Garden soil for raised vegetable beds

raised vegetable garden beds

Dried leaves and weeds are good composting materials

When it comes to filling the garden bed many people immediately think about buying soil and having it trucked in, but the truth is that it is the inferior option.  The main reason that raised garden beds can grow so more vegetables in the space than in-ground beds is that they are filled with compost rather than soil.

The principle of no-dig gardens is to fill the raised bed with layers of material then leave them for three months to breakdown into  a rich growing medium for the vegetables.  There are a couple of main differences to making compost, the first is that the material is not normally turned, to compensate for this layers of material are quite thin to allow the necessary interaction between the ingredients. The second difference is that the material is often not deep enough to reach the temperatures required for true compost.

 Ingredients for the growing medium

The main categories of materials used in this soil creation process are usually referred to as ‘carbon’, ‘nitrogen’, ‘minerals’ and ‘water’.  By carbon we mean plant materials which contain a high percentage of carbon in their structure, similarly ‘nitrogen’ refers to plant materials that are rich in nitrogen.  Minerals are derived from rocks and are present in small quantities, the most important being lime which is used to regulate the pH level of the bed.  The correct amount of water is needed for the microbes to work their magic on the mixture and turn it into our growing medium.

Carbon materials are brown

Brown plant materials are generally high in carbon, but be aware that many of them take far too long to decompose to be used in the vegetable garden.  Anything that is closely related to wood should be shredded, combined with some very high nitrogen products and composted for at least a year.  Good materials for your no-dig garden bed include the following:

  • straw
  • autumn leaves
  • newspaper (soak and use in small quantities)
  • dry grass clippings
  • wood ash
  • brown hay
  • peanut shells
  • peat moss  (soak and use in small quantities)
  • dried weeds
  • shredded corn cobs
  • sugar cane mulch

Nitrogen materials are soft and brightly coloured

Fresh plant materials are generally high in nitrogen, they are often green but other colours are common too, occasionally they are even brown.  As these materials dry out the nitrogen is often lost with the moisture.  There is a wider variety of materials high in nitrogen available and using a wide variety will increase your success.

  • green leaves
  • green grass clippings
  • fresh weeds
  • kitchen scraps
  • manures
  • blood and bone
  • hair
  • feathers
  • tea leaves
  • coffee grounds
  • seaweed
  • fish meal

 Minerals and other ingredients for compost

These materials are added in small quantities and do not contribute to the bulk of the garden bed, they add trace elements.  Generally these are not materials that you will have around the house and garden, and if your budget or source does not stretch to these additives don’t let that hold you up. They can be added as top dressing when you have them available.

The most common mineral materials are these

  • gypsum (increase calcium, breaks up clay)
  • dolomite ( increase alkalinity, calcium and magnesium)
  • rock dust (add iron, boron, chlorine, cobalt, copper manganese, copper, zinc and molybdenum)
  • potash (increase potassium)
  • sulphur (increase acidity)
  • Epsom salts ( increase magnesium)
  • egg shells (calcium)

Water

Most people fail to add enough water to their garden bed and this can hold up decomposition of the material for months.  You should be able to squeeze a few drops of water out of a handful compost materials with your fist.  Whilst good soaking rain is the best means of watering your garden bed, adding water to each layer as you apply it will help get the process started.

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Why Use Raised Vegetable Garden Beds?

raised vegetable garden beds

Raised Vegetable Garden Beds

Raised vegetable garden beds are raised above ground level with some sort of frame. Common materials for the frame are wood, bricks, corrugated iron, stone and concrete. This sort of garden bed is ideal for back-yards and other gardens that are tended by hand, but probably inappropriate for 1000 acre farms that are tended by large farm machinery.

There are many reasons for using raised garden beds here are the main ones for me.

No-dig Gardening

The very first veggie garden I remember starting was when I was 8 years old. There was a semicircular mound at one edge of the garden, and I begged my father to be allowed to use it as a veggie garden. Eventually he gave in.

The first step of course was to dig up the grass, which I started with enthusiasm. But I found that the job was long and hard. I ran out of enthusiasm long before the job was completed, unfortunately the disinterest continued long past the digging process. In the end someone else finished the digging for me; the final result was a semi-circular garden that was no longer a mound. I kept my disappointment about this aspect to myself, but I am sure it added to my disenchantment with the whole process of gardening.

More recently I had an elderly neighbour who was an expert gardener. Despite being disabled he managed to till his vegetable plots to a fine, even texture, like sifted flour. I don’t even know how to do that, let alone have that kind of stamina. So when I heard about no-dig gardening I realised this was the method for me.

Raised vegetable garden beds are the ideal situation for no-dig gardening. You simply build or place a frame on the ground and fill it with a suitable growing medium, and your garden is ready to grow.

Scientific reasons for Raised Vegetable Garden Beds.

Digging damages soil structure. Soil structure is the arrangement of the soil particles and the spaces between them. Naturally soil form layers with more organic material at the top, and firmer rock based elements, like sand and clay, in the subsoil. The spaces between the soil particles are created by worms and other creatures that live in the soil and they allow for water and oxygen to be available to the plants, as well as room for root growth. Digging and tilling destroy the layers and the worm tunnels.

Raised garden beds can be warmer than vegetable plots in the ground. If the frame is constructed from material that absorbs heat, such as metal or bricks, they will absorb extra warm from the sun warming the soil more quickly. If the frame is made of insulating materials such as wood, it will help the soil retain heat over-night instead of losing it to the neighbouring ground.

Australian soil quality is very low, and here in the Blue Mountains top soil very thin, previous gardens I’ve had here had no more than 3mm (1/8 inch) of top soil. Not enough for veggies! Raised beds are filled with a rich growing medium, ideal for the high demands of food production. The bed walls also ensure that precious compost and soil is retained within the garden bed itself and not spread across paths and other areas.

Raised beds allow for a clear distinction of paths from bed areas. Walking on gardens can compact the soil and so inhibit the growth of plants. By surrounding small raised beds with designated paths there is no temptation to walk of the growing areas.

Raised Garden Beds provide easy access

Finally raised garden beds make it easier to reach the plants. Less bending down for access means less damage to the gardener’s back and knees. Garden beds can be raised high enough for access to people in a wheel chair, although I don’t need this provision.

My Raised Vegetable Garden Beds

My intention is to try beds of different heights to compare the outcome and find the optimal depth for this location. I will also try different structural components and different growing mediums. It won’t be a fully controlled comparison, but I hope to have some results that give enough information to improve the production of my raised vegetable garden beds next year.

 

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