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Improving Garden Soil with Organic Amendments

Aged compost spread across garden beds in spring

You can improve your vegetable garden soil with organic amendments.

Organic materials such as aged compost will add nutrients to the soil. Organic matter will also help the soil retain moisture as well as help it to be well-drained.

You can add amend the soil with organic matter any time of the year. Good times to amend the soil with organic matter are in late winter or early spring a month or two before you sow seeds or set out transplants or in autumn after the harvest. You can do this by laying a layer of compost or other organic material across the soil—called sheet composting—and letting rain or irrigation carry nutrients deep into the soil. You can also add organic material to the garden during the growing season by side-dressing around plants with aged compost.

Sheet composting

Sheet composting is a method of adding organic material to the garden by spreading it across the top of the soil. Spread an inch or two of aged compost or organic planting mix (which is the name for commercial aged compost) across the bed then let it work itself into the soil naturally.

Turning the soil with a fork
Using a garden fork to turn the soil

Forking and turning the soil

An alternative to sheet composting is using a garden fork, rake, or shovel to gently turn the aged compost or amendment below the soil surface.

Tilling the soil

Some farmers and gardeners use mechanical devices to till the soil. This is a way to add amendments deeper into the soil. However, tilling too deeply can disturb earthworms and microorganisms at work beneath the soil.

“No dig” gardening

No dig gardeners do not till the soil. They use the sheet composting method to add organic matter to the soil layer-by-layer. By not digging or turning the soil they leave soil organisms undisturbed.

bucket of compost
Bucket of aged compost for side-dressing plants

Side-dressing during the season

Soil amendments also can be added to the garden during the growing season. Simply sprinkle or spread the amendments by hand around growing plants; this is called side dressing.

Soil amendments

Here are common organic soil amendments that will benefit your garden:

  • Compost. Compost is partially decomposed organic matter—leaves, grass clippings, dead plants, and vegetable kitchen waste (but not meats or dairy products). Compost is an important source of humus and is rich in nitrogen, phosphorus, potassium, and important trace elements necessary for plant growth. Compost has a crumbly texture and can be used as a soil conditioner and particularly when well finished—meaning decomposed–as a fertilizer. (Unfinished or partially decomposed compost can retard the germination and growth of some plants as it actually draws nitrogen from the soil to help the decomposition process.) A garden can never have too much finished compost.
  • Manure. Cow, horse, poultry, and rabbit manure are common sources of nitrogen and to a lesser degree phosphorus and potassium in gardens. Aged or rotted manure will turn to humus in short order. But don’t use animal manures too soon. Fresh animal manure contains salts which can burn plant tissue; “hot” (fresh) manure should be aged before being added to the vegetable garden. Dog and cat litter should be excluded from the garden or compost pile; that litter may contain disease organisms harmful to humans.
  • Green manure. Green manure is a crop that is grown in the garden and then incorporated into the soil to increase soil fertility or improve soil structure. Green manures are fast-growing plants. They include leguminous or pea-family plants (lupines, vetch, crimson or French clover which help fix or add nitrogen to the soil), cruciferous plants (mustard, turnips and winter rape which are grown for the bulk of their foliage), grasses (rye and Italian rye), and buckwheat. Green manure should be mowed or cut and turned into the soil just before flowering. Green manures decompose into humus. After the garden is harvested in the fall, planting a green manure cover crop will help prevent erosion, add nutrients to the soil, improve drainage, and choke out weeds.
  • Peat moss. Peat moss is partially decomposed mosses and sedges which are usually harvested or mined from boggy areas. Peat moss is not rich in nutrients and is slow to decompose but readily lightens heavy clay soil and helps dry soil retain moisture. Peat moss will not enhance soil fertility alone. The best peat moss is black and at an advanced stage of decomposition. ***Peat moss is a very slowly renewed resource and is best used when locally available not shipped to your garden from far away.
Aged compost in wheelbarrow
Aged compost

Aged compost advantage

Bulkier organic materials such as chopped leaves or straw and hay, fresh lawn clippings, or untreated finely ground tree bark will also help build the soil, but these materials decompose slowly. One inch of aged compost spread across the garden is equivalent to four inches (10 cm) of bulkier organic material. As a rule, add aged compost to your garden whenever you can.

Related articles of interest:

Your Vegetable Garden Soil

Soil Making the Kitchen Garden

Plant Nutrients

How to Improve Clay Soil

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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