..::Vegetable Gardening using Raised Beds::..

	Green Culture Singapore Feature Article for August 2006

	Online on 29 August 2006

	Text & Pictures by Wilson Wong
	Edited by Lynnette Terh

Clay, the most prevalent soil type in Singapore's built environment, is a nightmare to any urban vegetable gardener. The ground is often so badly compacted that drainage becomes a problem and the roots of plants cannot develop properly. Waterlogging is hence a frequent occurrence in areas with clayey soil. Whenever the soil is dry, the ground is rock hard and difficult to work with. On the other hand, clayey soil becomes extremely sticky when it is wet.

To make vegetable and herb gardening in water-logged and suffocating clayey soils easier and more rewarding, the best way to go about it is to construct raised beds. Do you know whenever you start digging on a patch of soil and then adding in organic materials to amend its structure, you are effectively building a raised bed?

After we have prepared a patch for cultivation, the soil will remain slightly raised above those that had not been worked before. Via this action, we are actually building a temporary raised bed. Loosening the soil breaks up its compacted nature and adds in more air pockets, leading to the improvement of soil structure and drainage. This process increases the volume of soil in the dug area and will appear to be raised slightly above the ground.

Above: A temporary raised bed has no
frames on the sides to hold back the soil.

Generous amounts of compost and other suitable organic matter must be worked into the raised bed to keep its structure "open" for a longer period of time. The addition of organic matter, which acts as a soil conditioner, helps to improve the structure of the soil by preventing the clay particles from compacting back again. This will not only improve the water retaining and drainage properties of the soil, it will also create a comfortable living environment for beneficial soil organisms.

The addition of organic material to a tilled bed will cause it to be raised further. The types of organic matter that one can use include animal manure, compost, sawdust, leaf mould and coco coir.

Very clayey soils may require as much as an equal part of organic material to be mixed into them. First, break up the soil of the exisiting ground by using a hoe ("changkul"), preferably up to 50 cm below the ground level. Next, mix the organic material into the ground thoroughly, until an even soil/soil conditioner mixture is obtained. Compress the sides of the bed using a hoe to reduce the incidence of erosion.

Take note that fresh and semi-decomposed materials, such as sawdust, leaf mould and coco coir, meant to be soil conditioners continue to break down in the soil. This process will sap the ground and from plants, nitrogen. Hence it is prudent to add nitrogen-rich fertilisers such as chicken manure or fresh grass clippings to meet this demand.

Above: Soil conditioners (from left) - compost, saw dust, leaf mould and coco coir.

Some soil conditioners feed the soil by releasing nutrients as they decompose in the ground. Except compost and animal manure, the other three soil conditioners mentioned here add minimal amount of nutrients to the soil.

You can also choose to pile in good, fertile soil purchased from a local nursery into an newly constructed empty frame. Raised beds offer the option for the gardener to use soil tailored accordingly to his/her crop's needs.

The bed should be raised at a height of at least 20 cm. The width recommended would be about 1.5 m so that one can reach the other side of the bed at an arm's length without having the need to cross over.

Above: A plank placed between temporary beds to help
keep foot traffic off them and make walking less muddy.

The length can be as long as you can manage. For practical reasons, one may want to consider a convenient length. This is to make it easy for one to cross from the other side of the bed to the opposide side easily, without having to walk from long distances or stepping over the raised bed.

Bear in mind that heavy equipment and foot traffic must be kept off the beds so that the soil structure will not be compacted down again.

The main disadvantage of constructing temporary raised beds is that they flatten with time due to erosion brought about routine watering or heavy rains. Good soil in the beds gets washed down and a mess will be created on the walkways between beds.

Permanent raised beds involve the construction of a framework to contain the soil found in a temporary raised bed. The construction of such a framework will entail substantial amount of labour and much expense, depending on the type of material chosen. The finished product will be a worthwhile investment as the effects will be lasting - the considerable amount of labour put into the rebuilding temporary raised beds and loss of good soil - will be reduced, if not, eliminated.

A frame for a permanent raised bed can be made from the most common materials such as thick plastic sheets, wooden planks, ceramic tiles or bricks. Two factors you need to take into consideration are cost and durability.

Permanent raised beds offer an additional advantage where it allows the higher raised beds to be constructed that are not obtainable in temporary raised beds. Taller raised beds, akin to a planter box, make gardening friendly for the elderly who may have trouble squatting or bending down. They will also make the hobby more accessible to the handicapped on wheelchair.

After each crop, remains from a healthy crop can be worked into the soil to add organic material. Diseased plants should be removed and disposed of promptly. Additional organic material such as compost should also be mixed into the soil to condition the soil further.

The bed should be tilled and soil beneath the raised bed should be exposed to the sun for at least a week to allow sterilisation and aeration to take place. So do not flatten the bed surface after digging!

Crop rotation is recommended and fertilisation should be performed to replenish nutrients that have been consumed by the previous crop. Organic fertilisers such as animal manure and/or a complete synthetic granular fertiliser can be used. Remember to follow the instructions given on the label.

Chin H. F. 1999. Malaysian Vegetables in Colour: A Complete Guide. Kuala Lumpur : Tropical Press, 1999