Building Up Soil for a Nutrient-Rich Raised Bed

All photos by David Ashwanden

A lot of permaculture involves utilising waste streams and turning problems into solutions, and I often bring these into practice by looking at what’s available around me and how I can use it effectively.

With this in mind, having come across some old bath tubs, I decided to create some raised beds, building up the soil using a layer mulch recipe rich in a mix of nutrients.

When creating soil using mulch recipes, I find that more important than following a recipe exactly is to remember than a healthy soil has two main ingredients — carbon and nitrogen — and that these need to be present in a C:N ratio of about 25-30:1, for optimal nutrition. Most gardens will have many choices for carbon and nitrogen, and normally each ingredient on its own does not have the correct ratio, so you need to intelligently combine them with the aim of achieving the optimum ratio. It is a bit of an ‘art’, but one which doesn’t take much time to become proficient in. The chart on this page, showing approximate C:N ratios for various ingredients, will be a helpful guide to this end.

Examples of materials rich in nitrogen include:

Green plants of any kind, but especially those with nitrogen-fixing qualities such as

• Clover (Trifolium spp.)
• Alfalfa (Medicago spp.)
• Vetch (Vicia spp.)

Green plants of any kind, but especially those with dynamically-accumulating qualities such as

• Nettle (Urtica spp.)
• Dandelion (Taraxacum spp.)
• Comfrey (Symphytum spp.)

For more extensive lists on plants which are good for adding to soil mixes, see for example Toby Hemenway’s Gaia’s Garden (1)

• Manure — ideally already rotted down

Examples of materials rich in carbon include:

• Cardboard — ideally already a little decomposed
• Straw — also, ideally already rotting somewhat
• Woodchips — be careful what type of wood it is (see step 5 for details)

Soil does need other nutrients in smaller quantities such as potassium and phosphate, and so if you can find materials which have these micro-nutrients present as well your plants will be all the happier.

Examples of high-potassium materials include:

• Wood ash
• Bone meal (if you happen to have some lying around)
• Eggshells

And some high-phosphate materials include:

• Fish bone and bone meal
• Soy bean, nut and seed husks and shells
• Banana peel

However, although these micronutrients are essential to your plants’ health (see for example 1), it is not absolutely necessary to add them into the layer mulch as you can always use a nutrient-rich plant feed once your crops are planted. Indeed, there is some argument that it is better not to add any more phosphorous to your garden (2, 3) as you can easily overload your plants, which only require trace amounts of it, and potentially create an imbalance in the local ecosystem.

To make your raised bed, you do not need a bath tub or the exact same ingredients as I have, but below is the method and materials I used, which you can adapt to whatever you have around you.

Materials I used:

• Bathtub
• Block of wood
• Small amount of compost
• Small amount of sand
• Small amount of gravel (enough to cover the bottom of the bathtub)
• Various mulching materials:
  • Ash
  • Woodchips
  • Straw
  • Manure
  • Green plants: nettles, clover
  • Cardboard

Tools I used: 

• Wheelbarrow for collecting materials
• Shovel for collecting materials
• Watering can
• Garden scissors
• Large bucket 
• Electric drill (optional)
• Gloves to protect my hands!

Method

Step 1: Siting your raised bed

In many ways, this is the most important step, as the more you consider all factors affecting where you will put your raised bed, the more productive your yield will be. It is important to think about what you will plant in your raised bed, and how much sun, water, shelter, etc., they will need.

It’s also useful to consider the different energies passing through the place where you are planning to put the raised bed. Where does the prevailing wind come from? Are there any channels of energy running through the site? These channels could be from water running along an already-marked way, animals regularly crossing the site, or pathways habitually used by other humans. It is probably not going to be very energy efficient to block any of these channels, as whatever you are blocking will become frustrated. If you do feel the best place for the raised bed is across a channel of energy — be it for water, animals, humans or others — it may be beneficial to redirect the channel instead of simply stopping it.

In my bathtub bed I am planning to grow courgettes and peanuts, both sun-loving crops, so I positioned it in a south-facing window of the open greenhouse on the site I am working in. Courgettes are climbers so putting the bed next to the window means that they can easily be trained up the frames once they are large enough.

I placed the bed next to a pathway regularly used by people, meaning it will create visual pleasure and be more likely to be looked after as it will be visited on a daily basis; but not in the path as this would be in the way.

Step 2: Ensuring proper drainage

This is not so important if you are building your raised bed from the ground up, but with the bathtub I wanted to make sure that water would drain out properly. Using the block of wood, I propped the bathtub up slightly on the opposite end from the drainage hole, to encourage water to run out of the hole.

If you are using a bathtub or similar item and are really concerned about drainage, you could also drill a few extra holes in the bottom, though this is probably not absolutely necessary.

I then gathered some gravel and put a layer on the bottom of the bath, covering it to about 5cm deep. This is to prevent the water from flowing straight out following the "three S" principle: slow, sink and spread.

Bottom layer – gravel for drainage

Step 3: Prepare the layers

You can do this step at any time; maybe even building up a supply of mulch materials from your wanderings around the area where you live over a period of time. However long it takes, you need to collect them, and I used a wheelbarrow and shovel for this.

When preparing ingredients of layer mulch it is always helpful to break down the ingredients as much as possible, as the larger the surface area there is, the faster the materials can decompose and begin mixing together. I use this same principle for making any good compost, worm food, fertiliser or indeed food for myself: the more we chop up the materials in the preparation, the faster they can begin to be digested, whether it is by the bacteria in a compost mix or in our own gut.

So, for example, when collecting green plants for the nitrogen layer, I placed them in a large bucket and, using my garden scissors, finely chopped the plants. If nettles are in the mix it is advisable to use gloves for this activity.

Adding manure — not too close to the top if it’s too fresh

Step 4: Apply the layers

Now you can start building up the mulch, one layer at a time. I have found that it does not matter too much which order you put the materials in, as long as you are maintaining the correct balance of carbon and nitrogen. Place the materials one at a time on top of your gravel, ensuring that each successive layer completely covers the one below it.

I watered each layer as it went in, to facilitate the decomposition.

Watering the woodchip layer

As one of my layers was manure which was fairly fresh (only a couple of weeks old), I ensured I placed this at the bottom of the bathtub as I did not want the fresh manure to burn the plants.

For the woodchip layer, I made sure that the woodchip was from trees which are not too acidic and which do not have allelopathic qualities.

The straw layer

Step 5: Compost on top

When making a sheet mulch on top of a garden, cardboard is a traditional method for suppressing weeds from growing up through your layers and interfering with your crops. Whilst this is not such a concern in a raised bed, I opted to use cardboard for my top layer just in case there were unwanted seeds present in the materials I put in (this is especially possible with straw, which can annoyingly cover your garden with grass if not properly controlled).

After watering the cardboard, I covered the top with the final layer of compost, ready to put plants in. This layer need only be a couple of centimetres deep, especially if you are sowing seeds direct into the bed: the idea is that by the time the plants have grown enough to begin reaching down through the layers, your materials will have begun decomposing and mixing enough to provide a nutrient-rich welcome for them.

Covering the cardboard with compost

Step 6: Ready to grow

Now your raised bed is ready for plant additions and there is no stopping you. And once you have done planting that one, why not look around for other things which you can use for raised beds? You may be surprised how easy creating an instant mini-garden can be.

Ready for planting

References

1. Hemenway, Toby, 2009. Gaia’s Garden: A Guide to Home-Scale Permaculture. Chelsea Green Publishing, Vermont.
2. Schindler, David and Vallentyne, John R. (2004) Over fertilization of the World’s Freshwaters and Estuaries , University of Alberta Press, p. 1
3. Busman et al, 2002. The nature of Phosphorous in Soils. Dept of Agriculture, University of Minnesota. https://www.extension.umn.edu/agriculture/nutrient-management/phosphorus/the-nature-of-phosphorus/ – retrieved 12/5/2014