VIII. DOUBLE DIGGING

Soil preparation is the key to a healthy, productive garden. You have little control over the texture of your soil – it is either sandy or clayey or something in between. But there are several things you can do to improve the structure of your soil. One of them is aerating the soil by double digging. Another is adding organic matter to the soil in form of compost.

The goal of double digging is to produce a “living sponge-cake” in the soil, to a depth of 24 inches, with 50% pore space for air and water. (The other 50% of the soil is mineral matter, including rock fragments and a small amount of organic matter). In a new garden the sponge-cake may turn out to be only 15 or 18 inches deep, but the microorganisms, worms, plant roots and water will usually cause it to become a little deeper each year. After a bed is completed it will typically last for three to four years, depending on the climate.

A Double-Dug Bed = A Lazy Bed:

The double-dug bed requires a high initial input of labor. Digging one bed can take up to 8 hours.
We are creating, however, a permanently improved soil, which will be much easier to work over time. A bed with two feet of loose soil allows plant roots to grow evenly and provides a steady supply of nutrients to the rest of the plant. Water is able to move through the soil freely, and weeds are easy to pull out. The plant roots have so much loose soil to grow into that more plants grow in an even area; this means more food from a smaller garden.

In conventional gardening people often grow their plants in rows and walk around them to cultivate, weed, harvest, e.t.c. continuously compacting the soil right on the roots of the growing plants. Each year they must re-dig working against compaction they have created during the previous growing season.

With BIA we have a permanent raised bed and a permanent path to walk on in the growing area. In this way we maintain the aeration of the bed. This is a major reason that working with permanently raised beds will become easier to work than a conventional garden. It may take 8 hours or so to double dig a bed the first time, but in later years it can take only 20 minutes to dig the same bed.

Soil Aeration vs. Soil Compaction:

Air is necessary in the soil for plant respiration. Both the roots and leaves of the plant are breathing. Any thing that puts weight on the soil compacts it, reducing its amount of air. Heavy machinery, heavy rains and irrigation, and walking on the soil, are all common sources of soil compaction. In heavily compacted soils especially wet areas, roots fail to get enough air, causing plant roots to suffocate. Compacted soil creates physical resistance to root growth; plants roots have to push harder to grow. This physical resistance is lessened when we aerate the soil.

When we dig, whether it is conventional or double digging, we bring air into the soil. Aeration is the major reason that we practice digging. While conventional digging often loosens the surface soil, leaving a hardpan of unaerated soil underneath, double digging breaks up the hardpan, and loosens the soil below.

Advantages of double digging:

Increased microbial life: In addition to plant roots, many soil microbes are dependent on air. The microbes are largely responsible for the texturing, structuring and cycling of nutrients in the soil. One cannot have good soil without good microbial life.
Deep growth of roots enables intensive planting: With compacted subsoil roots grow downward, hit the hard pan and then grow laterally. Each plant needs more lateral space in which to gather nutrients and water. With double digging plant roots strive deeply downwards and take up less space laterally. Therefore plants can be spaced closer together –with this intensive planting can we can get 4 -6 times the yield from the same space.
Water conservation: With the intensive planting that double digging allows, plant leaves just touch one another at maturity - plants shade the soil, creating a ‘living mulch’, so keeping the soil cool and conserving water
Increased water absorption: Water penetration improves because of the greater amount of pore space and looseness of the double-dug bed. As you irrigate or as the rain falls, water soaks easily and quickly into the soil. Compacted soils, on the other hand, are unable to absorb water well - heavy rains especially run off and are wasted for agricultural purposes
Less Soil Erosion: Soil erosion becomes less of a problem as there is less water run-off and not as much valuable topsoil carried within it

The best time to double dig is in the spring, just when seedlings are ready to be transplanted into the bed. Seedlings grow best in a newly loosened soil. If you are starting a new bed, it is also possible to single dig (to loosen the soil 12 inches deep with a fork) and sow compost crops. Then in the spring, the double digging will be that much easier.

Starting a new bed:

Tools: hoe, plank for standing (roughly 6ft or 2m), measuring string, and wooden stakes of approximately 75cm.

The area to be selected is most important as it will be a permanent gardening area.
A raised bed should be about 1 ½ meters, so that the middle can be reached from both sides
The bed length can be whatever desired, however if too long you can spend a lot of energy walking around beds
Paths should be as narrow as possible, to maximize the growing space- just enough space to walk is fine for the paths
A good bed size is 6 x 1 ½ meters with a 45cm wide path, roughly 100 square feet
This is a convenient area for calculating fertilizer applications and crop yields

Before starting a new bed, make measurements and put in stakes to mark each corner, connecting the stakes with a string. Depending on the condition of your soil, you may also need to do one or more of the following:

1.Remove any grass and weeds, including their roots, from the area before digging- this material can go into a compost pile.
2.If the soil is dry and hard, water it well, thoroughly soak the bed to depth of 2 feet or 65 cm. if water is not available, waiting for the rain is advisable or one can try dry digging. After the bed is soaked. Its best to wait until the soil is a proper moisture for digging two days after watering. The soil should not be sticky and muddy nor should it be so dry that it won’t form a ball when squeezed in the hand.
3.Loosen the soil 12 inches deep with a spading fork.
4.Water lightly for a day or two (5 minutes or so per 100 square feet), or even longer if very dry/ hard.
5.Let the soil rest for 1 day.
6.Spread a layer of compost or manure on the area to be dug 1-3 in or 2-8cm, depending on the soil fertility.

Digging Your Bed:

Stand on a digging board so that your weight is distributed evenly and does not recompact the soil.
1.Across the narrow end of the bed, dig a trench 1 foot wide and 1 foot deep with a spade. Put the soil into buckets or a wheelbarrow or pile it on the ground. It can then be put in a bin to use for making compost and flat soil, or it can go back into the bed after double digging is completed to fill the last trench. The last trench will not really need this soil because of the increased volume of aerated soil in the bed, while soil that is used in making compost will be returned to the bed as part of the cured compost.
2.Loosen the sol in this trench an additional 12 inches with a spading fork. Dig the fork onto its full depth (or as deeply as possible) and push the handle downward so the fork tines lever through the soil, loosening and aerating it. If the fork will not go through easily, pull it out a little and then push down. You should go only as deep as the tool will loosen easily. The next time you double dig that bed, you will be able to go a little deeper. If the soil in the lower trench is dry, water the loosened soil well before continuing. It is easier to get water down into the lower 12 inches of soil at this point than it is after the bed preparation has been completed.
3.Dig out the upper part of the second trench 1 foot deep and 1 foot wide with the spade-Dig the spade in to its full depth (or as deeply as possible), lift the soil out on the loosened, aerated soil into the upper part of the first trench. Try not to mix the soil layers – the less their living quarters are disturbed when the bed is dug, the more ready they will be to get on with their business of providing nutrients to the newly planted seedlings. Move each spadeful of soil forward in the same way until you have dug across the entire trench.
4.Loosen the lower 12 inches of soil in the second trench with the fork.
5.Continue in this way with the third trench and as many more trenches as you need to finish the bed.
6.After the third or fourth trench (and every 3 to 4 trenches after that), rake the accumulated soil forward and level the double dug portion of the bed. There will be less soil to move round when you reach the end of the bed and have less energy to move it! (You will not need the soil from the first trench to fill in the last trench, if you are using that soil for compost and flat mix).
7.When you have loosened the lower part of the last trench, rake the whole bed level. (Add the soil from the first trench, if you are not using it for other purposes).
8.Spread a 1-inch layer of cured compost over the surface of the bed.
Sift it into the top 2-inches with a spading fork. It is a good idea to put compost on your bed and plant your seedlings as soon as possible after double digging. If you cannot transplant your seedlings immediately, cover the double-dug bed with a shade net and keep the soil evenly moist to keep microorganisms alive. Put the compost on the bed just before transplanting.
9.Once the bed has been dug, don’t walk on it. One of the reasons for double digging is to put air in the soil.

By Hand?

Some people prefer to let a machine do their digging for them. But your garden soil will not benefit from rot tilling. A rot tiller destroys the earthworms and other soil creatures that help make your soil fertile. It also compacts the subsoil and destroys the soils structure. Dr. Robert Parnes, in Soil Fertility, notes that if we are to be sensitive to soil processes, we should avoid rot tillers.

How Long Should it Take?

An expert can double dig an established bed in 1-2 hours but the first time you double dig, it may take you the whole day to prepare a 100 square foot bed, especially if the soil has never been double dug before. As you become familiar with what double digging is all about, and as your garden gets more used to being double dug, it will gradually take less and less time and effort to dig a bed. The important thing is to take your time and learn to do it well. Increased speed will come from experience and skill – not from rushing which will only tire you out.

Taking Care of Your back:
When double digging is properly done, your whole body weight does most of the work, with a little help from your knees and arms. If you feel excessive pressure on your back, you should stop and think about how to put less
pressure on it. Use your body weight to push the spade and the fork into the soil.

Place your foot on the spade or the fork so that it is under your arch just in front of your heel. Your body weight is used more efficiently that way. Lift the spade only as high as you need to and let the soil slide off on its own as you tip the spade. When loosening the soil in the lower trench, use your body weight, rather than your leg and arm muscles, to push the fork through the soil.

If double digging really seems like it will be too much for you, you might also consider single digging and using much wider spacing for the plants.

There are a number of shapes to make your bed:
1.Flat top with sloping edges.
2.Rounded bed without any abrupt edges.
3.Flat bed with a raised lip around the edge. This is highly recommended for dry conditions. It will catch rain most effectively and make irrigation easier.


ANOTHER METHOD:

In the Arumeru district GSC has been training in an adapted method, as described below:

1.Select an area approximately 5 ft. by 20 ft. (the bed can be any length, but it must be 5 ft. wide so planting and harvest­ing can be done easily from either side of the plot). Ideally, choose an area close to the house, compost supply, and a water source. The area may need to be fenced off to prevent animals from enter­ing.
2.Mark the bed with sticks and strings. Remove grass and weeds from the top soil.
3.Loosen the top soil one foot deep and remove the soil, placing it on one side of the measured area.
4.Loosen the sub soil one foot deep and place this soil on the other side of a measured area. Now you will have a trench two feet deep.
5.With the third layer of soil now loosen one foot deep but the soil is not removed from the trench. Instead mix it with compost, at an interval of one bucket full of compost per meter length of the trench.
6.Now return the subsoil to the trench, and intermix it with compost at the same amount as in step five.
7. Finally return the top soil to the trench, intermixing it with compost of the same amount as previous layers.
8.The bed now will have the shape of a semi-cylinder. The shape obtained increases about 25% of an area. The bed is covered by cry grass (mulch).
9.The bed is ready to be planted with any crop.

NOTE: We may advocate ‘triple digging’ in Tanzania to provide extra aeration for soil compacted by tropical rains.

TERMS USED:

Extensive agriculture-agriculture requiring a large area to produce a crop for example maize, wheat etc.
Intensive agriculture-agriculture requiring a smaller area to produce the same amount of crop.
Aerate-to supply with air.
Compaction-to pack together, to make denser. In the compaction of soil we reduce the pore spaces and aeration.
Hardpan-a harder layer of compacted soil that may lie anywhere from 6 inches to 2 ft below the soil surface. A hardpan makes it difficult for plant roots and water to penetrate downwards.
Respiration-the process of breathing. Soil aeration affects plants ability to respire.
Mulch-a layer of material placed on the soil to consume moisture and hold down weeds. Living mulch applies to plants themselves shading, protecting, and holding down weeds in the bed.