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Your Garden’s Most Important Soil
On this page, find how to improve your garden’s soil, from adding to its fertility to helping it gain better texture and water retention, including:
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- Soil: what it is and what it contains.
- The layers that make up the topsoil of a typical garden.
- How to improve water drainage in clayey soils and help hold water in soils that are too sandy.
- Using soil amendments, compost, fertilizer, and additives.
- A step-by-step demonstration of how to double-dig amendments into garden soil.
Good Topsoil is Key for a Great Garden
The top foot (30 cm) of garden soil is the most important to your plants and is the most biologically active layer. It’s literally alive.
Each time that you till your garden soil you incorporate large amounts of air into it, fueling a population explosion in the microorganisms it contains. It’s always important to add compost or well-rotted manure whenever you till in order to give these microbes an ample food supply.
Remember that your garden soil consists of layers that extend from the surface’s topsoil to other layers deep within the earth, ending in bedrock.
Right beneath the topsoil is a layer of subsurface soil that, while rich in mineral nutrients, is mostly devoid of life. When you till, it’s important to loosen each layer independently, but avoid mixing them together.
Why Soil Amendments are Important
As you work your soil, it’s also a good idea to think beyond simply adding organic matter, especially if the site drains too quickly or slowly [see Percolation Test].
To improve deeper soil, it’s necessary to work amendments and organic materials into the bed’s soil deeper than a typical shovel’s blade, using the process known as “double digging” shown here and discussed below.
A loosening agent, such as the mineral gypsum, helps prevent clay particles from sticking together. Your soil may need added garden lime or sulfur to raise or lower its pH, making your soil more alkaline or acidic.
Adding clay and silt to soils that are too sandy improve its texture, while those that have too much clay and are too heavy will benefit from additions of silt and sand plus greater quantities of decomposed organic compost.
Study the process of amending soil with a mechanical tiller in detail [see Incorporating Fertilizers and Amendments]. Garden tillers typically mix the topsoil layer, seldom reaching more than 8–10 in. (20–25 cm) deep into the earth.
When the entire bed has been double dug, spread your amendments, compost, and fertilizer in uniform layers on top of the soil, then mix it into the previously loosened top layer with a shovel, working down the length of the bed at right angles to the original trenches.
Step-By-Step Instructions
In the following demonstration, you’ll find the traditional method of loosening topsoil and incorporating amendments. It’s called “double digging.”
Remember that the purposes of the double dig is to loosen all of the soil quite deeply, rather than mix the top of the soil with the heavy mineral soil beneath it. The top layer has most of the active beneficial microbes and animal life that your garden’s soil contains, so burying it deeply would kill them.
You want to loosen the compacted deeper layers to add air and, when that job is complete, add and mix your amendments and fertilizers into the active zone just under the surface where plant roots typically grow.
You’ll find the process rewarding, and so will your vegetables when you plant them.
The Double-Digging Process
To loosen the subsurface layer and create more topsoil in your garden’s beds, it’s necessary to double dig the bed using a shovel.
While the double-digging process is simple, it requires substantial effort if a large area is to be prepared for planting. Most gardeners tackle part of the task in the autumn, part in the spring. Double digging in autumn has the added benefit of incorporating fresh manure into the beds and allowing it to decompose naturally over the winter months.
To double dig your beds, follow these easy steps and plan to spread the strenuous task over several sessions:
How to Double-Dig a Garden Bed
Dig a trench 1 ft. (30 cm) deep across the width of your bed. Place the soil from the trench into a wheelbarrow or garden cart, reserving it for later in the process.
Next, loosen the soil at the bottom of the trench and remove any rocks or debris, working an additional 9–12 in. (23–30 cm) deep.
Start a second trench parallel to the first, turning the topsoil layer from the second trench into the first trench, filling it half full with soil.
Loosen the subsurface layer found at the bottom of the second trench, breaking up clods and removing rocks and any debris.
Continue digging, loosening and filling new trenches across the bed until only the final trench remains. Loosen and turn its subsurface layer. Finish by filling the last trench with the topsoil reserved from the first row.