Bed preparation and growing media guide

By Fiona Buining, Ainslie Urban Farm.

Introduction

Preparing garden beds before planting is very important. The soil is a living ecosystem that when well cared for will support plant growth and provide abundant harvests. Depending on the size of the garden bed and the soil you have to start with, this process may take some time. You don’t have to do the garden bed at once – you can get one part ready to plant and grow things while you work on the rest. Be patient.

The main aim is to build soil organic matter overtime and to keep replenishing it. Organic matter is anything that was once alive and is now dead. Examples are compost, fallen leaves, leaf mould, lawn clippings, lucerne, straw, paper and cardboard. Organic matter improves soil structure, improves water retention and drainage, feeds microorganisms and moderates soil pH over time. Microorganisms feed on organic matter releasing nutrients and producing humus. Humus holds and exchanges nutrients. The capacity of your soil to hold and exchange nutrients is called cation exchange capacity. Humus improves cation exchange capacity. Once your soil organic matter levels reach 10% the whole soil chemistry changes – for the better. This is a good thing to aim for.

Why add nutrients? Harvesting food essentially removes nutrients from the soil. Whilst many nutrients can be cycled and returned to the soil through the process of composting some are retained in the human body and some are excreted. Fertilisers, including manures, replace these “lost” nutrients and help build the soil and feed the plants. Where possible use products certified for use in organic gardens.

Breaking new ground or dealing with a weedy plot

Sheet mulching or no dig gardening is an easy way to start a new garden bed or rejuvenate a weedy plot. This method is great for growing pumpkins, zucchinis and cucumbers but will work for any plant. Sheet mulching may need to be repeated for several seasons to eliminate certain weeds such as couch and nut grass. Weeds can also be dug out and the topsoil shaken off into the garden bed.

Mow the grass or slash weeds. You can leave them on the surface or compost them.
Cover area with 6 or more layers of newspaper. Make sure the edges overlap by about 30%. Cardboard can be used, however, it takes longer to break down, and you need to remove any plastic sticky tape before putting down.
Place mounds of compost or boosted potting mix (see section below) on the newspaper. There will be one plant per mound therefore distance between mounds is based on the plant spacing.
Plant one seedling into the middle of each mound or if planting seeds plant 3 seeds per mound, then thin to the strongest seedling later.
Water in with seaweed solution.
Mulch area with spoiled lucerne, straw, pea straw, sugar cane mulch or fallen leaves.
Water regularly.
Plant roots will grow through the newspaper and eventually push through cardboard.

Double digging – French Biointensive method

This is a labour-intensive method to prepare a new garden bed or rejuvenate an existing one, that results in high yields in the first season. It is described in the book by John Jeavons called How to Grow More Vegetables. It is a method of deep soil preparation that is hard work. The first double dig takes 6-14 hours per 9.2 square metre bed. In subsequent seasons you can prepare the ground using a broad fork. Look it up if you are planning to use this method.

Esther Deans no dig method also called a lasagne garden

This method was developed by Esther Deans in the 1970’s in Sydney. It is an easy, efficient and effective way to prepare a garden bed. As with any sheet mulching method you may need to sheet mulch for several seasons to eliminate certain weeds.

Build a box with boards or bricks
Lay down 0.5 cm thick layer of newspaper, overlapping by 30 %
Lay down biscuits of lucerne hay
Sprinkle with organic fertilizer (blood and bone or Organic Xtra) or aged poultry manure.
Cover with a 20cm layer of loose straw
Sprinkle with fertiliser
Add 10cm layer of compost where you want to plants seeds or plants
Plant seeds or plants into compost
Next season add a layer of compost and or manure – no digging!

You can read more about Esther Deans method in her book: Esther Deans’ Gardening Book – growing without digging.

The next season and subsequent seasons

This procedure can be used after the first season of growing in any garden bed each time you “flip a bed” i.e. start new crops.

Use a broad fork or garden fork to gently aerate the soil. This process will loosen weeds. Resist the temptation to turn the soil over or break up clods of earth because this damages the soil structure.
Remove spent plants and weeds. Diseased plants and weeds such as couch, nut grass and Oxalis should NOT added to the compost.
Use a rake to form the bed into the desired shape.
Add lime, sulphur or gypsum if required (see below).
Add 2-5 cm layer of compost.
Add 250 grams per square metre of pelleted complete organic fertiliser.
Rake amendments evenly into the soil surface.
Plant seedlings and or seed.
Water in with seaweed solution.
Mulch to cover the soil surface and surround plants.
If you prepare the bed before you are ready to plant it is beneficial to cover the soil surface with mulch or old hessian sacks (ones that do not have plastic weaving). This is to reduce moisture loss and oxidation and volatilization of nutrients.

pH

Soil pH determines nutrient availability. Thus, if the pH is too high or too low plants cannot access the nutrients they require which leads to poor growth and deficiency symptoms. It’s a good idea to test the pH of soil, compost and mixes before using them. The ideal pH for growing most plants is between 6.0-7.0.

If the pH is less than 6.0 you can add agricultural lime or dolomite at the rate of between 100-400 grams per square metre depending on the soil type. Lime adds Calcium to the soil and dolomite adds Calcium and Magnesium.

If the pH is more than 7.5 you can add sulphur at the rate of 25-100 grams per square metre depending on the soil type.

If trying to change the pH, the quantities above will change the pH by 1 unit. Test the soil after a few months. Then you can add more lime or sulphur if required. Generally, it is easier to increase pH than decrease it.

Gypsum

If the soil pH is around 7.0 and you know that you need to add Calcium to the soil you can add gypsum without changing the pH,. Gypsum is also known as clay breaker. It helps the development of peds (soil aggregates) thereby improving the structure of the soil. Gypsum can be applied at the rate of 100 grams per square metre.

Soil testing

Soil testing can provide useful information about the base mineralogy of your soil which is useful in determining which amendments need to be added. For example, if your soil is naturally deficient in Boron it can be added thus avoiding Boron deficiency symptoms in plants such as beetroot, chard and brassicas. In addition, if you are eating a substantial amount food from your garden any plant nutrient deficiencies can over time manifest as human nutrient deficiencies.

Potting mixes, seed raising mixes

It is worth buying the best quality mix that you can find. These are generally labelled “premium”. Boosting potting and seed raising mixes by adding 250 grams or a good cup of pelleted complete organic fertiliser per 25 or 30 litre bag makes huge difference to growth. The organic fertiliser will release slowly, providing carbon to feed microbes, and nutrients to the plants. Of course you can make your own mixes based on compost.