CORVALLIS, Ore. — Adding organic matter is the best way to improve nearly all kinds of soils. If your soil dries and cracks in summer, drains slowly, is hard to dig whether wet or dry, or if rhododendrons and other shrubs wilt in heat even with irrigation, it likely needs amendment.
Organic additions boost the water- and nutrient-holding capacity of sandy soils. In clay soils, they improve drainage and aeration and help soil dry and warm more quickly in spring.
Organic additions boost the water- and nutrient-holding capacity of sandy soils. In clay soils, they improve drainage and aeration and help soil dry and warm more quickly in spring.
Good organic amendments include wood by-products (sawdust, bark mulch), well-rotted manure, straw (grass or wheat), and compost. Confirm that any organic materials were not treated with herbicides, which can persist in soil and injure crops.
Inorganic amendments — such as pumice, perlite, vermiculite and sand — primarily act as spacers to increase porosity and aeration. Sand alone does not hold water or nutrients well and can make silt or clay soils more compacted; pair sand with an organic amendment (e.g., sawdust or compost) if you use it at all.
Manure and composting basics
Manure can add valuable carbon and nutrients, but fresh manure can burn roots and should not be applied directly to established beds. Compost it first.
- Mix manure with a nitrogen source (e.g., lawn clippings, vegetable scraps).
- Build a pile roughly 3 feet high by 2 feet across (aerated, well-drained site).
- Turn the pile about every two weeks — or whenever it heats above and then cools below 145°F — to speed decomposition and reduce pathogens and weed seeds.
How to incorporate amendments
Tilling or discing can speed incorporation — but avoid over-tilling, which can create a dense layer that impedes roots and drainage. One or two passes are usually enough to mix amendments into the upper soil and jump-start microbial activity.
- Spread 1–3 inches of composted organic matter over the bed.
- Work it into the top 6–8 inches of soil with minimal passes.
- Mulch after planting to maintain moisture and continue building soil.
Fertility considerations
Organic matter supplies a slow, steady trickle of nutrients; you can supplement with fertilizers based on a soil test. Keep synthetic nitrogen away from tender roots at planting time; add soluble nutrients later as needed and water them in.
Try a green-manure cover crop (Western Oregon)
A simple way to build soil is to plant a winter cover crop. For Western Oregon, crimson clover is an excellent option.
- Seed at 12 pounds per 1,000 square feet by Oct. 1.
- Water to establish before cold weather.
- Till or disc under in late April; expect roughly 3–4 pounds of nitrogen per 1,000 square feet returned to the soil.
Choose region-appropriate mixes east of the Cascades; your county Extension office can recommend options for Central and Eastern Oregon.
Quick checklist
- Add composted organic matter annually.
- Avoid materials with herbicide residues.
- Limit tillage to 1–2 passes.
- Use cover crops to build soil over winter.
- Pair any sand use with ample organic matter.
Learn more
- Improving garden soils with organic matter
- Growing Your Own: A Practical Guide to Gardening in Oregon
Printed copies are available at county Extension offices.
Previously titled Add organic matter to improve garden soils
