Mulching at the next level

albedo, mineralisation, immobilisation, priming effects and more

Best to define it first then discuss it that way we know what I’m writing about:

A mulch is a layer of material applied to the surface of soil. wikipedia

A short definition yet one that brings up loads of stuff straight away. How deep a layer? What sort of material? What sort of soils?

In Permaculture we have the good old advice ‘comprehend and copy nature’. Nature in this context means the flows, processes, interconnections etc of natural systems. In the context of mulch we’re looking at something that mimics the process of leaf fall (plus petals, dead insects, bird droppings etc etc) and the litter layer especially:

• Their contribution to soil fertility through rainfall run-through (oxalic, fulvic and tannic acids)

• Their use by soil organisms as a resource

• the processes by which the material is broken down and shared

• the way the soil is protected from erosion (wind, sun, water)

We are mainly talking forest type systems here. Similar processes occur in prairie and savanha ecosystems but these tend to inspire the use of cover crops and strategies like roller-crimping and direct sowing into a standing crop more than mulch.

In general we are looking to cover the soil with something(s) that protects the soil from erosion, helps reduce evaporation and feeds soil life and crops in a similar way to leaf fall and the litter layer. It sounds simple but there are a few important parameters we have to understand.

Permaculture ethics

These ‘filter’ what we can and can’t use as mulch materials. Woven plastic or plastic sheets don’t care for the Earth nor people, it would be extremely difficult, if not impossible, to justify the use of these sheets in a Permaculture context. Nor could we adopt what is known as plasticulture as shown in the photo below.

Scale

This is a major factor, mulching an acre or hectare isn’t the same thing as mulching a small vegetable garden. There is no ‘one size fits all’, everything depends on the context, in this case the question would be ‘is mulching the best, most efficient strategy for bigger scale systems?’

Availability

This links in to the above point, what is available and in what quantities? If I’m going to grow that which I use as mulch then how much land do I need to do it? Can I grow a crop and use the residue as mulch? Cereal crops with the straw being used as mulch, orchards and woodland for woodchips and ramial chipped wood as examples.

I’m in a zone where there is no cereal culture so straw has to be imported from other places. We have plenty of trees but chipping wood involves a machine plus diesel or electricity so one can wonder if it is sustainable.

Sometimes it’s a good idea to think laterally about what we want to do and look around for unused resources. For example the price for wool has collapsed over the years yet this type of mulch has many advantages. The plants that grow in this mulch tend to be more resistant to pests, they have a better nitrogen content and are often more productive. Some people have also noticed that deer, deer, stags, etc. will not come towards strips mulched with wool. Other people have noticed much less damage from slugs.

Cardboard, newspapers and suchlike are an option, they do contain chemicals we might not want and come from an unsustainable economic system. That said they are quite effective in transforming a bit of lawn to a veg bed, “bad news to good food” as Mollison put it.

Stones? They have some strong points, soil protection as with other forms of mulch, and weak points, it is sometimes complicated to cultivate annual plants in stone mulch. We use this approach mainly around perennial plants, trees, shrubs, aromatic plants, etc.

Albedo and insulation

Albedo is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation). Wiki

A white shirt reflects some light and is cooler than a black shirt which absorbs it. This is the same with mulch materials, light coloured ones reflect more light and keep the soil cooler, darker ones absorb and warm the soil. So if I use a light coloured mulch in a cool-temperate climate I will be slowing the rate at which the soil warms up after the cold season.

We build houses using straw bales and one reason for this is that straw is a good insulator. The same is true when I cover the soil with it this can have the same consequence as the albedo. Wood chip have the same effect.

Climatic zones and the response of mulch materials

The above is just one climatic parameter. We also have to work out what impact a mulch material may have in a particular climatic and hydrological context. Geoff Lawton has a film on Youtube in which he explains how to make a forest garden. Unfortunately he doesn’t mention that what he is describing is for tropical zones. For example he recounts that 70% of the growing biomass is to be ‘cut and dropped’. In his context this material is rapidly broken down and then taken up by growing plants. In my context, temperate with hot summers the ‘cut and dropped’ material that Lawson uses would still be there years later, it breaks down much more slowly. The process here would be cut, chip and drop.

Carbon to nitrogen ratio

We talk about the ‘priming effect’ which is about how a material applied to a soil surface impacts the rate of decomposition of the soil organic matter

We have two processes, mineralisation, which is the decomposition of the chemical compounds in organic matter to soluble inorganic forms that can be used by plants. Immobilisation is the opposite, organic material is decomposed to organic forms which are used up by soil microbes and are thus unavailable to plants.

In a healthy soil the two process are both happening and the system turns around the carbon/nitrogen material of the litter layer or the mulch materials. Material that is high in carbon, like straw and woodchips (the effect is less with rameal chipped wood) stimulate microbial activity. When there are more than 30 parts of carbon to 1 part of nitrogen the microbes take up the nitrogen in a mineral form out-competing the plants which become nitrogen deficient, the so-called nitrogen gap, as the carbon is decomposed through respiration into Co2 the C:N ratio changes and when it falls below 25:1 then there is more nitrogen available than the microbes need and the excess is available to plants.

Mulch as habitat

The mulch materials can be a source of food for some forms of life and a habitat for others. Sometimes these can be beneficial, other times a total pain. Slugs love the cooler, damper conditions under mulch, Termites appreciate the cellulose material and the relative warmth of mulch layers during cooler seasons.

Designing

What I want is to protect my soil from erosion, maintain soil humidity, feed the soil flora and fauna, use locally available materials, grow crops and reduce my workload (irrigating for example).

I want to avoid, or mitigate, a nitrogen gap, avoid attracting those creatures that will destroy my crops and delaying post cold period soil warming. I also have to keep an eye on the weather, this year we have had exceptional amounts of rainfall and storms, a part of my mustard crop got literally washed away. In the 7 years I’ve been here we have had no trouble with slugs, this year they are everywhere. It is likely that heavy rainfall and storms will become the new normal for us so I have to adapt my system.

Flows

We have available materials and part of our design work is to decide how best to use them. In Permaculture we like to obtain as many benefits as possible and we do this by designing flows. For example:

vegetable waste → mulch (cold composting)

vegetable waste → hot composting → mulch

vegetable waste → biodigestor → digestate → vegetable gardens

vegetable waste → fish farm → eutrophied water → veg beds

vegetable waste → biodigestor → fish farm → eutrophied water → veg beds

The choice we make depends on our system, what is available and what we want. The opportunity cost of using the veg waste in a biodigestor is not having it to use as mulch. The benefits are methane gas and the soil enriching digestate, this lacks carbon so I will also have to find high carbon material to mulch the veg beds.

The nitrogen gap means delaying the flow of essential nitrogen to my plants. This means that the plants that will grow best in a new high carbon mulch are those which can fix their own nitrogen. These crops can then be followed by those that cannot. Lignin is mainly broken down by the fungi mycelium so I can use some material which is rich in mycelium as a first layer and then cover it with material that is rich in lignin.

Combining strategies

In temperate and cool-temperate areas we can grow an over-wintering nitrogen fixing crop such as winter field beans, forage peas or vetch. Lucerne can be used too, the top bit dies off from frost but the plant regrows in the spring. These cover crops protect the soil during the winter, fix nitrogen and their roots open the soil.

This cover crop can then be layed (flattened) and covered with a mulch layer. The dying cover crop will release nitrogen which helps to mitigate the nitrogen gap.

Mulching can be a really appropriate strategy, that said we must bear in mind other strategies that have similar advantages and be more adapted to a particular context. I have moved away from mulching and towards layed cover crops into which I plant or sow. The overwintering crop is dark green so helps my soils warm up, they have also been fixing nitrogen. I flatten them and then plant or sow into the dying cover crop. I also use overwintering clover to maintain a low plant layer into which I plant or sow after opening a farrow.

Lasagna growing beds are an interesting approach. They involves making several layers of different materials, high carbon layer followed by a high nitrogen layer. The entire lasagna bed acts to mulch out the lawn or whatever and can be seen as being a deep mulch layer.

Spot mulching as a strategy is something that we don’t seem to talk enough about. Instead of trying to cover an entire veg bed with mulch we create little ‘islands’ of mulch into which we can grow an appropriate plant guild.

As a final point it is worth combining mulch with other strategies that enhance soil fertility and the capacity of a soil to retain humidity. Biochar is an example of the latter but this brings us back to the opportunity costs! If I convert my woody waste into biochar I’ll need something else to mulch with.

The key points I suppose are that we must think before we act, get sufficient information, design our flows and reduce our workloads. Good advice for most things I suppose :-)