Sustainable Agriculture Extension Manual

Deep soil preparation
(double digging)

Double digging is a method of deep soil preparation in which the soil is fertilized with organic matter and is loosened to a depth of 60 cm (2 feet).

This technology, coupled with composting or manuring, is used to improve soil fertility and structure. It is recommended for use in compacted, heavy, or rocky soils, and soils whose structure has been degraded by chemicals and frequent ploughing.

Location

In areas with high population density, where the average farm size is shrinking, food production must become more intensive and efficient. Double digging allows farmers with very small farms to produce more food on less land. It is not suitable for use on larger pieces of land.

Advantages

• It increases the amount of space in the soil for air and water, which in turn improves crops growth.
• It improves the soil structure, especially when compost or manure are added.
• It promotes earthworms and the activities of soil microbes.
• It breaks up soil crusts and hard pans, and increases the speed that water that can seep into the soil.
• Used with enough organic fertilizer, it helps the soil retain water, allowing crops to survive longer in dry situations.
• It allows crops to planted close together, which can result in yields four times the conventional average.
• If the crops are planted close together in a diagonal pattern, their canopy suppresses weeds and acts as a living mulch, further decreasing the amount of moisture lost through evaporation.

Disadvantages

• Double digging is labour intensive, but usually needs to be done only once.
• It may damage the soil structure if not done properly or if the beds are not adequately fertilized with compost or manure.

Requirements

• Hoe or digging fork, rake, digging spade.
• Wheelbarrow or big buckets.
• Digging board (a plank of wood to stand on while you are digging, so you do not compress the soil).
• An area of at least 1.5 m x 6 m (5 feet x 20 feet) for each bed.

Procedure

1. Measure the area you want to double-dig, marking the corners with pegs. The beds should be 1.5 m wide, and can be as long as you want. Lay out beds across (perpendicular to) the slope if the ground is not level.

2. Spread several wheelbarrows of compost or animal manure on the area you have measured out. Cover the soil completely with a layer at least 5-8 cm (2-3 inches) deep (Picture 1).

3. Dig a narrow trench, about 30 cm (1 foot) wide, and about 30 cm (1 foot) deep (generally until you can see the subsoil). Set aside the topsoil you have dug out; you will need it later (Picture 2).

4. Using the digging fork, loosen the subsoil along the bottom of your first trench to a depth of another 30 cm (1 foot) (Picture 3).

5. Now turn a strip of topsoil 30 cm (1 foot) wide and deep into the first trench, mixing in the compost or manure, filling the first trench completely and forming a second trench next to it (Picture 4).

6. Loosen the subsoil below the second trench, as in step 4 (Picture 5).

7. Repeat this process until you reach to the end of your measured bed. You will notice that the level of the topsoil has been raised up from all the organic material being incorporated, as well as through breaking up the subsoil.

8. You will have an empty trench at the end of the bed. Bring the topsoil you set aside in step 1, and put it into this trench (Picture 6).

9. Shape the bed so it has a flat top and gently sloping edges.

10. Plant immediately. Plant the crops at a close spacing in a diagonal pattern so they cover the soil quickly and protect the bed (Picture 7).

11. If you are not yet ready to plant, mulch the bed (Picture 8).