Soil Amendments & Fertilizers

Create healthy soil

Fertile soil is the key to healthy plants – and easier gardening. It’s dark and crumbly and has a rich, earthy smell. It absorbs water like a sponge, breathes air like a lung and is teeming with life. A handful of healthy soil contains about 10 billion living organisms – far outnumbering the planet's human population! Soil fertility depends on these tiny creatures.

Build fertile soil with compost and mulch. Dig compost into the soil or use it on the surface as mulch. Other great mulches include arborist chips and autumn leaves. Organic matter like compost feeds worms, fungi, bacteria and other beneficial creatures that keep soil healthy and able to keep plants healthy.

Avoid soil compaction. Roots "breathe" and need air spaces. To improve compacted soil, mix organic amendments like compost into the soil. Create mounded or raised beds with walking paths in between and minimize tillage (especially when the soil is wet).

Soil Testing

Soil tests reveal the qualities of soil and what more it might need.

You can buy basic tests for major nutrients and pH, but a laboratory analysis is generally more reliable. Contact your local Cooperative Extension Service to find a soil fertility-testing lab.

Nutrients. Tests determine whether soil is deficient in any of the essential plant nutrients and what should be added.

pH. Most plants thrive between pH values 5 and 8. Northwest soils tend to be acidic (below pH 7) rather than alkaline (above pH 7).

Organic matter. Worms, insects and other soil creatures feed on soil's organic matter. Five percent organic matter in soil is ideal.

Biology. Specialized labs test soil for a wide array of beneficial soil organisms and suggest ways to increase their populations.

Choosing soil amendments (list)


The look, feel and smell are the best first indicators. Good compost:

  • Has a medium- to dark-brown color and a crumbly texture.
  • Isn't too wet or dry and may feel warm.
  • Has a mild, earthy odor indicating good maturity.

Other soil amendments

A soil amendment is any material mixed into soil to improve its chemical or physical properties.

Organic and inorganic soil amendments can both be useful. Organic amendments include barnyard manures, alfalfa meal, cover crops (green manure), grass clippings, compost, biosolids, sawdust and wood ash. They contain plant nutrients and act as organic fertilizers. Inorganic amendments are either mined or man-made and include vermiculite, perlite, crushed rock and sand.

Choosing fertilizers

Fertilizers provide essential plant nutrients including those listed below.

Nitrogen (N) promotes strong leaf growth. Buy slow-release nitrogen products called insoluble nitrogen. This is best for the plant; it assures a slower, steady supply of nitrogen.

Phosphorus (P) helps roots, flowers, seeds and fruits grow and develop.

Potassium (K) or Potash, is critical for overall plant health.

Secondary nutrients like calcium, sulfur and zinc foster strong stems and overall plant health.

Micronutrients such as copper and zinc contribute to plant health.

More is not better.  Too much fertilizer causes plant stress, and pest and disease problems. Fertilizers can run off into local waterways, polluting them.

Balance is the key. Don’t fertilize at all, unless plants or a soil test show a need. Only apply amounts recommended on the label.

Choose organic and natural fertilizers

Organic fertilizers are made from plant parts, animal wastes or byproducts, minerals or rocks. Look for "natural" and "organic" on the label.

Organic fertilizers usually have smaller N-P-K ratios (like 4-2-8 or 5-7-2). These products feed plants slowly and are more likely to contain essential micronutrients and vital organic matter.

Synthetic chemical fertilizers feed plants fast; they are often water-soluble and so get used up quickly or washed away. They are more likely to run off into lakes and streams or leach into groundwater, polluting the environment.

Time-release synthetic varieties pose less risk to the environment. Look for words such as "pelletized", "coated", or "slow-release" on the label.

Synthetic fertilizers often have higher N-P-K ratios (like 29-2-3 or 18-16-10) so it can be tricky to avoid over-fertilizing. They don't usually provide secondary nutrients or micronutrients.

Apply all fertilizers carefully. Some ingredients are serious water quality problems. Avoid fertilizing right before irrigation or rain so fertilizers don’t wash into storm drains, rivers, lakes or groundwater. Be careful not to get fertilizer on sidewalks, driveways or roads.

Grasscycle, it's free fertilizer!

Leave clippings on the lawn when you mow for a free supply of nitrogen and other nutrients. Save time and effort by not raking or bagging the clippings. Save at least 25% less on fertilizer too!

Lawn fertilizers

Lawns in the Northwest benefit most from a fertilizer with N-P-K ratios of 3-1-2, 3-0-2 or multiples of those. To help protect water quality, phosphorus in many fertilizers is banned in some locations. Look for "no phosphorus" lawn fertilizers with N-P-K ratios like x-0-x.

Weed and feed is a pesticide.

Herbicides are pesticides designed to kill weeds. Most weed and feed contains both a synthetic fertilizer and two or three different herbicides. Pull or spot-treat individual weeds instead of using weed and feed across your entire lawn. If you choose to use pesticides, pick a safer product using the brand-name product rankings in the Grow Smart, Grow Safe product tables.