dedicated to growing mulch and fertilizer for annual and perennial crops.
How does it work?
The comfrey is grown in raised beds for biomass and can be cut from 4-7 times each year with the material being used to make liquid fertiliser or used directly as mulch. The deep rooted comfrey mines nutrients deep in the subsoil that would otherwise wash away with the underground soil water or remain inaccessible to other plants. Some of these nutrients are relocated within the comfrey leaf that when cut and applied as mulch or converted into liquid fertiliser, the nutrients are delivered back to the top soil and made accessible to crops and other plants.
The nitrogen hungry comfrey are themselves fed with the biomass from nitrogen-fixing plants, that through a partnership with soil micro-organisms can convert atmospheric nitrogen into nitrogen fertilizers useful to themselves, but also becoming available to neighboring plants. For more on nitrogen fixation see here.
The pathways between the beds are sown with a nitrogen fixing ground cover and mown following a comfrey cut with the trimmings applied to the comfrey beds. The hedge composed of nitrogen-fixing shrubs running parallel to the comfrey beds, once mature, is trimmed at regular intervals (once or twice per year) with the trimmings deposited onto the comfrey beds.
Each time the path vegetation and hedge are cut, root tissue underground is shed in to the soil providing significant quantities of organic matter and nutrients to the plants.
In summary, the unique ability of the comfrey to feed deep and produce copious quantities of biomass is utilised to provide nutrients to main crops whilst the pathway ground cover and hedgerow's unique ability to fix nitrogen is utilised to provide nutrients to the comfrey.
Light demands - For optimal growth the polyculture should be orientated along the west to east axis and be to the north of any light demanding crops to reduce shading. The plants we have selected below will grow in partial shade and on other orientations, but will yield less biomass as a result.
Water - Adequate irrigation is a key to healthy and productive plants. This polyculture is not well suited to semi wetlands and areas with a high water table and will not thrive in very dry areas with no access to irrigation. In dry land/climate, selecting a position for the polyculture that requires as little irrigation as possible is essential and can be achieved by planting on contour and using simple earthworks to keep rain water around the root zones of plants.
Proximity to crops - Consider the distance between your beds and where you need to apply the mulch or prepare and store the liquid fertiliser. If growing comfrey for mulch it probably doesn't make much sense having to haul the material over large distances.
Species Selection - Species selection should take into account the following;
- Climatic compatibility with the site
- Drought tolerance
- Speed of growth i.e fast growing
- Tolerance of hard pruning
- Benefits to wildlife
If you have a heavy clay compact soil, it's best to double dig and incorporate plenty of organic matter (20-40L per m2) into the comfrey beds before planting.
Once you have cleared the whole area of weeds and forked it over to relieve compaction, mark out the bed shapes with string and dig out a 10cm layer of soil 50cm wide to create paths around the bed, applying the soil to the surface of the planting area and thereby creating a raised section of earth that will be your bed. A flat bottomed shovel is a good tool for this job.
If you are not sheet mulching, remove all weeds, fork over the beds, cut out the pathways and apply 20L of compost per metre length of the comfrey beds and 5L to the nitrogen-fixing hedge bed. The compost should be applied to the surface and topped with a 20cm layer of mulch.
The beds are now ready for planting and the paths for sowing. See below for details
Here's a closer look at each component in further detail based on a version of this design we have implemented in our Market Garden.
The Polyculture Components
- Nitrogen Fixing Hedge
- Nitrogen Fixing Ground Cover
- Comfrey Beds
1. Nitrogen-Fixing Hedge
The nitrogen-fixing hedge supplies a significant biological source of nitrogen and biomass, habitat for wildlife including a number of beneficial species, and makes an excellent living boundary/fence on the perimeter of a site or as a subdivision within a site.
The hedge is composed of three or more different species of nitrogen-fixing shrubs.
Planting out the Nitrogen-Fixing Hedge
Spacing - The diagram below shows the spacing between plants and distances from the edge of the beds. In order to increase nitrogen-fixation and produce biomass sooner, the young shrubs can be planted 50cm apart and thinned to 1m apart as they mature. If using mature potted starting plants they can be planted 1m apart. Generally it's much better to plant out younger bare root whips over old pot bound plants. The whips soon outgrow the older plants and are far more resilient once established, not to mention the considerable savings in cost.
Maintaining the Nitrogen-Fixing Hedge
Formative Pruning and Trimming - If planting out single stemmed whips, the following formative pruning is necessary during or after planting out. Cut up to 1/3rd off from the top of the whips. The cut should be made just above a node and at a 45 degree angle. This will encourage the plant to form a multi stemmed crown. The following year the tips of each of the multi stems can be cut again encouraging further branching. Once the plants have developed full crowns they can be trimmed to reduce width and to the desired height at least once per year. After trimming, the arisings can be raked onto the comfrey patch.
The best time to trim the hedge is early winter when the comfrey is dormant and mid summer after the comfrey has been cut. The summer cut avoids disturbing nesting birds and provides mulch during the dry season.
Feeding - These plants will not require any feeding, however mulching with a 30cm diameter mulch mat or card/straw mulch whilst the plants are establishing for the first 2-3 years will be beneficial both in reducing irrigation needs and preventing weed competition.
Irrigation - The first season after planting, irrigation should be applied when the soil beneath the mulch is dry. In following years irrigation is only necessary during very dry summers and should be applied before the plants reach wilting point. The plants will grow much faster and produce significantly more biomass if they have a good supply of water.
Native wild plants that emerge around the shrubs (apart from grasses) can be left to grow freely once the shrubs have matured and if they are not directly competing.
Nitrogen-Fixing Hedge - Species Overview
Caragana arborescens - Siberian Pea Tree / Pea Tree / Pea Shrub
A deciduous shrub originating from Central Asia belonging to the Fabaceae (legume) family growing to 5-6m high and 4m wide with an upright habit. It grows vigorously. Flowers are borne from buds on the previous year's wood and are typical of flowers from this family. Flowering occurs in May. Pollination is via bees, usually wild bumble bees. Pods develop from flowers - looking like small pea pods, they are 4-5cm long. The pods ripen to amber or brown from June -July onwards and seeds fall by August. The plant is extremely hardy tolerating winter temperatures of -40 Hardiness zone 2. Prefers a continental climate with hot dry summers and cold winters.
Uses: The young pods are eaten as a vegetable, lightly cooked. The pods become tough later in the season. The seeds are rich in fats and proteins (12% and 36% respectively) about the size of lentils and can be cooked and used in any way that beans are used (the cooked flavour is somewhat bland, so best used in spicy dishes). The young raw seeds have a pea-like flavour although it is not clear whether they should be eaten raw in much quantity. Widely used in windbreaks and shelter belts and used in wildlife-erosion control plantings stabilizing soil with an extensive root system. Good wildlife fodder and can be used to as poultry food. A fiber is obtained from the bark and used for rope making.
Nitrogen-Fixing Potential: The species is classified by USDA as being a medium nitrogen-fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m² or 0.014g /m2
Biodiversity - The shrubs will begin to flower in the 4th or 5th year after planting and are attractive to a wide range of pollen and nectar feeding invertebrates from Apil-May. In time as the hedge thickens up with regular pruning, suitable nesting habitat will form inside the lower part of the hedge. Birds such as Wren - Troglodytes troglodytes, Chiffchaff - Phylloscopus collybita and Robin - Erithacus rubecula are commonly found in dense low hedging. These birds can help to keep common vegetable pest populations low.
Planting Material – Five year old plants will provide an instant hedge effect but can prove to be expensive when planting out large areas. 2nd-3rd year whips are cost effective and with proper pruning and some attention during the first few years of development will quickly fill out.
These plants are easy to grow from seed. The first two years of growth are slow and they are best kept in nursery beds until approx 30-50cm tall when they can be planted out into their permanent positions.
We supply seeds and 2-3yr Caragana arborecens plants from our plant nursery. Click here for more info.
Cytisus scoparius - Broom / Scotch Broom / Common Broom
A hardy nitrogen-fixing shrub native to Europe growing to 2.4m by 1m at a fast rate. Its bright yellow flowers appear in spring, from May to June and attract a range of invertebrates. A versatile plant well suited to many soil types that can grow in nutritionally poor soil. Prefers a sunny position but tolerates some shade and will succeed in exposed conditions including maritime exposure. A deep root system means they are very drought tolerant once established and grow well on dry banks. Very tolerant of cutting, it regenerates quickly from the base.
Nitrogen-Fixing Potential: The species is classified by USDA as being a high nitrogen-fixer with estimated yields of +160lbs/acre or +72kg/4050m² or 0.018g /m2.
Propagation: The plant is very easy to grow from seed and large quantities of plants can be grown very quickly. Seed harvested in the summer can be sown straight after picking and overwintered indoors (or protected and planted out the following autumn). Seeds germinate better after soaking in warm water for 8-12 hrs prior to planting.
Elaeagnus umbellata - Autumn Olive / Autumn Elaeagnus
Uses: Edible fruit raw or cooked which is very tasty and can be made into jams, preserves etc. The fruit contains about 8.3% sugars, 4.5% protein, 12mg per 100mg Vitamin C. The harvested fruit stores for approximately 15 days at room temperature. It can be used as a hedge plant and tolerates maritime exposure succeeding in the most exposed positions. The nectar from the flowers is attractive to bees comprising 28% sugars. The plant is used as a nurse tree, when planted with fruit trees it is reported to increase the overall yield of the orchard by 10%. It can also be grown as a biomass crop on a three year rotation.
Nitrogen-Fixing Potential: The species is classified by USDA as being a medium nitrogen-fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m² or 0.014g /m2.
Propagation: Seed - best sown as soon as it is ripe in a cold frame. It should germinate in late winter or early spring, though it may take 18 months. Stored seed can be very slow to germinate, often taking more than 18 months. A warm stratification for four weeks followed by 12 weeks cold stratification can help. The seed usually (eventually) germinates quite well. Prick out the seedlings into individual pot as soon as they are large enough to handle and plant out when they are at least 15cm tall.
These plants can be grown from seed although may take up to 18 months to germinate. Once they do germinate they can be ready to plant out 7-8 months later and can starting providing good quantities of biomass within a few years.
N.B I'm not sure how the nitrogen-fixing potential of the above species are calculated and the relationship between regular pruning and N input into soils, but it's something I intend to look into further. If anybody can clarify this matter please comment below.
2. Nitrogen Fixing Ground Cover
A perennial nitrogen-fixing ground cover should be established quickly in the pathways and on the vertical edges of the bed to protect the soil from erosion and provide a further source of nitrogen input to the polyculture. I have experience using White Clover - Trifolium repens for this purpose but there may be other suitable species.
Sowing the Ground Cover
Following digging out the pathways, slightly loosen the surface of the path with a rake and hand sow the white clover seed onto the surface of the path and into vertical edges of the beds at a rate of 0.75g per metre length of bed. Tamp down with a shovel and water with a fine head sprinkler. The seed should germinate within 5-6 days. The best time to sow is late April - mid June. Keep a close eye on the emerging seedlings and irrigate when below the soil surface is dry. Keep foot traffic to an absolute minimum whilst the cover is establishing.
Maintaining the Nitrogen Fixing Ground Cover
Nitrogen Fixing Ground Cover Species Overview
Trifolium repens -White Clover, Dutch Clover, Ladino Clover
Uses: White clover has been described as the most important forage legume of the temperate zones. Besides making an excellent forage crop for livestock, clovers are a valuable survival food: they are high in proteins and although not easy for humans to digest raw, this is easily fixed by boiling the harvested plants for 5–10 minutes. Dried flower heads and seedpods can also be ground up into a nutritious flour and mixed with other foods, or can be steeped into an herbal tea. The plants ability to spreads aggressively by creeping stems makes is a good ground cover plant. The plant is also used as a companion plant when undersown with cereals or tomatoes.
Nitrogen-Fixing Potential: The species is classified by USDA as being a high nitrogen-fixer with estimated yields of +160lbs/acre or +72kg/4050m² or 0.018g /m2.
Other sources state up to 545kg of N per hectare per year is possible.
Propagation: Best propagated by seed. Pre-soak the seed for 12 hours in warm water and then sow in spring in situ. Division is also possible in the spring and autumn.
We supply Trifolium repens - White Clover seed from our plant nursery - Click here for more info.
3. Comfrey Beds
The comfrey beds provide copious amounts of biomass rich in nitrogen, phosphorus, potassium, calcium and many other valuable plant nutrients that the plants mine deep in the subsoil. The plants can produce 2m long roots that harvest these nutrients and relocate them into the plant biomass back on the surface. The biomass is used to feed crops in the garden
Planting and Maintaining the Comfrey Patch
Feeding - After you have cut the comfrey, mow the pathways between the beds and empty the contents to the base of the comfrey plants. Any mowings from the surrounding area can be used in a similar way. As the Nitrogen fixing hedge establishes, they should be trimmed to the required height and shape with the trimmings also applied to the comfrey. This can be done once a year for the first four years but biannually as the hedge develops. For extra feed and to increase yields undiluted urine can be applied to the plant at a rate of approx 1L per plant twice per growing season. Urine is an excellent feed for comfrey.
Irrigation - Comfrey will produce more biomass if irrigated and in dry climates it's essential to irrigate. Comfrey plants wilt very fast in hot conditions and will stop photosynthesising at this point.
20L m2 / week should be more than adequate. The beauty of biological systems are that if they are managed correctly, each year the soil in the beds will improve resulting in less water required and higher yields.
Comfrey Beds Species
Species - Symphytum x uplandicum - Comfrey 'Bocking 14'
The Biomass Belt in Our Market Garden
The hedge of fast growing nitrogen-fixing shrubs are planted into 70cm wide "no tread" bed rows that run parallel to 1.30m wide "no tread" bed rows of Comfrey 'Bocking 14'. In between the beds are 50cm wide pathways that are sown with a nitrogen-fixing ground cover such as Trifolium repens.
The beds run lengthwise west to east with the comfrey beds on the south and the nitrogen-fixing hedge on the north. This is to maximise plant exposure to sunlight. The beds and paths are laid out on contour with the beds elevated around 10-15cm above the surrounding pathways. We irrigate the beds by diverting stream water into the paths and dropping sandbags at various ends to control the level. We can raise the water level in paths to approximately 15cm resulting in the thorough absorption of water into the bed soil. Water is drawn up towards the surface of the bed via capillary action.
We produce liquid fertilser from one of the comfrey beds with the other bed used for harvesting comfrey root cuttings that we sell from our bio-nursery.
The liquid fertiliser provides a great supplemental feed in our young raised beds. As the garden beds develop overtime, soil conditions will improve and the liquid feed is less necessary. At this point the comfrey biomass will be applied directly to the beds as a mulch.
A word of awareness. This is not a tried and tested model, rather it has been formulated based on decades of working with plants and running successful productive ecosystems. and is largely based on extrapolating from my experience of what works on a small scale. We are keeping records of the polycuture yields to see how well they perform and will publish these at the end of each year.
Upcoming Development of the Polyculture Project
Acknowledgement and gratitude to Ken Fern for establishing the excellent plants for a future database that we use extensively both for rearing and caring for our plants and for writing our plant profiles.
Martin Crawford's Trees for Gardens, Orchards and Permaculture