Many beginning gardeners wonder, “How deep should I till my garden?” As with most gardening questions, there are probably as many answers on how to till a garden as gardens. However, there are some rules of thumb which can help you establish a thriving and prosperous plot from the start.
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Why is tilling important?
To understand why tilling is important, consider what plants need to grow and thrive. All plants need light, air, water and nutrients. The leaves above the ground furnish the plant with light and air. The roots below the ground furnish the plants with water and nutrients. The farther and faster a seedling can send its roots, the faster it can take up the water and nutrients it needs.
The first reason for tilling, then, is to allow your growing plants a good, loose environment into which they can grow roots. A plant can, eventually, drive its roots into the toughest, most compacted soil, but the time, effort and resources that a plant expends upon pushing roots into unfavorable ground is lost in developing good leaf structure, and ultimately, good fruits and vegetables. Yield will suffer, and getting a good yield out of your garden is the goal in the first place.
Besides breaking up the soil, good tilling can kill any weeds that have sprouted, although it may also serve to bring any weed seeds that are deeper underground to the surface where they can generate. It can also help remove stubble from the surface of the field, and help it decay into nutrients the plants can use.
Regular tilling also presents an opportunity. You can use the occasion to work in compost or manure, or other soil amendments. Time and effort spent improving garden soil is never wasted.
How deep to till a garden
Tilling depth depends upon three basic factors:
- The kinds of plants you intend to plant in the garden;
- The kind of soil you have in your garden;
- Whether the area has been used as a garden before.
Tilling depths based on kinds of plants
Different species of plants send roots to different depths. Since one of the reasons for tilling is to allow plants to develop good root structures, your garden plan and your tilling should take into account the plants, and the depth of loosened soil they require.
Plants with shallow roots should have tilled soil to a depth of between 6 and 10 inches. These plants include most of the vegetables where we harvest the roots, like beets, onions and garlic, or radishes. Other plants with shallow root systems include brassica, like cabbage, broccoli, cauliflower, and kale. Lettuce also falls in this category.
Plants with medium depth roots should have tilled soil to a depth of between 10 and 24 inches. These plants include nitrogen-fixing plants, like peas and beans. Carrots also fall into this category. Eggplants, peppers and tomatoes also benefit from tilling to this depth. Potato roots will also grow to this depth. The potato tubers, which we harvest, grow in a zone of 10 to 12 inches below the surface, and so this depth should definitely be loose soil.
Plants with deep roots should have soil tilled to a depth of at least 24 inches. These plants include most kinds of squash and pumpkins. Melons and cucumbers also send down deep roots, as do grapes.
Asparagus crowns constitute a special case for deep tilling, as they are best planted in a trench at least 6 inches deep, with 12 inches being the traditional ideal. The root system then grows below the crown, and so the soil below the trench also needs to be loosened. The crown, once placed, is then covered with enough soil to protect it. As the plant grows, more soil should be added until the trench is filled. Tilling the asparagus bed after planting will disturb the plants.
Tilling depth based on soil type
Some soils, with clay-like properties, are heavily compacted, and need deeper tilling to be broken effectively. Other soils, with more loam, are lighter and more easily broken. These soils can be effectively tilled at a shallower level.
The moisture level of the soil affects the depth of seeding. This, in turn, affects the depth of tilling. Fine, loose soil tends to draw water towards the surface through suction than heavier, compacted soil. Sandy soil tends to drain water downwards. Seeds can be planted at a slightly shallower depth in loose soil than in heavier soil or sandy soil. In general, seeds should be planted at the depth where soil moisture is at “field capacity.”
You can determine the moisture level at your proposed seeding depth by taking a bit of soil in the palm of your hand. If it is at optimum moisture, you should be able to form it into a compact ball with your hand. Soil with optimum moisture will also leave a bit of dampness in your hand when you hold it.
Preparation and tilling a garden bed for the first time
Opening a new garden bed for the first time presents special considerations for the gardener. While grass roots do loosen the soil to a certain extent, the underlying bed is usually quite compacted and can benefit from a deeper tilling than usual.
Since most vegetable plants have a difficult time competing with grass, you should either remove the sod first, or carefully take every bit of grass out of the bed to ensure that none of it re-roots. The grass should end up in your compost bin.
A good rule of thumb is to till a completely new garden bed deeper than usual, to a depth of about 8 to 12 inches. A tiller with an adjustable height can till to the desired depth, or you can use the French intensive gardening method, and double dig the soil with a fork to the desired depth.
In subsequent years, less effort to keep the bed in good shape is required, and ordinary tilling of between 4 and 8 inches should suffice.
Double digging and the French intensive garden method
The market gardeners around Paris and other major French cities are able to attain astounding yields through intensive cultivation of very small plots. The gardeners rely upon a variety of techniques to produce their crop, but deep tilling is a central part of the strategy.
A gardener using this method begins by laying out the desired bed size, at least 3 by 3 feet, but, as a practical matter, as long as one wishes. In lieu of conventional tilling, the gardener then sections the bed into one foot lengths. The soil is removed from the first section, to a depth of one foot, and put to the side.
The soil between one and two feet deep is then turned with a fork. The soil from the next foot of the bed is now loosened with a fork and put at the head of the bed. The process is repeated until the entire bed has been dug to a depth of two feet. The soil from the head of the bed is now put on the top layer at the foot of the bed.
Advocates of this method argue that the roots of plants can extend as deeply as they wish, and, with the other techniques of the system, yields are much higher than with conventional row gardening.
Drawbacks to deep tilling your garden.
Any sort of tilling disturbs and destroys the soil structure. This can disrupt beneficial organisms that improve soil like earthworms. It can also bring undesirable weed seeds to the surface.
Paradoxically, conventional tilling also alters the moisture holding capacity of the soil. Moisture spreads through soil by capillary action. When the soil is broken up, the capillary action is disrupted, soil moisture can be lost through evaporation, and moisture may be slow in reaching the seedbed where your plants germinate. In a dry year, this can affect your yields.
Many beginning gardeners wonder whether a garden should be tilled in the fall. Fall tillage can be useful to spread compost or to till stalks and other plant remains into the bed, where they can provide nutrients for the upcoming season. While most tilling is done in the spring, the garden can also benefit from fall tillage.
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