Organic matter is the key to amending less-than perfect garden soil. To fix mucky clay or sandy sand soil, add plenty of organic matter. You can't change the type of soil you have, but adding organic matter makes your soil more like loam, which is perfect for plant roots. Even if you have loam, you still should add organic matter every year.
Organic matter improves garden soil in the following ways:
It helps loosen and aerate clay soil.
It improves the water- and nutrient-holding capacity of sandy soil.
It provides the once-living material that attracts microorganisms, beneficial fungi, worms, and other soil-borne critters that improve the health of your vegetables.
How to work organic matter into soil
Work some organic matter into your soil before you plant each season. If you're using unfinished (raw) organic matter like leaves or undecomposed manure, add it to your soil at least one month before planting. That way it will break down before you plant. Add finished compost and manures just before planting.
Follow these steps to add organic matter to your garden soil:
Add a 1- to 2-inch layer of organic matter to the area where you intend to plant.
Go for the higher end (2 inches) if your garden is new or if your soil is heavy clay or very sandy. Use less if you've grown there for years or if your soil is loamy and fertile.
You need 3 cubic yards of compost to spread a 1-inch-thick layer over 1,000 square feet.
Work in the organic matter to a depth of at least 6 inches.
There's nothing glamorous about spreading manure. The best way to spread organic matter is with a wheelbarrow and a shovel. Work it into the soil with a shovel, iron fork, or rototiller.
The best organic material to add to your soil is compost. Composting breaks down yard waste, agricultural waste, wood scraps, and even sludge into a crumbly soil-like material called humus.
Compost is usually clean, easy to use, and available. You can buy it in bags or have it delivered by the truckload. Most waste disposal sites make compost and sell it relatively cheap. You also can make your own compost.
Before you buy compost, ask whether the compost contains any heavy metals, such as lead, and whether the compost is safe to use in a vegetable garden. Your local health department should be able to tell you what levels of lead and heavy metals are unsafe. The folks at the waste disposal site also may even be able to give you a precise nutrient content if they've performed any tests on the compost.
Using sawdust and manure
Using organic materials other than compost — such as sawdust and manure — is fine, but these materials present a few problems that compost doesn't. Here are some advantages and disadvantages:
Sawdust adds organic matter to your soil, which eventually breaks down and forms humus. However, the sawdust also robs the soil of nitrogen when it decomposes, so you have to add more fertilizer to compensate.
Livestock manure improves your soil's nitrogen level. However, livestock diets often include lots of hay that's full of weed seeds, which may germinate in your vegetable garden. Some manures (such as horse manure) add organic matter and some nutrients to your soil, but they're also loaded with bedding materials (like dried hay) that cause the same problem that adding sawdust causes.
If you use manure, make sure it has been sitting around for a year or two, so it's decomposed, and the salts have been leached out. Too much salt in the soil can be harmful to plants. Good quality compost or fully decomposed manure should have a dark brown color, earthy smell, and have little original material visible.