What To Know About Soil Amendments

Updated: Apr. 27, 2023

From our first-hand experience, learn how soil amendments work and which ones are right (and wrong) for your yard and garden.

In my new garden, with fast-draining sandy soils, I like to try any and every soil amendment available to improve it.

I visited a farm and picked up fresh chicken manure for my compost pile. What a stink.

I heard alpaca manure doesn’t need to be composted before use, so I tracked down an alpaca farmer and offered to clean out his stalls. Within a week, weed seeds exploded from this fresh manure. What a mess.

I heard how well biochar works. But I cheaped out, bought lump charcoal and tried mixing it into my garden soil. What a failure.

Good soil amendments can be magical. They can transform difficult soil from hard to soft, boost plant growth, make clay soils drain faster and help sandy soils hold more water. A really good soil amendment works like a slow-release fertilizer, gradually adding nutrients to the soil.

A bad amendment, however, turns soil to cement, pollutes it with herbicides and salts, or creates havoc with weed seeds. The following details will help you make the choice that’s right for your soil.

Inorganic Soil Amendments

Inorganic amendments like sand, silt and clay are simply rocks of various sizes. They will not decompose in your soil in your lifetime.


Sand is a commonly suggested amendment for clay soils. In my experience, adding sand to clay fails for two reasons.

First, adding enough sand to realistically change the texture of a clay soil is impossible. By definition, clay soil is 40 percent clay and 20 percent sand by volume. Perfect soils, like loams, contain only 20 percent clay and 40 percent sand. You do the math. It takes a huge load of sand to physically change a clay soil to loam.

Second, sand packs down and compacts easily. Like adding dry concrete mix to water, sand added to clay soil hardens in place.

Organic Soil Amendments

Over time, organic materials break apart in soil, releasing nutrients and adding stable organic matter as they decompose. Good organic soil amendments create better, easier-to-manage soil.


Compost made in your garden is the most valuable and available organic amendment. It’s ready to use when it’s cool to the touch and sifted to remove large particles.

Systems for home composting include piles on the ground and fancy rotating compost bins. I go with a Classic Speedibin composter because it’s easy to use and rat-proof. Naturally microbe rich compost will slowly acidify soil, feed plants and make soil softer as it increases the organic matter.


Worms are a natural part of most home-grown compost systems. But their manure, or castings, can also be purchased separately.

Sprinkled on top of the soil in a scant layer and worked in while planting or weeding, castings add minerals, inoculate soils with living micro-organisms and improve soil structure by boosting organic matter. Avoid buying pasteurized or sterilized castings; they’re biologically dead.


Wood of all kinds must be partially decomposed before adding it to the soil. Naturally low in nutrients when fresh, bark only releases its minerals after being digested by micro-organisms.

The best source for wood chips is a local arborist at work in your neighborhood. Find, pile and water fresh wood chips. Then they will heat up, turn brown and start decomposing. Wood will change soil chemistry, gradually acidifying it.


Manures, like wood chips, are better composted before use. “Hot” manures, or anything with lots of nitrogen (like fresh chicken or turkey), contain nutrients or salts that will burn sensitive roots. Rabbit and alpaca manures are low in salts, so theoretically can be added directly to the garden. But after my experience with alpaca manure and weed seeds, I’ve learned to compost every manure I bring into my garden. No exceptions.

Sadly, some manures contain herbicides. That’s because the cattle or horses ate hay from fields where Canada thistle was sprayed with Clopyralid herbicide. This chemical doesn’t break down in a cow’s stomach or in the composting process. Gardeners in Seattle discovered 20 years ago that even professionally composted materials still contain herbicides.

So what are the chances a home gardener can do a better job? Like compost, manures are naturally microbe rich and will slowly acidify soil. Manures also add nutrients, sometimes in a less than perfect balance, while increasing the soil’s organic matter. Use carefully.


Biochar is best when manufactured specifically as a soil amendment. Carbon-heavy materials like wood, corn stalks or straw are burned at specific temperatures over a measured amount of time. Correct production will retain the natural pore space of burned materials, producing a particle size perfect for amending soil and minimizing pollution.

Biochar improves soil moisture holding capacity, has a liming effect on soil, improves nutrient retention, softens soil and boosts harvests.

I mistakenly thought lump charcoal and specially prepared biochar were the same thing. The hardwood lumps are simply too large to amend soil and too hard to crush into tiny uniform pieces. My second mistake was forgetting to co-compost the biochar with organic amendments, so it connects to minerals in the compost instead of pulling them from your soil.

How Organic Amendments Work

Amendments do not change soil texture, but they can and do improve overall soil structure. Boosting biology by using unpasteurized composts, worm castings and co-composted biochar makes all soils softer and easier to work.