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Growing Fertility

Paul Alfrey
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We're extending the Polyculture Project to include experimental perennial polycultures on various plots of our newly acquired land. Our aim is to develop models that are low cost to establish and maintain, can produce healthy affordable nutritious food and will enhance biodiversity.

To view this post with tables and more info visit below the link


We've been looking into fencing our plots, and how to meet fertility demands of the establishing perennial crops such as fruits, nuts, herbs and perennial vegetables without relying on animal manures and imported compost, and have come up with a polyculture that may meet both of these needs that we call the biomass belt.

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 polyculture is composed of mineral accumulating comfrey in raised beds, Nitrogen fixing ground cover sown into pathways and a Nitrogen fixing hedgerow.

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 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.

What follows is a design guide covering how to select a site for this polyculture, a close look at each component of the design and the species within each component, and what I hope are clear and concise instructions to build and manage this polyculture. At the end you can find a summary table and a brief description of how we have set up this polyculture in ourPolyculture Market Garden.

Design Considerations

General - The polyculture can be situated to provide boundary hedging or subdivision hedging within a property whereby the Nitrogen fixing hedge provides a permanent living fence and the comfrey beds run adjacent to the hedge.
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.
N.B. All of the plants we have included in the polyculture are drought tolerant (Trifolium repens to a lesser extent) and will survive long periods without water once established, but will not produce high yields of biomass in these conditions.

Subsoil Mining - Siting the biomass patch, specifically the comfrey beds, below compost toilets/manure piles and areas that are likely to receive non toxic leachate will help prevent the loss of nutrients draining off site.

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
Ground Preparation

Raised beds are a major part of our fertility strategy and when managed properly overtime they retain water and nutrients very efficiently. This polyculture includes two 1.3 m wide raised beds surrounded by 50 cm paths and a 70 cm wide raised bed for the Nitrogen fixing hedge. It's important never to tread on the soil of a raised bed and the above dimensions allow reach within the beds from the pathways. The bed length can be as long as best fits your site and needs.

Example Perennial Polyculture - The Biomass Belt - Bed Layout

To form the beds the area should be cleared of all plants, best achieved in most situations by sheet mulching with ample organic matter 6 months prior to planting. Pernicious perennials such as wild brambles - Rubus fruticosus and couch grass -Elymus repens should be dug out before mulching and the soil should be forked over as deeply as possible with a strong garden fork.

If you have a heavy clay compact soil it's best to double dig and incorporate plenty of organic matter (20- 40 L m2) into the comfrey beds before planting.

The right shape shovel for forming paths
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 10 cm layer of soil 50 cm 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 20 L of compost per m length of the comfrey beds and 5 L to the nitrogen fixing hedge bed. The compost should be applied to the surface and topped with a 20 cm layer of mulch.

Raised Beds in our Market Garden
The beds are now ready for planting and the paths for sowing. See below for details

If you have access to a stream or river you can divert a water stream into your site and use the paths for irrigation. When establishing the paths you can alter the depth and gradient to facilitate the required movement of water within the polyculture. The slower the water travels along the paths the less erosion there will be. Capillary action will draw the water deep into the beds and under ground.

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
Perennial Polyculture

I've ran out of room here, to read the full post see


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