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.

Making a loaded garden bed

One technique that is used in sustainable vegetable growing is the idea of a loaded garden bed. This technique enables maximum nitrogen input with minimum disturbance to the soil and good moisture retention.

The way I have seen it used was when two lettuce plots were being harvested at one time. These were the steps taken to prepare the loaded garden bed:

1) All the outer leaves and root bases were discarded onto one of the beds.

2) The lettuce waste was ripped into smaller pieces with our hands.

3) A light layer of compost was spread over the leaves.

4) Some newspaper was soaked in water and then layered it in sections (approximately 5 pages thick) over the entire bed including the sides.

5) Straw was then added as a mulch over the top of the newspaper.

Once this process was complete we had the option of immediate pl

anting or leaving it for a few weeks. We planted immediately with seedlings that had been grown in individual pots. Using seedlings from pots was to ensure that the plant had enough soil in its immediate surroundings so that there was room for the roots to spread out. If we did not provide enough soil then the roots would come in contact with the green lettuce leaves before they had enough time to decompose and the roots would not be able to grow into it.

The lettuce leaves become a source of green manure or nitrogen fertiliser for the new crop.

Sustainable Agriculture

Saturday was the first day of a two day workshop at a local educational farm. The topic is sustainable agriculture, below is some information from about this. I do not claim to be an expert and am happy to respond to any comments or answer any questions anyone wishes to post.

Firstly, Sustainable - means that you can continue to do something over a long period of time using a set/or consistent amount of energy input. Sustainable agriculture then specifically refers to farming practices that do minimum long term damage to the environment whilst providing maximum yield and profit for the farmer. Meaning that the farmer can continue to grow produce on the land and the land can continue to support the growth. Sustainable agriculture does not have to be purely organic but they do tend to due to the energy that is required to manufacturing chemical pesticides and fertilisers.

Modern or Industrial agriculture is not considered sustainable as it is an input-->output system rather than a cycle. When land is first cleared and crops planted the soil is in typically (Mediterranean climate) good health and contains good quantities of organic matter and nutrients that support the development of crops. However, upon harvest the plants are removed from the land taking with them valuable nutrient that were once in the soil. The soil becomes nutrient deficient and chemical fertilisers are then input to enable growth of crops.

In addition to the loss of nutrients, soil also beings to lose its structure as topsoil can be eroded by wind and/or rain run off and heavy machinery can compact the soil. This means that further fertiliser may be needed to compensate for the loss of topsoil and mechanical ploughing may be needed to loosen the soil to allow growth.

Another issue is with pests as the mono-culture crops (one type of plant) also require large input of chemical pesticides. Having a large area of just one species of plant will attract and cause a boom in pest populations. It is basically like a kid in a candy store or finding an oasis in the desert. People start growing a pests food source on mass in one place then it is no wonder that the pest organism would move in and intend to stay. A loosing battle then begins to eradicate the pest population. Using chemical pesticides over a long period of time can then lead to the pest developing genetic resistance to the chemical and then further chemicals being needed to kill the pest.

In a sustainable set-up the goal is to make a cyclic system where soil health is the priority so that crops can continue to grow on the land long term. The main way to do this is to minimise soil disturbance and maximise return of organic matter and nitrogen to the soil. It is about recognising the inputs and outputs of the system and making the process more efficient so that nature does the work itself without requiring manufactured chemical additions.

However, soil health on its own is not enough to create a sustainable system as there is still the issue of pests and disease. Crop diversity (growing many species of plants together) and companion planting is one way to deal with this problem. It involves having different crop or non-crop plants planted in the same area so that large smorgasbords of pest food are not on offer and thus avoiding plague proportions of pests. Also, if you learn to plant as companions then some planting combinations will stop certain pests from attacking or minimise damage inflicted upon crop plants. For example: planting Rosemary with cabbages will repel cabbage flies. Crop diversity can also be of benefit to the farmer financially as not all their effort is invested in one plant that could have a poor season and income can potentially become year-round as plants could fruit or mature at different times meaning a more frequent sale of products.

One problem or limitation with sustainable practice is that it is labour intensive and hard to apply to large scale farms. This means for our food production to become sustainable we would need food to be produced on a more local level and for more people to be actively involved in the production of the worlds food supply.