Amendments change the soil’s body; fertilizers mainly add nutrients. Compost does a bit of both. This section makes more sense after you have read Introduction to Soil and Introduction to Fertilizers.

Garden stores sell both in the same aisle. Fertilizers are labeled with three numbers for nitrogen, phosphorus, and potassium — 4-6-4, for example. Soil amendments that bother to print N-P-K may read something like 0–0.5–0. They contain some of the same nutrients, in much smaller amounts. They are not an effective way to feed a hungry crop by themselves. They are how you build the soil that then holds and cycles those nutrients.

Why compost first — not raw waste

Amendments are made from a wide range of materials: cow or horse manure, vegetable scraps, grass clippings. The usual processing is composting. You might wonder why not dump manure and kitchen waste straight on the bed. Fresh manure is unpleasant, unstable (nutrients wash and evaporate), and can carry disease organisms that composting kills. Unprocessed high-carbon materials (leaves, wood, paper) can actually steal nitrogen from the soil for a while: soil life eats them and scavenges the nitrogen it needs from around the pile.

How much, and how to apply

Unlike fertilizers, which can be over-applied, compost is hard to overdo in a home bed — though a giant dump of unfinished material can tie up nitrogen for a while. Add at least a little to part of the garden every year, and to the whole garden if you can. Gardeners who make or buy enough compost sometimes spread as much as 2 inches (5 cm) across the whole plot each year.

Spread at the beginning of the season when you prepare a bed, or at the end when you put the garden to bed. Dump, rake to the thickness you want, then work it in with a spade or fork. It does not have to go deep — 8 inches (20 cm) or less is enough, where most feeder roots live. Get it off the surface so it breaks down where roots can use it. Compost on the surface still helps: it reduces disease-causing splash and feeds soil life.

1. Compost: the default. 1–2 inches on the surface, worked in or left as mulch.
2. Leaf mold: superb for water-holding.
3. Well-aged manure: nutrient-rich. Never fresh on food crops; never dog, cat, or pig manure in a vegetable bed.
4. Coconut coir or peat: lightens heavy soil. Coir is the less extractive of the two.
5. Lime (calcite or dolomite): raises pH. Test first.
6. Elemental sulfur: lowers pH slowly. Test first.
7. Gypsum: can help some clay soils; it does not substitute for organic matter.

Mulch can count as an amendment

Leaves, straw, newspaper, and cardboard used as mulch can be turned in when they are finished as a weed barrier. They add organic matter. They tend to be nutrient-poor — that is partly why they last as mulch. If you incorporate a lot of them and plant right away, add extra fertilizer so soil life is not eating the nitrogen your crops needed. Read this page with Buying and Applying Fertilizers. Long-season crops need to start in soil that is already decent — not in raw subsoil you hope to fix in June.