Soil - getting the mix right!

Last year my school purpose built two rectangular garden beds. The construction was done on top of concrete as most surfaces in Hong Kong are covered as such. This meant we went for a brick rectangle with inbuilt drains on either side. Inside the bed a layer of rocks was put down covered by a mesh to act as a filter and to help with drainage. Unfortunately, our clever design was let down by our ordered 'organic soil' which in fact turned out to be poor quality clay soil that set like a brick over the monsoonal summer.


Obviously, if you are putting in your own raised garden beds at your home or school then you can order in your soil mix - making sure that you choose the most appropriate mix. Different plants have different preferences for soil conditions but generally you want a combination of sand (for drainage), organic matter (for water retention and nutrient release) and silt.

When I was at university we would prepare our soil mix in the garden by selecting shovel loads from large storage piles of soil components and then combined them together with a cement mixer. If you had access to these kinds of stockpiles then we could focus on the perfect ration of components for each type of plant i.e. 1 shovel of this to 2 shovels of that. Most of us are not so lucky and have to try to work with what we have.

Probably more important than the exact composition of your soil mix are the following factors:

• How much drainage overall
• How many sunlight hours it recieves
• How acidic (the pH level) the soil is
  

The position, planning and soil choice of your garden bed will deal with how much sunlight and the drainage. Most plants have very specific pH ranges and getting it wrong can leave a plant more susceptible to disease. You can test the pH of your soil with a home testing kit or soil probe. You can make soil more alkaline easier than you can make a soil more acidic. To make soil more alkaline you can add wood ash or lime. To make soil more acidic you can add chemical mixes or go natural by using sphagnum peat. (http://www.thegardenhelper.com/soil.html)

In our concrete based, limited resource situation in Hong Kong we decided to look at a long term approach to soil improvement with an initial kick start.

To get our garden on track to better soil health, we removed around a quarter of the current, predominantly clay soil. Next we added three (4kg) bags of fresh-water river sand in a layer.

Then we spread organic, aged compost mix over the sand.


Lastly, we loosely mixed up the top layer. We still have to pH test to see if any bone meal etc. is needed.

This should be good enough to help this years students grow some crops. The plan is to plant in succession stages so that we minimise weeding, have good diversity and plant crops that replenish the soil. The students will plant some fast growing food plants, sow seeds for green manure plants and we will leave our nitrogen fixing plants in over the summer to recharge the bed.