Class I vs. Class II Compost: What’s the Difference and Which Should You Buy?

The Quick Answer

A1 Organics sells both Class I and Class II compost. Both are fully composted, stable, and mature. The difference comes down to what they are made from and how forgiving they are.

Class I compost is STA Certified compost made from plant-based materials like tree trimmings, leaves, and grass. It is lower in salts and passes a germination test, which makes it the safest choice for vegetable gardens, raised beds, containers, new plantings, and salt-sensitive plants.

Class II compost is generally made from manure-based materials. It carries more nutrients and more salts, so it works best on projects where the application rate is controlled.

Where Do Compost Classes Come From?

Unlike fertilizer, compost is not regulated for nutrient content, so two products labeled “compost” can be very different. To make products easier to compare, the Colorado Composting Council, the state chapter of the US Composting Council, publishes a Compost Classification Chart. It sorts finished compost into two classes based on lab testing using the Test Methods for the Evaluation of Compost and Composting (TMECC).

• Class I: USCC Certified STA Compost. Stable to very stable, generally made from plant-based materials.
• Class II: Fully composted, stable, mature compost, generally made from manure-based materials.

Products that are only partially composted, or raw materials like fresh manure, fall outside these two classes and are not ready for gardens, lawns, or landscapes.

For both classes, the producer must verify the compost was made in compliance with all local and state regulations, and must supply a current STA membership verification and a Compost Technical Data Sheet when requested.

How the Two Classes Compare

Why Salts Are the Deciding Factor in Colorado

Colorado State University Extension notes that root burn and plant loss from over-applying salty soil amendments is common, and that products made with manure or biosolids are often very high in salts. Compost made only from plant materials is usually lower in salts.

The classification chart reflects this. Class I has a maximum soluble salt level of 5, while Class II allows up to 15. That is also why Class II is not held to the germination test that Class I must pass.

For manure-based compost, CSU Extension recommends adding no more than 1 inch per season unless a soil test confirms salt levels, and avoiding these amendments in soils that are already salty or around salt-sensitive plants.

Which Compost Should You Buy?

Vegetable gardens, raised beds, and containers: Class I

Low salts and high stability make Class I the easy choice here. The classification chart lists vegetable gardens, raised gardens, and container mixes among its uses, and Class I can make up as much as 50% of the soil profile. Try EcoGro™.

Certified organic growing: EcoPlus™ Organic Compost

EcoPlus™ is OMRI Listed, which means it may be used in certified organic production under USDA National Organic Program regulations.

New lawns and sod: COMAND®

COMAND® is our STA Certified compost made specifically for turf. It’s a Class I compost, so it’s a low-risk choice for young grass and works well mixed into the soil before you lay sod or seed. A soil test before you order takes the guesswork out and helps you choose the right application rate.

Large commercial and agricultural projects: Call for a tailored evaluation

Large commercial and agricultural projects often involve unique soil conditions, bigger volumes, and specific growing goals, so a one-size-fits-all answer rarely fits. Call us for product recommendations based on your project.

Class Tells You the Type. Certification Tells You It’s Verified.

Because soil amendments are not regulated in Colorado, CSU Extension advises against assuming any bagged or bulk product is low in salts, and recommends asking suppliers for recent test results.

A1 Organics compost products are sampled and tested through the US Composting Council’s Seal of Testing Assurance (STA) program, which provides independent monitoring of compost quality. Test results are available on our Analytics page or by calling 800.776.1644.

How well any compost performs also depends on your soil, how much you apply, and how you work it in. The classification chart carries no implied warranty or performance guarantee.

Frequently Asked Questions