Do you need them?
If your soil is already dark, crumbly, earthy-smelling and full of worms, or if your plants seem happy and healthy, your soil may not need amendments. But most home gardens could use a little boost, especially for veggies, annuals and new plantings.
Test for nutrients and heavy metals
If you're not sure about your soil, or have any concerns about pollutants, The University of Massachusetts offers a range of relatively inexpensive quality testing. Contact the Oregon State University Master Gardeners to find out about additional options.
Compost is king
Compost improves soil structure – increasing drainage in heavy clay and water holding capacity in sandy soil. If not over-applied, it reduces polluted runoff into our rivers and streams. It's also teeming with beneficial organisms that make free fertilizer from air, rocks and decomposing plants. You can make your own compost from yard trimmings and food wastes, or buy it from the nursery. Here are some common ways to build better soil with compost:
- Mix 2 to 5 inches of compost into vegetable and flower gardens each year before planting.
- Make your own potting mix by using equal parts of compost and sand or Oregon pumice.
Other amendments help supplement compost
- Pumice and quarter-ten gravel are great for improving drainage. Both are locally produced and are better than perlite which requires a lot of energy to make, or vermiculite, which may have asbestos contamination.
- Aerated compost tea, available at some nurseries, or homemade compost extract add soil microbes without as much heavy lifting. Results depend on the quality of the compost the extract is made from.
- Mycorrhizal fungi is now sold in many nurseries and is thought to introduce beneficial microbes to your soil. In nature, these microscopic fungi attach themselves to plant roots, protecting and extending the root system, but their use in the garden is still somewhat experimental.
But some amendments may not be what they seem
- Polyacrylamide hydrogels are toxic and unnecessary. They contain small amounts of a nerve toxin called acrylamide. Hydrogels are often found in potting mixes, so look for ones without them or ones with yucca or cornstarch-based hydrogels.
- Avoid the peat moss/coir controversy. Peat moss is a non-renewable resource harvested from sensitive bog ecosystems, and its popular "greener" alternative is coir, a coconut husk fiber shipped from far away. Both are nice brown, fluffy seeding-mix-ingredients, but good compost is still king for amending garden soil.
Eggshells and lime
Eggshells are an easy and economical calcium source, and agricultural lime is good, too. These will add calcium, which is an important nutrient, especially for vegetable beds, and they help correct overly acidic soils. Dolomite lime also adds magnesium, another important nutrient.